The Examination of Prospects: A Mining Geology

C. Godfrey Gunther · 1912 · Mining Geology

How to geologically evaluate mineral prospects — systematic sampling, assay interpretation, structural controls, and forming professional opinions.

Public-domain full text preserved in the Mountain Man Mining Library. Original source: archive.org.

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The

Examination Of Prospects

A Mining Geology

By C. Godfrey Gunther, E. M.

Author Of "Blectro-Maqnetic Ore Separation"

First Edition Second IvFs.siQN-r-CojiREr:'i;E9

McGRAW-HILL BOOK COMPANY, Inc.

239 West 39Th Street, New York

6 Bouverie Street, London, E. C.

Thenl Pu

ASTOR, LENO: /.MD TILDEN FO;:.>iDATiONS n 1919 L

Copyright, 1912, by the McGraw-Hill Book Company

Printed And Electrotyped

By The Maple Press

York. Pa.

Preface

The purpose of this book is to present the practical side of economic geology concisely and in convenient form; established facts and the applications of accepted views are emphasized; theoretical discussions and questions of genesis are avoided. Coal, iron, and placer deposits are omitted; they are subjects of specialized study that are fully and concisely treated in other works. The reader is assumed to possess a knowledge of mineral- ogy, petrography, and of elementary geology.

The arrangement adopted is based on Kemp's theory of mag- matic waters, Lindgren's conclusions on hydrothermal and second- ary alterations, and on the theories of secondary enrichment enunciated by Emmons and Weed.

No general classification of ore-deposits is attempted, nor is any attempt made to fill by hypothesis the wide gaps, nor to explain the apparent contradictions, that rank economic geology among the most inexact of sciences. The present knowledge of the subject is too incomplete to warrant such broad generali- zations.

The present situation in mining in the United States may be summed up in the statement that the demand for good properties greatly exceeds the supply. While mines of the first rank will undoubtedly be discovered from time to time, it is probably true that a great proportion of deposits having outcrops of commercial grade or of evident promise have been recognized and explored. A review of mining conditions over long periods shows that the rich discoveries belong to pioneer days, and that as time goes on the more important developments are the result of lower working costs, improved metallurgical processes, and of an increasing knowledge of economic geology.

As engineers in search of developed mines no longer find properties having positive ore oi gresAeT neX, "Ociax.

vi PREFACE

price asked, so those in search of prospects should not expect to find proven ore-shoots awaiting their recommendation. There is usually abundant local capital for the preliminary development of a patently good prospect, and most of these are steadily worked from the time of their discovery until some apparently unfavor- able development shuts off the supply of local capital. A great majority of prospects have been examined again and again, pre- sumably by men who commanded a knowledge of sampling, the services of an assayer, and at least an elementary knowledge of geology. In order to pick a good prospect from those rejected by his predecessors, therefore, an engineer must base his hope of success upon superior geological training.

A careful search has been made of the voluminous bibliogra- phy of economic geology, and the results of this search in great part make up the present work. Individual credit is given in foot-notes wherever it may be assigned.

C. Godfrey Gunther.

Stratford, Conn., June, 1912.

Contents

Pagb

CHAPTER I. — Mining Examinations 1

Formal examinations — Preliminary examinations — Examinations for the rescue of badly expended capital — The examination of pros- pects — The examination of antiguas — Price and terms of sale — The exploration of prospects — Preliminary search of bibliography — Apex and title — Regularity of the deposit — Condition of hang- ing wall — -Necessity for an accurate survey — Refractory ores — ' Amount of exploration compared with results attained — Pre- paring a property for examination — A mine dressed for exam- ination — Past production — Low-grade ores — Large versus email properties — Sampling — The equipment for sampling — Marking the samples — Preparation to sample — Placing the samples — The size of samples to be taken — Salting — The shipment of samples — Resampling — High assays — The calculation of results — Stoping width — Hand picking — Metallurgical losses — The cost of mining and treatment — The estimation of ore reserves.

CHAPTER IL— Structural Geology 26

Stratification — Cleavage — Fissility — Schistose structure — Gneissic structure — Joints — Sheeting — Folds — The development of faults from folds — Fractures — Flexures — Faults — Exploration for the continuation of a faulted ore-body — Post-mineral fissures — Pre- mineral fissures — The expression of faults in topography — The importance of aereal geology — Exploration c!,nd development.

CHAPTER in. — Structural Features of Ore-Deposits 43

Veins, lodes and ledges — Fissure veins — Lodes — Ledges — Gash veins — Bed veins — Compound veins — Contact veins — Veins along dikes — Lodes along sheeted zones — Stringer lodes — Fault lodes — The mineraUzation of joints — Breccia lodes — Shear zones — Stock- works — Stocks — Pipes or chimneys — Branching veins — Linked veins — Conjugate veins — Overlapping veins — Systems of related veins — The persistence of veins in depth — The relation between depth and the number and character of veins — Mineral veins fol- low fissures of small displacement — The influence of country rock on vein structure — The distinction between intercalated and fissure veins in schistose rocks — Fissures and lodes formed subse- quent to the principal mineralization.

vu

viii CONTENTS

Pi

CHAPTER IV. — Primary Ores and their Distribution

Metallogenetic epochs and provinces — The distribution of ore

"fj deposits in individual mining districts — The association of ore

deposits with certain rocks — The depth to which primary ores

I persist — The criteria of primary ores — The minerals of distinctively

J primary origin — Distinctively primary minerals — Minerals both

primary and secondary in origin — Distinctively secondary minerals

— The primary associations of metals — The accessory minerals

that commonly indicate a segregation of values — Primary gold

ores — Primary copper ores — Primary silver ores — Primary lead

ores — Contact ores — Primary zinc ores — The depth of primary ore

deposition — Deposits formed at the surface — Veins formed near

the surface — Veins of deep-seated origin — Relative susceptibility

of hanging and foot walls to mineralization.

CHAPTER v.— Types of Primary Ore-Deposits

Magmatic segregations — Contact deposits — Pegmatitic deposits — Fahlbands — Regionally metamorphosed Oi-e-deposits — Deposits due to the filling of open spaces — Replacement veins — Replace- ment deposits— Disseminated mineralizations— Conglomerate beds — Bedded ore-deposits.

CHAPTER VI.— Primary Ore-Shoots 1

The factors that determine primary ore-shoots — The relative value of general and local data — Primary and secondary ore- shoots — Terms used to describe the dimensions of ore-shoots — The shapes of ore-shoots — Lenticular ore-shoots — The behavior of primary ore-shoots in depth — The decrease in value with depth — Predicting the depth to which ore-shoots may continue — The depth at which ore-deposits form — The structural features that influence ore-shoots — Ore-shoots due to available open space — Ore-shoots due to intersections--Ore-shoots due to impounding of solutions — Ore-shoots at the tops of anticlines, or "saddle- reefs " — The chemical influence of wall rocks on ore-shoots — Ore- shoots in veins of deep-seated origin.

CHAPTER VII. — The Primary Alteration of Wall Rocks . . . . 1 Metamorphic processes — Dynamo-regional Metamorphism — Con- tact metamorphism — Hydrothermal metamorphism — Propylitiza- tion — Sericitization — Alunitic alteration — Alteration to greisen along tin veins — Silicifi cation — Marmorization — Dolomitization.

CHAPTER VIII. — Alterations by Surface Agencies 1

The decomposition and ' ''xg of rocks — Leaching — Kaolini-

CONTENTS ix

Page zation — Oxidation — The migration and enrichment of metals — Upward migration — The influence of circulation channels on secon- dary alterations — The relation to oxidation and enrichment of topography and water* level — The irregularity of oxidation and enrichment — The zones developed by surface agencies — The depth of vein leached to form existing enrichments.

HAPTER IX. — Residual Ores and their Distribution. . . . 168 The precipitation of ores in the zone of oxidation — Oxidized ores of copper — Oxidized ores of lead — Oxidized ores of zinc — Residual shoots of gold ores — The distribution of silver by oxi- dation — The distribution of manganese by oxidation.

HAPTER X. — Secondary Ores and Ore-Shoots 173

The criteria of secondary ores — Secondary ore-shoots — The effect of structural features on secondary ore-shoots — The effect of the water level on secondary ore-shoots — The effect of chemically active wall rocks on secondary ore-shoots — The effect of por- osity on secondary ore-shoots — The effect of primary mineralization on secondary ore-shoots — Ores containing both sulphide and oxi- dized minerals — The enrichment of copper — Chalcocite enrichments — Disseminated chalcocite enrichments — Chalcopyritic enrich- ments — The enrichment of gold and silver with copper — The en- richment of silver — The enrichment of gold — The enrichment of lead — The enrichment of zinc — The enrichment of lesser metals — The migration of gangue minerals.

IIAPTER XI.— Outcrops 196

The relation between length of outcrop and persistency of vein in depth — The relation between size of outcrop and width of vein in dfepth — Breccia tion and post-mineral fracturing — Meandering of outcrops on hillsides — Down-hill creep — The topographical expres- sion of mineralization — Outcrops of deposits formed at slight depth — Porosity of outcrops — Casts in resistant gangue minerals — The composition of outcrops — The oxides of iron in gossan — The condition of outcrops indicative of secondary enrichments in depth — Rock alteration as a guide to ore-deposits — The outcrops of kao- linized rocks — The outcrops of contact deposits — Deposits of sur- face origin — Microscopical examination of specimens.

The Examination Of Prospects

Chapter I

Mining Examinations

Mining examinations are of several kinds and the scope of the investigation depends in each case upon the purpose for which the examination is made.

Formal Examinations. — formal examination of a developed mine is an expensive undertaking; from one to several months are allowed for the work according to the size of the property; many- samples are taken; investigation is made of the geological fea- tures, including, perhaps, a topographical and geological survey of the surface; the metallurgical treatment of the ore is studied; finally, a determination of costs is demanded.

Formal examinations are made for prospective purchasers or as a basis for the consolidation of several properties, and occa- sionally, on the owner's behalf, to verify the work of resident engineers, or to determine the readiness of a mine for equipment and the kind and capacity of equipment it shall receive. A formal examination of a mine should not be undertaken until a preliminary examination has shown that it is justified.

It is regrettable that some engineers are in the habit of prose- cuting long and exhaustive examinations of properties whose lack of merit should be, and perhaps is, apparent from the start, or of continuing their investigation long after an unfavorable result is seen to be inevitable. This wasting of a client's money is a species of trickery difficult to prove, and therefore the more contemptible.

Preliminary Examinations. — Preliminary examinations are

2 Examination Of Prospects

the advisability of making a formal examination. There is always a reason why a mine is offered for sale, which may or may not be known from the start; frequently the chief object of the preliminary examination is to determine this reason for selling.

A preliminary examination usually includes: the cutting of a few significant samples, the number depending upon the nature and the quantity of ore claimed by the owners; a geological recon- naissance, usually involving a map; a preliminary study of the probable metallurgical treatment of the ore; and a tentative estimate of costs. By significant samples are meant samples of the claimed ore reserves, taken at regular but longer intervals than would be allowable in a formal examination, and especially samples of the lowest workings, ends of drifts, and other points where a falling off in value may be suspected.

A preliminary examination should, of course, be planned so that the results obtained may be supplemented by, and need not be repeated in the formal examination that may follow. Data that is sufficiently reliable for use in a formal report is necessary for a reliable preliminary report; the work should not be slighted because it is to be checked if it yields a satisfactory result.

Examinations for the Rescue of Badly Expended Capital. — It frequently happens that an engineer is called in only after funds are exhausted, ore reserves depleted, metallurgical processes failures, and ruin appears inevitable. This is a situation that requires special aptitude; valuable clients are at stake as well as the recovery of a part of the ill-advised outlay; furthermore, what under the circumstances may be considered a flattering salvage is almost certain to appear small to the owners after the glowing promises of the mistaken promoters.

In examinations of this kind the work should be concentrated upon the most favorable showings, a careful review of the metal- lurgical processes should be made, and, in fact, every loophole that promises salvage should be investigated. Where large bodies of low grade, unpayable ore have already been developed, hand picking should be tried in the hope of securing a cona- mercial product.

Mining Examinations 3

The failure having perhaps been due to haphazard exploration, the geological relations of the ore should be carefully deter- mined, and all stopes should be surveyed, as it is probable that the best ore was mined and that nothing really good was left. This data, together with assay plans of the existing ore and all geological features, should be entered upon a large-scale map, upon which relations will become clear that otherwise would not suggest themselves. A little intelligent exploration based upon such a map will in many instances yield important results.

The Examination of Prospects. — The examination of a prospect is a very different undertaking from the examination of a mine; prospects are not expected to show ore reserves as a basis for purchase, and in the last analysis the recommendation of a prospect rests on an opinion rather than demonstrable facts.

The examination of a prospect requires that all significant samples shall be taken and a thorough geological investigation be made, which need not, however, be put in formal shape unless it yields a favorable result. The question to be answered in examining a prospect is: " What chance has it to make a mine?"

The same hesitation should not be felt in recommending that a prospect be acquired under option as is justifiable before advis- ing the purchase of a developed mine. It should be remembered that the majority of prospects have been examined many times, and that no brilliant showing of payable ore will be encountered ; any fairly consistent showing of geological promise is worth con- sidering, and merits the expenditure of a few hundreds or a few thousands of dollars in preliminary work, which, if it yields a favorable result, may be followed by more serious development.

Mining booms, even if founded oq mistaken estimates, fre- quently lead to the discovery of valuable properties; this is due to the fact that the excitement of a boom leads to much shallow prospecting, which often exposes conditions not evident from an inspection of the immediate surface. Test pits and trenches may be considered as halfway between examination work and exploration, and while inexpensive, they not infrequently yield important results.

4 Examination Of Prospects

The Examination of Antiguas. — The presence of ancient work- ings should not be assumed to indicate a valuable property. Those that have had much to do with antiguas in Mexico have respect for the ability of the pioneers to follow, to extract, and to treat ores. These properties were worked with slave labor; a certain proportion of the force being detailed to grow food for the miners, the labor cost of mining was practically nil; very low grade material, therefore, could be mined, as almost any recovery from the ores was profit. Similar conditions probably obtained wherever mines were operated by the Spanish, or by the ancients.

No antigua should be expected to show high-grade ore; the Mexicans, for example, are expert miners, and will be found to have removed all pillars or other payable ore that was left by the pioneers in any property now sufficiently open for examination.

The first step in the examination of an antigua is to make a survey and to map all the workings, placing on the map all the geological data obtainable. The pioneers did not understand faults, and never drove exploratory cross-cuts; not infrequently a detailed geological study of an antigua results in finding im- portant ore-bodies.

By means of samples, the grade of ore left in the mine by the pioneers can be ascertained, and this will' often determine whether or not the property may be expected to yield a profit with the application of modern metallurgical processes ; all ore above this minimum grade may be assumed to have been extracted.

The pioneer miners could not follow the ore very far below water level, and any antigua, the ore in which is primary, offers an attractive opportunity; in most cases, however, the ores mined were secondary, and probably do not continue far below the old workings before changing to low-grade primary material.

In the examination of antiguas the mode of bringing the pre to the surface should be carefully considered. The pioneers raised it on men's backs, and their workings — following the ore — are tortuous. It sometimes proves cheaper, where the tonnage is small, to pursue this method; the cost of a straight' shaft and

Mining Examinations 6

hoisting equipment must be apportioned against each ton of ore, and modern equipment may prove in the end more expensive than the ancient method.

If the pay shoots in an antigua are short and the stopes narrow, the chances are greatly against a profitable outcome for further exploration.

Price and Terms of Sale. — The final object of a mining exam- ination being a profit for the client for whom the work is done, the price and terms asked are considerations second to none; in the United States they are too often but slightly considered, and sales of undeveloped properties are made at prices that largely discount even a decidedly favorable outcome for the proposed development.

A great majority of mining examinations lead to unfavorable reports. This is owing to the fact that the demand for good properties greatly exceeds the supply, and to the great difference between the points of view of the owners of mining properties and the examining engineers. The owners generally suffer from extreme optimism, and many engineers from excessive profes- sional timidity, and neither is willing to meet the other halfway. Owners of prospects are usually brought to their senses after repeated unfavorable examinations, but many engineers never make a favorable report because of the risk of personal reputation. Mining is essentially a risky business, and he who declines to accept some risk will not make money for his clients.

No engineer should expect to find a mine having ore of a greater net value than the purchase price asked, unless the mine is admittedly bottomed, and has no possibilities beyond the ore already developed; this is a rare case, and most developed mines in addition to their ore reserves hold a certain promise for further tonnages of payable ore; each case must be considered on its own merits, and how much to allow for the value of prospective ore will differ with every engineer.

Mr. J. H. Curie* gives as his rule that 66 per cent, of the pur- chase price should be represented in net value of ore reserves, "The Gold Mines of the World/' p. 49.

/

z'

6 Examination Of Prospects

with the lower levels still looking well. On the basis of 66 per cent, of the purchase price to be represented in ore reserves, most sales are made at too high a figure, which is borne out by the fact that most mining ventures are not profitable, the balance being established by the occasional large success that repays its capital many times over. These considerations apply chiefly to the sales of properties in an advanced stage of develop- ment and not to prospects, with which this work chiefly treats.

The Exploration of Prospects. — In the exploration of prospects it should be expected that many ventures will prove failures, as the exceptional success will more than repay a number of unsuc- cessful ventures. The exploration of prospects should be undertaken only when the operators have abundant capital, and the pertinacity to acquire and explore several properties suc- cessively. In such a campaign it is evident that the costs of each exploration should be kept as low as possible, and that exploration should cease as soon as the chances of success are reduced below the apparent chances when the property was acquired for exploration; persistency beyond a certain point is a failing.

In order to keep the costs low, cash payments should not be made on prospects, nor should expensive machinery be installed, as such expenditures in an enterprise of this sort must be charged against the footage of work done. It is almost always advisable at the start to follow the ore, and with as cheap temporary equipment as will answer the immediate purpose.

While it is true that the speculative chance held by a good looking prospect has a certain cash value, only in rare cases should a cash payment be made; a monthly rental, or a " salary" to the owner, during the period of exploration, frequently offers a convenient compromise.

Preliminary Search of Bibliography. — If an examination is to be made in an established district, a search of the bibliography of the district is an important preliminary to the examination. Written statements are more likely to be accurate and corylserva- tive than spoken statements, and important data are freueKf

Mining Examinations 7

obtainable. If the library is well indexed, a search of this kind is not a long undertaking.

Apex and Title. — The questions of extralateral rights and title are of prime importance in the United States. A preliminary investigation by the examining engineer usually consists in questioning residents. In the event that a sale is to be made, these points should be investigated by a lawyer. The slightest chance of legal entanglement is a sufficient basis upon which to drop a negotiation, for if a mine proves successful, the number and backing of the contestants will be multiplied.

Regularity of the Deposit. — The regularity of a deposit together with its size determines the cost of development. Of two depos- its that contain a like quantity of payable ore, the regular deposit is by far the more valuable.

Unless the development of a mine is complete, which is rarely the case, the amount of ore it contains is not known until after it is worked out. A regular deposit is cheaply and rapidly developed and its value may be clearly indicated and realized through sale; a majority of irregular deposits, however, probably do not repay the cost of the development necessary to indicate their value, and must be worked by following and extracting the ore as it is found. An example of this type of property is the Butte Mine at Randsburg, California, which has produced $525,000, although it is said not at any time to have had more than $5000 worth of ore exposed; many lead deposits in lime- stone are of this type.

Examples are numerous of deposits whose regularity permits the working of very low-grade material : the so-called " porphyry- copper" deposits are of this class, as are also deposits of the type of the Alaska Tread well; the regularity of these deposits con- sidered as a whole (they are locally quite irregular) permits cheap development, which in turn permits the construction of the large plants necessary to treat such low grades of ore.

Certain mines in the Catalina Mountains, Arizona, are good examples of large but irregular deposits; these properties have

Bull 430, U. S. G. S., p. 40.

8 Examination Of Prospects

produced large quantities of payable ore, but at no time was sufficient ore exposed to warrant the construction of a IG-mile railroad connection, or a smelting plant: as a result, the ore produced must be hauled to the railroad in wagons and shipped to a custom smelter at heavy cost, and the mines are intermit- tently operated.

Condition of Hanging -Wall. — strong, firm hanging-wall is not infrequently a controlling factor in the operation of large, low-grade deposits, where the cost of timbering cannot be borne by the ore. The Granby, and other mines in the Boimdary District of British Columbia, probably could not operate if it were not for the fact that their large stopes remain open without artificial support: the mines of Douglas Island, Alaska, are other examples, and the firm hanging-wall is a factor of the greatest importance in the success of the Pilares Mine, at Nacozari. Sonora, and of many other properties that contain large deposits of low-grade ore. Most of the so-called "porphyry-copper" deposits have to contend with heavy hanging-walls of soft altered porphyry, and this disadvantage, it is thought, will become increasingly apparent as underground mining is con- ducted on a large scale.

Necessity for an Accurate Survey. — survey and map are imperative to a proper understanding of any but the simplest occurrence of ore. All the geological data should be placed on the map, without which they are likely to be unintelligible. For a preliminary survey, work done with a Brunton, or other pocket transit, is of sufficient accuracy. The writer has made satis- factory maps without an instrument, using a straightedge on a large sheet of paper fastened to a smooth surface, afterward taking off the angles with a straightedge and triangle.

Refractory Ores. — That an ore is refractory is often apparent on simple inspection, as, for instance, a lead-silver ore that carries much zinc, or copper in an ore for which cyanidati on would be natural treatment. A general knowledge of metallurgy is necessary to the examining engineer, and while a discussion of the details and costs of treatment are out of place in a report on a

Mining Examinations 9

prospect, the amenability of the ore to metallurgical treatment should be borne in mind.

In the examination of a property the ore from which is to be concentrated, the probable ratio of concentration should be con- sidered: if the valuable constituent occurs without accessory heavy metals, the ratio will, of course, be high; if the ore carries a large quantity of, for example, barren pyrite in addition to the valuable constituent, the ratio of concentration will be low, as will be also the grade of the concentrates. This condition is said to be a source of disappointment in certain large copper mines in Nevada.

Furthermore, ores whose valuable metals are present in both oxidized and su phide form are difficult to concentrate, because <3f the widely differing specific gravities of their valuable minerals.

In an ore in which a part of the valuable metal is contained in soluble form, the amount so present must be considered a net loss, and the assays must be correspondingly diminished: an example of this condition that is often met in arid regions is a copper ore of concentrating grade that contains a part of its cop- per as chalcanthite; where such an ore exists in large deposits, leaching may offer a profitable method of treatment.

Amount of Exploration Compared with Results Attained. — The relation between the amount of work done and the quantity of ore exposed is an important one; a given quantity of ore being exposed, the result may be considered satisfactory or unsatis- factory according to whether little or much exploration was necessary to develop it

In levels from a timbered shaft it is important to note whether they were driven at regular or irregular intervals; if the latter, it is more than likely that the widest and best parts of the deposit were selected in which to drive the levels, and that they do not, therefore, indicate the average character of the deposit .

Preparing a Property for Examination. — This is a subject that the owners of mining properties would do well to study. The fact that the surface offers a complete sectvox. oi ort-ijs''s.

10 Examination Of Prospects

and of their surroundings is often lost sight of; before attempting deeper work it is well to explore the surface thoroughly with trenches and test pits. If no ore is found by this work, and no indication found of residual conditions suggesting that ore once existed at this level, then deeper exploration is not warranted.

Where the ore or gangue mineral is harder and more resistant to weathering than the enclosing rock, the outcrops are usually bold and but little surface work is necessary, but where the ore or gangue is softer than the enclosing rocks, the veins or ore-bodies do not outcrop, and surface workings are necessary to expose the deposits for examination.

The surface is always a fair criterion of the conditions in depth, if interpreted in the light of the present knowledge of impover- ishment and enrichment due to surface agencies.

Development should follow the ore or vein; there is nothing more provoking than to be asked to examine a good surface showing somewhere beneath which a ''cross-cut" tunnel has been driven that throws no light on the conditions at that depth, but inferentially discourages further work.

In preparing a mine for examination, the ore should be cross- cut to its full width; no engineer will allow for ore that remains "in the wall.''

A Mine Dressed for Examination. — A mine that has been dressed for examination may be described as a trap for the ex- amining engineer. The favorable features are accentuated, and the unfavorable developments are concealed, — drifts are walled up, or are allowed, or encouraged, to cave, winzes are allowed to fill with water, and workings through poor stretches of the deposit are tightly timbered to hinder examination. In stoping opera- tions barren material is removed up to the best showings, which are left undisturbed, on the supposition that the engineer will assume that the material stoped was payable ore. Exploratory drifts are frequently stopped in good ore ; it is astonishing in how many cases a man who is familiar with the local ore-shoots can stop his drift just short of running into barren ground.

Mining Examinations 11

Vigilance and a suspicious attitude are the engineer's safe- guards against this kind of fraud. A method followed by the writer is almost sure to result in detection if trickery has been attempted. On going through the mine many questions are asked in regard to every point that suggests itself, and the answers are set down. The questions are repeated and answers noted from as many of the chief men on the ground as may be induced to answer them. After an interval the questions are repeated, but not in the same order, and the answers are again noted. Before leaving the property a third inquisition will yield the desired result. If the local representatives have been telling the truth, their answers will check up, but it is a very exceptional liar that can stick to a fabric of falsehoods three separate times with long intervals between.

Past Production. — study of smelter or mill returns from past ore shipments in connection with the amount of exploration done is often instructive in the examination of irregular deposits. The average value of past ore shipments, of course, is no criterion of the grade of ore left in the mine, as the best ore is invariably extracted first.

Low-Grade Ores. — A slightly explored property carrying low- grade ore that may be expected just to pay its way should be explored, as higher-grade ore may be found. Any large deposit of even very low-grade material should be given at least a pre- liminary examination, as the constant improvement in metal- lurgical processes is steadily rendering payable ores of lower and lower grade. The waste of 15 years ago is the ore of to- day, and the same advance may be reasonably expected in the future.

Large Versus Small Properties. — In general, a large body of low-grade ore is more likely to persist than a smaller body of high-grade ore. Small mines, unless containing shipping ore and requiring little equipment (and these are rarely purchasable at a reasonable price), are usually not profitable. The cost of all equipment must in the final analysis be charged against the

12 Examination Of Prospects

profit where two mines each containing one-half the tonnage of the same grade may both net losses. Furthermore, a small prop- erty is in a poor strategic position to secure good smelting or freight rates.

Sampling. — discussion of sampling, although fully treated elsewhere, cannot logically be omitted; inasmuch as proper sampling is often neglected, the subject will bear repetition.' Sampling is expensive work if properly carried out, and no other kind of sampling is of any value.

The ideal sample is a uniform groove, or channel, across the full width of the ore, and no more; how closely this may be approached in practice will depend upon the material sampled and upon the time and care given to the work.

The Equipment for Sampling. — hammer and moil are prefer- able to any other tools in cutting samples; a prospector's pick will do good work in soft, uniform ground, but in harder material, even if the po'nt and hammer-end are alternately used., is likely to have a selective effect, and samples taken with a pick are not above suspicion.

To catch the sample a cloth is best, spread out so as to catch all chips. If the ground is loose and masses are likely to fall, the sample is best caught in a box, which is also used where fine, rich material is likely to sift out of cracks and vugs and so find its way into the sample.

To break down samples a crusher is convenient, but two large, tough stones of barren rock such as may be found in any creek bed, one to lay the ore upon and the other to pound with, yield the maximum result for coarse breaking; unless the ore is very hard, the abrasion of the stones is negligible.

To cut down samples, rolling and quartering on flexible oil- cloth is a good method, but where many samples are to be taken,

Sampling and Estimation of Ore in a Mine," T. A. Rickard and others.

2 Credit cannot be given to all those that have written on this subject for the many practical points that come up as obvious solutions of the minor problems of mining examinations.

Mining Examinations

a Jones sampler with four pans, or a riflBle, is quicker and more certain to do accurate work.

A simple apparatus to permit the cutting of samples in an untimbered shaft has been used by the writer with great saving of time and expense over erecting platforms. This consists of a short seat, about 14 in. long by 6 in. wide, to which is rigidly fastened a pole in length about one and one-half times the width of the shaft to be sampled. This seat is fastened to the end of the windlass rope, and, straddling the rope, the sampler is lowered to the point where a sample is to be taken, meanwhile holding the pole parallel to the rope so as to permit the descent. Arriving at the point where the sample is to be taken, the pole is allowed to fall against the opposite i — i

wall, the end of the pole catch Notch A . ing in an inequality of the rock, the seat is hoisted a few inches, and the sampler, with feet braced against the face sampled, is held firmly and safely in position, with both

for Rope

Windlass Bope

Sid&Yieft:: - Fig. 1. — Sketch showing sampling

I uj. 1. pole should be bolted firmly to the seat.

pie may be caught m a bag

held between the feet, or a canvas receptacle may be rigged in front of the sampler, or the sample may be allowed to fall on a canvas spread over the bottom of the shaft, which should be protected from the impact of the falling ore by a few boards; the latter is usually not a safe method unless the men at the wind- lass are closely watched, as salting by them would be an easy matter.

Marking the Samples. — Durable tags carrying the number of the sample should be inserted in each sack; some engineers use metal tags, some wood, but the usual practice is to use tough paper rolled up tightly to prevent abrasion. The following sample tag is excellent, and should be made up in books contain- ing 50 sheets and numbered before going into the mine. The

R. C. Gemmel, "Sampling and Estimation oi Ot'ycl a.'vcifcj' ."SSSi

Examination Of Prospects

lower part of the tag is torn off, and after being rolled up, is inserted in the sack with the sample. Tearing the detached slip in half gives two nunAers for the duplicates when the sample is

Date

Sample taken

At point

From

Across

For Ft In

Measurement: At right angles to dip, vertical, horizontal.

To

Dip Strike

No

(perforation here)

No

No

cut down. On the backs of these slips the writer is accustomed to make a sketch in section of the place sampled, showing the shape of the drift and the sample cut by a dotted line; the geological features may also be indicated with colored pencils; and any remarks noted. The data is thus kept in convenient form for future reference and is invaluable in case of dispute; it fre- quently happens that an unfavorable report is questioned or disputed, and in any controversy regarding samples these books so kept will put the adversary to rout on sight.

The writer places the sample number on the outside of the sacks to permit ready identification without having to pour out the sample in search of the tag. This practice is objected to by some engineers on the ground that it permits an outsider to locate the samples with equal ease. If an outsider gets close enough to the samples, and the leisure in which to inspect the numbers, he is close enough to tamper with them, and this objection is not, therefore, considered as having weight.

Preparation to Sample. — Samples should be accurately referred to some permanent object, such as a cross-cut, winze, or survey

Mining Examinations 15

station. The intervals between samples should be measured along the center of the drift, as they otherwise will differ widely according as they are measured on one side of the drift or the other, and will therefore fail to plot correctly on the map.

It is poor judgment to mark the points at which the samples are to be taken in advance of the actual sampling; this amounts to an advertisement that a sample is to be taken along a certain line, and permits the evilly disposed to assist nature in the dis- tribution of values.

A face that is to be sampled should be thoroughly cleaned. If the ground is soft, a strip a few inches wider than the sample cut should be cleaned off with a pick; if the ore is hard, a brush or broom should be used either dry or with water. In driving any working, the fine material, often the richest, is powdered and thrown against th roof and walls, where a portion of it adheres; it is, therefore, of the greatest importance that the face to be sampled should be thoroughly cleaned. Irregular projections and loose pieces should be knocked off, to give, in so far as possible, a flat surface from which to cut the sample.

The face to be sampled should be examined carefully for soluble salts. In copper mines in dry climates an efflorescence of chal- canthite and other salts is usual, and the sampling of old workings is attended with considerable risk of salting from this cause. Samples containing these efflorescences, even after boiling in water, show blue crystals under the microscope. These efflores- cences are due to the evaporation of migrating solutions on the walls of the workings, and represent an enrichment not found throughout the mass of rock. The writer had occasion to re- sample a mine where there was much efflorescence. His samples were boiled in water with a little caustic soda and averaged about 3/10 per cent, copper; the sampling thus discredited averaged in excess of 2 per cent.

Where it is necessary to take samples from the floors of drifts it is best to cut large samples and to wash from them and dis- regard all fine material; fine particles of heavy minerals work into

Examination Of Prospects

method may give results somewhat too low, but is not so liable to serious error as would result from the inclusion of the fine material.

Samples should always be taken as nearly as possible at right angles to the lines of distribution of the minerals through the ore. Placing the Samples. — The interval between samples de- pends upon the regularity with which the valuable minerals are distributed. One extreme might be considered aji abso: lute'y uniform mass, of which one sample would suffice, and the other extreme, a segrega- tion of all the valuable mineral into a single mass; it is there- fore apparent that the proper interval between samples will differ with each exposure sampled.

In general, a 20-ft. interval will suffice for a large ore-shoot of uniform grade, a 10-ft. in- terval in average cases where many samples are to be taken from the same ore-shoot, and 5-ft. or leaser intervals where the ore is spotty. It is usually advisable to start with 20-ft. intervals in the examination of a property where no data are available, and resample at 10-ft. intervals, and perhaps again at 5-ft. intervals, where the results from the first scries indicate that such a course is advisable.

In sampling a wide vein or deposit it is best to divide the width into sections and to sample them separately, in order to determine the distribution of the values. These widths may be taken over even multiples of the total width if the deposit pre-

FiQ. 2 — Cross section showing ir regular exposure of ore in the roof o! a drift; the quantity of sample taken per foot should be less along A~B and C-D than along B-C, where the roof is ap- proximately at right angles to the vein.

Mining Examinations 17

sents a uniform appearance. If the vein or deposit presents a variegated or banded appearance, however, the several bands or zones should be sampled separately.

Where a section of ore is irregularly exposed, as is commonly the case with a vein in the roof of a drift, the sample must be cut deeper over the part that is at right angles to the vein than where the face is slanting, in order not to get an undue proportion from the slanting exposure.

The Size of Samples to be Taken. — The size of a sample should be limited to the least amount that will yield a true average of the exposure sampled. A few large samples are of little value as compared with many smaller samples, if the latter be well taken. Car-load shipments, ton-samples, and shooting down large samples are obsolete methods, as a bunch of rich ore is capable of salting the whole sample, and any sample that is too large to be sealed in a sack and properly protected against salting is a source of danger. Small mill runs are not satisfactory on any but very high-grade ores, as the clean-up will depend largely upon whether the plates are scraped clean or whether they are allowed to absorb amalgam.

The more uniform the ore the smaller may be the samples; where the ore is spotty, the samples should be large, as is also the case where the ore is loose and breaks irregularly, or is alternately hard and soft.

In cutting a sample the rich spots should be avoided if they are few in number; if there are many rich spots, the groove should include everything along its line.

It is a very difficult matter to sample correctly a spotty ore. In the case of an ore that consists of barren or nearly barren quartz carrying free gold, the average cannot well be determined by sampling; a majority of the samples of such an ore will be very low, or blanks, and a few will be very high; the average obtained will be more a matter of luck than a basis for an accurate estimate ; a large mill run is the solution of this problem.

a plan for safeguarding samples and never to rcvaVe

18 Examination Of Prospects

in carrying it out, whatever the circumstances; if an engineer trusts his judgment as to whether he is in safe company, his judgment is almost certain to be at fault at some time during his career, but if he always maintains the same vigilance he will never be salted. Salting is the result of carelessness, and is inexcusable.

The inclusion of waste samples is generally recommended as a safeguard, but like much good advice, is rarely carried out.

The writer insists that no one except assistants whose integrity is known to him shall approach a sample until it is placed in a new, clean sack, and sealed, with the top turned over and tied down to prevent the working in of fine particles at the mouth of the sack. As a check upon a series of samples so taken, it is well to save a portion of the fines from rejected quarterings and to pan them; if a black greasy scum appears, it is evidence of tellurides; if a string of colors appears, its origin should be ascertained.

After being sacked the samples should be locked in a mail sack, preferably made of leather, or in a trunk. If the sacks used are clean, a syringe cannot be used without detection through the stain left on the inside of the sack.

It is much better to offend the vendors by the precautions taken than to run any chances of being salted; those with honest intent seldom take offense at such precautions.

The Shipment of Samples. — Before shipping samples it is best if possible to grind them to a degree of fineness such that the assay er is sure to take a representative portion for assay; this is hard work, but it should not be left to the assistant of the assayer employed, who may not give proper care to this important work.

In shipping samples it is well to direct the assayer to reserve the pulp in case an umpire assay should be required, which will indicate to him that he is assaying against another man and will so induce accuracy. A duplicate set of samples should always be kept by the engineer. With low-grade ores it is best to in- struct the assayer to make crucible assays on two assay-ton lots. Samples should be packed for shipment in boxes in preference to

Mining Examinations 19

It should be unnecessary to state that local assayers should be viewed with suspicion, and that the man who is to run the samples should be as thoroughly known to the engineer as any assistant whom he may entrust with the sampling. In large examinations an assayer is usually included in the staff.

Resampling. — It is usually advisable to resample personally a certain proportion of the cuts as a check upon assistants, as well as to check high assays.

High Assays. — The treatment accorded abnormally high assays will vary with each property examined and with every engineer. The usual procedure is to reassay the sample one or more times, to determine if the high result is due to a rich speck in the pulp taken for assay. If the sample as a whole is found to be high, the cut should be resampled. If this result checks the first, some engineers recommend resampling halfway between the high sample and the adjacent samples, and using the average of these results in place of the high result; others advocate the omission of the high assay, using in its place the average of the other assays from the same exposure.

The most reasonable basis upon which to consider a high assay is in the light that it is due to the average grade of ore plus an extra amount of the valuable mineral, and to substitute for it the average of the higher samples from the same exposure the results of which have been accepted.

The Calctilation of Results. — The foot-ounce method is the one generally adopted in the calculation of ore reserves. Several elaborate and complicated methods have been put forward by various engineers, but it seems probable that the results of calcu- lation by the foot-ounce method are as accurate as the results of the samples themselves. In this method the length of each sam- ple is multiplied by its assay value; the products from aU the samples in the block under consideration are added, and this total divided by the sum of the lengths, the quotient being the average value.

All calculations should be made in dollars for gold, in ounces for silver, and in percentages for other metals, and NvliNfc

20 Examination Of Prospects

should not be translated into dollars per ton until the final result for a given block is obtained, when the market price used for the various metals should be set down also.

In calculating tonnages the specific gravity of the ore should be carefully determined; it is not unusual for engineers to assume an average specific gravity for the ore, a procedure that is likely to lead to serious error; this is apparent if it is remembered that the percentage error in the ore reserves is directly proportional to the percentage error in the guess at the specific gravity. With ores composed largely of heavy sulphides, the specific gravity may be determined by estimation, which requires that the percentage of iron be determined in addition to the other base metals; this method will yield accurate results and should be used where the specific gravity of the ore varies greatly, the calculation being applied to the individual sample results. The best method in most cases is to determine the specific gravity directly, weighing in air and ri terward in water several batches of material from different parts of the ore-shoot under investiga- tion. The method of packing a box with ore and determining the weight of a known volume of broken ore, and then introducing a factor to represent the relative volumes of broken ore and ore in place, is open to objection; the factor introduced is a guess, and the result depends upon the ratio of voids to ore and there- fore upon the tightness with which the ore is packed into the box.

The determination of the specific gravity of a porous ore is a difficult matter; it may best be accomplished by weighing in air and determining the volume of the pieces weighed by overflow of a vessel full of water. If this is done rapidly the result is correct, but does not take into account vugs or open spaces, which must be allowed for.

Stoping Width. — The relation between the width of a vein and the stoping width necessary to extract the ore, should receive careful attention in the examination of narrow veins. The minimum stoping width for machine drills is usually from 4 to 5 ft. ; the new air-hammer drills require less. For hand drilling, 30 in., or sometimes even less, is required.

Mining Examinations 21

The sloping width and the amount of waste broken with the ore varies with the relative hardness of the ore and wall rocks, and also according to the 'firmness of the hanging wall, which, if loose, will contribute waste by caving. In some veins the waste may be shot down first and the ore broken down clean afterward.

In the consideration of narrow veins the average amount and grade of ore mtist be dertemined as it stands, and the values, if any, that are carried by the wall rock; then, upon the assumption of a stoping width, the average value of the broken ore may be calculated. This average should be further corrected by the amount and value of the waste that may economically be sorted out, the final result being the grade that may be expected for the ore to be hoisted or shipped. These figures, in the case of an operating mine, may be checked by records of past production, where the volumes of material stoped can be determined, or where the amount and grade of the waste sorted out is known.

Hand Picking. — The results indicated by sampling are almost always diminished by slabs of wall rock that unavoidably become mixed with the ore. In deposits that carry heavy sulphides unevenly disseminated through the gangue, and in ores through which the values are irregularly distributed in such a manner that the richer portions are distinguishable to the unaided eye, the question of hand picking is of prime importance.

Hand picking is a very effective process, where properly arranged for and carried out on clean ore from which the fine material has been screened. Not only is the waste sorted out in hand picking, but clean mineral is saved as a high-grade product. The process yields two clean products, finished ore and clean waste, in one operation, and thereby saves not only mill capacity, and the cost of treating the waste, but also greatly reduces the metallurgical losses that the clean mineral would otherwise suffer. An inexpensive installation to permit efficient sorting will often result in the recovery of a payable product from an ore that could not otherwise be considered an asset.

Metallurgical Losses. — The question of metallurgical los>9e"& as important as that of the average grade oi t\ve vcv e.'bs*'

22 Examination Of Prospects

it should receive careful attention, and in formal examinations an exhaustive series of tests may be necessary. The mineralog- ical character of the ore is the criterion in the field work, unless laboratory tests can be made. Before a plant is installed or a developed property purchased, however, large scale tests are advisable.

The Cost of Mining and Treatment. — The cost of mining and treatment is a factor as important as grade of the ore, and is probably more subject to the personal equation than any other branch of an examining engineer's work: individual judgment based on experience is the final guide in an estimate of costs. This subject, which applies rather to the examination of developed mines than to prospects, is most thoroughly gone into in Mr. J. R Finlay's book on "The Cost of Mining.''

The results of past operations must be considered in the light of future probabilities, and it should be borne in mind that mining costs and selling prices of metals vary greatly with time, and, in some cases, with the seasons.

The larger the property and th more complete its development the easier becomes the problem of estimating costs. Those properties that most nearly approach the character of a manu- facturing enterprise are the easiest with which the engineer has to deal.

In the examination of an isolated prospect having no oit reserves, a detailed discussion of probable costs is out of place, as the costs will vary according to the tonnage developed. A tentative estimate based on experience is the best that may b.- offered in such cases.

The regularity of a deposit and its absolute size are important factors in the cost of mining; the angle of inclination of the de* posit with the horizon is important, also, as determining whether the ore when broken in the stopes will run or whether it will havJ| to be shoveled.

All the other attributes of a prospect must be considered in the light of its situation with respect to transportation; excessive distances, or a rugged topography without roads or trails, may

Mining Examinations 23

render valueless a property that would be valuable if better situated.

Water is a necessity for camp use and for metallurgical plants, and if there is no visible source of water near a prospect, the question at once becomes grave. Some gold mines in the Altar District of Sonora, and in many other desert regions, are commer- cially impossible on account of scanty water supply. Water in old workings should be investigated, and its source determined — whether it is due to seepage, or whether it comes from an under- ground channel. The amount of flow of water is, of course, of vital importance, and the question as to whether the flow comes from an underground reservoir which may ultimately be drained, or from a regularly flowing channel, is important. Examples are numerous where large flows of water have rendered impossible the mining of otherwise valuable ore-bodies.

Climate and altitude are not usually controlling factors in the United States, but become such in the high latitudes, or in the fever belts in the tropics. The high altitude has been a great detriment to the development of certain districts in southwestern Colorado.

Wages and supply of labor are factors for careful consideration; fuel and motive power also are factors of prime importance.

It is often advanced that a company, through large expenditure for equipment, may greatly better the working costs of a property that has been operating on a small scale; this is rarely true with small high-grade mines, where the small owner can secure working costs comparing favorably with anything that a large company can accomplish. This is the reason that small high-grade mines are usually poor purchases; their owners demand all that they are, or may become, worth. Good examples of this condition are offered by many mines in Mexico that produce ore as cheaply with primitive methods as could a large company with expensive installations.

The cost of equipment must finally be charged off against the tonnage mined, and for this reason large low-grade properties are likely to offer the best opportunities to the average investor ,

24 Examination Of Prospects

In determining the probable cost of mining, the development cost must not be forgotten; the footage of shafts, drifts, raises, and so forth must be considered in connection with the amount of ore developed, and the probable cost of future development must be included in the estimate of the mining costs.

Finally, the purchase price is a charge of which each ton must bear its share. These factors are often forgotten in America, where mining risks are too often accepted in a gambling spirit.

The Estimation of Ore Reserves. — Most engineers will agree fairly well in regard to the quantities of developed ore and of probable ore in any mine, but great differences are to be expected in estimates of possible ore, which, of necessity, are forced predic- tions of the future based upon insufficient data. A reasonable method by which to evade such predictions is suggested by Mr. J. H. Curie; it is to purchase mines on the basis of the ore reserves plus a royalty for all the ore afterward discovered.

The starting-point in a consideration of the probable depth to which an ore-shoot will persist is the determination whether the ore is of primary or of secondary origin; if primary, there is no genetic reason why the shoots should not continue to great depths; if secondary, little or no tonnage may be allowed below the lowest workings, unless the mineralogical character of the ore is such that it seems reasonably certain that the workings are still in the upper part of the zone of enrichment. Secondary ore-shoots, in general, are greater in horizontal than in vertical extent; primary ore-shoots, on the contrary, are commonly greater in vertical than in horizontal extent; the relation between the horizontal and vertical dimensions of exposed ore-shoots should be con- sidered in making estimates of the depth to which they probably continue.

If the ore exposed is the result of geological conditions unlikely to be duplicated in depth, the end of the mine is probably in sight.

It is certainly true that the tendency of primary as well as of secondary ore-shoots is to pinch out or to become low in grade

"The Gold Mines of the World, " p. 46.

Mining Examinations 25

with increasing depth; many primary ores, however, probab-y continue to depths below the limit of mining.

A factor of safety for the engineer's personal reputation has no place in a mining report, except in the margin of profit that he deems necessary to make the enterprise attractive.

Chapter Ii

Structural Geology

Ore-deposits are commonly divided into two classes, syngenetic and epigenetic, according to whether the ore was deposited to- gether with the enclosing rock or was introduced after its deposition or solidification. Epigenetic deposits, which are by far the more important class, owe their formation to the channels that permitted the ingress of their metals, and both classes are subject to great modification by post-mineral changes in the containing rocks. A discussion of the structural features of rocks, therefore, necessarily precedes any consideration of ore- deposits or processes of ore deposition.

Stratification. — The arrangement of sediments in parallel and approximately horizontal layers is called stratification, and is an original property of sedimentary rocks. Successive strata of the same sedimentary bed commonly differ from each other in some minor characteristic, such as texture or color, which results in a bedded or stratified appearance.

Cleavage. — The subjection of a rock to pressure develops within it parallel planes of weakness, called cleavage planes, along which the rock breaks into relatively regular slabs or blocks; the direc- tion of these cleavage planes, which bear no relation to stratifica- tion, is determined by the direction of the pressure that produced them. Where there are more than one set of cleavage planes, one of them is likely to be more prominent than the others. In sedi- mentary rocks, the cleavage may coincide with the stratification, but more commonly cuts across it. Cleavage has been defined* as the " capacity present in some rocks to break in some directions more easily than in others/' Cleavage, therefore, does not imply the existence of subdivisions, but rather the tendency to subdivide along certain planes.

Van Hise.

Structural Geology

Fissilily. — Fiasiiity is a " etructure in rocks by virtue of which they are already separated into parallel laminae. Fissility may be regarded as a development of the property of cleavage, and is commonly expressed along closely set parallel planes.

Schistose Structure. — The long-continued stresses of regional metamorphism with accompanying recrystallization and rock flowage develop a banded structure in both sedimentary and igneous rocks; the various minerals contained by the rocks are arranged with their longer axes parallel and form planes of weak- ness to fracture that differ from cleavage planes and that are independent of any original stratification. A rock that has suffered this change is said to have a schistose structure. Schists commonly exhibit cleavage that has no relation to their sehistos- ity, and occasionally, also, traces of the original stratification are visible.

Gneissic Structure, — A coarsely schistose structure is known as a gneissic structure. In gneisses the individual bands are more prominently developed than in schists, and the ease of fracture along these bands is relatively less.

Joints,— Planes of division through rocks that are the result of stresses insufficient to produce more than microscopic move- ment are called joints. Fragments of broken rock, both sedi- mentary and igneous, are commonly bounded in part by plana surfaces, which are joint planes. Joints are of several kinda, according to the nature of the stresses that produced them; they vary in expression from cleavage, orincipient jointing, to the well- developed planes that bound the prismatic columns of certain basalts. Joint planes frequently preserve their general direc- tions over long distances, and the angles between different joint systems are likely to be constant throughout large rock masses.

Where the joint planes are closely spaced and where the load of overlying rocks is light, they afford channels for the circulation of solutions and frequently become mineralized.

During the contraction that results from cooling and solidifiea- , igneous rocks separate into masses of roughly polygonal

lotion, ignei

Examination Of Prospects

section bounded by tension or contraction joints. Such joints are best developed in certain flows, which upon aolidififatioa divide into regular piiamatic columns.

Sedimentary rocks upon drying not infrequently suffer con- traction, which finds expression in similar, but usually less well- defined, planes of division. Fractures due to contraction are the result of internal strains in a rock maas, and do not pass outside of it into other rocks for any important distance.

Fia 3 — M tsonEkC kli obIo ngj t planoa at nearly ngM ang a to the bedding A/ter Ranso ne

The outer members of folded strata un lergo tensile stress during foldmg which occas onally results in tl e forn ation of tension joints that follow a ladial arrangement inward fiom the arc of the fold.

Sheeting.— Parallel planes of fracture, developed by compres- sive stresses, of relatively great continuity as compared with j oints, but of only incipient displacement, are called sheeting planes. Closely set and well-tieveioped sheeting pianeii often afford

30 Examination Of Prospects

channeb for the circulation of solutions, and, when mineralized, form sheeted lodea. Not infrequently systems of sheeting planes occur in pairs, parallel in strike, but intersecting in dip, such

Fig 5 — Ideal sectioii of the Fems-Haerty ore-body Encampment Wyoming showing convexity and the tmnerahzation of tension joints After Spencer

Fig 6 — Section of folded strata near Negaunee Michigan gn Gneiss g granite q quartzite s clay slate e iron beanng Negaunee strata, d dionte and diabase After Van Hiae and Bayley

Fig. 7.— Partly eroded anticline, Montpelii

fracturing being the typical result of compressive or torsional strains; these interdependent systems of sheeting planes are known as conjugate systems.

Structural Geology

Folds. — fold is a bend in a rock mass caused by compressive stress of insufficient intensity to produce a fault. A fold is called a syncline if its bend is concave above, and an anticline if its bend is concave below, A dome is an anticline whose length is zero,

Fio. 8. — Section through the ore-deposit at Meggen, Germany, showing a syDoIinal trough, k, Pyrite; s, barite; U and ha, slates; n, limestone. After Hundt.

and is best described by the usual meaning of its name. A basin is the synclinal equivalent of a dome. Folds are commonly persistent in strike, and domes or basins are of relatively rare A monocline may be described as a half-fold, as

Moaochne near Gallup, New Mexico. After Howell.

where strata asfume a terrace-like position, being parallel at different elevations either side of an inclined connecting part. The rock forming the convex or outer portion of a fold is, like the lower portion of a beam subjected to load, under tensile stress, which frequently results in a eeriea oi ItaaXAKea 'Oa.N. "sas

Examination Of Prospects

afford channels for the circulation of Bolutions ami so become

of ore deposition. The Development of Faults from Folds. — Where a fold ia

formed by conipreasive atreea beyond the capacity of the rock to withstand, a fault ia developed along the axis, or the plane bisecting the angle between the component limbs of the fold. Folds not infre- quently pass into faults along their strike, and in a region that has been subjected to

FiQ. lO:— Specimen of folded elate, Fig. 11.— A flyxuro. A/l<:r Black HillE, SoiH.h Dakota, Bliowing Beck.

a fissure developed along the axis of the fold. After Irving.

both folding and faulting these expressions of stress are likely to be parallel. Fractures. — As generally used, the term fracture denotes a

Structural Geology

break in a rock mass in importance intermediate between a joint and a fault, as these latter terms are generally used.

Flexures. — A flexure is a sharp bend in a series of strata or in a rock mass, without the development of a continuous fracture, the result being a displacement similar to that of a monocline. Flexures readily pass into faults.

Faults.— A fault is a fracture through a rock mass the opposite yralls of which have moved past each other, the word indicating lack of correspondence between opposite walls.

Fig. 12. — Normal faults: a recent fault near the surface, and the same

with the hanging dropped down. After Beck,

The strike of a fault is the direction of a horizontal line within the plane of the fault; the dip of a fault is the angle between the plane of the fault and a horizontal plane.

The distance measured on the plane of a fault between the new positions of two points that were originally opposite is called the total displacement, which distance may, for purposes of calcula- tion, be considered as the hypothenuse of a right triangle, whose sides represent the horizontal and the vertical movements, of which it is the resultant.

The terms throw and heave as commonly used refer respec- tively to the vertical and horizontal distances between the new positions of originally opposite points in a fault the. mover ment along which was directly down the dip; by offset is com-

J. E. Spun, "Geology Applied to Mining," p, ll'i.

34 Examination Of Prospects

monly meant the horizontal distance (perpendicular to the strike) between the two portions of a faulted vein, without regard to the direction or amount of total displacement.

In a great majority of faults the total displacement is not directly doivn the dip, but is the result of both horizontal and vertical movemeiita.

A normal fault is a fault along which the hanging wall lias moved downward on the foot-wall, and is the natural result of a drawing apart of the two rock masses, the least supported msa slippmg downward on the mass havmg the larger base ' Normal faults are commonh the effect of gravity, and due to tension,

r

Fig. 13. — Reverse fault, Coear d'.41eiies, Idaho, After Ranaome.

although they sometimes result from compressive streasea. Faults due to tension commonly find expression in a simple frau- ture and are not accompanied by sheeting.

A reverse fault is one along which the hanging wall haa moved upward on the foot-wall, which is commonly considered as having been forced under the overlying mass by compression. Such faults are less likely to be simple fractures than normal faulta, and are frequently accompanied by parallel sheeting, or by i folding of the strata or rock masses that form their walls.

Faults due to torsional stresses are commonly accompanied by differential movement, the displacement over one part of the fault being greater than over other parts, resulting in a tilting of the faulted block. Such faulta frequently follow curved lines of strike, and are often branching.

' J. F. Kemp, " Ore Deposits," p. 21.

Structural Geology

Faults are sometimes referred to as strike faults, or as dip faults, according to their direction as compared to the strike or dip of enclosing strata or associated veins.

A series of approximately parallel faults of similar class and displacement are called step faults; if of opposite displacement, they are called compensating faults.

Two faults, or two systems of faults, parallel in strike but inter-

nn

Fia. 14. — Section perpendicular to the Btrike of a eeries of step faidta at Leadville, Colorado: The total displacement of a fault that extends for a long distance through the region is here distributed among several parallel faults. After Emmons and Irving.

Becting in dip, are occasionally formed by the same compressive or torsional stresses, and are known as conjugate faults or fault systems.

Faults formed at slight depth below the surface are likely to be less regular and less persistent than deep-seated faults. The occurrence of many small, irregular fractures that become less in number and more regular in direction as depth amej4.,\a\ii&-

36 Examination Of Prospects

cative of faulting under light load and at slight depth; a further indication of shallow fonnation ia the presence of friction breccia along a fault rather than the pasty gouge that is characteristic ot deep-seated movements under heavy toads of overlying rocks. The upward branching of a fault, or a marked change in dip, is a sign of shallow dislocation. Surface detrital matter is occasionally found in a fault filling, conclusively proving a shallow depth at the time of faulting.

Fia. IS. — HorizoDtal sketch plan of a part of the Mizpah vein, Tonopah, Nevada, showing probable compensating faulting. After Spurr.

Exploration fortheContiauation of a Faulted Ore-body. — Where

an ore-body has been cut off by a fault the discovery of its con- tinuation beyond the fault is a problem of the greatest impor- tance. Numerous rules have been formulated to aid in a eearch for the continuation of a faulted ore-body, but it is safe to say that the great variety and complexity of the results of faulting render such rules valueless in most cases.

Where faulting dislocates a non-tabular ore-body that is contained in a homogeneous rock, the problem is insoluble, unless the fault carries a drag or trail of ore mixed with the fault filling.

The direction of the striations on the walls of a fault, in simple cases, indicates the direction of movement, the deeper ends of the furroughs being pointed in the opposite direction to the move- ment of the opposite walls. Occasionally an ore-body is de-

Structural Geology 37

formed where faulted, the deformation pointing in tlie direction of displacement and passing into a drag or trail through the fault filling.

Where a tabular ore-body or vein is faulted, the problem is likely to be simpler, as the recovery of any portion of the vein beyond the fault will lead to the discovery of the continuation of the ore-shoot. Where the enclosing rock is composed of a series of strata a knowledge of the succession of the strata is of the greatest assistance in the calculation.

The cases are rare where complete data for the calculation of the total displacement are available, as, for instance, where a fault cuts a dike that itself cuts a series of known strata, or an older fault, or any combination of intersections that permits the recognition of a point rather than of a plane on both sides of the fault.

In an endeavor to locate the continuation of a faulted ore-body the striations or slickensides should be studied, and also the fault filling for possible fragments of ore, or of country rock other than that of the immediate walls of the fault; a survey, map, and sections should be made showing the dip and strike of both the fault and of the ore-body, if of tabular form, and of every known stratum of the containing rock, igneous contact or dike, older fault or vein, or any distinguishing mark that may be recognized on both sides of the fault. The application of descriptive geome- try, or trigonometry, to this data will result in all the information regarding the displacement that the situation renders possible; complicated formulae, like rules of thumb, are of no assistance in the solution of such problems.

It is seldom that the direction and amount of total displace- ment may be measured directly, more frequently it is susceptible of calculation from related functions, and frequently it is not determinable imtil further exploration discloses the necessary data.

A comparison with known faults in the same district is fre-

38 Examination Of Prospects

out may become a relatively simple problem when again met with underground.

Post -mineral Fissures. — A line of weakness once established is likely to continue as such through long periods of time, the original gouge seam or zone of attrition material being preserved and offering a plane of easy relief to subsequent stresses; where a fissure has been completely healed .by mineralization, the result- ing vein of brittle quartz or other minerals is likely to be less resistant to fracture than the tougher enclosing rock. It is common, therefore, to find evidence of post-mineral movement along veins.

Post-mineral movement is likely to crush the ore and to mix it with waste to such an extent that its value is materially reduced and also to render the country rock loose and likely to cave during mining operations; not infrequently, post-mineral move- ment so complicates the structure as to render exploration difficult, or unremunerative.

A favorable effect of post-mineral fissuring, either along or across a vein or deposit, is in permitting the access of surface waters, which are thereby given opportunity to form secondary enrichments, as will be taken up in a later chapter.

A usual effect of post-mineral movement along a vein is the formation of false walls, or fissures carrying gouge, that cut the vein obliquely; these mask the part of the vein behind them, and also hide the juno ions of branch veins. False walls are frequently the cause of losing a vein in development, the tendency being to follow their well-defined fissures and to leave the vein to one side. Where such movements are known to have taken place, frequent cross-cutting may be necessary to make sure that the whole of the vein has been exposed.

Pre -mineral Fissures. — Gouge-filled fissures appear to be unfavorable to ore deposition, probably chiefly because the pasty gouge does not permit the ready passage of mineralizing solutions, and also because the continual movement along such

This subject is fully taken up by Mr. F. L. Ransome in P. P. 62, U. S. G, 8., p. 120.

fissures fends to close any minute channels tliat are permitted tQ form. Such fissures once formed probably persist as lines weakness and movement for long periods, and the occurrence a gouge-filled fissure in connection with an ore-deposit is proof that it is of later formation than the ore.

It is frequently seen in mines that such fissures act as efiicieni dams, the passage of solutions through them being as difficuH as the circulation of solutions along them ; they maybe considered therefore, as having frequently determined the limits of ore deposition through the impounding of mineralizing solutions.

The fact that an ore-body or vein is cut off by a gouge-filh fissure is no proof that a fault has displaced the ore-body or vei the continuation of which may never have existed beyond tl fissure. It is often difficult to prove whether such a fissure ii of later formation, and has faulted a vein, or whether it is oid( than the vein, which ends upon reaching it. The proof of fault is, of course, the continuation of the vein beyond it; this, however, is the object of the search. That such a fissure post-mineral fault may be indicated by the faulting of associated beds or dikes, or by a drag or trad of ore through the fault filling,

If such a gouge-filled fissure is older than and limits the ore, this may be indicated in a filled fissure, by a closing of lines of crustification against the fault, equivalent bands being connected, or by a change in the vein filling upon approaching the fault, or by a branching or widening of the vein upon approaching the fault.

A gouge-filled fissure older than the ore may through recent movement exhibit the characteristics of a fault that has dia placed the vein, though in reality limiting it.

The Expression of Faults in Topography. — The more recent a fault, and the greater the difference in resistance to erosion between the rocks of its walls, the more likely is it to find expres- sion in the topography; a fault of great displacement that finda no expression in the topography is probably not of recent origin. Faults may in some cases be recognized at the surface by feature other than the lack of correspondence between their walls, sucl

40 Examination Of Prospects

as a fault scarp, a saddle in a ridge, or the course of a canyon, or through the outcropping of a fault breccia that has become silicified and so rendered resistant to erosion. In most cases, however, erosion is the dominant feature in controlling the topog- raphy, and structural features are rarely represented at the surface, except through relative resistance to erosion.

The Importance of Areal Geology. — The surface affords a complete section of the geological features of any district, and in all but the simplest occurrences a geological map of the surface and a few vertical sections are invaluable guides in the examina- tion or exploration of any district. All significant outcrops of rocks, dikes, beds, veins, faults, shear zones and so forth should be determined and their strikes, dips, and elevations recorded on a large scale map, from which the data may be referred for study to a horizontal plane. Contour maps are useful in such work, but in most cases the expense of contouring is not justified, and the elevation of significant readings will be found to be sufficient.

A horizontal section thus prepared, with, perhaps, several vertical sections, will indicate through lack of correspondence of strata or other criteria, the existence and location of faults that otherwise might not be suspected. If the surface observations are abundant, they often afford data from which the displace- ments of faults may be calculated.

A geological map of the surface results in dividing a district into fault blocks, conditions within each of which may be con- sidered as constant, but which must be expected to change upon passing from one fault block to another. The importance of dividing a district into fault blocks will be appreciated on consid- ering that a prospect may be situated within a few hundred feet of an important mine, but actually separated from it, perhaps, by a fault thousands of feet in throw, or in a formation that is apparently similar, although actually unrelated, to the ore-bear- ing rock.

Exploration and Development. — In the opening of any property two distinct objects should be kept in mind — the development of known ore-bodies, and the exploration for further deposits.

Structural Geology

Where payable ore has been found, it should be followed with a view to the development of ore and also to gain knowledge in regard to its occurrence, each ore-shoot being opened individually without the testing of theories. In general, the time to go slowest in following an ore-body is when it shows signs of giving out;

Fia. 16. — Map showing the fault blocks at Tonopah, Nevada; the payable veins occur in the earlier andesite and are most prominent in the fault block bounded by the Burro, Mizpah, Stone Cabin, and Gold Hill faults. After Spurr.

every detail should then be studied and recorded before the rock has been dirtied by further blasting, as frequently small stringers or gouge-filled fissures will be found to lead from one ore-body to another.

In the exploration for further ore-shoots, however, the major

42 Examination Of Prospects

the general trend of the deposit should be cross-cut thoroughly. Exploration should be confined at first to cross-cutting the known ore-bearing horizon or zone of mineralization — perhaps a sedi- mentary bed favorable to the deposition of ore, a zone of crushing or brecciation, or a certain horizon known to be the most favor- able for secondary enrichments. Within these broad zones or horizons considered favorable to the existence of ore, the minor features, such as stringers of ore, low-grade ore, sheeted zones and so forth, may be considered important. Upon cross-cutting the apparent trend of the deposit at the horizon considered most favorable to the existence of ore, the most promising stringer or seam should be followed along its strike for an appropriate dis- tance, where another cross-cut is in order.

The plan to be followed demands detailed study in each indi- vidual case, and a state of mind is necessary that is receptive of new impressions as the work progresses. Preliminary explora- tion is frequently entrusted to a practical miner, when as a matter of fact, such work constitutes perhaps the most important field of the trained geologist.

Chapter Iii

Structural Features Of Ore-Deposits

Veins, Lodes and Ledges. — Many definitions have been advanced and many limitations advocated in the use of these terms. The following definitions appear to follow the best usage.

FiQ. 17. — Plan and section ot the Combination ledge, Goldfield, Nevada, showing the irregular oceurrenoe ot the ore within the ledge. After Ran-

Fissure Veins, — fissure vein is a mineral mass tabular in form as a whole, though often irregular in detail, filling or accompany- ing a fracture or series of closely set and intimately related par- allel fractures in the enclosing rock, tKe mmeTa\tiias?.V?i.NS3\w*s3s.

Examination Of Prospects

formed later than both the country rock and the fracture, either through the filling of open spaces along the fracture or through chemical alteration of the adjoining rock,'

Lodes. — lode is a zone of Assuring that contains roughly parallel mineral masses of the general type of fissure veins, usu- ally connected by cross veins and mineralized breccias to such a degree that over certain portions the whole width constitutes a single ore-body.

a, Upper MiasiBsippi Valley; the black shading indicatM galena. A/ler Kemp.

A ledge is an irregular mass of altered rock, containing ore bodies, the alteration of which is due to and characteristic of the action of mineralizing solutions."

Gash Veins. — gash vein is a vein of superficial character, widest near the surface and narrowing to extinction in depth. Gash veins are usually the results of solution and deposition

' Waldemar Lindgren. 'F. L. Eansome.

Structvhal Features Of Ore-Deposits

along joints or BmallfisBureB by surface waters, and are of second- ary origin. The term has been less correctly applied to lenticu- lar deposits that, prominent at the surface, die out in a similar manner in depth. The usual occurrence of gash veins is in sedi- mentary rocks. The typically short length of this type of vein should not lead to an expectation of continuity in depth, even if the origin is not suspected from study of the outcrop.

Bed Veins. — A bed vein is a vein that follows a bedding plane of an enclosing sedimentary rook, less frequently a plane between

FlQ. 19 — Section of a bed vein (copper) m the Snowstorm mine, Coeur d iJenes, Idaho A/ter Banaome

layers of volcanic rocks. Bed veins are commonly thought to be less persistent than veins that cut across the strata of enclosing rocks; many cases are known, however, where bed veins are both persistent and contain important ore-shoots. Blanket vein is often used as a synonym for bed vein, but actually refers to a horizontal or nearly horizontal position only.

A bed vein in unaltered rocks is sometimes distinguished with difficulty from a stratum whose mineralization was contempora- neous with its own deposition. In the examination of an unal- tered bed, an opinion may usually be based upon the closeness with which the mineralization follows the minute beddiu.?,i;;lwcie.

Examination Of Prospects

as well as the larger bed divisions; if the mineralization follows the larger bedding features only, and does not penetrate the relatively solid intervening atrata, it is prpbably later in origin than the bed itself. Fragments of country rock or the presence of croas or branching stringers are definite signs of a later origin, Furthev more, a mineralization that is not relatively continuous through a certam stratum but is transferred to strata above and below w ithout corresponding lateral extent is later than the contaming bed

Fig. 20. — Diagrammatic section of the Tonopah Extension vein, Toaepth Nevada, showing a compound vein. A, Altered wall rock; B, typical trUte vein of the earlier andesite period, containing silver sulphides and canjii'J values of several hundred dollars per ton; C, black, jaspery quart* of laW introduction than the original vein of which it carries fragments — the ¥fll of the black quartz and fragments variea from twenty to thirty doIIM per ton. After .

In the Coeur D'Alenes, Idaho,' a majority of the auriferous quartz veins are of the bed-vein type. These veins in some places occur singly, but more commonly they occur in groups, individual veins being separated by a few inches or a few feet of slaty country rock. An overlapping arrangement is common, one vein gradually pinching out while a parallel vein between

'F. L. Ransome, P. P. 62, U. 8. O. 8, p. 141.

mipm

adjacent beds becomes correspondingly thicker. Although certain individual veins persist for hundreds of feet without cutting across the planes of stratification, such crossings may be observed here and there, and cross-cutting stringers of quartz that link neighboring veins are numerous.

Compound Veins. — A compound vein is a vein that has been reopened after mineralization, and again mineralized, containing.

Fig. 21. — -Open pit of thi; Treadwell mine, Douglas Maud, Alaska, show- ing an aibite-diorite dike that haa been breooiated and impregnated, with ore. Afler S'pencer.

perhaps, two seta of ores. A frequent case is that of a vein composed chiefly of barren or low-grade filling that has received its valuable mineral upon being reopened.

Contact Veins. — -A contact between two rocks occasionally offers a line of weakness to fracturing that later mineralization may transform into a contact vein. Such a vein is rarely regular over long distances, unless its position aVou 'Oa'i <i'i'cA,%.'i\. Xi'e.

48 Examination Of Prospects ]

accidental, due to faulting. Veins of secondary minerals an likely to form along a contact where one wall offers an impervious barrier to migration of solutions and so induces precipitatioa along its course through impounding.

Veins Along Dikes. — Dikes of intrusive rocks, after solidifies- tion, are likely to remain lines of weakness along which new fractures readily form; veins frequently follow dikes, either aloi one wall or through the mass of the dike itself

Fid 22 — Settlor! ijf till Howard loiip t apple treek ( ok n-ado. showing a sheeted zone, the medial portion of the lode is closely Hhpet<(d, the sheet- ing planes becoming gradually farther apart m the foot- and hanging-walls. A/ter Lindgren and Ranaome

At Cripple Creek, Colorado, many of the veins follow phono- lite or basic dikes.

Lodes Along Sheeted Zones. — A sheeted zone 's a series of closely set parallel sheeting planes; they frequently afford chan- nels for mineralizing solutions and so become mineral lodea. Such lodes are commonly not as persistent as fissure veins, but mineral deposits may follow them for long distances, the mineralization

Structural Features Of Ore-Deposits 49

ing found sometimes in one and sometimes in another set of leeting planes, following, perhaps, an overlapping arrangement. X€eting planes are commonly parallel to the main fissures, and des are frequently made up of a mineralized fissure with one or ore mineralized sheeting planes nearby. There is usually no '"idence of movement along sheeting planes.

IVhere there has been much replacement along a lode the sheet- g planes are likely to be obscured, and the term is usually plied, therefore, to lodes that carry their minerals in a scanty .ngue. A typical lode along a sheeted zone is made up of ,Trow parallel veinlets separated by slabs of country rock. [Frequent cross-cutting is advisable in the exploration of a ineralized sheeted zone.

-At Cripple Creek, Colorado, sheeted zones consist of anum- r of narrow, approximately parallel, fissures, which collectively rm lodes ranging from a few inches up to 50 or 60 ft., or rarely, ft., in width. Within such wide belts of fracture, however, ro or more zones of concentrated fissuring may usually be stinguished that lie close enough together to be mined as a aole. In other words, the very wide sheeted zones are com- mnd sheeted zones. As a rule, the fissures are mere cracks, owing no brecciation, slickensiding, or other evidence of appre- ible movement of the walls; there are some notable exceptions

this statement, but the movement of one wall past the other 18 probably in few instances exceeded 1 or 2 ft. In general,the leeted zones are from 2 to 10 ft. in width. Among the numerous id important lodes that properly come under the designation of xeeted zone, several structural varieties may be distinguished, hich are sometimes exhibited in different parts of the same lode. L common form is that characterized by the presence of two main arallel fissures, usually 3 or 4 ft. apart, accompanied by less egular and less persistent fractures in the intervening and adja- ent rock. In another common type of sheeted zone the parallel ssures are more numerous, and are spaced with some regularity, 'here is usually a medial portion of the lode that ranges from a.

Waldemar Lindgren, P. P. 54, U. S. G. S., p. 10.

Examination Of Prospects

few inches to a foot or two in width, within which the rock is divided into a large number of very thin plates by fissures often less than an inch apart; this band of intense sheeting is accom- panied on both sides by parallel fissures that are spaced farther and farther apart, so that the sheeted zone as a whole merges gradually into the country rock. There are a large number of sheeted zones in breccia and in granite that are composed of many parallel or nearly parallel fissures, but that differ from the type just described in the absence of a well-defined medial zone and in the rather less regular character of the fractures.

Exit in Cliffs

H

5 Level

4 Level

k

i&yLevel

w/

, 5 Level

3Lev(

'

Feet

Soo 600

1 i —

—

Fig. 23. — Sketch plan of the underground workings of the North Star mine, Silverton, Colorado, showing the irregularity of mineralization of a stringer lode. After Ransome,

Stringer Lodes. — A lode in which the mineralization has fol- lowed a net-work of irregular curving fissures that have no general parallelism among themselves, but follow a general trend similar to that of a sheeted zone is known as a stringer lode. This type of lode rarely has definite walls, and the ore may be encountered at any point within the general zone of fissuring; as with sheeted zones, frequent cross-cutting is necegsary in the exploration of this type of deposit.

Structural Features Of Ore.Deposits

Fault Lodes. — A zone of faulting in which mineralization has -taken place irregularly through the crushed and comminuted :fsult material ia known as a fault lode. This type of deposit, -where the values are irregularly distributed either with or without s scanty gangue, is not uncommon in the areas of old schists in the desert region of the Southwest. These lodes are, difficult to foUow, and their exploration in advance of actual mining is usually unprofitable.

The Mineralization of Joints. — strongly jointed rock offers

Fia. 21. — Mineralisation of joints, Monte Cristo, Washington. AJter

many lines of weakness to fissuring stresses, and not infrequently a fissure that is well-defined in depth is dissipated through the joint planes upon nearing the surface. A mineralization that has taken place at relatively shallow depth, therefore, not infre- quently follows the joint planes and is distributed among them OB a system of reticulated veinlets. Such disseminations in depth are likely to coalesce into well-defined lodes. A system of mineralized joint planes is commonly the result of surface agencies; less frequently it is the result oi a 'p-rvoift.rj -aOTiiM-

Examination Of Prospects

zation at shallow depth in a region where erosion haa been slight.

Breccia Lodes. — A. zone of shattering in which the mineraliza- tion has cemented or replaced the brecciated mass of angular fragments and comminuted material is known as a breccia lode. This t3T>e of deposit is commonly irregular, the mineralization varying with the amount and degree of brecciation. In extreme cases the brecciation may have been out of proportion to the

a

fi

t

'"A

y

-t

-1

i.—— p-HiBr

A

fe

Wyfi-

Fio. 25. — Mineralisation of joints, Monte Criato, Wasbington. Ajter

quantity of mineralizing solutions, and have dissipated these solutions through so large a mass of rock that the resulting ore body is very low in grade.

Shear Zones. — zone of incipient fissuring or shearing that has been mineralized by impregnating solutions, commonly by replacement, and to a less extent, perhaps, by the filling of open spaces, is known as a shear zone. The passage of solutions through such zones is probably slow, and ample time is afforded for mineralization by replacement. While genetically a shear zone may be of any size, the term as generally used is applied to large, low-grade deposits.

Structural Features Of Ore-Deposits

Stockworks. — An area through which numerous veins traverse the rock in all directions, forming a net-work through mutual intersection, is known as a stockwork. In typical cases the individual veins are small and are considered collectively as a deposit.

. Stocks. — A stock is a deposit of irregular form due chiefly to replacement of the containing rock.

Surface

Surface

6th Level

Scale, 190 ft. lin.

7th Level

Fig. 26. — Ore chimneys in the Yankee Girl mine, Red Mountain, Colorado.

AJter Schwartz,

Pipes or Chimneys. — Certain deposits take the form of a pipe or chimney, and have marked vertical continuity, with very subordinate horizontal dimensions. It is difficult to conceive of a fissure whose only important dimension is approximately vertical, and these deposits were probably formed at the inter- section of fissures that throughout the remainder of their lengths

1 Beck-Weed, "Nature of Ore Deposits," p. 51.

Examination Of Prospects

did not permit the passage of mineralizing solutions, and so failed to be emphasized in connection with the main deposit; other deposits of this form appear to be due to the mineralization of fumarolic vents. Certain pipes in limestone appear to have been formed by solution, probably along a fracture, or intersec- tion of fractures: these are likely to be quite irregular in form.

At the Bassick Mine, Custer County, Colorado, an ore-de- posit that varies from 20 to 100 ft. in horizontal diameter has been

Fig. 27. — Plan of the productive lodes west of Silver Lake, Colorado, showing a branching vein system; the lodes farthest south are an example of an overlapping system. A/ter Ransome.

followed to a depth of over 800 ft. This chimney occurs in a volcanic neck and the ores have formed in concentric layers about boulders of the volcanic agglomerate.

Branching Veins. — Many veins send off branches into their hanging- or foot-walls, usually into the former; beyond such branches veins frequently lose in either width or value. A vein that branches along its strike may or may not unite farther on

Structural Features Of Ore-Deposits

tut, in general, the branching of a vein is likely to indicate the dying out of its fissure. If possible, therefore, development work should follow the direction in which the branches are converging. While no definite rule may be formulated, branch veins are likely

Fig. 28. — General sketch of the lode system of the Upper Harz, Germany,

showmg a linked-vein system. After Beck,

to diminish in both size and value with distance from the main vein, which usually formed the main channel of mineralizing solutions, and of which the branch veins were only lateral and dependant channels. Linked Veins. — A system of veins tYiat xA x'SiXisii

Examination Of Prospects

aOOLeval

Fia. 29. — Section of the Tomboy and Iron lodea, Silverton, Colorado. Afli HeTTon and Ranmme.

fm

w

w

/\m

y&fJh

W/y/

—Intersecting, or conjugate joint aystems mineralized Encam jnent, Wyoming A/ler Spencer

Structural Features Of Ore-Deposits 57

without crossing one another is called a linked-vein system. Where such a relation exists between veins, the arrangement appears likely to be persistent.

Conjugate Veins. — Fractures produced by compressive stress through homogeneous rocks are likely to form systems parallel in strike but opposite in dip, known as conjugate fractures; where mineralized these become conjugate veins, of which one dip is usually emphasized while the other is subordinate, or exists as a simple fissure only. Conjugate joint systems are of common occurrence. The intersection of two systems of intense jointing appear to be favorable loci for ore deposition.

Overlapping Veins. — It often happens that the stresses pro- ducing fractures do not find expression in a single persistent fissure, but in a number of more or less closely spaced parallel* fissures that partly overlap one another, forming a step-like system. Such a system is known as a system of overlapping fissures. Overlapping veins are probably the result of fissuring stresses that were exerted along a line other than the line of least resistance of the rock mass; while the lines of easiest cleavage of the rock were followed over relatively short distances, the final expression of the stress was along the average strike of the individual fissures taken as a whole, and parallel to the simple fissure that would have formed had the rock been homogeneous.

Many vein systems follow overlapping fractures, and this arrangement probably gave rise to the miner's rule to cross-cut '*into the hanging" or "into the foot" upon losing a vein, the overlapping being either persistently to the right, or persistently to the left, in most cases. An overlapping vein system is also spoken of as having an arrangement en ichelon. Blind veins, or veins that do not outcrop, frequently belong to an overlapping vein system, one of whose members outcropped and so led to the discovery of those that did not.

Systems of Related Veins. — A careful plotting of the veins of any mining district will often indicate a certain relationship among the payable fissures as regards strike, dip, or distribu-

Examination Of Prospects

frequently observed, and the important veins in many districts maintain a fairly parallel alignment. Mr. J. M. Boutwell,' has determined that at Bingham, Utah, over 84 per cent, of the payable fissures strike between N.5° E. and N, 43° E., and that

Fi<3. 31. — Diagram ehowing the trends of the barren fissures, the lean, and the payable veins and lodes of the Bingham District, Utah. A/ler

over 80 per cent, of them dip between 40° and 90° NW. In this

district all fisBures, barren, lean and payable, taken together,

strike about equally in all directions, but the northeast f

'J'. 38, U. S. a. S., p. 363.

Structural Features Of Ore-Deposits

appear to have been those that afforded channels for solutions at the time of mineralization. A striking instance of the parallelism of pay fissures is exhibited in the Bald Moimtain District, South Dakota.

A roughly radial distribution of veins about a common center has been noted in certain districts, and while well established in

Fig. 32. — Map of the principal ore-shoots of the Bald Mountain area, Black Hills, South Dakota, showing the marked parallelism of the payable fissures. A/ter Irving,

these districts, it is of rare occurrence. Cripple Creek, Colorado, affords perhaps the best known example. The veins at Cerro de Potosi, S. A., are said to follow a roughly radial distribution. While the recognition of a definite vein system is a valuable

Examination Of Prospects

N

A

/

I',

/

/

Lodes

i-\

Scale of Feet

1000 2000 3000

Fig. 33. — Plan of the principal veins of the Cripple Creek District, Colorado, showing a roughly radial distribution about a common center. After Lindgren and Ransome,

Structural Features Of Ore-Deposits 61

cate rather more clear relationships than commonly obtain among the veins of most districts.

In districts where the veins vary greatly in strike and dip it is easy to construct relationships that do not exist, and thus arrive at misleading conclusions.

The Persistence of Veins in Depth. — There is a distinct relation- ship between the length and depth of veins; in general, long, strong, wide veins persist in depth and short, non-continuous, irregular and weak veins die out at no great distance beneath the surface, perhaps to be followed by similar and roughly parallel veins in depth. While ore-bearing fissures are com- monly not important as faults, there is a relation between the amount of throw and the length and depth of a fissure; the greater the throw, the greater are likely to be these dimensions.

In the consideration of individual veins that have pinched in depth, a decision will rest upon the behavior of the vein in the parts already explored. If the proved length of the vein is con- siderably greater than the depth attained, and if the displacement along the vein is known to have been more than slight, the chances are good that the fissure will persist in depth. This has long been recognized by miners, who consider well-developed slickensides an indication of persistency in depth. . If a vein has been subject to local pinches above, it may be safely assumed that it will open out again with deeper exploration. In general, it may be said that the behavior of a vein in horizontal exposures is likely to be roughly duplicated down its dip. A possible change in the country rock should be borne in mind, however, as fissures and veins are likely to change markedly in structure upon passing from one rock into another.

A vein that is about to die out is likely to split into several diverging and diminishing stringers, and gradually to become lost in the country rock. This termination is more often noted in horizontal directions than in the dying out of a vein in depth, probably for the reason that few veins are followed in depth to their complete extinction.

The probabilities in regard to the ullVKvate de\Xv ol

Examination Of Prospects

N

A

f;

/

/

'' /

n

Scalo of Feet 1000 2000 300

Fig. 33. — Plan of the principal veins of the Cripple Creek District, Colorado, showing a roughly radial distribution about a common center. After Lindgren and Ranaome.

Structural Features Of Ore-Deposits 61

cate rather more clear relationships than commonly obtain among the veins of most districts.

In districts where the veins vary greatl}' in strike and dip it is easy to construct relationships that do not exist, and thus arrive at misleading conclusions.

The Persistence of Veins in Depth. — There is a distinct relation- ship between the length and depth of veins; in general, long, strong, wide veins persist in depth and short, non-continuous, irregular and weak veins die out at no great distance beneath the surface, perhaps to be followed by similar and roughly parallel veins in depth. While ore-bearing fissures are com- monly not important as faults, there is a relation between the amount of throw and the length and depth of a fissure; the greater the throw, the greater are likely to be these dimensions.

In the consideration of individual veins that have pinched in depth, a decision will rest upon the behavior of the vein in the parts already explored. If the proved length of the vein is con- siderably greater than the depth attained, and if the displacement along the vein is known to have been more than slight, the chances are good that the fissure will persist in depth. This has long been recognized by miners, who consider well-developed slickensides an indication of persistency in depth. . If a vein has been subject to local pinches above, it may be safely assumed that it will open out again with deeper exploration. In general, it may be said that the behavior of a vein in horizontal exposures is likely to be roughly duplicated down its dip, A possible change in the country rock should be borne in mind, however, as fissures and veins are likely to change markedly in structure upon passing from one rock into another.

A vein that is about to die out is likely to split into several diverging and diminishing stringers, and gradually to become lost in the country rock. This termination is more often noted in horizontal directions than in the dying out of a vein in depth, probably for the reason that few veins are followed in depth to their complete extinction.

Examination Of Prospects

N

A

f;

y

/—

Scale of Feet

1000 2000 3000

Fig. 33. — Plan of the principal veins of the Cripple Creek District, Colorado, showing a roughly radial distribution about a common center. AJter Lindgren and Ranaome,

Structural Features Of Ore-Deposits 61

cate rather more clear relationships than commonly obtain among the veins of most districts.

In districts where the veins vary greatly in strike and dip it is easy to construct relationships that do not exist, and thus arrive at misleading conclusions.

The Persistence of Veins in Depth. — There is a distinct relation- ship between the length and depth of veins; in general, long, strong, wide veins persist in depth and short, non-continuous, irregular and weak veins die out at no great distance beneath the surface, perhaps to be followed by similar and roughly parallel veins in depth. While ore-bearing fissures are com- monly not important as faults, there is a relation between the amount of throw and the length and depth of a fissure; the greater the throw, the greater are likely to be these dimensions.

In the consideration of individual veins that have pinched in depth, a decision will rest upon the behavior of the vein in the parts already explored. If the proved length of the vein is con- siderably greater than the depth attained, and if the displacement along the vein is known to have been more than slight, the chances are good that the fissure will persist in depth. This has long been recognized by miners, who consider well-developed slickensides an indication of persistency in depth. . If a vein has been subject to local pinches above, it may be sandy assumed that it will open out again with deeper exploration. In general, it may be said that the behavior of a vein in horizontal exposures is likely to be roughly duplicated down its dip. A possible change in the country rock should be borne in mind, however, as fissures and veins are likely to change markedly in structure upon passing from one rock into another.

A vein that is about to die out is likely to split into several diverging and diminishing stringers, and gradually to become lost in the country rock. This termination is more often noted in horizontal directions than in the dying out of a vein in depth, probably for the reason that few veins are followed in depth to their complete extinction.

62 Examination Of Prospects

veins, while of theoretical interest, have little or no practical significance. The depth at which the plasticity of rocks under great pressure no longer permits the existence of openings is far below the depth of possible mining operations.

The Relation Between Depth and the Number and Character of Veins. — In most mining districts veins are more numerous at and near the surface than in depth. Veins are much more likely to come together in depth than to divide as they go down, and many subordinate and weak veins die . out altogether within relatively slight depths below the surface. It is usual, moreover, for veins to become more regular in strike, dip and width in depth than they are nearer the surface, although they are commonly narrower in their deeper than in their upper parts. It appears probable that fissuring stresses are limited in their effect at considerable depth to a few fissures or to a single fissure, whereas near the surface the lesser burden of superposed rock permits a dissipation of the stresses among a number of more irregular fractures.

Mineral Veins Follow Fissures of Small Displacement. — Frac- tures through rock masses may be said to vary from planes of straining and incipient fracture to faults of great throw. Min- eralization may take place along zones of strained rocks, and while such conditions are structurally favorable to replacement, the passage of solutions must be slow at best. The other extreme, that of faults of great throw, appears to be unfavorable to the passage of solutions and mineralization.

Fractures through rocks are rarely plane surfaces; they com- monly follow curves over long distances, and through short distances are subject to many irregularities in strike and dip. The movement of one wall of a fissure past the other wall produces much finely ground material known as gouge, which in faults of large displacement commonly fills the fissure tightly, cement- ing the rock fragments and preventing any free flow of solutions.

Prof. Van Hise gives as his opinion that at depths of from 10,000 to 12,000 meters the weight of the overlying rocks is too great to permit the hardest rocks to retain their form.

Structural Features Of Ore-Deposits

Faults of small displacement are, in general, channels favorable to the ready passage of solutions. The irregularities of the walls not having been planed off, slight movement of one wall past the other brings the concavities opposite one another, thus forming open spaces; furthermore, the strained rocks bordering such fissures not having been ground up into gouge, permit the solu- tions to circulate through them and present conditions favorable to replacement. Most payable veins follow fissures of small dis- placement, although there are notable exceptions to this rule.

The Influence of Country Rock on Vein Structure. — A Assuring stress that produces an efficient circulation channel in one rock

Fia. 34. — Scattering of the Gottlob Vein, Freiberg, Germany, upon passing from gneiss into quartz-porphyry, g, Gneiss; p, quartz-porphyry; m, vein. AJter Beck.

may be too great, or too small, to produce a like result in a rock of

different character. The size, regularity and character of fissures vary according to the physical properties of the enclosing rocks, such as homogeneity, friability or plasticity.

A vein that traverses more than one country rock, therefore, is likely to vary greatly in structure in the several rocks. Fissures of small displacement, which are the sort commonly followed by mineral veins, may be strong and clean cut through a massive rock, but may fade out, and become lost in a plastic rock, where flowage or distortion is likely to take up the movement as a slight fold, without the production of a fracture.

Examination Of Prospects

Fig. 3S. — Horizontal plaa of the vein in the Maine mine, Georgetown, Colorado, showing thickening of the vein in pasfflng from gneiss into the harder porphyry. After Spun- and Garrey.

Fig. 36. — Horizontal plan of a part of the Seven Thirty v Colorado, showing diminution m the size of the vein in i granite into porphyry AJtirr Spurr and Garrey.

a, Georgetown, I passage from

Structural Features Of Ore-Deposits

vein that is persistent and regular in a homogeneous rock . passing into a brittle, unyielding rock, or into a rock mass is seamed with closely spaced joints, is likely to become Dated, and its mineralization scattered through so large a

. 37. — Diagrammado section of the Sheba ore-body, near Unionville la, showing the behavior of the vein in different rooks, o, Limestone; c, porphyry; d, ore. After Ranaome.

yrdaysclvase i' T\oXro.ce cpf ore

38. — Behavior of vein at Neihart, Montana, upon passing from Bchist into amphibolite. After Weed.

that the resulting deposit is of too low grade to permit iction.

!in8 that are contained in a large mass of igftoia Ytt, <it

66 Examination Of Prospects

reguiaj and more persistent than veins that traverse a series of bedded sediments.

Where it is seen, therefore, that a vein is to pasa from the rook through which it has been followed into a rock of different charac- ter, decision should be reaerved as to its probable continuity, unless abundant local evidence indicates that the change else- where in the district is without effect, or is favorable in character.

The behavior of a fissure at a contact between two rocks de- pends largely upon the angle at which it meets the contact.

39 — Sketch of a \em m the Gomer mine Idaho Spnnga, Colorado, showing the deflection of the vein upoa meeting a dike AfUr Spun

If the angle ts acute, the fissure may be deflected along the con- tact for some distance, and, upon crossing it, continue with a strike parallel to its original course If the fissure meets the contact at nearly right angles, it is likely to cross it without deflection, or, more rarely, to cease altogether.

In genera! shales, schi&ts and slates are more likely to absorb stresses without the production of defimte or continuous fissures, and so to permit a dissemination of solutions and resultant mineralization, than are rocks of non-fissile structure. Glassy rhyolites quartzites and other rigid and brittle rocks are likely

Structural Features Of Ore-Deposits 67

to be shattered by stress, and a mineralization that is regiilar in another rock may upon encountering them become disseminated through a large brecciated mass.

Soft, plastic intrusives are likely to adjust themselves to stress without fracture, and lodes upon passing into such rocks are unlikely to continue far into them; in this connection it should be borne in mind that the softening of igneous rocks through the metasomatic action of mineralizing solutions takes place during mineralization, and is not a factor in the formation of the fissures; such alteration indicates rather than militates against the continuity of associated fissures.

At Neihart, Montana, a series of steeply dipping metamor- phic rocks, consisting of feldspathic gneiss, softer bands of more schistose rocks, and occasional tough amphibolites, is cut by an irregular intrusion of diorite, and all in turn are intruded by a rhyolite porphyry. Well-defined fissure veins cross all these rocks, meeting the metamorphic strata at nearly right angles. The veins vary somewhat in width and in the relative abundance of included rock fragments upon passing from one belt of felds- pathic gneiss to another, and more markedly where they pass into the more schistose rocks, but the change is complete and abrupt where they meet the amphibolites; here the veins commonly narrow from a width of 7 or 8 ft. of good ore to a foot or more of barren gangue. Upon passing from the schist into the diorite, the veins invariably narrow, but continue well defined. In the rhyolite-porphyry the same fissures lose their compact character, and split into net-works of fractures through which the ore is dissipated; a well-defined vein upon passing into quartzite in the Big Seven Mine in this district splits into many small stringers.

The Distinction between Intercalated and Fissure Veins in Schistose Rocks. — This distinction is of great importance in the investigation of quartz veins through schists or gneisses, which frequently carry intercalated quartz lenses, or beds that are deceptive in appearance and not connected with the important mineralization of the district. W. H. Weed, Trans. A. I. M. E., XXXI, p.QT>

68 Examination Of Prospects

In a district where quartz lodes later than the schistosity are known to exist, and a quartz body conformable to the schistosity is found, an examination of the lode matter in thin section under the microscope will usually indicate whether the deposit is later than the schistosity, and perhaps connected with the important mineralization, or whether it is older than the mineralization and forms an integral part of the metamorphic series. Under the microscope minutely disseminated minerals, invisible to the un- aided eye, may be found which are characteristic of the commer- cially important veins, or, on the other hand, such minerals or inclusions may be lacking and the quartz under the microscope may show a schistose structure, indicating an origin contem- poraneous with that of the enclosing beds.

A careful investigation in the field will often disclose branching stringers, or minute veinlets, which indicate a later origin than that of the schists. Crustification is a clear indication of later origin.

Fissures and Lodes Formed Subsequent to the Principal Miner- alization. — In most mining districts the important mineraliza- tion was confined to one period, and where fissures and lodes of various origins are present, it is of the greatest importance to distinguish the commercially important series or system from the others. A careful plotting of all the fissures, whether ore-bearing or barren, will often indicate the prevailing trend of the pay fissures, and the general distribution of the pay fissures about some particular intrusive, or in some definite area or areas, may afford a basis upon which to decide whether or not a fissure is worthy of exploration. Lodes of different dates rarely carry the same vein fillings, and a distinction upon this basis is often practicable. The kind and degree of the alteration of the wall rocks along the veins affords another criterion.

Chapter Iv

Primary Ores And Their Distribution

Ore-deposits of commercial grade are local concentrations of great rarity when consiciered in relation to the area of unmineral- ized land surfaces, and they must therefore be considered as the products of exceptional and complex conditions. The data collected from a great number of individual deposits are sufficient to permit certain broad generalizations in regard to the conditions favorable to ore deposition which even numerous exceptions do not invalidate.

The broadest general relation of ore-deposits is with intrusive rocks, and while there are notable exceptions, the great majority of ore-deposits have a visible or closely inferred connection with intrusive rock masses.

Conditions that permit the dissipation of the ore-bearing vehicles do not permit the concentration of metals in deposits; the prime factor to consider in this connection is that structural conditions must be such as tend to concentrate and not to dissi- pate the ore-bearing solutions.

The disadvantage of extrusive rocks is at once apparent; any metals the magma may have contained are dissipated at the surface and are lost; with the exception of tiny vugs and cracks in quickly cooled flows, no concentrations of metals are known in extrusive rocks, and the lack of confinement and quick cooling apparently do not permit extrusives to exert mineralizing effect upon the rocks with which they come in contact. The close mineralogical similarity between extrusives and intrusives, and the great . difference in mineralizing power is a proof of the ease with which magmas part with their metallic content, and of the mobility of the ore-bearing vehicles in comparison with the. magma.

70 Examination Of Prospects

As Burface flows present the extreme conditions permitting dissipation, so laccoliths probably represent the extreme con- ditions favorable to the concentrated action of ore-bearii vehicles. The intrusion of a laccolith causes a domal uplift of overlying strata, with attendant Assuring of these strata; tie changes of volume during intrusion, cooling and contraction give rise to forces that reopen or keep open such fissures, over long periods of time, and afford vents for the mobile constituents ol the magma, which are the ore-bearing vehicles; in an extreme case the entire einanation from a laccolith may be conceived to pass through a single fissure.

Fia. 40. — Ideal section of laccoliths. A/Ur OUbert.

Between these two extremes of dissipation and confinement an infinite variety of intrusions are found in nature that give rise to an infinite variety of conditions under which ore deposition may take place.

In general, large bodies of igneous rocks do not carry ore deposits through their masses, the tendency being for the deposifs to form in adjacent rocks, or in the first cooled parts or edges of the intrusive itself; the stresses that fissure adjacent rocks are absorbed in the stJU hot and plastic parts of the intrusive, but not infrequently shatter the already solidified peripheries and afford therein channels for the passage of the ore-bearing solu- iJons and favorable loci for ore depoait\oiv.

Primary Ores And Their Distribution 71

Deep intrusives cut through and come in contact with many older rocks, and where these rocks exert precipitative action on the . ore-bearing solutions occur other favorable loci for ore deposition.

In general, prerequisites for ore deposition are conditions that permit the escape of the ore-bearing vehicles from intrusive magmas through restricted channels.

Metallogenetic Epochs and Provinces. — There are certain broad divisions in North America of which particular types of deposits are characteristic. These divisions in general may be assigned to different geologic epochs. Several prominent min- eral belts are well known. Gold quartz veins in schistose rocks are characteristic of a zone stretching for a long distance through California and to the north and south. A series of lenticular copper deposits occurs along the foot-hills of the Sierra Nevada in California. Many silver and silver-gold deposits east of the Sierra Nevadas present points of similarity and are referred to the early Tertiary; these deposits are characteristic of a broad zone that persists for a great distance to the south through the Cordilleras. Certain areas along the Appalachians in the Eastern States and in eastern Canada, and certain areas in the Western States, are characterized by deposits of pre-Cambrian age, which likewise present many points of similarity. The distribution of the disseminated chalcocite enrichments in the southwestern desert region is well known.

A majority of western ore deposits were formed in late Cre- taceous or in Tertiary times, and no great epochs of dynamic stress and metamorphism have affected them.

The Distribution of Ore-Deposits in Individual Mining Districts. — The valuable ore-deposits of any mining district commonly may be referred to a single period of mineralization, or to a single set of conditions; wherever possible, these governing factors should be determined and all exploration work should be planned with re- gard to them. Exploration by elimination, or the demonstration that certain areas do not contain ore-deposits, rarely yields

' Waldemar Lindgren, Economic Geology, Vol. IN ,

72 Examination Of Prospects

be directed to test the hypothesis that appears most reasonable in the light of a knowledge of ore-deposits in general, and of the particular mining district in which the work is undertaken.

The principal ore-deposits of any mining district are likely to be confined to a certain series of fissures, to some particular rock mass, stratum, area of altered rocks, locus of shattering, or to the vicinity of some particular intrusive or contact; not infrequently, the payable veins of a district fall into groups of similar strike, while the barren veins or lean veins fall into other groups. The significance of these relations is apparent in directing exploration.

The Association of Ore -Deposits with Certain Rocks. — There is a persistent connection between ore-deposits and monzonitic rocks throughout the Cordilleran region; the examples of this relation include many of the most important districts. That the converse of this relation — that ore-deposits may be expected where monzonitic stocks are found — is not true,- as is illustrated by the numerous monzonite masses through New Mexico that are not connected with any important mineralization.

The supposition that certain types of igneous rocks indicate the existence of ore is a fallacy that has been the cause of much fruitless exploration and loss. While a particular intrusive frequently controls ore deposition over a limited district or area, and exploration in that district is best confined to the sphere of influence of the intrusive, it does not follow that the same kind of rock elsewhere is a favorable indication of valuable deposits.

The association of tin and of tungsten with granite is well established, as is also the association between nickel, cobalt, platinum and chromium with basic rocks, commonly those that carry abundant ferro-magnesian silicates. Many other asso- ciations have been pointed out, but they appear to be persistent in restricted areas only.

The Depth to which Primary Ores Persist. — The outcrop or the exposure at any horizon of a primary ore-deposit affords a cri- terion of the value of that deposit to any depth above the zone of primary impoverishment; unless it can be shown that the deposit represents the root of a vein by far the gxeatex xoortion of which

Primary Ores And Their Distribution 73

has been destroyed by erosion, this is likely to mean any depth attainable by mining operations.

There is no genetic reason why the values of any primary deposit should not continue to great depths. Veins that are pockety, or whose ore-shoots are short and irregular, will prob- ably maintain these characteristics in depth, but the values, whatever their distribution, should be substantially the same at all horizons.

Secondary ores, however, being the results of surface processes, are limited to horizons near the surface; in mining geology there is no distinction of greater practical importance than that between primary and secondary ores.

The Criteria of Primary Ores. — In the investigation of any ore-shoot the first consideration is whether the ore is primary, secondary, or residual, as upon this rests all conclusions in regard to its persistency in depth. A primary ore is an ore that has undergone no change since deposition. A secondary ore is an ore formed by secondary, or surface, agencies. A residual ore is an ore that has remained after the solution and removal of asso- ciated minerals by secondary processes.

In distinguishmg between primary and secondary ores, the first criterion is the presence or absence of signs of oxidation; if an ore carries traces of oxidation, secondary action must be sus- pected, although it may be shown that it has had no effect in the distribution of values.

In thin section under the microscope the manner of intergrowth of primary minerals is characteristic, and in this way primary and secondary ores may usually be distinguished from each other; evidence of structural intergrowth is rarely visible to the unaided eye. The presence of two generations of sulphides, the richer being in general the later, and coating the other as if precipitated upon it, is usually clear evidence of secondary enrichment. The presence of seamlets of one sulphide, especially if the richer, through another sulphide, is likewise evidence of secondary enrichment. In deposits of massive pyrite that carry chalco-

74 EXAMINATION OF PkOSPECTS

latter mineral to occur as veinlets through the pyrite. This is probably due to the greater solubility and the later crystalliza- tion of chalcopyrite over pyrite under conditions of regional metamorphism, to which most of the deposits of this kind have been subjected.

The presence of fluid inclusions in an ore is evidence of primary origin unless secondary processes are seen to have been at work; a banded structure, or crustification, is another evidence of primary origin. While there are many doubtful cases where an ore may not be assigned definitely to either primary or secondary processes, in a majority of ores the typical associations of primary minerals, their manner of intergrowth, the presence of fluid inclusions or of crustification, indicate a primary ore, while the absence of oxidation, of secondary minerals, or of secondary rearrangement of the primary minerals, indicate that surface agencies have played no part in the distribution of values; such an ore may be expected to continue in depth to the zone of primary impoverishment, which in most cases is deeper than the limits of profitable mining.

No absolute rule may be formulated for field use, but an asso- ciation of the following sulphides commonly indicates a primary origin for any ore that carries no trace of oxidation or typical secondary structure: galena, zincblende or chalcopyrite with pyrite, pyrrhotite or arsenopyrite. The most common primary occurrence of gold is either as native gold or as a telluride. Probably the most common primary occurrence of silver is as argentite. Auriferous or argentiferous tetrahedrite is a common primary mineral.

The Minerals of Distinctively Primary Origin. — The minerals present in an ore frequently afford a basis upon which to judge its origin. Some minerals are distinctively primary, some dis- tinctively secondary, others, and among them are some of the most important ore minerals, are in some instances primary and in others secondary. The presence of a mineral of secondary origin proves the action of surface agencies; a mineral that is known to be sometimes of secoiidary oiim ttvat surface

Primary Ores And Their Distribution

encies may have enriched the ore under consideration, and so ,sts doubt upon the primary character of the ore containing it. enetic classifications of minerals have been made by Waldemar ndgren and W. H. Emmons, and their tables, which were ade for another purpose, have been freely consulted in the eparation of the following lists:

Distinctively Primary Minerals

Ore Minerals

Arsenopyrite

Pyrrhotite

Bismuthinite

Tellurides

Cobaltite

Tetradymite

Stibnite

(

Gangue Minerals

Albite

Orthoclase

Biotite

Rhodonite

Diopside

Rutile

Fluorite

Scapolite

Garnet

Specularite

Graphite

Spinel

Hornblende

Topaz

Ilmenite

Tourmaline

Muscovite

Minerals both Primary and Secondary in Origin

Argentite Proustite

Bomite (usually secondary) Pyrite Chalcopyrite (usually primary) Polybasite

Enargite Galena Gold Pyrargyrite

Economic Geology, Vol. II, p. 122. Economic Geology, Yo\. Ill, p. 625.

Sphalerite Stephanite Tetrahedrite Tennantite

76 Examination Of Prospects

Distinctively Secondary Minerals

Chalcedony Pyrolusite

Cuprite Chlorides

Chalcocite Sulphates of the heavy metals.

Covellite Carbonates of the heavy metals.

Kaolin Phosphates of the heavy metals.

Limonite Silicates of the heavy metals.

Opal Arsenates of the heavy metals.

The Primary Associations of Metals. — In primary ores some metals exhibit a tendency to associate themselves with certain minerals. Among the more prominent of these primary asso- ciations are:

Gold with quartz. Gold with pyrite. Gold with chalcopyrite. Silver with galena. Silver with copper. Silver with manganese. Copper with pyrite. Lead with barium.

Of these associations that of gold with chalcopyrite is probably stronger than its association with either quartz or pyrite, and the association of silver with lead is most marked.

The Accessory Minerals that Commonly Indicate a Segregation of Values. — Tetrahedrite is a guide to high silver and gold values in most deposits in which it occurs. In quartz veins, the presence of finely disseminated galena or chalcopyrite C'sulphurets"), or the presence of fluorite, are often indicative of high gold or silver values. In quartz veins that carry gold and silver it is frequent that quartz of a certain texture carries high values, while asso- ciated quartz of other textures is low grade or barren.

It is generally supposed that galena having curved crystal faces carries higher silver values than galena of cubical cleavage; Jt IS not unusual that where the cryataA. iak.ek oi ikexva, curved

Primary Ores And Their Distribution 77

fche associated minerals have a similar structure; the supposed relation, therefore, is not always a reliable guide. The fact is well known that a fine grained or granular galena is likely to carry more silver than the coarsely or well-crystallized mineral. Well- srystallized pyrite is usually quite lean in copper. It seems probable that the presence of silver in galena and of copper in pyrite tend to interrupt crystallization, and it is not unusual that information as to the content of these associated or contained metals may be gained by an inspection of the casts left in the outcrop after the solution and removal of the principal sulphide.

The relations between segregations of values and accessory minerals, or varying textures, are soon learned in the study of individual deposits, and often form valuable guides in exploration.

Primary Gold Ores. — At Cripple Creek, Colorado. — The characteristic feature of the ores is the occurrence of the gold in combination with tellurium, chiefly as calaverite, but partly also as the more argentiferous sylvanite, and probably to a minor ex- tent as other gold, silver and lead tellurides. Native gold appears to be absent from the telluride ores, except where set free by oxi- dation. Pyrite is widely disseminated through the country rock and also occurs in small quantities in the fissures associated with tellurides. Galena and sphalerite are sparingly present in the majority of the veins; tetrahedrite and stibnite are of frequent occurrence; molybdenite in small quantities is probably always present. The tetrahedrite is usually rich in silver, and also con- tains gold; possibly, however, the latter is due to admixed calav- erite, as the two minerals are often found in intimate intergrowth. The galena and zincblende rarely contain enough of the precious metals to form ore. Auriferous pyrite is often reported, but in the cases investigated the gold was found to be present as admixed tellurides. The usual minerals of the scanty gangue are quartz, fluorite, and dolomite.

At the Alaska-Tread well Mine, Alaska.' — The ore-bodies follow a dike of albite-diorite that carries a net-work of quartz and

Lindgren and Ransome, U. S. G. S., 'P. P. 54, p. 169.

A. C. Spencer, U. S. G. S. BvU. 225, p. 39, and Bull. 2&T,.

78 Examination Of Prospects

calcite veinlets. Pyrite occurs both in the veinlets and dis- seminated through the rock itself. Gold occurs in association with the pyrite and also native, and a large, though variable proportion of the value of the ore is saved by amalgamation. Visible specks of the gold are sometimes, though rarely, found. Associated minerals always present are pyrrhotite and magnetite; molybdenite is of common occurrence. Native arsenic, realgar, and orpiment have been noted. Arsenopyrite is suspected. Stibnite occurs in small amounts with the quartz. The bullion assays indicate small quantities of silver only.

At Grass Valley, California. — The primary ore is quartz that carries free gold in both fine and coarse particles, with from 2 per cent, to 3 per cent, of sulphides which also carry gold. Py- rite is the predominant sulphide; associated with it are galena, zincblende, chalcopyrite, and arsenopyrite. Subordinate acces- sory minerals are tetrahedrite and molybdenite. The quartz carries a little calcite. Fluid inclusions are abundant, and in many specimens are distributed in a manner dependent upon the distribution of the sulphides through the quartz.

Primary Copper Ores. — At Clifton, Arizona, the primary ores are unpayable disseminations that assay about 3/10 of 1 per cent, copper; from these low-grade ores the valuable deposits have been formed by secondary enrichment. The primary ore consists of sericitized quartz-mon zonite porphyry that carries veinlets of quartz and pyrite, disseminated pyrite, and a little finely divided chalcopyrite, zincblende, and molybdenite.

At Ducktown, Tennessee, theprimary ores consist of mass- ive pyrrhotite containing particles and stringers of chalcopyrite and pyrite, together with minute quantities of galena and zinc- blende. Calcite, zoisite and quartz, and occasionally bunches of garnet occur with the ore.

In Shasta County, California, the primary ores consist of

' Waldemar Lindgren, Seventeenth Annual Report U. S. G. S., Pt. II,

W. H. Weed, "Copper Mines of the World," p. 349. J. S. Diller, . 285, U. S. G. S., p. 173.

Primary Ores And Their Distribution 79

pyrite and chalcopyrite, . with some zincblende and galena, associated with quartz, calcite, and barite.

At Falun, Sweden, the ore is essentially a granular-crystal- line mixture of pyrite and quartz, with accessory magnetite, chalcopyrite, pyrrhotite, zincblende, and in rare cases, galena.

At Bisbee, Arizona, the primary ore consists of pyrite con- taining variable amounts of chalcopyrite and a little sphalerite Ekssociated with calcite, amphibole, pyroxene, garnet, chlorite, quartz, and vesuvianite; the metamorphic silicates are usually so tnely divided as to be indistinguishable by the unaided eye. The primary ore in general is unpayable.

Contact Ores. — Typical contact ores carry pyrite, chalco- pyrite, bomite, pjrrrhotite, specularite, and magnetite, with com- monly lesser amounts of galena and zincblende, associated with garnet, woUastonite, epidote, amphibole, pyroxene, vesuvianite, quartz and calcite. The gold and silver content is usually low. The distinctive association of minerals in contact ores is that of primary oxides with sulphides.

At San Pedro, New Mexico,* the contact ores consist of massive garnet replacing limestone and carrying chalcopyrite, Specularite, epidote, vesuvianite, woUastonite, quartz and calcite.

Primary Silver Ores. — At Lake City, Colorado, the primary ore minerals are galena, tetrahedrite, chalcopyrite, sphalerite and pyrite, associated with quartz, rhodonite, rhodochrosite, and barite. The silver is contained in the galena to the extent of 22 to 30 oz. per ton, and in the tetrahedrite, which is probably related to freibergite, in much larger quantity.

At the Granite-Bimetallic Mine, Montana,® the primary ore- consists of pyrite, arsenopyrite, tetrahedrite, and tennantite, with lesser quantities of galena and zincblende, in a gangue of

1 " Nature of Ore Deposits," Beck-Weed, p. 460. F. L. Ransome, P, P. 21, U. S. G. S.

Waldemar Lindgren.

M. B. Yung and R. S. McCaffery, Trans. A. I. M. E., XXXIII, p. 355.

J. D. Irving, Bull. 260, U. S. G. S., p. 81.

W. H. Emmons, Bull 315, U. S. G. S., p. 39.

80 Examination Of Prospects

quartz and rhodochrosite. Sparingly scattered through this ore are found small specks of pyrargyrite, proustite, and, rarely, realgar and orpiment. This ore carries from 20 to 30 oz. silver and from $1.50 to $3.00 in gold.

At Georgetown, Colorado, the primary ore consists of argentiferous galena and zincblende, with cupriferous pyrite, and chalcopyrite, in a gangue of quartz, and siderite, rhodochrosite, dolomite and calcite, in varying proportions; fluorite is present, but is rare.

At Tonopah, Nevada, the primary ores consist of argentite, polybasite, stephanite, and gold in a still undetermined form, associated with chalcopyrite, pyrite, and subordinate galena and zincblende in a gangue of quartz, adularia, sericite and carbonates.

At Park City, Utah, the primary ores consist of galena tetrahedrite, pyrite, chalcopyrite and zincblende in a siliceous gangue that carries a little barite and fluorite.

At Pachuca, Mexico, the primary ores where the veins become impoverished in depth consist of galena, zincblende, and pyrite in a gangue of quartz and rhodonite.,

At Guanajuato, Mexico, the primary ores con&ist of tetrahe- drite with zincblende and pyrite in a gangue of quartz, calcite, rhodonite, and fluorite.

Primary Lead Ores. — At Leadville, Colorado,* the primary ores consist of limestone replaced by quartz, pyrite, galena and zincblende, carrying a small quantity of silver sulphide. The ore contains about 1 per cent, manganese, in what form is not stated, as rhodonite and rhodochrosite are absent.

In the Coeur D'Alene District, Idaho, the primary ore consists of galena, pyrite, pyrrhotite, chalcopyrite, sphalerite,

' J. E. Spurr, P. P. 63, U. S. G. S., p. 136.

2 J. E. Spurr, P. P. 42, U. S. G. S., p. 22.

J. M. Boutwell, Bidl. 225, U. S. G. S., p. 147.

Srs. Aguilera and Ordonez, Trans. A. I. M. E., XXXII, p. 224.

5 Beck- Weed, "Nature of Ore Deposits," p. 265.

S. F. Emmons, Mono. XII, U. S. G. S., p. 32.

' F. L. Ransome, P. P. 62, U. S. G. S., p. 107.

Primary Ores And Their Distribution 81

Mid a little tetrahedrite and stibnite. Siderite and a little quartz form the gangue. Tetrahedrite is commonly accompanied by high values in silver.

Primary Zinc Ores. — The only primary ore of zinc of impor- tance is zincblende; in characteristic occurrences it is associated svith pyrite, and occasionally with galena and chalcopyrite.

The Depth of Primary Ore Deposition. — An important factor in bhe consideration of the probable behavior of a primary deposit in advance of exploration is the depth at which it formed. If it 3an be shown that a deposit was formed at the present surface, bhen it is evident that the type of ore prominent at the surface sannot be expected to continue in depth, as the conditions under which the ore was deposited — namely, surface conditions — were absent.

A deposit formed at relatively shallow depth is likely to grow more regular with deeper exploration; stringers and branch veins commonly consolidate in a single lode, or lodes, of relatively greater uniformity of dip, strike and thickness as compared with the more scattered units near the surface. This advantage of regularity is likely to be offset by a decrease in size in the deeper parts of the deposit.

A deposit of deep-seated origin, however, owes its discovery to a deep erosion, which has presumably removed the upper and irregular portions of the deposit, and exposed the deeper zone of relatively greater regularity. By regularity is meant the rela- tive regularity of different parts of the same deposit, and not the absolute regularity of a particular deposit as compared with deposits in general. Some primary deposits are probably quite irregular at all depths. A deposit of deep-seated origin whose values have not been redistributed by surface agencies is likely to be persistent in depth down to the zone of primary impover- ishment, and any section of such a deposit may be considered as indicative of its character at other horizons, in the absence of the factors that cause the localization of values into ore-shoots.

The deepest parts of ore-deposits, usually referred to as the

82 Examination Of Prospects

that have been referred to, and the data in regard to them are less satisfactory.

That increasing depths should give rise to transition types is to be expected, and it is not possible, therefore, to divide de- posits into sharply delimited classes on the basis of their depths of formation.

Deposits Formed at the Surface. — Primary mineral deposits formed at the surface by hot waters are rarely of economic importance; their primary condition is commonly obscured by the action of surface agencies. The sinters characteristic of surface-formed deposits are commonly made up of silica, as opal or chalcedony, and earthy carbonates. Calcite, fluorite, celes- tite, barite, and many other gangue minerals may also develop in crystallized form. Stibnite, pyrite, marcasite and cinnabar are known in crystallized form, and many other sulphides have been detected chemically in such deposits. Surface waters containing atmospheric oxygen are likely to have altered these deposits greatly, and limonite, hydrous oxides, carbonates, and sulphates of the heavy metals predominate among the ore min- erals, and kaolin, allophane and chloropal among the gangue minerals. The surficial formation of these deposits is commonly indicated by their structure.

Veins Formed near the Surface. — In typical examples these veins cut through beds of relatively recent volcanic rocks, and their depths at the time of vein formation may usually be deter- mined with fair accuracy. Structural features indicating a formation at shallow depth are: a greater number and width of the fissures near the surface; a branching of the upper parts of fissures; and fissures of changing dip, of which the deeper part is likely to have the flatter dip.

Metasomatic alteration of wall rocks is likely to extend to

greater distances from veins formed near the surface than from

deeper veins. In rocks of medium acidity a strong sericitization

. This and succeeding paragraphs are taken from Mr. Waldemar Lindgren's articles in Economic Geology Vol. II, p. 460, and Economic Geology, Vol. I, p. 34. Waldermar Undgren, P. P. 54, TJ. S. G. p. 11 .

Primary Ores And Their Distribution 83

is common immediately along the veins, and extensive pyritiza- tion is also frequent; the altering solutions have a tendency to spread from the veins through a large area of adjoining rocks, owing to more extensive Assuring near the surface, where, robbed of their most active ingredients, they effect a propylitic alteration over large areas. In very basic rocks this propylitization extends close up to the veins, where sericitization is likely to take its place; in large, irregularly altered areas, especially in siliceous rocks like rhyolite, extensive silicification is common.

In these deposits gold and silver prevail, and as compared with deep-seated veins of quartz gangue, silver is relatively more abundant, and free gold is commonly present in a more finely divided form; pyrite, zincblende, chalcopyrite, arsenopyrite, argentite, tellurides and stibnite are the prevailing ore minerals; among gangue minerals, quartz is most abundant, but it is often accompanied by chalcedony or opal; calcite and dolomite are moderately abundant in the vein filling; siderite occurs more rarely; barite and fluorite predominate locally. Magnetite and specularite, as well as all the silicates belonging to the greater depths of ore deposition, are absent. In these veins the filling of open spaces is an important process.

Veins of Deep-seated Origin. — These veins are divided by Mr. Lindgren into four classes:

(a) Contact deposits, which are discussed elsewhere.

(b) Cassiterite veins. The characteristic minerals of this type are cassiterite, pyrite, arsenopyrite, specularite, quartz, tour- maline, topaz, lepidolite, muscovite, apatite, fluorite, and wol- framite, with subordinate calcite and siderite. These veins are commonly poor in gold and silver, and the metasomatic altera- tion along their walls is likely to be intense.

(c) Apatite veins. The characteristic minerals of this type are apatite and other phosphates, scapolite, diopside, hornblende, biotite, specularite and pyrrhotite; strong metasomatic action is usual along the walls of these veins, and the introduction of chlorine and fluorine is characteristic; these veins are commoiibj poor in gold and silver.

Examination Of Prospects

(d) Deep-seated gold and silver veins. The characteristio minerals of this type are gold, pyrite, pyrrhotite, galena, Bine- blende, magnetite, specularite, ilmenite, quartz, biotite, touimv line, garnet, hornblende, chlorite, apatite, spinel, and epidote; calcite is present in small amounts; tlie replacement of the coun- try rock is usually strongly marked; amphibolites and micaceous schists are replaced by tourmaline, garnet, a green biotite and epidote; aoda-lime feldspars are unstable under the influence d vein forming solutions and alkali feldspars usually do not form. These veins commonly occur in, or close to, granite intrusives in schists.

s

Fio. 41. — Sectioci ii the v, ii f m jah \Biada, allowing upward branching i/tcr Spurr

Aa a rule, the walls of deep-seated veins are not altered through so great distances as is common with veins formed at shallow depths, and their vein fillings frequently bear evidence of having been subjected to the stresses of dynamometamorphism. The time of formation of the deep-seated deposits is likely to be remote as compared with deposits formed at ahallQ-w deaths.

Primary Ores And Their Distribution 85

Relative Susceptibility of Hanging- and Foot-walls to Mineraliza- tion. — In many veins the hanging-wall has been subject to brec- ciation and mineralization to a far greater extent than the foot- wall, and hydrothermal alteration more frequently extends into it than into the foot-wall. This is probably the result of the superior resistance of the foot-wall to fracturing as compared with the hanging-wall. The hanging-wall rocks readily adjust them- selves to stress through fracturing, while the foot-wall, under as, great, or greater, stress, remains massive, owing to the rein- forcement by underlying rock masses.

Chapter V

Types Of Primary Ore-Deposits

The classification here used is one of convenience only; it is not intended to include all known types of ore-deposits. The characteristic features of the several well-marked types of pri- mary mineralizations are described without reference to the ultimate source of their metals. The minerals of many deposits obscure origin were probably introduced through fissures that are now healed, or are due to the migration of metals from such deposits. A discussion of the genesis of these deposits is not justified by the present state of knowledge of economic geology.

Magmatic Segregations. — During the solidification of magmas under conditions that do not permit the escape of their metallic

Fia. 42. — Sketch showing the Drinkwater zone of quartz lenses in alaskite, Silver Peak, Nevada; the lenses are magmatic segregations. After Spun.

content, there is a tendency for like particles to form segregations probably through mass action, or the mutual attraction of like particles. Magmatic segregations are commonly made up of compounds of relatively low mobility.

Basic rocks appear to be the most suitable for the formation of segregations, the characteristic minerals of which are magnetite, ilmenite, chromite, pyrrhotite, pyrite, and pentlandite. The gangue minerals are those of the containing rock. Acid, or

Waldemar Lindgren, Economic Geology, Yo\. 11, 110.

Types Op Primary Ore-Deposits 87

quartzose segregations, however, are known. Gold and plati- num have been established as constituent minerals of rock masses, chiefly as sparse disseminations, unpayable in themselves, but important as the sources of placer deposits.

Magmatic segregations are rarely of economic importance as ores, and may usually be recognized in thin section under the microscope by the manner of intergrowth of the ore with the rock minerals. It is not unusual to find that rocks containing such deposits have become further differentiated into two or more parts of different mineralogical composition. Clearly established examples of magmatic segregations are more rare than was at one time supposed. The most generally accepted type is that of the titaniferous magnetite deposits.

In the Adirondack Mountains, New York, deposits of titaniferous magnetite are associated with basic intrusives in such a way as to indicate that the ore minerals were segregated from the containing rock as it cooled. No ore was formed along the contacts of these intrusives with the enclosing rocks, and the ore grades into the containing rock through a transition zone that shows a gradual change in the quantities but not in the kinds of the constituent minerals.

At Silver Peak, Nevada, auriferous quartz appears to have segregated from enclosing alaskite, which is a phase of the accom- panying granite; the ore bears the same relation to the alaskite that the latter bears to the granite. The alaskite is a granite without biotite; the quartz is an alaskite without feldspar. The segregated quartz is in this case of the same age and generation as the granules of quartz that make up a large proportion of the alaskite and granite.

Contact Deposits. — Contact deposits are deposits formed along the contacts between intrusives and their enclosing rocks, or in these rocks in immediate proximity to the intrusives. They are the result of direct emanation of mineral-bearing solutions from

1 J. F. Kemp, Nineteenth Annual Report, U. S. G. S., Ill, p. 392. J. E. SpuiT, P. P. 55. , U. S. G. S., p. g7.

88 Examination Of Prospects

the intruding magma. Contact deposits are usually limited to rocks that exert strong precipitative action. The characteristic minerals of contact deposits are: specularite, magnetite, bomite, chalcopyrite, pyrite, pyrrhotite, and more rarely, galena and zincblende, associated with garnet, wollastonite, epidote, Uvaite, amphibole, pyroxene, zoisite, vesuvianite, quartz and calcite, and rarely, fluorite and barite. The unique feature is the associa- tion of the oxides of iron with sulphides. The sulphides fre- quently carry gold and silver, but usually in small quantities only.

Contact deposits are rarely of commercial importance, although there are certain notable exceptions. Many deposits formerly included in this class are now more correctly considered as replacement deposits.

Contact deposits are usually quite irregular in form, due largely to the common irregularity of the igneous intrusion, and when the ore is lost, it is generally recovered with difficulty; while there is apparently no genetic reason why contact deposits, which are of deep-seated origin, should not be persistent, it is well known that few contact deposits are jmined at more than shallow depths.

In most cases certain beds of the intruded rocks exert a greater precipitative action, or are more easily replaced, than other beds of the series, and contact minerals develop much more abundantly in them than elsewhere. It is necessary in the examination of contact deposits to trace these favorable beds and to determine their probable thickness beneath the better exposures; not infre- quently, the important mineralization is confined to one or more such beds, to which, therefore, exploration should be limited, and below which the mineralization should not be expected to extend.

Most contact minerals are resistant to weathering, and erosion is often halted at the horizon where the contact metamorphic silicates reach their largest development. Large outcrops do not, therefore, afford a reliable criterion of the extent of such

WaJdemar Lindgren, Trans,, A. I. M. E., Vo\. XXXV, .

Types Of Primary Ore-Deposits 89

deposits in depth. A large development of contact metamorphic silicates does not necessarily indicate the existence of valuable ore any more than a barren quartz vein indicates the presence of gold. Whether or not such an outcrop contained sulphides before oxidation may usually be determined by a search for and examination of the casts left in the resistant silicates upon the solution of contained sulphides; this feature is taken up in the chapter on outcrops. The ores of contact deposits, unless of smelting grade, offer serious metallurgical difficulties, owing to the high specific gravity of their gangue minerals.

The phenomena of contact metamorphism are considered in the chapter on Primary Alterations of Wall Rocks.

At Mokenci, Arizona, important contact deposits occur in limestones and shales along intrusions of quartz-monzonite por- phyry. Wherever the porphyry came in contact with granite or quartzite, little alteration is observed, but wherever the porphyry meets the limestones or shales of the Paleozoic series extensive contact metamorphism has taken place. The whole Paleozoic , series is affected, but more particularly the pure limestone of the lower Carboniferous, which, for a distance of several hundred feet from the contact, has been converted into an almost solid mass of garnet. The shales have suffered less from this metamorphism, but near the porphyry are likely to contain epidote and other accessions. Wherever alteration by surface agencies has not masked the phenomena, magnetite, pyrite, chalcopyrite, molyb- denite, specularite and zincblende accompany in various pro- portions the contact-metamorphic minerals, which here comprise garnet, epidote, diopside, tremolite and quartz. In form, the deposits in limestone are irregular, but in many cases they assume a tabular shape, due to the accumulation of the minerals along certain favorable planes of stratification or along the walls of dikes. These contact deposits differ from most examples in the absence of woUastonite and vesuvianite, and in the metamorph- ism of the shales; instead of the knotty schist or hornfels usually produced from shales there is found at Morenci a -

Waldemar Lindgren, P. P. 43, U. S. G. S., p.

90 Examination Of Prospects

fels, with much amphibole (tremolite), epidote, pyrite and magnetite.

Pegmatitic Deposits. — The characteristic minerals of pegmatitic deposits are magnetite, bornite, arsenopyrite, molybdenite, cassi- terite and wolframite, associated with quartz, muscovite, alkali feldspars, tourmaline, apatite, fluorite, spodumene,* and more rarely, hornblende and soda-lime feldspars. These deposits, which are of deep-seated origin, contain little gold and silver and except where associated with stockworks and cassiterite-bearing impregnations they are irregular in value and of slight economic importance, except for mica and minerals of the rare earths. Pegmatite dikes carrying wolframite are known at many places in the western United States; pockets of rich ore are occasionally found in them, but these deposits apparently have not repaid exploration.

Fahlbands. — Beds of schist that have been impregnated with sulphides and subjected to dynamo-regional metamorphism are known as fahlbands. The sulphides occur disseminated through the schist intergrown with the principal minerals of the rock, and also along the planes of schistosity; the association is such as to render obscure their origin and mode of introduction. These deposits, which are rarely of economic importance, are apparently confined to areas of pre-Cambrian rocks. Fahlbands frequently persist over long distances, but it is rare that their mineralization is sufficiently concentrated to form ore. In certain European localities, they have exerted an important influence upon the segregation of values in veins that cross them.

In the Upper Pecos District, New Mexico,* an amphibolit, probably produced by regional metamorphism from a dioritic or diabasic rock, carries disseminated chalcopyrite and zincblende that contain a little gold and silver, associated with a green bio- tite, tourmaline, and veinlets of dark quartz, and intergrown with the principal minerals of the containing rock.

Waldemar Lindgren, Economic Geology, Vol. II, p. 111.

3 J. F. Kemp, "Ore Deposits," p. 73.

Lindgren Graton and Gordon, P. P. 68, U. S. G. S., p. 60,

Types Of Primary Ore-Deposits

Regionally Metamorphosed Ore-Deposits. — Deposits that were formed in remote geological ages are likely to have been deeply buried, and to have undergone rearrangement under the stresses of dynamo-regional metamorphism. The recrystallization result-

E3 B

SchlsC On Zoat Gouan Cbalcocita On

Fia. 43. — Section of the ore-deposit in the Mary mine, DucJitown, Tennessee, showing a regionally metamorphosed ore-deposit. AfUr Emmons and Laney.

ing from these processes may so completely change the enclosing rocks to crystalline scliists as to hide completely their original character, it being difficult to distinguish schists that result from the metamorphism of sedimentary beds from those that were originally igneous rocks. Under these ccm.i!vAQMl cA.

92 Examination Of Prospects

heat and flowage the small amount of moisture contained in the rocks appears sufficient to accomplish a complete rearrangement of the minerals. Ore-deposits contained in these rocks have imdergone complete transformation, and their original relation- ships have been obscured to such an extent that their origin is not determinable; they are, therefore, best described as a separate type.. Rearrangement under these conditions brings about a segregation of ore minerals into homogeneous bodies, and maybe considered a process of primary concentration, the ores resulting from rearrangement without accession of material from without. Minerals that are stable under the conditions of dynamo-regional metamorphism and which therefore are characteristic of these deposits are: pyrite, chalcopyrite, pyrrhotite, magnetite, quartz, jnuscovite, biotite, epidote, hornblende and albite.

Regionally metamorphosed deposits usually occur parallel to the schistosity of the enclosing rock, or cross it at low angles, in accord with the law that tabular bodies under stress and flowage, tend to orient themselves in the direction of least pressure.* The relations of contact deposits to igneous rocks are likely to have been destroyed by the metamorphism, and the fissures and chan- nels that produced replacement deposits are likely to have been healed. The usual guides in exploration for these deposits, therefore, are obliterated. The most marked structural features of regionally metamorphosed ore-deposits are their lenticular form, and the frequent distribution of such lenses in an over- lapping series. The occurrence of these primary ore-bodies is discussed under 'Xenticular Ore-Shoots'' in a succeeding chapter. The individual lenses of deposits of this type are occasionally of large size, but not infrequently the mineralization is confined to a single lenticular mass, and expectations of future ore are not justified beyond the probable content of the lenses already exposed. These deposits, being thickest at their centers and narrowing to extinction toward their peripheries, often permit approximate estimates of their content to be made in advance of

Waldemar Lindgren, Economic Geology, Vol. II, p. 127. W, H. EmmonSf Economic Geology, "Vol. IV, 17.

exploration. The section presented by the surface is likely to be J & fair criterion, of the distribution of deposits of this type in-J depth. In exploration for further deposits cross-cuts should be J driven near the extremities of the known lenses to expose possible A overlapping bodies; in most cases little other exploration i justified.

. The Vermont Coppeh Belt' contains three districts — Corinth] I Copperfield and South Strafford. The deposits occur along i I due north-south line, which corresponds to the general directioaS of the schistosity of the rocks. The rocks are micaceous Bchist*.'! and gneisses, formed from sandstones and shales by regionalij metamorphism. The original bedding, though obscure, is fl occasionally in evidence, and does not correspond with the folia- I tion. Intrusions of granite are common in the region, but do not 1 occur in the immediate vicinity of the ore-deposits. The bodies are lenticular masses which simulate bedded deposits, since they appear to conform to the banding of tlie enclosing ' schists. At each locality only one workable lens has been found outcropping; in the deep mines the outcropping lens wedges out in depth, but is found to overlap the tapered upper end of another lens in the foot-wall. The deposits have no gossan cap, sulphides appearing at the surface. The ores consist of massive pyrrhotite, chalcopyrite, pyrite, and a little sphalerite mixed with variable quantities of quartz and actinolite; in the leaner ores, garnet anda biotite are present.

Deposits Due to the Filling of Open Spaces. — The filling ofV open fissures or preexisting cavities in rocks is a process of great importance in ore deposition; while many veins are due to thiff] process alone, a majority of mineral veins are probably thftJ result of both replacement and the filling of open spaces. Open T spaces are caused by irregular fissuring accompanied by moderate movement, sufficient to bring projection opposite- projection and concavity opposite concavity, and thus cauHefl pinches and swells in the fissure and resulting vein. It is clear that a unifoi'm open space of large extent cai :.H. Weed, 22.'-., U. S. G. S., p. 199.

94 Examination Of Prospects

remain open along a fissure for any length of time, but must soon be closed by the pressure of overlying rocks. Fissures, therefore, that remained open for sufficient lengths of time to become filled and mineralized are commonly irregular in croas- section, the open stretches being separated by tight portions where the wall rocks came together and formed the buttresses that permitted intervening portions to remain open. It is A common fallacy that filled fissures are characterized by uni- formity of direction and thickness; in most veins, which are the result of both filling and replacement, the latter process tends to counteract and to obscure the irregularities of the origni open spaces.

Fig. 44. — Section of a crustified vein near the London shaft, Silvertoo, Colorado, a. Country rock; 6, quartz and choicopyrite; c, tetrahedril*; d-<i', quartz; e, galena. After Ranaome, '

Enlargement by solution is thought by many to play a large part in the formation of cavities that are subsequently filled with ore. Except in limestone; perhaps, such solution takes place contemporaneously with and constitutes a part of the process of replacement.

Open fissures and cavities in rocks are more abundant near the surface than in depth, because of the less pressure of overlying rocks that tends to close them. While a majority of deep-seated veins are replacement veins, open cavities frequently persist to

Types Of Primary Ore-Deposits 95

trc great depths, especially in veins that pinch and swell as before Tk described.

rA. According to C. R. Van Hise the zone of flowage from pres- I sure of overlying rocks, in which no cavities can exist, is reached r at 1625 ft. in soft shales and at 32,500 ft. in firm granites.

Fissuring accompanied by sufficient movement to form open spaces in one rock, may produce a tight fissure in a more plastic rock at the same depth, and veins that are the result of the fiilling of open spaces may be expected to vary in width according to the character of the rock traversed; a vein that cuts a series of different beds may be expected to vary markedly in passing through them.

If the valuable mineral in a filled fissure was among the earliest deposited, it will be found near the walls, and may be expected to persist longitudinally along the vein; if it was among "the last deposited, it is likely to be limited to the spaces at the center that remained open at the time of its deposition, and, therefore, to be more markedly confined to shoots.

The usual criterion of a filled fissure is the arrangement of the filling in crusts or bands parallel to the walls, similar bands occupying the same relative positions on either side of the center line, which is frequently marked by interlocking combs of crystals. The bands nearest the walls represent the minerals first deposited, and the central part the latest deposition. In cases where the vein after being filled has been reopened, the additional crusts deposited will not be symmetrical with respect to the original arrangement, and occasionally veins are seen that in this way exhibit a record of repeated opening and filling.

While in filled deposits evidence of crustification is often visible, not infrequently the filling is homogeneous or quite irregular, and the method of vein filling is not apparent, unless disclosed by the examination of thin sections under the micro- scope. The presence of radiating clusters of crystals is indica- tive of deposition in an open space, as is also a lack of alteration of included fragments of the wall rocks. Not infrequently,

Sixteenth Annual Report U. S. G. S., I, p. 312.

96 Examination Of Prospects

replacement veins exhibit a banded structure; this is the resuK of a thin sheeting of the country rock, the lines of which are repeated and preserved in the arrangement of the vein minerals, the replacement having taken place along the sheeting planes first, and later, under different conditions, having penetrated the intervening slabs. A reopening of a fissure during or after mineralization may give rise to a structure in close imitation of crustification.

The lines of demarkation between ore and wall rock are commonly well-defined in filled deposits, while in replacement deposits the ore is likely to merge gradually into the walls. That the action of the solutions was confined to the open spaces in filled fissures is not invariably the rule; frequently the ore is found along a well-defined wall, beyond which the ore minerals do not penetrate, but the rock bordening the vein is altered, evidently by the action of the vein-forming solutions. It is probable in such cases that the walls of the vein acted as dialyzers, restraining the passage of certain elements and compounds, which precipitated within the vein, but permitting the passage of the solutions that altered the adjacent rocks.

At Pings Altos, New Mexico, the process of open fissure fill- ing is well illustrated in the Pacific vein. Five distinct bands may be counted. Proceeding from each wall inward to the center, each of these bands has an almost perfect counterpart on the opposite side of the vein. The first band, that next to the wall, contains quartz and pyrite; its inner edge is outlined by the crystalline terminations of quartz prisms, a beautiful example of comb structure. The succeeding band is composed of zinc- blende and chalcopyrite; the chalcoprite grows more abundant toward the inner edge and, in fact, forms two subsidiary "bands separated by a thin band of sphalerite. The next layer, a thin band, contains quartz and chalcopyrite. It is followed by a narrow band of sphalerite, which in turn is followed by a thicker band of quartz that contains fine grains of disseminated chalco- pyrite, and locally fails to join with its corresponding band on

Sidney Paige, Bull 470, U. S. G, S., p. 114.

Types Of Primary Ore-Deposits

the opposite eide, leaving an open crystalline cavity at the center.

On one wall of the vein is a narrow secondary vein, evidently a reopened fissure Its walls are outlined bj narrow bands of quartz (with a little chalcopynte and galena) between which is a pinkish cream colored mass of iron and magnesia carbon-

Mi IMl

Fig. 45. — Specimen from the Pacific vein near Pinos Altos, New Mexico, showing crustificatiott. a, Chalcopynte; 6, pyrite; c, zineblende; d, quartE; e, aerioitiied porphyry;/, carbonates, quartz, and iron oxide. A/ter Paige.

ates, and quartz. The narrow quartz bands forming the walls of this little vein have locally been broken, and pieces of the wall now lie at varying angles across the vein, embedded in the vein filling.

On the opposite wall of the main vein a fragment of country rock is included in and surrounded by the vein material of the large vein.

98 Examination Of Prospects

From this data the history of the mineralization of this partic- ular vein may be deduced. A fracture was formed in the country rock and filled by solutions carrying zinc and iron sulphides. Fracturing continued and cross fissures on a sniall scale were opened. The forces, of whose presence this first fracturing waa a preliminary, finally succeeded in producing an open fracture measured by the width of the vein described, and solutions carrying silica, iron sulphide, a trace of zinc sulphide, and lead, circulated through the open spaces thus afforded. Along both walls quartz and pyrite were precipitated simultaneously, and continued to be precipitated, apparently, until solutions ceased to circulate, or ceased to carry sulphur, iron and silica, for the boundary between the first band and the succeeding one is sharp both in demarkation and in mineral content. When mineralizing waters next flowed past the walls, zincblende and chalcopyrite were deposited, and it is evident that, although copper, sulphur and iron were present during the remainder of the history of the vein, though growing markedly less toward the end, the zincblende and silica content fluctuated, first a layer of one and then of the other being precipitated. Parts of the vein along the center were probably never completely filled, not be- cause there was a lack of material, but because deposition fortui- tously isolated geode-like open spaces within which the circu- lation ceased. The small vein at the edge of the large one points to a recurrence of fracturing, and the advent of carbonated waters, carrying silica also; it marks a distinct change in the solutions, with the cessation of which the mineralization closed.

Replacement Veins. — A majority of veins are in part, at least, the result of the replacement of their walls by mineralizing solu- tions, and in many cases the process of fissure filling was probably so subordinate as to be practically negligable; the original fissures of replacement veins, which were probably narrow, acted chiefly as channels for the passage of the replacing and mineralizing solutions. While in a filled fissure the width of the vein repre- sents the width of the cavity filled, and the line of demarkation between ore and wall rock is sharp, in a replacement vein the

TYPES OF FRIMAIiY ORE-DEPOSITS

99"

-width bears no relation to the size of the original fissure, and the line of deraarkation between ore and waJl rock is commonly ill- defined.

Replacement veins vary greatly in size according to the ease

of solubility or replacement of the rock traversed, and their

values are likely to be localized by the chemical precipitative

action of the different rocks traversed. Replacement veins are

J. co mmonly accompanied by metasomatic alteration of their 'walls

ft -will be described in a succeeding chapter; fragments of fiu

Fio. 4 — r rpplaoement, Oia

wall rocks included in replacement veins are likely to be parti or completely replaced by ore; the original outlines of su< fragments are usually preserved in the arrangement of the replacing minerals, which may enclose cores of freah or partly altered rock, and thus conclusively prove the process of replacement.

Replacement Deposits. — Metasomatic replacement of country rock by mineralizing solutions is a process of importance in ore deposition, and there are probably few epi|

mm

irtJy i of

Examination Of Prospects

netic deposits in the formation of which it has not played some part. In a classiBcation of ore-deposits for the purpose of study and practical correlation it appears beat to discuss under this head those replacement deposits that arc relatively homogeneous, taking up the other types accordmg to their most prominent characteristics.

Replacement deposits are most common m the more easily soluble and replaceable rocks, among which limestone is most prominent. That the preeipitative action of the replaced rock

FiQ. 47. — Replacement of limestone by copper ore, Bmgbam, Utah, show- ing the greater replacement of selected beds. After BoutJvelL

is not the controlling feature of the process is shown by the occurrence of large replacement deposits in quartzite, shales, schists and other rocks as well as in limestones. The relative replaceability of a rock varies, of course, with the chemical composition of the mineralizing solution, certain solutions attack- ing limestone with greatest activity, while others replace quartzite with equal ease; in a givep deposit, however, a rock that has been extensively replaced must be considered the ore-bearing rock, as it is unlikely that a different rock will have yielded equally

Types Of Primary Ore-Deposits

the attack of the same mineralizing solutions; important lOBits in a bed of limestone, for example, may not be expected continue into underlying quart zite or granite. It often ipens that one bed only of a seriea of apparently similar rocks offered a favorable horizon for ore deposition, other beds

. 48. — Replacement of limestone by argentiferous galena, Bingham, Utah. After Boutwell.

ag lean or barren. In such cases a study of the stratigraphy

Imperative.

leplacement deposits commonly occmt m Xj'n.e N\ft'ai&

lougJi not adjacent to, intrusive TOcAta, a-ii wie t-Mws

Examination Of Prospects

closely related to contact deposits; in many instaaces, however, replacement deposits occur without visible sociation with intrusives, the mineralizing solutions having gained access to the replaced beds or rocks through fissures. Replacement deposits unless occurring along a prominent fissure, are likely to be as irregular in distribution through the mineralized horizon as they are individually in form. Replacement deposits are frequently

Fig. 49.Sectioii through the ore-bodies at Sierra Mojada, Coahuils, Mexico, showing irregular replacement deposits in limestone. Aflir Malcolmson.

connected with each other or with the main circulation channel, by feeders, or veinlets, which often are quite barren of minerak and with difficulty traceable. Such obscure feeders are often the only guides in exploration for new deposits.

The degree of shattering, brecciation or straining is, next to the repl a ceability of the rock, the moat important factor in determining the position of replaeemeiA ifeoft\\.%-, "Oii igcwiSa

Types Of Primary Ore-Deposits

the area of the surfaces exposed in relation to the mass of rock, the more rapidly and thoroughly does replacement take place, and the existence of these deposits may occasionally be pre- dicted through tracing the shattered or strained zones. The passage of solutions through rocks is seen under the microscope to take place even where the existence of cracks or minute fssures ia not discemable, and the invading mineral may form grains that are apparently completely surrounded by fresh, solid Tock; a zone, therefore, within which the rock has been subjected

FiQ. 50 — Sections showing mineralizing fies ire- limestone strata by siliceous gold ore Black IIiUs bouth Datota AJter Irving.

to slight strammg of the particles onlj without rupture may afford access to the replacing solutions Shattering is a less evident factor in the formation of replacement deposits in lime- stone than m other rocki probably because of its read} solubility.

Replacement deposits are rarely of great vertical extent, and very often the rock above them affords no indication of their existence. Outcrops of these deposits, therefore, are accidental, and relatively scarce, and the existence of one such body having been established, blind exploration for new deposits in the same horizon is more often justified than is the case with deposits of other tjTJes.

The boundaries of replacement depoaYa ate vvsi&'S -owj

Examination Of Prospects

defined, the ore merging gradually into the enclosing rock, where the boundary between ore and waste becomes a question of assay only. Occasionally, one mineral has penetrated the rock more easily than the others, and so preponderates in the outer parts of the deposits; in exploration, therefore, upon meeting such a lean, or perhaps barren, mineralization, it may indicate the existence of payable ore beyond.

Replacement ores frequently reproduce the structure of tie replaced rock, certain bedding planes being, followed in preference to others, and where this condition obtains the bedding planes are usually parallel to the greatest dimension of the ore-body.

The principal alterations of limestones as an accompaniment of mineralization are silicification, marmorization, and to a less extent, the development of a disseminated pyrite mineralization, any of which may afford clews to the existence of ore-bodies. The primary alterations that accompany mineralization will be discussed in a later chapter.

In the Coeur D'Alenes, Idaho,* large replacement deposits have formed along shattered zones in quartzite. In the Bunker Hill mine the ore consists of galena and siderite, with small quantities of quartz, zincblende and pyrite, the gangue being the enclosing quartzite. It appears that siderite first replaced the quartzite, and that later argentiferous galena partially replaced the siderite, though the direct replacement of quartzite by galena is occasionally noted. The best ore consists of rather fine- grained masses of galena with subordinate siderite, which grades into ore in which the siderite exceeds the galena, and this into barren quartzite. The ore is principally a replacement of the Revett quartzite, but the replacement is closely connected with fissuring, and some of the galena was deposited in open spaces. In some of the important stopes, quartz and pyrite are usually most conspicuous in the transition zone from ore to country rock. The zone of fissured quartzite in which the ore-bodies occiu* has a maximum width of 300 ft. measured perpendicularly to the Bunker Hill fissure. Within this zone, here in contact with the

i?! L. Ransome, P. P. 62, TJ. S.G.., p. l'i.

Types Of Primary Ore-Deposits 105

foot-wall, there separated from it by barren quartzite, are numer- ous irregular ore-bodies, usually without definite walls or bound- aries. Individual ore-shoots reach 500 ft. in length, 100 ft. or more in width, and 300 to 400 ft. in depth. The whole fissured zone may, in a broad sense, be regarded as a single great lode, within which the partly overlapping and partly connected ore bodies are not uniformly distributed, but are grouped in at least four fairly distinct shoots. That no ore should have been de- posited beneath the persistent seam of dark gouge characteristic of this -fissure is remarkable, as the quartzites of the foot- wall, which have been well explored, are identical in character with those of the hanging wall, and are in places extensively fissured and broken, though usually to a less degree than in the hanging wall.

In the Highland Boy Mine, Bingham, Utah,* important re- placement deposits of copper ore occur in limestone. The limestone is commonly a coarsely crystalline marble, in general more cherty toward the base where it rests upon quartzite, more massive and crystalline above, and locally siliceous. The ore bodies lie within the main body of the limestone well above the underlying quartzite along a zone of fissuring and mineralization. Localization of ore has resulted in the formation of three well- defined lenses or shoots, the largest of which reaches a width of 400 ft. and a thickness of 100 ft.; the shoots are approximately conformable to the bedding. In the few instances where cross- cutting has exposed the quartzite, the ore does not make down to it. The walls of the ore-bodies are commonly slip planes or beds of siliceous or crystalline limestone. In some instances the upper parts of the ore-bodies become progressively leaner until they pass into the barren limestone that forms the hanging-wall; laterally, the ore bodies pinch tto thin, irregular seams. The ore consists, of pyrite, chalcopyrite with some bornite and chalco- cite (secondary?) associated with small quantities of galena, specularite, marcasite, enargite and zincblende; galena is practi- cally restricted to fracture zones. The ore carries aixill

'J. M. Boutwell, P. P. 38, U. S. G. S., p. 21 .

106 Examination Of Prospects

commercially valuable quantities of gold and silver. In the exploration for ore-bodies in this district (Ibid., p. 154) on approaching a shoot of copper sulphide ore that lies within barren marble, lean ore may be observed in certain beds. This grad- ually becomes larger in proportion to the barren rock, and higher in grade, until the entire bed or beds are ore. In the extreme outer parts of these shoots bands of country rock alternate with bands of lean ore, and within the shoot the original bedded character is sometimes preserved by bands of barren siliceous material, and in some cases the massive ore itself preserves the bedded structure.

In Shasta County, California,* large masses of pyritic copper ore have formed as replacement deposits in alaskite porphyry and also occasionally extend into shale; the largest of these deposits, the Iron Mountain, probably originally contained 20,000,000 tons of pyritic ore. The ore-bodies are, in general, roughly tabular, and although irregular in form, may best be referred to as lenses. The ores, which consist of pyrite, chal- copyrite, and zincblende with subordinate galena associated with quartz, calcite and barite, chlorite and sericite, commonly merge gradually into the surrounding rock. The ore-bodies occur in zones of highly shattered and comminuted rock, and this con- dition is apparently the determining factor in their localization. That the chemical composition of the enclosing rock was not the controlling factor in deposition is proved by the replacement in different deposits of alaskite, shale, and of a basic dike.

In the Black Hills, South Dakota, in the Bald Mountain District, large replacement deposits of siliceous gold ores have formed in limestone. Long, narrow, restricted fissures exhibit- ing, in general, a common trend, pass upward through a series of shales, limestones and quartzites. Where these fissures intersect the limestone, the ore, consisting of pyrite and quartz carrying gold and silver, replaces the rock for considerable distances either side of the mineralizing fissure; where many fissures are grouped

1 L. C. Graton, Bull. 430, U. S. G. S., p. 89. J, D. Irving, Economic Geology Yo\. Ill, p. 14.

Types Of Psimabt Ore-Dbposits

m

together tlie mineralization from them has coalesced to fornj flat masses of great lateral extent. The mineralization along thra fissures themselves is commonly slight, and is often absent, ancft the fissures are so small as to be detected with difficulty in manjj instances.

At Santa Edlalia, Chihuahua, Mexico,* the ore-depositi form great maSaeB of irregular form in a limestone dome, followin|a and in part limited by, the stratification planes. The ore-bodie( are in some instances connected by fissures, by films of red clay, od by limestone checked and netted with minute fractures filled witlifl iron oxide. The ores are for the greater part oxidized, and con-- sist of more or less impure cerussite, sometimes containing cores of residual galena. The replacement of the limestone is indicated by lines of chert through the ore that correspond to similar lines in the unaltered limeatone walls, and by the presence of silicified fossils in the ore. The district is one of the most important in Mexico.

Disseminated Mineralizations. — .\n important class of deposits is that in which the valuable minerals occur as minut(l particles, or narrow seamlets, or stringers, throughout a larga mass of enclosing country rock. The number of such mineralizap tions whose primary ore is of payable grade is probably amallM but these deposits, especially those that contain copper, are C the greatest importance where enriched by secondary processes Disseminated mineralizations are probably due in great part t metasomatic replacement, but the occurrence of their mineral as sparse disseminations, or impregnations, wiiich are structural terms, is sufficient to warrant their description as a separated type.

Disseminated mineralizations are most frequent in schists am in intrusives; these rocks appear to favor disseminated mineral!*! isations in much the same way that limestone appears to induce segregation and localization of introduced minerals. A char- acteristic feature of disseminated deposits is the occurrence of the most thorough mineralization in the areas most fissure

' W. H. Weed, "Nature of Ore DepoBits," fieck-Weed, p. 573.

108 Examination Of Prospects

or shattered. In many cases the mineralization accompames reticulated quartz veinlets through the shattered rock, and often, especially where the dissemination occurs in the mineral- izing intrusive, the introduced minerals are abundant along joint planes as well as fracture planes, and occur in much less quantity in the interior of the masses bounded by such surfaces. Disseminated mineralizations are commonly closely associated with intrusives, and where important, extend through very large rock masses.

At Bingham Canyon, Utah,* a great mass of intrusive monzon- ite carries throughout an irregular but persistent mineralization of finely disseminated pyrite and chalcopyrite that carry low values in gold. In the fresh monzonite these minerals occur as minute grains scattered through the rock, and along joint planes, and also embedded in irregular quartz veinlets. A correlation of assays with structure indicates that the values are highest where the fissuring and veining are most pronounced. Through secondary enrichment this deposit has yielded copper-deposits of the first rank.

At Clifton, Arizona, intrusive monzonite-porphyry carries a disseminated mineralization of pyrite and chalcopyTite,with subordinate zincblende and molybdenite, associated with quartz and a sericitization of the containing rock. This mineraliza- tion is most intense in and along certain veins through the monzonite-porphyry, but extends as impregnations through the rock, along joint planes, and associated with quartz veinlets, for long distances either side of the mineralizing veins. The containing rock is here, also, the mineralizing intrusive. Large areas of the intrusive carry sulphides, but the mineralization is apparently most intense in the vicinity of the centers of intrusion at Morenci and near Metcalf . The primary ore is unpayable, but through secondary enrichment it has yielded important deposits.

In the Burro Mountains, New Mexico, an intrusion of mon- zonitic porphyry through granite has been accompanied by a dis-

1 J. M. Boutwell, P. P. 38, U. S. G. S., p. 259.

Beminated mineralization consisting of minute grains and a lets of pyrite and chalcopyrite associated with quartz. The! mineralization is most intense along certain fracture zonea, and 1 appears to be proportional to the amount of ahattering of the I encloaing rock. The primary mineralization ia impayable, but J has at two localities yielded important deposits through second- I ary enrichment.

At the Hopeful Mine, Nogal District, New Mexico, a much I altered rock, probably sericitized and later kaolinized, carries a J disseminated pyritic mineralization containing gold but no-i copper. The pyrite occurs as grains through the rock, which. J L. C. Graton states to be probably a monzonite, and is most! abundant along joint planes and certain ill-defined fissures. ' The I gold, which is stated to vary between $1.00 and S3. 50 perJ ton, is said to be uniformly distributed, and to be approxi-B mately equal in the oxidized and in the sulphide ore.

Conglomerate Beds. — While not a numerically import anrf type, mineralized beds of conglomerate form the ores of two qn the moat important mining districts in the world. The origin of these deposits ia not clear, and while to a certain extent similar, 1 they posses features that render difficult their classification with the more comnion and better understood deposits. In both the Michigan copper deposits and the gold depoaits of the Rand the ore consists of native metals in the cementing material of J conglomerate beds, and in both districts the mineralizations arw remarkably persistent over great areas, and to great deptha.

In the Witwatersrand, Socth Africa,' beds or "reefs" conglomerate ai'e intercalated with quartzitic sandstones andJ more rarely, slates; there are eight groups of these reefs, certainj of which have been exploited over a length of 48 miles. InJ thickness the reefs vary between a maximum of several metera to a complete wedging out. The thinner reefs are commonly th6 richer. The intervening strata are practically barren, although exceptions to this rule are known. The reefs are composed of pebbles, commonly ranging from a hazel-nut to a hen's egg in

' Beck- Weed, "Nnluri- cit Ore Deposits," p, 512. J

110 Examination Of Prospects

Size, of quartz and quartzite, more rarely of siliceous schist, and occasional rounded pyrite granules; the pebbles are often deformed, being flattened or splintered, and themselves rarely carry any mineralization. The cementing material is composed of small quartz granules and pyrite with associated particles of gold. The pyrite occurs in rather irregular distribution, and frequently forms crusts around the pebbles of the conglomerate, and occasionally is concentrated in delicate films parallel to the stratification. The mineralization, while exhibiting local irregu- larity, is relatively uniform and persistent over long distances both in strike and in depth.

On Keweenaw Point, Michigan,* beds of copper bearing conglomerate occur interstratified with sandstones and sheets of diabase, both compact and amygdaloidal, and with melaphyre; certain beds of strongly altered diabase scoriaceous in character are known as ash beds. In the conglomerates, the copper has replaced the finer particles so as to appear as a cement; the boulders themselves, or particular minerals in them, are often permeated with copper, which occasionally occurs in large masses. The copper is associated with chlorite, epidote, and abundant zeolites. The associated amygdaloidal rocks carry copper in their small cavities, and in certain shattered areas it occurs irregularly, occasionally in fragments of large size. The rich parts of the beds occur as shoots, in some instances several thousands of feet in length; these shoots continue somewhat diagonally down the dip of the beds to great depths without essential diminution or change in mineralization. The distri- bution of the copper in the amygdaloidal sheets is much the same as in the conglomerate beds.

Bedded Ore -Deposits. — Where mineralization has proceeded contemporaneously with the deposition of the enclosing bed the resulting deposit is known as a seam, or bedded deposit; in this class are also included those replacements of similar occurrence the origin of whose mineralization is not apparent. The criteria for distinguishing between bedded deposits and intercalated veins

J. P. Kemp, " Ore Deposits," p. 204.

Types Of Primary Ore-Deposits 111

axe given in a preceding paragraph. Beck makes a further distinction between interbedded deposits, which are overlain "by other strata, and superficial deposits where there are no over- lying beds, as, for example, beds of bog iron ore.

Bedded deposits are occasionally recognizable by contained fossils, which may have become mineralized. These deposits, of \srhich a majority contain iron or manganese, are commonly of large horizontal dimensions as compared with thickness, and where the strata are folded, they follow all the sinuosities of the containing bed; in occurrence they are comparable with coal seams. A study of the stratigraphy of the bed containing the ore and of the associated strata is imperative in the investigation of such deposits.

Certain types of bedded deposits terminate through wedging out around their peripheries, others toward their edges become gradually poorer through the occurrence of barren partings, -which increase in proportion to the ore until the mass becomes unpayable. Bedded deposits whose mineralization was con- - temporaneous with the deposition of the containing stratum are commonly persistent over large areas.

Bedded deposits whose mineralization is later than the de- position of the containing bed are less likely to be persistent over large areas, and their contained mineralization is likely to have been controlled by some constituent of the bed, such as carbonaceous material, and to fail over such parts of the bed as did not contain such preciptants. Where two or more bedded deposits occur in the same series they are persistently parallel through all the sinuosities of the associated strata.

The Clinton Ore Measures of the United States, are geo- graphically persistent in extent, and wherever they outcrop they almost invariably contain one or more beds of red hematite intercalated with sandstones and shales. In occurrence the ore varies somewhat, at times being the replacement of fossils, again as small oolitic concretions, and in places constituting a highly

Beck- Weed, "Nature of Ore Deposits," p. 49.

J. F. Kemp, "Ore Deposits/' p. 115.

Examination Of Prospects

ferruginous Umestoae. In some cases it can be shown tliese beds result from the weathering near the surface of of ferruginous limestone, and are thus secondary or residi character. They constitute deposits of great economic in ance in several states.

measures, Clinton, New York, 6 After C. H. Smith.

Near Mansfeld, Germany,' a stratified copper deposit ei over an area 120 miles long and 60 to 90 miles wide; the aA thicknesa of the bed varies between 18 and 23 inches; the c bearing member is a blackish, bituminous shale which li< conformably upon red sandstones and conglomerates, i overlain by limestones and dolomites. The copper o

' Beck-Weed, "Nature ot Ore DepositB," p, 488.

Types Of Primary Ore-Deposits 113

i?m of chalcopyrite, bomite and chalcocite, associated with rrite, galena, zineblende and other sulphides, and contains a .-tie silver. Although the entire bed is copper-bearing, the Liable ore is commonly confined to a layer from 3 to 6 inches thickness, and appears to be associated with the bituminous sterial, the bed becoming leaner in its lower portion where ,e carbonaceous material becomes less in quantity; replace- ents of fossilB, especially those of fishes, are frequent. The rerage value of the ore appears to be from 2 to 3 per cent, pper, with some silver.

10 52 — Section ot a partly eroded anticline near Manrfeld Germany. showing the Manafeld copper shale as a black hne A/ter Schroder

The Red Sandstone Beds op the Sodthwestehn United Itates.— These beds at many places carry copper as chalcocite rith a little pyrite and chalcopyrite replacing organic matter, uch as tree trunks, leaves, bark, and aflsociated finer particles. Jccasionally, the kaolin or calcite of the cementing material be- ween the grains of the sandstone is replaced. These deposits ippear to be unconnected with fissures and, except at Tularosa, i. M., are not in association with intrusives. The chalcocite, which commonly carries a little silver, is evidently of .later origin han the containing beds, but the source of mineralization is not .pparent. These deposits, while frequently confined to a single led, are commonly present in several beds of the sandstone eries. The mineralization is extremely irregular, and these leposits have not yielded satisfactory rea\iU% oti e.'x.'OTft.'Cvsa..

Chapter Vi Primary Ore -Shoots I

A preceding paragraph treats of the irregular manner in which ore-deposits occur and the complex factors that control their distribution; the occurrence of metals in ore-shoots in individual deposits is equally irregular, and the factors that control such segregation are equally complex. While the dimen- sions of ore-shoots in individual deposits may not be foretold with any degree of accuracy, the occurrence of ore in shoots is: not accidental, but is controlled by laws some of which are understood. Any one familiar with permutations and combina- tions who stops to consider the variety of factors that control ore deposition, and their varying relative importance, will admit that the science of economic geology must advance greatly before the occurrence of ore-shoots may be predicted accurately in advance of exploration.

The Factors that Determine Primary Ore-Shoots. — The segrega- tion of minerals in any ore-shoot must be referred to some con- dition, or combination of conditions, local to the ore-shoot compared with the remainder of the deposit of low-grade oi barren minerals. Among the localizing factors that haveb recognized are, in filled fissures, the amount and distribution ol open spaces available for ore deposition, and in replacemeni deposits, the degree of brecciation giving access to the replacinj solutions; the intersection of veins with other veins, dikes, sheeted zones, or porous strata; the impounding of solutions by impervious strata; and the differing precipitative influences of enclosing rocks. A large proportion of important and well- defined ore-shoots, especially in deposits of deep-seated origin, are not assignable to any of these factors.

The Relative Value of General and Local Data. — While a

knowledge of the character of ore-shoots in general is important, a knowledge of the behavior of the known ore-shoots in the prop- n erty or in the district under investigation is even more signifi- cant. Accurate and complete assay and geological maps of the J areas already mined are essential to an intelligent investigatioiJ of ore-ahoota, and it is strange that such maps are so rarely to b found at even important mines.

The distribution of ore in shoots is the greatest factor in t risk of mining, and the factor of most vital importance to thM mine owner and mining engineer. Not infrequently, a practica3 man from long experience in a particular deposit can predict il behavior in advance of exploration with a valuable percentage of accuracy; this reliability of pradiction would in many cases be greatly increased by a technical study of complete assay and geological maps, upon which relations become definite that otherJ ' wise would not suggest themselves, I

Ore-shoots vary with commercial as well as with natural conditions, and material left in place as waste may in later years constitute a valuable ore. Furthermore, while the difference between assays of 20 cents and $1.00 is not of immediate commercial importance as regards the marketability of an c it may be of vital importance as a guide to a new ore-shoot. It! is natural to extract ore and to be satisfied as long as the ore li but all ore-shoots come to an end, and then, when a search ii commenced for further reserves, the data of most value in eon-J ducting that search will have been lost unless carefully recorded on a map, and the exploration must be ctmducted by a process* of elimination which may or may not yield results.

Primary and Secondary Ore-Shoots.In the study of any! deposit the first question to decide is whether the ore-shoots a due to primary segregation of values or to secondary enrichment by surface agencies.

It is much easier to recognize a secondary ore with oertaintjn than to determine an ore to be certainly primary, for the r that the criterion of a primary ore is the known lack of secondarjH additions. With a majority of ores microscopic study in thin

Examination Of Prospects

flection will reveal primary character, but there are manyinfltan where the assumed primary nature of an ore is open to questi especially with ores of gold or silver.

Even below the zone of recognized secondary enrichment, di exploration usually shows a falling off in value in the prim; ore in depth, and the actual influence of surface agencies n extend to greater depths than is generally supposed. It often h. pens that commercially valuable ore ceases at the depth read by secondary agencies, and the data is likely to be scanty

Cross Section

Longiiadin

al Section

Sh

—

Levell

—

Width or

Level 2

/

Fig. 53. — Diagram illustrating the terms used to describe the dimension! ore-shoots. A/ter Lindgren and Ransome.

regard to the primary distribution of values where not obscui by surface agencies. Secondary ore-shoots will be considered a succeeding chapter.

Terms Used to Describe the Dimensions of Ore-Shoots. — 1 most convenient terms for use in describing ore-shoots are the suggested by Messrs. Lindgren and Ransom.' The longi dimension of a shoot is called the "pitch length"; the horizoni length the "stope length"; the width at right angles to t

'P. P. 54, U. S. G. B., p. 205.

Primary Ore-Sboots

Examination Of Pbospects

pitch length is tenned the "breadth"; thickness is measured at right angles to the plane of the pitch length and breadth.

The Shapes of Ore-Shoots. — Probably no mineral lode ap- proaches the regidarity of a seam of coal, although as compared with the majority of mineral veins some are notably regular. The Smuggler vein at Telluride, Colorado, and the older, East-West,*

Fig. 55. — Primary ore-shoots, Grass Valley, California. After lAndgnn.

veins at Butte, Montana, are remarkably regiilar and uniform in mineralogical character; the more prominent of these veins have been stoped for thousands of feet along their strike, showing little, it any, disposition to develop shoots; the gold reefs of the Rand, South Africa, are markedly regular over long distances as compared with most lodes. These examples, while of marked general regularity, show considerable variation in value at different points. The other extreme, that of irregular- ity, is more common, and instances are numerous where the ' R, H. Sales, Economic Geology, Vol. Ill, p. 327.

Primary Ore-Shoots 119

"valuable metals occur in small high-grade masses so irregularly

idistributed through the gangue that they may not be developed

*Ahead of extraction, but must be discovered by chance and

mined out as found. Between these two extremes may be classed

a majority of ore-deposits, where the payable ore occurs in fairly

definite shoots, the size and shape of which depend upon the

; market prices of metals and the cost of mining as well as on

— geological conditions.

' The broadest generalization that can be made in regard to the

ape of primary ore-shoots is that their vertical dimension is

-likely to exceed, even considerably exceed, their horizontal

# dimensions: in secondary ore-shoots the reverse is the rule.

P There are many instances where primary ore-shoots are as well

ll defined in vertical as in horizontal extent, as, for example, are

i[inany of ore-shoots at Cripple Creek, Colorado; here the ratio

between pitch length and breadth varies from 1 1/2 to 1 to 5 to 1

ai in the shoots that have not been truncated by erosion.

Some generalizations have been attempted in regard to the jielative size of ore-shoots at the surface and in depth: this applies with some force to secondary ore-shoots and to those primary ore i shoots whose upper parts have been" leached. It is evident, however, that most primary ore-shoots were formed at very considerable depths and that the present surface must be con- sidered accidental and therefore not a factor in determining the primary distribution of metals. Where primary ore-shoots are definitely limited in vertical extent they are likely to be of roughly lenticular form, and other shoots are likely to appear in the projection of their pitch lengths in depth. In some districts primary ore-shoots are persistent to great depths, although of well-defined breadth.

No valuable generalizations can be formulated, but there is often an approach to regularity in the relationship among the ore-shoots, of individual mines or districts, as regards pitch, continuity, relative pitch-length to breadth, and in the regularity of sequence of different shoots. The effect of structural con-

Waldemar Lindgrea and F. L. Ransome, P. P. 5A:A3..

Examination Of Prospects

ditions upon ore-shoots and the chemical eflfect of the wall rocb in inducing precipitation will be taken up in a succeeding pan- graph. Where these factors do not appear to have had effect in the distribution of metals, the causes of their segregation into shoots are not understood, unless they are referred to maa action, where a precipitation once started, by whatever cauae, becomes a continuous process until the final result is an ore-shoot

Fig. 56.— Sections of the Silver Bell ore-shoot, Silverton, Colorado. AJle.

Ransome,

A certain type of ore-shoot gradually fades out around it peripheries, the values becoming progressively less from th< center toward the edges, where the final boundary of the shoo is determined by the limiting cost of mining and treatment This fading out may be accompanied by a visible change in th mineralogy of the shoot, or the same gangue and acessory mineral aj- persist beyond the payable vsXm-, >Jcvfe nsoi tsi'b: xiasJ

Primary Ore-Shoots 121

out with the diminishing values, or may continue as strongly beyond them. Another type of shoot presents sharp outlines, the grade of ore being maintained up to the boundaries of the shoot.

In many wide, compound lodes or shear zones the ore-bodies occur with great individual irregularity, either connected with each other by stringers or apparently unconnected, and lying either along the foot- or the hanging-wall; taken collectively, however, they are likely to show a general alignment, or arrange- ment in shoots, and the lode in the direction of their collective pitch lengths may be considered promising territory.

Ore-shoots not infrequently follow an overlapping arrange- ment where each shoot overlaps the one succeeding on one side and the one following on the other. Where there are several parallel fissures an ore-shoot on one vein is likely to be succeeded by a slightly overlapping shoot in the adjoining vein.

Lenticular Ore-Shoots. — Lenticular masses of ore in schistose rocks constitute another class of primary ore-shoots; here the original ore-body, of whatever origin, has been transformed by regional metamorphism into lenticular masses oriented with their plane of greatest dimension roughly parallel to the schistos- ity. These lenticular masses are likely to follow one another in overlapping sequence, but often occur singly.

The lenticular form is often well-defined, the central part being thickest, and the ore gradually diminishing in thickness toward the edges, where the lens dies out in the shist.

During the formation of these lenses recrystallization appears to destroy the lines of schistosity through the mass of the ore, but the schistose structure is commonly present near the bound- aries, where the ore is often mingled with parallel bands of the minerals of the enclosing schist. In pyritic lenses carrying copper, a part of the chalcopyrite is likely to be segregated and to occupy veinlets through the pyrite.

In many cases lenticular ore-bodies appear to have controlled

122 Examination Op Prospects

W. H. Emmons* states thsit where such deposits are separated to form overlapping lenses, there is some evidence to show thai they separate where the ore is moat siliceous, for a mixture of sulphides and quartz seems to be less capable of resisting stresses than the more massive pyrite. In such cases a thin fissure with the sehistosity parallel on either side will probably be found to connect the broken ends.

Fio. 57. — Section showing the lenticular massea that make up the Conti- nental vein, Encampment, Wyoming. After Spencer,

The Behavior of Primary Ore-Shoots in Depth. — A majority of the ore-deposits of our Western States are believed to be the result of ascending mineral-bearing solutions which deposited their burdens where diminishing temperature and pressure

permitted precipitation upon nearing the surface.

Assuming a uniform fissure through a homogeneous rock, it would seem, therefore, that the changes to be expected in depth would be those due to changes in pressure and temperature, the most easily precipitated mineral, or the mineral in greatest excess, extending the deepest, with the other minerals following in regular sequence until near the surface the most soluble mineral would be found. This is partly borne out by recorded facts, but no absolute succession of minerals may be distinguished,

'Economic Geology, Vol. VI, p. 781.

Primary Ore-Shoots 123

for the reason, possibly, that the solubilities of the different elements and compounds vary greatly according to the presence of other compounds in solution, and also by reason of the interference of structural conditions and the chemical effects of enclosing wall rocks. The usual order is, nearest the surface, compounds of mercury, then galena, blende; chalcopyrite or cupriferous pyrite, with pyrite the lowest. These zones all grade one into the other, and in many cases but little change is noted in the relative abundance of these minerals over con- siderable vertical distances. Gold appears to be precipitated at all depths, free gold and auriferous pycite extending the deepest, and the compounds of gold of greatest mobility, such as tellu- rides, extending the highest. The position of silver appears to be higher up in the series. In the absence of arsenic, anti- mony, bismuth and tellurium, silver is usually associated with galena, but in the presence of these elements, it is likely to be higher up in the series and combined with these elements in preference to lead. The primary character of certain of the silver minerals is open to question. This sequence is partially corroborated by the typical associations of the metals. Of the six elements, silver is most commonly associated with galena, galena with zincblende, zincblende with chalcopyrite (to a less marked degree), chalcopyrite with pyrite, and pyrite with gold.

The general tendency is for ore-shoots to become smaller, but more regular, with increase in depth.

The Decrease in Value with Depth. — Primary ores commonly show a progressive decrease in value in depth. Mr. Waldemar Lindgren says* "This decrease is likely to be rapid near the original apex of the veins, but below this it is in most cases very slow, extending over a vertical range of many thousand feet." As examples, he gives the Grass Valley Mines, California, where the North Star vein produces a gold ore from a vertical depth of 1600 ft., which corresponds to 4100 ft., on the incline, equally as rich as that found at higher levels. The saddle reefs

' Economic Geology , Vol. I, p. 45.

124 Examination Of Prospects

at Bendigo, Australia, contain payable ore at a depth of 4l56 ft. In the Coeur D'Alenes, Idaho/ the lead-silver ores appear to maintain their tenor to depths of 1800 ft. below their outcrops without sign of diminishing values.

If any generalization may be made, it would seem that silver, lead and zinc are less likely than gold to be persistent over great vertical distances.

Predicting the Depth to which Qre-Shoots may Continue.— The primary character of an ore being established, and the effect of structural and chemical precipitants not being apparent, the chances are good that an ore-shoot will maintain its values in depth. A shoot whose partial development indicates a widen- ing tendency, or fairly uniform breadth through several levels, has, of course, a much better chance of vertical persistency than one that is definitely narrowing as it goes down, which probably belongs to the type of ore-shoot that has definite vertical as well as horizontal limits. If the variation in value along the dip is no more pronounced than along the strike, there is no basis upon which to presuppose the absence of ore in the deeper portions of the vein, where further exploration may disclose other ore-shoots, most likely in the projection of the pitch length of a shoot higher up on the vein. It has been established, how- ever, that in many instances where the values are maintained in depth, the absolute quantity of ore grows less with increasing depth, which may be a condition more apparent than real, on account of the greatly increased expense of deep exploration.

The Depths at which Ore -Deposits Form. — Ore-deposits have been classified on the basis of the depths, or the conditions, under which they were formed. Deposits of igneous, pegmatitic or of contact origin, and of abyssal, moderate and of shallow depths exhibit characteristic mineral associations, which fre- quently permit the investigator to assign an ore-deposit to one of these classes.

A knowledge of the depth at which an ore was formed gives an

F. L. Ransome, P. P. 62, XJ. S. G. B., p. liQ-

Primary Ore-Shoots . 125

idea of the amount of erosion that took place to expose the deposit at the surface, and permits an intelligent corelation to be made with deposits of similar origin elsewhere.

The Structural Features that Influence Ore -Shoots. — With the exception of magmatic segregations and deposits due to contact metamorphism, a prerequisite of mineralization is the existence of a fissure or fissures to give access to the mineralizing solutions; the mineralizing effect of these solutions is probably controlled to a large degree by the character and changes of these circu- lation channels.

' One extreme of fracturing may be considered the solid, un- broken rock, through which solutions work their way very slowly. The other extreme is a crushed or ground-up condition where the fineness of comminution is sufiicient to produce a gouge or clay-like mass, which is also relatively impervious to the passage of solutions. Between these two extremes lie the favorable conditions for ore deposition.

Ore is deposited from solutions either in open spaces or by replacement of the fissured rock. In replacement deposits the degree of mineralization is frequently proportional to the degree of brecciation, as replacement proceeds most rapidly where it has the greatest area of rock surfaces on which to work, until the point is reached where the fineness of comminution commences to retard the passage of the solutions. There is, however, a point where brecciation ceases to be an advantage — where it is so extensive that the solutions are dissipated through a large mass, and the resulting mineralization is too scattered to yield a commercially important deposit.

It sometimes happens that two or more systems of fracturing or brecciation have effected the rocks; here the time of the fissur- ing becomes important. Fissures heal and become closed in time, and so cease to afford circulation channels; the most favorable time of Assuring or brecciation may be considered that just before the introduction of the solutions — perhaps due to the same igneous disturbances to which the solutions owe their origin. Post-mineral fracturing is, of qI

126 Examination Of Prospects

primary effect, but may be of the greatest importance in working secondary changes.

In the investigation of a mineralized area, therefore, the & tribution and extent of the brecciated masses, and the degree and relative time of brecciation should be studied; mineralizatioa is frequently co-extensive with or confined to the brecciated areas. The brecciated structure of an unmineralized rock is sometimes not visible on fresh fractures, but becomes plain (m weathering, or upon being wetted.

The degree of brecciation varies greatly in different rocb; massive or rigid rocks yield breccias where soft or plastic rocks yield to strain without brecciation. The continuity of a brec- ciated zone from one rock into another is, therefore, uncertain.

The degree and character of brecciation is of the greatest importance in the study of the surface exposures where the existence of disseminated copper ores is suspected.

At Bingham, Utah, Mr. J. M. Boutwell, referring to the dis- seminated mineralization of the great laccolithic mass, states that: '*In general, underground observations tend to show that chalcopyrite and pyrite occur in greatest quantity where the country rock has been most broken."

At Aspen, Colorado, referring to the silver lodes, Mr. J. E. Spurr states that: "A microscopic study of limestone in the process of replacement by dolomite, silica, and sulphides shows that the rock was first profoundly strained and crushed in the vicinity of faults, so that many tiny passages were opened to solutions, which finally worked through and through the strained material, replacing it. The amount of straining, which regu- lated the area of surface offered to the solutions, usually deter- mined whether there would be little or much replacement; where the circulation zone along a vein is wide and is filled largely with finely ground material which is not so pasty as to prohibit free circulation through it, the replacement of earthy materials by metallic minerals as well as their accoinpanying

P. P. 38, U.S. G. S., p. 131. P. P. 63, U. S. G. S., p. 160.

Primary Ore-Shoots 127

gangue goes on very much more rapidly than elsewhere along the vein, where the rock is hard and is only sliced by fracture planes. "

Ore-Shoots due to Available Open Space. — In deposits formed by the filling of open spaces the distribution of these open spaces is usually the controlling factor in ore deposition, and the segre- gation of metals into ore-shoots bears the closest relation to the vein structure.

The pinching and swelling of veins in fissures of small dis- placement have been discussed in a preceding chapter; where this condition can be shown to exist, the pinching out of an ore- shoot may be reasonably expected to be followed by at least a widening of the vein, which may or may not contain ore.

A vein that is the result of both replacement and of the filling of open spaces is likely to vary in mineral content according to which of the two processes took place during ore deposition, the open spaces perhaps contributing the ore-shoots while the replacement of the walls where the fissure was tight is represented by intervals of barren or low-grade filling.

In a crustified vein, if it is determined that the valuable mineral was among the first formed, and relatively near the walls, the ore is likely to be persistent as compared with a mineral that was among the last to form and so confined to such spaces as were kept open the longest.

Ore -Shoots due to Intersections. — Ore-shoots are frequently developed where a vein is intersected by another vein or by a fissure, dike, sheeted zone, or branch vein. This association is more common in deposits that have been formed near the surface than in the deposits of deep seated origin.

Intersecting members appear to affect the conditions of circu- lation, or to introduce chemical or physical changes that acceler- ate the deposition of ore minerals along such junctions.

An. intersection may afford, through increased local crushing, a more open channel for the passage of solutions, and so lead to

See Economic Geology, Vol. I, p. 43, Waldexoa.! "Lvada.

Examination Of Prospects

Fig. 58. — Longitudinal section along the Neu-Hoffnung vein, Him- melatahrt, Germany, showing the ore-shoots along the interseotions with several other veins. AJter Beck.

Primary Ore-Shoots 129

the passage of a greater quantity of the mineralizing solutions through the part of the vein affected by the intersection; moreover, a partial stoppage of circulation appears to induce precipitation, stagnation, or partial stagnation, being favorable to precipitation. In some instances the intersecting veins carried solutions of different metal content, as is evidenced by their different vein fillings, and through a mingling of the two solutions chemical precipitation was brought about that formed ore-shoots: furthermore, a mingling of solutions under different conditions of temperature or pressure, appears likely to disturb the equilibrium of the solution, and so to cause new or increased local precipitation. In some cases the intersection is with a mineralized vein, and both veins along their intersection and for some distance back from it carry ore-shoots; in other cases, the intersection is with a barren, or unmineralized fracture, which appears equally likely to cause a segregation of values.

The relation between intersections and secondary ore-bodies is of perhaps even greater importance than in primary deposits.

Dr. Richard Beck states that the localization of values is most marked where veins intersect at acute angles, because the intersecting vein walls are close together for longer distances and also because the mass of crushed and permeable rock is likely to be greater in acute than in more nearly rectangular intersections.

In the Tornado-Mogul Siliceous Ore-Shoot, Black Hills, South Dakota, the large north-south ore-body is joined on the east by northeast ore-shoots; at the junctions the ores carry more gold and silver than do any of the shoots away from the intersections.

At Cripple Creek, Colorado, the smaller ore-shoots are quite generally associated with intersections; the large ore-shoots appear to be independent of intersections.

Beck-Weed, "Nature of Ore Deposits," p. 391. ' J. D. Irving, Economic Geology y Vol. Ill, p. 153.

Waldemar Lindgren and F. L. Ransome, P. P. 54, \3. p. 210-212.

130 Examination Of Prospects

In the Georgetown Quadrangle, Colorado,' a large pi

portion of the important ore-shoots occur at intersections wi branch veins.

At Tonopah, Nevada,* pre-mineral cross fractures ai

QuarU-momciDi

Fig. 59. — Section of the rocks and diagram of the ore-bodieB in i American Nettie mine, Ouray, Colorado, showing the localization of below an impervious stratum due to the impounding of solutions, Aj

cross walla appear to have been the cause of localizing vali in ore-shoots, which, in general, pitch in a direction paral to the intersections of these minor fissures with the main v( system. To quote Mr. Spurr: "To these cross walls, mt

' J. E. Spurr, P. P. 63, U. 8. G. 8., p. 159.

J. K Spurr, P. P. 42, U. 8. G. 8., p. 85, and 119

Primary Ore-Shoots

less pronounced, was due the division of the water circu- ion into columns of unequal importance, and the minerali- ion accomplished by these waters was correspondingly alized," 3re -Shoots Due to the Impounding of Solutions. — A numerically

Blanht linustana

Biaclishth

Sandt/fae Sandy shah

—Diagrammatic section across a lode and ore-body formed beneath an imperviouH stratum, Rico, Colorado. AJter

3ortant type of ore-body appears to be the result of the im- inding of mineralizing solutions beneath impervious cappings. i'issures are formed only in rocks that are relatively rigid, and likely to die out upon encountering a stratum or mass of rock ,t yields to the forces that elsewhere prodMiie ?isa\6, "Jijssi.

132 Examination Of Prospects

and the only rock of wide distribution th?it yields in preference to fracturing is shale; as shales are frequently intercalated with limestones and quartzites, ore-bodies due to the impounding of solutions are usually replacments of these rocks and the develop- ment of the ore-shoots may be considered as the effect of both structural and chemical causes.

It appears that where a mineralizing solution is impounded, or its circulation arrested, the increased time during which the mineralizing agents are in contact with the rock is an important factor in localizing deposition. The fact that the farther upward passage of these solutions or mineralizing agents was almost, if not quite checked, indicates that the flow of solutions from below could not have continued long; large ore-bodies formed beneath impervious strata would seem to indicate, therefore, that the mineralizing agents possessed greater mobility than liquid solutions, and perhaps rose through such stagnant solutions to perform the work of ore deposition.

In the Ouray District, Colorado,* fissures of small displace- ment that are well developed in limestone and quartzite strata become lost upon meeting black shales, where they are repre- sented by slight distortions of the beds only. Beneath these impervious shale beds important replacement deposits have formed in the limestones and quartzites, apparently due to the impounding of rising solutions.

In the Black Hills, South Dakota, the vertical fissures pass through limestone into impervious shales: the ore-bodies, which are replacements of the limestone, are widest just beneath the shales, and narrowest in their downward extension, apparently from the impounding of rising waters. In the great Tornado- Mogul ore-shoot this occurs on a large scale. Here a phonolite dike forms the west wall of the ore-body, and not only has the upward progress of the solutions been prevented by overlying shales, but they have been confined laterally also by the por- phyry wall.

J. D. Irving, Bull. 260, U. S. G. S., pp. 58, 62.

J. D. Irving, Economic Geology, HI, i. Avd P . P ,2fi,U, S. G. S.

Primary Ore-Shoots 133

n Grant Codnty, New Mexico,* silver, and silver-gold ores ur in limestone immediately below strata of shale, which, by jounding solutions, have had a distinct effect in the localization

'la. 61. — Sketch showing narrow miaerahziDg fissures and the replace it of limestone by ore beneath a stratum of impervious shale, Alameda le, Portland, South Dakota. A/ler Irving.

I. 62. — Section along the West Side Vein, Tombstone, Arizona, showing the ore-bodies developed at the tops of anticlines. After Church.

the ore-bodies at Kingston, Lake Valley, Hermosa, and other

nps.

a H. Gordon, P. P. 68, U. S. G. S., p. 269.

Examination Of Prospects

Ore -Shoots at the Tops of Anticlines, or 'Saddle Reefs."— In some districts the ore-deposits occur persistently at the tops of anticlinal folds. In many cases this appears to be the result of local impounding of ascending solutions at the tops of anticlines that are capped by strata of impervious rock. Fractures and open spaces form in the convex side of folded strata somewhat before the production of a continuous fissure, and the largest

Fig. 63. — Plan of (we-bodies, Tombstone, Arizona, showing their relation

to anticlinal folds, AJter Church.

open spaces along fissures through folded rocks are often found at such points; these open spaces appear to be favorable to the segregation of ore minerals. Rising solutions, therefore, are likely to form ore-bodies at the tops of anticlines. The same relation appears to hold true for the localization of secondary ores in synclinal troughs by descending solutions.

At Tombstone, Arizona, the deposits lie in the anticlines, in some cases on the flank, in other cases at the apex, but the synclines are barren. Ore-shoots in the veins are associated with anticlinal folds of the wall rock.

'John A. Church, Trans. A. I. M. "E., XXXIW,

Primary Ore-Shoots

At Bbndigo, Australia,* quartz veins traverse highly folded shales and sandstones. The ore-depcits are local developments at the tops of the anticlines, and in typical cases a series of these so-called '-saddles" are superposed; they are usually connected by stringers with underlying and overlying saddles, or with the mineralizing fissure.

Fia. M — Section through a saddle reef Bendigo Victona A, Sand- stone B shaly sandstone C quartz ore After RtcLaTil.

In the Nova Scotian Gold Fields. — Mr, E. R. Faribault'has described a condition similar to that at Bendigo, Australia. In the example given, of 21 arches of strata on the anticlines, 20 contain gold ore. The cause of the localization appears to

' T. A. Hickard, Trans., A. I. M. E., XX, p. 480. ' Trana., Can. M. L, 18S9; Beck-Weed, p. 412.

136 Examination Of Prospects

have been opening or parting due to slipping, as shown by slickensides, along the contact planes between quartzite and slate. That the parting and deposition was gradual and pro- gressive is shown by the crustified structure of the ore.

At the Elkhorn Mine, Elkhorn, Montana/ the principal deposit occurs as a saddle-shaped mass along the axis of a steeply pitching anticlinal fold. The ore occurs at a contact between altered shale and a massive crystalline limestone. The bedding planes are ore bearing only where the general dip of the rocks is disturbed by flexures. In addition to the ore formed along the contact, a number of very large ore-bodieB have been found at some little distance from it in the dolomite, but always in the same structural position.

The Chemical Influence of Wall Rocks on Ore -Shoots. — The chemical effects of different wall rocks upon the segregation of minerals into ore-shoots is well established by many clear examples.

A vein that indicates by a well-developed banded structure or crustification its origin through the filling of open spaces is unlikely to vary in mineral content in passing from one country rock into another, except as the structural character of the original fissure varies in the different rocks. In veins that were formed by a replacement of the walls of their fissures, however, the mineral content is likely to vary markedly in different rocks, in proportion to the precipitative action, permea- bility, and solubility of the several rocks encountered.

As the chemical activity of any rock mass must vary with the character of different mineralizing solutions, being greater or less according to the kinds and amounts of the dissolved materials, no general rule may be formulated to apply to all veins, nor to any given type of country rock. In general, however, where a vein passes from a massive and insoluble rock to one that is relatively pervious, soluble, and replaceable, the probabilities are that the usual vein filling will expand and a relatively large replacement body will be found in the favorable rock. Such

W. H. Weed. Trans,, A. I. M. E., XXXI, p. 647.

Primary Ore-Shoots

enlargements frequently extend outward for considerable dis- tances from the mineralizing vein, which may indeed be an insignificant and unmineralized fissure, known only through its effect in having transported solutions to a rock horizon suitable for the precipitation of their mineral content.

A distinction is difficult between the practically very different but genetically similar types whose extremes are represented by

Fig. 65. — Section through the Dolcoath mine, Cornwall, showing the copper fitopes where the vein is in schists, and the tin stopes where the vein travereea granite. AJier LeNeve Foster.

replacement deposits on one hand, and by typical veins whose ore-shoots are the result of chemical action of the wall rock on the other.

The relative precipitative action of the various rocks upon mineralizing solutions- may not be stated definitely, but there are certain general relations that may be considered well demon- strated in spite of exceptions. The sedimentary rocks are, in general, more active chemical precipitants and more easibi fc- placeable than are the igneous rocks, aWikoM "a.Q'v.

138 Examination Of Prospects

variably the case. Limestones are among the most ehemicallj active rocks in precipitating minerals from solutions, and aw most easily replaceable by mineralizing solutions of usual com- position. Next to limestones come calcareous shales, Band- stones, quartzites and conglomerates.

In each case the chemical and physical character of the rock and of the mmerahzing solution determines the most faTorable locus for deposition, and local data, as usual, is of more value in predicting the most favorable horizon for exploration than u the best authenticated general data. Where several beds of lie

Fig. 66. — Sections showing the behavior of veins in paseing throng porphyry dikes, Cornwall, England; the values are segregated where tbe porphyry forms the walla of the veins. A/ler De la Beche.

same sediments are present, it is usual that one particular bed has been the most active chemically, and carries the ore-deposita to the perhaps complete exclusion of tbe other beds, of the series. Carbon is an active precipitant of minerals from solu- tions. The rock or bed that will be most active chemically in precipitating minerals and will contain the richest or largest ore-bodies may occasionally be predicted from its being known to contain carbonaceous matter. Most sedimentary beds con- tain iron, and not infrequently this combines with sulphur contained in carbonaceous material to form pyiite, which is an active precipitant of other sulphides and of gold. This con- dition is typical of shales. The influence of carbon, contained in waJ] rocks, upon the precipitation, ot oTea,\.W\MwiQubtedlr

Primary Ore-Shoots 139

the controlling factor in some districts, has probably been over- rated; thd amount and distribution of carbonaceous matter is wholly incapable of explaining the occurrences of many large replacement deposits; its chief function may be, perhaps, to start a precipitation which is carried on through mass action in the easily replaceable rock.

The selective precipitative action of different igneous rocks is also well established, but although the different effects of various rocks in segregating values may be apparent, the differing characteristics that produce these effects are not usually notice- able, and in advance of local information, no one rock may be considered more favorable for ore deposition than another. Many veins pass through several igneous rocks without apparent change in value or mineral content.

In the exploration of a vein in depth a cessation of payable ore should not immediately be assigned to primary impoverishment due to depth alone, for it has been demonstrated that such impoverishment frequently, or even usually, takes place gradu- ally and over great vertical distances. In cases where the segre- gation of ore into shoots may be referred to the precipitative action of wall rocks, the possibility of a reccurrence of the favorable wall rock at greater depths at once suggests itself. If the exposed ore is due to geological factors unlikely to recur in depth, then deeper exploration is unwarranted; if the geo- logical conditions appear to be as good in depth as in the horizon of known productivity, then deeper exploration for primary ore may be justified.

In thp secondary concentration of metals by surface agencies the character of the country rock is of the greatest importance, some rocks actively precipitating metals and not permitting the migratioil necessary to form enrichments.

Where it is seen that a vein is to pass into a different rock, the change will be awaited with anxiety unless it is known that the vein has been formed through the filling of open spaces, when a change is much less likely than in a vein formed partly by re-

Examination Of Prospects

he expected to accompany better ore, unless the expectation is based upon abundant local evidence.

At Nbihabt, Montana/ the primary ore conEiists of galena zincblende and pyrite in a gangue of carbonates of lime, iron and manganese; the veins carry ore-shoots between walls of pink or white feldspathic gneiss, but are barren where they pass through dark colored gneiseee, amphibolite and diorite. The mineralisbg solutions appear to have reacted with the feldspars and not with the ferro-magnesian silieatea

Fig. 67. — Diagram (plan) showing the rocks traversed by the Ntihart vwm and their ore content in the various racks. After Weed.

At Butte, Montana,' a aeries of copper veins intersect quartz-monzonite, aplite, and quartz-porphyry. The veins were formed through replacement of the wall rocks, which are much altered; in the monzonite they are commonly rich in copper; in the aplite they are equally wide and strong, but are lean, beit composed chiefly of quartz with comparatively little pyrite and copper. In the porphyry the veins are narrow and lean. The pyrite contained in the veins where they pass through aplite and the quartz-porphyry is noticeably poor in copper.

W. H. Weed, Trans., A. I. M. E., XXXI, p. 645. ' W, H. Weed, Trana., A. I. M. E., XXXI, ?A1.

Pbimaby Ore-Shoots

In this district the ore-shoots favor the more basic rock. The silver veins at Butte exhibit a marked crustification, and are clearly the result of the filling of open fissures. They traverse both the monzomte and the aplite but there is no perceptible difEerence m their character or tenor where they pass through the different rocks

FiQ 68 Ideal plan of a copper \en, Butte, Montana, showing the impovenahment whereaphfeforoiB the walls ot the vein; the ore is indicated in black. 4fter Weed.

At Gympib, Eastern Australia,' the precipitative influence of a slate band is so marked that it is followed in exploration in- stead of the veins. The main slate band is 200 ft. in thickness, and the several veins carry ore where they pass through it, but are elsewhere unpayable.

At Aspen, Colorado, the mineralizing solutions appear to have risen through the underlying siliceous formations (granite and quartzite) with but little precipitation of ores, and through the overlying dolomitic and calcareous formations with much pre- cipitation, and finally, upon encountering the overlying shales and the prophyry sheet that was intruded at their base, the solutions were impounded, and the shales reacted powerfully with the solutions for the production of large deposits.

At Ballarat, Australia,' the prevailing country rocks are

' J. H. Curie, " Gold Mines of the World," p. 188.

J, E. , Economic Geology, IV, p. 310.

T. A. Rickard, Trans., A. I. M. E., XXX, p. WW.

Examination Of Prospects

slates and sandstones that carry as intercalated members a series of black slate bands, which contain carbonaceous matter and pyrite. These bands are persistent over long distances and while typically very thin, being from a fraction of an inch to a few inches in thickness only, they have had a remarkable effect

Fig. 69. — Plans of the " Indicator" slate seam near Ballarat, Australia, showing the segregation of gold in the intersecting quartz veins. After Dunn,

in segregating the gold content of a series of quartz veins that intersect them. The chief of these slate seams is known as the ''Indicator" and along the intersections of the quartz veins with it, as well as with the other seams, the ore-shoots have formed.

Primary Ore-Shoots 143

i At San Javier and Tecoripa, Sonora, Mexico, an extreme 1 case of the precipitative action of carbon upon mineralizing solu- a tions is evident. Here the solutions have followed steeply dip- ti ping and much altered coal seams; the carbonaceous matter has in great part been altered to graphite, with which are associated the rich silver ores.

In the San Juan Mountains, Colorado,* the distribution of the primary ore in veins that cut a series of volcanic beds appears to be due to the greater precipitative effect of certain beds over others. The veins are commonly low in grade in the rhyolitic layers, and of good grade, frequently of high grade, in the andes- itic layers. It is said that in the San Juan andesitic breccias the value of the ore varies from layer to layer. Ore deposition here appears to have been greater in the basic than in the sili- ceous rocks.

In the Plateau Region of Arizona and New Mexico, there are many districts where tree trunks contained in red Triassic sandstones have by replacement been transformed wholly or in part to chalcocite, which appears in this connection to be a primary mineral. The quantity of ore being limited to the car- bonaceous material, which was scanty in amount and erratic in distribution, these tree trunk deposits have yielded small com- mercial returns.

Near San Bernardo, Sonora, Mexico, a small quantity of high-grade ore was mined where a tree trunk had been replaced by chalcocite and silver glance. The whole tree was extracted, including several branches, when the "mine" came to an end.

Ore -Shoots in Veins of Deep-seated Origin. — In many veins of deep-seated origin the ore occurs in well-defined shoots which, apparently, are not connected with any of the factors discussed in the preceding paragraphs; to this class belong some of the most important ore-shoots known.

Inasmuch as a localization of values must be assigned to a local difference or cause, these shoots are probably best referred to either mass action, or to a change in the predominant circula-

C. W. PuringtoB, Economic Geology 1, p.

144 Examination Of Prospects

tion channels at different times in the formation of the vein or deposit. In the former case, a precipitation of a metal once started, by any cause — and all precipitations niiist have a local start — is continued progressively, the mineral formed attracting to itself additions of the same kind from the solutions until the result is a localization of values, or an ore-shoot. In the latter case, it is difficult to conceive that a fissure at all times permits the passage of solutions over its entire length with equal facility; therefore, at any given time certain parts of a fissure probably constitute the main channels for the passage of solutions and so give rise to the localization of values in ore-shoots. It is prob- able that veins, constituting lines of weakness, have been opened and reopened many times during ore deposition; the ore- shoots, therefore, may be considered to represent the circulatioa channels at the time of passage of the richest solutions.

Many ore-shoots are of lenticular shape, being widest at the center and decreasing in thickness toward their peripheries, where they die out in stringers and fade into barren material. The study of fissures themselves indicates for them a similar form, inasmuch as they are strongest over their central portions, and die out at their ends in stringers or fade put in the rock where the stresses that formed them were absorbed without fracture. This type of shoot may, therefore, be referred to a reopening of the vein in a manner similar to that of the formation of fissures through rocks.

Certain ore-shoots of great depth as compared to width may be due to incipient folds on the plane of the vein, such lines of weakness in the vein as were most nearly vertical affording the most favorable channels for rising solutions.

Chapter Vii The Primary Alteration Of Wall Rocks

Metamorphic Processes. — The term metamorphism as com- i monly used means any change in a rock either in form or com- : position, from whatever cause. By metasomatism is meant a I metamorphism that involves a change in the chemical composi- tion of a rock by the addition or subtraction of substance.

Certain types of metamorphism are due to the action of miner- alizing solutions, and are closely connected with ore deposition; they form, therefore, valuable guides in the exploration for ore- deposits. Other types of metamorphism are due to regional processes, and are not connected with ore deposition.

Dynamo-regional Metamorphism. — By dynamo-regional met- amorphism is meant a primary alteration of rocks over large areas under conditions of pressure, flowage, and heat. This process produces schists and gneisses from either sediments or igneous rocks. It is thought that there is little migration of minerals or of elements in dynamo-regional metamorphism, and that the changes are due chiefly to recrystallization and rearrangement of original minerals in bands with their major axes oriented normal to the direction of greatest pressure. Under these conditions limestones are commonly altered to marble, and sandstones to quartzite, basic igneous rocks to greenstone or serpentine, while rocks of normal or acid com- position commonly yield the usual types of schist or gneiss. Dynamo-regional metamorphism, extending over large areas, has no connection with mineralizing processes, and is, therefore, no guide to ore-deposits; while this process has certain features, such as characteristic mineral associations, in common with contact metamorphism, the results of the two processes may usually be distinguished without difficulty.

1 Waldemar Lindgren, Trans., A. I. M. E., XXX, p.

146 Examination Of Prospects

Contact Metamorphism. — By contact metamorphism is meant the change in structure and composition of the enclosing rocks in the immediate vicinity of and due to accession of substance/ .from igneous intrusions. The rocks most affected by this process are limestones, shales, and sandstones, which are altered respectively to marble, hornfels, and quartzites with the further addition of new minerals; igneous rocks are affe.cted to a less extent by contact metamorphism, having themselves been formed under igneous conditions. The characteristic contact met- amorphic minerals are garnet, epidote, woUastonite, p)rroxene, amphibole, magnetite, quartz and sulphides; the unique feature of contact metamorphic deposits is the primary association of Oxides with sulphides. While contact metamorphism is frequently associated with ore deposition, the development of a large zone of contact minerals does not presuppose or indicate the existence of ore-deposits.

The distance to which contact metamorphism effects the altered rocks varies greatly in dififerent occurrences, and also in dififerent beds of a sedimentary series, certain beds being altered for long distances from the contact while others may persist unchanged up to within a short distance of the intrusive. Con- tact metamorphic minerals and products are commonly tough and resistant to erosion, and are, therefore, likely to form prom- inent outcrops.

Hydrothermal Metamorphism. — By hydrothermal metamor- phism is meant the alteration of rocks by hot ascending solutions; it is distinctly a metasomatic process and is a frequent accom- paniment of ore deposition.

The walls of veins are commonly fractured or strained by the stresses that produced their fissures, especially the hanging walls, and in this way ready access is afforded to the solutions that accomplish their alteration. The alteration is usually intense

Waldemar Lindgren, P. P. 43, U. S. G. S., p. 124. Waldemar Lindgren, Trans., A. I. M. E., XXXI, p. 227. ' The present knowledge of metasomatic replacement and of hydrothermal metamorphism is chiefly due to Mr. 'Waldeisiat lAndea..

The Primary Alteration Of Wall Rocks 147

immediately along the veins, but is likely to diminish rapidly in degree with distance from them. It appears that the walls of a vein tend to keep within them the heavy bases, and gangue minerals, but permit the passage of the depleted solutions that effect the rock alteration. Not infrequently, disseminated =Bulphides are found in the altered wall rocks, although these iminerals commonly carry low values in gold and silver as co.m- Epared with the sulphides deposited within the veins. Pyrite is -the most widely disseminated sulphide in wall rocks, and is frequently the result there of the alteration of the rock minerals themselves, in which case it does not carry values in the precious txmetals. Zincblende and galena are more rarely found as IB disseminations.

2: The results of hydrothermal metamorphism are frequently

valuable guides in the search for ore-deposits, especially where

'they are confined to the vicinity of the veins or stocks. In

atoany cases, however, and especially where mineralization and

y attendant primary rock alteration have taken place at slight

if depth below the surface, the porosity and shattered condition

of the wall rocks has permitted a wide distribution of the alter-

I'ing waters, and the altered areas are likely to be large, and so to

lose their value as aids in the search for ore-bodies.

r In many parts of the arid regions of the western United States

and Mexico there are large areas of brilliant red hills, the rocks of

c which have suffered hydrothermal metamorphism; the iron

r mineralization of these red hills is likely to be more apparent than

r real, the surface being stained a deep red, yellow or brown, but

upon fresh fractures the rock is seen to carry little iron, present

in most cases as pyrite, resulting from the alteration of the rock

minerals.

The alteration of wall rocks is likely to be less along deep- seated veins than in deposits of shallower origin; it is also likely to reach a less development where it accompanies deposits resulting from the filling of open spaces than in connection with replacement deposits.

The most common effect of intense aWjeYXKoTL

148 Examination Of Prospects

along veins is sericitization and the introduction of pyrite, whidi alteration is likely to grade into propylitization with greater distance from the veins, the latter process being the less intense and more widespread; in basic rocks, propylitization is likdy to persist close up to the veins, there to give way perhaps to sericitization. While the degree and extent of alteration by oue set of solutions is likely to vary markedly in dififerent rocks, it not infrequently happens that the final products of alteration and recrystallization are similar, and it becomes a matter of difficulty to distinguish one rock from another. The chief chemical effects of hydrothermal metamorphism are an increase in potash with attendant loss in soda, silicification, and the introduction of sulphides, chiefly pyrite.

The effects of hydrothermal metamorphism may usually be distinguished readily from the effects of hydro-metamorphism, or the alteration accomplished by surface waters: the latter alteration is rather more likely to be uniform, and is essentially a process of oxidation, hydration, and solution, while the former is characterized by sericitization and silicification. Kaolin a the chief guide in making this distinction, being a typicaBj surface product, often formed from sericite as well as directly from feldspars through the action of sulphuric acid solutions set free by the oxidation of pyritic sulphides.

Propylitization. — By propylitization is meant a hydrothermil metamorphism that involves the development of chlorite, epidote and pyrite from the dark rock-making silicates, and of quarti, calcite, and epidote from the feldspars. Rocks that have been subjected to propylitic alteration are commonly greenish-gray in color and carry bright green stains of epidote; the barren pyrite developed is present in well-developed crystals, and upon oxida- tion is likely to color the surface red, brown or yellow.

This type of rock alteration is frequently present over large

areas in mineralized districts, and is rarely a guide to ore-deposits,

except as indicating in general that a district has been subjected

to metamorphic action.

Propylitization is thought to \\a.Ne l.ce at no great

The Primary Alteration Of Wall Rocks 1491

depth beneath the surface, where extensive fracturing and' I ' jointing permitted easy access by solutions, and although! BBsociated in a general way with ore deposition, is rarely tol be referred to any particular fissure or aeries of fissures. In M I rocks that have suffered propylitic alteration the feldspars have! usually become duil, if not more completely altered, but thej principal pliyaical features of the rocks, such as texture, are pre-B served. Propylitic alteration is most prominently developed! in rocks of intermediate or basic composition.

Sericitization. — By sericitization is meant a hydrothermail metamorphism that results in an almost complete loss of soda J and a large gain in potash, silica, and commonly also of pyrite, with occasionally lesser quantites of carbonic acid and fluorine. The typical result of complete sericitization is a finely granular aggregate of sericite, quartz, pyrite and calcite; after thorough. J sericitization it is difficult to distinguish between rocks that were originally quite dissimilar.

Sericitization is a common form of hydrothermal metamor- phism, and is an intimate accompaniment of the mineralization of moat ore-deposits in igneous rocks; the alteration is commonly , most intense immediately along the veins, and decreases with I distance from them, sometimes passing into a propylitic altera- tion, which latter transition is likely to take place within i ahort distance of the veins in basic rocks; sericitization not in-jj ' frequently persists over large areas in rocks of intermediate c acid composition,

Sericitized rocks are commonly white to light yellow in color, and are usually soft; the minute scales or plates of sericite are usually distinguishible in hand specimens and often form ; means of distinguishing a sericitized from a kaolinized the two minerals are likely to occur together, however, the lattei being formed from the former as well as from feldspars by thCT action of sui'face waters carrying sulpliuric acid. The pyrite ii troduced in sericitization is sometimes accompanied by minu quantities of chalcopjTite and zincblende, less frequently 1 galena or arse n op y rite.

lar

latM

ar- on

eFiii.H

150 Examination Of Prospects

At Bisbee, Arizona. — The porphyry of Sacramento Hill and the adjacent schists have undergone sericitic alteration. The results of alteration of both rocks are similar. The original quartz has recrystallized into quartz aggregates, the feldspars have become areas of sericite, quartz and pyrite, with occasion- ally a little epidote, chlorite, zircon and rutile. Under the action of surface agencies the pyrite has oxidized, and much of the sericite has been altered to kaolin. The surface rock con- sists chiefly of an aggregate of quartz, stained and streaked by iron, and containing nests of sericite and kaolin at the surface. Much of the kaolin has weathered out, leaving cavities. The outcrops are brownish-red in color, but the rock in most ex- posures is white on fresh fractures.

Alunitic Alteration. — The occurrence of alunite as a primary mineral has been established at Goldfield, Nevada. It is here associated with gold, sulphides, tellurides, sulphantimonites, and kaolin, as a primary result of hydrothermal metamorphism. Kaolin is elsewhere regarded as a typical product of the action of surface waters. The alunitic alteration at Goldfield is wide- spread, and is intimately associated with ore deposition. Alimite occurs massive, as a crystalline constituent of the altered rocks in the ore-bearing areas, and intercrystallized with pyrite, gold, tellurides and other minerals in the ores.

Alteration to Greisen Along Tin Veins. — The wall rocks of tin veins are characteristically altered by hydrothermal metamor- phism to greisen, the alteration consisting of the replacement of feldspars and of biotite by quartz, lepidolite, topaz, tourmaline, I and fiuorite, commonly associated with cassiterite, wolframite, chalcopyrite and arsenopyrite. The altered rock in this case frequently constitutes payable ore. Greisen is commonly referred to as altered granite. Where the altered walls are other rocks, the same minerals are likely to be developed.

Silicification. — Silicification of wall rocks by hydrothermal metamorphism is a common form of primary alteration that is

F. L. Ransome, P. P. 21, U. S. G. S., p. 150. 3 F. L. Ransome, P. P. 66, U. S. G. S., p. 192.

The Primary Alteration Of Wall Rocks 151

frequently associated with ore deposition. The tendency toward silicification is probably greater in acid than in basic rocks, but the process is common also in limestones, where it is likely to

Fia. 70. — Diagram showing the alunitiaed areas and the known productive areas at Goldfield, Nevada. A/Ur Ransome

form a reticulated structure, siliceous seams enclosmg partially altered or fresh fragments. Quartz formed m this manner is commonly of the cherty variety; the replacement of calcite by , silica often reproduces closely the orimaX a\,r\it\,Nise , Vwsve.

152 Examination Of Prospects

difference in hardness between the altered and the unaltered parts offers the best means of distinction.

Silicification frequently accompanies other forms of hydro- thermal metamorphism, where it is likely to represent the ex- treme of alteration, and thus serve as a valuable guide in a search for ore-deposits.

Harmorization. — The alteration of limestone to marble is frequently accomplished by contact and by hydrothermal as wdl as by regional metamorphism. A large, thoroughly marmo ized area is probably without significance of mineralization, but local or irregular marmorization along contacts with a mineral- izing intrusive, or along fissures, is not infrequently associated with ore deposition.

Dolomitization. — The replacement of a part of the calcium in limestone by magnesium is called dolomitization, and the resulting rock, dolomite. This change is commonly effected over large areas, and has no connection with ore deposition. Dolo- mitization is occasionally, however, of local occurrence, and as such may in some cases be the accompaniment of ore deposition. This alteration is accompanied by an important shrinkage in volume, and may thus indirectly play a part in ore deposition.

Chapter Vitt V

Alterations By Surface Agencies I

By hydrometamorphism is meant the alteration of rocks, oref and minerals by atmospheric waters. In its broadest sense, iifl includes the varied processes of weathering, oxidation, hydrationB the leaching of rocks and ores, and the solution, migration anjfl enrichment of metals.

Rain water, as it strikes the earth, is practically pure water except for small quantities of dissolved atmospheric gases; upon* coming in contact with ore minerals, especially sulphides, how-1 ever, it is transforiiied into solutions of great chemical activity,.B which attack, transform aijd rearrange the minerals of ore— J bodies, and so effect enrichments of the greatest economiel importance- Primary deposits so low in grade as to be withoutfl commercial value are frequently thus transformed into depositiM of commercial importance. It is necessary in the examinatioi™ of any ore-body, therefore, to determine whether the valuable is of primary or of secondary origin; if primary, its values mayB be expected to coutinue indefinitely in depth to the zone of primaryM impoverishment; if secondary, the values are known to be coO'-B trolled by surface agencies, and the valuable ore may be expectedfl to continue to such depths onlj' as have been reached by surface W&ters. fl

The Decomposition and Weathering of Rocks. — The decompoai- 1 tion and weathering of rocks, unlike the changes undergone hyM ore-bodies, are accomplished by water carrying as dissolved con- Btituents chiefly atmospheric oxygen and carbon dioxide, to- gether with vegetable acids. The chemical changes wrought are principally hydration, oxidation and the formation of car- bonates. The tendency is towai-d the solution of the more easily dissolved minerals, which is attended by the formation i concentration of the more resistant minerals,

154 Examination Of Prospects

among which are quartz, aluminous clays and limonite; complex minerals are thus broken down into simpler constituents more resistant to weathering, which remain at the surface.

The weathering of rocks is characteristically regular over large areas, and is rarely mistaken for the results of hydrothermai alteration or kaolinization. The residual minerals from rock decomposition are commonly soft, and are readily carried away by erosion, which is likely constantly to present fresh rock sur- faces to the attack of the chemical alteration above outlineA Even in regions of heavy rainfall, however, a combination of slight slopes and abundant and active vegetable acids may result in deep accumulations of residual minerals, as frequently occurs in tropical countries.

Weathering attended by slight or partial decomposition results chiefly in the staining of the surface rock red, brown or yellow by iron oxides set free by the alteration of basic silicates, or magnetite, and in the alteration of amphibolitic rocks to serpentine.

Where atmospheric waters percolate to depths considerably below the surface, their oxidizing power is diminished, and hydra- tion is the principal alteration; the minerals thus formed through rearrangement of original constituents, and to a less extent through introduced substances, are epidote, chlorite, serpentine, pyrite, zeolites and quartz.

Leaching. — Leaching, usually accompanied by kaolinization, is a process of solution exercised by surface waters by which the soluble minerals are removed and the resistant minerals left as residual products. The minerals resistant to leaching and which commonly make up the leached upper parts of ore-bodies are quartz, kaolin, limonite and oxide of manganese. The com- pleteness with which minerals are removed from the leached zone depends upon their relative solubilities and also upon the presence or absence of effective precipitants in that zone, which latter may cause residual masses of oxidized ores to form in the leached zone at the expense of the enrichments below.

In many ore-bodies the presence of active oxidizing pre-

Alterations By Surface Agencies 155

cipitants causes the metals to be precipitated immediately or shortly after undergoing solution, a process known as pre- cipitation in situ; where this process predominates, secondary enrichments are prevented or are of relatively slight importance.

Under certain conditions limonite is completely removed, and the leached zone, white in color, is made up of quartz, kaolin and unchanged sericite. Where oxidation and solution of the accessory minerals proceed more rapidly than the solution and migration of the valuable metal, as often happens in deposits containing gold, the reduction in mass may result in a residual concentration; this form of enrichment is not uncommon, and is very different from the secondary enrichment due solution and migration.

Kaolinization. — Kaolinization is typically the result of surface agencies, except, perhaps, where hydrothermal solutions were acid in character. This process consists in the decomposition, and solution of feldspars and other minerals with the formation of kaolin as the residual product. A thorough kaolinization destroys the original character of the rocks attacked, and diverse rocks upon kaolinization yield products so similar that their original nature is distinguishable with difficulty, if at all. This process of alteration appears to depend upon the acid solutions set free by the oxidation of pyritic sulphides.

The presence of any considerable amount of kaolin is likely to indicate that sulphides have existed in the kaolinized area and that they have been removed in solution, perhaps with the formation of enriched sulphides in depth; kaolinization, there- fore, often constitutes a valuable guide in a search for secondary ore-deposits.

Theoretically, kaolinization is attended by a shrinkage in volume of from 12 to 15 per cent, which is partly offset by in- creased porosity. Kaolinization readily attacks feldspars, of which the soda-lime varieties appear to be the most susceptible, and microcline the most resistant; sericite also is readily decom- posed, and dark silicates are bleached through the removal of iron.

156 Examination. Of Prospects

soft, earthy mass, in which no minerals may be distinguished by the unaided eye except kaolin, quartz and limonite, and in which the structure of the original rock is completely destroyed. A rock that exhibits the outlines of the feldspar phenocrysts cannot be considered completely kaolinized, and such incom- plete alteration rarely overlies important bodies of enriched sulphides. Kaolinization rarely extends into the wall rocks of veins, except where these rocks themselves originally carried a sulphide mineralization.

Oxidation. — Oxidation changes sulphides, arsenides, antimo- nides, tellurides and allied compounds into oxides, native metals, and salts containing oxygen, which, according to conditions of solubility and local precipitants, either remain in sitUy or migrate to be redeposited under favorable conditions lower down in the deposit.

Access by oxidizing solutions is the controlling factor in this process, which reaches its most complete development in deposits carrying abundant sulphides, which through solution and removal, become progressively more porous under the action of the solutions and so help to bring the process to completion. Sulphides that are locked up as small grains within massive gangue minerals are rarely altered by oxidation, unless intense post-mineral shattering affords access to the solutions.

Oxidation tends to obscure primary structural relationships, and also to segregate the newly formed minerals into larger masses, either of enriched oxides or enriched sulphides.

Oxidation, through the more ready circulation of solutions, is likely to proceed to greater depths along veins than through the mass of the enclosing rocks; the country rock, therefore, is likely to exhibit signs of oxidation in the vicinity of veins, which is a valuable guide in underground exploration.

The order in which the various sulphides are attacked by oxidation varies according to their relative abundance, and also according to structural conditions and the character of associated minerals: Weed states that the general order of

' Trans,, A. I. M. E., XXX, p. 42.

Alterations By Surface Agencies 167

attack by oxidation is directly as the relative affinities of the several metals for oxygen, and inversely as their aflSnities for sulphur. The order in which the common primary sulphides are attacked, therefore, is arsenopyrite, pyrite, chalcopyrite, zincblende and galena.

Chalcopyrite in certain occurrences is quite resistant to oxida- tion, and pyrite varies in this respect markedly in. different local- ities, occasionally remaining quite fresh for many years in mine dumps that are completely permeated by sulphate solutions, while elsewhere it has so strong an aflSnity for .oxygen as to become highly heated when wetted. Tellurides appear to be ready subjects of oxidation. Among the secondary sulphides, chalcocite is readily dissolved by acid solutions, as is illustrated in the various leaching processes. Tetrahedrite is a complex mineral of variable composition, and is irregular in its suscep- tibility to oxidation and solution. Most silver minerals are readily attacked by sulphate solutions, but become fixed in the presence of chlorine or its compounds.

The results of oxidation are likely to differ widely according to the mineral associations present, and are to a large extent sub- ject to the influence of the enclosing rocks; deposits in lime- stone are likely to contain carbonates as oxidized ores, while silicates or oxides are likely to predominate in acid rocks. Dur- ing the oxidation of sulphide deposits in limestone, acid solu- tions reacting with this rock produce gypsum, a relatively soluble but strongly combined compound that resists change or precipitation; it is probable that the soluble products of oxida- tion that are not precipitated leave but little trace for present observation. Contact minerals are quite resistant to the attack of oxidizing solutions, and frequently enclose unaltered sul- phides at the surface, while associated bodies of originally more compact sulphides are oxidized to great depths. The oxidation of sulphide deposits in limestone is likely to be quite irregular, and to effect far-reaching rearrangements of minerals, as this rock is soluble and permits surface waters to form replacement

1 Waldemar Lindgren, P. P. 54, TJ. S. G. p. *1W

168 Examination Of Prospects

deposits to an extent not approached in igneous rocks or quartzite.

A zone of rich oxidized ore is not uncomnion immediately above the zone of sulphide enrichment, having been formed through the oxidation of the upper part of that zone. Not infrequently, and especially in desert regions, a deep zone is formed of partly oxidized, partly enriched sulphide, ore, which in most instances is not underlain by any definite zone of sul- phide enrichment. In ores of the base metals an association of oxides with sulphides must usually be of smelting grade, as their widely differing specific gravities render concentration difficult.

The minerals that result from the oxidation and hydration of primary minerals are: Chalcedony, opal, kaolin, limonite, pyrolusite, hematite, cuprite, the sulphates, carbonates, phos- phates, silicates, and arsenates of the heavy metals, chlorides, and native gold, silver and copper.

The Migration and Enrichment of Metals. — During the oxida- tion of a deposit where one or more metals go into solution under conditions that permit migration, the metals commonly descend to the water level, where they are precipitated as second- ary sulphides. This process of enrichment is probably most thorough and accomplishes its most important results in deposits whose primary ore consists in part of pyritic sulphides, which upon oxidation yield solutions of sulphates and free sulphuric acid to continue the solution and rearrangement. In deposits that contain important quantities of heavy sulphides the lower limit of the zone of enrichment is commonly well-defined.

The completeness of secondary concentrations of metals is usually proportional to the relative solubilities of theil" sulphides. Galena is relatively insoluble, and commonly does not migrate far, while copper, whose sulphate is readily soluble, frequently migrates through important vertical distances, and is commonly concentrated in well-defined zones.

The actual depths of secondary enrichments, and their

' Waldemar Lindgren, Economic Gcologij, 11, t. Vi.

Alterations By Surface Agencies 159'

relative depths in different parts of the same deposit, are deter- xuined by the position and character of the circulation channels and by the water level. Along the main channels of under- ground water circulation oxidizing and enriching solutions often penetrate to considerable depths below the ground-water level.

The waters that accomplish solution and enrichment of metals are commonly acid in character, and where observed, owe their activity to contained sulphuric acid and sulphates of iron. There is no lack of da,ta, however, to show that moat of the common sulphides are soluble in pure water, whose solvent action is greatly increased by small amounts of dissolved salts or acids, prominently among which may be mentioned carbonic acid, which in solution attacks pyritic sulphides.

Discussions of the reactions of solution and precipitation are of uncertain practical value, on account of the complex and constantly changing conditions under which they take place.

The character of the natural waters that flow or aeep through the rocks is frequently a criterion upon which to base an opinion in regard to the probability of secondary enrichments in depth. Where the mine water is acid, or where it carries dissolved metals, the conditions are known to be those under which migration and enrichment take place: where the mine waters contain no acid or dissolved metals, and where there is evidence of precipitation in situ, there is but little likelihood of the existence of secondary enrichments in depth. Where water seeps through the rocks and appears in the workings, samples may be gathered for testing; where the rate of evaporation is greater than the seepage, efflorescences form on the walla of workings that may likewise be tested, Effloresences of the sulphates of iron, copper, zinc, magnesium, sodiura and aluminum are of common occurrence on the walls of mine workings, especially in arid climates.

Pyrite and other primary sulphides are probably the most active precipitants of secondary minerals. The theoretical order in which metals arc precipitated by sulphides from sul- phate solutions is directly as their relative affinities for sulphur. The relative affinities of the common metals for sulphur iH

160 Examination Of Prospects

according to Schuermann, mercury, silver, copper, bismuth, cadmium, antimony, tin, lead, zinc, nickel, cobalt, iron, arsenic and manganese. In a secondary rearrangement of a primary ore containing silver, copper, lead and zinc therefore the second- ary sulphides should be arranged in the following order: uppe most, argentite, then chalcocite, bomite, chalcopyrite, galena, zincblende, and at the bottom pyrite. The relative solubilities of the sulphates of these metals usually interfere with such an arrangement; lead sulphate, for example, is relatively insoluble, and commonly does not migrate far before being precipitated, while the more soluble metals descend farther before being precii- tated. Organic matter is an active precipitant of metallic salts, and may be important as such in some deposits: precipitation by kaolin through adsorption appears also to be an important process in the formation of secondary ores.

Upward Migration. — The upward migration of solutions due to capillary action is occasionally a factor in the rearrangement of values in the upper parts of ore-deposits. Limonite is some- times formed at the surface through the oxidation and evaporation of solutions carrying sulphate of iron, and occurrences are known where the chloride of silver owes its distribution to upward migration.

At Leadville, Colorado. — At a point where an ore-body had been to a large extent removed by erosion the loose wash for a distance of 10 ft. above the solid rock was found to carry chloride of silver; several hundred tons of this material were shipped that carried from 40 to 60 oz. of silver. This deposit was found at a depth of 150 ft. below the surface.

The Influence of Circtilation Channels on Secondary Altera- tions. — Secondary alterations are in every case controlled by the channels that permit surface waters to circulate through the rocks and ores. In massive ores in districts where heavy rainfall and steep slopes give rise to an erosion more rapid than percolation and alteration, these processes are of no eflfect, and

Waldemar Lindgren, P. P. 43, U. S. G. S., p. 182. Max BoehmeVy Economic Gcologij, 111, p.

rr

Alterations By Surface Agencies 161

primary ores and minerals appear at the surface. Where post- simeral fracturing permits surface waters to pass through the ores, however, a thorough rearrangement of the minerals and values may be accomplished, even in districts of heavy rainfall, fteep slopes, and rapid erosion. Solution is a characteristic 3ifeature of the action of surface waters upon. the upper parts of awnre-bodies, which commonly become more and more porous under ifiheir attack and through seepage a supply of water is thus main- iftained under hydrostatic pressure for the continuance of the Mtprocess, even in districts of scanty rainfall. It is thus seen that conditions of heavy or slight rainfall, with attendant rapid or lif dow erosion, may be balanced or counteracted by the presence ifof good or poor circulation channels, and enrichments may aiform under either set of conditions. Under conditions of heavy I rainfall, sulphide enrichments are likely to form without import- ed ant zones of oxidized ores; under desert conditions, the oxidized B acne is likely to be deep and important though characteristically irregular, often merging irregularly into the sulphide enrich- ment. The subject of circulation channels will be further dis- cussed in the chapter on secondary ores and ore-shoots. IT The Relation of Oxidation and Enrichment to Topography and Water Level. — close relation exists between the depth of second- i. ary alteration and the ground water level. Oxidation com- ij monly reaches to or somewhat below the water level, and en- Lj riched sulphides commonly occur at or just below this horizon. While not a imiform relation, the water level commonly follows i the topography.

The ground water may be considered to fill a complicated p but connected system of porous rock masses, fissures, joints, and other open spaces. In most instances the water level reflects the surface in a modified form; being deepest below the middle slopes, and becoming shallow again in the foothills, where the Water reaches the surface in springs. In the Southwestern States, the valleys are commonly filled with d6bris; the appar- ent foothills, therefore, are often the middle slopes as referred to

the outlet of ground water, and oxidation '

3f

162 Examination Of Prospects

portant depths beneath them. The ground-water level dependi upon the permeability of the rocks traversed as weU as upon the height of outlet. The water level should be horizontal beneath a hill of loose conglomerate, for example, but might easily reach the surface in a tight rock that does not permit seepage.

In investigating the probable depth of the water level in a new district, it should be borne in mind that springs may exist at levels considerably above the general ground-water level, being the mouths of accidental drainage channels that permit a more easy escape of waters than does continued seepage. A number of springs at approximately the same elevation is commonly a sign of the true water level.

Where there has been much faulting, a district may be divided into fault blocks, each of which constitutes a separate hydro- static basin, the water being impounded to a ceratin extent by the impervious gouge along the fault planes. The ground- water level is likely to be deep in arid regions, and shallow ii regions of heavy rainfall, and the same relation holds for the depths to which oxidation may be expected to penetrate.

Where the water level is permanent, well-defined, and not too deep, oxidation and enrichment tend to form relatively regular zones; where the water level is irregular or practically lacking, oxidation and enrichment are likely to be quite irregular. The water level in any district is likely to change with time, and the level at some past epoch may have been the determining factor in the location of enrichments as now found.

The Irregularity of Oxidation and Enrichment. — large mass of primary ore uniformly mineralized and brecciated, or of uniform porosity, should . yield upon oxidation and enrichment bodies of secondary ores of uniform depth that bear a like definite relation to the topography. This result, rarely attained, finds its nearest expression in certain disseminated chalcocite enrichments.

Oxidation and enrichment persist deepest along the more prominent zones or lines of fracture, and it is not unusual to find tongues of secondary ores deep within the primary zone. Fur- thermore, iiomogeneous masses ol Tvovaxy sulphides

L Alterations By Surface Agencies 163

r

)

if are often found in the oxidized zone, having been protected by T. their massive structure against attack by oxidizing and dissolv- '& ing solutions. In a search for secondary ores, therefore, the % post-mineral fractures should be carefully studied, and explora- L tion should be directed to expose their intersections with the It zones of primary mineralization.

It occasionally happens that the r6le played by fracturing in 4.' the formation of secondary enrichments is emphasized by the occurrence of such enrichments along post-mineral fissures i or joint planes, the precipitation having been accomplished by c absorption, or other agencies.

The Zones Developed by Surface Agencies. — The ideal result

: of the rearrangement of a primary ore-body by surface waters

would be the formation of a surface zone of resistant minerals,

; leached of their metals, beneath which would be found an

- oxidized zone of low grade, and in turn at successively greater

I depths, a zone of rich carbonate and oxide ores, and a zone of

secondary sulphide enrichment resting upon the primary ores.

The complete series is occasionally developed, but most deposits

; exhibit irregularities due to structural features, relative rapidity

of erosion as compared to solution and concentration, differing

solubilities of the minerals present, and the precipitative action

of associated rocks or minerals.

The oxidized and leached zones are usually deep in arid regions, and an important proportion of the valuable ore is likely to occur in the oxidized zone; in wet climates, the leached and oxidized zones are commonly slightly developed, and the major proportion of the valuable ore is likely to be found as sulphide enrichments.

In most cases the enriched ores are not the result of the leaching of the present oxidized zone, but are derived from a great vertical extent of vein or deposit, the residual minerals of which have been removed by erosion; the present enrichments may, therefore, be considered to represent repeated reconcen- trations.

Examination Of Prospects

districts, and in different parts of the same district, or even in same deposit, depending upon the local conditions of wt circulation. The depths at which changes commonly occui other districts do not form reliable guides in the examinat

Fia. 71. — Diagram showing the occurrence of oxidized, enriched,

primary ore in the Granite vein, Phillipsburg, Montana; the horiio dimensions of the areas show approximately the relative amounts of i type of ore at various depths, but are without significance as to their j tions in the vein. After W. H. Emmam.

of a new deposit. The several zones occasionally are shar marked, and the change comes without warning, but commo the zones grade gradually one into the next, and so give warn of the impending change.

Alterations By Surface Agencies 165

While it is not possible to predict the depths at which the several zones will occur, it is possible and of great practical importance to recognize the zone exposed, and so to appreciate the changes that may reasonably be expected with deeper ex- ploration. The relative importance of the several zones depends in large measure upon the minerals and metals of the primary ore. Copper, for example, is frequently completely leached from the surface zone and is found as chalcocite enrichments without overlying residual ores. Where oxidation proceeds more rapidly than solution and enrichment, as for example, in gold deposits, the oxidized ore commonly contains the bulk of the values of the primary ore, perhaps in concentrated form, while the zone of enriched sulphides is poorly developed, or lacking. The upper part of the enriched zone is commonly the richest horizon of a secondary deposit.

In general, the completeness with which any ore-body is rearranged by oxidation is proportional to the relative solu- bilities of the metals it contains in the presence of existing precipitants, and the degree of post-mineral fracturing, or of porosity due to abundant sulphides as compared with resistant gangue minerals.

In the upper zones of deposits that have been subject to oxidation and enrichment, kernels, or residual masses, are likely to be found that represent the character of the zones below. Residual nodules of oxidized ores are likely to occur in the leached surface zone, nodules of enriched sulphides in the oxidized zone, and masses of residual primary ore in the zone of sulphide enrichment. A study of such residual masses may yield in- formation in regard to the character of the lower zones not yet exposed by exploration; a free milling oxidized gold ore, for example, may contain masses of unaltered and refractory pyritic Ore, indicating that such will be the nature of the primary ore when reached. Residual kernels of galena frequently carry higher values in silver than does this mineral in the primary ore.

At Phillipsburg, Montana, the Granite Vein affords a good

W. H. ISmmona, BuU. 315, U S. G S., p. 4a.

166 Examination Of Prospects

example of the rearrangement of values by surface agcies. The vein has suffered post-mineral movement and much of the ore is fractured, permitting ready access to percolating surface waters. The primary ore consists of pyrite, arsenopjrrite, tetrahedrite, and tennantite, with some galena and zincblende. Sparingly scattered through this ore are small specks of pyaragy- rite, proustite, and lesser amounts of realgar and orpimeni The primary ore carries from 20 to 30 oz. of silver and from $1.50 to $3.00 in gold; the gangue is quartz and rhodochrosite. The secondary or enriched sulphide ore, resting upon the primary ore, consists of bands of quartz and rhodochrosite alternating with bands of rich sulphides, comprising argentite, proustite, pyrargyrite, tetrahedrite, tennantite, pyrite and arsenopyrite. Galena and zincblende are locally abundant; chalcocite and bornite are of rare occurrence. This ore carries from 50 to 1000 oz. of silver and from $4 to $8 in gold. Probably more than one- half of the silver in this ore is present as dark ruby silver, or pyrargyrite, with some proustite, occurring in minute veinlets or seams filling cracks in the vein, as films on the outside of crushed vein material, or as relatively large crystals lining cavities. This ore is responsible for a large proportion of the dividends that the mine has paid. The enriched oxidized ore occurs just above the enriched sulphide ore; it is composed of quartz stained by the oxides of iron and manganese, and less commonly by copper carbonates. Cerargyrite and native silver occur as thin seams cutting through the quartz and as films on the outside of quartz fragments. Associated with this ore are small quantities of argentite and pyromorphite, and residual galena, zincblende, pyrite and chalcopyrite are of local occur- rence. This ore carries from 300 to 400 oz. of silver and from $5 to $16 in gold. The poor, or leached, oxidized ore forms the upper zone; it is composed of quartz, commonly broken and stained by iron and manganese oxides. It carries small quantities of lead carbonate, malachite, azurite, chrysocoUa, pyromor- phite, and residual pyrite and galena; it carries less than 40 oz. of silver, and but little gold. TYv. Q.QTLtavaed by the

Alterations By Surface Agencies 167

L oxidized ores are remnants that have escaped oxidation, their massive structure having prevented the access of the oxidizing solutions; low-grade pyrite occurs locally as druses of

. secondary origin.

The Depth of Vein Leached to Form Existing Eniichments. — The metallic content of the enriched ores in any deposit is rarely due to the leaching of the present depth of the oxidized and

'-. lieached zones, but is probably derived from a large vertical

- extent of vein now removed by erosion. It is occasionally possible to calculate approximately the depth worked over by surface agencies necessary to have produced the existing enrichments.

At Phillipsburg, Montana, the Granite Vein yields data upon which to calculate the depth of primary ore worked over necess- ary to account for the enrichments found. Here the primary ore carries about 25 oz. of silver. The enriched ores, about 400 ft. in vertical extent in one part of the mine, average 175 oz. of silver. On the basis of these figures, assuming a constant width, the depth of primary ore necessary to produce the enrichment, must have been about 2400 ft., provided that all of the contained silver found its way into the enriched ores.

W. H. Emmons, Bull 315. U. S. G. S., p. 43.

Chapter Ix

Residual Ores And Their Distribution

The Precipitation of Ores in The Zone of Oxidation.— In

deposits in which the secondary zones are well-defined a layer of rich oxidized ore is frequently found to immediately overlie the enriched sulphides, from which it is 'derived by direct oxida- tion in place. In deposits that contain oxidizing percipitants particles and masses of residual oxidized ore are likely to be scattered through the leached zone, having been precipitated during migration before reaching the zone of enrichment. This precipitation hinders secondary enrichment and in extreme cases prevents the formation of such concentrations.

Residual ores are precipitated by various reagents. Car- bonates are formed through the action of the carbonic acid con- tained in surface waters, and also directly through the replace- ment of calcite, especially where the containing rock is limestone. Native metals, such as copper, gold, and silver, are formed by the action of reducing agents, among which organic matter and ferrous sulphate are prominent. Kaolin, gouge, and certain shales occasionally act as powerful precipitants through adsorp- tion. Silver is often found as residual chloride, formed through precipitation by the chlorine contained in surface waters.

Residual ores are often the result of incomplete solution, the relatively insoluble minerals being left behind during the migra- tion of associated metals; incomplete oxidation and solution often leave residual masses of unaltered, or partly altered, sul- phides in the oxidized zone. Such residual particles or masses have commonly been enriched by additions from circulating, metal-bearing solutions.

During oxidation under conditions that permit reprecipitation, such as the oxidation of sulphide deposits in limestone, a scat-

Residual Ores And Their Distribution 169

tered primary mineralization is probably often segregated into masses of rich oxidized minerals without important vertical migration. In the zinc and lead deposits of the Mississippi Valley the lead is usually found as residual enrichments of galena, above the water level, while the zinc and iron sulphides have in great part been removed by solution and redeposited at and below the water level.

Oxidized Ores of Copper. — Native copper is the last stage in the reduction of copper compounds, and is frequently found as pellets or j&lms in the upper oxidized zones of copper deposits. It is commonly associated with cuprite, from which it is probably derived in most cases. Cuprite is often found just above the chalcocite zone, where it is formed from the chalcocite by direct oxidation. Both native copper and cuprite are commonly indicative of long and thorough oxidation, and are frequently found above important chalcocite enrichments. Tenorite, or melaconite, is intimately related to and associated with chalco- cite. ChrysocoUa is a common residual ore of copper, and is more likely to be abundant in siliceous rocks than in limestone, where carbonates are likely to prevail. The silicate of copper is fre- quently associated with manganese in black compounds of indefinite composition commonly called copper-pitch ores. Malachite and azurite are among the most important oxidized copper minerals; they are relatively resistant to oxidizing pro- cesses and are frequently found as residual ores. Azurite appears to be the more resistant of the two. Malachite and azurite frequently occur with limonite, an association that is explained on the supposition that siderite was formed with the copper carbonates, but subsequently, being more susceptible to altera- tion, decomposed to limonite. Brochantite is occasionally an important oxidized ore of copper, being often a transitional step in the formation of carbonates. In desert climates, ataca- mite may constitute an important ore, though its easy solu- bility confines it to regions of extreme aridity.

Oxidized Ores of Lead. — The only important primary ore of

170 Examination Of Prospects

frequently found near the surface. Upon oxidation it alters slowly to sulphate and carbonate, relatively insoluble products that commonly do not migrate far before being precipitated. Under the attack of sulphate solutions galena alters to anglesite, which may remain as a residual ore; more frequently the angle- site is altered to cerussite, which is the less soluble compound. In certain districts, as for example, the Coeur d'Alenes where galena is in primary association with siderite, the latter mineral upon alteration to limonite sets free abundant carbonic anhydride, and the galena passes into cerussite apparently without the formation of anglesite.

Oxidation occasionally proceeds to great depths in lead deposits, and the bulk of the ore is found as residual particles or masses that have been added to somewhat by enrichment through solution and precipitation. Pyromorphite, and the oxides of lead, plattnerite and massicot, are of less common occurrence, and are usually characteristic of the upper parts of the oxidized zone, in which lead chromate and molybdate also are occasion- ally found.

Where residual masses of galena occur in the oxidized zone they frequently carry silver due to enrichment by migrating solutions, and are higher in grade than the primary galena unaffected by surface processes.

Oxidized Ores of Zinc. — Sphalerite, or zincblende, the only important primary zinc mineral, yields readily to oxidation with the formation of zinc sulphate, a very soluble salt that commonly migrates far before being precipitated.

It is rare to find any traces of zinc in the upper parts of oxi- dized ore bodies; lower down in the deposits, zinc is frequently precipitated as calamine or smithsonite replacing limestone or other sedimentary rocks. These minerals are difficult to iden- tify, as they usually reproduce with great fidelity the structure of the replaced rock, and their low specific gravities fail to call attention to their high metallic content.

F. L. Ransome, P. P. 62, U. S. G. S., p. 132.

Residual Ores And Their Distribution 171

At the Horn Silver Mine, Utah/ a secondary distribution of metals has taken place under conditions of partial oxidation. The upper 400 ft. of the ore-body, whose gangue is chiefly quartz and barite, carries lead-silver ores, the lead as galena, cerussite, anglesite and oxides, and the silver as cerargyrite and as ruby silver; zinc and copper are scanty or lacking. At 700 ft., large bodies of zinc ores occur as carbonate and silicate associated with some lead. From 650 to 750 ft. copper comes in, occurring as chalcocite, with which is associated galena.

Residual Shoots of Gold Ores. — The upper parts of gold de- posits are commonly richer than the underlying primary ores. This residual enrichment in the zone of oxidation is in large degree owing to the solution and migration of associated min- erals, the reduction in mass giving rise to a concentration of the undissolved gold. A further factor in this concentration is the actual solution, migration and enrichment of gold.

The solution and migration of gold takes place more slowly in most cases than the migration of associated metals; this results in the zone of gold enrichment being left at a horizon above the enrichments of associated metals. The minerals that commonly occur with gold in the upper parts of oxidized ore-bodies are quartz and limonite; the limonite is often removed by solution, leaving the gold in a porous mass of quartz, the result being a still further concentration of the gold with respect to the containing gangue.

A surface enrichment frequently takes place in gold deposits during the disintegration of th,e outcrop on weathering; the lighter grains of gangue are washed or blown away and the gold particles tend to work downward into superficial cracks, forming enrichments that usually extend for very short distances only below the surface. Where erosion or glaciation proceeds more rapidly than oxidation, the soft upper parts of ore-deposits are removed and primary ores outcrop at the surface.

Shoots of residual gold ores must be considered as surface phenomena. Their greatest dimension is more likely to be

S. F. Emmons, Trans., A. I. M. E., XXXI, p. b%.

172 Examination Of Prospects

horizontal than vertical, being confined to the zone above the water level, or the depth to which oxidation has penetrated.

The Distribution of Silver by Oxidation. — Silver ores are af- fected in various ways by oxidation. The sulphate of silver, being readily soluble, migrates freely, and may precipitate as sulphide enrichments at the water level. Precipitation of silver as chloride is frequent, however, in the zone of oxidation, especi- ally in arid regions where the residual ores are likely to be the most important: finally, silver chloride is slightly soluble, and, migrating through relatively short distances, is likely to produce local enrichments in the oxidized zone. A residual concentra- tion of silver as chloride and native silver may also take place through the removal by solution of relatively soluble gangue min- erals. Minerals that are frequently associated with cerargyritein oxidized ores are native silver, the bromide and the iodide.

The residual ores of silver, being confined to the zone of oxida- tion, commonly show a distribution related to the topography.

The Distribution of Manganese by Oxidation. — The oxides of manganese are frequently found as residual concentrations in the oxidized zone. Indeed, it is sometimes dijB&cult to explain their abundance in the oxidized parts of certain deposits whose primary ore contains but little of this element. The oxides of iron and of manganese frequently occur with residual concentra- tions of both silver and of gold. Manganese oxide usually is most abundant in the upper part of the oxidized zone.

The relative abundance of manganese in the oxidized zone as compared with the primary zone often gives an idea in regard to depth of leaching and erosion necessary to produce such concentrations.

Chapter X

Secondary Ores And Ore-Shoots

The Criteria of Secondary Ores. — Secondary minerals are the result of a process of concentration and enrichment and are commonly richer than the primary minerals of the same deposit. Secondary ores that contain abundant sulphides are commonly distinguishable in the field from primary ores; this distinction is more difficult with ores that carry a small quantity only of sulphide minerals, and microscopical examination of the ore in thin section is advisable in all doubtful cases.

It is frequently difficult, and in some cases impossible, to determine the primary or secondary character of quartzose gold ores. The safest guide is the presence or absence of gaseous or fluid inclusions in association with the ore minerals; such primary association coupled with a lack of traces of oxidation is indicative of primary origin.

Any ore that carries traces of oxidation, such as iron stains, dendritic oxides of manganese, kaolin and so forth must be con- sidered to have been acted upon by surface agencies, and secondary enrichment must be suspected. Further examination, however, may indicate that" the contained values are primary, and, there- fore, persistent in their vertical distribution.

The manner of occurrence of secondary minerals is frequently characteristic; secondary minerals, being due to the action of surface waters, are commonly found in cracks through earlier minerals, or as crusts lining vugs or surrounding nucleal masses, or in general, in some distribution later than and not conform-* ing to the arrangement of the primary minerals. Secondary sulphides are often found as soft black pulverulent masses or coatings, although they frequently occur well-crystallized or as compact masses of metallic luster. Where tVv \V}ij5rs:ss5ck

Examination Of Prospects

'2- — Pliof.i>"licrograpll of secondsiHly ciirii-lir-ii (iro from the low

part of the clialcocite zone, Morunci, Arizona, showing tl.e ooourreiice of the

iatei and richer sulphide throuEt the pyrit. Datk gray - secondBrr

chaJoocite, developing by replacement in ©arj —

Secondary Ores And Ore-Shoots

It?

may be determined, secondary ores are always found in connec- tion witli post-mineral fractures, or beneath a porous capping. The form of an ore-deposit often indicates its secondary origin, Tliis criterion, however, is not available until after the deposit has been partly explored. An ore-deposit formed above and resting on an impervious stratum ia probably the result of descending waters and is of secondary origin, A deposit that

I f I J rite cryatais si ovnng secoudary clialcooite along t] edgta A/Ut Paige

branches as it is followed downward, leaving the channel of primary mineralization to become enclosed in the wall rocks, is certainly of secondary origin.

Most veins are lines of weakness likoly to be reopened by post-mineral fractures; surface waters frequently find their way to great depths along such fractures and secondary processea may be active to any depth to which they penetrate. The range of influence of these processes may be appreciated upon considering the number of deep mines that are wet.

A list of the minerals that are formed by oxidizing processes i given in a preceding chapter. Many minerals that are found a

176 Examination Of Prospects

secondary enrichments are also formed by primary processes, and their occurrence, therefore, may not be accepted as proof of secondary origin. Minerals that are secondary in a great majority of their known occurrences are covellite, chalcocite, kaolin, chal- cedony, and the sulpharsenites and sulphantimonites

Minerals formed in Lower Ground-water Zone

(Zone of Secondary Sulphide Enrichment*)

Quartz Bomite

Chalcedony Covellite

Opal Chalcocite

Kaolin Argentite

Gold Pyrargyrite

Silver Stephanite

Pyrite Polybasite

Galena Other complex sulpharsenites

Chalcopyrite and sulphantimonites.

Secondary Ore -Shoots. — Secondary ore-shoots are the results of surface agencies and bear a definite relation to the surface, and in any district they are likely to be found within certain limits of depth. In shallow deposits, the vertical distribution fre- quently reflects the topography, while deep enrichments com- monly occur in more nearly horizontal zones. The greatest dimension of secondary ore-shoots is, in general, horizontal, while in primary ore-shoots the reverse is the rule. Furthermore, in secondary deposits the change in values is likely to be most pronounced vertically, while in primary deposits the best defined changes in value are commonly in the direction of their strike.

Secondary processes affect in like manner all types of primary deposits, the degree of alteration and reconcentration depending upon the original constituents of the deposits and upon the structural features that control the passage of waters. In the

' Waldemar Lindgren, Economic Geology , 11, Vl.

Secondary Ores And Orb-Shoots 177

f consideration of secondary ore-shoots the primary distribution

of values must not be lost sight of; aecondary enrichments

- commonly occur at certain horizons within the primary ore-

' shoots, although cases are not rare where secondary enrichment

has affected veins for long distances along their strike, the

primary mineralization of which was probably irregular.

" In exploration for flat-lying or bed-like enrichments, such as the

common type of disseminated chalcocite enrichments, the

workings should be vertical, so as to pass through the successive

. Eones. In exploration for possible tongues of secondary ores

Fig. 74. — Lontudinal aeotion of Oi part of thp Humboldt lode, Moreotd, Arizona, showing the relations of the leached ennched and primary /oues. After Lindgren.

along post-mineml fractures in the zone of primary mineraliza- tion, however, the workings should be honzontal, vertical exploration within the zone of primary ores is futile.

In the investigation of a secondary ore-deposit it is wise to average all samples taken from each level; a comparison made between the different levels will often indicate the trend of in- crease or decrease in values as depth is attained, and 90 perhaps afford a basis upon which to form an opinion of the continuity of the ore-shoots in depth.

At MoHBNci AND Metcalf, Arizona, the important chalcocite enrichments all occur high up on the ViiWa; cioAq-cv ''iifiOT* "wi have proceeded faster than leaching and eMYftViaeaX. , b.'A "Cosi

178 Examination Of Prospects

canyon bottoms are all in rock carrying a primary pyritic minerali- zation only. The elevation of the surface is here a guide in exploration; the higher slopes are considered promising ground, while explorations from the canyon bottoms cannot be expected to yield results.

The Effect of Structural Features on Secondary Ore -Shoots.— Post-mineral fractures are the best guides in a search for second- ary ore-deposits; these deposits are in every case due to surface waters, which circulate most abundantly along fractures, faults, brecciated zones, joint planes, or other openings. Any search for secondary ore-bodies, therefore, should be directed to explore the intersections of post-mineral fractures with the general . trend of the primary mineralization. Solid, impervious masses of primary ore are not subject to enrichment, and commonly carry their primary mineralization unaltered close to the surface. Thorough reopening of fractures during oxidation constitutes the most favorable condition for the formation of secondary enrichments. The general association of secondary ores with fractured, shattered, or permeable rocks cannot be too strongly emphasized.

The Effect of the Water Level on Secondary Ore-Shoots.— In evenly shattered or uniformly permeable deposits precipitaticMi and enrichment generally take place at the water level, where chemical conditions change and precipitants in the form of primary sulphides are likely to be met. Where, however, a prominent fracture cuts a primary ore-deposit, and forms an important charmel for descending solutions, secondary enrich- ments are likely to persist deep into the zone of primary ores. The most favorable conditions for large secondary enrichments are a repeated reopening of the fractures coupled with a gradual sinking of the water level.

The Effect of Chemically Active Wall Rocks on Secondary Ore- Shoots. — In deposits that contain active precipitants, or that are contained within rocks having like effect, migration and enrichment do not take place, the metals being precipitated j

Secondary Ores And Ore-Shoots 179

indifferent rocks one of them may permit migration and enrich- jment, while the others may impede or stop these processes. Acid igneous rocks and acid gangue minerals appear to be favorable to migration and enrichment, while sedimentary rocks : and carbonate gangue minerals appear to be unfavorable. Con- liiact deposits, on account of their resistant minerals and their commonly strong precipitative action, rarely contain secondary , enrichments, although such enrichments are not rare in associated -Sntrusives.

. ; At Clifton, Arizona. — The veins where contained in porphyry ;,carry important secondary chalcocite enrichments, but where ey enter limestone or shale, they become impoverished. Tlie Effect of Porosity on Secondary Ore-Shoots. — Most fresh x:rocks are massive and impervious, except certain sediments, Whose active precipitative action prevents migration and en- ichment. Alteration, usually involving kaolinization, is there- ore a usual prerequisite of enrichment. The ease with which cks or gangue minerals alter to a permeable mass is an important ctor in secondary enrichment, aside from the existence of the cessary post-mineral fracturing that gives access to the surface aters. .K- The Effect of Primary Mineralization on Secondary Ore- jphoots. — Disseminated enrichments are commonly confined to yftreas whose outcrops bear traces of abundant primary minerali-

nation. The tendency in vein-like deposits, however, is toward " -the confinement of descending and enriching waters within the 3vein, and thus produce relatively long enrichments whose dis-

;tribution is not so definitely controlled by the primary ore-shoots.

. Vein deposits are commonly more persistent vertically than

T)recciated zones, and enrichments in veins may be due to the ' leaching of many thousands of feet of the vein now rerboved by

erosion, resulting perhaps in a great concentration, and a wide f difference in value between the primary and the secondary ores.

In disseminated deposits there is usually less difference in value r between the primary and the secondary ores. Waldemar Lindgren, P. P. 43, U. S. G. S., p. 2.04.

180 Examination Of Prospects

Ores Containing Both Stilphide and Oxidized Minerals.— In arid regions where the water level is deep and the supply of surface water is irregular, a zone is frequently developed in which the valuable metals occur in both sulphide and oxidized form, witli, in general, slight enrichment only of the primary values. The occurrence of such ores through a considerable vertical range indicates incomplete solution and enrichment, and in most cases, the absence of any well-defined zone of secondary sulphides at greater depth.

The Enrichment of Copper. — Primary copper ores commonly contain abundant pyrite, and are actively attacked by oxidation; the sulphates thus formed are very soluble, but yield their copper readily as secondary sulphides upon coming in contact with fresh pyrite at the water level. As a result of these con- ditions, copper is peculiarly susceptible to solution and enrirh- ment, and the best defined types of secondary deposits are those of copper.

Chalcopyrite is the usual starting-point, being the most abundant primary copper mineral, and is the form in which the copper of cupriferous pyrite is supposed to exist. Bomite, enargite, tetrahedrite, tennantite and occasionally chalcocite are primary copper minerals of lesser importance. The secondary copper sulphides are chalcocite, chalcopyrite, enargite, bomite and covellite; the last named being probably exclusively secondary in origin.

On account of the strong precipitative action of calcite, chalcocite enrichments are more rare in limestone than in other I rocks, except where the primary sulphides occur in large masses. I

Chalcocite Enrichments. — The final stage in copper enrich- ment is chalcocite, which is also economically the most importact copper mineral. Kaolin and quartz are the typical associates of secondary chalcocite, which commonly also encloses residusl pyrite. The upper part of the chalcocite zone is commonly the richest.

The zones developed in massive pyritic deposits are usually well'deRncd, and the changes ixortv \,qi Os\k,Quite, [

SECOlfDARY ORES AND OBBSBOOTS ISl

and from chalcocite to pyrite are usually sudden; tlio enrich- ment is often a shallow zone of nearly pure chalcocite. Pyritic masses that have been shattered commonly carry their secondary chalcocite in seamlets, or as coatings on nucleal masses, and en- richment in this form may persist through considerable depths.

At Bisbee, Arizona/ the primary mineralization of lean cupriferous pyrite occurs in large bodies inlimestone; the country rock is commonly impregnated with pyrite, and minute particles of contact minerals in the vicinity of the ore-bodies, and also in places exhibits a partial sUicification. Secondary processes have transformed and enriched these primary deposits, resulting in the formation of ore-bodies of great economic importance. In typical occurrences a core of lean cupriferous pyrite 's surrounded by a shell of pyrite carrying secondary chalcocite, commonly as seamlets and coatings on the pyrite grains; this shell is surrounded in turn by an envelope of ferruginous clays containing oxidized copper ores. Where prominent fractures cut the pyritic masses they are accompanied by chalcocite, and where several such fractures occur close together large and rich ore-bodies result. In exploration, when a drift through one of these pyritic masses shows a gradually rising copper content, it is known that the periphery of the deposit is being approached.

At Ducktown, Tennessee, the outcrops consist of hydrous iron oxide associated with kaolin and quartz. This material, which is a valuable iron ore, extends to a maximum depth of about 100 ft. Below this iron ore there la commonly a few feet of chalcocite ore, which in most of the deposits lies like a floor between the gossan and the underlying primary sulphides; the primary ore consists of pyrrhotite, pyrite and chalcopyrite associated with lesser amounts of zincblende, bornite, specularite and magnetite, with a gangue of lime silicates, quartz and marraorized limestone. The enclosing country rock is schist.

At Clifton, Arizona, t.he Copper King vein exiiibits well the several zones produced by surface agencies. The outcrop of

' F. L. Ransome, P. P. 21, U- S. G. S., p. 155.

W. H. Emmons and F. B. I.juiBy, BvM. 470, U. S. G. S.,

J

182 Examination Of Prospects

this vein is white quartz, showing in a few places casts of origiD- ally contained sulphides, but for the greater part, the quartz is massive, and unstained by iron. This outcrop carried, where underlain by the ore-body, a little malachite and chrysocolla as stains on post-mineral fractures through the quartz. Beneath the hard capping, kaolin is mixed with the quartz, which carries copper stains and small patches of residual carbonates and silicate of copper, which gradually increase in quantity until the vein becomes ore. Beneath this oxidized ore, chalcocitc, associated with kaolin, quartz, and cuprite, comes in, graduaDj changing to chalcocite, associated with pyrite and quartz, whick form a very hard and tough ore for several hundred feet in depth. The pyrite in this ore becomes more prominent as depth is at- tained, until upon the disappearance of the chalcocite, the chief values in the pyritic material are as chalcopyrite, which in tuii becomes less in depth, and in the lower part of the deposit the primary ore is exposed, consisting of pyrite, associated withi little chalcopyrite and zincblende in a gangue of quartz anl quartz-porphyry. The pyritic ores carry low values in gold.

Disseminated Chalcocite Enrichments. — Disseminated chalco- cite enrichments, popularly called "porphyry copper deposits," now form one of the principal reserves of known copper oitti many millions of tons of such ores averaging perhaps 2 per ceat copper have been developed at different camps in the South* western States.

The primary ore of this type of deposit is disseminated cujrf erous pyrite, usually associated with quartz and accompanie' by sericitization of the containing rock. This mineralization ii commonly introduced into crushed or sheared zones, the minenk forming veinlets through the rocks; it also frequently folloU joint planes, and occurs as scattered particles through theroi between such fractures.

Upon the oxidation of the pyrite, solutions containing th

sulphates of iron and of copper and sulphuric acid seepdownwu

through the mineralized mass, kaolinizing the rock and leachi"!

Secondary Ores And Ore-Shoots 183

contact with unaltered pyrite at the water level. This process, while very irregular over short distances, produces enriched deposits that are relatively uniform when considered in large masses. The enrichments commonly bear a definite relation to the depth below the surface and to the ground-water level.

In some instances a part of the iron is left behind in the leached zone as limonite, while in others the iron is completely removed with the copper, the leached capping being a porous mass of white kaolin and quartz. In the upper oxidized zone, where not completely leached, large bodies of low-grade residual ores are frequently found, in which the copper occurs as chrysocolla, carbonate, or partly oxidized particles of sulphide minerals, and also in some cases, as native copper and cuprite. The low grade of these residual ores coupled with the presence of perhaps an important proportion of the contained copper as sulphate results in serious metallurgical difficulties, due to solution and to the low specific gravity of some of the copper compounds.

Secondary chalcocite enrichments, while varying greatly in their geological associations, present a remarkable similarity to casual observation, the secondary processes having tended to produce like results from dissimilar primary deposits. Deposits of disseminated copper minerals are in many cases associated with monzonitic intrusives.

Important features in the consideration of these deposits are the depth at which they occur, whether susceptible to stripping and steam-shovelling, and the strength of their walls, which are commonly kaolinized, soft, and likely to cave upon the removal of any large quantity of ore.

Inasmuch as these deposits are commonly of horizontal tabular form, exploration should be carried out vertically. The zone of enrichment is expected beneath a thoroughly leached and kaolinized capping, and is underlain by primary pyritic ores, into which it is futile further to continue vertical exploration.

The primary ore may usually be recognized by the freshness of the containing rock, as well as by the typical primary minerals.

184 Examination Of Prospects

beneath pyritic material, but always where this pyritic material was disintegrated and the associated or containing rockwdl kaolinized, the pyrite having been leached of its copper without complete oxidation of its sulphur.

At the Inspiration Mine, near Miami, Arizona, the primary mineralization occurs as an impregnation of schists. The average depth of the leached capping is 367 ft. The average thickness of the enriched ore is 155.5 ft., and its average grade is 2.00 per cent, copper.

At Morenci, Arizona, the upper limit of the chalcocite zone In the disseminated deposits under Copper Mountain is represented by a curve somewhat less convex than the contour of the surface. The thickness of the chalcocite zone is somewhat over 200 ft, although directly below the summit it reaches 300 ft. in thick- ness. The depth of leached material in this district varies from a few feet only to over 200 ft. The average grade of the ore now being mined is probably between 2.00 and 2.5 per cent, copper.

Chalcopyrite Enrichments. — Chalcopyritic enrichments are not of so common occurrence as enrichments of chalcocite, but form important deposits in certain districts. Secondary chal- copyrite is commonly associated with bornite, and also with limonite.

At Bingham, Utah, the enrichment of the Highland Boy ore- body is chiefly chalcopyritic. Here the carbonate and oxide ore passes into an enriched zone characterized by chalcopyrite, tarnished and coated with bornite and seamed with limonite: covellite is occasionally an associated mineral. Below this zone of sulphide enrichment the primary ore is found, consisting of lean cupriferous pyrite. The transition between the enriched ore and the overlying oxidized ore is gradual, as is also the change to the underlying primary ore.

At San Antonio De La Huerta, Sonora, the primary solu- tion-breccia ore-bodies appear to have received a chalcbpyritie

Henry Krumb, "First Annual Report of the Inspiration Copper Ce," Waldemar Lindgren, P. P. 43, U. S. G. S., p. 204. 'J. M. Boutwell, P. P. 38, U. S. G, S., p. 223.

Secondary Ores And Ore-Shoots 185

enrichment. The chalcopyrite is here associated with limonite and small quantities of copper carbonates and silicates; the en- richments occur at slight depth below the surface.

The Enrichment of Gold and Silver with Copper. — Under certain conditions, depending upon the presence of proper solvents and the absence of precipitants, gold and silver migrate and form enrichments in company with copper. Surface waters contain- ing chlorides precipitate silver in the oxidized zone as chloride in many instances, and the enrichment of silver with the second- ary copper sulphides is thus in part prevented. Not infre- quently, however, only a part of the silver is precipitated as residual ore, while the remainder descends with the copper to the water level. The enrichment of gold in copper deposits is like- wise variable, and, perhaps, takes place less frequently even than the enrichment of silver, being left in most cases as residual ore in the oxidized zone. Theoretically, silver should be precipitated higher up in the zone of secondary enrichment than copper, as its ajffinity for sulphur is greater. The copper in enriched copper- silver-gold ores is commonly in the form of chalcocite, although it also occurs in such association as chalcopyrite and enargite.

At Rio Tinto, Spain, the oxidized zone, from 30 to 150 ft./ deep, carries from 35 to 50 per cent, iron with traces of copper/ arsenic and sulphur. Directly beneath the gossan and resting on the pyritic ore in certain deposits there occurs a bed from 4 to 8 in. thick of earthy, porous material that carries from $10 to $20 gold and about 40 oz. of silver. The upper part of .the pyritic zone is enriched by a net-work of seamlets of chalcocite, bomite and chalcopyrite, which minerals occur in progressively less quantity with increasing depth. The pyritic ore directly below the gossan carries from 4 to 5 per cent, copper; at a depth of from 200 to 230 ft. the copper averages about 2 per cent. ; at 330 ft. about 1.5 per cent. ; at from 425 to 460 ft. about 1 per cent. The primary ore consists of pyrite with a little quartz and chal- copyrite.

At Silverton, Colorado, the enrichment of silver with cop-

Beck-Weed, "Nature of Ore Deposits," p. 4&5>.

186 Examination Of Prospects

per IS well illustrated. According to F. L. Ransome: "The ore first struck, in some cases at the surface, consisted chiefly of argentiferous galena. At a depth varying somewhat in diffe ent mines, but which appears commonly to have been less than 200 ft., the galena ore changed to an ore consisting chiefly of highly argentiferous stromeyerite, silver and copper glance. At a still greater depth, commonly at about 500 ft., the stro- meyerite changed to argentiferous bornite, still deeper to chal- copyrite and pyrite, and finally to low-grade auriferous and argentiferous pyrite. These changes were more or less irregular and overlapping, pyrite, for example, was found at nearly all levels, and bunches of galena were met with far below the leads at which this mineral ceased to be the dominant ore. Small, rich streaks of bornite were also found, with chalcopyrite and pyrite, below the levels at which it occurred in large masses," According to Schwartz the rich parts of the ore-bodies were in every case associated with open, water-bearing fissures.

The Enrichment of Silver. — The primary ores of silver aie readily attacked by oxidation, and in the presence of acid sul- phate solutions, yield their silver as soluble sulphate; in this state silver may descend to form secondary enrichments at the water level, where it is readily precipitated by other sulphides. The leaching of the upper parts of silver deposits is similar to the corresponding alteration of copper deposits, with the added factor that a certain proportion of the silver is likely to be pre- cipitated in the oxidized zone as chloride, or as native silver.

The complex sulphur, arsenic and antimony compoimds of silver occur as both primary and secondary minerals, and it is often difficult to determine to which process they should be as* signed. Where copper occurs with silver in deposits worked over by surface agencies the two metals frequently migrate and pre* cipitate together, although the tendency is for a certain propo tion of the silver to be left behind as residual minerals in the oxidized zone and for the secondary sulphide of silver to occur

F. L. Ransome, BuU. 182, U. S. G. S., p. 137. ''Trans., A. I. M. E., XVIU, p. 144.,

Secondary Ores And Ore-Shoots 187

near the top of the zone of enrichment. Many of the great bonanzas of silver ores in arid climates are probably secondary enrichments.

Stibnite is of frequent occurrence in silver ores in many dis- tricts in Mexico; it is typically a primary mineral, and the man- ner of intergrowth of the silver minerals with it may indicate their primary or secondary origin.

In the investigation of deposits of silver ores where secondary enrichment is suspected, it should be borne in mind that a relatively small body of rich silver ore may yield important economic return; no such general leaching of rocks or complete alteration of primary sulphides, therefore, should be required upon which to base an expectation of important deposits as is the case with secondary copper ores.

At the Promontorio Mine, Durango, Mexico,* the primary ore consists chiefly of quartz, galena, and zincblende, a little pyrite, and subordinate chalcopyrite. The oxidized vein-filling con- sists of quartz, kaolin, hematite, wad, and limonite, with occa- sional films of malachite, linarite and the remains of sulphides. The minerals that have contributed to secondary enrichment are native silver, chalcocite, and a little chalcopyrite. The second- ary enrichments are contained in the oxidized parts of the vein, and in the country rock of the walls and horses. The primary ore-shoots are distinguishable by their comparatively high con- tent of sulphides, by their lack of secondary minerals, and by their habit of being cut off by faults unless occurring in unf aulted parts of the vein. The secondary ore-shoots are recognized by their low content of sulphides, by the presence in their richer parts of the secondary minerals, native silver, chalcocite and chalcopyrite, and by their tendency to follow closely well-de- fined faults. Primary ore-shoots are dominant in the lower levels and the secondary ore-shoots in the upper levels.

At Georgetown, Colorado, the silver lead deposits show evidence of rearrangement by surface agencies. The zone of

F. C. Lincoln, Trans., A. I. M. E., XXXVIII, p. 740. a Spurr and Garrey, P. P. 63, U. S. G, S., p. \Aa.

188 Examination Of Prospects

complete oxidation in these veins usually extends from the surface to a 'depth of from 5 to 40 ft. only. The oxidized material is a brown clay; it is, in general, rich ore, containing several hundred ounces of silver. Below this oxidized material, and mixed with the lower part of it, occur friable, locally powdery, black sulphides and bunches of secondary galena. These pulverulent sulphides are rich ores also, containing, relatively large quan- tities of silver and of lead, and more gold than occurs at greater depths. The soft sulphides are found chiefly in cracks and along water courses, and are of secondary origin: they persist to considerable depths below the surface, but in decreasing quan- tities. Below the zone of soft secondary sulphides, and irregu- larly within its lower part, occur secondary polybasite, argen- tiferous tetrahedrite, and ruby silver, which in quantity diminish steadily but irregularly with increasing depth. The best ore in most of these veins has been found within 500 ft. of the surface, but locally it extends down to 700 or 800 ft., and in one case to 1000 ft. below the surface. Much secondary pyrite and siderite occur in the upper parts of these veins. The pri- mary ore carries from 20 to 30 oz. of silver and the enriched ore from 200 to 300 oz. per ton. The primary ore consists of galena, zincblende and cupriferous pyrite in a gangue of quartz, and the carbonates of iron, manganese, magnesia and lime.

At Georgetown, Colorado/ in the Bismark ore-shoot, the oxidized ores of brown clayey material carry several hundred ounces of silver and extend to a depth of about 40 ft. Mixed with these ores and extending from 200 to 300 ft. deeper, most prominently along water courses, soft black secondary sulphides are found. These ores consist chiefly of galena and zincblende, and carry several hundred ounces of silver per ton. The primary ore is composed of zincblende and galena with some p3rrite, in a gangue of quartz, siderite, subordinate barite, and calcite.

At Lake City, Colorado, the primary ore of the silver- bearing fissure veins consists of galena, tetrahedrite, chalcopy-

Spurr and Garrey, citing B. B. Lawrence, P. P. 63, U. S. G. S., p. 190. 2 J. D. Irving, BtilL 260, U. S. G. S., p. %1.

Secondary Ores And Ore-Shoots 189

rite, zincblende and pyrite in a gangue of quartz, rhodonite, rhodochrosite and barite. In the upper parts of the veins secondary ruby silver and argentite are found associated with anglesite, cerussite, limonite and pyrolusite. The values in the upper parts of the veins are commonly high, and bonanzas of ruby silver are found.

At Broken Hill, N. S. W., the oxidized zone is largely made up of "kaolin ore," which carries oxidized minerals of iron, manganese, lead and copper, and relatively low silver values. In the lower part of the oxidized zone occur the " dry ores,'' which carry the antimonial and arsenical sulphides of silver, polybasite, stromeyerite, dycrasite, proustite, pyrargyrite, and stephanite. Beneath the dry ores occurs a thin layer, from 3 in. to 6 ft. in thickness, of sooty black sulphides, which carry as much as 250 oz. of silver and 12 per cent, copper. This layer rests upon the primary ores, which consist of an intimate mixture of argentif- erous galena and blende, in a gangue of quartz, garnet, rhodo- nite, and feldspar, with chalcopyrite, arsenopyrite, wulfenite and fluorite as accessory minerals. The primary ores carry from 5 to 36 oz. of silver, 7 to 50 per cent, lead, and 14 to 30 per cent. zinc.

At Neihart, Montana, superficial alteration is not marked, and no great zones of carbonate or oxidized ores ore found: the deepest general oxidation in the district reaches lYO ft. below the outcrop, but locally oxidation extends as pipes and along drainage fissures to greater depths. The secondarily enriched ores carry polybasite, pyrargyrite and argentite as crusts lining cavities, as thin seamlets through the primary ores, and as sooty sulphides associated with maganese oxides in the oxidized zone. The secondary minerals also occur in the fractures in the shattered country rock. The primary ore contains galena, zincblende, and pyrite in a gangue of quartz and barite.

The Enrichment of Gold. — Gold is readily soluble in the pres- ence of nascent chlorine, which is produced by the action of

S. F. Emmons, Trans., A. I. M. E., XXX, p. 204, quoting Jaqjiet. 2 W. H. Weed, Trans., A. I. M. E., XXX, p. \Zh.

190 Examination Of Prospects

sulphuric acid upon chlorides in the presence of the oxide of manganese, all of which compounds are frequently present in the zone of oxidiation. Gold is also soluble in solutions of ferric hydrate, sodium sulphide, sodium carbonates and other reagents formed in nature. Gold is precipitated readily by pyritic sul- phides and by organic matter, and when in solution with ferric hydrate is precipitated upon the reduction of this compound to the ferrous salt, a change likely to take place at water level. Gold also migrates and precipitates with the antimonial sul- phides of silver.

The relatively small bulk of gold as compared with its asso- ciated gangue, commonly resistant quartz, tends to protect the gold from solution and also renders it diifficult to distinguish a primary gold ore from one of secondary origin. The association of rich ores with post-mineral fissures, the presence of films of oxidized minerals through the ore, and the shape of the ore shoots are the most reliable criteria of secondary originl Second- ary ore-shoots are likely to be longer horizontally than they are deep in deposits of uniform permeability, while primary ore shoots are commonly deeper than they are long.

A majority of gold deposits are larger and richer near the surface than in depth, and their ore-shoots often carry films of oxidized minerals, such as dendritic oxide of manganese, or an incipient oxidation of the associated sulphides, far below the zone of weathering. The high-grade ores found under these conditions are probably the result, in part, of secondary enrich- ment. Gold is less easily dissolved under average conditions than are most other metals, and is frequently left behind in the upper parts of ore-deposits in which secondary enrichments of other metals have formed at the water level.

It is not uncommon to find rich bodies of gold ore at and just beneath the outcrops of gold veins, due probably to the several processes of mechanical concentration during weathering, resid- ual concentration through the removal of soluble constituents, and also to some extent to solution, migration and enrichment.

Secondary Ores And Ore-Shoots 191

At the Ruby Mine, Montana/ an important secondary ore- body, in which the values were about eqiial in gold and silver, occurred along a post-mineral fault. The gold and the silver appear to have migrated together in antimonial sulphides, notably pyrargyrite, which occurred as coatings on boulders of decomposed rhyolite, and lining cavities in the zone of enrich- ment.

In the Granite Vein, Mont an a, which is described in a pre- ceding paragraph, the low-grade ore of the leached zone carries but little gold; the enriched oxide ore which occurs at the base of the oxidized zone carries from $5 to $16, the underlying enriched sulphide ore from $4 to $8, and the primary ore from $1.50 to $3. The accompanying silver values are concentrated in greater degree than the gold.

At Monte Cristo, Washington,' there is no zone of complete oxidation and the sulphides frequently outcrop. Partial oxida- tion extends to a depth of perhaps 10 ft. The distribution of the sulphide minerals indicates rearrangement by surface agencies. The upper zone is characterized by galena, gold, and silver, and its ores average $19.00 in gold and 12 oz. of silver. The lower limit of the galena zone follows the contour of the surface at depths of from 100 to 150 ft. Below this occur less regular, but still definite, zones that are characterized respectively by zincblende, chalcopyrite, and arsenopyrite with pyrite. The ores of the last-named zone average $12 gold and 7 oz. of silver.

The Enrichment of Lead. — Lead is less easily soluble than most other metals: upon oxidation it alters first to the sulphate, anglesite, and then to the carbonate, cerussite. The carbonate, apparently, may also form directly from the sulphide. Owing to the relative insolubility of these minerals lead is dissolved slowly and is readily precipitated and the process of enrichment

W. H. Weed, Trans.y A. I. M. E., XXX, p. 433. 2 W. H. Emmons, Bull, 315, U. S. G. S., p. 39.

'J. E. Spurr, Twenty-second Annual Report, U. S. G. S., and "Geolo Applied to Mining, " p, 280.

192 Examination Of Prospects

is a slow one. Long continued action of surface waters, how- ever, with repeated solution and precipitation may produce a zone of lead enrichment higher up in a deposit than the enrich- ments of associated metals such as copper and zinc, whose second- ary sulphides are commonly found at the water level. Galena is slightly soluble in water, and in solutions containing sodium suljAide. The sulphate of lead, anglesite, is slightly soluble in water, and the carbonate, cerussite, is soluble in carbonated waters. Galena is precipitated by hydrogen sulphide produced by the reactions of solutions upon sulphides, by organic matter, and probably when migrating as the sulphide, by direct replacement of caleite.

While in most deposits the upper lead bearing zone is in large part made up of residual galena, anglesite and cerussite, migra- tion and enrichment have probably assisted in concentrating these minerals. The migration of lead in the presence of acid sulphate solutions is apparently more difficult and less com- plete than in certain limestone deposits where carbonated waters have probably acted as the solvents.

The examples of secondary enrichments of galena are neither numerous nor clear, the usual occurrence of lead minerals in deposits worked over by surface agencies being as residual minerals.

At Monte Cristo, Washington, as described in a preceding paragraph, the upper part of the sulphide zone is characterized by galena, the lower limit of which follows roughly the contour of the surface, indicating a concentration by surface agencies.

At Georgetown, Colorado,* in the veins of the Freeland group galena was more abundant in the upper few hundred feet, while but small quantities of this mineral is contained by the characteristic pyritic ore. In the zone of relatively abundant galena, brown carbonates (siderite, rhodochrosite, and some barite), were abundant, as was also tetrahedrite, which occurred in cracks through the older sulphides. Both the carbonates and tetrahedrite were in places clearly of secondary origin.

Secondary Ores And Ore-Shoots 193

In the Upper Mississippi Valley deposits of zincblende and galena associated with marcasite, pyrite and subordinate chalco- pjrrite occur in unaltered, flat lying dolomites and limestones far from any known igneous rocks, and apparently, in most cases, unconnected with any important fissure. In form these ore-bodies occur in vertical crevices, in irregular ore-bodies at the juncture of an ore bearing horizon with a vertical fissure, in pitches and flats, and in thin, flat lying disseminations. The ore-bodies are closely related to certain very shallow structural . basins, and to certain shale beds known locally as oil rock,

Fig. 75. — Diagram illustrating type of "pitch and flat" deposit of the Upper Mississippi Valley, a, Flats; 6, pitches; c, vertical crevice. After Grant,

which contain a large percentage of fossil gum. These ore- bodies are believed by many to represent concentrations by descending waters of sparsely disseminated mineralizations; I their genesis must be considered doubtful. The common verti- t cal distribution of the several ore minerals is: galena in the '- uppermost zone, below which is found zinc carbonate ore, under- lain in turn by zincblende ores below the water level, while the lowest zone is pyritic; the several zones commonly merge into each other. The localization of the ores appears to be con- nected with the water level; lead ores occur above the water level, zinc carbonate from slightly above to slightly below, and the zincblende below but near to the water level. A second- ary distribution is thus indicated. ' H. F, Bain, BvU. 294, U. S. G. S., p. 46,

194 Examination Of Prospects

While scanty occurrences of zincblende and galena are known in both the overlying and underlying rocks, the important mineralizations are confined to the Galena dolomite, and in those districts where this rock has been largely eroded they also occur in the underlying Platteville formation: while ore-deposits are found in all horizons of these beds certain strata are locally more likely to contain ore than others. The ore-deposits are in the main confined to very shallow synclines. The aforemen- tioned " oil rock " is a brown or black shale varying in thicknea from 6 to 8 ft. down to a few inches; it contains thin lenses of dolomite and the shaly material commonly does not form single bands more than one foot in thickness. This bed contains fosd gum that appears to have been active in precipitating the ore minerals. Pitches and flats are commonly developed above the oil rock, which itself frequently contains disseminated ores. The oil rock is seldom seen in any considerable thickness outside of the mines, though it is not of such a nature as to be especially subject to weathering.

The Enrichment of Zinc. — Zincblende is readily attacked by oxidizing solutions and the resulting sulphate is very soluble. Zinc is usually completely leached from the oxidized zones of most deposits, but sulphide enrichments in igneous rocks are comparatively rare; in sedimentary rocks, however, migrating solutions carrying zinc frequently replace certain beds of the country rock with oxidized zinc minerals. Such deposits, even when of important extent, are difficult to detect, as the replacement commonly retains the structure and to some ex- tent the color of the replaced rock, while the low specific gravity of these minerals does not call attention to their presence. Zincblende, under certain conditions, apparently migrates with ease in limestone.

In the Upper Mississippi Valley secondary enrichments of zincblende are common in the deposits in limestone and dolomite. The zincblende occurs just below the water level and is usually associated with marcasite and some galena. This zone passes in depth into ores that carry ab\iiidaii\)T[iTe.\,tb.but little zinc.

Secondary Ores And Ore-Shoots 195

At Leadville, Colorado, the zinc is commonly leached from the oxidized ores near the surface and forms large replacement de- posits of oxidized minerals in depth, the existence of which was not suspected for many years.

At Pachuca, Mexico,* the silver veins are impoverished in depth by large quantities of barren zincblende.

The Enrichment of the Lesser Metals. — Under certain con- ditions, nickel, cobalt, arsenic, antimony, tin and cadmium appear to be capable of solution, migration and enrichment.

The Migration of Gangue Minerals. — The solution, migration, and occasionally the precipitation, of gangue minerals are im- portant processes in the rearrangement of ore-deposits by sur- face agencies. The residual concentration of gold in the oxidized zone is the result of the solution and removal of associated gangue minerals and sulphides.

The carbonates and alumina appear to be especially susceptible to solution and removal, while under certain conditions quartz is a mobile mineral.

The oxides of manganese are apparently soluble in solutions of ferrous sulphate; the presence of dendritic oxide of man- ganese in many deep deposits is probably the result of the migration of this compound in solution.

Waldemar Lindgren, Trans., A. I. M. E., XXX, p. 650.

F. W. Clarke, quoting F. P. Dunnington, BidL 330, U. S. G. S., p. 458.

Chapter Xi

Outcrops

In the examination of an undeveloped prospect a decision must be arrived at from an inspection of the outcrops and the exposures in a few shallow pits. Prospects that are offered for sale rarely expose any important quantity of payable ore, work having usually been stopped when the immediate exploration no longer yielded favorable results. It should be borne in mind, furthermore, that a great majority of prospects have been ex- amined many times, and if of conspicuous promise, would have been acquired for development. In most cases the problem for the engineer, therefore, is to discover the traces of a valuable mineralization that has disappeared through solution, or the conditions that indicate the possibility, or probability, of under- lying secondary enrichments.

The available data being meager, every feature should be the subject of careful study — the condition of the outcrop, the structural relationships, the associations of minerals, the altera- tions of wall rocks, the chances for underlying enrichments, and so forth, as well as the actual assay value of the material exposed.

It is frequently advisable to spend the time and money neces- sary to have trenches dug at various significant points along a promising outcrop ; such exposures, even if the depth attained is slight, often disclose conditions that are not apparent at the actual surface, as for example, the existence of soft minerals and the distribution of the various minerals through the mass of the deposit. Trenches also permit samples to be taken from points that were not accessible during former examinations.

The Relation between Length of Outcrop and Persistency of Vein in Depth. — Strong, persistent outcrops of uniform width may be taken to indicate the probable size and character of the underlying vein, whose persistency m Aa-XJa. Ukely to be

19

Outcrops 197

proportional to the length of its outcrop. Fissures that may be traced for long distances on the surface are commonly found to be equally persistent in depth, while short, branching and irreg- ular outcrops are usually indicative of similar irregularities in the underlying deposits. Outcrops that comprise a series of large irregular masses, perhaps connected by narrower veins, are in general likely to become smaller in depth.

The Relation between Size of Outcrop and Width of Vein in Depth. — In a vein or deposit that is harder and more resistant than the enclosing rocks, erosion is likely to be halted at the widest part, that part offering the greatest resistance to erosion and the greatest protection to the adjoining rocks. Such out- crops, therefore, are likely to be larger than the average of the underlying deposit. The converse of this condition is also true. A vein or deposit that is softer and less resistant to erosion than the enclosing rocks is likely to become larger in depth. Erosion tends to remove the surface of such a deposit faster than the adjoining wall rocks, forming a depression, the walls of which tend to close together upon the removal of the intervening soft material; occasionally, a narrow and inconspicuous gouge-filled fissure is the only surface indication of an important vein of soft minerals.

Brecciation and Post-mineral Fracturing. — In disseminated deposits the most intense primary mineralization is likely to be connected with a thorough brecciation, as are also disseminated enrichments. The study of post-mineral fracturing, therefore, is of as much importance in the investigation of these deposits as is the study of the minerals of the outcrops themselves.

Meandering of Outcrops on Hillsides.-— The outcrop of a vertical vein is a straight line, whatever the slope of the surface, and the outcrop of a horizontal bed follows a contour around all hillsides. A vein of intermediate dip, however, outcrops to the right or to the left of its strike at any horizon in accordance with its dip and the slopes of the surface. An idea may be obtained in the field as to the dip of a vein by paralleling a book or other plane

Vss Examinatu

while in mapping, simple trigonometric calculations or graphic solutions will give tlie heights or horizontal positions of the vein at any desired point: contouring is expensive and is usually

Down Hill Creep. — An outcrop situated on a steep hillside is

likely to overturn in the direction of the slope of the hill; loose fragments of the partially disintegrated outcrop become

Fro. 76. -Outcrop of a vein : iiig and dot

Bullfrog, Novadfl, ehowing surface ovi i-hill creep. After liatisomc.

mingled with the surface matCTial, and when at some distance from the parent mass are widely separated and constitute "float." The apparent dip of a vein that outcrops along a steep hillside is, therefore, likely to be flatter than its true dip below the me- chanical influence of erosion.

The Topographic Expression of Mineralization. — Mineraliza- i/on occasionally finds locaV expTtssio-o. topography.

Hp

Silicified areas and resistant outcrops may form prominent ridges or knobs, and soft or altered rocks may result in depressions or saddles in the ridges. Faults are occasionally prominent topographic features where one 'wal! is notably more resistant to

Fig. 77. — Fault plane developed iuto a acurp liy erosiou, Globe, Arizona! the rook on the left (hanging- wall) is quartzite, that on the right (foot-wall) ia granite. After Ransome.

erosion than the other, but, in general, unless of great displace- ment, faults are not likely to be represented in the topography. Different kinds of rocks upon weathering produce characteristic topographic outlines, as, for example, tke \a\:i4'a-iiSs.\i.Sss,

200 Examination Of Prospects

steep talus slopes of horizontal sedimentary beds, or the ragged outlines common in areas of volcanic rocks. While topographic relief is rarely significant in the examination of mining properties, it may be a valuable guide to the prospector, as many mineralized areas are connected with low, rounded foot hills of notably different outline from the general relief of the region.

Outcrops of Deposits Formed at Slight Depth. — Deposits formed at slight depth below the surface are characteristically irregular in their upper part, and tend to consolidate and to become structurally more regular with greater depth.

Porosity of Outcrops. — The outcrops of ore-deposits that have yielded to oxidation are commonly porous and cellular, owing to the removal of certain of the original constituents in solution. Such outcrops are favorable in that they indicate solution and possible secondary enrichments of the dissolved substances, or, at least, the original presence of easily dissolved minerals which in their unaltered state may have been valuable. A massive, tight outcrop, on the other hand, is usually indicative at slight depth of the typical value of the deposit, which has presumably been but little affected by secondary agencies.

Casts in Resistant Gangue Minerals. — Upon the oxidation and leaching of an ore composed of sulphide minerals in a resistant gangue, the outcrop is likely to retain the casts, or open spac, left by the removal of the sulphides. In many instances out- crops exhibit abundant casts of minerals that have completely disappeared, and whose presence originally would not be sus- pected without a close examination for such evidence.

The crystallization of pyrite appears to be interrupted when it contains a notable proportion of copper. Pure, barren, pyrite commonly crystallizes as cubes or other isometric forms, while chalcopyrite is commonly distributed in an irregular manner; the crystallization of cupriferous pyi'ite, in general, is wavy and irregular, although occasional cubes may be noted. A less strongly emphasized but still marked relation exists between pure, barren galena and highly argentiferous galena. This mineral is less likely to be well crystallized when it carries an

Outcrops 201

important proportion of silver. An examination of the casts of these sulphides in a leached outcrop, therefore, is likely to yield evidence to some extent in regard to the value as well as to the kinds of the sulphides originally present.

The Composition of Outcrops. — The minerals present in out- crops are those primary constituents of the ore most resistant to solution, and the most resistant of the products of alteration. The most resistant of the primary minerals is usually quartz, which commonly forms a large proportion of the outcrops of the deposits in which it is an important constituent. Magnetite and specularite are also relatively resistant minerals. Of the products of oxidation and hydration kaolin and limonite are the most resistant, and are commonly found in or close beneath the outcrop. Sericite likewise is resistant, but is likely to be changed to kaolin where sulphuric acid solutions are prominent in accom- plishing the surface alteration.

The oxides of manganese are resistant, and are frequently found in large quantities in the oxidized parts of ore-deposits, even those in which manganese forms a quite subordinate pro- portion of the primary ore.

Gold is a resistant mineral, and is frequently concentrated at the surface and in the oxidized zone, as is discussed in a pre- ceding paragraph.

The more resistant of the ore minerals are galena and its oxidation products, anglesite and cerussite, chloride of silver, native silver, cuprite, native copper, chrysocoUa and to a lesser degree the carbonates of copper, all of which minerals are likely to be left behind as residual ores during solution and migration.

In copper deposits that have been subjected to thorough leach- ing and alteration, the outcrops are likely to consist of soft white kaolin and quartz, carrying at the surface, perhaps, trifling quantities of limonite, manganese oxide, and chrysocoUa as stains. Such outcrops are most favorable for the existence of chalcocite enrichments in depth.

The thorough alteration of an intensely mineralized mass may

202 Examination Of Prospects

rocks that received a scanty mineralization only and so remain relatively unaltered, often retain traces of the primary minerals and so furnish a clue to the original nature of the principal deposit.

The Oxides of Iron in Gossan. — The oxides of iron, either massive or as stain, are frequently the most prominent con- stituents of outcrops and of the superficial parts of ore-deposits that have suffered oxidation. The condition and occurrence of these minerals are often indicative of the character of the mineralization in depth.

The final product of the oxidation and hydration of iron minerals is limonite, and the other oxides of iron on thorough alteration yield this mineral. Magnetite in an outcrop is commonly present as an unaltered primary mineral, and may not be taken to represent the alteration product of a sulphide mineralization. Specularite, of characteristic crystal form, is likewise a primary mineral, and not the result of the alteration of sulphides: micaceous hematite should be distinguished from specularite, as it is frequently found as the alteration product of sulphides, the micaceous structure having been developed by stress exerted after its formation. Certain gangue minerals, such as lime-iron garnet, yield limonite upon oxidation; in most cases limonite of this origin occurs as soft earthy masses, mixed with unaltered contact minerals as, for example, partially decomposed epidote, and is commonly distinguishable from limonite resulting from the oxidation of sulphides.

In many outcrops in arid regions whose underlying ore de- posits have been explored, the limonite resulting from the oxidation of pyrite occurs as a massive brown mineral, while the chalcopyrite of the original ore is represented by seamlets and patches of soft, bright red hematite.

The Condition of Outcrops Indicative of Secondary Enrichments in Depth. — Features of outcrops that indicate the possibility of enrichments in depth are the residual indications of a good primary mineralization together with a porous or brecciated structure, or the presence of post-mineral fractures.

Outcrops 203

A majority of enrichments of secondary sulphides are prob- ably due to the migration of sulphate and sulphuric acid solutions; these solutions frequently leave traces through the presence of a kaolinitic alteration of the associated rocks, or of kaolin in the out- crop or oxidized zone. Resistant minerals that remain after a thorough alteration of this kind are kaolin, limonite and quartz. Feldspars are completely altered and sulphides are absent. The presence of unaltered feldspars or of most of the other usual gangue minerals except quartz indicates a partial alteration at best, and the presence of sulphides indicates an incomplete solu- tion. Galena is a resistant mineral as compared with other sul- phides, and not infrequently overlies important enrichments of other metals, but the presence of unaltered pyrite, chalcopyrite or zincblende renders it unlikely that important enrichments will be found through deeper exploration.

The outcrops of suspected disseminated chalcocite enrichments should contain little else than kaolin, limonite and quartz, together with, perhaps, some unaltered sericite, itself a product of primary altering agencies. The presence of pyrite in such ah outcrop or in the oxidized zone is a most unfavorable sign; cases are known where pyrite was found above secondary chalcocite enrichments, but in every case it was partly altered, crumbly, and was contained within thoroughly kaolinized rock, the supposition being that the copper was leached from the pyrite under conditions that did not permit a complete oxidation of the sulphur. Where the containing rock shows the outlines of the feldspars the kaolinization must be considered unsatisfactory.

Efflorescences of soluble salts in the outcrops or at slight depth below them are frequently instructive. An efflorescence of copper sulphate is indicative of the solution of chalcocite, and may or may not be mixed with iron sulphate. An efflorescence of iron sulphate with but little copper is commonly indicative of pyrite at no great depth. Sulphur and a yellowish-green sul- phate of iron are probably not formed except close to oxidizing pyritic sulphides. Other efflorescences frequently found are alum

204 Examination Of Prospects

any zone of chalcocite enrichment; complex sulphates of alumi- num and other bases are also formed in the leached zone.

Where large quantities of pyritic sulphides have oxidized and I have in part been removed in solution it is not uncommon to find the conglomerates of stream beds cemented by limonite, and, in some instances, carrying oxidized copper minerals; the presence of such conglomerates may be taken to indicate con- ditions under which migration and enrichment may have taken place within the deposits themselves.

The elevation of an outcrop as compared with the drainage level, or the elevations of the known enrichnients of the district, are important factors in the consideration of possible secondary enrichments.

Disseminated chalcocite enrichments appear to be confined to regions of slight rainfall: important enrichments of copper and other metals are found in veins that have suffered post-mineral fracturing under all climatic conditions, except where rapid ero- sion or glaciation continuously exposes fresh surfaces of primary ores and thus does not permit the operation of surface agencies.

Rock Alteration as a Guide to Ore-Deposits. — The several types of rock alteration are discussed in a preceding chapter where their close relationship with mineralizing processes is emphasized. In the field it is usually possible to distinguish between a primary or hydrothermal alteration of the rocks and the results of ordinary weathering, and also between either of these and kaolinization, but where such distinction is doubtful, slides should be cut and examined under the microscope, when the type of alteration will become apparent. A '' highly altered condition" means nothing unless its type and probable relation to mineralization are understood.

The Outcrops of Kaolinized Rocks. — Upon the kaolinitic altera- tion of rocks carrying pyritic mineralizations the products of the alteration are likely to be similar whatever the original nature of the individual rocks. On Shannon Mountain, Arizona, granite, porphyry, and shales have all suffered intense kaolinitic alteration, and the resulting mass of kaolin, sericite, and quartz

Outcrops 205

with associated limonite and chalcocite may rarely be differen- tiated in the field into parts representing the original types of rocks. In granitic rocks that were not sericitized before suffering kaolinization the quartz phenocrysts remain clearly distinguishable, and in hand specimens tend to obscure the degrfee of alteration; a kaolinitic alteration, however, that has not obliterated the outlines of the feldspars cannot be considered thorough, and outcrops of this nature are rarely underlain by important enrichments. Thorough leaching and kaolinization usually removes the iron as well as the other bases, and the dumps of workings in the leached zones are commonly white in color, in sharp contrast with the brown or red color of the surface. In deposits that originally carried abundant pyrite, much limonite may remain in the leached zone. The line of demarkation between the leached and the enriched zones in such cases is well marked, the former being stained by iron, while the latter is white in color.

The Outcrops of Contact Deposits. — The minerals developed by contact metamorphism are, in general, resistant to oxidation and erosion, and are likely, therefore, to form conspicuous out- crops. The tightness of these minerals and the slowness with which they decompose under the influence of surface agencies protect the sulphides contained within them and so in large measure prevent migration and enrichment. A further hinder- ance to secondary enrichment in deposits of this nature is the usual presence of active precipitants. As a result of these con- ditions secondary enrichments are rare in contact deposits ex- cept where the sulphides were present in large masses; in all other cases the outcrops of contact deposits are likely to be in- dicative of the values contained by the deposit at all depths.

Deposits of Surface Origin. — Certain deposits of surface origin present close imitations of the outcrops of ore-deposits, but are not underlain by valuable minerals. Bog iron ore (limonite) is a familiar example of surficial deposit not connected with any underlying mineralization, and many deposits of the

206 Examination Of Prospects

cretions of manganese oxides that form on the sea bottom are likely to be concentrated into such superficial deposits, as is also, upon erosion, the manganese contained in snaall quantities by many rocks. Bog iron ores commonly contain casts of v;e- table remains, sand, and silt, and may also be recognised by their bedded form. Surhcial deposits of limonite and mangaoeea oxides may generally be distinguished from the outcrops of mineral deposits through their lack of associated minerals char acteristic of outcrops. Furthermore, after slight exploration their structure is revealed and their superEcial nature becomes apparent. Pyritic deposits are occasionally met with where the sulphides have replaced roots, or other organic matter, and whose superficial formation is evident.

Fia. 78. — Section of open cut, Bertha Zino Mines, lrginia, showing the superficial position of the residual ore. AfUr WaUon,

Microscopic Examination of Specimens. — The investigation of

an outcrop is largely a search for residual conditions indicating that an important mineralization has been removed from the surface by oxidation and solution. It is often advisable to have slides cut from specimens of an outcrop and its associated rocks, and to have them examined under the microscope. Information is thus gained of the character and degree of the alteration, and in many cases of the mineralogical nature of the original ore and of the distribution of the several minerals. Specimens taken for this purpose should be chosen carefully, and a record of them kept in the same way as is the custom with samples taken for assay: furthermoTe, it is beat always to Teser stttimen,

Outcrops

preferably part of the same piece that is sent away, for purposes comparison upon receipt of the results of the examination, HicroBCopic examination ia of great value, if only for corrobo- ration of the evidence gathered in the field, but may also yield information and indicate posaibilities that without it would not be suspected.

Descriptions of Outcrops

At Nacozari. Sonoha, Mexico, the outcrops of the Pilares solution-breccia are prominent. The country rocks of this dis- trict are andesitic and rhyolitic porphyries whose surface over an area of several square miles is stained red, yellow or brown by the oxides of iron resulting from the oxidation of a disseminated cupriferous pyritic mineralization. Shallow leaching and attend- ant unimportant enrichments of disseminated ehalcocite and chalcopyrite are present at many points in the district, but the depth and degree of alteration and enricVmciA a.Tt Via

208 Examination Of Prospects

slight to yield ore-bodies of commercial importance. The m of the gulches frequently show efflorescences of chalcanthite usually associated with ferric sulphate and sulphur, the ear marl of a near-by unaltered pyritic mineralization: a white, probabi] aluminous, precipitate is present at many points where solutioi are oozing out of the rocks, and this, apparently, is also a sign shallow alteration. In the vicinity of the Pilares ore-body, the] only important copper deposit in the district, the rock is strong] sericitized, and in this respect differs from the remainder of the district seen by the writer. The outcrop of this ore-deposit is composed of a striped breccia, seamlets of ore minerals being separated by long, thin slabs or sharp angular fragments of the country rock that frequently show a curving parallel arrangfrj ment of ore- and rock-areas.

The chief ore mineral is chalcopyrite, with some bomite, as-! sociated with pyrite and quartz. In the outcrop, the seamlets' residual after the ore are made up principally of micaceous hematite and quartz, with small particles of copper carbonates. Through the brown micaceous hematite, and as individual seams, occurs bright red hematite, in distribution following that of the chalcopyrite in the ore of the sulphide zone, and evidently-] residual after this mineral. The outcrop is of small extent as, compared with that of the deposit as developed in the deq)] levels.

At Jerome, Arizona, the conditions exposed in the open pitrf| the United Verde mine are indicative of intense mineralization. Here the principal country rocks appear to be schist and

This structure is considered by Mr. S. F. Emmons to indicate mineni' zation attended by a splitting off of the coimtry rock in concentric shdli under the influence of the mineralizing solutions; this type of deposit ii termed by him a " solution breccia." The writer has seen the same typi of mineralization at several other districts, all of them in Sonora; in every case deposits were contained in deep flows, or sills, of andesitic or rhyolitit porphyry. At San Antonio de la Huerta the copper deposits are of thi type; at La Trinidad, Sahuaripa, the lead silver deposits occur in a solution breccia, as do also the silver deposits at Guadalupe; the gold deposits netf Bacoacbi are said to be solution bxeccvas.

Outcrops 209

h(i|lltlier basic granite-porphyry that carries a heavy pyritic leralization. The residual minerals remaining in the walls the pit from which the surface ores were mined are white >liny quartz, limonite, and oxides of manganese in which jur copper carbonates and silicates. The outcrop is said to kve carried gold values. Sharp lines of demarkation exist kween bands of unaltered schist and the mineralized and Itered parts of the deposit. Kaolinization does not appear have affected the wall rocks. The principal ore minerals in .epth are auriferous chalcopyrite and bornite. At Morenci and Metcalf,, Arizona, the lode and the dissemi- ited deposits in porphyry have inconspicuous outcrops. The .odes are represented at the surface by quartz-filled fractures trough sericitized and kaolinized porphyry. A light iron stain common at the surface throughout the intrusive areas, but, "Sn general, the iron as well as the copper has been removed 'Srom the mass of the leached zone by solution. Except where tie secondary ores actually outcropped, as is said to have been iihe case with the Metcalf ore-bodies, there is but little indica- tion of copper at the surface, although a little chrysocoUa is occasionally seen, and light efflorescenses of chalcanthite are common on the walls of shallow workings driven into the por- phyry. The most promising areas for disseminated chalcocite enrichments are indicated by abundant casts, residual after an jpntense primary pyritic mineralization, together with ramifying ?' quartz veinlets associated with a thorough kaolinization of the rock. All of the important enrichments in this district occur high up in the hills; the canyons are in every case in lean pri- mary ore, and extensive exploration of the lower hill slopes has been without result.

At Bisbee, Arizona, one only of the large pyritic ore bodies outcropped at the surface; in this deposit the usual oxidized copper minerals were present in quantities sufficient to con- stitute ore. The limestone in the vicinity of many of the ore- bodies is fractured and carries disseminated pyrite. The rusty F. L. Ransome, P. P. 21, U. S. G. S.,

210 Examination Of Prospects

outcrops of this pyritic mineralization have in some instaiMHrabi indicated the presence of underlying ore-bodies; slight deiniMdut sions were noted at the surface above certain ore-bodies, probat)l&lo< due to subsidence upon the removal in solution of certuAt p constituents of the deposits. Many of the most important ( the ore-bodies of this distinct, however, are not represented mki any way at the surface. The intensely mineralized porphyijfca of Sacramento Hill taken in connection with the fracturiiigt silicification and marmorization of the limestone are surfacAe conditions patently favorable to the existence of ore-deposits, w

At Bingham, Utah/ the outcrops of the lead-silver, and the Bi copper deposits of the Old Jordan group are inconspicuouB. I The containing limestone is irregularly altered to white or gray marble, often carrying structureless patches of decosrposed powdery limestone; silicification is extensive, qua/? oeouning as granular masses, as fine compact replacements, and as cherfcy bands and patches. The outcrops of the lodes, which are trace- able with difficulty, are belts of shattered and discolored rock; the fissure fillings are marked by crushing and are stained by the oxides of iron and copper carbonates. The replacement deposits are represented at the surface by shattering, silicification, and discoloration of the limestone. At many points the rusty, siliceous outcrops carried gold values. An upper zone of car- bonates and oxidized ores here is commonly underlain by enriched sulphides.

At Park City, Utah, the ores occur as lode and replacement deposits in limestone; outcrops are scanty or lacking. Marmor- ization and silicification of the limestone are present in places, and shattering and discolorization are the chief surface indica- tions of the important deposits.

In the Coeur D'Alene District, Idaho, the outcrops of the important lead-silver deposits in limestone are inconspicuous. Soil and vegetation cover the hillsides, and the material of the lodes is neither so superior to the enclosing rock in hardness or

J. M. Boutwell, P. P. 38, U. S. G. S., p. 238. 2 F. L. Ransome, P. P. 62, U. S. G. S., p. 1*2..

Outcrops 211

ibility as to form bold outcrops, nor so easily eroded as to luce trenches or saddles in the topography. The courses of lodes may not be followed at the surface without the aid of pits or trenches. The ores at the surface carry galena with Lttle cerussite, limonite and copper carbonates. At Metcalf, Arizona , large contact metamorphic deposits the summit of Shannon Mountain, their resistant minerals lying halted erosion at the horizon of greatest development. Wke limestone strata have been largely altered by contact meta- iorphism to magnetite and garnet with abundant pyrite and jociated chalcopyrite.

Oxidation has worked over these deposits thoroughly; while of the copper has been removed by solution, in the sedi- ments most of the copper originally present probably remains as ddual ores; the magnetite and garnet have been partly altered limonite and quartz. The primary pyritic mineralization Wai sufficiently intense to have permitted access by surface ' wattTs and thorough oxidation. The residual ores are chiefly azurite, malachite, chrysocolla, and brochantite, associated with *limonite, hematite, and the oxides of manganese. In the por- phyry areas the surface is kaolinized and leached, and underlying enrichments of chalcocite are found in large deposits, both as disseminated enrichments, and in lode-like deposits along fissures. Near the top of the enriched zone further oxidation has produced cuprite and some native copper from the chalcocite. The shales, in part altered to hornfels by contact metamorphism, received in places a disseminated mineralization of pyrite and chalcopyrite which have probably been somewhat enriched by circulating solutions.

At San Pedro, New Mexico, copper occurs as chalcopyrite associated with garnet in extensive contact deposits. These deposits are confined to the lower part of a laccolithic roof of limestones. The shaly limestone is in places altered to hornfels and carries tremolite and diopside as coarse crystals: this type of altered rock carries no ore; above these beds occur garnetized Waldemar Lindgren, P. P. 68, U. S. G. p.

212 Examination Of Prospects

beds from 50 to 100 ft. thick and of great horizontal extent, through which the ore is irregularly distributed. The primary ore is composed of chalcopyrite, with some gold, in a yellowish- garnet, chiefly andradite, calcite, and lesser quantities of tremo- lite and wollastonite. Oxidation has had but slight effect on these deposits, in which apparently there has been no migration or enrichment.

At Virginia City, Nevada, the Comstock Lode outcrops for a long distance as siliceous masses and quartz veinlets cementing a fractured zone. This lode, unlike most deposits, occupies a fault of large displacement. The country rocks are propyUtized andesites. The lode is continuous over, its central part, but branches at either end ; while broad and scattered at the surface, it becomes more regular in depth. The principal values at the surface were as chloride of silver. The bonanza ores contained stephanite, polybasite, argentite, and native gold, associated with small quantities of galena and zincblende in a quartz gangue.

At Tonopah, Nevada the important veins showed prominent and continuous outcrops of white quartz; the first. samples broken from the veins, although rich, showed no ore minerals, and were of unpromising appearance. The quartz has in places a purplish color, due to minute particles of argentite. The ores near the surface carried chloride, bromide and iodide of silver associated with limonite and oxides of manganese. The coxmtry rock is a sericitized andesite.

At Silverton, Colorado, the outcrops of the stock deposits of the Red Mountain area form prominent siliceous knobs composed of silicified andesite carrying finely disseminated pyrite, sericite and kaolin. These knobs are thoroughly fractured and contain vugs, cavities, and ramifying caves. The residual ores were found chiefly in the caves, as beds of sandy or clayey material, and on their walls associated with porous, spongy masses of quartz. The oxidized ores carried argentiferous cerussite and anglcsite associated with siderite, barite, "oxide of iron" and

J. E. Spurr, P. P. 42, U. S. G. S., p. 122.

2 F. L. Ransome, Bull. 182, U. S. G. S., p. 233.

Outcrops 213

)lin. At slight depth argentiferous galena formed the principal i; this gave way in depth to argentiferous enargite, chalcocite, 9K>mite and chalcopyrite, which in turn were underlain by lean .: J)rimary pyritic sulphides.

- - At Cripple Creek, Colorado, the telluride gold ores occur with

'% ery scanty gangue, and the outcrops of the veins are not

conspicuous. "As elements of geological structure, the lode

fissures at Cripple Creek are exceedingly inconspicuous. They

"iire marked neither by bold outcrops of quartz nor by superficial

.ands of ferruginous gossan. They seldom fault perceptibly

the structures that they traverse, and are not sujSiciently different

from the enclosing rocks as regards resistance to erosion, to have

influenced perceptibly the topographic development of the

district. It is this obscurity that retarded the discovery of the

ore-deposits, and that to-day renders it impossible to follow the

veins over the surface without first stripping off the soil and loose

rock, or sinking test pits."

The surface ores here contain dull native gold associated with tellurites in ferruginous clays, kaolin and alunite. Hydrother- mal metamorphism was an inconspicuous process in this district. In the Black Hills, South Dakota, the large Homestake ore-bodies outcropped as iron stained chloritic slates, quartz, and porphyry, which in places carried as high as $16 in gold. These altered gold-bearing rocks were not sufficiently resistant to ero- sion to produce prominent outcrops. In the open cuts it is not possible for the unpractised eye to distinguish the slates that are sufficiently mineralized to constitute ore from those that are practically barren. In general, the rocks within the ore zone appear to be somewhat more completely silicified, and to carry more iron stain than the country rocks, and also to have been subject in greater degree to fracturing and folding. In the sur- face ores the quartz occurs in three phases: as thin silicious layers intercalated in the slates, as thin seams that frequently follow the lamination and bedding, but occasionally cut across

Waldemar Lindgren and S. F. Emmons, P. P. 54, U. S. G. S., p. 153. a J. D. Irving, P. P. 26, U. S. G. S., p. 57.

214 Examination Of Prospects

them, and also as veinlets independent of the structural featurei of the containing rock, which last are the most important. The gold in these deposits occurs in finely disseminated particles, and appears to be of later origin than the quartz and rock gangue.

In THE MoGOLLONEs DISTRICT, New MEXICO, the coimtry rocks are a series of flows, f ragmental beds and tuffs, composed of soda- rhyolite, andesite and basalt. The rocks in the mineralized areas exhibit propylitic alteration; silicification, resulting in a greai- ish-gray hornstone, is prominent along the veins. Sericitic alte ation is absent. The veins are partly filled fissures and partly the result of replacement of the walls along fractured zones. The topography is rugged, and the veins being harder than the altered wall rocks, form bold outcrops. The veins carry native silver, argentite, chalcocite, pyrite, chalcopyrite, and bornite associated with specularite in a gangue of quartz, calcite and fluorite. In certain of the veins in which copper is present in small quantity the ore minerals are finely disseminated through the gangue. In another type of vein copper forms a large proportion of the total value of the ore and sulphides are abundant. Oxidation haa reached a slight depth only in these veins and secondary enrich- ment does not appear to have effected any important rearrange- ment of the values. The water level in this district is deep, and the slight depth of secondary alteration and oxidation must be explained by the geologic youth of the region, erosion having proceeded more rapidly than oxidation. The degree to which the rocks have been shattered and the presence of abundant druse- lined cavities in the veins, together witli the type of rock altera- tion, indicate a slight depth at the time of ore deposition, and that no great depth of rock has been removed by erosion since the veins were formed. The oxidized ores found at the surface contained chiefly malachite, cerargyrite, and gold, associated with limonite.

1 L. C. Graton, P. P. 68, U. S. G. S., p. 195.

Index

Adirondack Mts., deposits of, 87 Aereal geology, 40 Aguilera, J. G., 80 AlaskarTreadwell mine, 7, 47, 77 Alteration, alimitic, 150

contact, 146

decomposition, 153

depth of, 167

dynamo-regional, 145

to greisen, 150

hydrometamorphic, 153

hydrothermal, 146

influence of channels on, 160

kaolinitic, 155

leaching, 154

metasomatic, 82, 84

migration, 158

oxidation, 156

primary, 145, 204

propylitic, 148

secondary, 153

sericitic, 149

silicification, 150

by surface agencies, 153

of wall rocks, 145

weathering, 153 Alunitic alteration, 150 Anticlines, 31

ore-deposits at tops of, 134 Antiguas, examination of, 4 Apatite veins, 83 Apex, 7

Appalachians, deposits of, 71 Aspen, Colo., 126, 141 Associations, of metals, 160

of minerals, 76

Bain, H. F., 193

Bald Mountain, S. D., 59, 106

Ballerat, Australia, 141

Basin, 31

Bassick mine, Colo., 54

Bayley, W. S., 30

Beck, R., 32, 33, 53, 55, 63, 79, 80,

109, 110, 112, 128, 129, 185 Bed vein, 45 Bedded ore-deposits, 110 Bendigo, Australia, 124, 135 Bertha mines, Va., 206 Bibliography, search of, 6 Bingham, Utah, 58, 108, 126, 184,

Bisbee, Ariz., 79, 150, 181, 209 Black HiUs, S. D., 32, 103, 106, 129,

132, 213 Blanket vein, 45 Boehmer, Max, 160 Booms, mining, 3 Boutwell, J. M., 58, 80, 105, 108,

126, 184, 210 Branching veins, 54, 61, 81 Breccia lodes, 52 Broken Hill, N. S. W., 189 Bullfrog, Nev., 198 Bunker Hill mine, Ida., 104 Burro Mts., N. M., 108 Butte, Mont., 118, 140

Carbon, precipitation by, 138 Casts in resistant minerals, 200 Cassiterite veins, 83

Index

Chalcanthite, 9, 15 Chalcooite, enrichments, ISO

disseminated, 182 Chaloopyritic enrichments, 184 Chinmeys, 53 Church, J. A., 133, 134 Clark, F. W., 195 Classification of ore-deposits, 86 Cleavage, 26

Clifton, Ariz., 78, 108, 179, 181 Clinton ore measures, 111 Coeur d'Alenes, Ida., 46, 80, 104,

117, 124, 210 Compensating faults, 35 Comstock lode, Nev., 212 Compound veins, 46, 47 Concentrations, surface, 171 Concretions, oolitic. 111 Conglomerate beds, 109 Conjugate faults, 35

joints, 56

veins, 56, 57 Contact deposits, 87, 88, 157

metamorphism, 146, 152

ores, 79, 124

veins, 47 Contouring, 40 Copper, enrichment of, 180

oxidized ores of, 169

primary ores of, 78, 79

secondary ores of, 176 Copper King mine, Ariz., 181 Cordilleras, deposits of, 71, 72 Cornwall, England, 137, 138 Cost of mining, 22 Cripple Creek, Colo., 48, 49, 59, 60,

77, 119, 129, 213 Crosscutting, 9, 42, 49, 57 Crosscut tunnels, 9 Crustification, 39, 74, 94, 96, 97,

Crystallization, interrupted, 77, 200 Curie, J, H., 5, 24, 141

Darton, N. H., 29 Decomposition of rocksi 153 Deep-seated veins, 83 Deformation, 37 De la Beche, 138 De la Mar mine, Utah, 171 Development, 40 Devil's Tower, Wyo., 29 Diller, J. S., 78 Displacement, 33, 34, 37 Disseminated enrichments, 182

mineralizations, 107 Dolcoath mine, England, 137 Dolomitization, 152 Dome, 31 Drag, 36

Dressing a mine, 10 Ducktown, Tenn., 78, 91, 181 Dunnington, F. P., 195

Efflorescent salts, 15, 159, 203 Emmons, S. F., 35, 80, 171, 189,

Emmons, W. H., 75, 79, 91, 92, 122,

164, 165, 181, 191 Encampment, Wyo., 30, 56 Enrichment, chalcocitic, 180

chalcopyritic, 184

of copper, 180

depths of, 163, 165

disseminated, 182

of gold, 189

of gold and silver with copper,

irregularity of, 162

of lead, 191

of lesser metals, 195

of metals, 158

relation of, to topography, 161

relation of, to water level, 161

of silver, 186

of zinc, 194

Index

Epigenetic deposits, 26 Examinations, of antiguas, 4

formal, 1

preliminary, 1

preparing a property for, 9

of prospects, 3

for rescue of capital, 2 Exploration, 71, 139

amoimt of, compared with results attained, 9

and development, 40

for faulted ore-body, 36

of prospects, 6 Extrusive rocks, 69

Fahlbands, 90 Falun, Sweden, 79 Faribault, E. R., 135 Faults, 33

blocks, 40

compensating, 35

depth of formation, 35, 36

development from folds, 31

expression in topography, 39,

lodes, 51

normal, 34

reverse, 34

step, 35

torsion, 34 Filled fissures, 39, 74, 93, 95, 96, 97,

Filling of open spaces, 93 Finlay, J. R., 22 Fissility, 27

Fissures, gouge filled, 39 Fissuring, time of, 68 Flexures, 32, 33 Folds, 31

Formal examinations, 1 Foster, Le Neve, 137 Fractures, 32 Freiberg Germany, 63

Gale, H. S., 30

Gallup, N. M., 31

Garrey, G. H., 64, 66, 187, 188, 192

Gash veins, 44

Gemmel, R. C., 13

Geological maps, 3, 4, 8, 37, 40

sections, 37, 40 Geology, aereal, 40

structural, 26 Georgetown, Colo., 64, 80, 130, 187,

Gilbert, G. K., 70 Globe, Ariz., 199 Gneissic structure, 27, 67 Gold, enrichment of, 185

distribution of by oxidation, 171

primary ores of, 77, 78 Goldfield, Nev., 43, 151 Gordon, C. H., 90 Gossan, 202 Gouge filled fissures, 39 Granby mine, B. C., 8 Granite mine, Mont., 79, 164, 191 Grant Co., N. M., 133 Grant, U. S., 193 Grass Valley, Cal., 78, 123 Graton, L. C, 90, 105, 109, 214 Guanajuato, Mex., 80 Gympie, Australia, 141

Hand picking, 2, 21 Hanging wall, relative mineraliza- tion of, 85 strength of, 8 Harder, E. C, 207 Harz, Germany, 55 Heave, 33

Helena-Frisco mine, Ida., 117 Hermosa, N. M., 133 Herron, J., 56 High assays, 19

Index

Himmelsfahrt, Germany, 128 Homestake mine, S. D., 213 Horn Silver, Utah, 171 Howell, E. E., 31 Hundt, R., 31 Hydrometamorphism, 153 Hydrothermal metamorphism, 146

alunitic, 150

distinction from weathering, 1 48

to greisen, 150

propylitic, 148

sericitic, 149

silicification, 150

Idaho Springs, Colo., 66

Inclusions, fluid, 74

Indicator seam at Ballerat, 142

Intercalated veins, 67

Intergrowth, structural, 74

Intrusive rocks, relation of minerali- zation to, 69, 70

Irving, J. D., 32, 35, 79, 105, 129, 132, 188, 213

Jerome, Ariz., 208 Joints, 27

compression, 27

conjugate, 56

mineralization of, 51, 56

tension, 28, 31 Jones sampler, 13

Kaolin, precipitation by, 168

Kaolinization, 154, 155

Kemp, J. F., 34, 44, 87, 90, 110,

Keweenaw Point, Mich., 110 Kingston, N. M., 133 Krumb, H., 184

Laccoliths, 70

Lake aty, Colo., 79, 188

Lake alley, N. M., 133

Laney, F. B., 91, 181 Large vs. small properties, 11 Leaching, 154, 159 Leaching (process), 9 Lead, enrichment of, 191

oxidized ores of, 169

primary ores of, 80 Leadville, Colo., 35, 80, 160, 195 Ledges, 44 Lincohi, F. O., 187 Lindgren, W., 44, 49, 71, 75, 77, 78, 79, 82, 86, 88, 89, 90, 92, 108,116,119,123,127,129, 145, 146, 157, 158, 160, 176, 179, 184, 195, 211, 213 Linked veins, 55 Local data, better than general

data, 37, 114, 138 Lodes, 44

breccia, 52

fault, 51

sheeted, 48

stringer, 50 Losses, metallurgical, 21 Low grade ores, 11

McCaffery, R. S., 79

Magmatic segregations, 86

Malcolmson, J. W., 102

Manganese, distribution of, by oxi- dation, 172

Mansfeld, Germany, 112

Marmorization, 152

Meandering of outcrops, 197

Metals, associations of, 123, 160 migration of, 158 vertical sequence of, 123

Metallogenetic epochs and provinces,

Metallurgical losses, 8, 21

Metamorphism, contact, 146, 152 dynamo-regional, 91, 146 YidTcAJaeravl, 146, 148, 150

Index

Metasomatio alteration, see altera- tion, metasomatic.

replacement, 99 Metcalf, Ariz., 108, 177, 209, 211 Miami, Ariz., 184 Michigan, copper deposits of, 110 Microscopic slides, 73, 206 Migration of metals, 158

of gangue minerals, 195

upward, 160 Mill runs, 17

Mine dressed for examination, 10 Minerals, accessory, 76

both primary and secondary, 75

primary, 74, 75

secondary, 76 Mining, cost of, 22 Mining examinations, 1

formal, 1

for rescue of capital, 2

preUminary, 1

of prospects, 3 Mogollones, N. M., 214 Monocline, 31

Monte Cristo, Wash., 51, 191 Montpelier, Ida., 30 Monzonite, association of ores with,

Morenci, Ariz., 89, 108, 174, 177, 184, 209

Nacozari, Mex., 8, 207

Negaunee, Mich., 30

Neihart, Mont., 65, 67, 140, 189

Nieggen, Germany, 31

Nogal, N. M., 109

Normal faults, 34

Nova Scotia, gold deposits of, 135

Offset, 33

Old Jordan mine, Utah, 210 Odlitic concretions. 111 Open spaces, 95

Ordonez, E., 80 Ore deposition, loci of, 69, 70 Ore-deposits, associated with cer- tain rocks, 71 behavior of in depth, 81 deep-seated, 81 distribution in districts, 71 epigenetic, 26

formed at shallow depths, 81 lenticular, 92 pegmatitic, 90

regionally metamorphosed, 91 replacement, 99 syngenetic, 26 Ores, contact, 79, 88 low grade, 11 oxidized, 169, 170 primary, 69

criteria of, 73, 115 decrease in value of, in

depth, 123 depth to which persistent, 72, 81, 124, 139 refractory, 8 residual, 73 secondary, 173

criteria of, 174 minerals of, 176 Ore reserves, calculation of, 19, 24 Ore-shoots, distinction between pri- mary and secondary, 115 primary, 24, 114

at tops of anticlines, 136 deep-seated, 24, 124, 143 due to impounding, 130 due to intersections, 127 due to open spaces, 127 factors determining, 114 influence of wall rocks on,

irregularity of, 114, 118 lenticular, 120, 144

.i*

Index

OreHshoots, primary, shapes of, 118 structure, influence of, on,

terms used to describe, 116 residual, 171 secondary, 24, 173, 176

effect of porosity on, 179 effect of primary mineral- ization on, 179 effect of structure on, 178 effect of wall rocks on, 178 effect of water level on, 178 lenticular, 92 shapes of, 190 Ouray, Colo., 90, 130, 132 Outcrops, 196 casts in, 200 composition of, 201 conditions of indicating enrich- ments, 202 of contact deposits, 205 down hill creep of, 198 of kaolinized rocks, 204 meandering of, 197 porosity of, 200

relation of length to depth, 196 size of compared with size of

vein, 197 of superficial deposits, 200, 205 Overlapping veins, 57 Oxidation, 156

minerals resistant to, 157 irregularity of, 162 I elation to topography and water level, 161

Pachuca, Mex., 80, 195 Paige, Sidney, 96, 175 Park aty, Utah, 80, 210 Pecos, N. M., 90 Pegmatitic deposits, 90, 124 Phillipsburg, Mont., 164 Pilares mine Sonora, 8, 207

Pinos Altos, N. M., 96

Pipes, 53

Porphyry copper deposits, 8, 182,

Porosity, effect of, on secondary

ores, 179 Post-mineral movements, 38, 125,

Preliminary exam in ations, 1 Pre-mineral fissures, 38, 125 Primary alterations, see alterations,

primary, minerals, 74, 75 ore-shoots, see ore-shoots, pri- mary, ores, of copper, 78, 79

depth of, 81

of gold, 77, 78

of lead, 80

of silver, 79, 80

of zinc, 81 Promontorio mine, Durango, Max.,

Propylitization, 83, 148 Prospects, examination of, 3, 196 exploration of, 6, 9, 36, 40, 71,

Purington, C. W., 143 Pyrite, precipitation by, 138, 159

Rand mines, South Africa, 109, 118

Eandsburg, Cal., 7

Eansome, F. L., 28, 34, 38, 46, 50,

60,77,79,80,104,116,119,

124, 129, 150, 170, 181, 186,

198, 209, 210, 212 Ratio of concentration, 9 Red beds, 113, 143 Red Mountain, Colo., 53 Refractory ores, 8 Regional metamorphism, 91, 145 Regularity of ore-deposits. 7, 22,

Si

Index

Heplaoeinent, oonditionB favorable to, 103, 125, 138

deposits, 100

imitating banding, 96

selective, 101

veins, 98 Besampling, 19 Besidual kernels, 165

ores, 168

orenshoots, 171 Resistant minerals, 201 ' Reverse faults, 34 Rickard, T. A., 12, 141 Rico, Colo., 131 Rio Tinto, Spain, 185 Risk of mining, 5 Roots of veins, 81 Ruby mine, Mont., 191

Saddle reefs, 134 Sales, R. H., 118 Salting, 17 Samples, breaking, 12

cutting down, 12

interval between, 16

marking, 13

placing the, 16

shipment of, 18

size of, 17 Sampling, 12

equipment for, 12

preparation for, 14 San Antonio de la Huerta, Mexico,

San Javier, Mex., 143 San Juan Mtns., Colo., 143 San Pedro, N. M., 79, 211 Santa Eulalia, Mex., 107 Schist, alteration of, 89

veins in, 67, 68 Schistose structure, 27, 121, 145 Schwartz, T. E., 53, 186

Secondary alterations, see altera- tions, secondary, ore-shoots, see orenshoots, sec- ondary.

Sections, geological, 37, 40

Segregation of values, indications of, 76

Segregations, magmatic, 86

Selective precipitation, SS 139

Sericitization, 82, 149

Shafts, sampling, 13

Shasta County, Cal., 78, 105

Shear zones, 52

Sheeted zones, 49

Sheeting, 28, 34

Sierra Mojada, Mex., 102

Sierra Nevada, deposits of, 71

Silicificationl50

Silver, distribution of, by oxidation, enrichment of, 185, 186 primary ores of, 79, 80

Silver Lake, Colo., 54

Silver Peak, Nov., 86, 87

Silverton, Colo., 50, 94, 120, 185, 212

Size of ore-deposits, 11

Smith, C. H., 112

Snowstorm mine, Ida.,

Solution, enlargement of cavities by,

Solution breccias, 207, 208

Specific gravity, determination

Spencer, A. C, 30, 47, 56, 77

Spurr, J. E., 33, 80, 87, 126, 130, 141, 187, 188, 191, 192, 212

Step faults, 35

Stocks, 53

Stockworks, 53

Stoping width, 20

Stratification, 26

Striations, 36

Stringer lodes, 50

Index

Structural geology, 26 Structure, gneissic, 27

schistose, 27 Surficial deposits, 205 Syncline, 31 Syngenetic deposits,' 26 SystenA of related veins, 67

Tecoripa, Mex., 143

Telluride, Colo., 118

Terms of sale, 5, 6

Throw, 33

Title, 7

Tomboy mine, Colo., 56

Tombstone, Ariz., 134

Tonopah, Nov., 36, 41, 80, 84, 130,

Topography, expression of faults in, 39, 199

of mineralization in, 198 Trail, 36 Tularosa, N. M., 113

Veins, distinction between inter- calated and fissuie, 67

fissure, 43

follow faults of small displace- ment, 62

formed near the surface, 82

gash, 44

in schist, 68

intercalated, 67

linked, 55

movement along, 63

overlapping, 57

persistence of, in depth, 61

pinching and swelling of, 127

pockety, 73

relation between depth and num- ber of, 62

replacement, 98, 136

structure, influence of country rock on, 63

systems of, 57 Virginia City, Nov., 212

Union ville, Nev., 65

United Verde mine, Ariz., 208

Values, minerals indicating a segre- gation of, 76 Van Hise, C. R., 26, 27, 30, 62, 95 Veins along dikes, 48

bed, 45

blanket, 45

branching, 54, 61, 81

compound, 46, 47

conjugate, 56, 57

contact, 47

deepnseated, 83

Watson, T. L., 206 Weathering, 153

minerals resistant to, 88 Weed, W. H., 67, 78, 79, 93, 107, 136,

140, 156, 189, 190, 191 Witwatersrand, South Africa, 109

Yung, M. B., 79

Zinc, enrichment of, 194

oxidized ores of, 170

primary ores of, 81 Zones developed by surface agen- cies, 163

; .

'.I

'i

Steei Structures

A Practical Work On The Duties Of A Building Superin-

Tendent For Steel-Frame Buildings And The

Proper Methods Of Handling The

Materials And Construction

By

Edgar S. Belden

Vice-Fkbsidsnt, Qeorgb A. Fuller Construction Comfant

Kansas City, Missouri

<t4

Illustrated

American Technical Society

Chicago

Xa6063

A51C. Lev-x AND

TlcD*N

Coptrioht, 1917, Bt

American Technical Society

Coptriohted In Great Bbitain All Rights Reserved

T

Introduction

rCE problems of superintendence of steel frame structures ai-e so different from those which arise in connection with other types of buildings that it has been necessary for men to make a specialty of building superintendence for steel buildings. The knowledge of the best types of design, the proper methods of fabri- cation, the tests which should be connected for quality of steel, and finally the proper methods of erecting the steel, all call for special training apart from the usual building superintendence methods.

It is with the idea of giving engineer and layman the most au- thoritative information on this important subject that this little voliune has been published. It does not attempt to go into the theory of design of steel structures, but confines itself to the prob- lems of superintendence alone. The author is abundantly quali- fied to speak on this subject as he has erected many steel buildings for one of the biggest contracting firms in the country. He has given the reader the benefit of his experience as a superintendent by outlining the duties of this office, and making clear the engineer- ing, legal, and practical knowledge required. Then he goes into detail regarding the inspection of the steel material in the fabrica- tion shops and the proper method of storing it until needed. The problems of erection are all treated — equipment required, f oundar tions, the handling of the steel, riveting, and painting.

The author closes the article with some advice as to the proper organization of his force, how the superintendent should work with architect and owner and what qualities a good superintendent should possess. Altogether the article should prove a valuable addition to the technical Hterature in this field.

Shop inspection 30

Amount of inspection varies with work 30

Drawings in shop 31

Shop processes 31

Reports 38

Inspection and superintendence of erection 38

Kinds of structure 38

Different methods of erecting steel 39

Capacity of equipment 40

Omission of small items 41

Necessary risks 41

Adequate equipment . 41

srricks used in steel erection 42

Classes of derricks 42

Types of derricks 48

Cableways 60

iperintendent's authority over equipment and work 60

Interference with contractor ill-advised 60

Honest and dishonest contractors 61

Overloading structures 61

Hoisting engines 62

ickle 62

Classification 62

Chains 63

Cordage 64

Wire rope 65

igines, power, etc 67

Kinds of power 67

Size of engine 67

Types of engine 69

Hand powers 70

Loads 71

Tackle blocks, shackles, hooks, and wire-rope fastenings 71

nes and levels and how to establish them 71

Importance of correct location 71

Employment of licensed surveyor 72

Superintendent's check onlines and levels 72

Preserving marks 72

Locating foundations 73

Setting foundation shoes 73

Locating grillage beams 75

Grouting shoes 75

>undations 75

Soil 75

Types of foundations 77

Proper location 77

Importance of foundations 78

Concrete and other masonry 78

Foundation steel 79

Pile foundations 79

Caissons 80

Settlement of adjacent structures 80

System in handling eteel 81

Steps in erection process 82

Temporary plank floors 83

Plumbing and alignment 83

Shims 84

Floor beams 84

Small castings 84

Field riveting 85

'ainting 89

Object of painting 89

Concrete as preservative 89

Kind of paint 89

Inspection of paint 90

liscellaneoiis problems 90

Large organizations 91

Field organization 91

Proper size of force 92

Proper use of organization 92

Contact with employer 92

Necessary qualities for superintendent 93

System and speed 94

Proper sequence of work 94

Building Superintendence

Steel Construction

Introduction

Classes of Structures. Steel structures are practically dividec into two classes: first, those that are built as part of buildings; and second, all those used for other purposes, such as bridges, viaducts railroads, etc. Steel structures are now usually designed by engi neers who have specialized in one of the two classes. The details o design and methods employed in the fabrication or in the manufac ture of the parts of steel structures are somewhat different for th( two classes.

Steel for bridges, etc., and in a limited way for buildings, ha been used for a great number of years, but the modern practice o employing steel skeletons has been developed entirely since 1883 when the first structure of this kind was erected in Chicago, namely the Home Insurance Building at the northeast corner of LaSall and Adams streets.

Structural Steel. Manufacturing Processes. Steel, before i reaches the site of the structure ready for erection, goes througl several different processes and stages of manufacture. First, th' iron ore is smelted and made into pig iron; the pig iron is then con verted into steel billets; these in turn are rolled into what are calle( structural shapes, such as plates, bars, angles, tees, channels I-beams, etc., all of which is done at what is usually termed "th< mill". The structural shapes are next taken to the fabrication sho] where they are cut, punched, assembled, riveted, and bolted together and otherwise manufactured according to the specifications, into th different parts of the structure, such as columns, girders, etc., an< made complete so as to be easily erected at the site of the building

Standard Sections. Certain methods of procedure and detail of construction have been standardized for both dass structures. Tie standards used in steel buWdm to

L/ixui/c;xc;u. ixx vixc ixiaiXiLuaiv;i>LU; cbxiu. c;j.c;vi>xuu. v/x ou\;u. vvvrxivy pxxxi-

dly to lower the cost and to facilitate the speed of completion. Ileal departure from standards in the steelwork means delay and ra expense. A rolling mill with a large business in making stand-

stock shapes dislikes to stop its machinery to make special pes, and does so only when paid handsomely for the extra trouble Dived. Fabrication shops have expensive machinery built for idard work and organizations of men trained accordingly. It ts money and time to change the machinery and to educate the 1 to departure from the system of standards.

Good Design. Good designing of steel structures allows a rimum amount of assembling work to be accomplished at the p, leaving a minimum amount to be done at the site of erection, tending always upon the limitations of transportation of materials n the shop to the site and upon those of the machinery available use in the work of erection. Railroads are limited as to the size [ weight of the pieces which they can handle, and it is better and re economical to employ machinery and equipment that can be d for several jobs, than that which is limited to one only. There- J, the separate pieces of steel should come to the site in shapes [ sizes adapted to standard erection equipment.

Divisions of Work. General Dimsions. The duties of the ineer and of the architect divide themselves into what are called 3e work and field work. The office work consists briefly in mak- the design; in preparing the contract, drawings, specifications, I other papers; and in receiving the bids and awarding the con- its for the job.

The field work performed by an engineer or his subordinates ermed the superintendence of the work. It consists of inspec- i; examination; testings; and supervision, primarily to see that work in the mill, the shop, and at the site conforms to the con- 3t requirements. It also includes making reports of progress, , forming estimates of amounts due the contractors from time ;ime, and other duties chiefly of a business nature.

Superirdendent, The engineer or his subordinate who under- es to superintend the work of erecting a steel structure must 'e good health and steady nerves; he must also be a good climber.

at great heights, in order to give the work proper and adequate inspection. He must be a maa of good judgment and he should never forget that he is part of an organization or machine whose object should be the completion of the structure in the shortest time, with the least confusion, and the smallest expenditure of money consistent with the result desired. The superintendent must remember that he is a cog in this machine. If the cog wants to go the wrong way, or if it does not fit into the other cogs, great loss of effort and sometimes great damage may be occasioned. His prm- cipal duty b to see that the work conforms to the contract require- ments, and this must be done in a helpful way. A supermtendent who does not know his business or who has a disagreeable deposi- tion may so hamper the work and hinder the contractor as to delay the completion of the structure, and thus defeat the whole object of the operation. An owner primarily wants his structure erected accordmg to the contract requirements, but he is also more than likely to want it completed as soon as possible. Often the entire success of the owner's plans is dependent upon the work being done in a short time; therefore the superintendent is not working in the interests of the owner if he, for any reason whatsoever, unnecessarily impedes it. He is also a poor manager if he allows inferior work- manship and materials to enter into the structure, or if he permits the contractors, or anyone else connected with the operation, to delay it imreasonably. In fact, he must do his own work properly, promptly, and at the right time, and must see that all others inter- ested in the operation do the same. "All others interested" includes not only the contractors and the men under the superintendent but those over him as well. To be able to accomplish all that is demanded of him, a superintendent must be diplomatic, and, it goes without saying, must know the details of his business. We shall later discuss more at length some of the duties required of a super- intendent.

Designing Engineer. The engineer who specializes in the design of bridges, etc., is usually supreme in authority in his realm of work and, because of the nature of these structures, he demands the greatest care and accuracy both in the maiiuiiet\H& cjv'OoaTj%x\ and in their erection.

design created by the architect, who is usually supreme in authority both as regards all matters of design and as regards the procedure at the building.

In modem practice, the work of the designing engineer and that of the fabricator and of erector of steel structures are entirely sepa- rate and distinct, the one designing, and the others contracting to manufacture and to erect the structure as designed. Sometimes the designer and the erector are employed by the fabricator, but though each of the three more often performs his share of the work separately, yet each is more or less dependent upon the work of the others.

The designer deals principally with the theoretical construction and the economical use of the materials entering into the work, so that the owner who employs him may have a structure to be used for certain purposes at a minimum expenditure of time and money.

The fabricator deals mostly with the economical use of shop machinery, methods, and the employment of workmen in the shop, all to the end of making the cost of the shop work as small as possible.

The erector is concerned with a similar problem at the site of the structure.

The designer, although he must necessarily know a great deal of the methods of shop practice and of those employed by erectors, does not dictate to the fabricator or to the erector altogether as to how the desired results are to be obtained. He does specify, how- ever, certain processes which he wishes employed by the fabricator or erector and he should do so; for example, while he may desire that the rivets be driven by use of power riveters of given capacity, he allows the others to determine what make of riveter shall be used and what kind of power shall be employed — whether steam, air, electric, or other. Again, while conditions may require the designer to specify that the erector shall allow no smoke to be made at the site, he permits the latter to determine just how this result can best be obtained.

The designer should always be supreme [in authority as to the results desired. He should, however, take into consideration the limiting conditions of shop and erection, and the less he dictates to the fabricator and to the erector as to the met\iod?> to

I the designing engineer should not be too general in his specifications I of requirements, because It is right and desirable that bidders for I certain work should bid on essentially the same thing and that there I should be no uncertainty as to the results desired. The specifica- tions of the competent engineer must be a happy medium between those too exacting and those too general.

1 General Superintendence Problems J

I Reconciling Theory And Practice H

Value of Forethought. Theoretically, a knowledge of the con- tract requirements and of the construction details, together with authority to reject all work that does not conform to them, is all that a superintendent needs in the way of equipment to make an expert. Practically, he needs much more.

Forethought Is most Important. It is proper to exercise author- ity to reject work, but it Is far better to use forethought so that the work will need no rejection. It will be found that work once com- pleted is not always as easily corrected as might be supposed. The best of men dislike to take dowTi work already finished, and, when ordered to do so, often make great efforts to avoid doing it, thereby creating much confusion, delay, and dissatisfaction. If rejection of work becomes necessary, It must be done promptly and decisively; the matter must be followed up and the correction forced without delay. Much expensive alteration is often caused by lack of atten- tion to defective work at the right time.

Judgment in Handling Mistakes. The superintendent must not be too lenient regarding mistakes, nor too credulous. On the other hand, he must not be too exacting. He must know what is right and act accordingly, with justice to all. A thorough knowl- edge of the practical side of the shop and field work gives him the assurance to decide correctly a.nd to stand by his decision. Too much purely theoretical and too little practical knowledge often tends to make a superintendent severe and unjust. This has a tendency to work not only against the interests of the owner, his eiala'et but against his own as well.

result in a structure as near theoretical perfection as it can be unda the conditions imposed upon it. This force deals largely with the theory of design, the theory of strains and stresses, etc. The con- tractor has more largely to do with the actualities. The trainiiig of a contractor is different from that of the engineer. The forma is likely to find out earlier in his career what can be done with the forces of nature, and what he may expect to happen if these forces are defied; therefore his knowledge is of a more positive character.

The superintendent soon learns that it is one thing to put down on paper something that it is desirable to execute and quite another thing to accomplish this result in the field. One important reason for this lies in the fact that the human element enters largely into the work after it leaves the engineer's office and that it is with this element that the superintendent must deal, to a great extent. Tie good superintendent must have the ability to manage men, to get them to do the right thing at the right time.

Problem of Handling Men. Inspection involves the intelligent examination of work and materials and a report as to whether or not these conform to specifications or to contract requirements, while superintendence includes not only inspection, acceptance, and rejection of work and materials, but also to some extent supervision of the work and the men. This supervision, however, must not encroach upon or interfere with the supervision which is the duty or the right of the contractor.

Characteristics of the Workmen. A superintendent of structural steel construction comes in contact in the field with the following men: the owner, the architect or the engineer, the contractors, the contractors' superintendents, the foremen, the sub-foremen, and the workmen who are called "structural steel men" or "bridgemen". The structural steel men, or bridgemen, are as a class, strong, plucky, and fearless, and they must necessarily be so. It requires steady nerves and strong muscles to enable a man to climb to dizzy heighits, to lift heavy loads while there, with but little support, and to guide the heavy steel into place. These bridgemen are usually men of strong likes and dislikes. They are clannish and will take great risks and fight death itself to help each other or to help those they

ana reaay ngniers, rougn spuKeu, quicK Lemperea, aiiu used lu danger; but when properly approached they are easy to handle. Most of these men are of a roving disposition, going from town to town and following the work. They are usually of a type who know their business thoroughly, or think they do, and dislike extremely to be ordered about. Consequently the contractor, his superin- tendent, and formen, must have tact, nerve, and physical strength, in order to control and guide these men successfully. It is part of the duty of the engineer or superintendent to study and understand the characteristics of all the men with whom he has to deal and to set them a good example.

Personal Relations. As in most practical business life, it will be found that success in the field work of the construction business is to a large extent a question of personal relationship. In order that the work may be carried on with the least amount of lost effort, the different persons connected with it must maintain their proper relationship one to the other, and they must "get along" together. The superintendent is in a position to do much toward preserving harmony and he should seize every opportunity to do so. He may quietly and diplomatically say a word now and then to ease or alter the feelings of the men toward one another.

While the superintendent must maintain a certain dignity, he must, nevertheless, be tactful enough not to consider it beneath him to be friendly with the men. He must realize that his job is an important one which cannot be slighted, but if he assumes the attitude that his specific knowledge is something sacred, only attain- able by the favored few, he is likely to antagonize those with whom he works, who have not had his advantages. Such a mistaken position will surely cause him to lose much of his influence over the men. The latter usually know fairly well most of the things concerning the work which the superintendent knows; they have had these things taught them by hard knocks and generally form shrewd opinions as to the accuracy of the superintendent's knowl- edge. These practical men in the field are very likely to take advan- tage of any weakness of the superintendent and em'Vo' .a own gain. Conceit is a dangerous weakness in a. su'x\.XK.'cAew

u iuu;n:;9ta.

A tactful superintendent insists in a friendly but dignified upon the substantial fulfillment of the terms of the contract they actually are and not as someone thinks they are. He also nonstrates that his presence is a factor that aids materially in jping harmony, speed, and continual "push" in the work.

A good superintendent is not petty, but looks at things in a )ad-minded way, with an appreciation of what is essential and at is nonessential. It is difficult to put down on paper the rules follow or the methods to use in handling men. A man must m these things largely by experience. One can get along best ii some men by treating them kindly, with others by using strict cipline. Some men one must praise; others one may ridicule and iter. Some men need encouragement, while others are too codc- e. Each man is a separate problem and the more important the n, the more important is it that the problem should be solved xectly.

Progress Charts. The superintendent also soon learns that the st carefully formed plan of procedure, mapped out at the begin- ig of the work in the field, can seldom be followed very closely, the reason that unforeseen conditions are continually arising at J site. A flood may carry away the falsework of part of the dge; certain cars of material may be lost on the way to the site; ; owner, when he sees the building actually begun, may change his id, and often does, as to certain pre-arranged requirements, setting completely the carefully prepared theoretical progress charts

Generally, it is the unexpected that happens. The capable Ider successfully solves the problems from day to day as they 3e, and is quick to act and to take advantage of opportunities the sake of getting the entire job completed on or before a stated le. A good superintendent is a help to the builder in pointing : opportunities that may appear.

Shifting Character of Contractor's Organization. By the term ganization" we mean the combination of men, machinery, and lipment brought together to build a certain structure, and this ;gests a difficult problem in construction work. It usually takes ae time to create a good organization and, irv work of this char-

pleted, after which there isnolonger aneedf or thatparticmar orgamza tion. Men have different capabilities and characteristics; some wor] well under one foreman while they cannot get along with another; i man operates one machine better than he does another, and so on Practically, the contractor cannot afford to keep a complet organization standing idle, and very seldom do the jobs com along in such sequence as to enable him to transfer his organizatioi in its entirety from one job to another. He therefore loses hi men; they obtain work with other contractors; and the next tim they are wanted, they cannot be obtained. The superintenden must understand this element of practical work for the reason thai although the contractor may be capable and desirous of doing th right thing, he often has unknown men working for him, who things contrary to his wishes and instructions. The superintenden must be able to detect this condition and have it corrected. H may find, if he is alert, many opportunities to save the contractor as well as the owner, much annoyance and expense caused by th non-efficient, careless, and thoughtless workman. While the con tractor is bound to correct the mistakes made by his employes, h wili be grateful for timely information which will save him th expense of correction, and the prompt detection of errors further the advancement of the job, which is, of course, to the interest o the owner.

Handling Business Details

Value of Business Methods with Business Men. It is essentia that the superintendent should have knowledge of the busines methods of the community in which he has to work. Scientifi- training generally is limited to a study of the laws of nature and thei residts. Business training teaches one how to deal with men an< money and how to understand the laws relating thereto. Th owner and the contractor are usually thorough business men, use< to business methods, and they cannot work in harmony with thos not similarly trained. Engineers and superintendents, to convinc the business man of the importance of recommendations and decis ions, must talk to him in terms which he can xeadJ xwAtwzcw that is, they must use the language oi buamesia.

jtrict observance of this rule will save much time and annoyance.

Daily Records. The superintendent should keep a daily ord of the work under his supervision. This record is sometimes led the 'log". It should state the condition of the weather each y; the approximate number of men employed on the dififerent inches of the work and, briefly, what they are doing; the time en the different kinds and parts of the work were conunenced i are to be completed; all unusual occurrences coming to the super- endent's notice, such as accidents, mishaps, delays, together th their causes; the visits to the work of prominent people con- :ted with the operation. It must be accurate, complete, and arly stated, so that anyone taking it up in the future, after these ngs have been forgotten, may have an adequate idea of what lly happened at the time the log was made. A good attitude d tid when writing up the log is to assume that it will be needed in awsuit at some future time, the decision for which may restXipon 5 record contained therein. This log, however, must not ISe a lanufactured" one; that is, it must contain a statement of facts they really happened, not as someone might wish they had ppened.

Contractor's Payments. The engineer and the superintendent ist know in a general way the values of the different branches of 5 work coming under their supervision; it is by means of their tificates that the contractor is paid by the owner, and these pay- outs must be just, neither too large nor too small. The engineer d the superintendent must keep an accurate account of all pay- that have been made and of those that are due to the differ- t parties concerned.

Superintendent's Rulings* In making rulings or in rejecting, rk, the superintendent should take the matter up with the proper son in authority. It is not enough to give orders to the work- n; in fact, important ones should never be given directly to them, e dealings should be with the foreman, the contractor's super- endent, or with the contractor personally, depending upon the portance of the matter. It should be an iron-clad rule that all rk which has been refused, or is liable to Teect\oxv, o\A5L\\caaftdv

man next highest in authority who can be reached without delay.

Purchases. The engineer should know enough of the laws o business to be able to purchase materials and other things cheaply and without being imposed upon.

Legal Points Encountered

Importance of Legal Knowledge* The engineer and the super intendent must recognize the existence of a rigid framework of lega principles upon and around which all the affairs of the business world are carried on. To refuse to acknowledge this or to act in i manner contrary to these principles is to invite disaster. Th< engineer finds that there are forces at work in the business work which he is compelled to meet, conquer, and use; and that they aD almost as irresistible as the forces of nature with which he deal; when designing the structure. It is easy to imagine what woul( happen to his work if the engineer, in designing it, should attemp to defy the law of gravity; it is not so easy to see what would happei to it if the laws of business were defied; yet the result is sometime just as disastrous.

All business is at bottom principally a matter of contracts therefore it is essential that the engineer and the superintenden should know something of the law relating to contracts, and als< something of the law of agency.

In olden times, personal right was a question of might. A civilized life became more complex, the principal method of enforc ing right was changed from might to the recognition of a system o rules regarding personal and property rights. These rules are noi; known as "the law". As the construction business becomes mor complex, it is found that careful compliance with the law become more and more important.

Rudiments of Law. There are certain underlying principle recognized by all authorities that may be termed the rudiments o the law. Other principles are not so clearly stated or xmderstoo< and therefore authorities differ regarding them. We emphasize th< necessity and the importance of the engmeer and the superintenden having a clear understanding of the rudiments ol XJoa Vkw's* community in which they work. TYie \iaxvdY\xv% ol cssoss

if uut uic ULiurt; uic parucs tu oii a2ix;ciiicub uiiuciisuuiu cuiu iuuuw

5 rudiments of law, the less will they need the services of the omey and incur the consequent delay and expense.

Field of Private Law. The engineer and the sui)erintendent Qe in contact largely with that branch of the law known as private or the law of contracts and the law of torts. Contracts are cements of any nature. Torts are private wrongs not covered

contracts. Injury inflicted upon the property or body of one

son by another, which injury is not a breach of contract, is a tort.

rts and crimes often overlap. Contract rights are obtained only

agreement. A tort, in distinction, has to do with one's nat-

il rights, it is the violation of such rights, independent of contract

An engineer or a sui)erintendent will probably learn early his career, sometimes to his discomfitiu'e, that all agreements, that certain provisions of a contract, even though they are a i; of a written and signed document, may have no force in the irts, if either party should choose not to abide by the terms of J so-called agreement or contract. The reason for this is that !h terms are not in accordance with the law of the conomiunity i will not be enforced by the courts.

Parties may enter into any kind of agreement they choose, the provisions and conditions are legal. There are, however, at is known in law as "impossible contracts".

Contracts. A contract has been defined as "an agreement :ween two or more competent parties, enforcible in a court of jy and based upon a sufficient consideration to do or not to do )articular thing." The essentials of a contract are briefly: firsiy rties competent at law to make an agreement; second, something agree upon; and third, a sufficient consideration for the bargain.

Other definitions of a contract have been stated as follows:

"A transaction in which each party comes under an obligation the other and each reciprocally acquires a right to what is prom- d by the other."

"A convention by which one or more persons obligate them- ves to one or more other persons, to give or to do, or not to do nething."

Consideration. One of the importaiA tSom xcvxYtQi m

can be shown to the satisfaction of the court that the consideratio named is not a proper one, the contract is not valid. It must I understood that consideration means compensation. There i of course, the money consideration which is the most common on but considerations at law are not limited to money. The theoi of the law is that both sides of the contract shall, in the opinic of the contracting parties, be equivalent or equal in value.

Competency of Person Making Contract. Another thing require in a contract is that the person or persons making the agreemei be competent at law, or be legally qualified to make it. In th connection the engineer most often takes care to see that persoi binding or attempting to bind a corporation are authorized to c so. Generally, it will be found that only certain of the high< officials of a corporation, such as a president or a vice-presiden has any proper or original authority to sign contracts for the co poration. The corporation does, however, have agents in varyir capacities who can with authority bind it in a limited way. Anotb thing that must be looked out for, is to see that the signatiu attached to an agreement or other document, such as a receij payment, is complete. The name of the principal, that is, th corporation, person, partnership, or organization, for which tt person may be signing, must be written first. Under this shoul be written the name of the person signing, and beneath this h title or office, such as agent, cashier, president, or whatever it ma be. If a man should sign simply his own name followed by h title or office, he would not bind his principal but only himse personally. Again, the mere name of the corporation, partnershi] or organization is not sufficient; it should be followed by the sign ture of an authorized officer or agent.

Relations of Partnerships and Corporations to Contract. Pan nerships may be defined as the combining of two or more persor by agreement in an enterprise for common profit. The law pei ' taining to partnerships is different from that pertaining to coi porations. A partnership may be created by mutual consent c the partners for the transaction of any kind of business which a individual has the right to transact, the onVy

Auviiviviuat paxi/Xicx xo xxauic xui av;u3 \ji i/xxc vtxici' paruicrs. .

I may bind the other by contract. However, if the partnership sulopted a firm name, a contract made in the name of one of ndividual members does not bind the partnership. It cannot 3und by any name other than its own.

A. corporation must have i)ermission from the government y business. It cannot be formed for every purpose. An indi- Eil or a partnership can engage in certain lines of business which ienied to a corporation. A partnership no longer exists when of its members dies or when a change in membership is made, 5 a corporation is not affected by the death of some of its mem- nor by any change of members, but has a continuous existence ig the term for which it is created? As stated before, each ler is the recognized and authorized agent of the partnership, 3 in a corporation only those appointed in a manner prescribed aw and by the rules of the organization, can act as agents. Subject Matter of Contracts. The subject matter of a contract : be a lawful one; any agreement to do an unlawful uiing would 3 the contract null and void.

To know whether or not the subject matter or the statements le contract are legal — in other words, whether or not the terms le contract can be enforced in the courts — often requires a derable knowledge of legal relationships and need not be dis- ;d here except to mention briefly some of the more important which are known to be illegal.

A contract cannot contain agreements which violate some or federal statute, or which are contrary to the rules of what lown as common law, or which are forbidden by public policy, ites and the rules of common law are well defined, but the :ines of public policy are somewhat elastic. It has been said: enever any contract conflicts with the morals of the times contravenes any of the established interests of society, it is as against public policy". A United States Court has said, wed from the standpoint of morals, square dealing, and com- ial integrity," such and such a thing cannot be approved. I few instances of general practice m tYie coutl ow p the general trend of the courts' inteTpxetatAoxi ol

litigation, to restrain freedom of trade, to give excessive or highly arbitrary powers to an architect or an engineer, to bargain away the contractor's legal rights, or to tend to wrest from the courts their proper jm-isdiction. An agreement that deprives the parties of their legal right to have their disputes and grievances heard by a properly conducted tribunal, such as a court of law, is held to be illegal, but an agreement which has arbitration clauses in it is held to be legal; in the latter case, if either party does not like the decision of the arbitrators, there are usually ways of taking the matter into the courts. Clauses that stipulate that the engineer or the architect shall have the sole power to fix the price of work that may be added, omitted, or altered from the contract work are generally held valid at law, and his decision will, in the absence of fraud or collusion, be enforced by the courts. The final word in case of dispute over the compensation can be given only by the courts. In some States it is held that clauses are void at law which provide that the architect or the engineer shall be the sole judge in deciding all matters in the contract, or that he shall be an arbi- trator between owner and contractor.

Mutual Understanding. The agreement must be a mutual understanding between the parties. Usually it is held at law that, when a party signs his name to a written agreement, he admits the understanding of all the clauses, thereby making it a mutual one. It is essential, however, that there be a meeting of the minds of the contracting parties, for if there is a mutual mistake on their part, their minds do not meet and there is no contract. A mistake on the part of one of the parties only generally does not make the contract void.

Performance PreverUed, If either party to a contract does or does not do something, and such act or failure to act prevents the other party from performing his agreements, the latter is excused from the performance of them.

Offers to Pay. An imconditional offer to pay in legal money, so as to stop interest and costs, is equivaleat m ?3\sisec- The law also declares what constitutes \ega\. Ta.oxv% checks, silver certificates, and bank notes axe

VAAV/ tJfX VAV/O

p-eement. If one party notifies the other party that he will

to cany out the contract, the other party may stop the mance of his part of it, but he is entitled to and supposed able to collect the cost to him of work perf onned, and in some iie profit that would be his if the entire contract had been com- . Profit, however, is not often a tangible thing, and its ice is often hard to prove in the courts. If one party refuses form his part of the contract, the other party cannot compel do so. The second party is, however, entitled to damages

are usually actual and rarely punitive, and can collect them, ne party to a contract cannot claim damages for breach of ,ct if the other party refuses or neglects to do some unim- it thing, unless the thing is clearly stated in the contract

reasonable, because the law does not recognize trivial things, ognizes substantial performance as actual. ailure to Complete Contract on Time. In all construction ,cts the time of completion should be agreed upon and clearly . It is usual also to include in the contract, at the time of g it, what the amount of the damages shall be if the work ; completed at the time mentioned. These damages must med in the contract as "liquidated damages'' and must be easonable amount in order to have the courts enforce their :ion. gency* Few business deals are completed or can be com-

solely and personally by the parties to a contract; therefore rincipals to a contract must have representatives or agents p them. A corporation has no identity of a personal nature P necessity performs all its acts through its officers and other i. There are recognized rules and laws in all communities

govern the relationship existing between principals and epresentatives, and which are known as the 'laws of agency". appointment of Agents. Any person, corporation, or party, as the legal right to enter into a contract, can appoint agents ; instead. Almost any one, except a very young child or a

who may have interests opposed to t\ios ol \)afe

and the other responds with a nod of his head, this is held to be enougl to make the first person an agent of the second to perform the par ticular thing mentioned. If a person does something unauthorizec by another and the latter ratifies the act, then the first persoi becomes the agent of the second, at least in respect to that act,

Agenfs Avihority and Responsibility. If one man knows tha a second is actmg for him and permits the second to do things a his agent, the first one is liable for the acts of the agent just as i he had performed the acts himself. When it comes to the knowledg of a person that another is assuming to act as his agent, such persoi must elect without delay either to repudiate the acts of the allege( agent or else accept the responsibility for them.

A mere assertion on the part of a person that he has the author ity necessary to act as, the agent for another does not make hin the agent, because only the principal's consent can make him so

An agent cannot do for his principal anything which the prin cipal cannot lawfully do for himself. While it is true that a thin person dealing with an agent is required to ascertain at his peri whether or not the agent has actual authority to act for his principal the third i)erson has the right to assume that the agent has tb authority to perform the duties which are customarily done persons acting in the same or similar capacity, unless the principa should expressly call the attention of the third party to the contrary For example, the duties of a superintendent are ordinarily under stood to give him the power to do certain things for his employe and unless the employer openly states to the contrary (so as no to deceive the third person), then the third person has the righ to assume that the particular superintendent with whom he i dealing has all the authority which it is customary for all superin tendents to have.

When a third person, before dealing with the agent, ascertain;

that the agent has received authority of a certain charaet

the principa], then the third party may Te\y oxv. \)afe

apparent powers conferred and need not be

pected or secret limitations upon sucli powera.

QS are of such a nature as to deceive a third party.

An agent can be held personally responsible at law for all the mgful acts which he may commit; it is no excuse that was ing as the agent for someone else.

An agent cannot, ordinarily, without the expressed consent his principal, transfer to another his authority to act. However, ere mere mechanical or clerical work is to be done, the agent I employ others to help him.

Liability Law, It is well that a superintendent know something the laws governing liability for injury received on the work, has been said that a principal, or master, is obliged to furnish agent or servant with a reasonably safe place in which to work, i with reasonably safe tools and instruments. If the principal s to do these things and the servant is injured through no neg- mce or carelessness on his part, what is known at law as a tort happened and the principal is liable to the servant in damages any injuries. The agent or servant must, however, assimie the cs which naturally belong to the work in which he is engaged. I workman carelessly steps off a scaffold he cannot collect damages m the contractor, but if the scaffold should be constructed in :h a manner as to be unsafe and, falling, should injure the workman, in the latter can collect damages.

In most localities there are laws, such as factory laws, workmen's npensation laws, and the like, which govern what a contractor ill or shall not do toward insiwing the safety of his men and of J public. The superintendent should make it his business to dy these laws and to see that they are substantially obeyed.

Building Laws* Most communities also have regulations called Iding laws which govern the erection of all kinds of structures, viously the superintendent should be familiar with these laws. He )uld obtain copies from local authorities and study them carefully.

Lien Laws. These laws vary in the different States. They )vide that in case a workman employed on the job, or anyone nishing materials used in it, is not paid for his work or for the provided this is done in the prescribed V7ay axvd VcVm

proved to be correct, then the owner of the property will be com- pelled to pay the workman or material man the amoimt of the lien, notwithstanding the fact that he has already paid the contractor for this same work or materials.

It is important, therefore, that the superintendent study the lien laws of the State in which he works, and satisfy himseU before issuing certificates for payment that the owner is protected against liens of all sorts.

Application of the Law. Complex Questions. The foregoing statements are only a few of the rudiments of the law. In applying them to the work, the superintendent should use common sense always referring the more complicated points of law to the attorneys The good sense of the engineer or superintendent will often be shown by referring a really doubtful question of law to an attorney instead of attempting to pass upon it himself. A certain amouni of legal knowledge is necessary that the superintendent may the proper relationship of things and the rights of all parties con- cerned; also that he may know what the terms of a contract anc specifications really mean, in other words, that he may interprei them correctly. The use of common sense in these matters usually does more for the job than does the technical enforcement of laws Substantial justice to all parties should be the object sought.

What the Law Expects of Superintendents. The law require! that an engineer, architect, or superintendent, who agrees to direc the work, shall be on the job a sufficient part of the time to enabb him to give it prompt and adequate inspection. This means tha he must, of course, know defective work when he sees it and tha he will discover and examine it before it is hidden. WTien defective work is discovered or in any way called to his attention, the super intendent must without delay take action to effect its correction If he is to reject any work he must do so promptly, for he has m right under any circmnstances to conceal his discovery and thei reject the work after much time has elapsed. Sometimes, however the contractor or his men intentionally conceal defeetw an attempt to deceive. The superintendeiit tqw&\* X-t \ja5 such a contractor or workman see that it la xvot toVv owx sjXi to do that kind of work.

kftsman and the superintendent must have a technical knowledge the work. The draftsman's work is largely accomplished hy owing how to make a good drawing; that is, he records his ideas 1 those of others in a concise, clear and intelligent manner hy .king a series of lines and letters on paper* The sui)erintendenty ivever, performs his task, for the most part, by handling the men. 3 conceptions and those of others are recorded in lasting sted similar materials and not merely on paper. The draftsman sonally makes the lines on the paper. The superintendent must npel others to make the record. It cannot be emphasized too ongly that it is just as important for the superintendent to know w to deal with men as it is for the draftsman to know how to draw.

Knowledge of Drawings Important. The more a superintendent ows about drawings and how they are made, the better. It is luty for him to interpret the meanings of the different drawings, deifications, and other contract papers. In fact, he should know tter than anyone on the job what the contract provisions are d this before the work is executed in the structure. If anyone s actually had a hand in making anything, the better will he be le to understand the process of its manufacture. It, therefore, is edless to say that the more experience the superintendent has d in the drafting-room, the better grounded he is in the abbrevia- ►ns and conventional signs that are standard practice in the making drawings, and, hence the more practical is his knowledge of the intent of the drawings.

One who has perfected himself in the reading of drawings ds in the rush of work and in the noise and confusion, that this Ips him to avoid mistakes, and gives him more time to devote watching the progress of the work.

While it is possible for one who has never had experience in e actual making of drawings to perform successfully the duties a superintendent, he is able to perform his duties better and more sily if he has had such experience.

Study of Drawings. It should be tVie duty ol \!aa axtojl- Jentj when be £rst gets the contract draVm axvd peSiGKsyDa,

mentSy and those that are peculiar to this particular piece of work He must not, however, depend entirely upon this first study; froc time to time he must refresh his memory, especially of particula parts of the structure as they are about to be erected, so tiiat h may have a correct and positive knowledge of the work as it i done. To illustrate: We are assigned to a twenty-story oflSo building. Supplied with the drawings and specifications, we begii at once to study and ponder over them, and to ask our superior about certain points that may not be quite clear, until we have ai excellent idea as to what the contract includes. The actual wori on the structure begins. While the foundation work is going oe it is not necessary for us to give the roof drawings any special atten tion; but a short time before the roof is reached, we should devot some time to studying again the drawings for this part of the worl

A man can imdoubtedly do more work and better work b; using some system and not by trying to retain too much in hi mind at one time. The less the brain is occupied with the non essential things, the more it can occupy itself with the essential.

Accuracy of Drawings, It is presumed, when the drawing and specifications are turned over to the job, that they are complet and accurate, but the superintendent should not take this too mud for granted. He should be on the lookout for errors and omission and correct them before they affect the progress of the work.

Supplying Workmen with Drawings. The superintendent mus see that the different workmen are supplied with a sufficient numbe of copies of the drawings. A squad of men putting steel togethe on the twentieth story are very likely to go wrong in the work i the erection drawings are kept in the office on the first floor. Th drawings must be located so that the workmen actually doing th work can refer to them continually and easily.

Handling Drawings. The particular drawing needed by th men may be tacked to a light drawing board which can easily b taken from place to place. This method preserves the drawings saves the foreman time in handling it, allows tJba eJaaJska work iaoaa high structure.

;uiuea urawings.

General Drawings. The general drawings are called by different imes, such as assembly, framing, or setting diagrams. Each awing is supposed to show as much of the structure as it can early, with the different parts assembled in the same relation ' one another that they will bear in the structure. These drawings e usually on a comparatively small scale and constitute the ans, elevations, and sections of the building. They are in e nature of diagrams, which usually do not attempt to show e details of the connections of the different members, but ther to indicate the proper relationship of the members. The embers are usually shown by a single line designated by some stinctive mark. '

Marking System Necessary. It is very important that each ece that goes into the structure should be indicated in some simple anner on these general drawings by a mark that is different from ose on all the other pieces.

There are several good systems of marking. An example of le sometimes used is as follows: Each piece of steel that belongs L a certain floor has a mark, the first character of which is a figure rresponding to the number of that particular floor; after this ;ure is a letter designating whether the piece is a beam, a girder, separator, etc.; then follow other figures in numerical order given some systematic way up and down or across the drawing; each ece that goes into a structure is given, at the shop where it is bricated, the same mark as the one shown on these general draw- gs or setting diagrams. By this systeiA, when a piece of steel received at the building, marked "2B30", the erection gang lows immediately that this piece is beam 30 for the second floor; marked "5G 18", it is understood that it is girder 18 for the fifth or, and so on. The last numbers of the marks having been pAaced L the drawing in some regular order, the squad boss, or person terested, can turn almost directly to the place on the drawing lich shows where the piece is to go.

It Is understood that the fabricator usviaW sewL

of the same size and of exactly the same detail located on ail ttie floors from the basement to the roof, one can readily see that it would be cheaper to run them all through together, and to ship them at once so as to save the space in the shop.

Unless the erector has used some forethought and provided in the contract for the sequence in which the steel is to come to the job, he is likely to find that the fabricator cares very little how much sorting is required at the site. It is for this reason and for others of equal importance that some good and intelligent system of marking should be devised and used on the work; otherwise there is bound to be a great loss of time, which is equivalent to a loss of money.

Bridgemen are highly paid, and one can readily see the loss incurred, if, while workmen stand around idle, the boss of the gang has to make long search through a number of large-sized drawings, in order to tell where each little piece belongs.

Shop Detail Drawings. Shop details are drawings of each individual piece on a scale large enough to show without confusion all the information required to fabricate and erect this particular piece. These details give the men in the shop the exact knowledge needed to lay out) cut, pimch, assemble, and rivet the piece shown, as well as to give it the proper erection mark. It is just as important that the men erecting the structure shall have a sufficient number of copies of the shop details on the erection work as it is for them to have the general drawings.

Conflict in Requirements. Sometimes there is a conflict between the requirements as stated on the drawings and specifica- tions, and the formal contract. The contract is usually considered the most important paper because it is usually more carefully drawn than the others; the specifications are next in order, inasmuch as the person who writes them has more authority tlian the drafts- man; the drawings are of least weight. It is, therefore, customary to rule that the specifications have precedence over tlie, dxsSsi'g. It is welJ, however, to know that the courts 4.o -aoX. -ss; this practice, because tliey make the real iuteixl ol "Owt '4.%ais3c6. matter how it is shown, the primary consideialAoti..

Classes of Inspections. The three of inspection, so lied, that are given to the materials and workmanship of a sted ructure are the "mill", the "shop", and the "erection*\

It is not customary for the same man to make all of the inspec- )ns, one reason being that each inspection requires a somewhat fiferent sort of knowledge. Another reason is that by having le man inspect a number of jobs at the mill, a second man inspect number at the shop, and a third man inspect the erection, a great ving of time and money is effected, particularly in the matter i . transportation.

There are now large corporations, as well as smaller concerns, lo make inspection of all kinds their sole business. These con- rns are able to do this at a reasonable price because they place en in the different large mills and shops and keep them there [ the time. These men become very familiar with the workings the particular institution to which they are assigned; consequently ey know where to station themselves to do the most efficient )rk. They can watch a number of jobs just as easily as they n watch one.

Mill Inspection

Knowledge Necessary for Mill Inspector. The mill inspector ould have a knowledge of metallurgy, particularly in its applica- )n to iron; he should also have a knowledge of chemistry and lysics, inasmuch as his work deals largely with the composition id strength of materials. The materials entering into the work mmonly termed steel structures, are cast iron, wrought iron, and 3el. Each material has several grades. All differ from one lother, chiefly in the amount of carbon they contain, but also in e quantities of other substances generally considered impurities, ch as phosphorus, sulphur, manganese, and silicon. Wrought )n comes the nearest to being pure iron; it sometimes contains ) carbon, and seldom over one-fourth of one per cent. Cast )n contains the most carbon, sometimes as m\ic\v as five per cent;

and cast iron.

and white iron by rapid cooling. Gray iron should be used where strength is needed. It is soft and tough, melts at a lower heat than the white; remains fluid a long time; can be planed, turned, and drilled; is red when molten; and makes good castings. The fracture is granular, of a gray color, and has a metallic luster. White iron should be used where hardness is required. It is hard and brittle; is not easily melted; thickens rapidly; cannot be worked; and is white when fluid. The fracture is white in color, crystalline-, with a vitreous luster.

Defects in Castings. The principal defects in castings are blowholes; honeycomb caused by confined-air cavities; flaws caused by the collection of foundry dirt and other impurities, and by unequal contraction while cooling; imeven thickness caused by the dis- placement of the cores; and cold-shuts, or weak seams, caused by the chilling of the iron where the molten metal is poured from different ends of the casting. The mold so chills the iron that it does not properly mix and unite when it comes together in the mold. Castings should remain in the mold until cold; the slower the cooling the better, for irregular and too rapid cooling seriously injures castings, particularly where different thicknesses of metal occur, by causing strains that often result in rupture under a small load.

Inspection of Castings. The inspector can test castings roughly by the use of a hammer. Honeycomb, blowholes, sand holes, etc., cause a dullness in the sound when the casting is struck. Gray and white cast iron can often be distinguished by the blow of a hammer on the edge of the casting. The one is soft enough to be slightly indented, while the other, being hard and brittle, chips off. In order that the flaws, shrinkage cracks, and blowholes may be detected, all castings should be inspected carefully when they first come out of the molding sand — after they have been thoroughly cleaned with steel brushes or in some other way but before they are painted and "doctored up".

ifjiould be drilled into the different sides so t\iat ol Jcv'OttvssxR

n should show even and close grained when broken; the color 3iild be a light bluish gray; and the fracture should have con- ierable metallic luster. Both texture and color should be uniform, the fractiu should show crystalline patches, or should be mottled her with dark or light iron, the casting may be imsafe; blowholes U make it still more imsafe.

Tests of Castings. In important work, where it is essential at the castings should have a given and uniform strength, tests 3 made. Test bars are poured before and after the castings are ide, at least one for each two thousand poimds of castings, or

such manner as the specifications demand. These bars are ually made 1 inch by 3 inches by either 14 or 26 inches. They 2 placed narrow side up on supports either 12 or 24 inches apart, d are loaded in the center until they break, record being kept

the deflection and breaking weight. The bars that are to be jted for tensile strength are turned down by a machine to a given curate diameter and then pulled apart in a testing machine.

Wrought Iron. Wrought iron, when perfect, is simply pure without carbon or impurities of any sort. It has a property lich neither cast iron nor steel has — it can be welded in the old- jhioned way. That is, if two pieces are firmly pressed or hammered gether when nearly at white heat, they adhere and make one 3ce. Cast iron and steel can now be welded by submitting them

certain recently discovered processes, such as the oxy-acetylene ocess.

Qiudities, Good iron is ductile, tough, and fibrous, free from ws, blisters, cinder pockets, buckles, and cracks along the edges.

is readily heated, soft under the hammer, and throws out few arks. When broken gradually, it shows long silky fibers of iden-gray color which twist and stick together before breaking, hen broken rapidly, it has a crystalline appearance in the fractiu'e. Wrought iron which is brittle when cold or which cracks when nt double, contains phosphorus. This is called "cold-short" )n and is indicated, when broken, either by a coarse grain with colored spots, or by a fine grain of steely appearance. Wrought )n which cracks when bent at a red heat Wt eowldewhte

does not crack when bent 180 degrees around a bar with a diameter twice the thickness of the piece, or, when heated to a working tem- perature, it is bent sharply to a right angle. When cut slightly on one side and then bent, the fracture is nearly all fibrous.

Wrought iron which is not rolled or hammered after it has been brought to a white heat is injured. When the hot iron is suddenly cooled in water, it hardens and, if the load is gradually applied, the breaking strength increases, but the iron is more likely to snap suddenly without warning. When the metal is allowed to cool gradually, it softens and the breaking strength is reduced.

Rivets are usually made of soft, tough wrought iron, or of good soft steel, neither of which should crack when bent cold until the sides come together.

Tests of Wrought Iron. In the mill inspection of wrought iron, all tests are made after the rolling process, for, on account of the way this material is manufactured, accurate tests cannot be made before.

Tests for tensile strength, ductility, and elasticity are made by placing test bars, cut from the full-sized bar, in the testing machines, and recording the diuerent weights observed. These test pieces are usually made 1 inch wide, by the thickness of the piece from which it is taken, by about 18 inches long.

Steel. Steel can usually be determined or distinguished from wrought or cast iron by the property of temper which it possesses, although the soft steels do not take a temper. When steel is suddenly cooled after being heated to a high temperature, it hardens and the degree of hardness or softness can be accurately regulated by the degree of temperature to which the steel has been heated. This is called giving it a temper.

Another method of determining steel is by the nitric-acid test. A drop of the acid produces on steel a dark gray stain.

Varieties of Steel. Steel is made by many processes, either by adding carbon to wrought iron or by removing a portion of the carbon in pig iron. Those most conunonly employed in making steel for structural purposes are the Bessemer and oe\Nr\>kaasJa.

process, the lining is made of calcined dolomite containing lime and magnesia, or some refractory substance which contains prac- tically no silica.

Besides the varieties of steel given there are those known as puddled, blister, shear, and natural steel.

The three different grades of steel commonly spoken of are mild or soft, medium, and hard. Steels with less than 15 per cent carbon are soft; with from 15 per cent to 30 per cent carbon are medium; with more than 30 per cent carbon are hard.

For boiler plates and rivets, or where high ductility is desired, soft steels are used. For general structural purposes the mild steels are also used, while for axles, shafts, tools, and good wearing surfaces, the hard steels are preferred. When greater hardness and tenacity is desired, steel alloys are made by adding to the molten steel, small quantities of some metal such as nickel, man- ganese, chromium, or tungsten. These steel alloys are not com- monly used in structural work.

When the metal is taken from the Bessemer converter, or from the open-hearth, it is run into ladles and poured into molds of uniform and specific sizes, usually holding more than 15,000 pounds of metal, termed ingot molds. As the steel comes from the molds in the form of ingots, it is rolled into the different shapes used in the trade, such as plates, bars, angles, channels, and I-beams.

Inspection of Steel. The mill inspector's work at a steel mill is largely confined to the examination and testing of the ingots.

The following defects are likely to be encountered in the inspection of ingots: segregation, which means the gathering together by themselves during the cooling of the ingot, of certain constituents, such as carbon, phosphorus, sulphur, and sometimes manganese and silica; cracks, both internal and external, caused by too rapid cooling; blowholes, caused by gas escaping during the cooling; pipes or cavities of conical shape, usually in the top of the ingot and caused by the outside cooling more rapidly than the inside. The ingot should be a solid mass of metal of regular shape when it reaches the rolls, in order that seams, laminations, laps that do 77ot weld, cracks, pits, and other defects may iiotocc\a\5xt!£L&ca<5A

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from the mold so as to leave a cavity — bled ingots — should I rejected.

After a man has had considerable experience, he can judj somewhat of the quality and grade of steel by the appearance the fracture; as the fracture, however, may be affected by the mann< in which it is made, this test is usually found to be uncertain.

The quality and grade of the ingot steel is determined t testing samples of each heat or blow, obtained by running a sma quantity of the molten metal into molds usually about 4 incht square, and afterward rolling them down to f inch round; also t taking drillings directly from one of the ingots. These samples -ai usually tested for chemical analysis, for elastic limit, and f( ultimate strength.

Marking and Recording. It is essential that each ingot teste should be clearly marked in some permanent way, either by stamp or by painting, so that the bar itself and the shapes it subsequent! assumes can be readily and accurately identified at any time.

A complete record of each furnace, or converter full of melte steel should be kept, stating the character of the raw materia that went into the melt, the size and niunber of ingots producec the number rejected, and the reason for their rejection.

Inspection of Rolling, The inspector of the rolled steel shoul watch for any defects which prevent the rolled shape from beir a solid, uniform mass of steel, without cracks, seams, lamina tions, pits, cavities of any sort, or any other fault that will injui its strength and durability. The shapes should also be inspecte for the proper size and dunensions, and for the straightness an trueness of the different pieces.

Tests of Rolled Steel. The inspector should select certai places in the rolled steel where test pieces shall be cut, properl mark them, and keep a record of the places. The testing of th samples is done usually in accordance with the engineer's spec fications, which often direct minutely how the tests shall be mad< The more common tests are the tensile, bending (both hot axwicokVi

(where the material is placed in dilute sulphuric or nitric acid for a certain period, and, upon removal, its appearance noted). In making these tests, all materials not conforming to the specified requirements should be rejected.

Steel Specifications. It is needless to say that the engineer, in preparing his specifications, should be very careful with regard to the qualities and characteristics of the materials he demands and the tests they should stand. Usually, consultation with prac- tical manufacturers of steel is necessary, in order to get desired results. The specifications should be consistent. Uidess the engineer is an expert steel-maker, he should not attempt to specify both the exact chemical formula and the physical requirements, for by so doing he may demand that which cannot be obtained in one piece of steel. He should, however, fix certain limits, liberal rather than rigid, beyond which the hurtful elements should not go.

Necessity of Mill Inspection. Except in the more important work, where the engineer in his design has utilized his materiab up to a high limit, and a uniform given strength is of the utmost importance, mill inspection may not be required. It will be found that commercial grades of materials of known uniformity can now be purchased from several different reputable makers and furnished in prompt deliveries, thus making it unnecessary to delay the work by insisting on the mill inspection. It is enough to bind the man- ufacturer to stand back of the materials he furnishes.

Shop Inspection

Amount of Inspection Varies with Work. As in other things, it is well for the work and for the inspector if the fabrication is given into the hands of shops which are properly equipped with machinery, men, and management, and capable of performing the class of work desired.

The degree of accuracy of the shop work and the rigidity of inspection should be governed by the character and the nature of the structure into which the finished work is to go. For bridges, especially heavy bridges of long spans, and other structures where the engineer of necessity has strained the matetiala to the limit,

in the design, or where the failure of one piece will not necess jeopardize the entire structure, less exactness can be reqi It is needless to say that the more exacting the requirements greater is the cost of the shop work.

The specified requirements and the inspection should be tical and not too theoretical; they should be made commens with the amount of money that the owner wants to and oug spend for work of the character ordinarily furnished for si structures.

Drawings in Shop. One of the first duties of the shop insp is to see that detail drawings which have the approval and nature of the designing engineer are being followed. Somel the shops make their own details and it is important that ' have the signature of approval. The inspector should be sup with a complete set of these drawings together with a copy o: specifications and a bill of materials. He should have also a pr place where he can keep these papers for his own use. li specifications call for mill inspection, the shop inspector st be supplied promptly with a copy of the mill inspection n and he should see that all materials coming to the shop to be on his work have been inspected and passed by the mill inspe

Shop Processes. The various processes through which materials are passed in the shop will now be considered some in detail.

First Straightening of Materials. The steel and wrought plates, bars, and other structural shapes coming to the shop 1 the rolling mill are carefully straightened and made true to si by passing the materials through straightening rolls and c machmes of different types. It is a matter of great momen see that the work is properly done, because upon this depends right distribution of the stresses for the different portions of completed built-up members. Generally, the shop officials re nize the importance of having the materials straight and 1 because they know that work performed in this operation is e than offset by the saving in labor and time in the assembliuj the different pieces in the built-up meioibei.

to the templates. Cuts are marked with a sharp hard mstrument, and holes to be pwiched or drilled are indicated by center pmiches.

Punching. The next process is the pondiing. It is important that the machines, the pimches, and the dies be of proper size and design, and that the edges be sharp and mibroken; otherwise, cracked and ragged holes will occur which should not be tcerated imd any circumstances. The diameter of the die should never be more than A inch larger than the diameter of the punch.

Second Straightening. After the pimching is completed, the materials should again be straightened and trued up, because the process of punching causes more or less buckling. If not straight- ened, the several pieces to be riveted cannot be properly brought together and fitted, and often there will be enough spring between them to distort the rivets, so that many will be found loose when cooled.

Assembling. The next process is an important one. It is the assembling of the various separate pieces to form the built-up members, the pieces being held together temporarily by means (rf holding or assembling bolts. The inspector must insist there shall be a suflScient number of bolts to hold the work properly while it is being riveted; oftentimes the lack of a few assembling bolts causes a serious defect in the built-up member. The inspector should also watch to see that the several pieces are put together properly; that right-hand members are not put where left-hand ones belong; that certain pieces are not turned end for end; and that other mistakes of the kind do not occur. All surfaces riveted together which cannot be painted after riveting should be painted before they are assembled.

Reaming. When the holes are to be reamed, they should be punched somewhat smaller than the size of the finished hole. Punch- ing tends to distress the metal on the edge of the hole, and thereby impairs the strength of the member to a greater or less degree. The extent of this distressed material varies with the thickness of the metal punched and with the size of the punch used. Reaming is the removal of this distressed material by the use of a sharp

splices and at important connections, but it is almost always specific for all holes in bridge and railroad work. If desired, it should I definitely specified. Reaming can be done in the separate piec< before assembling; it is better, however, to do it after assemblini for then the holes in the different pieces are sure to correspon exactly and are "fair" for riveting, a most desirable thing. 1 fact, where reaming is not particularly mentioned, and where pimchc holes are supposed to come together accurately but do not do s the inspector should insist that a reamer be used to correct the erro When this is done, a larger-sized rivet than the one specified wi be required properly to fill the enlarged hole. The finished ho should be about inch larger than the diameter of the rivet specifier Reaming, of course, improves the work, but adds some to its cos

Drifting. The shop men generally make a good deal of uj of the driftpin, which is a cigar-shaped hardened-steel pin, used 1 make the holes in the different pieces come together in the assen bling process, and to make the holes "fair" or true for the rive This latter process should never be permitted except in the cheapei kind of work or in unimportant places. Drifting distorts the materi and in good work is never allowed after the assembled pieces ai bolted up and some of the rivets have been driven. In fact, it sometimes dangerous to allow it, because of the unequal straii it produces in the different pieces of the member.

Many first-class shops use the reamer without being told, ( they find that the extra cost in so doing is offset by the saving i the cost of riveting. With perfectly true holes, the riveting goi much faster than otherwise. All burrs should be removed fro] the edge of holes before the rivetmg is done.

Riveting. After the reaming comes the riveting. In additio to seeing that the rivets are made of good material, there are tvi things which are essential in good riveting — the rivets should I properly heated, and they should be properly driven. The heatir must be especially watched, because after the rivet has been drive there is no way of telling whether or not it has been burned. Tl head may look as it should, while the shank may be muck and weakened. Tie heating forge shoxAd \ie 'a.c covsmsgnkc

they are not heated to the red color, for then they do not always fill the hole and the rivet metal is injured. Steel rivets require more watching than wrought-iron rivets, to see that overheating and imderheating do not occur. They should be driven just as rapidly as can be, after they have reached the proper heat If too many are placed in the forge at one time, they are likely to become overheated and spoiled.

Power riveting machines are used almost exclusively nowadaj for driving the heated rivets. The pressure required to fill the holes properly is from 50 to 150 tons to the square inch of rivet section.

Loose Rivets. It is important that the right length of rivet be selected for each hole. The rivet must completely fill the hole, that is, the heads must be concentric, and fit absolutely tight aD around. The riveting machine should make no marks in the metal of the heads, and the heads, when finished, should also be without cracks.

All loose rivets or those loosely driven should be rejected abso- lutely, and fresh ones driven into the holes. The defective ones can be detected by hitting a sharp blow on each side of the head with an inspector's hammer. This is a special tool, weighing one pound or less, with a handle quite small at the shank, to absorb at this point some of the spring of the hammer. Practice soon enables the inspector to tell the loose rivets by the action of the hammer. Sometimes they are made to appear tight by the use, after they are cold, of a calking iron, but the marks of the iron can generally be detected. The more modern way of concealing loose rivets is by squeezing the cold heads with a smaller die, or by striking the cold heads on the sides with the riveting machine. These last two methods are not so easily detected after the men are through with the work. Re-driving cold rivets by any method, and calking rivet heads, should not be permitted.

Rejecting Rivets. When the inspector rejects a rivet, he should mark it by striking it a blow with the stamping end of his hammer or by some punch. He should also mark the metal close to the rivet in the same way so that the new rivet can easily be

riveted together shall be securely and snugly held by a suffi< number of temporary bolts before the riveting is started.

As in other things, an inspector can go to extremes in the tei of rivets and thereby do more harm often than real good. In best shops, where high-powered machines and modem rivet-he£ furnaces are used, there will be comparatively few rivets di that really need to be rejected. The inspector soon learns w to draw the line. The importance of the structure should go somewhat the exactness required. In large bridges and rail work, the riveting should be nearer to perfection than in buil work, although a good job should always be required on coin going into a building or any other structure. While poor Vi manship should never be accepted, there are degrees of good y; manship. In a bridge the engineer depends almost entirely i the different members to resist the loads, and he must of necei strain his materials to a high limit. A failure of one connec in a bridge is likely to cause the collapse of the entire struci but in a building such a failure merely causes a local injury does not essentially damage the structure as a whole. It wil readily recognized that in a building some members are of i importance than others; particularly is this the case with the colu] because if one colmnn near the foundation were to give way, g harm would be done; the collapse of a floor beam, however, is likely to result in any great damage. In a building other mate are used in the construction which help the steel but which designing engineer usually ignores in determining the size of steel members, mainly because he cannot know definitely the pn way in which these other materials will be placed. Howeve is very seldom that the steel in a building is left uncovered; it be encased in concrete, brick, fireproofing, etc., all of which actually stiffen the structure and act as partial supports to horizontal members.

Another side of this question of rejection is the moral e that will sometimes be produced by a judicial action witk to a tew rivets. An inspector should never \mi9Let tos:? cknxsosS

finds that the shop men are inclined to take advantage of his leniency. Again, the shop crew, while not actually doing bad work, may become somewhat lax in their efforts and need a spur to keep them up to the standard. No set rules can be given as to what is the right thing to do in these cases; each problem must be solved when it presents itself by the liberal use of common sense, good judgment, and knowledge of human nature.

Planing and Miliing, After each member has been riveted, it is taken to the planers and drill presses where the ends are made square and true, where all bevels are cut and the piece made the proper finished length, and where the pin and other holes are drilled. The inspector should see that all cuts and holes are located in the right places and in accordance with the drawings.

In making measurements, nothing but a good quality of sted tape should ever be used and this should be tested from time to time with a standard measure and correction made if necessary.

When the piece leaves the planer and drill press, it should be thoroughly inspected to see that all the requirements of the speci- fications and drawings have been fully carried out.

Painting. It is necessary for the inspector to pass on the work before it is painted, because the paint so covers the work that it is diflBcult to detect many of the errors.

The inspection of the painting should be carefully made. Paint- ing is regarded as a preservative of the steel, but to be effective it must be properly done. The inspector should satisfy himself that the paint used is the brand and quality specified. Adultera- tions and substitutions, while not often attempted by the good shops, are easy to make, and when made are for the purpose of lowering the cost, not of improving the paint. Whenever there is a suspicion of imfair dealing, chemical analysis of the paint will aid in the detection.

Before the paint is applied, the surface of the steel must be carefully brushed with steel brushes to remove all loose rust and, especially, all loose scale. The surface must also be dry, as water and dampness prevent the paint from adhering, and thus make it possible for rust to start. It is best to have the painting done

Daily Record. The inspector should keep a carefully prepare< daily record of the work while it is going through the shop, par ticularly if there is a time limit to the contract. Such a record i usually kept on printed forms with headings so as to simplify th< process as much as possible. A good example of a form used i one that has vertical rulings with headings at the top, reading fron left to right as follows: Number of Drawing; Name of Piece Date; Punched; Assembled; Reamed; Riveted; Milled; Painted Shipped; Remarks.

These records should be made in triplicate, or as many copie supplied as may be required, so that the designing engineer and al those entitled to them may have copies without delay.

Size of Drawings. It is customary to make the shop detail; on sheets not much larger than specification paper so that th< inspector can easily carry the details around with him througl the shop. However, if the drawings are large and bulky, th< inspector finds his work simplified by entering in a notebook th< principal dimensions and other information regarding the differen pieces. This notebook should be with him on his inspection tour and should often be referred to.

Loading. From time to time, the inspector should see hov the finished steel is being loaded on the cars for shipment. The men will be tempted to throw it in the easiest way possible so a: to fill the car quickly. Often the steel is so loaded that the movemeni and jar of the car seriously injures the metal, causing a delay ir the work while it is being repaired. The inspector should insisi that the loading be done so that the steel will not be twisted oi bent while in transit.

System. As in other things, the more system that the inspectoi can put into his work, the more and better work he can do. Th< records suggested above will furnish a means for keeping his superior infonned of the progress of the work, and they will also relieve hi; mind of numerous details and, consequently, of a certain amoim of worry and uneasiness.

One thing that an inspector often, overlooks is getting the worl out of the shop in a sequence that conforms soixi'V \Jsia

tu puaii uuc wuiA. Liiiuuju tiic siiup tills uiebttcr ui sciuciii:!; is a

vital one. The records which the inspector makes greatly aid him to see that no piece is overlooked entirely. Many times the whole building is held up, awaiting the receipt of some small member which has been delayed in the shop or in transit? Having on the site ninety per cent of all the steel of a building going above the first floor does not aid the erector very much if he is short a few of the basement columns. This state of affairs often creates confusion on account of the lack of room to store the material at the site.

Reports. The man who is to superintend the erection of the structure should be supplied promptly with copies of the shop inspector's reports; and if the specifications require mill inspection, with copies of the mill inspector's reports. These should show that all material coming to the site has been accepted by the shop and mill inspectors.

Inspection And Superintendence Of Erection

The remainder of this article will deal with the inspection and superintendence of the erection of the structures after the steel and other materials have been delivered at the site of the building.

Kinds of Structure. Some of the different kinds of steel struc- tures that a superintendent encounters in his work are the following:

(1) Steel skeleton buildings, where the entire load of the build- ing, both of walls and of interior, is carried on the steelwork or frame.

(2) Wall-bearing steel buildings, where the interior loads of the building are carried on steel coliunns, girders, and beams, but the walls are self-supporting and carry the outer ends of the girders and beams.

(3) Bridges of every type and description, divided into two general classes — highway and railway.

(4) Viaduxits, either for highways or for railroads.

(5) Subways under railroads or forming tunnels under city streets for cars or for traflBc of any sort.

(6) Elevated railroads in cities, of colunm and girder con- struction, built for fast-moving trains above the traffic on the streets.

Different Methods of Erecting Steel. Various methods used in the erection of steel structures include the diverse ways of applying power, such as steam, electricity, air, or gasoline, through cables, ropes, etc., which are supported on some kind of a derrick, crane, traveler, or similar appliance, the power being increased by the use of pulleys, sheaves, and blocks. The determination of the proper 'method to be used in erecting a steel structure resolves itself into a separate problem for each individual case. It is here that the skill of the contractor and his erecting engineer is brought into play.

If the steel structure extends upward, usually a derrick or a system of derricks is adopted, of a style and size that can be easily and quickly elevated as the structure progresses. This method applies to tall buildings and towers.

If the structure extends horizontally, like an elevated railroad, a bridge, or a train shed, then some kind of a traveler or system of derricks that can be moved with facility in a horizontal direction is usually adopted.

Before the proper method can be determined, the contractor must ascertain accurately the different conditions that enter into the problem. First the weight and size of the largest and heaviest individual pieces to be erected must be known; then the required speed of erection must be determined, and this will decide the number of derricks or other machines to be used. It can readily be seen that such machines can make only so many moves a day, and that if the pieces are too heavy to be lifted by hand, no matter how many men the contractor may put on the job, the limit of the erection speed of the structure is the limit of the speed capacity of the machine adopted. It is important that a sufficient number of derricks be arranged for, and the superintendent should satisfy himself that the contractor has made no mistake in this respect. Each derrick requires only a certain number of men to operate it properly and economically; therefore the proper number of men employed on a given piece of work is determined by the number of derricks or machines used on that work. The reach or length of boom must also be decided upon. The longer the boom of a given cross-section, the less load the derrick can pick up. It is advisable tcio'jMiSL

Derrick Layout. If the structure to be erected is a building, a convenient way of arriving at a good derrick layout is to make a plan to scale of the derrick which the contractor would like to use and cut this from cardboard. Place this on the general setting plan of the typical floor, and move it around until the best location is determined upon. In doing this, it must be kept in mind that proper supports for the mast must be provided for, and also proper places of sufficient strength to which the guys or ends of stiff legs can be anchored. If these supports and anchor places can be found in the structure already erected, then a minimum of such work as shoring is required, and an expense in the moving of the derricks saved.

Bridge Traveler. Often when the structure to be erected is an important bridge, the engineer who designs it also designs the traveler or whatever method is to be employed in the erection. By doing this, he makes sure that the contractor will not put undue and unnecessary strain upon the structure during the construction.

Locomotive Crane. It is usual to employ a locomotive crane or derrick car whenever the work is of such nature that it can be reached from a railroad track already in place or which can easily be put in place. A locomotive crane, however, is an expensive piece of machinery and only the large contractors can afford to use one on small jobs. Of course a job may be important enough to make the purchase of a crane desirable.

Capacity of Equipment. It is a very important duty of the superintendent to see that the derricks, travelers, and other machines which the contractor proposes to use in the erection of the work are of a capacity adequate to the work. To determine this, the superintendent must know something of the theory of strains and stresses in materials, and of the theory of design of structures. If he does not have this knowledge, or is in any way doubtful about the strength of the machinery for erection, he should refer the matter promptly to his superior or to some engineer who is capable of advising him correctly. Structures have been known to collapse during erection because of the contractor's carelessness or the contractor's or superintendent's lack of knowledge regarding these matters.

nificant or trivial to the men, but really important if the structur is to be made safe for holding the equipment.

It is an old and trite saying that a structm is no stronge than its weakest point, but this is a very important fact for ever; superintendent and every other person connected with the wor to remember. Many a collapse has been caused by the omissioi of small things — a pin, a few bolts or rivets — or by the neglec of a worn-out cable or rope.

To repeat, then, the superintendent must watch the importan little things ceaselessly, particularly in a job that is being rushed where the men are striving to make as big a showing as possible The superintendent ought to keep in mind that not only the safet; of the structure itself but the preservation of the lives of the mer and of his own life, are dependent upon such watchfulness an knowledge.

Another thing that it is well for those in charge of construt tion work to appreciate is that while it may not take much effoi or extra material to keep a thing from starting to move, it take an enormous amount of effort to stop it after it has once atarte — so great an effort that it is generally impossible to prevent th collapse of the structure or its paHs.

Necessary Risks. It is necessary to take some chances i: the building business, but one need not be foolhardy or reckless As in other things, the superintendent must be so thoroughly familia with this aspect of the work, that he can tell whether or not th contractor and his men are takmg legitimate risks. A superir tendent who is too inexperienced or too fearful can often seriousl, delay the progress of the work by his unnecessary objections an lack of confidence, while another can by his ignorance and can lessness allow the work to be carried on in a positively dangerou manner. However, it is better to err on the safe side, and be to careful, rather than too careless.

Adequate Equipment. Every good contractor knows, to often because of bitter experience, that there is no economy i trying to do work with machinery, tools, or equipment of any soi that is too light or in any way inadequate for the work to be pel

has to deal, they invariably make allowances for this weakness, at a greater expense in wages paid than a good derrick would have cost K the contractor b trying to get along with a twenty-horse- power engine when the work requires one of twenty-five horsepower, he will lose money; he will make it, however, by installing an engine of thirty or thirty-five horsepower. On the other hand, if the machinery and equipment is larger, more elaborate, or more complicated than is necessary, the erection cost will be unjustifiably large. The capable contractor uses machinery and equipment of size and capacity slightly larger than is required for the work. His rig also is comparatively simple and, as a general rule, he lets others do the experimenting with new rigs and devices.

Good Contracting Engineering. This question of having the rig of proper size and capacity is, of course, nothing more or less than that of good engineering, which means good business. It doeaTnot require a very clever man to design an equipment that is either too weak or too strong for the work, but it does require one of skill to have it just strong enough and not too complicated or elaborate. It takes ability to employ men and materials of any sort economically, to know the proper amount of work each should do, and to see that the work is done.

As with machinery, so with workmen; the capable contractor has the right number of men at work. There is no more economy in trying to do a piece of work with too few men than in endeavoring to do it with too many.

Here is another place where it will be seen that the work of the designing engineer differs from that of the contractor and the erecting engineer. The designer concerns lumself largely . with the economical use of materials, while the contractor is concerned with the economical use of men in handling the materials.

Derricks Used in Steel Erection

Classes of Derricks. The derrick is the steel setter's best friend in the way of machinery, and some form of it is found on every job. It is therefore essential that the superintendent know how derricks are designed and the advantages of the different types.

durable and is growing in popularity.

Timber derricks are those which have the prindpal parts b of wooden timbers connected by fittings maUe of cast iron, mallei

Fi(. I . Typw ot Struotuial Stei Used fi

StiS-Lf in All-Steel Deninks

iron, or foldings. These are the cheapest to build. Some c tractors think they are better than steel derricks because t have more elasticity, withstand knocks better, and ace U:!

where it comes in contact with the fittings; such defects as dry rot may affect the strength of the derrick. Good irons and fittings for timber derricks can be purchased from a number of reliable manufacturers in the United States. All such material should be of a size and strength sufficient to withstand the violent strains which are often imposed upon the machine. The irons and fittings should always be somewhat stronger than the timbers on which they are used, and so designed as not to put unnecessary stresses upon the timber. The method of fastening the fittings to the timbers is a most important part of the design of the derrick.

It has been learned that any column carries more if the load is balanced on top of it. A fitting fastened to a timber so that it tends to bend that timber when the derrick is carrying a load is not well designed. The best irons are those which tend to strain the timbers concentrically and not eccentrically. The fittings should also be fastened so as not to split the ends of the timbers.

Steel derricks are those that are built entirely of steel. In Fig. 1, A is upper end of mast, B the middle interchangeable section, C the lower end of mast, D the upper end of stiff leg, and E the lower end of stiff leg. Steel derricks are now used more than formerly. They can be of stronger construction than the timber ones, and more easily designed so that the strains may be applied concentrically to the different members. Sted derricks do not withstand as hard side shocks as the wood, because the wood springs, while the steel becomes deformed so as to impair its strength. Steel derricks are comparatively expensive to build, but if properly kept up and painted as often as necessary, their life is indefinite; another advantage they have over the wood is that the different members, such as the boom and the mast, can be made in interchangeable sections, which enables the user to lengthen or shorten these members at will with but little trouble.

Capacity of Derricks. The capacity of a derrick depends upon a number of things. The quality of the timbers or of the steel must be considered; only tough, clear, straight-grained sticks, or a good quality of tough mild steel, should ever be used. The size and length of the timbers, or the amount of steel and its dis-

feet long, does not cany nearly the load that a 16- by 16-mch boom, 30 feet long, carries. Again, the spread of the metal in the cross-section of a steel member affects its capacity. For instance, if we have two booms, each 80 feet long and built up of four angles of the same size and shape laced together, one having the angles spaced back to back 15 inches and the other 30 inches, the latter will be found to carry much more than the former.

Other things influencing the capacity of the derrick are the ratio of the length of the boom to the mast, the ratio of the spread of the guys to the length of the mast, the length of the sills to the length of the mast in a stiff-leg derrick, and so on. The shorter the mast is for a given length of boom, the smaller the capacity; the nearer the guys of a guy derrick are anchored to the foot of the mast, the smaller the capacity; and the shorter the sills are for a given length of mast in a stiff-leg derrick, the smaller the capa- city.

The capacity also depends upon the strength of the irons and fittings, and in the manner in which the loads are applied to the timbers through these; the way in which the fittings are fastened to the tim- bers; and the number of bolts and rivets used.

The capacity of a boom, mast, etc., can be increased by means of a 4-rod truss. Fig. 2. Boom with 4-Rod

Fie. 2. Courtesy of Clyde Iron Wc

Siuipit; way ui turivuig at lux: appruAimatc ouiaiAio ixunjacu. upuu

a guy and upon a stiff-leg derrick is given below. It must be kept in mind, however, in applying the results thus obtained,

that the capacity of the derrick will be mod- ified by some of the items mentioned above. (See Figs. 3, 4, 5, and 6, which are diagrams of a derrick showing various positions of the boom.)

Let M length of mast ; B length of boom ; L length of topping lift ; Cr= length of guy or stiff leg; length of sill; vertical distance from top of mast to a line drawn at right angles to mast through end of boom; if horizontal distance along this line from mast to end of boom; W=\osLd in pounds; and restrain in pounds on tie- down or anchor.

Then (1) the compression strain in pounds on the mast

Lwm

Fig. 3. Strain Diagram for Guy and Stiff-Leg Derrick — Boom Horisontal

to which

Vw

Fig. 4. Strain Diagram for Guy and Stiff- Leg Derrick — Boom Angle with Horizontal Less than 45 Degrees

must be added if F is below the top of the mast, or subtracted if F is above the top of the mast. If V above the mast becomes long enough, the strain on the mast will become tension instead of compression. (2) The compression

strain in pounds on boom LW

Bw

M

tension strain in pounds on top-

Lwo

ping lift=-r7-; tension strain in pounds on guy or stiff leg

Ms'

lU 13

Fig. /). Strain Diagram for Guy and Derrick — Boom Angle 45 Degre

Strain m pounds on the tie-down or anchor=

(strain in pounds on guy or stiff leg) X Af

Note that if this strain is directed in any direction other than at right angles to sill, the amount of strain will vary with the angle of direction. The student should apply the above form- ulas to a mmiber of derricks of different proportions and with the boom in various positions — from the hori- zontal almost to the vertical — and note how the strain different members change as the proportions and the pos the boom change. If the student does his work rightly, observe the following facts:

(1) When the boom is in a horizontal position, directly opposite to a stiff leg or guy and in line with it, all the members of the derrick in line with the boom are sub- jected to the greatest strains that can be put upon them by the ordinary method of loading, with the exception of the compression strain on the boom, which does not change for any of the vary- ing positions the boom may be in. However, a horizontal not so strong as a vertical one because its own weight tends it at the middle. Therefore it will be seen that the limii capacity of a guy or stiff-leg derrick is the load that can s; lifted by it when the boom is horizontal.

Fig. 6. Strain Diagram for Guy and Sti Derrick — Boom Angle with Horizoni Greater than 45 Degrees

boom is made loir or the mast shorter, these strains increase.

(3) The length of the sill does not affect the strain on the sill itself; as it is made longer, however, less strain is put upon the 3ti£f leg, its tie-down or anchor, and upon the mast, but this stram does not affect those on the other mem- bers. As the sill is made shorter, a greater strain is put upon the stiff leg, its tie-down or anchor, and upon the mast, but this does not affect those on the other members.

(4) The greater the angle made by a guy and the mast, the less is the strain on the mast and on the guy, if the weight of the guy itself is Ignored. This angle of the guy to the mast does not affect the strains in the boom and topping lift but does affect the pull on the anchors. Types of Derricks. The principal types ot derricks used in the erection of steel structures are pole derricks, or gin poles; builders' or house derricks, some- times called setters' and breast derricks; guy; stiff-legv full-circle stiff-leg; com- bined stiff-leg and guy; A frame; crane derricks; and tower derricks.

Pole Derrick. A pole derrick. Fig. 7, more often called a gin pole by the bridgemen, is merely a pole or single stick, guyed at the top to keep it from tipping over, with some kind of a block or a sheave fastened at the top, and a crab or a snatch block fastened at the bottom, the purpose of the snatch block being to lead the hoisting rope to the hoisting engine, or whatever power is used. This Idnd of a derrick Is light and can be erected by hand, but its range of action in one place is lunited to practically one lift, and if moved, that must lie done by hand. It has been found that the cost of

a day, make3 this derrick an expensive tool for setting steel generally. It is, however, almost always used in one form or an- other in the setting-up of the larger derricks, as it can be put in place by the men without the use of other power.

Builders' or House Derrick. A builders' or house derrick, J'ig. 8, sometimes called a set* ters' derrick and also breast der- rick is an improvement upon the gin pole. It needs to be guyed in two directions only, front and back, the derrick being stiil enough to brace itself sideways; the gin pole should have at least four guys. The house derrick is somewhat easier to move than the gin pole, as it can be rolled sideways on the two small wheels fastened to the sill. Lite the gin pole, it is generally light and can be set up by hand, but it has the same limitations as the other in that It must be moved practically each time a new lift is made. The capacity of this kind of derrick is comparatively small; it is found mostly on the smaller jobs, though it is useful in a limited way on almost every job. It is also em- ployed in settmguptbe larger der- ricks. Stone setters, however, make more use of it than steel men.

from four to eight in number, made of ropes or steel cables; it is pivoted top and bottom, which allowa it to revolve. The boom is fastened to the mast by means of a pin, which enables it to take any position between the horizontal and the vertical. It revolves with the mast. It will therefore be seen that a lift can be made at any point within a circle which has for its radius the length of the boom, without changing the location of the derrick. The boom should never be allowed to drop beyond the horizontal, as this

tends to lift the mast out of its seat, and so to cripple the derrick. Many a serious accident has resulted from this cause. The guy derrick is a common style used by steel men. It will cover a com- plete circle, and it is easy to raise from one story to another, without the use of a gin pole or a house derrick. To do this, the boom is unshipped from the mast after it has been brought to a vertical position; it is then lifted by means of the mast to the new level, guyed like a gin pole, and used to lift up the mast. A guy derrick must always have a mast at least ten feet longer than the boom, in order that the boom shall go under the guys. The great draw- back to the use of this style of derrick on a building is usually the

always found that the extra time consumed in topping up ai lowering the boom, in order to get under the guys, more than offse its advantage of easy elevation.

Bull Wheels. Bull wheels. Fig. 10, are used on guy derricl and on other styles also, in order that the derrick may be slui by the same engine that operates the hobting ropes. A bull whe

Staff-Leg Derrick

is seldom used on a building job, however, because it interfer somewhat with the raising of the derrick from one floor to anothf

Stiff-Leg Derrick. The stifF-leg derrick, Fig. 11, is probab the most common one used today as it has several advantag over the guy derrick, although it has its disadvantages as we This type is like the guy derrick, except that in place of the guys it h two slanting members called stiff legs, placed at right angles one another to keep the mast in an upright position. The st legs are subjected to both tensile and compressive strain, dependii on the portion of the boom.

One advantage of this derrick is that only two anchoring plac are needed. Another is that the boom can. be nwaVi. Vsw 'Ca

three-fourths of a circle while the boom is loaded. The boom without load can be slung in behind the stiff legs by lifting one stiff leg while the boom passes by, the other being guyed tem- porarily. This is often done where only one derrick is used but two or more derricks are generally needed, in which case they are placed so as to help each other, especially in moving up from one story to another. It is generally found, also, that on account

"f

SiU

V,

Fig. 12. Plan of Layout for Two Sti£f-Leg Derricks

of the longer boom which may be used, as much area can be covered as by a guy derrick, if not more, and there are no guys to interfere with the movement of the boom.

Where the job is large enough for two stiflP-leg derricks, a good arrangement is that shown in Fig. 12. It will be noticed that a complete circle is covered by the two derricks and because of the longer booms this circle is larger than if one guy derrick were used; consequently the work can go on much more than twice as rapidly. The derricks are in such position that they can lift one another up from level to level.

tion> a Btnun is exerted teoding to lift the mast. Generally, i Bttings of the derrick are not designed to reast this stnuo,

sill below the seat ot tHe mast. Ibey taiie up tbe vertical strain and prevent the mast from leaving the seat. The superintendent

should always insist that they be used, as the lack of them often causes serious accidents.

Full-Circle Stiff-Leg Derrick. This type of derrick, Fig. 13, and the combined stiff-leg and guy derricks. Fig- 14, are mod- ifications of the stiff-leg and the guy derricks. They are but seldom used iu steel erection and only where special conditions make them advantageous. These derricks are not so easily erected as other derricks and therefore are usually adopted on jobs where a minimum of moving from one location to another is required such as in a storage and sorting yard.

and easily moved horizontally. It is almost always mounted with the hoisting engine on a flat car or some movable platform. This form of rig is well adapted to jobs in which the steel work is not very high but covers considerable ground.

Where sufficient width of room is available and where a traveler of some kind is needed, two stiff-leg derricks, Fig. 16, fastened together and mounted on a movable platform make a good rig.

RoiLs on jufhtch iroveler mo

Platform Traveler

tig. 16. Plan of Layout for Two SUiff-Leg Derricks on Movable Platform

Crane Derrick. A crane derrick. Fig. 17, is composed of a mast held in an upright position by guys or full-circle stiff legs. The boom is fixed permanently to the mast in a horizontal position near the top. On the boom is a trolley which shifts the load in the horizontal direction. This style of derrick is comparatively limited in capacity, but is useful where a large number of light loads have to be handled. It requires less power to move the loads horizontally, as is readily seen, for the trolley can be moved more quickly and with less effort than is required to top up a long boom attached to a guy or a stiff-leg derrick, especially when this

can dear them as it swings past. In building work where t goea up to some height, sometimes a trestle work or a 1

erected first and a crane derrick located on top of it. In t the machine needs to be set up but once to complete the the building within its reach. It is estimated that the cos trestle or tower upon wluch the derrick is placed is more thf

Tower Derrick Buildioi icon Uoitl aiJ Derrick Cumpanu, St.

Tower Derrick. Tower derricks, Fig. 18, as the name implies, are fastened to a tower and need no guys, stiff leg, or other support to keep the mast in an upright poaition. Where these derricks are used, a tower must be built ahead of the structure. The derrick

within its reach. This form is seldom used except on jobs where

there 13 no place available or convenient in the structure on which to locate a guy or a stiff-leg derrick. Some contractors, however, believe that a tower is the cheaper method, because they estimate

is designed so that it can easily be taken apart and used on other work, and the first cost can thus be divided among several jobs. Tower derricks are commonly used in connection with the travdera employed in the erection of bridges.

Cableways. In some kinds of work it will be found that there is no feasible way of setting up a derrick of any kind and in such a case some sort of a suspension cableway can be used to great advantage. A cableway, Fig. 19, is composed of a strong steel cable suspended over the work and between towers which may be of the fixed, semi-portable, or traveling type. The load is moved along the cable by means of some kind of a trolley. The rapidity of erection with a cableway is usually much less than where a derrick is used, because the latter can make so many more moves in a day than can the former.

Superintendent's Authority over Equipment and Work

Interference with Contractor lUAdvised. The superintendent ordinarily has little authority over a contractor's equipment. It wUl usually be found that the contractor is capable of selecting a good method for doing his work. A superintendent has no right to dictate what equipment a contractor shall use, or to interfere in any way in the matter, unless he is convinced that the chosen equipment is positively dangerous, or can prove that it is insuflBcient to perform the work within the time limit mentioned in the con- tract. It is usually a somewhat delicate matter to interfere with the methods adopted by a contractor, and for this reason it is most essential that the superintendent shall have a thorough knowledge of contractors' equipment in general. This knowledge will enable him to stand firm and to prove his contention in any case where he finds it necessary to interfere. It is not suflBcient reason for interfering that the superintendent simply thinks he has a better way than the contractor; the contractor may, however, welcome a good suggestion, as a suggestion. Only when the superintendent is convinced that the equipment is positively inadequate to perform the work properly within the given time, is he justified in protesting. It must be kept in mind that the law will hold in a great majority

tated to him what methods he should employ. If the engine dictates the methods, he must also relieve the contractor of responi bility for the results. It is not generally wise to dictate; it is mu better, particularly where the contractor is reasonably capab to make him responsible for certain results and, at the same tir to leave him free in his choice of the methods. In this connectic it is well to remember that the contractor's practical training mai him generally a better judge than the superintendent in his p£ ticular field. It is also reasonable to consider that it is vital to the contractor's own interest to select the best and safest wa On the other hand, the superintendent may be called upon to de with a dishonest contractor, or one who is ignorant regarding certa parts of his business; in that case the superintendent should n hesitate to interfere and to make his interference felt.

Honest and Dishonest Contractors. We are assuming, ho ever, that the superintendent is called upon to work with contractc who are competent, reasonable, and honest. In most cases whc the engineer or superintendent shows these characteristics, t contractor will be found trying to do what is right according his judgment. It is good policy to assume that a contractor any other man is honest until proved otherwise. It cannot, ho ever, be denied that there are men in the contracting busine who are trying to make money by dishonest methods. Whene\ the superintendent comes in contact with such a man, he mi take especial care always to be technically correct, because the is no man who is more likely to take advantage of a superintenden technical mistakes and profit by them, than is the dishonest co tractor, especially where it has been necessary for the superintende to find considerable fault with the work.

Overloading Structures. In addition to the superintenden duty of ascertaining whether or not the contractor's equipme is adequate, there is the duty of seeing that the structure is n overloaded. In the rush of work, steel men are prone to overlo; some portion of the structure, either by providing insufficient supp for their equipment or by piling too much material in some pj

is necessary to rectify the state of affairs. He must always insist, however, that it be corrected without delay.

A guy derrick should have good support under the shoe of the mast and ample strength in the places where anchors are to be fastened. The stiff-leg derrick should have three places of support, namely, under the mast and under each of the ends of the two stiff legs, these last two supports being sufficient to withstand not only the compression strain but also the tension strain which may be placed upon the stiff legs, depending upon the position of the boom.

Hoisting Engines. Location. The hoisting engine should always be located in some place where the vibration of it when in action is not transmitted to the structure. The continued vibration of a reciprocating steam engine, air compressor, or similar machine puts undue strains on the structure, tending to make the operation xmsaf e. To save the structure from this vibration the hoisting engine is left in the basement while the derricks are raised with the structure.

Bracing. It is well to remember that the hoisting engine must be properly braced and anchored, otherwise it is likely to go skidding over the lot when an attempt is made to pick up a load. The steel men are not often negligent in this matter of sup- ports and anchors for the engines, but nevertheless the superin- tendent should not overlook the inspection of them. Supports and anchors should always be placed in the direction opposite to that of the pull of the hoisting rope. If the engine is located directly under a derrick, then there must be enough weight to hold the engine down; otherwise, instead of lifting the load, the engine will lift itself. The superintendent more often finds that the steel men forget some small things, such as a few rivets in the connections of the beams and girders, which to them seem insignificant. Never- theless, there is little use in providing a strong and expensive derrick if the support for it is neglected, even in the little items.

Tackle

Classification. Tackle comprises the cordage, ropes, cables, chains, and all fastenings, connections, and other fittings, also a// blocks, pulleys, and sheaves, which are iec\>xvie m \

iiwactviis tvt 7ccaiiviatu vitaiiio

Size

Center of One

Outside

AvEBAGB Weight

Ayeragb

Of

Link to Center

Length

OF Chain per

Safe Work-

Chain

OF Next Link

OF Link

Foot

ing Load

(in.)

(in.)

(in.)

(lb.)

(tons)

Ih

From 2 to J

4f

From 8 to 1(

3J

From 19 to %

From 32 to 3(

From 50 to 5(

7J

From 64 to 7i

There is nothing in all the contractor's equipment that is often deceptive concerning its quality and condition as the tackl New-looking rope may not necessarily mean that it is strong( than other rope of the same size which does not look so new, the strength of the rope depends upon what it is made of, ho old it is, and how it has been kept.

Chains. Chains, the least dependable part of tackle, ai considered treacherous by some men. No chain, unless it has be< made by a capable and reliable manufacturer, and has been proper tested, should ever be permitted upon an erection job. It w: be seen that because of the way a chain is made — each link welde separately, usually by hand — that it is difficult to get uniformil in the strength of the different links. The strength of the cha: depends upon the quality of the iron, its composition, the degn to which the iron has been heated (iron that has been burned weakened), what the temperature was during the process of weldin and how carefully the welding is done — all of which may vary degree, thus affecting the strength of the finished chain.

No chain should be expected to do more than it was designs for. Every time a chain is overloaded beyond a certain limi it is weakened.

All chains in actual use are subject to deterioration, apparei and invisible; the apparent being the wear of the links where th( come in contact with each other or with other things, the invisib being an alteration in the nature of the material or of its fibe caused by shocks, strain, and frost, wlaida. 'pioAvsie.'s* This crystallization can be remedied by iiecvi'ew. \x£ie8i&Ck%>

weignt and btrengtn oi manna ana Msai Kopes

DiAMKTSR

OF Rope (in.)

Length of Rope

Weiqhino One

Pound

(ft. or in.)

Average Strength

OF New Manila

Rope

(lb.)

Average Strength

of New Sisal

Rope

Ob.)

: . %

m

u

H

13i ft. 6 ft. 4 ft.

3Ht.

26 in. 18 in. 12 in. 10 in.

is done by heating the chain to a cherry red and allowing it to cool slowly. A chain should be annealed at least twice a year.

The size of a chain is that of the diameter of the iron out of which the chain is made. Table I gives some of the characteristics of chains.

Cordage. Cordage is made principally out of the various grades of hemp, jute, sisal, and cotton, all of which are jfibers from different fibrous plants.

Manila Hemp, The strongest rope is the Manila rope, made from the best quality of INIanila hemp, a product of the Philippine Islands. There are on the market from twenty to thirty different grades of Manila hemp varying from a specially fine grade of long, clean, white, strong fibers, to an inferior grade with short, dark fibers. Cheap rope has short fiber and heavy weight; often weight- making adulterants are added.

Real Hemp. Real hemp is a native of central and western Asia, but is extensively cultivated in many countries. There are various plants which have fibers of similar nature and which are called hemp but which in reality are plants of other genera. Manila hemp is not a real hemp, as it comes from one species of the musa plant, another species of which is the banana plant.

Jute, Jute comes from a plant called Jews' mallow, is a native of Bengal where most of the jute used in commerce is produced. It is of inferior value for ropes for it does not stand moisture well.

Sisal. Sisal comes from sisal grass, sometimes called sisal

and for that reason is sometimes preferred. Table II gives variou facts relative to the weight and strength of Manila and sisal ropes

Cotton Rope. Cotton rope is seldom used by steel setter except in the small sizes, for signal ropes, etc.

Wire Rope. There are many different kinds of wire rope varying in quality of material, lays and number of strands, an twists and number of wires in the strands. Only that kind of wir rope adapted to the work it has to perform should be used. It i false economy to use cheap rope for such purposes as hoisting because the cheaper rope has to be replaced so often that the tote cost equals the cost of the best grade. A wire rope may be mad of 114 wires, but if one of these wires becomes broken, the rope i imsaf e for further use in connection with pulleys, sheaves, and blocks as the broken strand may unravel and rap itself around the sheave If the wire happens to entangle itself in the sheave or block, ani the power is not shut off promptly, there is great danger that th block or sheave will be torn from its support and drop the load.

Wire rope is usually made with a hemp center. This centei essential to the pliability of the rope, acts as a cushion around whicl are laid the strands. Where the rope is to be used without bending or is not to be moved from one location to another, rope with i wire center is sometimes used.

Wire rope is usually made of 6 strands, which in turn are com posed of 7, 9, 12, 19, or 37 wires, each, a finished rope having, thus 42, 54, 73, 114, or 222 wires.

Ropes made of 6 strands, each strand of 19 wires, are bes adapted for hoisting purposes. Those made of 6 strands, eacl strand of 7 or 12 wires, are best adapted for guys, rigging, am straight haulage purposes.

As usually constructed, the wires in the strand are laid o: twisted in one direction, while the strands composing the rope an laid in the opposite direction. Where the rope will probably b subjected to great crushing force or pressure, one constructed witl themselves will wear longer.

specincaiions lor o x iieni

lJ-viiit)r

M.K.C„

Pboper Loao roR

DirrRDNT

"'Sf'

Wkiqbt

D1Am.Ter

(lb.)

Abvibed

Sloe"

il"J.

'sn'

Crudible aitel

Iron

t

ft

0,50

0,80

820O

Iiooo

3,55

Wire rope should not be run over too small aheaves, for these tend to break the strands and increase materially the wear on the rope. It wears out faster when run at great speed, and therefore should be replaced more often when the speed is high.

Deterioration in wire rope is more often caused by rust than by the wear of constant use. To preserve it properly it should be kept well lubricated with a proper lubricant — one which will pene- trate between the wires, prevent friction between them, and cover them so as to prevent corrosion from moisture.

Use of Galvanized Rope. Sometimes aa a preventive against rust, the rope is made up of galvanized wires, but this kind of rope should never be used for running ropes — that is, for ropes that pas.s through pulleys and blocks — as the coating of zinc wears off very quickly, after which the rope rusts with great rapidity. It has also been discovered that in the process of galvanizing, the steel in the wire is likely to be burned, which materially weakens the rope. It is better to keep the rope saturated with a good lubricant.

Kinds and Strength of Wire Rope. The different materials commonly used in the construction of wire rope are patent, plow, special, crucible cast steel, and Swedish iron. Table III gives the proper working loads on hoisting wire ropes of various kinds, ajid Table IV the safe loads on standing Vne loea.

Diameter

Of

Rope

(in.)

Averaob

Weight

PER Foot

(lb.)

Proper Load for Different Grades of Wire

(lb.)

Patent Steel

Plow steel

Special Steel

Crucible Cast Steel

Swedish Iron

t

Splices. Wire rope can be spliced so as to make the splic imperceptible. The diameter will not be altered nor the strengtl materially decreased. A smoother and better splice can be mad in wire rope than in Manila rope. The splices for running rop are all of the kind known as the long splice, and should be at leas twenty feet in length.

Kinks. Wire rope should never be coiled like hemp rope it should always be kept on a reel or a drum of some sort. Whei not on a reel it should be rolled on the ground like a wheel or ; hoop to prevent kinking or twisting. Great care should be exerciser to see that wire rope is never kinked as this breaks the wire and s destroys the strength of the rope. Kinking, while not uncommon is a consequence of carelessness and rush work, and can be easil; avoided by a little watchfulness.

Engines, Power, Etc.

Kinds of Power. Steam and electric power are used mos often in the erection of steel structures; gasoline and compresses air are rarely used.

Size of Engine. To determine the size of a steam engin

required on a given piece of work, first determine the proper siz

of the hoisting cable or rope and select an ewYcva

uiKiuuu Lu ueuiLeuuy uit:;uuuiui::L'uipari!iuirupe ubuuiuuimcLiioii with the blocks. That is, if a f-inch patent steel cable desigoed for a working load of 8,000 pounds is used running through a single and a double block, thus making four parts, the load to be picked up should not be over 32,000 pounds and the engine should be

of a capacity to pull at normal speeds about 8,500 or 9,000 pounds on a single line. This means that a steam hoisting engine with two 9- by 12-inch cylinders, commonly rated as a 40-horsepower engine, will be required. If the 40-horsepower engine and |-inch cable be used In connection with two double blocks, thus making five parts, a load of 40,000 pounds can be lifted ; if used in connection with a double and a triple block, thus making six parts, a load of

Type of Engine. The hoisting engines most commonly fount on a steel-setting job are made with two hoisting drums — om to operate the topping lift of tlie derrick and the other to operat the fall that picks up the load — together with a reversible drun to operate the boom-swinging mechanism on the derrick. Thes drums are connected to the engine through strong frictions, ab held by powerful brakes. This arrangement allows one drum t

be used while the other is idle or holding the load, and vice versa A standard type of steam hoist is shown in Fig. 20.

Steam vs. Electric Hoists. The distinction between a stean and an electric hoisting engine is tbat of the kind of power used ii operating the drums. In choosing an electric hoist to do the sami work as a steam hoist, the motor selected should have a capacity at least 50 per cent greater than that of the steam engine. Thi is because of the different natures of the power. In the stean engine, the power is primarily produced by the expansive energ]

With a steam engine it is possible to reduce the speed of the drum, although increasing the pressure of the steam, and yet exert the same amount of power as would be exerted if the engine were running at a higher speed but with less pressiwe. The faster the armature in a motor moves, other conditions remaining the same, the more power the motor produces. It will be easily seen that in setting steel, the speed of the hoist must be varied from slow to fast, and vice versa. The speed of the steain engine is regulated by the amount of steam that is let into the cylinders, while the speed of the electric motor is regulated by means of a resistance box which takes more or less of the energy in the current away from the motor itself. The greater the current that is sent through the resistance coils, the more slowly the hoist moves but also the less power it exerts. It is to overcome this loss of power in the slow speeds that larger-sized motors are required. The same effect is sometimes obtained by using interpole motors and special controllers by which the torque of the armature can be increased about 50 per cent at slow speeds.

Advantages of Electric Hoists. There are several advantages which the electric hoist has over the steam, which offset the disad- vantage mentioned above. Some of these advantages are as follows: The electric hoist consumes nothing when it is idle; the cost of maintenance is less; it is ready to start at any time; there is no freezing of water in the winter time; there is no delay of work to raise steam; there is much less danger of fire and none from explosions; it makes no smoke, a great advantage in work where the smoke would injure the finished work. An electric hoist is shown in Fig. 21.

Hand Powers. Where speed is not essential, and on small jobs where the expense of setting up a hoisting engine is relatively large, hand powers, sometimes called crabs, and hand-operated winches are used. These are implements made in a great variety of shapes, sizes, and capacities, which increase the lifting capacity of a man or of several men by means of a system of gears operated by winch handles. They can be bought from implement houses out of stock, with lifting capacities on single line up to 10,000 pounds;

Loads. It is a good practice often of use in the rush of tl work, for the superintendent to fix in his mind how much steel takes to make a ton. He should have mental "short cuts" and pra tice them so that he can, by looking at a load being lifted, tell vei closely how much it weighs. It is easy to remember the foUowii rough estimates of what constitutes a ton: a little over 4 cub feet of steel; 80 lineal feet, or five 10-inch I-beams 16 feet Ion and 60 lineal feet, or four light 12-inch I-beams 15 feet long.

Tackle Blocks, Shackles, Hooks, and Wire-Rope Fastening Everyone of these items should be carefully designed and considen for strength, quality, and condition. As stated before, all tl preparation for a strong derrick, a strong engine, and a strong ro] is of little avail or of no avail if any one of the small details- block, a shackle, a hook, or one of the fastenings — is not stroi enough to carry the load that the large things carry.

Cheap blocks, cheap shackles, and cheap hooks should nev be used. They are false economy, especially in these days wh( the responsibility for accidents, so often causing loss of life, fal upon the owners of the defective machinery. Great care should 1 taken to purchase only the best of all these small parts.

Tackle blocks used by steel setters should be made of ste< They should have sheaves of a diameter large enough to prevei undue wear of the cable or rope. The pins of the sheaves shou be of size sufficient to carry the load and should be fitted with sel lubricating bronze bushings. These blocks should have stra] as well as shackles and hooks of strength ample to sustain the lo£ without breaking; especially should the swivel hook on a blo( be considered, as this one part is often the weakest point of tl whole rig. All of the other small items should be gone over ar the details carefully considered, with the object of seeing that th( are strong enough to perform as much work as the balance of tl rig, if not more.

Lines and Levels and How to Establish Them Iiiq)ortance of Correct Location. The establishment of tl lines and levels of a structiu is one of the most important phas< of the work. The superintendent is not often called upon to c

lot, but where encroachment on his next-door neighbor's property may occasion great loss of time and money. It is therefore a vital matter that the utmost caution be exercised in the location of the structure. There have been cases where the owner of a building has lost his entire fortune through the erecting of a portion of his building on another person's property.

Employment of Licensed Surveyor. It is often expensive to correct a mistake in location unless the error is discovered early. To insure the owner against loss incurred thereby, it is customary for him to employ a licensed surveyor to establbh the lot lines and principal levels. The surveyor can be held accountable for his mistakes and be made to pay for the expense of their correction.

Superintendent's Check on Lines and Levels. After the marks have been placed convenient for reference, and after the surveyor has provided the contractor with a plat showing where these marks are to be found, the contractor locates the intermediate lines and levels. It is the superintendent's duty to check all of the naeasure- ments, lines, and levels which the contractor has made, with the marks that are placed by the surveyor and also with the drawings, to see that no mistake has been made.

In establishing and in checking lines and levels, both the con- tractor and the superintendent should use a surveyor's transit and a surveyor's level, both of which should be of first-class work- manship. It is a risk to do this work with anything but accurate instruments. The transit should be frequently tested for vertical alignment, as many sights must be made with the telescope pointing up or down.

Preserving Marks. The most important lines to mark are the boundary lines of the property. These should be permanently marked in places which are not likely to be disturbed during the building operations. There are various ways of preserving these marks: they can be cut into adjacent buildings by means of a cold chisel, or upon sidewalks, curbs, street-cai tT/ika, or other con- venient places; they may also be pteserved'b>f ixieax

Locating Foundations. An experienced contractor exercise great care in locating the foundations accurately, both as to heigb and horizontal location. He knows that the more accurate th work is here, the less trouble and expense he will have in setting th steel above. In a tall building, the other parts of the work, sue as walls, partitions, elevators, and stairs, are all placed with referenc to the steel skeleton. Therefore, if the steel skeleton is not properl,

F. 22. . Lmyout foe LochUoe Marks id Betting Faundstion Shoe*

located, much trouble is experienced in making not only the stec but the other parts of the work fit together properly.

Setting Foundation Shoes. On top of the foundation ar generally set shoes of some kind on which the columns rest directly If these are accurately placed, it is reasonable to suppose that th balance of the structure will be properly located. Th aie c two general types — those that are secured in place by means c anchor bolts into the masonry bdow, ani iJooafc ?sai upon the masonry, being held in place bj \iisa owel

The following is a good method of setting foundation shoes. 1 By means of a transit, points intersecting the center lines of each fomidation are marked on the four sides of the foundation, and close to it. These marks are located on batter boards, or something stable, about one foot above the top of the shoe, Fig. 22. From these points are stretched and securely fastened, cords or mason's lines indicating the center lines of the foundation in both directions parallel to the edges of the shoes. In the meantime, the correspond- ing center lines are marked on the top of the shoe. If anchor bolts are being located, a wooden template is made with holes in it for the bolts and the center lines marked on the template instead of on the shoe. Then the shoe, or template, is moved from side to side until the center lines marked on either are exactly under the center lines marked by the cords, this condition being determined by the use of one or more plumb bobs dropped from the cords.

Setting Shoe at Proper Height. The shoe is set at its proper height by the following method: A wooden stake is driven as close as can be to each foundation in a place where it will not be disturbed, and by means of a surveyor's level the top of the stake is set at the level which the top or, if more convenient, the bottom of the shoe, is to have. When the masonry foundation or the steel grillage below the shoe is built, it will be left with its top one or two inches below the bottom of the shoe in its permanent position.

Anchor bolts, if used, are first located, by means of a straight- edge and hand level, at the right height with reference to the above- mentioned stake. In the same way — that is, by means of the straightedge, hand level, and stake — two thin narrow strips of wood or steel are located and bedded in mortar on top of the masonry, so that the tops of the strips are level and at the elevation of the bottom of the shoe. After the mortar holding the strips has hardened the shoe is placed on them and requires no additional leveling. This method saves much time and patience because, while it is comparatively easy to locate the shoe either horizontally or ver- tically, it is more often difficult to locate it in both directions at the same time.

the strips. The top of the shoe must m all cases be turned accurately milled in a lathe and be set absolutely level. If n the column above will rest imevenly on the shoe, thus putting strai on the shoe that may cripple it.

Locating Qrillage Beams. When grillage beams are to be plaa the method just described may be followed in locating the Grillage beams are usually fastened together with separators a bolts. Unless the beams are very heavy, it is well to fasten th< together before placing them in their permanent positions.

Grouting Shoes. After the grillage beams and shoes ha been properly located and the location checked by the superi tendent, strong rich cement grout should be forced into all of t spaces between the beams, shoes, and masonry, so as to make solid, homogeneous foundation. Care should be exercised to see th no load is placed upon the shoe until after this grout is fully set.

Foundations

The determination of the kind of foundation best suited a: most economical for any job is so largely a matter of good judgme and experience that only a few general suggestions are offered guides in solving the problems as they arise.

Soli. Examination. The superintendent is called upon n only to see that the foundation design is carefully carried out ai the workmanship and materials of proper quality, but also to exami the soil or other material on which the foundations are to be bui and to determine correctly whether or not this soil or other materi has sufficient bearing power to carry the load to be placed upon by the foundations as designed. The designing engineer's inform tion concerning the nature and character of the materials lyii below the surface at the site of the building, may have been sufficie neither in amount nor in accuracy to enable him to make a desig adequate to the work. Test borings may be made, but these a very often unsatisfactory, and only when the foundation hole h been opened so as to allow a close inspection of the material a accurate knowledge of the soil be obtained.

ability to sustain loads placed upon t\iem* TVv'sfc

After the soil has been exposed, the first thing to determine is its bearing power. It may be that from past experience and past tests of materials similar to that imder consideration the safe bearing power can be correctly decided upon without further investi- gation. If any doubt should exist regarding it, however, especially where the loads of the structure are to be great, accurate tests should be made. These tests can be made by placing a box about two feet square on the material to be tested, and loading it until settlement is observed, keeping a record of the loads placed and the corresponding settlements.

In common practice it is considered that ordinary soils safely sustain loads from 2 to 4 tons per square foot without settlement; that soft and treacherous soils carry not over 1 ton per. square foot, probably less; while for rock, loads up to 50 tons per square foot may be imposed with safety.

Clay, Clay is one of the most deceptive materials upon which to build. When dry, it is firm and reasonably strong, but when wet it becomes elastic and unreliable. It has a great tendency to mix with water. Sometimes it is foimd combined with sand or with marl, a mixture which when wet is especially treacherous. If the foundation is built on clay, great care should be used to see that good drainage is secured, both before and after it is completed. The effect of frost on clay is very great, and all foundations on it should be started well below the frost line.

Sand. Sand forms an excellent material on which to build so long as it can be kept from shifting. It has, however, no cohesion and has the fluidity of water when water is added to it. Therefore it is of great importance that sand be well drained both before and after the work is begim.

Wet Materials. The construction of foundations in quick- sand, under water, and on compressible soils is one of the most diflBcult with which the engineer and contractor have to contend. Such soils are likely to cause a great deal of trouble, anxiety, and expense, and it requires the greatest skill on the part of the engineer o/ the contractor to build them, TVxe ai sA

piles, freezing, and otiiers.

Types of Foundations. There are three general types foundations in common use — surface, sometimes called spi and also floating foimdations; pile foundations, either of w or concrete; and caisson foundations, which are columns of cone or other masonry carried down through the soft upper strati materials to the solid rock or hard pan located some distance be In digging the caisson, two methods are used, one called the o] and the other, in which compressed air is required to keep out soft material, called the pneumatic process. In some places, noU Chicago, round holes or open wells are sunk through the uj materials to solid rock without the use of air, the holes being fi with concrete.

The bearing power of the foundation material determ the type of foundation to be adopted. Where the material high bearing power and will not be disturbed, the surface f ounda may be the better one to use. Where the soil has little bea: power, pile or caisson foundations may be the cheaper. In S( cases where the structure is an important one, the best foundati that can be built will be decided upon, regardless of cost.

Proper Location. In all types of foundations, excepting course where the columns come directly on top of the solid r( it is of great importance that the column be located in the cei of the artificial foundation, or, where this cannot be done, that load should be equalized by means of cantilever girders or s( similar method.

The superintendent should be alert to detect any atter made by the contractor or his men to conceal mistakes made the location of foundations. This is sometimes done by covei the lower portions with dirt, which hide the fact that the top la although itself in the right location to receive the column footi is ofiF the center of the foundation below. It is human nature try to save the expense involved in the correction of such ern but sometimes by doing so a dangerous condition is incurred. 1 writer once beard of a case wliere a coTictX. csSssjsrpc "jixrsfc one hundred feet to rock, was located m \5afc oto\v% sJssRfc

thousand dollars which correction would have involved, the con- tractor had the top of the caisson secretly removed and a new top placed in the right location for the whole caisson, thus making the center of the colunm come on the outer edge of the caisson below. Fortunately this condition was discovered in time, or else a serious accident, perhaps involving loss of life, might have occurred.

The case just cited is an extreme one, but it serves to illustrate what some men do when tempted. It also shows that the superin- tendent of this building was negligent in his duty, or ignorant of his business; otherwise he would have detected the mistake before the contractor had had an opportimity to yield to the temptation.

Importance of Foundations. The stability and endurance of any structure depend largely upon the character of its foundations. The superintendent should realize that it is of the utmost importance that he give this part of the work special attention. He should see that all the requirements of the specifications and all instructions issued by the engineer or architect are faithfully carried out, and he should report without delay to his superior any probable source of failure he may detect. Besides the quality of workmanship and materials in the artificial foundations, there are two things which are sources of failure and which must be guarded against — inequality of settlement, and lateral escape of the supporting material.

Concrete and Other Masonry. Nowadays concrete enters largely into all foundation work chiefly because of its cheapness and adaptability. The inspection and superintendence of it is like that of any other kind of concrete and masonry work. It is a study in itself and is not to be dwelt upon here.

The most important things that the inspector is called upon to watch are the quality of the materials, the proportion of the ingredients, the thoroughness of the mix, and the method of placing, so that a compact and homogeneous mass is produced. The cement should be properly tested. In the best practice, it comes to the job with the test tags attached to the bags, and there should be

The crushed rock or the gravel should be clean and of good qualit The water used in the mix should be clean and free from all grea or acid; the mixing should be thoroughly done by some good m chanical mixer so that all ingredients are well mingled. Care shou be taken that the right amount of cement is used. Cement is t most expensive ingredient, so that the temptation is to put in smaller quantity than is called for by the specifications; and becau of its nature, it is harder to detect a shortage of it than of the oth elements in the mixed concrete. Concrete should be thorough rammed in place; it should not be so dry as to prevent its flowii into all parts of the foundation. Usually, the specifications coverij the work describe fully the proportion and quality of the materia and the method of mixing and placing. The inspector has litl difficulty in seeing that a good job of concrete work is done, if 1 carefully studies these specifications.

Foundation Steel. In all the different types of foundatio: used for steel structures, steel of some shape and design is general to be found.

The common practice among engineers is to bed this f oimdati( steel in concrete without painting it, knowing as they do that ceme: is one of the best preservatives of steel. When steel is so burie care should be exercised to see that it is cleaned of all dirt, mud, loo rust, and especially loose scale; in other words, prepared so th nothing comes between the steel and the concrete. To preser the steel, the cement must touch it; therefore, the concrete shou be so rammed and spaded that all parts of the steel are thorough covered by the cement.

Pile Foundations. Where pile foundations are used, tl inspector needs only to follow the engineer's specifications govemii the work, using common sense and good judgment in the applicatic of them. He should always be on the job and watch with his eyes the progress of the work. He should never take for grantc that the work is being done properly. If the piles are of wood, tl inspector is required to see that each is straight, sound, and

that all the piles are driven in the right location.

Caissons. Caisson work is a specialty in itself and the con- tractors undertaking it are nearly always e3q)erienced and equipped with especial knowledge as to methods and procedure. The inspec- tion of it consists chiefly in seeing that the caisson is plumb, m passing upon and accepting the bottom when this is reached, and in ascertaining whether or not the concrete or other masonry is properly proportioned, mixed, and placed.

Caisson work is almost always carried on continuously, twenty- four hours a day. This must be done when soft material is encountered, for otherwise the caisson would be inclined to fill during the idle hours. The superintendent or inspector should hold him- self in readiness at all times of the day or night to inspect and pass on the bottoms, when notified by the men that these are ready. It is generally dangerous to leave a caisson standing idle long; it is also expensive to keep it clean and ready for concrete for any length of time.

Settlement of Adjacent Structures. One important duty to be performed by the superintendent where the caissons are being sunk near other structures liable to injury by settlement, is to see that no voids are left outside of the caisson shell or lagging. The men are likely to become careless in this respect, particularly where the work is rushed. Such neglect is one of the principal causes of settlement in surrounding structures; it can readily be seen that if the soil has nothing to hold or support it, it will be squeezed out of place by any heavy weight and will move imtil it encoimters something that stops it. Movement causes settlement and this is a dangerous thing. The squeezing of water out of the soil also leads to the same result. It is sometimes practically impossible to prevent some settlement, in which case the safe and the cus- tomary thing to do is to shore up the adjacent or adjoining structure, placing it upon jackscrews so that, as the foundations settle, the superstructure can be held to its proper level. Shoring work is in itself a special trade, and all such work should be given into the properly and salely handled.

HI pittucy auu. tiic uciiiuisjs iitjistiiig ciigiiics, auu. utucr parts ui tJ

erection equipment are set up ready for work, the steel for tl superstructure will commence to arrive.

Covering the erection of the superstructure, we quote fro the standard specifications of the United States Navy Departmei Bureau of Yards and Docks, which state the matter tersely follows:

Field Work

Unloading, Storing, and Handling. Material shall be unloads stored, and handled in such a manner and with such appliances and care to prevent the distorting and injuring of the members; material which is injui shall be repaired or replaced, if necessary, as may be required by the ofl&< in charge and at the expense of the contractor.

Erecting. All field connections shall be riveted. The various memb forming parts of a completed frame or structure, after being assembled, sh be accurately aligned and adjusted before riveting is begun. All requiremei specified for shop work which are applicable shall apply to the field work.

System in Handling Steel. Sequence in Arrival. Systc in any work means speed and economy, and this is particular true in the erection of steel. Theoretically, the steel should cor to the structure in such sequence and with just enough rapidi so that it can be taken from the car or wagon and placed in its p( manent position without rehandling — an ideal arrangement almc never attained. Under no circumstances should any quanti of material be allowed to accumulate at the building site, whi cannot be put in place at once. The cost of moving materia and the consequent delays in getting them out of the way of co struction work, in the long run amount to a considerable sui Then, too, every time the material is handled it is subject to injur and in some poorly systematized jobs the steel is moved so oft that most of the paint is worn ofiF.

Sorting Yard. When the steel comes from any distance the shop, especially where it has to be shipped in over railroac the experienced and capable contractor will provide a sorting ai storage yard handy to the job. As the steel ames, 'iSi and stored so that it can be easily picked, up m \ as needed in the erection work.

SHJillC WAlllC MCAUAC MSS %AJ km UOVL bAAV ik/UAAVAXXAg* axami.7

are discovered early, and this tends to insure the erection work against delays.

A quantity of steel ready for the work, provided it is not piled in a confused mass, acts as a stimulus to the men, for they usually put forth greater efifort if they can see work ahead of them. If the material is coming to the job in an imcertain manner, the men will be inclined to "nurse" their jobs so as to avoid a lay-oflF and consequent loss of wages while awaiting the arrival of more steel. Then again, the men usually like to vie with each other in the speed of their work, provided are moving smoothly and the material is coming to them in the right quantities. This spirit of rivalry not only adds zest to the job, but causes it to be finished more quickly. The best men like to work on something that is being handled properly; they like the excitement in the rushed work; they like a foreman who, while treating them fairly, also pushes the work; and, on the other hand, they dislike a job that is poorly handled and one that drags along.

It costs something to operate a storage and sorting yard, but this is often found to be money well spent and a saving in the long run.

Injured Steel. When steel is bent or twisted out of shape through unskilful and careless handling, either during transportation or at the site, it must be carefully straightened and repaired. If any piece is very badly out of shape, the best thing to do is to have it replaced with a new one; if the bend is not serious, however, the piece may be straightened after heating it to a red heat;* after this it should be again heated all over to a red heat and then left to cool slowly. This process is called annealing and is for the purpose of restoring to the steel all of its original strength and of securing homogeneity of the structure of the metal that is supposed to be injured by imequal heating or by the manipulation attending the straightening process.

Steps in Erection Process. Steel structures, as they are designed in these days, are very rapidly erected. In a tall building as many

redlieat.

The diif erent steps taken in the erection of a tall building are about as follows: The columns, girders, beams, and all the large pieces are assembled in place by means of the derricks, etc., the pieces being held together by temporary or erection bolts. Usually the steel men use just as few of these bolts as they can, as it takes time to put them in place. A gang of men now follow who put in the small pieces, such as the separators, tie-rods, and short and light pieces of beams, and do other work which can be handled readily without the use of heavy equipment. In the meantime another gang is busy plumbing up the structure by means of guys in which are inserted turnbuckles, and by means of shores or wooden timbers set diagonally between the foot of one column and the top of the next. These shores are tightened by means of either wedges or jackscrews. After the structure has been assembled and plumbed, aligned and adjusted, the riveting is commenced. It is of great importance for the safety of the structure that the riveting be started as early as possible; it should never be farther than four stories behind the derricks.

Temporary Plank Floors. The factory laws in some States require that temporary plank floors covering the entire area of the structure be placed on the steel as fast as it is erected, thus providing a safe place for the workmen and affording protection against the dropping of bolts, rivets, etc., on persons working below.

Plumbing and Alignment. The shores and guys which the plumbing-up gang have placed should be left in position until they are in the way of the masonry or other parts of the work. The riveting should never go ahead of the plumbing-up. Whether or not the structure is plumb is determined by dropping a heavy plumb bob on the end of a long string fastened to a stick secured to the top of the column, the string being three or four inches away from the column. Then with the aid of a pocket rule, the space between the column and the string is measured at different heights of the column; if these distances are found to vary, the column is pulled by means of the guy or pushed by means of the shores until they are the same.

floors after tiiey flave been erected because, unless an e/ror nas been made in the fabrication of the columns, the floors f ollo\7 the levels of the shoes. Therefore it is customary to consider a building m proper alignment after it has been made plumb. If the top of the base plates and the ends of the columns have been properly planed or milled ofiF, and if the base plates have been set level, there is little difficulty in making the structure plumb.

Shims. Where careless work has been done, the contractor sometimes tries to use shims in the joints between the successive column sections. If the shims are of such a nature that the column loads are concentrated on a smaller area of the column section than called for by the engineer's design, the superintendent should not allow them to remain. Shimming columns is the easy and quick way of making the structure plumb. If permitted at all, the shim should be made of a tapered plate that will cover the entire bottom or top of the column.

Floor Beams. In a steel building where fireproof arches are to form the floor construction, care should be exercised to see that all floor beams are set parallel. Where both ends of the beams are connected to girders, there will be little difficulty in this unless the shop has fabricated the steel wrong. It is where the ends of the beams rest upon masonry walls that the most care should be taken in making them parallel. If the floor construction is to be of fireproof tile arches, the superintendent should also watch to see that the tie-rods are all in place and their bolts drawn tight.

Small Castings. The superintendent should always see that wall plates are put in place imder all beams and girders that rest on masonry. He should also make certain that all cast-iron and other separators are properly placed and that none are omitted. These separators and many other small items are looked upon by the men as of little consequence; however, if the designing engineer had not thought they were necessary, he would not have included them in the plan of the structure, and it is safe to assmne that he knows best what should go into it. Moreover, it is not the concern of the erection gang whether these items are needed or not; if the drawings and specifications call iot tYveca, \sv5Sfiaftsa A men to put them in place.

lucHL or some [imt'e wiiere me viomiion wui iiui aiiecL uie sirufn: ia geoerally large enough to supply air to several hai at one time. The air is piped to the hammers, the last length of the pipe beingrubbersoastoallow the hammer to be moved iithout delay or trouble.

Necessarily, because of the size of the equipment, the same amount of pressure cannot be brought to bear upon the field rivets as upon the shop ri\-cts, for the shop machines are larger and mor

powerful. It cannot be expected, therefore, that as good a job cS field riveting can be done as of shop riveting, but there are certain things in the field work that should not be permitted, viz, loose rivets, those with small and badly formed heads, burned rivets, and those which are driven up at a blue heat, that is, a heat ranging from 430 to 600 degrees.

In the field work the noles are not likely to come together 80 well as do the holes in the shop, and a certain amount of drifting by the use of the driftpin ia permitted. If the holes are very bad, Aowever, so that the driftpin doea not le&tfs 'OnKm.

Gup of

RiTBT

(in)

U U

DiAiiiTBB or Shank (in.)

f

Length or Shank (in.)

u

2f

2f

3j

4j

Si

5t

2J

3J

4f

Note. The lengths of shanks for countersunk heads will be the lengths given a reduced by from 75 to 100 per cent of the diameter of the shank.

they should be reamed out and a larger-sized rivet used than t called for.

Before any rivets are driven up, the steel plates should be dra together as tightly as possible by means of the erection bolts.

Sizes of Rivets. The size of a rivet is described by the diame and length of the shank in even eighths of an inch, when it is c and before it is driven up. The diameter of a rivet should not over inch greater after it is driven than it is before it is heat The rivet should, however, fill the hole completely, and to do t it should be heated all over to a red heat in daylight. It shoi not fall into the hole but require a slight pressure to force it

The height of the head of a snap rivet, which is one wit! conical head in contradistinction to a countersunk rivet, shoi be about two-thirds the diameter of the shank, and the diame of the head should be from one and one-half to two times the dia eter of the shank. The grip of a rivet is the total thickness of t

To this must be added about 9 per cent of the length to allow for filling the rivet hole, which is usually ife inch larger than the rivet. In Table V is given the data for plain rivets of various diameters. Heating Rivets. The heating of rivets should be carefully watched. Those made of iron are not as liable to injury from burning as are those of steel. The rivet should be heated all over to a red heat in the daytime; the men will try to slight the work by heating only the ends on which the head is formed. Any steel and wrought iron is rendered brittle and its strength impaired if it is worked, that is, hammered, etc., while at a blue heat, from 430 to 600 degrees. It will be seen, therefore, that rivets must be driven up quickly after they have been heated to the right tem- perature and that they must not be hammered too long after they are in the holes. The forge in which they are heated should be placed as close to the rivet hole as is practical.

Loose Rivets, All loose rivets should be cut out and new ones driven into their places. The attempt to make them seem tight by the use of the calking iron or by re-cupping with the hanmier should never be allowed. The inspector should examine each with the inspector's hanmier. If the place into which the rivet is to be driven is diflBcult to reach, he should look at the rivets before the staging used by the men is removed. When this is done, how- ever, care must be exercised to see that the rivet is cold to the touch before it is tested or disturbed.

Riveting Gang. A riveting gang is generally composed of four men — the heater, the passer, the bucker-up, and the riveter; the last named drives the rivet and is boss of the gang. When there are a number of gangs there is a boss over all the riveters who reports to the general foreman. There are also boys who carry the cold rivets from the storeroom to the heaters and run other errands.

Bolts. In some places in the structure it is found impossible

to drive up rivets properly, and the holes must be filled with bolts.

If the connections are not important, that is, if the bolts are not

depended upon to carry the load from one piece to the next, ordinary

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the hole and require driving to put into place. In all cases whe

bolts are used, after the nut has been turned up as far as it will g

the screw end should be riveted cold by hammering it until tl

threads are deformed. This prevents the nut from working loos

which it is likely to do, especially if there be some vibration in tl

structure.

Painting

Object of Painting. The primary object in painting ste is to preserve it. All steel and iron absorb more or less oxyg< when exposed directly to the air and the surface soon becom coated with rust, which is the resultant chemical change caus< by the oxidizing process. Rust is fonned very rapidly; it can 1 for a time interrupted by painting, but the process goes on slow even under the paint, which in time will peel off, together with layer of rust. Rust has the peculiar quality of spreading ai extending from a center, if there is the slightest chance for it do so; a small point of rust on the metal may grow imder the surfa of the paint. Steel and iron are entirely destroyed in time by tl action of oxygen. It is therefore of the greatest importanc especially where the metal is buried so that the paint cannot 1 renewed from time to time, that the metal be protected from tl action of the oxygen, either by keeping out the oxygen or by neutrali ing it chemically.

Concrete as Preservative. Lime in any of its forms, or cor bined with other materials to make cement, seems to neutrali the action of oxygen on iron or steel. When these metals are be entirely encased with concrete, a slight amoimt of red rust do not affect the lasting qualities of the metal because the lime countc acts the action of the oxygen. However, no scale or loose ru should be left on the metal when it is buried in the concrete.

Kind of Paint. Paint is supposed to be a waterproof ai air-tight covering that keeps out the oxygen. There are mai brands on the market which are sold for this purpose. The pri cipal duty of the superintendent or inspector is to satisfy hims< and quality of paint, and furthermore to ee tJaaX.

is of course the most important; this coat is, however, seldom applied at the building. Some paints when used for the priming coat seem to aid the oxidizing, producing rust rather than preventing it. These, of course, must be most carefully avoided.

Paint for steel must have the property of expanding and con- tracting in about the same ratio as the steel itself; otherwise it cracks and leaves the metal exposed to the air.

Inspection of Paint. A good way to determine whether or not the right paint is being used is to make the contractor show his receipted bills for the materials. Most good and reputable manufacturers and dealers of high-grade paints will aid in the detection of substitution. They are usually compelled to do this to protect the reputation of their paint. The manufacturer, when asked, keeps the superintendent informed as to how many square feet of siuace the paint should cover and also what quantities of paint are being purchased for the particular job in question. With this information, together with the amount of surface that is being covered at the job, the superintendent can soon tell whether or not the right paint is being used. This method cannot always be depended upon, however, and when any doubt exists, other methods of detection should be used. A chemical analyst will sometimes aid in this. Any chemist will also tell the superintendent of some rough test which the superintendent can himself make, such as the burning of lead paints, and what the results will be for pure and impure paint.

In order to see that the required number of coats of paint are given to the work, it is a good plan to have each coat made of a different color or of a different shade of the same color.

MISCELLANEOUS PROBLEMS Superintendent and Contractors' Organizations

There are many kinds of contracts, and contractors to handle them. Each contractor has his own opinion regarding his organiza- tion. An organization varies with the amount of work the con- tractor executes in a year and also t\ve sVl ol \!aa ot \obs that he has. The small contractor VitYi smaYL -wotV ViO !&\5et

OVU 1/JJ.C WAXX V'JJJXVrjr XXKI VrUXAV/A AJIV/A. TT tAJVfV T X/A AAV/ TT AAA V

his office "in his hat'*. A concern or an individual contractc handling a medium amount of work has a small organization t assist in carrying out contracts, while a large firm or contractc has a very complete organization to handle all the details of larg contracts in the most complete manner possible.

Large Organizations. The large contracting firms have th following general officers: president, vice-president, general mar ager, chief engineer, superintendent, treasiu'er, auditor, piu'chasin agent, and others who have charge of the different parts of the worl Sometimes the duties of more than one of the officers are performe by one person. The work which these officers have to overse resolves itself into different departments, such as the executiv( engineering, contracting, purchasing, auditing and accounting superintending, storehouse, and transportation departments.

Field Organization. The work in the field will probably h in direct charge of a contractor's superintendent, or if the job i small of a foreman simply. The contractor's superintendent wi have a foreman or several foremen under him, who have charge c certain areas or parts of the work. These men will have as assistant a number of sub-foremen usually called "straw bosses" who are i direct charge of the men.

The theory on which such an organization is based is tha any one man has the time and capacity to handle and deal wit only a certain number of men. It would manifestly be impossibl for one superintendent to direct two thousand workmen doing man different kinds of work, unless he had someone to help him. If b attempted it alone, he would not have time to watch each mai and as a result the men would be inclined to take advantage c him, and not to work when his back was turned. Furthermore one man could not plan the work so as to keep all the workme busy and at the same time direct them as to the manner in whic the work should be done. The superintendent, however, hs time to plan the work, if he has foremen to see that the me do it as he outlines it. In this way, the superintendent has t deal with .only a few, instead of dvTeclVy Vtfa wisaxA he can hold the foremen accountable iox liJcve -TorcMxsRfc

In addition to the foremen and the sub-foremen, the superin- tendent will have one or more timekeepers and material clerks, depending upon the size of the job. When the job is of medium size, one boy can both handle the timekeeping and also order and check up the material as it arrives. On small jobs, both of these things are done by the foreman.

Proper Size of Force. There is always a proper size for the gang any one man should handle. If fewer men than this number are in the gang, the foreman does not have to work as hard as he should and the cost of his salary per man is excessive. If there is more than the required number, then the foreman does not get the maximum amount of work out of them, and loss occurs in that way.

Every organization must be modified to suit the conditions and the work. A very easy and common mistake is to make an organization top heavy with so many foremen and sub-foremen that there is not enough work to go around to keep the different ones busy. This method makes the general expense run beyond that which is economical or possible from the standpoint of cost. A job is properly organized when each man from the head down to the water boy has to work hard and continuously, but not so hard that the work suffers on account of lack of time.

Proper Use of Organization. Early after the contract is let, it is well for the superintendent to inform himself as to the con- tractor's organization, if he has not already come in contact with it, in order to know who is the proper person with whom to deal in regard to certain matters. Like any tool, the superin- tendent must know how to use the organization properly.

Superintendent and Superior Officers

Contact with Employer. If the owner is a corporation or other large concern, it will be the duty of the superintendent to find out enough about the organization so that when called upon he will know through what channels to act. Ordinarily, however, if the superintendent is employed by the engineer or the architect,

work. Making people start and tinisti tiieir tasks on time requires all the diplomacy, tact, and force of which a superintendent may be possessed.

Importance of System

System and Speed. The speed with which buildings and structures are finished depends largely upon system. Every set of mechanics with their materials must be ready to start at the proper time. No department should wait for any other. The architect, engineer, superintendent, contractor, all sub-contractors, and the different mechanics have their certain duties to perform, and all must work hand in hand to accomplish the result. It is just as harmful to the work for the architect or the superintendent to neglect something at the critical time, as it is for the contractor, and sometimes the results are more injurious. The great secret of success in construction work, as in other work, is constant, unre- mitting "push" m all departments.

Proper Sequence of Work. All persons in construction work soon learn that there are certain times to do certain things, and that, just as in other walks of life, there is a psychological moment in the progress of the work which should be seized to insure success. If these moments are neglected, the chance to do the thing in the shortest and most economical way will be lost forever, and great additional effort, worry, and delay will result.

Many illustrations of the workings of this truth can be given. Suppose that in a tall office building, when the erection derricks reach the eighth floor ready to put in place all the steel of that floor, that for some reason, either because the engineer did not make the design at the right time, or because the shop did not fabricate the piece when it should have done so, one of the large heavy floor girders cannot be put in place. True, the balance of the steel can be erected and the building go on up, but the point is that while the derricks were in place to erect the eighth floor, the girder could have been erected in five minutes' time and at a com- paratively small cost. Having to raise it into place afterward will doubtless take hours, perlaaps day, aivd times the original expense, besides interfeniigmt\votXieToTVxAia

reaching the delay may be.

The writer has in mind another example of the mis doing things at the wrong time. The superintendent o: required that certain supports holding up the forms of a i roof of a building nearing completion be taken down tc The result was that the entire rear portion of the building ci into the basement, and the accident wiped out the owner' fortune.

Not only must the superintendent do his own work pr but of all men on the job he is the one who should know t best time for doing each piece of work.

Index.

A A-frame derrick

B

Builders' or house derrick

Building superintendence 1

material, inspection for steel work

steel construction

BuU wheels -

Cableways -

Caissons -.

Cast iron

Chains

Cordage --

Cotton rope

Crane derrick

. D

Derricks

capacity of —

types of

F

Field organization

Field riveting

Floor beams

Foundation shoes

Foundations

importance of

pile

Full-circle stiflp-leg derrick

G

General superintendence problems

business details, handling of

contractor's organization

designing engineer vs. actualities of contractor

drawings, duties regarding -

forethought, value of

handling men, problem of

legal points encountered

progress charts 8

Grillage beams :.- 75

Grouting shoes 75

Guy derrick 50

H

Hemp - 64

Hoisting engines 62

Jute - - 64

M

Manila hemp 64

MiU inspection 24

P

Painting 89

concrete as preservative 89

object of - 89

painty inspection of 90

paint, kind of 89

Pole derrick .- 48

Preventer guys 53

S

Shims 84

Shop inspection 30

drawings in shop 31

reports 38

shop processes 31

Sisal... 64

Steel construction 1

good design 2

structural Bteel 1

structure, classes of_ 1

work, divisions of 2

Steel work, erection of 24

adjacent structures, settlement of 80

cablewaya 60

caissons 80

cordage

derricks 42

engines, power, etc 67

erectloa process, fitepsin - -—