Economic Geology and the Bulletin of the Society of Economic Geologists 1928-12: Vol 23 Iss 8

1928 · Historical

Economic Geology and the Bulletin of the Society of Economic Geologists 1928-12: Volume 23 , Issue 8. Digitized from IA1518511-02 . Previous issue:…

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Economic Geology

VoL. XXIII DECEMBER, 1928 No. 8

Dolomitization And Ore Deposition.’

D. F. HEWETT. Introduction Summaries of occurrence United States—Mississippi Valley Oklahoma—Bromide District Virginia and Tennessee Colorado—Aspen District Colorado—Red Cliff District Utah—Tintic District Nevada—Goodsprings District California—Cerro Gordo District Alaska—Kennecott Mine Other districts FEurope—England Italy—Sardinia; Alps Germany and Poland—Upper Silesia Sweden Greece—Laurium Other districts Summary of features Occurrence Physical properties of the dolomitized limestone Chemical properties of the dolomitized limestone Quantitative elements of dolomitization Geologic range of dolomitized limestones Local relations of sulphides and dolomite Regiona! relations of sulphide ore bodies and dolomite Relation of dolomitization to intrusive rocks Silver content of sulphides Relation of magnesite to dolomitized limestone

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822 D. F. Hewett.

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Dolomitization as a guide to ore deposits 862 INTRODUCTION.

For many years the search for metallic mineral deposits was directed largely toward the discovery of mineralized outcrops or to the debris from their disintegration. Later, the products of weathering of commonly associated minerals were sought, such as the iron gossans of gold and copper deposits. More recently, repetitions of the structural associations of known bodies have been explored to find bodies that did not outcrop, and the latest methods involve the determination of physical constants of un- explored lands in the hope that some comparisons may be made with other lands known to contain certain minerals or metals.

It has seemed to the writer that a systematic review of the nature and extent of the mineral changes in the wall-rocks ap- parently characteristic of some mineral deposits may facilitate the search for unknown deposits. Having seen several lead and zinc deposits in the United States in which the limestone wall- rock has been altered to dolomite, I have recently reviewed the literature concerning this class of deposits and, during a trip to Europe in 1926, visited several well-known localities where such deposits were known to exist or probably existed. This paper presents the essential facts gathered in this review and tentative conclusions concerning the paragenetic relations of the metallic minerals and the altered rocks and the sources of the magnesia. The subject is shown to be more complex than first thought and although the method of study may be profitably applied in search of ore in some places, it must be used with caution. This paper will have served its purpose if engineers and geologists exploring lead and zinc deposits in limestone regions will observe carefully and record the wall-rock alterations and their relations to ore- bodies.

Dolomitization And Ore Deposition. 823

General Statement.—The occurrence and mutual relations of limestone and dolomite in stratified rocks have absorbed the atten- tion of numerous geologists for many years and there is an exten- sive literature on the subject. One of the best recent summaries ? of the origin of dolomite, outlines the theories that have been ad- vanced to explain its origin, as follows:

I. Primary deposition theories a. The chemical theory b. The organic theory c. The clastic theory

II. Alteration theories a. The marine alteration theory b. The ground water theory c. The pneumatolytic alteration theory

III. Leaching theories a. The marine leaching theory b. The surface leaching theory

Those who study the occurrence of dolomite in any region will do well to consider the possible application of each of these theo- ries. It will be pointed out later that another theory resembling that of pneumatolytic alteration seems to explain most of the cases discussed in this paper, but several cases seem to demand others, resembling the ground-water and chemical theories.

The presence of dolomite in many ore deposits has long been recorded, but as a characteristic alteration of limestone adjacent to metalliferous deposits, attention seems to have been first di- rected in the United States to its occurrence in Missouri by Schmidt.* Since then it has been observed and studied at other localities, notably at Aspen, Colorado by J. E. Spurr.*

2 Van Tuyl, F. M., “ The Origin of Dolomite.” Iowa Geol. Survey, vol. 25, p. 257-406, 1914.

3 Schmidt, A., “On the Forms and Origin of the Lead and Zinc Deposits of Southwest Missouri, St. Louis Acad. of Sci. Trans., vol. 3, pp. 246-252, 1875.

4Spurr, J. E., “Geology of the Aspen Mining District, Colorado,” U. S. Geol. Surv. Mon. 31, pp. 206-226, 1898.

824 D. F. Hewett.

Summaries Of Occurrence.

It is not so much a purpose in the following summaries to pre- sent the details of occurrence of dolomite in each district as to quote, so far as possible from the author’s summaries, statements concerning the extent of dolomitization and its relation to the process of deposition of metallic sulphides. For several European districts, the outstanding geologic features will be stated briefly. In the general discussion of the process which follows the de- scriptions of districts, there is repeated reference to local details recorded in the literature but not specifically mentioned earlier. It is assumed that those who are interested in these or other de- tails will refer to the literature.

United States—Mississippi Valley Region.—Bain, Van Hise and Adams state: °

In the Mississippi Valley, the close association of the metals with dolomite and magnesian limestone is very marked... . In practically every instance where the ores are mined, dolomite occurs either in intimate mixture with the ores or in the immediate vicinity of the ore bodies. No- where in the Mississippi Valley are zinc and lead ores found in quantity where there is not evidence of more or less dolomitization (p. 209).

The first suggestion is that a close chemical relation exists between magnesia and the metals concerned (lead and zinc), but of this there is no good evidence. . . . The very widespread presence of lead and zinc compounds in the magnesian limestone strongly suggests that the original introduction of lead, zinc and magnesia in the rocks was simultaneous and doubtless dependent upon chemical interaction (p. 211).

In 1906 Bain wrote: °

The manner of preservation of the fossils, the forms which are normally preserved in limestone occurring in the reverse, or as casts, in the dolomite, seems to indicate that the rocks as originally deposited were not dolomite but limestone, and that they were changed later. . . . In the Mississippi Valley, as elsewhere, dolomitization has been both local and regional. Local dolomitization is definitely related to particular fractures and is

5 Bain, H. F., Van Hise, C. R., Adams, G. I., Preliminary Report on the lead and zine deposits of the Ozark uplift. U. S. Geol. Surv., 22d Ann. Rept., Pt. 2, pp. 209, 211, 1901.

6 Bain, H. F., ‘‘ Zinc and Lead Deposits of the Upper Mississippi Valley,’ U. S. Geol. Surv. Bull. 294, p. 30, 1906.

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Dolomitization And Ore Deposition. 825

believed to be due to the action of ordinary underground waters. Dolo- mite found in connection with the Joplin ore deposits affords an ex- cellent example of local dolomitization. The attempt has been made to refer the regional dolomitization of the Upper Mississippi Valley to similar agencies, but the necessary reduction in bulk, estimated as Io to 1, is greater than the field evidence indicates has occurred in this region. A much more probable explanation is that the change occurred while the rocks were still beneath the sea, after the limestone was formed, but before it was elevated above the water or completely buried by succeed- ing beds (p. 30).

Of conditions in the Granby area of southwestern Missouri, Buckley and Buehler wrote:

As a rock, dolomite is not abundant in this area. Beds of fine-grained gray dolomite, having much the appearance of limestone, were observed in the Blue Jacket No. 2, in the Cuba shaft and upon the dumps of the Sunset ground. The beds are thought to be dolomitized limestone and of secondary origin (p. 52).

Of southeastern Missouri, Buckley wrote: ® y

It is my opinion that the magnesium contained in the Bonneterre dolo- mite was introduced after the sediments were deposited, and that when first laid down, they were probably as free from this constituent as the marble boulders (found in some places). . . . The dolomitization of the Bonneterre limestone may have occurred either while it was beneath the sea or subsequent to its emergence. The evidence furnished by these formations tends to confirm the theory of the secondary origin of the dolomite so common in the Cambrian dolomite (p. 44).

Reviewing the special report on lead and zinc in Kansas bv Haworth, Crane and Rogers, Bain wrote:

. the argument for secondary introduction of magnesia built up on the distribution of the dolomite along cracks and fissures, is believed to be good, and sufficiently excludes the dolomite in question from the regional dolomite of earlier age, or even certain bedded dolomities found in the Carboniferous, south ef Carthage. It is in the main true that dolomite is not present in the very important ore bodies found in the

7 Buckley, E. R. and Buehler, H. A., “ Geology of the Granby Area, Missouri,” Missouri Bur. of Geol. and Mines, vol. 4, second series, 1906.

8 Buckley, E. R., Geology of the Disseminated Lead Deposits of St. Francois and Washington Counties, Missouri, Mo. Bur. of Geol. and Mines, vol. 9, pt. 2, pp. 40-121, 1909.

826 D. F. Hewett.

flint. If, however, magnesia were carried in the ore solutions, it might well fail of precipitation when in contact with a chert, and be precipitated in volume where limestone formed the country rock.®

Later, Siebenthal wrote:

Dolomite occurs in three forms in the Joplin region. A fine grained buff variety evidently resulted from the dolomitization of the limestone in place without alteration of the bedding, for such beds of dolomite may at places be followed directly into unaltered beds of limestone. Though nowhere associated with areas of mineralization, this form of dolomite is usually found where the rocks have been considerably disturbed by solu- tion. This kind of dolomite is similar in texture to all the widespread formations of dolomitic limestone. A second variety of dolomite, usually called “gray spar,” occurs in large bodies of massive coarse granular texture, which lie adjacent to the ore in many of the ore-bodies of the area. The crystals of dolomite in this variety of the rock are many times larger than those in the variety first mentioned, and show the curved faces characteristic of the mineral. The third variety of dolomite is known as “ pink spar ” and occurs in veins in both the foregoing varieties, or forms the lining of cavities or pockets inthem. This variety and calcite line cavities in the ore bodies in many places where the massive varieties of dolomite do not occur, as, for instance, in chert ground.

(P. 15) The deep well waters of the region, derived from the Cambrian and Ordovician dolomite, contain much more magnesium in proportion to the calcium that do the waters derived from the Mississippian limestones or from the Pennsylvanian shale, though they contain less silica. The deep well waters resemble sea water which is the most common agent ot dolomitization. These waters are therefore probably the agents of dol- omitization and siliceous replacement, as well as of mineralization.

Oklahoma; Bromide District——The alteration to dolomite of the limestone adjoining a manganese deposit near Bromide, Okla- homa, has been studied by the writer." In this region, several manganese deposits containing hausmannite and manganiferous carbonates replace Ordovician and Silurian limestone along per- sistent faults. Analyses of the wall rock and vein material show a progressive increase in the magnesia, iron and manganese con-

® Bain, H. F., Review in Econ. Geot., vol. 2, pp. 190-191, 1907.

10 Siebenthal, C. E., “Origin of the Zinc and Lead Deposits of the Joplin Region,” U. S. Geol. Surv. Bull. 606, pp. 15, 183-184, 1916.

11 Hewett, D. F., Manganese deposits near Bromide, Oklahoma. U. S. Geol. Surv. Bull. 725, pp. 311-325, 1921.

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Dolomitization And Ore Deposition. 827

tent of the wall-rock limestone toward the fault. From a light gray earthy limestone, the alteration has produced a darker, harder and denser rock. Doubtless, the alteration has been pro- duced by magnesia-bearing solutions rising from considerable depth along the faults. No igneous rocks that may be related to these deposits have been found in the region.

Virginia and Tennessee.—According to Watson,” “ Dolomite is much more common than calcite over the Virginia area. Like calcite, it occurs largely as composing the country rock and in crystallized massive form filling spaes between the broken lime- stone fragments.” In Tennessee, the country rock, surrounding the lead and zinc deposits, commonly fine-grained gray dolomite, is replaced by or cemented by secondary white dolomite, which contains the sulphides of zinc and lead. Localiy chert replaces both varieties of dolomite,** and some dolomite is deposited after the sulphide minerals.

On the basis of a recent brief visit to the zinc mine of the American Zinc Lead and Smelting Company of Mascot, Ten- nessee and to other mines near Jefferson City, Tennessee, Mr. E. T. McKnight of the U. S. Geological Survey has concluded that the ore-bodies of those areas are enveloped in secondary dolomite which replaces limestone.**

Colorado, Aspen District—In 1898 Spurr wrote:

Along these main faults and fractures, however, the blue limestone at the top of the Leadville formation has been altered irregularly for vary- ing distances on each side of the fractures into dolomite, microscopically identical with that formed at the earlier period. This later dolomite forms wherever the rock has been open to circulating waters (p. 208).

This dolomite (the bedded dolomite) as also that which originated later, always contains some iron and silica, which were introduced probably at

12 Watson, T. L., “Lead and Zinc Deposits of Virginia,” Va. Geol. Surv. Bull. I. Pp. 42, 1905.

13 Secrist, M. H., “ Zinc Deposits of East Tennessee,” Tenn. Dept. of Education Division of Geology, Bull. 31, p. 165, 1924.

14 Personal communication.

15 Spurr, J. E., “ Geology of the Aspen Mining District, Colorado.” U. S. Geo! Surv., Mon. 31, pp. 206-216, 1898.

828 D. F. Hewett.

the same time as the magnesia. On the other hand, analyses of the locally dolomized zones which traverse the blue limestone show all transi- tions from a pure dolomite into a pure limestone, the dolomization grow- ing less as the distance from the fracture along which the dolomizing solutions apparently flowed, increases (p. 209).

The local dolomization almost invariably accompanies the ore. Even when the latter is in blue limestone, there is usually a sort of envelope of dolomite around it, which in turn is surrounded by limestone. In rare cases, when the ore is directly enclosed in blue limestone without such an envelope, its analysis shows the presence of magnesia, while the lime- stone is almost pure (p. 210).

In the Aspen district, Spurr also observed the widespread deposition of silica (jasperoid) along faults and fractures by replacement of the earlier limestone and dolomite. Both the jas- peroid and later dolomite are ferruginous and the conclusion is reached that magnesia, silica and iron oxide were deposited es- sentially simultaneously during the period of ore deposition.

Colorado, Red Cliff —At the Eagle Mines in the Red Cliff dis- trict,*° a nearly horizontal group of rocks, quartzites, magnesian limestones and sandstones, have been intruded by a sill of quartz- monzonite porphyry. The sill is underlain by a tabular body of sulphide minerals, blende, galena, and pyrite, which replaces dolomitic limestone and merges with an enveloping shell of man- ganiferous siderite. This, in turn, is surrounded by a less definite shell of coarsely crystallized gray dolomite which merges with the enclosing magnesian limestone. The authors conclude that both dolomite and siderite replaced the limestone after brecciation.

Utah, Tintic District—The deposition of secondary dolomite in carbonate rocks in parts of the Tintic District, Utah, is re- corded by Lindgren and Loughlin." To quote:

The distribution of dolomite (bedded dolomite) has no relation to degree of folding, to any kind of fracturing, or to ore deposition. An apparent

16 Crawford, R. D., and Gibson, R., “ Geology and Ore Deposits of the Red Cliff District, Colorado,” Colo. Geol. Surv., Bull. 30, pp. 55-56, 75-79, 1925.

17 Lindgren, W., and Loughlin, G. F., “Geology of the Ore Deposits of the Tintic Mining District, Utah,’ U. S. Geol. Surv. Prof. Paper 107, p. 91, 159, 184.

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Dolomitization And Ore Deposition. 829

exception may be the occurrence of veins and impregnations of white to pink dolomite spar, but these are distinct from the dolomite beds, are limited to the zone of mineralization, are found in dolomites and argil- laceous limestones alike, and are clearly of later origin and contem- poraneous with the ore deposits (p. 91).

In some of the outlying mines in North Tinctic and East Tintic dis- tricts galena is found in direct intergrowth with secondary dolomite or replacing limestone or dolomite. These occurrences are interpreted as nearly the last phase of mineralization at places where the depositing solutions have become weak and relatively cool. At these places the galena generally contains but little silver (p. 159).

Dolomite being less soluble, was deposited in fractures surrounding the ore zone. In the peripheral parts of the district, dolomite and calcite, with some galena and zinc blende and only minute amounts of quartz, were the only minerals deposited (p. 184).

Briefly stated, the Tintic district presents a thick section of Paleozoic limestone, bedded dolomite, sandstone and shale, folded and faulted, then deeply eroded prior to the extrusion in Tertiary time of great masses of rhyolite and latite, and the intrusion of one large and several small bodies of monzonite. Contact meta- morphism is shown both in the monzonite and near-by sediments, but the outstanding rock alterations include silicification of the sediments, sericitization of the monzonite, and sporadic dolo- mitization of the carbonate rocks in a zone several miles distant from the outcrops of the monzonite. Both the sericitization of the monzonite and the silicification of dolomite have driven off their contained magnesia, and it seems plausible that these are the principal local sources of the magnesia that accomplishes the dolomitization observed.

Nevada, Goodsprings District—The study of this district by the writer during 1921-1922, showed that large masses of pure limestone that form beds in Devonian, Mississippian and Pennsyl- vania formations, have been converted to nearly pure dolomite, most abundantly and conspicuously near the ore bodies. The evidence of alteration includes (1) change in color from light or dark gray of the limestone to cream-colored dolomite, (Figs 1, 2) (2) change in texture from dense porcelain-like limestone to

Fic. 1. View of a ridge near the Silver Gem Mine, Goodsprings Dist., Nevada, showing distribution of dolomitized limestone. The area shown in Fig. 2 lies in the bottom of the ravine.

Fic. 2. Close view of dolomitized zone (light) in limestone (dark) Ravine near Silver Gem Mine, Goodsprings District, Nevada. The thin section represented by Fig. 3 was collected from the limestone on the right; that represented by Fig. 4, from the center of the white zone. The limestone contains 94 per cent. calcium carbonate and 5.9 per cent. quartz grains; the dolomite contains 2.0 per cent. calcium carbonate, 93.0 per cent. dolomite molecule, and 5 per cent. quartz.

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DOLOMITIZATION AND ORE DEPOSITION. 831 crystalline dolomite, (Figs. 3, 4) (3) change in composition from material containing 95 to 98 per cent. calcium carbonate to material containing 90 to 95 per cent. doiomite molecule. AAl- though the areas of alteration are sporadic, both areally and stratigraphically, it is estimated that the quantity of magnesia added is sufficient to make a layer of magnesium carbonate about 100 feet thick throughout the 225 square miles examined, ob- viously a tremendous quantity. The principal masses of intrus- ive rocks in the area are sills and dikes of granite porphyry, largely localized in three areas, each scarcely a mile in. diameter.

Of the ore bodies of seventy-five mines examined in the dis- trict, only two were found which were not enveloped in dolomite and these were on the outer edge of the district. Most of these ore bodies contain only lead and zinc minerals but a few contain copper, cobalt and nickel minerals. From the standpoint of para- genesis, the sulphides of lead, zinc, and copper largely lie in breccias of dolomitized limestone but in part replace the dolomite, so that dolomitization has preceded deposition of sulphides. There is little hypogene silica in the district.

California—Cerro Gordo District—According to Knopi dolomitization of Carboniferous limestone is found near several ore-bodies of the Cerro Gordo mine and marblization of lime- stones is found nearby.

The west wall-rock of the Union (or China) stope is distinctive and is locally known as “altered lime.” Instead of the dense white marble or limestone that generally incloses the ore bodies, the wall rock consists of a moderately coarse-grained curved dolomite spar more or less stained with manganese. In places, owing to the development of the dolomite in distinctive crystals, it resembles a porphyry closely crowded with white feldspar phenocrysts. The dolomite contains a few limonite pseudo- morphs after pyrite. Similar dolomite rock is found in the storehouse drift of the 400 level. This dolomitization of the ‘marble wall rock evidently accompanied the primary mineralization—it is not improbable that such dolomitized rock may serve as a guide in the search for bodies of argentiferous galena.

18 Knopf, A., “ Geologic Reconnaissance of the Inyo Range, Calif.,” U. S. Geol. Surv. Prof. Paper 110, p. 114, 1918.

832 D. F. Hewett.

Fic. 3. Thin section of limestone near Silver Gem Mine, Goodsprings Dist., Nevada. The section shows bryozoa, foraminifera and quartz grains. See Fig. 2. White line is 1. millimeter.

Fic. 4. Thin section of dolomitized limestone near Silver Gem Mine, Goodsprings Dist., Nevada. It contains 2.0 per cent. calcium carbonate, 93.0 per cent. dolomite molecule and 5. per cent. quartz. See Fig. 2. White line is 1. millimeter.

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Dolomitization And Ore Deposition. 833

The intrusive rocks of the area include monzonite porphyry, quartz diorite porphyry, and diabase, all considerably altered. The diabase shows much sericitization. The outcrops are not ex- tensive in the district.

Alaska, Kennecott—According to Bateman and McLaughlin, the Chitistone limestone is dolomitized near the chalcocite bodies of the Kennecott mine.*®

The gray limestone is fairly pure, yielding 3.9 per cent. MgCO3, whereas the sparkling gray dolomite rock averages about 30 per cent. MgCO3. The dolomitization was not connected with the immediate ore deposition, for it is a widespread feature; neither is it a stratigraphic phase, for the contact between the dolomite and gray limestone is an irregular surface, in some places cutting across bedding planes, following them in others, or terminating abruptly against small faults, along or normal to the bedding planes (p. 9).

The dolomitization of the limestone is not related to the ore formation for it is a widespread feature, whereas the ore is local. . . . Dolomitiza- tion is considered to have preceded the ore formation.

Intrusive rocks that may have played a part in ore-deposition are not recorded near the Kennecott Mine.

Other Districts—The descriptions of several other districts where the ores occur in carbonate rocks are not sufficiently de- tailed to permit a conclusion whether dolomitization is present or not.” According to Lindgren,” the process of contact metamor- phism at Bingham has involved an addition of silica, iron, sulphur, magnesia, alumina and soda, to the limestone as much as 500 to 2,000 feet from intrusive contacts.

The published descriptions of Mexican lead and zinc deposits consulted by the writer, which are largely in early Cretaceous limestone, do not refer to dolomite. The writer is advised by

19 Bateman, A. M., and McLaughlin, D. H., “Geology of the Ore Deposits of Kennecott, Alaska,’’ Econ. GEOoL., vol. 15, pp. 1-80, 1920.

20 Richards, R. W., “ Notes on Lead and Copper Deposits in the Bear River Range, Idaho,” U. S. Geol. Surv. Bull. 470, pp. 177-187, 1911.

Jenkins, O. P., Lead Ore Deposits of Pend d’Orielle and Stevens Counties, Wash- ington. Washington Dept. of Conservation and Devel., Div. of Geology, Bull. 31, Ppp. 127-130, 1924.

21 Lindgren, W., ‘“ Contact Metamorphism at Bingham, Utah,” Geol. Soc. Amer. Bull., vol. 35, pp. 507-534, 1924.

834 D. F. Hewett.

W. F. Foshag, who recently visited a number of these districts, that the lead and zinc deposits at the Los Lamentos mine in north- ern Chihuahua are enveloped in a dolomitized zone of the Cre- taceous limestones. The alteration was not observed at several other localities visited.”*

Europe, England—There is a large record of the occurrence and genesis of dolomite in Great Britain, but only meager ref- erence to dolomitization near metalliferous deposits, so far as the writer can find. The recent summaries of the occurrence of lead. zinc and copper in Great Britain contain the description of a single occurrence, that at the Boltsburn mine, Durham, England.*

22 Personal communication. 23 More important recent references are given below. a. Strahan, A., ‘‘ Geology of the South Wales Coal Field,’ Memoirs of the Geol. Survey, Part VIII, pp. 10-20, 1907. b. Trechman, C. T., “On the Lithology and Composition of Durham Magnesian Limestones,” Quart. Jour. Geol. Soc., vol. 70, pp. 232-265, 1914. c. Woolacott, D., ‘‘ The Magnesian Limestones of Durham,” Geol. Mag., Dec. vol. 6, pp. 452-465, 458-498, I9I19. -d. Parsons, L. M., “ Dolomitization and the Liecestershire Dolomites,” Geol. Mag.., Dec. 6, vol. 5, pp. 246-258, 1918. e. Parsons, L. M., Dolomitization in the Carboniferous Limestone of the Midlands,” Geol. Mag., vol. 59, p. 51-63, 104-117, 1921. 24 Memoirs of the Geological Survey, Special Reports on the Mineral Resources cf Great Britain, 1921-1923. a. Vol. XVII., The Lead, Zinc, Copper and Nickel Ores of Scotland,” by S. V Wilson. b. Vol. XIX., “ Lead and Zinc Ores in the Carboniferous Rocks of North Wales,’ by Bernard Smith. c. Vol. XX., “Lead and Zinc: the Mining District of North Cardiganshire and West Montgomeryshire,” by O. T. Jones. d. Vol. XXI., “ Lead, Silver-lead and Zinc Ores of Cornwall, Devon and Somer- set,’ by H. Dewey. c. Vol. XXII., “Lead and Zine Ores of the Lake District,” by T. Eastwood. f. Vol. XXIII, “Lead and Zinc Ores in the Pre-Carboniferous Rocks of West Shropshire and North Wales,” by B. Smith and H. Dewey. x. Vol. XXV., “Lead and Zinc Ore of Northumberland and Alston Moor, by S Smith. h. Vol. XXVI., ‘“‘ Lead and Zinc Ores of Durham, Yorkshire and Derbyshire,” by R. S. Carruthers and A. Strahan. See also Louis, H., Lead Mines in Weardale, County Durham,” The Mining Magazine, vol. 16, pp. 15-25, 1917. i. Vol. XXVII., “ Copper Ores of Cornwall and Devon, by H. Dewey.

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Dolomitization And Ore Deposition. 835

During a visit in 1926 to several of the lead-producing dis- tricts of England, the writer was able to study and confirm the presence of dolomitization at the Boltsburn mine and to establish its presence at the Millclose mine in Derbyshire but was unable to detect its presence at several other mines; Hudgill, Haggs and several mines near Nenthead, Cumberland ; Killhopeburn, Sedling, and Groverake mines in Durham, and Allenheads mines in North- umberland. In the following summary, statements concerning the regional geology are based upon the reports of the Geological Survey referred to, but sketches and descriptions of the dolo- mitized rocks at Boltsburn are based upon the writer’s observa- tions. For permission to examine the workings the writer is in- debted to Prof. H. Louis and other officials of the Weardale Lead Company.

The lead and zinc deposits of Alston Moor and Weardale are found in a group of shales, sandstones, and limestones, about 2,000 feet thick, which lie near the top of the Lower Carbonifer- ous measures of that region. These rocks strike northwest and dip northeast at less than 10°. The only intrusive rocks nearby include two sills of basalt which antedate the ore. The deposits include (1) simple, nearly vertical, filled fissures along which there has been little movement, and (2) “flats” which extend outward from the fissures into the limestone beds, principally the Great Limestone, 65 feet thick, near the middle of the section. The fissure filling is largely fluorite with less barite, quartz, and siderite. Dolomite, largely deposited by replacing the limestone, is the most abundant mineral in the “ flats ” at the Boltsburn mine, but the nearby fissure, 2 to 6 feet wide, is filled with fluorite and minor quartz and galena. Few veins have been explored more than 600 feet below the outcrop. The records indicate that more than half the lead output of the districts has come from the “flats” of a few mines (Allenheads and Weardale) and the re- mainder from the many veins.

In the Boltsburn mine, the Great Limestone is dark gray and the texture is porcelain-like except for sparse crystalline fossil fragments. There are meager traces of bedding underground.

836 D. F. Hewett.

’

There are three “ flats” of which the upper is the largest and most productive; locally, it is 8 to 10 feet thick and extends out- ward both ways from the vein from 25 to 50 feet. This “ flat ” (Fig. 5) is a tabular mass of dolomitized limestone breccia in

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Fic. 5. Generalized vertical cross-section of Boltsburn vein and upper “flat,” showing relation of dolomitized limestone to vein.

which there are several major flat slipping-planes and nearly verti- cal joints. The dolomite is uniformly light gray in color and coarsely crystalline; on the borders, the zone of transition to dark gray limestone is 1 to 2 feet wide. Galena occurs along the flat slips, vertical joints and sporadic open cavities, and locally re- places the dolomite breccia. In many places, a thin layer of siderite lies between the galena and the dolomite. No galena was found in the limestone. In the flats, fluorite forms perfect lilac- tinted cubic crystals as much as 4 inches in diameter confined to sporadic open cavities. Here and there, minute quartz crystals cover the fluorite.

Without describing additional details, the writer concludes that the limestone was sporadically shattered by movement along the vein, then replaced by dolomite and brecciated by further move- ment. This replacement may have produced some pore space The deposition of fluorite, galena and quartz followed the dolo- mite, essentially in the order named.

Igle eng wor area Fig. und nine larg

25 , et me F, an Sarda Eng. Iglesi Ital. I Iglesic

Dolomitization And Ore Deposition. 837

At the Mill Close mine in Derbyshire, only material on the dump was ‘examined. Here, somewhat as at the Boltsburn mine, galena and blende are found in vertical veins and in adjacent “ flats” which locally seem to be large open solution cavities, in a bed of limestone between shale above and a basalt flow below. The material on the dump showed a thin zone of porous dolomite separating the normal gray to cream limestone and the veins and masses of mixed fluorite and galena. A few specimens showed veinlets of barite and pyrite replacing the limestone without the layer of dolomite.

Italy, Sardinia.—The Iglesias district in southwestern Sardinia presents a very impressive display of dolomitization of large masses of limestone in the neighborhood of lead and zinc deposits. Through the courtesy of Sigs. P. Stephani of the Portusola Co. and A. Binetti of the Monteponi Co. the writer was able to de- vote four days during early May, 1926, to an examination of the surface geology and some of the underground workings of the Montiponi and San Giovanni mines.

There are many reports on the geology and mines of the Iglesias district by Italian, French and German geologists and engineers.” The recent reports by Novarese are based upon field work in 1919 that involved the preparation of a geologic map ot area 9 by 11 kilometers, a generalized copy of which is given in Fig. 6. This work shows that a large part of the district is underlain by Middle Cambrian and Silurian rocks, separable into nine units, but there are also several small areas of Permian and larger areas of Tertiary and Quaternary rocks. In Fig. 6 the

25 A good summary of these reports is presented by De Launay,” Gites mineraux et metalliféres,” vol. III., p. 211, 1913. More recent references follow: Sartori, F. and Testa, L., “ Sulla struttura della dolomia metallifera,” Associazione minereria Sarda, Anno. 19, pp. 25-27, 1914. Wright, C. W., ‘“ Calamine Mines of Sardinia,” Eng. Min. Jour., vol. 100, pp. 625-628, 1915. Novarese, V., I1 Cambriano dell’ Iglesiente, Attic.,” R. Accad. Lincci, Rendic., Ser. 5, vol. XXIX., part 2, pp. 56-58, 1920. Novarese, V., “Cenni Sommarii Sul Paleozoico dell’ Iglesiente,” Soc. Geol. Ital. Bull., vol. XLI., pp. 316-325, 1923. Novarese, V., “ Contributo alla Geol. dell’ Iglesiente, La Serie Paleozoico,” Boll. R. Uff. Geol. Ital., vol. XLIX., pp. 1-107,

838 D. F. Hewett.

Permian rocks are omitted and the younger rocks shown by a single pattern. The oldest Cambrian rocks are reddish and green sandy shales and sandstones, which occupy a central belt and do

\NY Mort

SS . : *GENNARUTA aS. So~- IGLESIAS S

AGRUZIAU MONTEPONIg 5 bi ac Aa

Ht

Yy’ So San Giorgio

Yj Yj SAG

Gonnesa

5 ta LEGEND MIDDLE CAMBRIAN MIDDLE CAMBRIAN CUYMIDDLE CAMBRIAN (7 SILURIAN, PERMIAN RED AND GREEN LIMESTONE AND SHALE AND YW Serre’ (ss DOLOMITE AN CONGLOMERATE 1]; QUATERNARY. DIABASE SECONDARY UNDOLOMITIZED PRINCIPAL DIKES SILICA CE LIMESTONE /FAULT RF MINES

Fic. 6. Geologic map of the Iglesias district. After Novarese.

not contain any ore deposits. The overlying ore-bearing lime- stone, about 2,000 feet thick, is separated into three units, a shaley limestone 100 feet thick, overlain by cream and gray dolo- mite, in the lower cream-colored half of which there are numerous sporadic masses of dense gray limestone. Novarese mapped only these three units, but the writer was impressed that the upper gray dolomite is distinctly separable for mapping purposes and has had a different origin than the lower cream dolomite. The upper gray dolomite shows persistent bedding planes and bears evidence that

it is wer sma tion mite crea brok depc have have the ¢ rock glom depo stock way, but t soutl they there are p cordi after forms No by th tion, dolor a 3 ki and i1 on the of lin brian to the limesti

Dolomitization And Ore Deposition. 839

it is a bedded dolomite, very little if any altered since the beds were buried and consolidated. It contains very few and relatively small ore deposits. By contrast, the distribution and border rela- tions of the gray limestone masses in the lower half of the dolo- mite unit show clearly that it was originally limestone. The cream dolomite is without persistent bedding planes but is much broken by joints and fractures, and it contains most of the ore deposits of the region. At least several Italian geologists who have studied the region, Zoppi, Sartori, Testa and Novarese, have stated this conclusion and have been deeply impressed by the extent of dolomitization. The higher Cambrian and Silurian rocks are largely sandstones and shales with thin layers of con- glomerate and limestone. The only intrusive rocks near the ore deposits are small dikes of diabase (Fig. 6) but there is a small stock of granite about 10 miles northeast of Iglesias. Ina broad way, the bedded rocks form a sharp anticline which trends west, but there are local deviations from a simple pattern. Along the south limb, the lowest Cambrian rocks dip nearly vertically but they dip less steeply on the north limb. Near Monteponi mine there appears to be a local syncline that trends northwest. There are probably more faults in the area than appear in Fig. 6. Ac- cording to DeLaunay, the ore deposits assuredly were formed after the Hercynian deformation and, in part, may have been formed in early Tertiary time.

No observer of the Iglesias district can fail to be impressed by the greater number, great range in size and shape, distribu- tion, and border relations of the limestone masses in the cream dolomite. The writer examined about twenty masses that lie in a 3 kilometer belt near the Monteponi mine along the north limb and in a similar belt on the south limb from San Giovanni mine on the west to San Georgio mine on the east. Clearly, the masses of limestone are confined to the lower half of the Middle Cam- brian dolomite, and where determinable, the bedding conforms to that required by the local structure. The contacts of the limestone masses with the surrounding dolomite are sinuous in

840 D. F. Hewett.

Ls

Bes, Ss Fic. 7. View of contact of limestone and dolomite resulting from its

alteration. Monteponi Hill, Iglesias District, Sardinia. A Brunton com- pass rests on the contact.

Limestone sytheta. wy a

Fic. 8. View of contact of limestone (light) and dolomite (dark) result- ing from its alteration. Monteponi Hill, Iglesias District, Sardinia.

W' nc pe

Dolomitization And Ore Deposition. 841

larger outline as well as in detail, and commonly are sharp and well-defined. The accompanying photographs were taken on the north side of the largest irregular body southwest of the Monte- poni mine (Figs. 7,8). Startling as it may seem, the writer can- not avoid the conclusion that these limestone masses are all that re- mains, scarcely 10 per cent. of the original volume, of a zone of limestone about 1,000 feet thick and at least 8 miles long. Al- though no analyses of the limestone are at hand, Sartori and Testa present 6 analyses of the resulting dolomite, in all of which the dolomite molecule exceeds 96 per cent. If, as it appears, the alteration of the limestone has taken place without appreciable change in volume, and extends a kilometer below the present sur- face, a simple calculation will show that approximately 1.25 cubic kilometers of magnesium carbonate have been added to the area by displacing a part of the calcium carbonate.

The sulphides, galena, and blende, of the ore deposits of the region are largely altered to oxidized minerals near the surface, so that only the deeper work reveals the detailed relations of the sulphides to the dolomitization. At the Monteponi mine, oxidized zinc minerals persist from the outcrop, about 340 meters above sea level, to slightly below sea level. According to local report as well as the printed record, most of the weathered ore bodies occur in or near fractures in the dolomitized limestone, but a few do occur in unaltered limestone. The writer was able to see such bodies on the lowest level of the Monteponi mine and in the main tunnel of the San Giovanni mine. The large open cut on Monte- poni hill explores numerous shoots of oxidized zinc minerals in a shaley zone of the dolomitized limestone.

Italy, Alps —The belt of Upper Triassic dolomites that extends for more than 300 miles along the south slope of the Alps in northern Italy contains numerous lead and zinc deposits, the most important of which are localized at Bleiberg, Raib!, and Auronzo; others are found as far west as Belluno on the Adda River.”

26 DeLaunay, L., “La metallogenie de I’Italie,” pp. 22-26, and map, 1906. Mexico.

842 D. F. Hewet1.

There are many descriptions of these deposits and much has been written concerning the origin of the dolomite in the beds in which the deposits are contained.** In April, 1926, through the courtesy of Mr. C. W. Wright of Merano, the writer was able to visit two mines near Auronzo, and through the courtesy of Mr. C. C. Williams of Bewick, Moreing & Co. spent a day under- ground at Raibl.

According to Kraus, the Triassic beds near Raibl show the following succession, beginning with the lowest and oldest:

Lower Triassic: 1. Werfener shales. 2. Buchenstein beds ; conglomerate, sandstone and gray shale.

Upper Triassic: 3. Wengen beds. 4. Ore-bearing dolomite, several thousand feet thick. 5. Raibl beds, cherty limestone and shale.

. Principal dolomite.

On

These beds trend west and dip south about 45°. The Seebach Valley trends north and therefore exposes a complete dip cross- section of the beds on both sides of the valley. A small intrusive mass of felsite-porphyry outcrops at Kaltwasser, a mile north of Raibl. Most of the ore deposits, as well as the mine workings, lie under Konigsberg, a mountain on the west side of the valley.

27 Posepny, F., “Die Blei-und Galmei-Erzlagerstatten von Raibl in Karnten,” Jahrbuch der K. Kénig. Geol. Reichsanstalt, vol. 23, pp. 317-420, 1873.

The summary by Hupfield, Zeit. fiir. prakt. Geol., vol. 5, pp. 233-247, 1897, con- tains an exhaustive bibliography from 1783 to 1896. More recent important ac- counts follow: Posepny F., ‘‘ The Genesis of Ore Deposits,” Amer. Inst. Min. Eng., vol. XXIII, p. 289, 1893. Geyer, G., “ Zur tectonik des bleiberger Tales in Karn- ten,” Verhand. der K. K. Reichsanst., pp. 338-359, 1901. Kraus, M., Das Staat- liche Blei-Zinkerz-Bergbauterrain bei Raibl in Karnten, Berg- und Huttenmanni- sches Jahrbuch, vol. 61, pp. 1-83, 1913. Mackay, R. A., “ The Influence of Super- imposed Strata on the Deposition of Certain Lead-zinc Ores,” Inst. Min. and Met. Bull., 254, p. 14, 1925.

28 See especially Skeats, E. W., ‘On the Chemical and Mineralogical Evidence as to the Origin of the Dolomites of Southern Tyrol,” Quart. Jour. Geol. Soc., vol. 61, pp. 97-141, 1905.

Dolomitization And Ore Deposition. 843

The unweathered ore bodies are found only in the ore-bearing dolomite and have two general structural associations: (1) the less important bodies lie along several northward-trending faults; Abendblatt, Morganblatt, Vincenzi-Aloisiblatt ; (2) the more im- portant bodies are roughly tabular but lie nearly parallel to the bedding, principally between the Abendblatt and Morgenblatt, 100 feet or more below the overlying Raibl beds. The recently dis- covered Josephi ore-body is roughly parallel to this second group but nearly 1,000 feet below the overlying Raibl beds.

At an early period it was thought that the tabular bodies were true beds but since Posepny’s work of 1873, most have agreed that the sulphides were deposited in open cavities adjacent to minor fractures that are connected with the major faults. Posepny, as well as later observers, recognized that dolomite was deposited before, during and after the metallic sulphides, and that the earliest dolomite replaced the wall-rock limestone whereas the latest filled the open spaces remaining. All have recognized the presence of white crystalline dolomite in the orebodies and agreed that it was the latest mineral to be deposited in the open spaces.

Prior to the writer’s visit, according to C. C. Williams, Mr. Malcolm MacLaren also concluded that the dolomitization of the wall-rocks was related to ore-deposition, and that dolomite formed an aureole around the ore bodies, and advised the determination of magnesia content in exploration workings. After investiga- tions extending over more than a year, it has been found that the magnesia content of the wall rock decreases outward from the Abendblatt and Morgenblatt but is nearly constant in the 50- meter zone between them. At these faults, the rock is nearly pure dolomite, but the magnesia content decreases steadily out- ward for 20 to 40 meters, where the rock is nearly pure limestone. Nearly the same conditions surround the Vincenzi-Aloisi fault, 400 meters east of the Morgenblatt, where the magnesia content decreases outward from the west wall but the east wall is lime- stone.

In examining the stopes and several thousand feet of work- ings with Mr. Williams, the writer completely confirmed the

844 D. F. Hewett.

conclusions above stated. by testing the wall-rock with hydro- chloric acid. In contrast with the opinions of Kraus, Posepny and others, however, who assumed ore deposition in solution cavities, the writer reached the conclusion that the sulphides were deposited in open breccias, that they were later brecciated and the spaces filled or nearly filled with white dolomite. In places (Aloisi-blatt), the white dolomite forms from 20 to 30 per cent. of the mined orebodies. It was extremely impressive to note the relation of dolomitization to major fractures and ore-zones and to find this relation practically used in exploration for unknown orebodies.

The writer’s examination of the two mines near Auronzo was unfortunately too brief to permit more than the general conclu- sion that the orebodies were entirely enveloped in dolomite and that limestone was present in similar beds remote from the ore- bodies. It was not possible to conclude, as at Raibl, that dolo- mitized rocks adjoined the fissures through which the metallic sulphides entered the ore-zones.

Concerning the dolomites of the southern Tyrol, Skeats wrote:

In the dolomites of the Tyrol, however, the dolomitization is not local, but on a very extensive scale. The Schlern dolomite (essentially the ore-bearing dolomite of Raibl. D. F. H.) extends over many square miles, and in places exceeds 3,000 feet in thickness, so that no local cause can explain its production over such large areas. The rock was with- out doubt originally a limestone composed entirely of organisms and was subsequently converted into dolomite. There can be no question that the magnesium was obtained from the sea water. Chief interest is centered in the conditions under which this partial replacement of calcium carbon- ate by magnesium carbonate took place (p. 133).

The writer cannot object to the general conclusions brought forth by Skeats but, on the basis of the recent work at Raibl, thinks that local dolomitization, long subsequent to burial of the beds, may be an important factor in many areas in the southern Tyrol.

Germany and Poland, Upper Silesia—The famous lead and zinc deposits of Upper Silesia are found in the lower Muschelkalk

Dolomitization And Ore Deposition. 845

beds (Triassic) about 120 meters thick.** Viewed broadly, these beds form a simple shallow syncline about 20 miles long which trends slightly south of east. Beuthen, Germany, lies near the west end, but the eastern part extends into present Poland. The same rocks extend irregularly northward beyond Tarnowitz where they are also ore-bearing. The western part of the field is broken by a number of small faults which trend north and northwest. Some of these contain the sulphide minerals and are therefore pre-mineral. The nearest intrusive igneous rocks are small stocks of melaphyre and porphyry about 40 miles southeast of Beuthen. Through the courtesy of the owners of the Bleischarley mine on the Polish border, 2 miles east of Beuthen, the writer was able to spend three days underground during July, 1926.

Near Beuthen, the stratigraphic section includes 50 to 80 meters of ore-bearing gray dolomite which is underlain by 1 to 4 feet of soft gray clay (vitrioletten acid-forming clay, because it contains disseminated iron sulphide) which in turn overlies 40 meters or more of thin-bedded gray shaley limestone, (sohlen- stein). The sulphide minerals, blende, wurtzite, galena, and marcasite, are concentrated in tabular breccia zones in the dolo- mite over the vitrioletten The average combined zinc and lead content is about 16 per cent., but it is locally higher. The recent German authors agree (1) that the ore-bearing dolomite was once limestone but was altered to dolomite before the sulphides were deposited, (2) that the sulphides were deposited from solutions rising along the faults.*°

Further it may be regarded as established that the ores were deposited from solutions, which, circulating within the Muschelkalk beds, effected

29 There is an extensive literature concerning these deposits. For complete bibliography prior to 1891, see Althans, R., “ Die Erz-formation des Muschelkalks in Oberschlesien,” Jahrb. der K. Pr. geol. Landesanstalt, vol. XII., p. 37, 1891. Also Beyschlag, F., “Uber die Erzlagerstatten Oberschlesien,” Deutsch. Geol. Gesell., vol. 54, pp. 9-10, 1902. Luchs, A., “ Die Bodenschatze Schlesiens.” Bar- tonec, F., “ Uber die Erzfuhrenden Triasschichten west-Galiziens,” Oester. Zeit. f. Berg.-und Huttenwesen, vol. 54, pp. 645, 650, 665-669, 1906. “ Handbuch des QOberschlesischen Industrie-bezirks,” Obersch. Berg. und Hutten. Verein. Band II., der Festschrift, pp. 42-52, 1913. Seidl, K., “ Die oberschlesische Zinkerz- lagerstatte, Zeit. d. Oberschl. Berg. und Hutten. Verein, pp. 762-776, 1927.

30 Beyschlag, Krusch & Vogt. Truscott translation, pp. 728-729.

846 D. F. Hewett.

their dolomitization. . . . The most probable of all, however, is the as- sumption that the solutions rose from depth along fissures, from which, following fractures and cracks, they spread laterally through the perme- able beds of the Muschelkalk.

In the several square miles of the explored area examined by the writer, the sulphides are confined to breccia zones at the base of the dolomite, and the layers of sulphides nearly surround sharp-angular fragments of dolomite. The vitrioletten was not evenly layered, but lay cn an undulating surface cut across the underlying limestone (sohlenstein). It was thin on high points and thick in the hollows, so as to suggest that it represented un- dissolved clay once part of beds of sohlenstein. Further, it seemed as though the brecciation of the dolomite and consequently the quantity of sulphides was greatest where the vitrioletten was thickest. These relations indicated that the dolomite had been formed from limestone before sulphide deposition, and that the breccia zones represented collapse breccias,” due to solution of the underlying sohlenstein, also before sulphide deposition. In the several square miles of workings examined hastily by the writer, no undolomitized limestone was found in the ore-bearing dolo- mite, but German authors state that the beds are limestone in nearby areas that yield no lead and zinc.

The following statement is quoted from a recent summary of the geologic features of the district :

The tectonic disturbances initiated the dolomitization and mineraliza- tion of the rocks. The fracture systems of the Triassic, as a concomitant of the tectonic disturbances, are to this day the paths of an extensive ground-water circulation. This same ground-water circulation has also in former times participated to a pronounced degree in the transforma- tion of the limestones into dolomites, and in the distribution, deposition, redistribution and alteration of the ores brought to the dolomites from depth. Through the combined operation of the most varied factors, the seemingly haphazard present picture of the Upper Silesian ore deposits originated ; yet their development, broadly has been simple.

31 Locke, A., The Formation of Certain Ore-bodies by Mineralization Stoping,”

Econ. GEoL., vol. 21, pp. 431-453, 1926. 32 “ Handbuch des Oberschesisches Industrie,” p. 48.

oc hal

sul

of lon

zon sev are roc bas the at loca zon: sills gabl thot tion and

zinc been the : othe:

33 B Miner

34 T Erzlag

hausti 35 B to it.

Dolomitization And Ore Deposition. 847

weden.—According to Vogt the lead-zinc deposits at Sala occur in the dolomitized part of a limestone body surrounded by halleflinta and granite. The author concluded that the dolo- mitization of the limestone preceded deposition of the metallic sulphides.

Greece-—Laurium: The silver-bearing lead and zinc deposits of Laurium are sporadically distributed over an area 12 miles long by 6 miles wide, that nearly coincides with a broad anticline in Cretaceous rocks. The local geologic section includes three zones of limestone separated by layers of shale, aggregating several thousand feet in thickness. The unoxidized orebodies are largely tabular and lie at three zones in the succession of rocks. The most productive zone or “contact” lies near the base of the lowest limestone and is explored at Camaresa near the crest of the anticline. The highest “contact” is explored at Plaka, 4 miles north on the east flank of the anticline. The local igneous rocks include a stock of unaltered quartz mon- zonite 1,500 feet in diameter near Plaka; sporadic thin dikes and sills of a similar rock, “ eurite,’ and some sills of serpentinized gabbro largely localized on the east side of the district. Al- though several authors record the presence of dolomitization in the district,” the writer was unable to detect it by close examina- tion of several thousand feet of underground work at Camaresa and Plaka shafts and the nearby surface.

Other Districts—In a number of other districts where lead and zinc occur in carbonate rocks, the process of dolomitization has been observed; in some places, the observer has concluded that the alteration is related to the process of sulphide deposition ; in other places, that there is no relation between the two. The cir-

33 Beyschlag, F., Krusch, P., and Vogt, J. W. L., ““ The Deposits of the Useful Minerals and Rocks,” Translation by Truscott, S. J., pp. 771-772, 1916.

34The account by Lepsius, “ Die geologischen Verhaltnisse der Laurischen Erzlagerstatten,” Zeit. Prakt. Geol., vol. 4, pp. 152-157, 1896, contains an ex- haustive bibliography, 1635-1805.

35 Beyschlag, Krusch and Vogt, loc. cit., p. 748. De Launay does not refer to it.

848 D. F. Hewett.

cumstances under which this summary is prepared do not warrant a review of the local conditions. Dolomitization of the wall rocks is probably present in several districts in Asturias, Spain the Vielle-Montagne-Moresnet district on the Belgian-German border and in Baden,* and in other localities in France, Algiers, Tunis and Morocco.

Summary Of Features.

Occurrence.—The foregoing summary shows that the limestone walls of many zinc and lead and of a few copper deposits are altered to nearly pure dolomite. Doubtless with further research record could be found of similar alteration in numerous other districts. It should be noted also that in some districts where the alteration might be found, a careful search has failed to reveal its presence (i.e., Laurium). Similarly, in most districts, notably Goodsprings, Nevada, and Iglesias, Sardinia, although most of the ore-deposits are surrounded by an aureole of dolomitized lime- stone, there are a few which assuredly are enveloped by limestone.

Physical Properties of the Dolomitized Limestone-——In most places, the process of dolomitization produces changes in the color, texture, and porosity of the limestone. The color of lime- stones ranges from nearly white through gray to black, as well as from cream through yellowish to brick red. Within the writ- er’s experience, the color of the replacing dolomite on fresh frac- ture is nearly uniform, the range being from cream to pale brown- ish, the darker colors being observed on weathered surfaces. There is generally, therefore, a striking contrast in color between a dark limestone and its dolomitized product; in many cases how- ever, the lighter limestones show only slight change in color after dolomitization. With regard to texture, the dolomite resulting from the alteration of limestone is commonly more uniformly and more coarsely crystalline. At one place near Goodsprings,

36 DeLaunay, L., loc. cit., vol. III., pp. 198-199.

37 DeLaunay, L., loc. cit., vol. III., p. 199.

38 Beyschlag, Krusch and Vogt, Joc. cit., pp. 737-738. DeLaunay, L., loc. cit., PP. 199-206.

alt In

stc sec dol tha nut suc

dol

cor

irot

lect

orig

con

Scie

Dolomitization And Ore Deposition. 849

Nevada, a dark, dense limestone has been altered to a nearly white holocrystalline dolomite largely made up of crystals 5 to 10 mm. in diameter. At Raibl, Italy, however, both the limestone and its alteration product are finely crystalline. Concerning po- rosity of the rocks, there is little precise data. The range in porosity of six limestones from Goodsprings is from 0.36 to 1.49 per cent., and the range in the porosity of six dolomites resulting from their alteration is 1.69 to 3.20 per cent. The determined porosity of the dolomite does not properly reflect the conditions, however, for polished sections as well as analyses show that they contain sporadic patches of clear calcite which fill drusey cavities in the dolomite. The calcite was deposited after the alteration to dolomite was complete, and if it were dissolved, the porosity would range from about 3 to 6 per cent., probably an average range for most secondary dolomites.

Commonly, the zone of transition between limestone and its altered product is narrow and sharply defined. (See Figs. 1, 2). In a few places, the transition zone is poorly defined (Bromide, Okla. and Raibl, Italy) and as much as 50, or even 100 feet wide.

The microscopic properties of only a few dolomitized lime stones have been studied by the writer. Figs. 3 and 4 show thin sections of a Pennsylvanian limestone from Goodsprings and its dolomitized equivalent. These show what is widely recorded, that the process of alteration commonly destroys all traces of mi- nute fossils, such as foraminifera, and most of the larger fossils such as corals, brachiopods and gastropods.

The writer has not determined the optical constants of any dolomitized limestones.*°

Chemical Properties of the Dolomitized Limestones—It is re- corded in many places that dolomitized limestones contain more iron than the original rock, and the analyses of specimens col- lected by the writer confirm this. In the Goodsprings district the original limestones. contain mere traces of iron but the dolomite contains from trace to 0.26 per cent. This is less than the com-

39 Ford, W. E., “ Studies in the Calcite Group,” Trans. Conn. Acad. Arts and Sciences, vol. 22, pp. 211-248, 1917.

850 D.F. Hewett.

mon range, however. In a few places (Bromide, Okla.), small, as well as large, percentages of manganese also are added during the process of alteration to dolomite.

A comparison of the analyses and the polished sections of speci- mens of dolomitized limestones from Goodsprings, Nevada, shows that they are mixtures of almost pure dolomite (CaCO,: MgCO, =1:1) and a little calcite, rather than isomorphously mixed calcite and dolomite.

Quantitative Elements of Dolomitization—There will be dis- cussed here (1) the quantities of magnesia contributed per unit volume of limestone under several different assumptions and (2) the general extent of the dolomitized masses in area and depth. If it were possible to convert a cubic decimeter (1,000 c.c.) of compact limestone (specific gravity 2.715) into dolomite (specific gravity 2.870) without loss of any lime, the resulting volume would be 1,743 c.c. or an increase of 74.3 per cent. So far as the writer has observed or can learn from the literature, evidence of increase in volume has not been found at any place. On the contrary, large bodies of dolomitized limestone show no disturb- ance of bedding-planes or other evidence of increase in volume. To convert a cubic decimeter of limestone into dolomite without change in volume, involves the addition of 628 grams of MgO and 178 grams of CO, and the loss of 646 grams of CaO. If the dolomite has any porosity, however, the quantities of MgO and CO, added are diminished proportionately. According to the writer's experience, dolomitized limestones near ore deposits are commonly slightly porous but the porosity is rarely more than 5 per cent. It is clear therefore, that for each 1,000 c.c. (2,715 grams) of limestone converted to dolomite under these circum- stances, about 600 grams of MgO have been added, or per cubic meter of limestone, about 600 Kgr. of MgO.

Of the districts which have come within the writer’s observa- tion, the Iglesias district, Sardinia, presents the greatest extent of dolomitized limestone, but this is closely followed by Good- springs, Nevada, and Upper Silesia. A simple calculation will

sho

Dolomitization And Ore Deposition. 851

show that of the productive area of the Iglesias district more than go per cent. of a mass of rock 12 kilometers long, 400 meters thick, and 1 kilometer deep (assumed), or about 5 cubic kilo- meters, has been converted to dolomite. The magnesia necessary to make this change, calculated as solid magnesium carbonate or magnesite (specific gravity 3.08) would have a volume of roughly 1.25 cubic kilometers. This calculation gives a measure of the tremendous quantity of material for which not only a competent mechanism of transfer but also a competent source, must be imagined.

A preliminary review of the mineralogic changes of wall rocks that appear to have some relation to epigenetic ore-deposition in- dicates that a number of elements are added and that, since the changes in volume of the rock are small, a corresponding quantity of other elements is eliminated. As shown by Lindgren,*® the kind and degree of alteration probably depend largely on the temperature and pressure prevailing at a given place or zone, on the character and concentration of the solutions, and in part on the character of the nearby wall-rock. Although much is known concerning the elements added and elements eliminated and their quantity per unit of volume, little is recorded concerning the areal and vertical extent of the altered masses. The elements most commonly added include hydrogen and oxygen combined as water, silicon as silica, carbon as carbon dioxide, sulphur as sulphides and sulphates, fluorine, boron, potassium, magnesium, iron and the many other metals and metalloids. If the quantity of the ele- ments added per unit of volume and the apparent areal extent of alteration be considered, it seems that the quantity of magnesia added during dolomitization attains the same order of magnitude as that of silica, potash, etc., in areas where silification and sericit- ization are found.

Geologic Range of Dolomitized Limestone-——The process of alteration of limestone to dolomite near lead and zinc deposits has been observed in rocks that show a wide range in age. The fol-

40 Lindgren, W., “ Mineral Deposits,” pp. 478-487, 549-562, 653, 659-664, 1910.

852 D. F. Hewett.

lowing table presents a summary of the age of the beds mentioned in this article.

Age of Beds. Localities. BNGASNC teeta ese erie. tans oo Italian Alps Upper Silesia, Poland BORNSUINARIAN yo wider win 6 chelagionyesewes Goodsprings, Nevada DRESSISRIDDIAD, 6 5.5. a stes one ge chaste Aspen, Colorado

Tintic, Utah

Goodsprings, Nevada

Cerro Gordo, Cal.

Southwest Missouri

Central and northern England

EERO ee oe es el da. Goodsprings, Nevada

ST teh aera aome Se ea Bromide, Oklahoma ORAIGVIOIAN: =.usooae Shs eeenc eet 2 see Tennessee, Virginia, Oklahoma CATH EET 0 ee a AR es ee cesta. Southeast Missouri

Iglesias, Sardinia

In most of these districts, the process of dolomitization here considered is much younger and related to an epoch of ore-deposi- tion.

Local Relations of Sulphides and Dolomite —\Vithin the writ- er’s experience, the sulphides both replace dolomitized limestone and are deposited in open spaces which are largely breccia zones. In the Goodsprings district, both associations were observed but deposition in open breccias is more common. Oxidation has de- stroyed the evidence in many deposits however. In the districts of northern England, as well as in Upper Silesia and at Raibl, deposition of lead and zinc sulphides in the open spaces of angular breccias appears to prevail. Replacement is common in the Iglesias district and may be present in many mines. In most places, but especially at Raibl, Goodsprings, and Tennessee, the final mineral after the sulphides is dolomite, which commonly fills all remaining open space. Dolomite was not so noted in Upper Silesia.

From these associations the conclusion is reached that broadly the process of dolomitization of limestone seems to distinctly pre-

cede deposition of sulphides, even though here and there some dolomite is deposited later.

Fk a fe witl Siss (Uy sigr miti Igle few pro sulp and mag

k Ker be ¢ less dep rela neat ther othe posi dolo sma nort ther lies min ston succ

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Dolomitization And Ore Deposition. 853

Regional Relations of Sulphide Ore Bodies and Dolomite.—In a few regions, the record indicates that no ore bodies are found without an envelope of dolomitized limestone or dolomite (Mis- sissippi Valley) or in extensive masses of dolomitized limestone (Upper Silesia). In some other regions, and this seems to be significant, most of the sulphide bodies are enveloped in dolo- mitized limestone but a conspicuous few (Goodsprings and Iglesias) are assuredly enclosed in unaltered limestone. These few exceptions in areal relations raise the question whether the process of dolomitization coincides precisely in time with that of sulphide deposition. If the two processes do not coincide in space and in time, the question is then raised whether the source of the magnesia is the same as the source of the metals.

Relation of Dolomitization to Intrusive Rocks —Except in the Kentucky-Illinois fluorspar field, no igneous rocks are known in the entire Mississippi Valley or in Tennessee or Virginia that can be considered as competent to promote the circulation of, much less supply, the waters that deposited the zinc, lead and copper deposits of the region; it is widely believed that they have no relation to igneous rocks. Similarly, no igneous rocks are known near the productive parts of the Upper Silesian field although there are melaphyre intrusions nearly 40 miles southeast. On the other hand, intrusive masses of porphyries of intermediate com- position are sporadically distributed through the belt of Triassic dolomites south of the Alps, within which zinc and lead deposits are known. The only intrusives in the Iglesias district are sparse small bodies of diabase, although granitic bodies appear 10 miles northeast. In the Northumberland-Durham field of England there is a persistent sill of dolerite (Great Whin sill) which under- lies much of the productive ground and is probably earlier than the ore-bearing fractures. In central England, Derbyshire, many mine workings penetrate a sheet of basic igneous rock (toad- stone), but this is regarded as a surface flow laid down in the succession of sediments.

In the Goodsprings district there are numerous small dikes and

854 D. F. Hewett.

several larger sills of granite porphyry which were intruded before ore deposition. Dolomitization is widespread near the in- trusions but there are several large areas of dolomitized limestone as much as 18 miles distant from outcropping bodies of intrusive rock. It is possible that such rocks may be buried at no great depth. Near the areas of dolomitized limestone the intrusive is highly altered by sericitization.

At Aspen, Knopf “ has recently recognized five successive types of intrusive rocks, all of which precede ore deposition. These intrusives range in composition from diorite porphyry to aplite, and all except the latest show considerable alteration, mostly of the sericite type. They outcrop over a small part of the district but the actual distribution of dolomitized areas is not recorded.

At Cerro Gordo, California, the outcrops of the intrusives are meager in extent. They range from quartz diorite porphyry to diabase. All are considerably altered and the diabase shows much sericite.

A review of the districts in which dolomitization of limestone has taken place near metalliferous deposits leaves the writer with the impression that outcrops of pre-mineral intrusive rocks are either sparse and small or wholly absent. The varieties range from the most siliceous and least magnesian to the least siliceous and most magnesian, but the most common is near quartz mon- zonite. In most districts also, the intrusive rocks show extensive alteration to sericite or similar minerals.

It is noteworthy that there is no record of dolomitization in a number of districts such as Ely, Nevada, Bisbee and Tombstone, Arizona, Park City and Bingham, Utah, where there are rather sparse intrusives into thick sections of carbonate rocks. In most of these districts, copper is the most abundant metal sought and the intrusives are considerably altered. In some of these dis- tricts (Ely and Park City, as well as Cerro Gordo) marbleization of the limestones is recorded; possibly closer search might show that some of the marble is dolomite.

41 Knopf, A., “ Recent Developments in the Aspen District, Colo.,”’ U. S. Geol. Survey Bull. 785, p. 1-28, 1926.

Mi: Mi: Mis Mis Ter Nev Col Eng Ital Ital Ital Ital Upr

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Dolomitization And Ore Deposition. 855

Silver Content of Sulphides—The following Table I. sum- marizes the approximate or average silver content of unweathered and unenriched galena or metallic lead from ore-deposits in several regions showing dolomitization of the limestone wall-rocks. There is not complete uniformity in the tabulated figures but the writer is impressed that if allowance is made for several excep- tions, such as Aspen, there is a decided tendency for the lead ores in dolomitized areas to contain much less silver than those in rocks otherwise altered, and this is regardless of the peculiarities of the assumed source of the solutions that deposited the sulphides. The situation at Aspen is not simple, since in addition to dolomitiza- tion there is present also ferration (sideritization?) and silicifica- tion of the limestones.

Table I.

Ounces Of Silver.

é Per Ton Per Ton Locality. Gabns ead. Remarks. Missouri “ southwest... . . Trace to 4.0 — Missouri southwest... . . 0.75-1.75 Missouri central. Trace to 3.75 — Missouri southeast 0.205-2.065 — Product of 4 mines. Tennessee “ ee None — Several samples Nevada,* Goodsprings. . . . — Largely 2.0-30.0 Colorado, Aspen High Many silver minerals in ores.

England, Alston Moor 2.3-30.0 Average about 6 oz. Italy, Pertusola Co.*’ — 15.0-19.0 Italy, Montiponi Co.*" 13.0-24.0 Die ae ST Cr 0.5-1.0 — Italy, Auronzo® 1.0-2.0 a IDEAL SERIA s 5. os 556 — 6.0-8.0

42 Winslow, A., “ Lead and Zinc Deposits of Missouri,” Missouri Geol. Survey, vol. 7 (Sec. 2), PP, 454-455, 1894.

43 Buckley, E. R., “ Geol. of the Disseminated Lead Deposits of St. Francois and Washington County,” Missouri Geol. Survey, vol. 9, pt. 1, p. 250, 1909.

44 Secrist, M. H., “‘ Zinc Deposits of East Tennessee,” Dept. of Education, Div. of Geol., Bull. 31, p. 28, 1924.

45 Unpublished data, D. F. Hewett.

46 Spurr, J. E., loc. cit.

#7 Santmyers, R. M., “ The Lead Industry,” U. S. Bur. Foreign and Dom. Com- merce, Trade Inf. Bull. 371, p. 14, 1925.

48 Personal communication, C. W. Wright. 49 “ Handbuch der Oberschlesischen Industrie,” p. 452.

856 D. F. Hewett.

It is true that the silver content of the galena in some other mining districts is rather low. At three outstanding mines in the Linares-La Carolina district, Spain, where the ores form simple veins in slates and granitic rocks, the range is 8 to 20 ounces of silver per ton of lead. In most districts where siderite or silica is important in either the gangue or wall rock however, the silver content of galena commonly ranges from 40 to 100 ounces per ton.

Relation of Magnesite to Dolomitized Limestone —In a sum- mary of magnesite deposits and their origin. Bain has noted that where magnesite appears to replace limestone near its contact with intrusive igneous rocks, especially in Styria, Austria, Stevens County, Washington and Argenteuil County, Quebec, there is a zone of dolomite between the limestone and the magnesite which lies next to the intrusive rock. At present there seems to be gen- eral agreement that the magnesia which replaces the limestone has been derived from deep sources and is closely related to the in- trusives. These intrusives show a wide range in composition but basic types are most abundant.

Magnesite is uncommon, if not very rare, near zinc and lead deposits, and similarly zinc and lead minerals are rare or unknown near the larger replacement magnesite deposits. Viewed broadly, such magnesite deposits are very uncommon, whereas dolomitized limestone near zinc and lead deposits is rather common.

Summary.

Dolomitization of limestone wall rocks is most commonly found near zinc and lead deposits, but in a few places (Kennecott, Good- springs) is near copper deposits. In some places (Aspen, Red Cliff, Alston Moor) it is accompanied by sideritic alteration of the rocks, but in many places this is absent. In some places (Missouri, Upper Silesia) dolomitization is not accom- panied by outcropping intrusive rocks, but in most places it is,

50 Bain, G. W., “ Types of Magnesite Deposits and Their Origin,” Ecox. GEot., vol. 19, pp. 412-434, 1924.

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Dolomitization And Ore Deposition. 857

although the masses rarely have great areal extent. The related sideritic alteration seems to be confined to areas where there are some outcrops of igneous rocks. One might conclude from the foregoing relations that, even if the process of dolomitization is not confined to areas containing intrusive rocks, it is at least pres- ent only in a surface zone remote from large intrusive centers where high temperatures and pressures did not prevail.

In some areas (Goodsprings, Tintic, Cerro Gordo, Iglesias) where the limestones are altered to dolomite, the intrusive rock is much altered, generally sericitized, but on the other hand, there are numerous areas in which the intrusives are highly altered but which appear to lack dolomitization of limestone (Bisbee, Arizona, and Ely, Nevada. The following proposition does seem to hold however ; that if there are intrusive rocks in an area show- ing dolomitization of limestone, the intrusive rock is altered, gen- erally sericitized.

From the standpoint of details the fact that most dolomitic ore deposits show dolomite breccia cemented by sulphide minerals, seems to indicate that rock alteration precedes deposition of metal sulphides. Considered with the supplementary fact that, in dis- tricts showing intrusive rocks and dolomitized limestone, most ore-deposits are enveloped in dolomite but a few deposits are in limestone, it seems as though the two processes are separable in time, even though the source of the solutions, as well as the mag- nesia and metals, be the same. If the solutions have a magmatic source, they were first rich in magnesia before they were rich enough in metals to deposit them.

Sources of Magnesia.—For the purpose of this discussion one may imagine five general sources of magnesium:

1. The waters of the sea.

2. The shell of sedimentary rocks.

3. The underlying crystalline rocks such as gneisses. 4. The shallow bodies of intrusive rocks.

5. The deeper magma reservoirs.

858 D. F. Hewett.

1. As the result of the classical studies of Judd at Funafuti, it is now widely believed that the magnesium of sea-water may partly replace limestone before burial and produce dolomite. Also certain animals withdraw from sea water the magnesium in their hard parts, but this is not sufficient to form dolomite. The writer cannot imagine that the magnesium of sea water is a direct source of magnesia for the alterations considered here.

2. Most limestones contain some magnesium, and there are ex- tensive beds of dolomitic limestones, not to mention other sedi- ments that contain magnesium. Many investigators believe that such magnesium may migrate locally in such limestones, shortly, if not long, after burial, to form segregated bodies of dolomite in the midst of limestone. As noted above, it is Siebenthal’s idea that the magnesium needed to dolomitize the limestone of the Joplin region has been derived from remote bedded dolomites, transported by deeply-migrating ground-water. The circulation therefore, is essentially artesian, the source of the magnesia being near the intake of the water, and the site of deposition being near the outlet. Though the concept is attractive, and has much to commend it, it is not without difficulties. Among these, it may be mentioned that, considered broadly, the waters should selec- tively dissolve more magnesia than lime. Also, if the process took place on a large scale, the region of intake should show ex- tensive effects of solution.

Doubtless if dolomitized beds are underlain, regionally, by bedded dolomites and an artesian circulation can have taken place, such deeper beds may be a possible source of magnesia. In some districts, however, such as Goodsprings, Nevada, and Iglesias, Sardinia, the structural situation is distinctly unfavorable to artesian circulation and therefore to a remote lateral source of magnesia.

The term “ dedolomitization ’’ has been applied to the process by which the magnesia of a limestone has been segregated and combined with silica and other elements to form silicates such as diopside, thereby leaving the limestone almost free of magnesia. It has been noted in those masses of carbonate rocks which have

mA As yay A TT of DH S&S 0

Dolomitization And Ore Deposition. 859

been in contact with or enclosed by intrusive igneous rocks.” It is a useful term and might well be broadened to include the proc- ess of elimination of magnesia from limestone or dolomite by replacing it with other elements, such as iron or silica. For in- stance, large bodies of manganiferous siderite have been formed in the Leadville district, Colorado, by replacing the lime and magnesia of dolomite with iron and manganese. Obviously, in this process, whatever may be the source of the iron and man- ganese, magnesia is driven away.” As large bodies of manganif- erous siderite replace dolomitic limestone near Red Cliff, Colo- rado,”* Silver City, New Mexico,™ Pioche, Nevada,* all districts containing lead and zinc deposits with sparse surface outcrops of intrusive rocks, considerable magnesia must have been driven away from the replaced rock. The silicification of a dolomite such as is noted widely at Tintic, Utah, would yield magnesia as well as lime, and the magnesia could accomplish the alteration to dolomite of any limestone through which the solution later passed. In the opinion of the writer, this source of magnesia deserves serious consideration.

3. Ancient crystalline rocks show a wider range in composition than igneous rocks, but all contain appreciable magnesia. The common gneisses and schists contain from 0.3 to 3. per cent, magnesia, and there are many varieties of less siliceous and more magnesian gneissic rocks. In looking upon these rocks as possi- ble sources of magnesia as weil as the younger intrusives, the writer is impressed that they deserve consideration but it seems doubtful that the magnesia could be effectively removed and trans-

51 Eskola, P., ‘On Contact Phenomena between Gneiss and Limestone in Western Massachusetts,” Jour. Geol., vol. 30, p. 287, 1922.

52 Emmons, S. F., Irving, J. D., Loughlin, G. F., “ Geology and Ore Deposits of the Leadville Mining District, Colo.,” U. S. Geol. Surv. Prof. Paper 148, pp. 151- 154, 1927.

53 Crawford, R. D., and Gibson, R., “ Geology and Ore Deposits of the Red Cliff District, Colo.,” Colo. Geol. Surv. Bull. 30, pp. 55-56, 1925.

54 Wells, E. H., “ Manganese in New Mexico,” New Mex. Sch. Mines Bull. No. 2. PP. 44-49, 1918.

55 Westgate, L. S., and Knopf, A., “ Geology of Pioche, Nevada and Vicinity,” Amer. Inst. Min. and Met. Eng. Trans., vol. 75, pp. 834-835, 1927.

860 D. F. Hewett.

ported by an artesian circulation. Two of the serious problems met in considering all deeply-buried massive rocks as sources of magnesia or any other elements are the processes of release and transportation. Even though we know that under the influence of weathering near the surface, composite rocks break down into a few simple minerals, it seems clear that deeply buried rocks only set free a part of the ingredients either by recombination or by displacement through interaction with new substances.

4. The intrusive rocks exposed at the surface show a wide range of magnesium content as well as other constituents. Many granites and aplites contain as little as 0.2 per cent.; the quartz- monzonites, from 0.3 to 2.5 per cent.; the intermediate andesites and diorites, from 2.0 to 5.0 per cent. A few basic rocks con- tain as much as 45 per cent. magnesia. In most metal mining districts, considerable masses of the intrusive rocks show a high degree of alteration which is reflected by the development of new minerals. In the western United States the sericitic or propylitic types are common and by these processes magnesia as well as lime and some other ingredients are commonly eliminated. Good quantitative data of the kind and degrees of alteration of the in- trusives are on record for a number of districts—Nevada City and Grass Valley, California,°* Wood River, Idaho,°* Tonopah,* Goldfield®® and Ely, Nevada,® Breckenridge, Colorado, as well as some foreign districts, notably Huaraki, New Zealand.” In

56 Lindgren, W., “ The Gold Quartz Veins of Nevada City and Grass Valley Dis- tricts, Calif.” U. S. Geol. Surv., 17th Annual Rept., Part II., pp. 146-157, 18906.

57 Lindgren, W., “‘ The Gold and Silver Veins of Silver City, Delamar, and other Mining Districts in Idaho,” U. S. Geol. Surv., zoth Annual Rept., Part III., pp. 218-231, 1899.

58 Spurr, J. E., “Geology of the Tonopah Mining District, Nev.,” U. S. Geol. Surv. Prof. Paper 42, pp. 207-252, 1905.

59 Ransome, F. L., “ Geology and Ore Deposits of Goldfield, Nev.,” U. S. Geol. Surv. Prof. Paper 66, pp. 176-186, 1909.

60 Spencer, A. C., ‘“‘ The Geology and Ore Deposits of Ely, Nevada,” U. S. Geol. Surv. Prof. Paper 96, pp. 55-64, 1917.

61 Ransome, F. L., ‘Geology and Ore Deposits of the Breckenridge District, Colo.,” U. S. Geol. Surv. Prof. Paper 75, pp. 95-101, 1911.

62 Findlayson, A. M., “ Problems in the Geology of the Huaraki Gold Field,” Econ. GEOL., vol. 4, pp. 632-645, 1909.

Dolomitization And Ore Deposition. 861

all but one of these districts as well as many more that have been less thoroughly studied, the alteration of the intrusive rock has caused the elimination of most of the magnesia that it once con- tained.

To the writer, it seems that this magnesia, driven out of the intrusive rock by its alteration, may be considered as available to accomplish dolomitization in the higher limestones.

5. Magmatic sources of many elements that accomplish rock alterations are attractive because they appear to impose no further burdens in the way of speculation or inquiry. The writer be- lieves that magmas are a probable source of water and many other substances but that it is unwise to look to them until all other possible sources are thoroughly considered. The impres- sion prevails widely that the emanations from magmas differ in kind and quantity but bear some relation to the composition of that part which later becomes intrusive igneous rocks. One might expect therefore that if the magnesia which accomplishes dolo- mitization is largely from a magmatic source, the basic rocks would be more conspicuous than the siliceous in regions which show this kind of rock alteration. Actually, the outcropping igneous rocks in these regions show a wide range in composition and the more siliceous and less magnesian varieties are probably more abundant than the basic. From this the tentative conclu- sion is reached that little of the magnesia added to limestone to form dolomite, in the districts here described, has had a direct magmatic origin.

Hypotheses of Genesis—When the features of districts char- acterized by dolomitization of limestone are reviewed, as to orig- inal stratigraphy, igneous intrusions, structure, and ore deposits, it seems doubtful whether the magnesia has been derived from similar sources and whether the same mechanism of transfer has been effective in each district. The explanation offered by Siebenthal of the source, method of transfer and -deposition of magnesia in southwestern Missouri seems to be satisfactory there, but doubtfully applicable to many other regions. Possibly the

862 -D. F. Hewett.

explanation applies to Upper Silesia also, but it is quite inap- plicable to Iglesias, Goodsprings, and other regions that have been greatly disturbed and contain intrusive igneous rocks.

For regions that show dolomitization of limestone and sparse but altered intrusive rocks, it is proposed that an important if not the principal source of the magnesia may be the large more deeply buried masses of igneous rocks from which magnesia has been driven off during alteration, probably commonly sericitization. If this is true, such dolomitization is a shallow type of alteration, inherently connected with other types of alteration of more deeply- buried rocks. Viewed quantitatively, to provide magnesia to con- vert a unit mass of limestone into dolomite containing 21 per cent. magnesia will require all of the magnesia contained in about 10 times the mass of an igneous rock (quartz monzonite) con- taining 2 per cent. magnesia. Thus far, no district has been studied sufficiently closely to determine whether such quantitative relations exist or probably once existed.

One objection may be raised to the proposed hypothesis; if it applies widely, dolomitized limestones should be found in many regions where limestones have been intruded by igneous rocks, now altered, but for which the existing records do not describe such alterations. The writer has found that dolomitized lime- stone has been called “ marble” in several localities and believes that it occurs much more widely than descriptions indicate.

Dolomitization As A Guide To Ore Deposits.

The summary presented above of the occurrence of dolomite near ore deposits shows that the extent of dolomite differs from one district to another as well as from one deposit to another in most districts. In some districts where dolomitization is present a few deposits are found in the unaltered limestone.

In order that an alteration of the wall rocks shall be most help- ful as a guide to ore, it is desirable that it be present as an aureole around most of the deposits, say 80 to go per cent., as at Good- springs, and that the extent be neither too great nor too small, but range say from 10 to 50 times the size of the deposit. If the

Dolomitization And Ore Deposition. 863

extent of the altered rock is about equal to or less than the deposit, it will be as hard to find as the deposit and if the extent of altered rock is many hundred times that of the deposit, it may be as hard to find as if there were no alteration whatever. Thus, in Dur- ham and Derbyshire, England, where the altered rock is only sporadically distributed and small in extent, it offers slight aid. In the Iglesias district, Sardinia, also, where the altered rock underlies a number of square miles and some important ore bodies are known to lie outside the zone of alteration, it is not a useful guide. In some places, such as Goodsprings, however, where the areal extent of the altered zone is fairly large and most of the known ore bodies lie in it, explorations for ore in unaltered rock should be undertaken cautiously. Probably there is a habit of occurrence for each district, and if it can be determined, and the quantity of dolomite lies between the two extremes of abundance

and sparseness, it can be used as a local aid in searching for ore bodies.

U. S. GeorocicaL Survey, WasuincrTon, D. C.

PHYSICO-CHEMICAL FACTORS CONTROLLING MAG- MATIC DIFFERENTIATION AND VEIN FORMATION.’

Clarence S. Ross.

Introduction.

RECENT petrologic research has produced much new information about the late mineralizer-controlled stages of magmatic differ- entiation; the deuteric reactions in igneous rocks, the formation of dikes, and the early antecedents of vein formation. At the same time a large fund of information has become available about the phase relations of silicate melts and the part played by water in certain of these systems. The effort to understand vein-form- ing processes has led to an attempt to correlate the genetic proc- esses in dikes and veins with the later stages in the crystallization of igneous rocks, the deuteric reactions in such rocks, and the concentration and expulsion of a water-rich residuum. Such a study has required a careful review of physico-chemical investi- gations that might throw light on the problem, and the results may be of general interest to students of mineral deposits.

Geologists have long recognized that mineralization processes often occur in several stages, and this relationship is now receiv- ing detailed study. The writer has been engaged in a study of the copper-bearing pyrrhotite veins of the southern Appalachian region, which have a complex rather than a simple genetic history and have been developed by several successive stages of mineral- ization. Most of the minerals present have replaced country rock or earlier vein minerals through the action of changing hydrothermal solutions. The initial episode in some veins was the intrusion of an aplitic or pegmatitic magma; this was fol- lowed by high-temperature hydrothermal solutions that deposited quartz and ferromagnesian minerals, and the vein-forming process usually ended with the introduction of sulphides.

1 Published by permission of the Director of the U. S. Geological Survey.

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Magmatic Differentiation. 865

Schaller ? has shown that most pegmatites have had a complex genetic history. The first episode was the introduction of a magma that crystallized into rock consisting dominantly of potash feldspar or of potash feldspar and quartz, but in many pegmatites this pyrogenic stage was followed by hydrothermal stages. Solutions rose through the pegmatite, and the earliest of these contained soda and silica that more or less completely replaced microline by albite and quartz, and at the same time micas may have been formed. In some pegmatites successive waves of hydrothermal solutions replaced earlier minerals by tourmaline, beryl, lithium minerals, rare-earth minerals, radio-active minerals, and garnet or other ferromagnesian minerals.

Shannon has made a very thorough study of the diabase of Goose Creek, Virginia, in which he traced differentiation from pyrogenic to hydrothermal stages with unusual completeness. Recently Gillson has described interesting deuteric processes in the granodiorite of the Pend Oreille district, Idaho. All these investigations show that many mineral deposits have had a com- plex history and that reactions between different minerals or be- tween minerals and residual magma material are of common oc- currence.

The problems of magmatic differentiation have been treated in a notable series of papers by Bowen,’ who has also described re- actions between magma and solid materials or between these materials and residual solutions.° ‘The phase relations and geol-

2 Schaller, Waldemar T., “ Mineral Replacements in Pegmatites,”’ 4m. Miner- alogist, vol. 12, pp. 59-63, 1927.

3’ Shannon, Earl V., “The Mineralogy and Petrology of Intrusive Diabase at Goose Creek, Loudoun County, Virginia,” U. S. Nat. Mus. Proc. No. 2539 (vol. 66, art. 2), pp. 1-86, 1924.

4 Gillson, Joseph L., “ Granodiorites in the Pend Oreille District of Northern Idaho,” Jour. Geology, vol. 35, pp. 1-31, 1927.

5 Bowen, N. L., “ Crystallization-differentiation in Silicate Liquids,” Amer. Jour. Sci., 4th ser., vol. 39, pp. 175-191, 1915. “ The Later Stages of the Evolution of Igneous Rocks,” Jour. Geology, vol. 23, Supplement to No. 8, pp. 1-91, 1915. “The Behavior of Inclusions in Igneous Magmas,” Jour. Geology, vol. 30, Supplement to No. 6, pp. 513-570, 1922.

6 Bowen, N. L., “ Genetic Features of Alnoitic Rocks at Isle Cadeux, Quebec,” Am. Jour. Sci., 5th ser., vol. 3, pp. 1-34, 1922. “ The Reaction Principle in

866 Clarence S. Ross.

ogic bearing of silicate melts in which water and other volatile substances are parts of the system is of fundamental importance to the petrologist and student of ore deposits. The most out- standing investigations of such systems have been those made by Morey."

Definition Of Terms.

The process of vein formation can be accurately described only when the meanings of the terms used are clearly understood, and therefore it seems desirable to outline the meanings of some of the terms used in this paper.

The terms “magma” and “hydrothermal” have been used with widely differing meanings, which have occasioned much misunderstanding in the sciences of petrology and geology. It is believed that there exists a continuous transition from nearly anhydrous magma through concentrated hydrothermal solutions down to the most dilute thermal solutions. Therefore any di- vision between magma and hydrothermal solutions must be purely arbitrary. Most geologists would probably agree about the es- sential physical differences of the materials lying at the two ends of this continuous series, but there is possible doubt as to the proper term to be applied to an intermediate material that par- takes of some of the properties of both magma and hydrothermal solution.

As most commonly used in the past, “ magma” has signified the molten material which owed its liquidity primarily to heat and which on solidifying gave rise to rock—that is, magma is rock- forming material in a heat-induced liquid state which on cooling forms an igneous rock. It is recognized that all magmas prob- ably contain some mineralizers, and these will modify and may be sufficiently abundant in some magmas to profoundly modify the temperature of crystallization, but this does not change the fact Petrogenesis,” Jour. Geology, vol. 30, pp. 177-198, 1922. “The Fen Area in Telemark, Norway,” Amer. Jour. Sci., 5th ser., vol. 8, pp. 1-11, 1924.

7 Morey, G. W., “ The Ternary System H,O-KSiO,-SiO.,” Amer. Chem. Soc. Jour., vol. 39, pp. 1173-1229, 1917. “ The Development of Pressures in Magmas

as a Result of Crystallization,” Washington Acad. Sci. Jour., vol. 12, pp. 219-230,

p

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Magmatic Differentiation. 867

that heat is the main factor that produces liquidity. By definition “molten ’’ means “ reduced to the fluid state by heat,” and this definition of a magma has been accepted by Schaller.* Ina recent paper Phemister ° cites Teall,?® Geikie,"* Chamberlin,’? Rastall,?* and Gregory as authorities in support of the definition of magma as molten rock-forming material and concludes: “ It is evident, therefore, that the molten character of a magma is per- haps its most distinguishing feature. . . . If it is proved that in the ore-forming agency aqueous constituents were present in marked degree, then the term magma must be dropped and the phrase ‘hydrothermal solution’ substituted.”** The term “magma ”’ usually has a much more restricted meaning than the term “ magmatic,” which has often been used to signify any ma- terial or process even remotely derived from or related to a magma. Therefore “magmatic” has become too inclusive a term and should be avoided where exact ideas are to be conveyed.

The term “ molten” does not in itself carry any meaning ex- cept liquidity produced by heat and does not commit the user to any theory about the physical chemistry of the “ molten ” system. It is to be kept in mind, however, that magmas are heat-produced liquid solutions, and the use of “ molten” or “ molten magma,” does not imply any oversight of that fundamental fact. Thus Iddings says: “‘ Since the magma from which an igneous rock has solidified was in a liquid state at high temperature, and its composition was not simple, the molten magma must have been

8 Schaller, Waldemar T., “ Mineral Replacements in Pegmatites,” Amer. Min- eralogist, vol. 12, p. 59, 1927.

9 Phemister, T. C., “ Igneous Rocks of Sudbury and Their Relation to the Ore Deposits,” Ontario Dept. Mines, Thirty-fourth Ann. Rept., vol. 34, pt. 8, 1925.

10 Teal, J. J. H., “ British Petrography,” p. 438, London, 1888.

11 Geikie, A., “ Structural and Field Geology,” p. 33, New York, 1920.

12 Chamberlin, T. C., and Salisbury, R. D., “ Geology,” vol. 1, p. 384, New York,

13 Rastall, R. H., “The Geology of the Metalliferous Deposits,” p. 13, Cam- bridge, 1923.

14 Gregory, J. W., Chem. Soc. Trans., vol. 121, p. 755, 1922.

15 Phemister, T. C., op. cit., p. 52.

16 Iddings, Joseph P., “Igneous Rocks,” vol. 1, p. 91, New York, John Wiley & Sons, 1909.

868 Clarence S. Ross.

a solution from which minerals crystallized to form the rock.” Bowen says: “It is now many years since petrologists first began to think of the crystallization of a molten magma in terms of the physico-chemical principles governing the behavior of solutions.”

A water-rich differentiate is one of the end products of a crys- tallizing igneous mass, and after its escape it becomes the material that forms dikes, veins, and a large variety of mineral deposits. At different stages in its history it will contain different propor- tions of water, and have different temperatures. It seems evi- dent that at successive stages it will be normal magma, water-rich magma, a magma-rich solution, a concentrated hydrothermal solution, a dilute hydrothermal solution, and possibly end its career as a thermal spring. The degree of concentration of solid elements implied by the term “hydrothermal solution” is not always clear. To some geologists it means only a very dilute solution, but others understand it to include fairly concentrated solutions of solid materials. In this paper we shall consider the physical and chemical properties of the dike and vein forming materials that have been derived from a differentiating magma. The most significant of these properties are due to the presence of water and other mineralizers, and are similar in character although not always in degree, in dilute and concentrated solu- tions of solid matter in water. For this reason it will be neces- sary to consider concentrated hot solutions and dilute solutions together ; although it is recognized that distinct names may be desirable. Thus Sederholm** has applied the name “ ichor ” to concentrated magma residua of the intermediate type (“‘ granite juice ’’).

The term “‘ hydrothermal solution” means hot water that con- tains dissolved materials, but it implies nothing about the source, nature, or degree of concentration of these materials. Hydro- thermal solutions derived from a magma, however, contain resid-

17 Bowen, N. L., “ The Reaction Principle in Petrogenesis,” Jour. Geology, vol. 30, p. 177, 1922.

18 Sederholm, J. J., ‘On Migmatites and Associated pre-Cambrian Rocks of Southwestern Finland,’ Comm. Geol. Finlande, Bull. 77, p. 89, 1926.

er

Magmatic Differentiation. 869

ual magma materials ; materials redissolved from the parent rock and often from other rocks traversed. The same is true of concentrated solutions, but these will be richer in solid elements, and have higher temperatures.

The term “ pneumatolytic”’ originally denoted gas-controlled processes,” but it has been used to describe processes that were not the result of a gas phase, and recently its meaning has been further attenuated by the formation of a group of hybrid terms. For these reasons ‘‘ pneumatolytic ” and its derivatives have lost all definite meanings, and the terms “ gas phase” and “ gas phase reactions ” will be used exclusively in this paper to describe proc- esses of this type.

In the following discussion, the principle of closed and open systems will be applied to the products of differentiation. A closed system is understood to be one where no material is added or escapes during the course of the reactions, and an open system is one where there is addition or removal of material—that is, there is interchange of an essential part of the material during the course of the reaction. It is recognized that no system is ever completely closed within the earth’s crust, and that pressures are usually maintained that prevent the rapid or explosive escape of materials, so that an open system does not show perfect free- dom. It is evident, however, that there is a pronounced differ- ence between a system in which the final products are completely or almost completely representative of the initial materials and one in which the final products are entirely unlike the initial materials.

The radicals H.O, Cl, F, B2O;, P20;, SO;, CO2, H.S and re- lated compounds play a fundamental part in differentiation and vein formation, but the best name to apply to the group is ques- tionable. The term ’ is probably most used, but it is objectionable because it is based on the suppositious property

‘ mineralizers ’

19 Bunsen, R., “ Annalen der Physik und Chemie” (Poggendorff), vol. 83, p. 238, 1851.

20 Niggli, Paul, “ Die leicht-fliichtigen Bestandteile im Magma,” Leipzig, 1920.

21 Schaller, Waldemar T., “ Mineral Replacements in Pegmatites,” Amer. Mtn- eralogist, vol. 12, p. 59, 1927.

870 Clarence S. Ross.

of inducing the formation of certain minerals and not upon the real character of the “ mineralizers.” Fenner has called these materials “ volatiles,’ but this term seems hardly applicable to systems in which the materials are present not in volatile form (gas phase) but in a liquid phase. Shand has called the ma- terials that escape from crystallizing magma “ fugitive constit- uents.” This is undoubtedly a good term to describe the mate- rials that escape from a magma, but it would include all the dis- solved solids that escape with the water and other mineralizers and so would not be confined to the water and the allied dissolved gases. Niggli* has called them “ leicht-fliichtigen Bestandteile,” and this designation probably describes these elements more com- pletely than any English term in common use. It is undesirable to propose any new names in this paper, and so it will probably be best to use “ mineralizers,” a word that seems to carry the de- sired meaning to the greatest number of geologists, even though it poorly describes these materials.

Physical Differences Between Magma And Hydrothermal Solutions.

A primary magma undergoes profound changes in composition during differentiation, and products that are quite dissimilar may result, for there may be filter press action that squeezes off inter- stitial magma after crystallization is partly complete, and there may be crystal settling. Very commonly, however, an igneous rock is approximately cooled and solidified magma, and the chemical composition of the two is essentially similar. Thus many cooling magmas are approximately closed systems, although never completely closed. Probably almost all magmas contain some water and other mineralizers, but a primary magma con- taining as much as 3 per cent. would probably be considered rich in water. This water fails to become an important part of the

22 Fenner, Clarence N., “ The Katmai Magmatic Province,” Jour. Geology, vol. 34, No. 7, pt. 2, p. 696, 1926.

23 Shand, S. J., “ Eruptive Rocks,” pp. 31-48, London, Thomas Murby & Co.; New York, D. Van Nostrand, 1927.

24 Niggli, Paul, “ Die leicht-fliichtigen Bestandteile im Magma,” Leipzig, 1920.

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Magmatic Differentiation. 871

solidified rock, but it forms so small a proportion of the original magma that its loss and the loss of the dissolved gaseous and solid materials (the fugitive materials) which it may carry with it when it escapes will usually not profoundly modify the chemical similarity between the magma and the rock derived from it. In other words, an igneous rock usually gives a fair though not necessarily a complete record of the chemical composition of the immediate magma from which it was formed.

Water escaping from a cooling magma would probably not remove much more than its own mass of solid materials, and so a magma containing 3 per cent. of water would probably not lose much more than 6 per cent., as a maximum, of its original ma- terials. It is evident that a very wet magma might be materially modified by the escape of water and the dissolved materials that escaped with it.

A solution containing water and other mineralizers is an open system where part of the material, and usually a large part, eventually escapes. The crystallization of a hydrothermal solu- tion or even a concentrated water-magma solution results in a mineral assemblage with a chemical composition unlike that of the solution from which it crystallized. A large and usually an unknown part of this solution has failed to be deposited at any one place and has passed out of the immediate system, possibly to form an entirely different mineral assemblage elsewhere. Dur- ing the course of crystallization the solution has usually been in constant migration, and it may have changed in composition dur- ing the course of its depositional history.

The Source Of Heat Energy In Vein-Forming Materials.

The introduction of new vein-forming and ore-forming ma- terials into a pre-existing rock, the replacement of the old materials in rocks that usually were not at high temperatures prior to their invasion, and the removal of the replaced materials are processes that can be accomplished only by the expenditure of considerable amounts of heat and chemical energy. It is there- fore essential to examine the chemical and physical processes that

872 Clarence S. Ross.

are most likely to furnish this energy and to evaluate so far as possible the energy relations in vein-forming and ore-forming materials.

The superheat (heat above the temperature at which crystal- lization begins) of most magmas is usually considered to be small, as has recently been strongly emphasized by Bowen.** Most theories of the mechanism of differentiation of a magma postulate the separation of a liquid from a crystal phase through partial fusion, crystal settling, straining off of residual magma, or other process. These postulates imply that at some stage in its history the magma residuum (either magma or solutions) has been in equilibrium with a crystal phase, and therefore that the liquid fraction can have no superheat at the time of its splitting off from the parent source; but changed conditions after the liquid fraction has separated from the crystal fraction may modify these relations. If the liquid fraction is intruded into higher levels the hydrostatic pressure in the new environment is lower, and the inclosing rock is commonly cooler than the rocks at deeper levels. The lower pressure lowers the temperature at which crystalliza- tion begins and so results in some superheat, but the cooler en- vironment favors a more rapid loss of heat and so promotes crystallization. The specific heat of silicate melts is low, and that of sulphide melts is even lower. All these physical relations combine to give a vein-forming material of the magma type (“ vein dike”) a comparatively small supply of heat energy.

Bowen has made a notable contribution to geologic thought in a paper on “ The reaction principle in petrogenesis,” and an- other on “ The behavior of inclusions in igneous magma.” In the first paper he says: ‘‘ The minerals making up the rocks of an igneous sequence can be arranged as a reaction series, and it is the existence of such a series that controls the crystallization and differentiation of the rocks of the sequence.” The reactions take place in a definite sequence and according to definite phase relations, and when fragments are included in a magma exother-

25 Bowen, N. L., “ The Behavior of Inclusions in Igneous Rocks,” Jour. Geology, vol. 30, pp. 513-570, 1922. 26 Bowen, N. L., Jour. Geology, vol. 30, pp. 177-198, 513-519, 1922.

Magmatic Differentiation. 873

mic reactions will tend to take place. Thus the resolution of minerals or mineral replacements will be favored that tend to add keat to the system. Where a vein is formed by several successive stages of mineralization it is undoubtedly an example of a reac- tion series, in which the relations are more complex and there is a greater variety of reactions than in igneous rocks. Here, as in igneous rocks, the tendency toward exothermic reactions will promote certain reaction series, and this source of energy is un- doubtedly a factor in vein formation. However, the phase re- lations between minerals and hydrothermal solutions are so little known that in general it is not yet possible to determine the heat relations in such a system and to determine on that basis the reactions that may have taken place.

The Part Played By Energy In Vein Formation.

In Magmas.—The almost complete absence of recognizable contact-metamorphic effects in the wall rock of even the relatively hot magmas that have formed basalt dikes or diabase sills is note- worthy. Hypabyssal intrusive magmas, even those that are known to have had high temperatures, were in general able to exert little effect on most inclosing rocks by means of their heat energy unassisted by hydrothermal or gaseous emanations, and without doubt vein-forming materials are cooler than basalt magma.

During the development of many veins, including the copper- bearing pyrrhotite veins of the southern Appalachian region, the vein-forming materials did not gain access through large open spaces but penetrated through fault planes, fault breccias, small fractures, and capillary and subcapillary spaces. Veins of this type would present a very large radiation surface and thus favor a rapid dispersion of heat which would quickly cool the incoming material. If hot diabasic magma rapidly introduced into rela- tively large spaces is commonly able to effect little change in the country rock, even less would much cooler vein-forming magma, introduced through small spaces, have the necessary heat reserve to accomplish the profound replacements that are recorded in many types of veins.

874 Clarence S. Ross.

The rate of penetration of capillary spaces is a function of the viscosity, and the ultimate degree of penetration is a function of the surface tension of the penetrating liquid. Both these factors give hydrothermal and even very concentrated solutions much greater power than a magma to penetrate capillary spaces. The superheat and total heat energy of a magma is low, and the physical conditions are favorable for the rapid dispersion of heat. All this promotes rapid cooling, rise in the viscosity, and decrease in ability for movement, followed by crystallization, which would promptly put an end to the intrusive and replacing activities of a magma. Therefore it seems quite improbable that magma as heretofore defined could be introduced into veins and accomplish the physical and chemical work that has been recorded by exten- sive replacements.

If the introduction of magma presents difficulties, the removal of replaced material by the action of a magma presents still greater difficulties, for the incoming vein-forming materials have commonly made way for themselves by replacing older materials in a great variety of mineral deposits. In the copper-bearing pyrrhotite deposits and the pegmatites that have undergone hydro- thermal alteration a large quantity of replaced material was re- moved through small openings, and most of it was transported so far that there is no remaining record of the place or mode of its disposal. Moreover, a vein-forming material of the magma type must have done this work after expending all the energy and ac- complishing all the work outlined in the preceding paragraphs. Such action is clearly quite beyond the power of magma which owed its energy primarily to the heat that kept it liquid, but whose genetic history precluded the presence of any great amounts of superheat and whose total heat energy was compara- tively small.

In hydrothermal solutions —With either concentrated or dilute hydrothermal solutions as the active transporting and replacing agents the physical relations are quite different. Water and mineralizer bearing solutions, like the end magmas, have differ- entiated while in equilibrium with a crystal phase and so would

Magmatic Differentiation. 875

normally possess no large supply of superheat, but at the same temperatures their heat energy is very much greater than that of any magma. The specific heat of water is approximately 1 for all temperatures of vein and ore deposition, but that of rock melts averages about 0.25 and in sulphide melts it is usually much lower. The richer the system becomes in water the higher will be its specific heat, as the specific heat is dependent upon the water concentration. The specific heats of the glasses of a number of rock-forming silicates are given by White,” and these approxi- mate the specific heats of magmas much more closely than the specific heats of crystals. These physical relations show that the heat energy of a dilute solution is about four times that of a dry silicate melt and as much as six times that of a dry sulphide melt at the same temperature, and that of a concentrated solution is two or three times as great as that of a magma low in water. The hot solutions, unlike a magma near the crystallization point, are able to keep up a steady, long-continued migration through small open spaces. Under the conditions of vein formation a slight chilling of a magma would produce crystallization that would block the channels of ingress, but the cooling of a hydro- thermal solution would produce a much less complete precipita- tion of crystals, and the channels would not be blocked until the process had been long continued. The first solutions would be quickly chilled and would deposit part of the material carried in solution, but they would at the same time heat the inclosing rock, and later solutions would therefore cool more slowly and so de- posit the material they held in solution more slowly. The heat- carrying capacity, the powers of reaction with pre-existing min- erals, the power to penetrate capillary spaces, and the ability to introduce and remove material are more potent and more per- sistent in hydrothermal and even in hot concentrated solutions than in magmas. The migration of solutions enables them to draw continuously on the heat supply of the crystallizing parent magma and to build up heat gradually in the surrounding rocks in a way that is usually impossible for a magma and would be

27 White, W. P., Am. Jour. Sct., 4th ser., vol. 47, pp. 1-54, 1919.

876 Clarence S. Ross.

quite impossible where it had to penetrate small open spaces and capillary spaces.

The Physical Condition Of Vein-Forming Material.

The question has often arisen, do the mineralizers given off by an igneous mass escape from the parent source as a liquid or a gas phase?®* The volatile elements of vein-forming materials have often been compared with volcanic emanations, and this is necessary because these emanations are the only ones that can be directly observed. Volcanic gases no doubt give a general idea of the mineralizer or volatile elements that take part in mag- matic processes, but the physical relations in a volcanic magma are entirely different from those in a deep-seated and slowly crys- tallizing magma, and this difference must profoundly modify the chemical composition of the mineralizers and the entire nature of their activities. Fenner says: “We have some qualitative knowledge of the chemical nature of the volatiles; what little quantitative knowledge we have applies almost wholly to surface lavas at a stage just a little prior to solidification, for it is only then that collections can be made. It is surely unjustifiable to assume that this information serves as a reliable basis of estimate of the relative concentration of volatiles in a freshly risen lava during a great volcanic eruption and still less to the concentration at profound depths.”

Gas-phase Reactions—A lava may pass quickly from an en- vironment of high heat and pressure to a cool environment with low pressure, and many hypabyssal intrusives must cool near the surface under slight load. During the rise from depths-and ex- trusion of a magma the pressure may be quickly released, so that the volatile elements escape rapidly and often with explosive violence. As they escape from solution in the hot magma they pass directly into the gas phase,*° and it seems quite improbable

28 Lindgren, Waldemar, “Hot Springs and Volcanic Emanations,” Econ. GEOLOGY, vol. 22, pp. 189-192, 1927.

29 Fenner, Clarence N., “ The Katmai Magmatic Province,” Jour. Geology, vol. 34, No. 7, pt. 2, p. 739, 1926.

30 Day, Arthur L., and Allen, E. T., “ The Volcanic Activity and Hot Springs of Lassen Peak,” Carnegie Inst. Washington Pub. 360, pp. 163-165, 1925. Allen,

E. T., “ Chemical Aspects of Vulcanism, with a Collection of Analyses of Vol- canic Gases,” Franklin Inst. Jour., vol. 193, pp. 29-80, 1922.

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Magmatic Differentiation. 877

that any liquid phase rich in water and other mineralizers is present unless part of the gases recondense. The reactions will be of the gas-phase type, and solid materials must be segregated, transported, and deposited by a gas phase. This gas-phase genesis of minerals appears to be confined to certain types of deposits formed at rather high temperature or near the surface and to volcanic emanations, as at Katmai and Vesuvius.™

The concentration of solid elements in a gas phase must be very low, and gas phases seem quite inadequate to transport the large quantity of material found in many dikes and ore deposits of other types. It seems evident that great masses of quartz could not be the result of gas-phase reactions at the moderate tempera- tures that must have existed during vein formation, because the work of Morey (see p. 883) shows that “ it is improbable that, no matter how high the total pressure, the SiO, content of the vapor can be appreciable.’ For these reasons it is believed that water- rich solutions provide the only mechanism that could introduce such materials as feldspar, quartz, silicates, carbonates, and sul- phides as they are found in most veins and could remove the re- placed material, often to such a distance that there is no present record of its place of disposal.

The Origin Of Water-Rich Residual Solutions.

At some depth where pressure can be maintained crystallization may commonly go forward without any sudden change in en- vironment, and the formation of anhydrous minerals will cause a concentration of water and other mineralizers in the residuum. The load of overlying rock tends to retain these elements within the magma, and so the later stages of crystallization take place in the presence of concentrated mineralizers and at much lower temperatures than would prevail in a volcanic magma. The

31 Allen, E. T., and Zies, E. G. “ A Chemical Study of the Fumaroles of the Katmai Region,” Nat. Geog. Soc., Katmai series, No. 2, 1923.

Zies, E. G., “ The Fumarolic Action in the Valley of Ten Thousand Smokes,” Idem, No. 3, 1924.

Lacroix, A., “ftude minéralogique des produits silicatés de l’éruption du

Vésuve” (Avril, 1906); Mus. hist. nat. Nouv. archives, 4th ser., vol. 9, fase. 1,

878 Clarence S. Ross.

residual magma differentiate formed under these conditions may be rich in water and at a late stage in the crystallization process may be itself a concentrated or even a dilute hydrothermal solu- tion. It may produce deuteric reactions within its parent rock that are similar to many of its reactions after its escape and dur- ing its vein-forming career. If residual material escapes from the parent igneous mass it will be escaping from a crystal system and not from a melt, as where the mineralizers escape in the gas phase from volcanic magmas. That is, ore-forming and vein- forming materials are likely to be magma residua which are de- veloped under physical conditions that allow them to exist as solu- tions rich in solid elements or even as hydrothermal solutions, and there are no physical laws that demand that they be gas- phase magma emanations. Progressive crystallization of the original magma has gradually and by a continuous train of proc- esses concentrated mineralizers and produced a water-rich solu- tion, and its mere migration under pressure from its place of origin will make it a vein-forming, replacing, or ore-forming solution. If it escapes at a high temperature to a region of low pressure it will undoubtedly change to a gas phase, and its further reactions will be of the gas-phase type. If, on the other hand, it is under sufficient pressure and escapes slowly through suf- ficiently tight channels, pressure may be maintained, and the water rich dike-forming and vein-forming materials may remain in the liquid phase throughout their mineralizing career. Critical Temperatures in Magmas and Solutions—Some recent work by Morey has shown that critical temperatures are seldom attained under geologic conditions and that liquid phases may exist at almost any temperature. Solid and gaseous elements will be retained or be taken into solution in the magma residuum and will raise the critical temperature of solutions under pressure within the earth’s crust. Morey says: “ Sulphur, boric acid, hydrofluoric acid, and other volatile compounds, as well as water, are present in magmas, and these will prevent critical phases and insure the presence of a liquid phase. Critical end points will

32 Morey, G. W., “ The Réle of Water in Magmas.” Paper read before Wash- ington Petrologists’ Club.

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Magmatic Differentiation. 879

not need to be considered in geology.” Thus it is evident that the critical temperature of water does not set a limit on the tem- perature of water-bearing residual solutions.

Solubility of Mineralizers in Magma.—The effect of pressure to keep volatile mineralizers in solution in the residuum is em- phasized by Shepherd and Merwin in a discussion of the gases of Pelée. They say:** “At depths as the concentration of volatiles proceeds, the pressure produced keeps the volatiles in solution so that bubbles do not form. Conceivably crystals might form fast enough to produce supersaturation around their faces and thus induce bubble formation, but the excess pressure would tend to slow down or stop the reaction.” They estimate that under the conditions at Pelée, with a temperature of 1200° and at a depth of 1 kilometer, where the pressure would be about 100 atmospheres, a maximum of 1,800 c.c. and a minimum of 75 c.c. of volatiles (mineralizers) could remain in solution in 1 gram of magma.

The Development Of Pressure In A Magma Residuum.

Morey has presented a paper on “ The development of pres- sure in magmas as a result of crystallization,” and Day has used the principle to explain certain phases of volcanic activity. The increase of pressure with decreasing temperatures and advancing crystallization plays an even more important part in the develop- ment of dikes, veins, and ore deposits than in volcanism. The crystallization of anhydrous minerals in a magma results in a concentration of water and other mineralizers in the liquid residuum. This lowers the temperature of crystallization, but the vapor pressure is thereby increased. Morey says:

The eutectic between K,Si,O, and SiO, lies at the remarkably low temperature of 520°. If a mixture of K,O, SiO,, and H,O, containing

33 Shepherd, E. S., and Merwin, H. E., “The Gases of the Pelée Lavas of 1902,” Jour. Geology, vol. 35, PP. 114, 115, 1927.

34 Morey, G. W., Washington Acad. Sci. Jour., vol. 12, pp. 219-230, 1922.

35 Day, Arthur L., “Some Causes of Volcanic Activity.” An address before Franklin Inst. Philadelphia, 1924. 36 Morey, G. W., op. cit., pp. 225, 226, 230.

880 Clarence S. Ross.

g.1 per cent. of H,O, with the other ingredients in the molecular ratio SiO,/K,O 4.26, be cooled from a high temperature, the vapor pressure of the mixture will fall as the temperature falls. The mixture will not begin to freeze until it has cooled to 500°, when crystals of quartz and the ternary compound KHSi,O, will separate. The vapor pressure of the solution at this temperature is 160 atmospheres. On further cooling, the substances continue to crystallize, and the pressure increases rapidly. When the temperature has fallen 20°, to 480°, the water content has in- creased to 10.2 per cent., and the pressure to 180 atmospheres. When the temperature has fallen to 420°, the water content has increased to 12.5 per cent., and the pressure to 340 atmospheres; more than double the pressure at 500°... .

It is evident, then, that as a magma containing water and other volatile components cools, with consequent crystallization, the pressure will rapidly rise from its initial value, and as the cooling continues the pressure will increase until the temperature of maximum pressure has been reached, or until the pressure is relieved by escape of the volatile material. In the first case, which is that in which the liquid cools under a crust of suf- ficient weight and strength to withstand the internal pressure, the liquid will solidify as an intrusive mass. In the case of an actual magma the fact that water has a critical temperature at 374° C. has no signifi- cance, because of the probability that enough material will remain in solution to raise the critical temperature of the mixture the requisite amount. The water, containing in solution residual material such as dis- solved gases, boric acid, sulphur, and probably some alkalies, will be available for metamorphic processes. .. .

This increase [in pressure] is rapid whether measured in terms of de- crease in temperature of the three-phase equilibrium or in terms of the content of volatile material in the solution. From the latter fact it fol- lows that in systems of the type of magmas, in which the nonvolatile material is composed of such substances as the silicates and in which the pressure required to retain any considerable proportion of water in solu- tion must be large, a comparatively small amount of crystallization will result in a large increase in pressure.

The water-vapor pressure developed by the magma will be proportional to its water content and will reach its maximum at some temperature well above the critical temperature of water. If the magma residuum is at high temperature and under high pressure it will retain in solution a large proportion of the silicic- alkalic fraction of the magma and may escape to form dikes,

veins, “ lit-par-lit intrusions,” or injection gneisses. At a slightly

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MAGMATIC DIFFERENTIATION. 881 later stage after crystallization of the feldspar and part of the quartz-forming material there will be a higher concentration of mineralizers, and the residuum will be a water-rich solution that may contain CO., CO, Cl, F, P:O;, B.O;, H.-S, or SO. and SOs, and possibly H. In addition it will contain dissolved solid ma- terials whose nature and concentration will depend upon the char- acter of the rock from which they are derived and upon the tem- perature, pressure, and chemical composition of the solvents. A slight reduction of the pressure will allow the most volatile ma- terials or even a part of the water itself to change to the gas phase. The liquid phase plus the new gas phase will have a greatly increased volume that will cause migration to any region of reduced pressure and act to expel the hydrothermal solutions, just as the oil in an oil well is expelled by the expansive force of the gas that was in solution in the oil.

The pressures developed during the crystallization may have no connection with the original intrusive force of the invading magma but develop through cooling and become effective when crystallization has put a stop to the power of movement and the intrusive force of the magma has spent itself. The water-rich magma residuum and hydrothermal solutions are thus ready to take up the work laid down by the magma and carry it forward with a new and more effective force—a force that the magma may not have possessed even in the early stages. This shows that the end products of crystallization are not necessarily inert but may be active agents for the transportation of the products of differentiation and mark the stage of the most active vein forma- tion and metamorphism in the entire history of the intrusive mass.

Concentration Of Alkalies In The Magma Residuum

The profound influence that the formation of feldspar and especially plagioclase plays in the differentiation of an igneous rock has been emphasized by Bowen.** The first crystals of plagioclase that develop in a magma will be much more calcic

37 Bowen, N. L., “ The Melting Phenomena of the Plagioclase Feldspars,” Am. Jour. Sci., 4th ser., vol. 35, pp. 577-596, 1913.

882 Clarence S. Ross.

than the melt from which they crystallize. The next plagioclase to crystallize will be slightly richer in soda, but the melt at the same time has become even more sodic, and as the process con- tinues the molten residuum tends to become more and more sodic until crystallization is complete. Slow cooling will permit reac- tion between crystalline feldspar and the melt that will tend to equalize the composition of the two, but there is a strong tendency for incomplete equalization and for the concentration of sodic material in the magmatic residuum. As long as lime is available, plagioclase normally precedes potash feldspars in the crystalliza- tion sequence of a magma, but the sodic members of the plagio- clase series may remain in solution in the residuum after potash feldspars begin to crystallize. This still further concentrates soda in the residuum of a soda-potash magma, and near the end of the feldspar-forming process the residuum of many types of magma may contain silica, alkalies (often largely soda), water, and the other mineralizers. If calcium is abundant, as in gabbroic rocks, soda tends to combine with calcium in calcic plagioclase that is formed rather early in the crystallization process, and the small proportion of potash in the original magma tends to be concen- trated in the residuum instead of soda. If the late residuum is deficient in alumina through its fixation in early products of crys- tallization, feldspars will fail to form in the later stages, and alkalies will be further concentrated in the residuum. The mechanism of concentration of lithium, strontium, the rare earths, and related elements is possibly similar to that which promoted the concentration of sodium. These elements are usually so sparse in the original magma that they do not reach appreciable concentration till late in the pegmatite stage of a few unusual rocks.

Water-rich solutions may become alkalic through the hydroly- sis of alkalic minerals. In discussing the character of volcanic emanations Day and Allen say: “If, however, it were possible for a magma or a batholith to give off liquid water, the solution

38 Day, Arthur L., and Allen, E. T., “ The Volcanic Activity and Hot Springs of Lassen Peak,” Carnegie Inst. Washington Pub. 360, p. 165, 1925.

Magmatic Differentiation. 883

would always be alkaline, for we know that the principal reaction of an igneous rock with water at 100° is the hydrolysis of the silicates, a reaction which should move nearer completion at the higher temperature of the batholith. This reaction is obviously favored by the solubility of the alkali hydroxides, though it would doubtless be unimportant if the water were given off by the batholith in the form of steam, on account of the slight volatility of the hydroxides.”

The Effect Of Alkalies On The Phase Relations Of A Magma Residuum.

The part played by an alkali in the water-silica residuum is pointed out by Morey, who says:

In the system H,O-SiO, we have an interesting relation. . . . The solubility of SiO, in H,O is very slight—so slight, in fact, that its amount is not enough to affect the ordinary fixed points in the binary system H,O-SiO,, such as the triple point, boiling point, or critical point. Hence, as we heat up a mixture of H,O and SiO,, at the critical tem- perature of the former, 374°, the solution will show critical phenomena; the amount of SiO, in both liquid and vapor phases at the critical point is approximately zero. The first critical end point in the system H,O- SiO, therefore coincides with the critical point of H,O.

Similar relations are met with when we start at the triple point of SiO, and add H,O. When molten SiO, is placed in an atmosphere of water vapor, a certain amount of the latter will dissolve and will depress the freezing point of the SiO,. If the pressure of H,O vapor is in- creased, more will be dissolved, and a greater temperature lowering will result; the solution will follow the melting point or solubility curve for the binary system H,O-SiO,. This process, however, can not go on indefinitely ; a point will be reached where, no matter what the pressure of H,O vapor, enough H,O can not be held in the liquid to cause a further depression of melting point—in other words, to keep H,O in the liquid phase. The solution will then have reached the second critical end point in the system H,O-SiO,. ...

. at the critical end point both phases must have the same com- position, and since it is highly improbable that no matter how high the total pressure the SiO, content of the vapor can be appreciable, it follows that both phases must approach pure H,O in composition. But in this

39 Morey, G. W., “The Ternary System H.O-—-K.SiO,-SiO.,” Am. Chem. Soc. Jour., vol. 39, PP. 1173-1229, 1917.

884 Clarence S. Ross.

case the temperature must be fairly low; 550° seems, indeed, to be a prob- able upper limit. This assumption, also, is in harmony with certain geological evidence.

Addition of K,O will cause an immediate disappearance of the critical field; this field will therefore be very small—so small, in fact, as not to occupy an appreciable area in a projection. .. . The pressures in this region will be enormous, however, and it is doubtful if it can be experi- mentally realized. This is evident from a consideration of the variation of pressure with increasing SiO, content along the isotherm when quartz is the solid phase. For example, at 600°, when we increased the SiO,/H,O ratio from 4.49 to 4.76—that is, the mol. fraction SiO, in the binary system K,O-SiO, from 0.741 to 0.752—-the pressure increased from 93 to 560 atmospheres. This is the highest pressure determined in the system. What the magnitude of the pressure will be when we ap- proach the binary system H,O-SiO, can only be imagined.

The temperature of inversion of quartz to tridymite is 870°, and so all quartz has formed below that temperature, and the quartz in dikes and veins probably considerably below it, but the quartz of most mineral deposits seems to have formed not far above, and much of it below, the inversion temperature of quartz at 573°. If, as Morey concludes, the concentration of SiO, is almost zero at 550°, the residuum in a silica-water system would necessarily be dominantly water at all the probable temperatures of quartz formation in dikes and veins, and the concentration of water would not be high even at temperatures of 800°. Morey shows that the introduction of a small proportion of K.O will cause a disappearance of the critical point—that is, the solution containing dissolved material may be under pressure so high that liquid phases will not disappear—and the cooling curve can reach the H.O-K.SiO;-SiO, field. A very small amount of alkali per- mits the formation of very concentrated silica solutions and their crystallization within a few hours into coarse quartz.*° All this shows that water and alkali are normally both necessary to insure a system even moderately rich in silica with temperature low enough to crystallize into quartz. The unstable phase of col- loidal silica could not long exist in such a silica-alkali-water sys-

40 Morey, G. W., Paper read before the Petrologists’ Club of Washington.

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Magmatic Differentiation. 885

tem, and it seems evident that colloidal silica is not a factor in the formation of quartz veins from hot solutions, although it may play a part in deposits formed at very low temperature. Morey’s quoted experiments involved a SiO.-KO, + H.O system, but NazO would in general have a similar effect. Physical and geologic relations show that the residual alkali is normally Na,O in granitic rocks but may be K,O in gabbroic rocks. That is, the magma residuum at the time quartz begins to form is most likely to be dominantly a SiO.-H.O-alkali system, but in addition it may contain a very great variety of other mineralizers and solid elements in solution. As the quartz crystallizes, the water, alkalies, and other elements will be still further concentrated in the residuum.

The genetic history of residual magma solutions seems to in- sure their alkalic character, but, as Bowen has pointed out, a gas phase released from a liquid phase by decrease of pressure may be acid, even though escaping from an alkalic solution. Thus chlorine in the solution would be in combination, possibly with sodium, but under certain temperatures and conditions it could escape in the gas phase as hydrochloric acid. Thus the character of these released gas phases may be similar to that of gases escaping directly from a magma, which are very commonly acid, as has been shown by the work of Day and Allen.”

Physico-chemical investigations reveal many of the processes that have controlled the formation of dikes, veins, and a large variety of mineral deposits. This work is beginning to give a clear picture of the manner in which some of the materials that produce mineral deposits are concentrated from the parent magma, the way in which they are expelled, transported, and deposited. Some of the broader physico-chemical laws that have been determined apply to all types of mineral deposits, but the study of phase relations should be extended to a large variety of systems. The success of the work that has been done serves to

41 Bowen, N. L., Informal communication to Washington Petrologists Club. 42 Day, Arthur L., and Allen, E. T., op. cié., p. 125.

886 Clarence S. Ross.

emphasize the urgent need for similar work on other systems that bear on ore geology, and especially on those that include car- bonates and sulphides. The methods of work, and many of the problems have been outlined by the recent progress in physics, chemistry, and petrology, and further investigation of phase sys- tems that are significant is one of the pressing needs of ore geology.

U. S. GEoLocicaL SuRVEY, WasuinectTon, D. C.

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ORIGIN OF WHITE CLAYS AND BAUXITE, AND CHEMICAL CRITERIA OF PENEPLANATION.’

W. G. Woolnough.

In reading a paper on the “ Origin of White Clays of South Caro- lina”’ by Fred. R. Neumann, in a recent number of Economic GEoLocy,” the writer was struck by the close resemblance, in some respects, between these deposits and some of the clays of Western Australia. Without wishing to dogmatize regarding the clays of South Carolina, it appears to me that an outline of the conditions observed in Australia, and of certain deductions therefrom, may be of interest to American readers.

The problem of the genetic association of very pure white clays, with cappings of concretionary ferruginous bauxite, all over Australia, but particularly in the western State,** has led the writer to the recognition of certain “chemical criteria of pene- planation.* Such chemical criteria have been implicitly inferred in numerous publications; but, so far as I have been able to find, have not been put forward explicitly.

Peneplanation.—In all descriptions of the mechanism of pene- planation, it is insisted that the base-level of sub-aérial erosion is a surface, gently curved and inclined, generally lying almost at sea level. When erosion of an uplifted land surface commences, the earlier stages are carried out with extreme rapidity, geologi- cally speaking. The mechanical effects of abrasion and transpor- tation are dominant; chemical action is subordinate. The me-

1 Published by permission of the Honourable Minister for Home and Territories.

2 Neumann, Fred. R., “ Origin of the White Clays of South Carolina,” Econ. GEOL., vol. 22, 374-387, 1927.

3 Simpson, E. S., “ Laterite in Western Australia,” Geol. Mag., 1912, 399-406.

*Woolnough, W. G., “The Physiographic Significance of Laterite in Western Australia,” Geol. Mag., 1918, 385-393.

5 Woolnough, W. G., “ Presidential Address to the Royal Society of New South Wales, May 4, 1927,” Jour. and Proc. Roy. Soc. N. S. W., 1927, \xi, 1-53.

888 W. G. Woolnough

chanical forces remove the rock particles as soon as they are loos- ened by incipient chemical decomposition, and thoroughly rotted rock is almost a rarity. The streams are the most potent agents in the work; and, as their gradients become lessened the rapidity of the work falls off. The final stages of base-levelling are al- most inconceivably slow. This is the reason why an ideally per- fect peneplain is seldom, if ever, recognized.

Usually, long before peneplanation has been carried to its logi- cal limit, earth movement has supervened and a new cycle of ero- sion has been initiated. Even during the penultimate phases of the processes, as ordinarily carried out, there is still an effective run-off of streams, sluggish though that action may be.

It is possible to conceive of a set of circumstances in which crustal stability has been so long maintained that even the most sluggish run-off has practically ceased. Lateral drainage has come to an end, and whatever run-off there is, is so extremely lei- surely that scarcely even the finest silt can be transported me- chanically. Colloidal suspension may still persist ; and such emul- sions, together with true solutions, are alone responsible for further lowering of the land surface.

Hence, when peneplanation is forced to its extreme limit, chemical action completely dominates over mechanical processes of erosion.

In such conditions, the subsoil becomes completely saturated. Contact between rock-minerals and meteoric solutions is long maintained, and rock-weathering is profound. Removal of the insoluble products of weathering is inhibited, and retention in situ of such products is an outstanding feature of peneplanation carried to the limit.

Peneplanation in Pluvial Climates.—In pluvial climates, satura- tion of the weathered material is complete and permanent, rain- fall soaks downwards and laterally, and little or no ascent of sub- surface waters can take place. Consequently, the downward ex- tension of oxidation, carbonation, and hydration, that trinity of reactions mainly responsible for rock-weathering, is hindered. The effective depth of weathering is somewhat restricted.

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Origin Of White Clays And Bauxite. 889

On a dead-flat surface in a humid region the mat of vegetation hinders even the slight run-off that would take place otherwise. It filters out the fine sediment, and, by contributing plant acids to the descending solutions, aids in the work of chemical attack on the materials of the subsoil.

Excessively slow though it may be, there must always be a seep- age of ground water towards the drainage bottom. This is ef- fective in removing the soluble products of rock decomposition, and also some of the suspensoid systems. Given sufficient time, the soil and subsoil must be leached completely of all soluble ma- terials. All the alkalies, alkaline earths, and magnesia are re- moved. Most of the iron and some of the alumina are removed in the form of soluble salts, and still more in colloidal form. Some of the silica, also, is dissolved by the alkaline solutions, and some goes into colloidal suspension. Only insoluble minerals stable under sub-aérial conditions remain in the residue. Silica, chiefly as quartz, gibbsite, diaspore, koalinite, and such minor constituents as zircon, rutile, xenotime, cyanite and so on, build up the bulk of the residue in situ.

It must be remembered that the results described above are those which are to be expected when peneplanation has reached absolute perfection. As already stated, such perfection is very rarely at- tained, and the results halt at some point short of ideal complete- ness. In particular, the removal of the iron and alumina is apt to be imperfect.

Peneplanation in Regions of Seasonal Rainfall—tIn contrast with the pluvial conditions outlined above, we may consider the case of perfection of peneplanation in a region with a climate marked by strongly contrasted periods of saturation and desicca- tion. Such alternations may be annual, as they are at the present time over a great part of the coastal portions of Australia; or they may be of longer duration, as in the interior sub-arid regions of most continents. In either case the general effect is the same, though there are doubtless differences in detail.

In such circumstances the mat of vegetation is less permanent and continuous than is the case in a pluvial climate, a condition

890 W. G. Woolnough.

slightly more favorable to surface run-off. Nevertheless, the re- moval of solid matter in suspension in running water is very in- significant, since the gradient of the streams is, by hypothesis, negligible: During wet seasons meteoric water is greedily ab- sorbed by the parched soil and subsoil. So rapid is the sinking in of the water, assisted as it is by a network of cracks, large and small, that the gases dissolved from the atmosphere are carried immediately to considerable depths. There is not, as in pluvial climates, so gradual an abstraction of oxygen and carbon dioxide from the surface downwards. On the other hand, these active constituents are fairly uniformly distributed throughout a con- siderable vertical zone. During their contact, long or short, with the rock minerals, the waters carry on their processes of decompo- sition. These effects are cumulative, and the final result is de- composition at least as profound as that which is effected in a pluvial region. It is in the subsequent destinies of the solutions that there is a marked difference.

In a region with seasonal rainfall, the period of saturation is succeeded by one of desiccation. As the surface soil becomes dried, capillarity supplies moisture from the deeper levels of the soil and subsoil. Thus, upon the slow lateral seepage of solutions characteristic of the other type of climate, there is superimposed a movement vertically upwards, of a relatively extremely pronounced and active type. Large amounts of dissolved material are drawn towards the surface. The more soluble constituents may reach the daylight, there to be deposited as an efflorescence if the desic- cation is sufficiently complete. During each successive wet season these soluble salts are moved laterally in the direction of the drain- age, the sluggishness of the current being immaterial when dis- solved load, only, is considered. In most regions of this type, specialized types of vegetation have been evolved which are capable of dealing effectively with the salt solutions; and any noteworthy accumulation of salt is prevented, unless quite exten- sive “‘sumps”’ occur along the line of drainage. If, however, the mantle of vegetation is removed, this natural safeguard is in- terfered with, the rate of run-off is accelerated, and very serious

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Origin Of White Clays And Bauxite. 891

“salting ” of the lower levels may occur. Such a process may be seen at work in parts of the Wheat Belt of Western Australia.

Under normal conditions, however, the most soluble, saline fraction of the products of weathering (chiefly salts of alkalies and alkaline earths) is removed and carried seawards in much the same way as it is in a pluvial climate. It is the less soluble con- stituents, and particularly those of a colloidal character, which behave in markedly different fashions under the two sets of con- ditions. Produced in the subsoil under conditions of complete saturation, and of definite hydrogen-ion concentration, they en- counter conditions rapidly altering in every respect as they mi- grate towards the surface during the dry season. Usually, long before they reach the open air, the character of the solvent has changed to such an extent that precipitation of the colloids super- venes. This generally takes place around a number of isolated nuclei in the subsoil. Hydroxides of aluminium and iron are most colloidal when precipitated in the cold and from dilute solu- tions.

During the next ensuing period of saturation the descending solutions which wash over the precipitated colloids approximate much more closely, in hydrogen-ion concentration, to those which produced precipitation than they do to those favoring solution. The latter conditions exist just before the groundwater starts on its upward journey at the beginning of the dry season. While, undoubtedly, there is some re-solution of the precipitate, it is likely to be subordinate in amount. A large proportion of tHe colloidal matter is left in the subsoil, and accumulates there progressively. In this way there originates a subsoil deposit of mineral matter, amorphous because derived from colloidal suspensions, and pre- dominantly nodular or concretionary in habit because of its pro- gressive accretion. Such substances as hydrated silica and hy- drated oxides of aluminium and iron are most abundant and con- spicuous in such deposits.

Intermediate between the soluble salts, completely removed in solution, and the colloidal precipitates, largely retained in the sub- soil, are the carbonates of calcium, magnesium and iron, which

892 W. G. Woolnough.

may accompany either the salts or the colloids, or may be divided between them according to circumstances. On the whole, they belong rather to the former than to the latter category.

Chemical Criteria of Peneplanation.—I wish, then, to suggest that one essential criterion of a high degree of perfection of peneplanation is that the rocks of the area show evidence of very deep and very complete chemical alteration by meteoric waters.

If the residual material consists entirely of the most insoluble products of rock weathering, a uniformly moist climate may be postulated during the last stages of peneplanation.

If, on the other hand, there is a crust of concretionary, amor- phous material, chiefly alumina, iron oxide or amorphous silica, resting upon a substratum of insoluble residual constituents, the final stages of peneplanation took place under climatic conditions marked by sharply defined alternations of saturation and destc- cation.

At the risk of wearisome repetition I wish to emphasize the fact that absolutely ideal conditions of perfection in peneplanation must be extremely rare. When the conditions are nearly, but not quite perfect, we get a close approximation to the results outlined above. There still remain, however, slight residual differences of elevation of the land surface, which, on the one hand, provide posi- tive features whence a certain amount of mechanically transported sediment is removed, and, on the other, negative features in which such detritus accumulates as beds of freshwater sediment. If, then, we encounter patches of such sediments genetically associated with the characteristic chemically formed coatings, the phe- nomenon need cause no surprise, nor need it be taken as disproof of the thesis advanced above.

Owing to the development, probably in Miocene time, of an ex- tremely perfect peneplain over almost the whole extent of Au- stralia, and the contemporaneous existence of a climate character- ized by marked alternations of dry and wet seasons, there has originated, in almost every part of Australia, a hard crust or “armor-plate”’ of chemically formed material. This crust may

be aluminous, ferruginous, siliceous, or calcareous ; but always re- flects in its composition the nature of the underlying bed-rocks.

-

Origin Of White Clays And Bauxite. 893

I claim that this chemically formed crust, produced under such well-defined conditions, is of formational rank, and I have sug- gested for it the name of duricrust.

That it is not due to any transportation of material by mechani- cal means is shown by the fact that, in the granite plateau of the Darling Range of Western Australia, a thick coating of concre- tionary “ laterite” is underlain by a deeply and completely decom- posed mass of kaolinized granite. This has suffered no me- chanical transportation whatever, since thin veins of muscovite- bearing pegmatite in the underlying granite can be traced up- wards, without a break, through the kaolinized rock and into the “laterite.” In these veins, only the quartz and muscovite have escaped decomposition. The feldspars are completely kaolinized and the materials of the vein are quite incoherent. They could not possibly have withstood the action of even tiie feeblest current of water.

Similar effects are apparent in other places where the bed-rocks are crystalline schists or sediments; and the thesis that conditions of perfect peneplanation favor chemical weathering, and are un- affected by mechanical transport, appears to be well established.

Neumann brings out this point very specifically in his description of the clays of South Carolina; but has not added the corollary of complete leaching of all soluble materials from the subsoil. I suggest that this process of leaching provides a sufficient explana- tion of the absence of iron from clays derived, by mechanical transportation, from such a deeply weathered surface, during a subsequent period of uplift and denudation.

The important point which has not been emphasized by Neu- mann is that, given a sufficiently long continuance of perfect peneplain conditions, practically the whole of the iron will have been removed in solution. There is thus no need to invoke selec- tive sorting out of the iron minerals from a red regolith, like that forming at the present time over the Piedmont Belt of South Carolina, to account for the whiteness of the sedimentary clays. These were deposited during the period immediately following upon the complete peneplanation of the area in Middle Cretaceous

894 W. G. Woolnough.

time, at which time the regolith was probably not red, but was completely leached of all its iron.

Neumann has postulated a pluvial climate at the time of for- mation of the clays. As has been indicated above, the complete removal of the iron in solution appears to be a necessary conse- quence of perfect peneplanation under such conditions. Even if the climate had been markedly seasonal as regards the incidence of the rainfall, the whiteness of the transported clays would still be preserved. In the case of Western Australia, the concretionary “ shotty”” character of the “ laterite’? ensures a complete me- chanical separation of this material from the pure white clay, whenever the “ duricrust ” and its subjacent clay stratum are sub- ject to transportation by running water. The ferruginous ma- terial is deposited at the inner edge of the piedmont in the form of alluvial fans; whereas the pure clays, carried further afield, come to rest in any deep basin competent to receive them.

The writer wishes again to disclaim any intention of criticism of the work of Neumann. The conditions met with in Australia are so clearly defined and so relatively simple, that it is thought that they may throw light on structures in other countries where certain complications may obscure the facts to some extent.

Canserra, F, C. T., AUSTRALIA.

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Certain Magmatic Titaniferous Iron Ores And Their Origin.

Part Ii.

Freleigh F. Osborne.

CONTENTS—continucd.

Goncordant bodies in anorthosite: os... 02sec cs Mek e eee 895 Deposits along Calamity Brook. 2.502.442.4025 er eee 896 Meponite: Cheney ‘Bond .)...1. 855s cai fan oe eee. 898 Genesis of the concordant bodies. ccccecccee 899

Contondanta hediess in. Sabre cs, és. 35 0a an ak Sere doe aw Ss gol KGeRIOSISSONT THE LEDOSIES. actn.s ino saeco in cae Sea go2

Titaniferous.aton).ofe in permatites: <.<.0c¢.2-<. 00ers. 904

Mineratopraphys 20 ae ns scat es eee Bere Oh See's 905 Relations of magnetite, hematite, ilmenite, spinel. 905 Occurrence of magnetite, ilmenite and hematite 914 CIO NC FECT aI gE ER sn SGP Mate ees rh a Ec aD g15

Relationship of magmatic injections of titaniferous iron ore to some

non-titaniferous magnetite deposits. eee eee 915 Pneumatolitic vs. magmatic origin of the titaniferous iron ores 917 Suminany And CONCHISIONS?) /6:<..c)2sOr1. Sei: 4 s6ikaic Skee bee Seas cose 919

Concordant Bodies In Anorthosite.

In contrast to the discordant bodies of ore, are other bodies parallel to the observed structure in the rock that fall under the class of concordant bodies. They vary from nearly pure iron ore to pyroxenite. This variation is important because it may help to give an idea of the processes that produced the pure ore by show- ing intermediate products.

In the vicinity of Lake Sanford there are several iron ore de- posits which belong to the concordant group, for example, those exposed along Calamity Brook and the west side of Henderson Lake and the ores near Cheney Pond. On account of the differ- ence in composition the last deposit will be described separately fromi the first two.

896 Freleigh F. Osborne.

Calamity Brook.

Along Calamity Brook above its junction with the Hudson, there are exposures of several types of iron ore bodies; some are composed almost entirely of magnetite and ilmenite, others consist predominantly of silicate with a small amount of metallic min- erals. Near the junction of Calamity Brook with the Hudson, and on the north bank of the brook, there is a small body of ore that belongs to the discordant class. A little farther upstream is a medium-grained gabbro dike, which contains considerable iron ore. It shows a nearly vertical contact with the anorthosite and a blocky pseudo-columnar jointing. On the south side of the brook at the point where it joins the Hudson, there are a number of nar- row pegmatite dikes, cutting the anorthosite. Some consist of twinned feldspar only; others have plagioclase, pyroxene, magne- tite, and titanite. The three types of ore bodies just described are clearly intrusive into the anorthosite, but the relationships of the remaining bodies of gabbro, norite, and websterite exposed along the brook to the anorthosite are not so clear. They are sill-like bodies, which follow the structure of the anorthosite. A map made of the Calamity Brook area shows that for 800 feet along the brook they have a consistent north-south strike, and a dip of 25° east. About one quarter of a mile from the outlet they have the same strike but the dip is 20° west and they have the same attitude on the ridge 800 feet north of this point. The contacts of these bodies and the anorthosite are usually sharp, and both rocks are coarse-grained up to the very margins. The anorthosite on the footwall and hanging wall side of the bodies is often differ- ent but the same variation is found in the anorthosite along the valley of the Hudson where there are no ore bodies. Singewald,”* on account of the sharpness of the contacts, considered them in- trusive. Specimens from the Bushveld complex in South Africa show contacts between anorthosite and more basic rock which are no sharper than those formed along Calamity Brook, and the

95 Singewald, J. T., Jr., “ Titaniferous Iron Ores in the United States,” U. S. Bureau of Mines, Bull. 64, p. 65, 1913.

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Magmatic Titaniferous Iron Ores. 897

banding of the Bushveld Lopolith is ascribed to a Stratiform ar- rangement due to the sinking of crystals. Recently Reunnig ” has suggested that the banding of the lopolith is due to successive intrusions of magmas derived by differentiation at depth. He be-

Fic. 8. Ilmenite websterite from Calamity Brook showing the interstitial character of the iron ores and the manner in which they corrode and embay the silicate minerals. The pyroxenes are both diallage and hypersthene with intergrowths of the other very common. Nicols crossed. XX 13%.

Fic. 9. Ore from Sanford ore body, Lake Sanford showing the etched surface of magnetite with one ilmenite lamella (white, I) which is out- lined by spinel. Other spinel lamellz are visible on the etched sur- face. XX 45.

lieves that even the ores were magmas that intruded the country rock. ‘Thin sections of Bushveld rocks appear to lend confirma- tion to this view because they show textures that are very difficult to explain by accumulation of crystals alone.

96 Wagner, P. A., “ The Magmatic Nickel Deposits of the Bushveld Complex in the Rustenberg District, Transvaal,” 1924.

97 Reuning, E., “ Verbandsverhaltnisse und Chemismus der Gesteine des Bush-

veld Igneous Complex, Transvaals und das Problem seiner Entstehung,”

Neues Jahrb. f. Min., Beil. Bd. 57, 631-664. 1927.

898 Freleigh F. Osborne.

On account of the doubt regarding the actual genetic connec- tion of the sill-like bodies to the anorthosite the writer prefers to group them under a purely morphological group as “ concordant bodies in anorthosite.” Rocks similar to those along Calamity Brook are found on the west side of Henderson Lake where they strike north-south and dip 15° west. They are poorly exposed, and it is possible that they are an extension of the Calamity bodies. Rocks of a similar type are also found among diamond drill cores from the north end to the Sanford Hill ore body, but their relations to the Sanford Hill type could not be ascertained.

Petrography.—All the sill-like bodies are composed of essen- tially the same minerals; the difference is in their relative propor- tions. The following rock types are found most commonly: gabbro, norite, pyroxenite, ilmenite, and websterite. The optical properties of the minerals of the rocks are generally the same and their relationships to one and another similar; on this account a complete petrographic description need not be given of each rock. it is believed that the rocks are all products of the same process of differentiation, since all gradations are found between them.

The minerals of the concordant bodies are plagioclase, diallage, hypersthene and iron ores. The plagioclase crystallized first and was followed by the pyroxene with the iron ore minerals last. The pyroxenes are present in complex intergrowths of various types which increase in complexity with the amount of pyroxene in the rock. (See Fig. 8.) In some of the rocks there is a tendency for diallage or hypersthene to form a poikilitic texture. The ore bodies of this type show considerable variation in the character of the iron ores. Those, in which there is considerable plagioclase usually consist of magnetite and ilmenite, although in many the magnetite is only in small amounts. In the rocks high in pyrox- ene there is a tendency for ilmenite and hematite to form the iron ores. Where magnetite is absent the well-developed reaction rims around the plagioclase are lacking.

Cheney Pond.

The Cheney Pond ore body is on the east slope of the valley extending from the south end of Cheney Pond. The locality is

he e nc he Bl pc sh th ra nc an C:

ho

co It

re

Magmatic Titaniferous Iron Ores. 899

heavily mantled with drift and vegetation with relatively few exposures. The rocks show a well-defined banding which strikes north and south and dips from a few degrees to 20° west. A hornblende gneiss and a gabbro are the commonest rocks found. To the west the nearest outcrop is an anorthosite with a few ex- posures of gabbro near it. Singewald says that the drill cores show that the ore and gabbro grade into anorthosite. Some of the old drill cores are still on the ground but they are so disar- ranged that the relations between the ore body and anorthosite can- not be confirmed. The contacts between the ore-bearing gabbro and the hornblende gneiss are sharp, resembling those of the Calamity bodies.

Petrography.—The hornblende gneiss consists of a dark green hornblende with a little interstitial hypersthene and iron ores. The gabbro that occurs below the hornblende gneiss locally be- comes sufficiently rich in the iron ore minerals to be called an ore. It consists of labradorite, hypersthene, hornblende, and apatite. About 200 yards east of the deposit syenite is exposed, but its relationship to the anorthosite or the ore body is uncertain.

Genesis of the Concordant Bodies in Anorthosite.

The iron ore bodies associated with the gabbro, norite, and websterite series are so intimately tied up with them in origin that any explanation of the one will explain the other. There are a number of features for which any explanation must account: (1) The bodies are sill-like; (2) they are of wide extent; (3) the minerals composing them are the minerals of late crystalliza- tion in the country rock; (4) they are in sharp contact with the anorthosite; (5) the texture shows that the rocks crystallized under static conditions; (6) when feldspar is present the amount of iron ore increases with the amount of pyroxene. Although the writer is in doubt about the actual relationships of the bodies to the anorthosite in the field, it appears most probable that the bodies are intrusive, and are filter-pressed differentiates from the anor- thosite.

98 Singewald, J. T., Jr., “ Titaniferous Iron Ore in the United States,” U. S. Bureau of Mines, Bull. 64, p. 63, 1913.

900 Freleigh F,. Osborne.

Xvidence for the rather long interval separating the periods of formation of the minerals of the concordant bodies may be de- rived from the presence of ophitic and poikilitic textures. This tendency is readily apparent in those rocks in which the mafic sili- cates are prominent. Experimental work by Fouqué and Lévy ” indicates that ophitic texture is produced in artificial melts if the plagioclase is allowed to separate first, leaving an interstitial liquid consisting of pyroxene. The pyroxene forms a coarse-grained ag- gregate enclosing the plagioclase, which has a lath-shaped outline. The thin sections lend support to the idea that the pyroxene and iron ore were molten after the plagioclase had crystallized. The conditions for the formation of ophitic texture in a rock are not known, but they are probably connected with the composition of the magma, the conditions of cooling, and the content of min- eralizers. This texture is commonly found in plutonic and hyp- abyssal rocks, but has been described from a thin lava flow. Whatever are the conditions necessary for its occurrence, the separation of the pyroxene at a stage following the plagioclase is essential. It is noteworthy that in the rocks in which an ophitic texture is developed the iron ores also are usually later than the plagioclase and commonly than the pyroxene.

With the foregoing evidence in mind it is possible to account for the variety of sill-like bodies found along Calamity Brook as filter-pressed differentiates from an original anorthosite magma.

The gabbros with the low content of mafic could be accounted for as a restmagma pressed from the original anorthosite when part of the plagioclase had crystallized and while the pyroxene and iron ores were still molten. The norites with their higher content of mafic constituents would represent a differentiate at a still later stage. ‘The websterites would be formed when all the plagioclase had separated and the restmagma had the composition of pyrox- ene and iron ore. The pure ore represents a final filter-pressed product derived after most of the pyroxene had separated. This hypothesis accounts for the types intermediate between iron ore

99 Zirkel, F., “ Lehrbuch der Petrographie,” Bd. 1, p. 827, 1893. 100 Waters, A. C., “A Structural and Petrographic Study of the Glass Buttes, Lake County, Oregon,” Jour. Geol., 35, p. 446, 1927.

and stitu the

A ore : tion at tl anor the 1 ther mig!

catec that that belo:

U in w enclc oreti conc closii coun possi they type is kn tario 101 ] matic South 103 / Areas,

MAGMATIC TITANIFEROUS IRON ORES. 9goI and anorthosite, for the concentration of the last crystallizing con- stituents, and for the increase in the iron ore with the pyroxene in the plagioclase-bearing types.

A feature that is not easy to explain is the localization of the ore along the primary structure planes of the anorthosite, whereas the coarse-grained discordant type cuts across it. Such localiza- tion is probably the result of the tendency of the rock to bow up at the center due to a force originating within the mass of the anorthosite. The filter-pressing action would be intense close to the margins, but in the area in which the bowing had taken place, there would be loci of lower pressure into which the magma would migrate.

The process of filter-pressing on a large scale has been advo- cated by Foslie to account for certain dikes in norite. From the literature it is rather difficult to find descriptions of deposits that may be of the same type as these just described. It is possible that some of the deposits associated with the Bushveld Lopolith belong here.

Concordant Bodies In Gabbro.

Under concordant bodies in gabbro are included those deposits in which the iron ore has crystallized later than the silicates of the enclosing rock and conforms to the primary structure of it. The- oretically there are two possible types: one in which the iron ore concentration is contemporaneous with the formation of the en- closing rock, and another in which the iron ore is injected into the country rock parallel to the primary structure. It is almost im- possible to distinguish between the two types in the field, therefore they are grouped together. The only example of the concordant type in gabbro that the writer has had the opportunity of studying, is known as the Pusey Mine and is in Glamorgan County, On- tario. The ore is in bands parallel to the direction of the primary

101 Foslie, Steinar, “ Field Observations in Northern Norway Bearing on Mag- matic Differentiation,’ Jour. Geol., 29, pp. 701-719, 1921.

102 Wagner, P. A., “ The Iron Deposits of the Union of South Africa,” Union of South Africa Geol. Surv., Mem. 26, 1928.

103 Adams, F. D., and Barlow, A. E., “ Geology of the Haliburton and Bancroft Areas, Ontario,’’ Can. Geol. Surv., Mem. 6, pp. 163-166, 1910.

902 Freleigh F. Osborne.

structure of the rock. In some places the banding is rather thin: 4 cm. of ore may alternate with 2 cm. of rock, but thicker ore bands and wider silicate bands are common. The ore, even in the hand specimen, may be seen to be interstitial to the silicates. The ore is of too low a grade and too high in titanium to be com- mercial.

Petrography.—The specimens collected by the writer show a titaniferous or ferriferous augite, generally in idiomorphic crys- tals, with a plagioclase which is quite strongly zoned. The iron ore is interstitial to all the other constituents and well-developed reaction rims surround it. Where the iron ore is in contact with augite a dark brown hornblende is formed, where it is in contact with plagioclase there is a rim of light-green hornblende. All the specimens contain magnetite and ilmenite ; although in some places magnetite is present in very small amounts.

Genesis of the Deposits —Foye considers the Pusey iron ore to be of contact metamorphic origin, due to the intrusion of a nepheline syenite into the gabbro. The writer could not find the deposits along the contact, but from Foye’s description, and from analogy with the definitely magmatic ores above the deposit, he would conclude that they are magmatic concentrations, in which the surrounding gabbro has been somewhat scapolitized due to the intrusion of the nepheline syenite. The reaction rims, which Foye ascribes to the contact metamorphism, are similar through- out the whole thickness of the gabbro and appear to have been formed by reaction between liquid iron ore and the silicates, with which they are in contact.

The origin of the iron ore bodies is intimately connected with the origin of primary banding in igneous rocks. Grout has summarized the processes which may produce primary banding: (1) Partial assimilation of inclusions forming schlieren, (2) Lit-par-lit or fluidal gneiss, (3) Deformation during crystalliza-

104 Foye, W. G., “The Relation of the Titaniferous Magnetite of Glamorgan County, Ontario, to the Associated Scapolitized Gabbro,” Econ. GEOoL., 11, p. 662,

105 Grout, F. F., “Internal Structure of Igneous Rocks; Their Significance and Origin; with Special Reference to the Duluth Gabbro,” Jour. Geol., 26, p. 452, 1918.

ferent tion, (8) C iron ¢ consid rich z the ir for th sunk ¢ tallize dence experi immis does n for ba Aft the D count filter-f tallizec bowin; the fo layer. form : metho 106 Da physical “ Eviden Jour. Ge Pp. 66-6 a Silicat W., BD Pp. 364, 1 Pp. 1-42 been pre of Agate 107 Bo

p. 418, 1

ene

Magmatic Titaniferous Iron Ores. 903

tion, (4) Deformation just after solidification, (5) Streaked dif- ferentiation with reference to rhythmic cooling or intrusive ac- tion, (6) Successive intrusions, (7) Heterogeneous intrusion, (8) Convection during crystal differentiation. In the case of the iron ores, number (1), (2), (6), (7) may be dismissed from consideration because of the frequent repetition of the iron ore rich zones. The texture seems to eliminate (5) and (8) because the iron ores lack idiomorphic outlines and they form a matrix for the silicates. It is, of course, conceivable that they may have sunk or have been carried down as immiscible drops, which crys- tallized later. However, the lack of experimental and field evi- dence for immiscibility seems to preclude this possibility. The experimental evidence shows the possibility of there being some immiscibility of oxides in silicate melts of a composition which does not resemble that of any igneous rock. The third mechanism for banding is now left for consideration.

After Grout had minimized the efficacy of crystal settling in the Duluth gabbro and postulated two-phase convection to ac- count for the banding, Bowen suggested the possibility of filter-pressing as an explanation. When the magma has crys- tallized sufficiently for the mass to begin to transmit stress, the bowing of the basin may cause a separation of the bands and the formation of an actual or potential space beneath the stronger layer. The still fluid part of the magma may migrate there and form a layer of the last crystallizing part of the magma. This method would give alternating layers of quite different composi-

106 Day, A. L., “Immiscibility in Silicate Liquids,” Ann. Rept. Director Geo- physical Laboratory, Year Book, No. 25, pp. 61-62, 1925-1926. Tanton, T. L., “Evidence of Liquid Immiscibility in a Silicate Magma, Agate Point, Ontario,” Jour. Geol., 33, pp. 629-641, 1925; “‘ Emulsions of Silicates,” Amer. Jour. Sci., 15, pp. 66-68, 1928. Bowen, N. L., “ Concerning Evidence of Liquid Immiscibility in a Silicate Magma, Agate Point, Ont.,” Jour. Geol. 34, pp. 71-73, 1926. Bain, G. W., “ Diffusion in the Agate Point Vitrophyres,” Amer. Jour. Sci., 11, pp. 74-88, P. 364, 1926. Greig, J. W., “ Immiscibility in Silicate Melts,” Amer. Jour. Sci., 13, PP. I-44, PP. 133-154, 1927; 14, P. 472, 1927. “On the Evidence which has been presented for Liquid Silicate Immiscibility in the Laboratory and in the Rocks of Agate Point, Ontario,” Amer. Jour. Sci., 15, pp. 375-402, 1928.

107 Bowen, N. L., “ Crystallization Differentiation in Magmas,” Jour. Geol., 27, p. 418, 1919.

904 Freleigh F. Osborne.

tion. Benson has concluded that both Bowen’s filtration method and convection are inadequate to explain the primary structure, as, for example, in the Bushveld Complex.

Though the author favors filter-pressing to account for some of the other deposits, in this case it seems inadequate. At the Pusey Mine the iron ore, the last constituent to crystallize, forms bands from % to 1% inches thick, separated by a few inches of silicate minerals. Crystal bridging near the bottom of a magma chamber seems improbable, and a bridging producing bands of the thick- ness and spacing of those found would seem particularly unlikely. Mead has suggested that closely spaced planes of shearing in a partly solidified magma may form, and that a residual magma may migrate to these planes, forming a rock with gneissic struc- ture. Proof of the operation of this process in nature is yet lack- ing. There does not appear to be any adequate explanation for this type of occurrence of the iron ores for the primary structure in igneous rocks.

Titaniferous Ore In Pegmatites.

The pegmatites of many undersaturated rocks contain titanifer- ous iron ore. Iron ores from the corundum-bearing pegmatite at Craigmont, from the sodalite deposit at Bronson, Ontario, and from a gabbro pegmatite near the mouth of Calamity Brook were selected for study. All these ores are essentially the same in min- eralographic character. They contain magnetite, which is cut by irregular tongues of ilmenite apparently caused by replacement. The ilmenite contains intergrown hematite lamellae. The magne- tite is altered to hematite parallel to the octahedral parting, and, in the deposits from the corundum and sodalite pegmatites, the ilmen- ite is altered to rutile. The character of the normal magmatic ores will be described later, but those in which mineralizers have been active are readily distinguished from them. It is of interest to note that in the gabbro pegmatite, titanite is found as crystals associated with apatite, suggesting that with the increase in vola-

108 Benson, W. N., “ The Tectonic Conditions Accompanying the Intrusion of Basic and Ultrabasic Igneous Rocks,” Nat. Acad. Sci., vol. 19, Mem. I, p. 73, 1926.

109 Mead, W. J., ‘“ The Geologic R6le of Dilatancy,” Jour. Geol., 33, p. 607, 1925.

til

th

ol th

MAGMATIC TITANIFEROUS IRON ORES. 905 tile constituents silica may be freed to combine with the TiO, to form titanite, leading one to postulate a stage in the derivation of the non-titaniferous ores from a basic magma.

Mineralography.

The methods of investigation of the iron ore minerals have been outlined in a previous paper,”*® hence it is unnecessary to treat them here.

The Relations of Magnetite, Hematite, IImenite, Spinel.

Several investigators have used the reflecting microscope to study magnetite, hematite, and ilmenite; also there have been chemical investigations of the magnetite and hematite relation- ships ; consequently, there is considerable literature on the subject. The most recent and valuable paper is by Ramdohr."* He sum- marizes most of the available literature on the subject and gives results of his own experimental work in rendering homogeneous by heating intergrowths of magnetite and ilmenite, and ilmenite and hematite.

Ilmenite and Spinel—rThe so-called titaniferous magnetite usually consists of an aggregate of magnetite, ilmenite and subor- dinate spinel. The appearance of the ore in the hand specimen suggests that the ilmenite completed its crystallization after the magnetite. The ilmenite from most localities is homogeneous even when examined by high magnification (1200), but in some places it shows inclusions of a non-opaque mineral in small tablets, which when viewed by polarized reflected light are seen to be arranged parallel to the base or at right angles to it. The tablets are not so continuous as those of spinel in magnetite.

Magnetite and Spinel—The magnetite commonly contains many inclusions of various kinds. One invariably present, in the experience of the writer, is a spinel, usually pleonaste or hercynite,

110 Osborne, F. F., “ Technique in the Investigation of Iron Ore,” Econ. GEOot., 23, PP. 442-450, 1928.

111 Ramdohr, Paul, “ Beobachtungen an Magnetit, Ilmenit, Eisenglanz und Ueber- legungen iiber das System FeO, Fe,0., TiO,,” Festschrift zur 150—Jahrfeier der

Bergakademie Claushal, 1925, pp. 307-341. Reprinted and enlarged, Neues Jahrb. f. Min., Beil. Bd. 54, pp. 320-379, 1926.

906 Freleigh F. Osborne.

as shown by the clear green color in thin section. This non- opaque mineral will be referred to simply as spinel. In some sec- tions it appears as dashes and round dots; in others as well-formed narrow lamellae, and in still others as broad and irregular lamellae. The inclusions of spinel are arranged parallel to the faces of the cube, so that in most sections they appear as two sets of lamel- lae intersecting one another at nearly right angles. Ina few places the ilmenite and spinel lamellae appear to be parallel to one an- other (Fig. 9), indicating that some spinel may separate parallel to the octahedron, or that some ilmenite may separate parallel to the cube. The amount of spinel in magnetite is variable, but the maximum may be estimated at about 4 per cent; usually it is much less.

The presence of spinel is believed to be due to the unmixing of a once homogeneous solid solution of spinel in magnetite. The unmixing probably took place at high temperature. Ramdohr found that in Taberg magnetite heated for 12 hours at 1000°, the spinel is dissolved completely. After 24 hours at goo° in an atmosphere of nitrogen, part of the spinel dissolved, and some solution is apparent after 12 hours at 800°. These experiments suggest that the magnetite held the spinel in solid solution and that unmixing occurred at a temperature near 800°. The solubility of spinel in magnetite is probably also dependent on the amount of ilmenite already dissolved.

Magnetite and Ilmenite-—One of the most interesting features of the ore from a commercial and scientific point of view, is the ilmenite that is intergrown with the magnetite. In the case of the Adirondack iron ore the presence of this intergrowth was pointed out by Singewald and Warren.*”

112 Granigg, B., “ Zur Anwendung metallographischer Methoden auf die mikro- skopische Untersuchung von Erzlagerstatten, VI, Ueber die Titanomagnetit von Smaland Taberg,” Metall und Erz, 17, p. 57, 1920. Schneiderhohn, H., “ Anleitung zur Mikr. Bestimmung, etc.,” p. 258, Berlin, 1922.

113 Ramdohr, Paul, op. cit., p. 371.

114 Singewald, J. T., Jr., “ Titaniferous Iron Ore in the United States,” U. S. Bur. of Mines Bull. 64, 1913.

115 Warren, C. H., “ The Microstructure of Certain Titanic Iron Ores,” Econ. GEOL., 13, Pp. 430, 1918.

In direct to the show

Fic. HCl. contain the pho

Fig. netite differen octahed the mas

are du erick © of lam lamella to be s also px

116 Sin

117 Bre Pp. 686, 1

Magmatic Titaniferous Iron Ores. 907

In the magnetite the ilmenite shows as lamellz in three or four directions. Singewald says, with the ilmenite arranged parallel to the octahedral faces of magnetite that a random section will show the lamellae in three directions, and that a greater number

Fic. 10. Ore from the Split Rock Mine, showing surface etched with HCl. The ilmenite (I) and the gangue (G) unetched. The magnetite contains lamelle of ilmenite and spinel which are indistinguishable in the photograph. & 49.

Fig. 11. Kent Mine Lincoln Pond. Etched surface of a coarse mag- netite ore showing the etching effected on three grains of magnetite of different orientation by HCl. Note that the first solution is along the octahedral directions. The bright dots in the dark sector are spinel in the magnetite. 45.

are due to twinning which will develop five directions. Brod-

erick however, has shown that there should be four directions

of lamellae as a maximum. In the opinion of the writer, when

lamellae are present in more than four directions, some are likely

to be spinel, which may easily be confused with ilmenite, or it is

also possible that the unmixing of the ilmenite may take place 116 Singewald, J. T., Jr., op. cit., p. 31.

117 Broderick, T. M., “ Titaniferous Iron Ores of Minnesota,” Econ. Grox., 12, Pp. 686, 1917.

908 Freleigh F. Osborne.

parallel to the cube as well as to the octahedron. (Figs. 10 and 11) Warren has noted that many of the rods and dots called ilmenite by Singewald are really spinel. The writer confirmed this observation by the use of polarized reflected light. All but a very few of the dots are composed of non-opaque mineral, and many of the lamellz in the magnetite are of the same mineral.

The ilmenite in the lamellae is generally tabular parallel to its base; thus basal plates of ilmenite lie parallel to the octahedral planes of magnetite. In some ores, notably those that belong to the pegmatite type, broad tongues of ilmenite cut the magnetite. The ilmenite in the tongues is not oriented with respect to the direc- tions in the magnetite, and the manner of occurrence suggests replacement.

The magnetite-ilmenite relationships in the titaniferous iron ores have been the subject of considerable study and several hy- potheses have been advanced to account for them. It was early recognized that the grains of magnetite do not consist of homo- geneous magnetite, but are intergrowths of two minerals. Hus- sak’s investigation of the titaniferous magnetite from Brazil was among the earliest pieces of work on polished surfaces of ores by reflected light. He suggested that the intergrowth is due to the solidification of a eutectic. Warren and Singewald express somewhat the same idea. Warren believes that the non-opaque lamellae may have originated by secondary growth and that the ilmenite lamellae are due to unmixing. There seems to be one in- superable argument against the magnetite and intergrown ilmenite being considered as a eutectic, 7.c., the magnetite appears to have completed its crystallization before the ilmenite, whereas if it were a eutectic it should have been altogether liquid until the last stage. Lindley believes that magnetite and ilmenite can form a con-

118 Warren, C. H., op. cit., p. 430, 1918.

119 Singewald, J. T., Jr., op. cit., pp. 122-123.

120 Hussak, E., “ Uber die Mikrostruktur einiger brasilianischer Titanmagnetit- eisengesteine,’ Neues Jahrb. f. Min., 1904, I, p. 108.

121 Warren, C. H., op. cit., p. 440.

122 Singewald, J. T., Jr., op. cit., p. 34.

123 Lindley, H. W., ‘“‘ Mikrographie der Eisenerzmineralien oberhessischer Besalte,” Neues Jahrb. f. Min., Beil. Bd. 53 A, p. 356, 1925.

tinuot if the ordin: place. of the tite a the m netite result tempe Th matic ilmen a less ilmer octah stanc has b prob: netit amot of F a mc cause Tl colo1 of il: thos be ol is of histc sam mag a

t & to

Magmatic Titaniferous Iron Ores. 909

tinuous series of mix-crystals which may remain homogeneous if they are cooled with sufficient rapidity, although under the ordinary conditions of cooling in plutonic rocks unmixing takes place. Ramdohr believes that at the temperature of separation of the ores, there is considerable mutual solubility of the magne- tite and ilmenite, and that there is an unmixing of ilmenite from the magnetite on cooling. He found that he could make the mag- netite with intergrown ilmenite homogeneous by heating. As a result of this work he believes that the unmixing took place at a temperature from 700-800 degrees.

The evidence seems to indicate that at the temperature of for- mation of the ore minerals there is considerable solubility of ilmenite, in magnetite, and the same is also true of spinel, but to a lesser extent. On cooling, by reason of decreased solubility, the ilmenite separates from the magnetite parallel to the face of the octahedron, and the spinel parallel to the cube. The apparent con- stancy between the amount of TiO, and Fe in the magnetite which has been suggested to be due to the formation of a eutectic is more probably the result of a limited solubility of the ilmenite in mag- netite at the temperature of formation of the minerals. The amount of spinel is less constant since it depends upon the amount of FeO, MgO, Al.O, in the magma. These oxides constitute a more complex group of variables than the TiO, and FeO, be- cause they may go to form silicate minerals.

The magnetite in the titaniferous ores is of a rather constant color. Ramdohr has noted that with an increase in the amount of ilmenite dissolved in magnetite, the properties tend to approach those of ilmenite. He found that with an increase of ilmenite, the color becomes more strongly “ braunrosa.” As far as could be observed, the magnetite in the deposits examined by the writer is of nearly constant color, probably because it had nearly the same history after solidification and therefore has approximately the same amount of ilmenite in solution. Lindley notes that the magnetite with the high content of ilmenite is red brown under

124Ramdohr, P., op. cit., p. 374.

125 Ramdohr, P., op. cit., pp. 335-336. 126 Lindley, H. W., op. cit., p. 343.

gIO FRELEIGH F, OSBORNE.

polarized light, is weakly anisotropic, and is more resistant to acids than normal varieties.

A number of investigators have noted the natural occurrence of a mineral with some properties intermediate between those of magnetite and hematite. It has been identified as a ferromag- netic form of ferric oxide and has been formed artificially by sev- eral investigators. The natural compound appears to have been

Fic. 12. Ore from drill cores from the north end of Sanford Hill, Lake Sanford. Showing the hematite etched from ilmenite (I). Gangue (G) is unetched. X 16.

Fic. 13. Polished surface of ilmenite-hematite intergrowth showing the two generations of lamellz. Note the polysynthetic twinning cutting across both ilmenite and hematite in the upper right. The section at the lower left is nearly basal as shown by the width of the hematite lamellz. Crossed nicols. 18.

first identified by Hesemann as a magnetic ferric oxide. New- house and Callahan have described a form of oxidized mag-

127 Hesemann, J., “‘ Die devonischen Eisenerze des Mittelharzes,” Abhandl. zur prakt. Geol. u. Bergsw., Bd. 10, pp. 38-42, 1927.

128 Newhouse, W. H., and Callahan, W. H., ‘‘ Two Kinds of Magnetite,” Econ. GEOL., 22, pp. 629-632, 1927.

on set of

of

ge firs ere

MAGMATIC TITANIFEROUS IRON ORES. gII

netite from a number of localities which may be of the same type. Wagner has found a mineral in the oxidized portions of certain of the titaniferous ore bodies in South Africa, that may be the same. He proposes the name “ maghemite.” In the titaniferous magnetite studied by the writer, there is no evidence of variation in color due to oxidation.

Ilmenite and Hematite-——The relationships between ilmenite anid hematite are, in general, simpler than those between magnetite and ilmenite. Ramdohr and Lindley believe that there is continuous mutual solubility of hematite and ilmenite. The writer’s results indicate that there is considerable miscibility, but the occurrence of hematite interstitially to ilmenite grains indi- cates the possibility that, under some conditions, there may be an excess of hematite.

The hematite separates from the ilmenite parallel to the base and in parallel crystallographic orientation with it (Figs 12 and 13). A noteworthy feature of the ilmenite-hematite intergrowths is the constancy of the ratio between the ilmenite and hematite that obtains in any one deposit. The same spacing is found in isolated grains as in the main ore body regardless of grain size. Their relative amount varies in the different deposits.

In many localities there are two sets of hematite lamellae, one large and one small. According to Ramdohr the larger set, which he calls the first generation, is due to a lower solubility of the hematite in ilmenite than existed at the time of formation of the original homogeneous mineral. The lamellz of the second generation, which are smaller and occur within the lamellz of the first generation, are due to a change in the symmetry of the min- erals. At ordinary temperatures hematite is rhombohedral-hemi- hedral, and ilmenite is rhombohedral-tetratohedral. At high tem- peratures they both have the same symmetry and, when the in- version point is reached at which the symmetry becomes different,

129 Wagner, P. A., “ Changes in the Oxidation of Iron in Magnetites,” Econ. GEOL., 22, pp. 843-846, 1927.

180 Ramdohr, P., op. cit., p. 374. 181 Lindley, H. W., op. cit., p. 356. 182 Warren, C. H., op. cit., p. 434. 188 Ramdohr, P., op. cit., pp. 350-351.

gi2 FRELEIGH F, OSBORNE.

the unmixing takes place. By heating the specimens Ramdohr ob- served that the lamellae of the first generation disappeared at about 700° and those of the second at 500°—600°. Some experimental work has shown that hematite undergoes an inversion at 675° but that the temperature is considerably modified by the presence of foreign substances.

The disposition of the ferrous iron in excess of that required for ilmenite is one of the problems presented by this type of ore. Lindley observed magnetite arranged parallel to the rhombo- hedron in ore from Eckernsund which showed hematite arranged parallel to the base. Warren has also described lamallae of magnetite in ilmenite. The writer failed to find any magnetite in the ilmenite-hematite intergrowths from the deposits which he visited. If such existed it should have been apparent after a very brief etching of the surface with HCl. However, much of the hematite of the intergrowths is ferromagnetic, and crushed grains of the intergrowth yield a magnetic fraction with a magnet of low intensity, such as a needle or knife blade. Furthermore the hematite of the lamellae is more readily attacked by HCl and HF than the specular hematite used as a standard. The anomalous behavior may possibly be explained by the presence of a certain amount of magnetite in solution in ilmenite.

Magnetite and Hematite—The question of the solubility of magnetite in hematite is still an unsettled problem. Sosman and Hostetter postulated solid solution of magnetite in hematite over a considerable range. Their conclusion was attacked by Broderick and others on the ground that the intermediate types that might be expected did not occur in nature, but that the

134 Forestier, H. and Chaudron, G., Compte rend., 180, pp. 1264-1266, 1925.

185 Lindley, H. W., op. cit., p. 349.

186 Warren, C. H., op. cit., p. 423.

137 Sosman, R. B. and Hostetter, J. C., “‘ The Ferrous Iron Content and Suscepti- bility of Some Artificial and Natural Oxides of Iron,” Trans. Amer. Inst. Min. Eng., 58, pp. 409-433, 1918.

138 Broderick, T. M., “ Some of the Relationships of Magnetite and Hematite,” Ecox. GEOL., 14, pp. 353-366, 1919.

139 Gilbert, Geoffrey, ““ Some Magnetite-Hematite Relations,” Econ. GEOL., 20, pp. 587-596, 1925. Gruner, J. W., “ Paragenesis of the Martite Bodies and Magnetites of the Mesabi Range,” Econ. GEOL., 17, pp. I-14, 1922.

MAGMATIC TITANIFEROUS IRON ORES. 913 hematite containing ferrous iron could be seen to contain micro- scopically visible magnetite. Ruer and Nakamota found that the solubility of magnetite in hematite even at high temperature did not exceed four per cent. Sosman’s present attitude in the matter may be seen in the following quotation :

Sosman and Hostetter’s assumption of limited solid solution of Fe,O, in Fe,O, is questioned by Gruner and by some European investigators, on the ground that it is not shown in certain natural ores containing ferrous iron. The case is not a perfectly plain one, because the decrease in oxy- gen pressure in equilibrium with the oxides from Fe,O, toward Fe,O, is small and is brought out only by a logarithmic plot, but it is undoubt- edly present and there is yet no proof that it can be due to other causes than solid solution.

To the writer, it seems possible that solid solutions may exist. The material called hematite shows a considerabie variation in properties: the streak shows notable variation in color, and be- havior with etching reagents is not constant. Some microscop- ically homogeneous hematite is attacked more readily than other homogeneous specimens by HCl and HF, and its behavior to- ward boiling KOH solution varies. A specimen of hematite, which superficially resembles magnetite or ilmenite, gives a black power after passing a 200 mesh screen, gives oniy a slight red tinge on the streak plate, and by qualitative test yields some fer- rous iron. A complete analysis of this material is not available, but the writer hopes to be able to present it later. In view of the uncertainty regarding the occurrence of a solid solution, the writer feels justified in suggesting that the magnetic properties and anomalous etching behavior of the hematite in the ilmenite are due to the solid solution of magnetite in the hematite. The mix-crys- tals retain the symmetry of the hematite and the white color, which serves to distinguish them from ilmenite, but they are ferro- inagnetic and less resistant to acids than hematite.

140 Ruer, R. and Nakamota, M., “Uber Eisen und Kupferoxyde,” Rec. de trav. Chim. de Pays-Bas, 42, pp. 675-682, 1923.

141 Sosman, R. B., “‘ The Common Earths,” Ann. Surv. of Amer. Chem., vol. 2, pp. 123-129.

gI4 FRELEIGH F. OSBORNE.

Conditions of Occurrence of Magnetite, Ilmenite and Hematite.

The conditions governing the occurrence of the three minerals have been given earlier in this paper, therefore only a brief re- view need be given here. In a magma, if there is any FeO avail- able it will combine with TiO, to form ilmenite. The relative rarity of occurrence of rutile and perovskite as pyrogenetic min- erals is evidence of the ease of the formation of this compound. If there is an excess of ferrous iron after the formation of the metasilicates and orthosilicates, it will combine with the Fe,O, to form magnetite. Thus the occurrence of hematite or magnetite is governed by the amount of iron that has gone into the forma- tion of the silicates. A small excess of iron might form magnetite which would go into solid solution in the hematite. The occur- rence of the ores in the pegmatite seems to be due to a different process. The magnetite was deposited first and then was replaced by ilmenite containing hematite in solid solution, which later un- mixed and formed the tiny hematite blebs in the ilmenite. The later occurrence of ilmenite in the deposits of the pegmatite type parallels the observation of the later occurrence of the ilmenite in the pyrogenetic iron ores and helps to confirm the conclusion that the ilmenite-hematite intergrowths originate at a lower tem- perature than the magnetite-ilmenite.

Hematite and Magnetite——Where hematite and magnetite oc- cur as microscopically separable bodies they appear invariably to be associated with hydrothermal alteration. In the pegmatite type, besides the hematite unmixed in the ilmenite, there is com- monly hematite formed along the octahedral parting of the mag- netite, obviously as the result of the hydrothermal alteration.

Rutile —Rutile is probably also in some occurrences a hydro- thermal mineral. In the ore of the pegmatite type it occurs as aggregates along the margin of some of the ilmenite grains. The rutile in urbainite has been considered of primary origin but there is a possibility that it arises because of an unusually high content of volatiles in the magma. The greater alteration of the anortho- site surrounding this deposit and the presence of a typical meta- morphic mineral, sapphirine, suggest that there was an unusual

facto ilmer Fe,O move

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MAGMATIC TITANIFEROUS IRON ORES. 915 factor operative here. The rutile may be due to the reaction of ilmenite with hematite according to the reaction FeTiO; + Fe,O; TiO, + Fe,0O,. The magnetite so formed may be re- moved in solution.

Conclusions.

The more important conclusions derived from the mineralo- graphic study of the ore may be summarized briefly. The lamellz of ilmenite and spinel in magnetite, and of hematite in ilmenite, are due to the unmixing of solid solutions which were stable at the temperature of formation. Further study confirms the obser- vation of Warren that spinel is often abundant in the magnetite and that some investigators have confused it with ilmenite. This conclusion has commercial and theoretical bearing, because it means that a greater amount of titanium than previously thought is dissolved in the magnetite or else that ilmenite is present as submicroscopic lamellae. In any case it shows that a certain amount of ilmenite cannot be freed from the magnetite by fine grinding.

The experimental work of Ramdohr indicates that the unmix- ing of the original solid solutions was the result of slow cooling and was probably complete at a temperature above 500°. In the pneumatolytic ores the occurrence of irregular, unoriented ilmen- ite in tongues cutting the magnetite is probably the result of replacement. The occurrence of hematite along the octahedral parting of the magnetite and rutile in ilmenite is also probably the consequence of hydrothermal alteration.

RELATIONSHIP OF MAGMATIC INJECTIONS OF TITANIFEROUS IRON ORE TO SOME NON-TITANIFEROUS MAGNETITE DEPOSITS.

The genesis of the non-titaniferous ores of the type that oc- curs at Mineville is still an unsettled problem. In the field, the writer was impressed by the resemblance between certain magne- tite-bearing pegmatites in the anorthosite, and some of the ore and gangue from Mineville. In the pegmatite, the magnetite is

cut by tongues of ilmenite, which are apparently of replacement

916 Freleigh F. Osborne.

origin. However, much of the TiO, has combined with silica and lime to form titanite, which appears in the pegmatite along with biotite and apatite. The ores at Mineville are ordinarily thought of as non-titaniferous, and this is true in so far as tita- nium in the form of metallic minerals is concerned, but in many places the ore contains considerable titanite, and in certain pits, assays as high as 4 per cent. titanium may be obtained. The titanite is dark brown, almost black, and it may easily be mis- taken for magnetite with strongly developed parting, except that the streak is non-metallic. Kemp believes that the ore has been derived from the gabbro by action of pegmatites, but the relationships in the vicinity of the deposits are so complex that there is a possibility that emanations from more than one rock body are present. The presence of zircon and allanite, minerals that are typical of acidic rocks, suggests that some of the ore may be derived from a granite. The plagioclase-bearing peg- matite associated with the ore may have been derived from the gabbro because of the presence of volatile constituents. The TiO, formed titanite, whereas if volatiles had not been active ilmenite might have been formed.

The non-titaniferous magnetite deposits of southeastern New York have been studied by Colony.’** He believes that the mate- rial that forms the pegmatites and the magnetite is the end stage product of a basic magma. The material that gives rise to the pegmatites and iron ore is supposed to have been differentiated into the two final products of pegmatite and magnetite. He be- lieves that the magnetite deposits are “ magmatic-replacement de- posits, or replacement deposits of deuteric origin.” ‘These deposits are similar to the magmatic injection types in that they are the last products of crystallization of the magma, but in the titanifer- ous magnetite deposits the effect of volatile constituents is not great and evidence of deuteric effects in rock alteration and re- placement is not apparent. In the magmatic injection deposits the

142 Kemp, J. F., “‘ Geology of the Elizabethtown and Port Henry Quadrangles,” N. Y. State Mus. Bull., 138, pp. 126-132, 1910.

143 Colony, R. J., “‘ The Magnetite Iron Ore Deposits of Southeastern New York,” New York State Mus. Bull., 249-250, p. 70, 1923.

Magmatic Titaniferous Iron Ores. 917

minerals are all of pyrogenetic character. There is not much evidence for replacement in the titaniferous deposits. Colony has described ore from one deposit of the ‘‘ magmatic-replacement type” which contains 1.01 per cent. TiO,. According to him it contains ilmenite, but no ilmenite was found in a specimen kindly lent the writer by Professor Colony. At any rate the analysis shows that some ore of this type may contain TiO.. However, in most cases there is no danger of confusing magmatic injection deposits with “ magmatic-replacement ”’ deposits since the former contain only pyrogenetic minerals and the latter are accompanied by rock alteration.

Non-titaniferous magnetite has been described as forming in- jected bodies in ore deposits of other than the Kiruna type. Clapp in discussing the genesis of a magnetite deposit of con- tact metamorphic origin suggests that the magnetite has been in- jected as a “‘ virtually magnetite magma.” The appearance of the ore body is the only evédence for injection. The ore outlines blocks of limestone, but in detail there is abundant evidence for replacement by dilute solutions.

The criteria for distinguishing magmatic injection from pneu- matolytic ore bodies are not very definite. However, it is be- lieved that some criteria may be brought out by a consideration of the reasons why, in the opinion of the writer, the titaniferous ores are not of pneumatolytic origin. Most of the evidence has been presented before, but it is summarized here.

Puneumatolytic vs. Magmatic Origin of the Titaniferous Iron Ores.—A pneumatolytic or hydrothermal origin for the ore min- erals of the magmatic injection deposits was entertained as a working hypothesis. The principal argument in its favor is, of course, the fact that the iron ores crystallized later than the sili-

144 Colony, R. J., op. cit., p. 115.

145 Clapp, C. H., “ Southern Vancouver Island,” Geol. Surv. Can., Mem. 13, pp. 192-193, 1912.

146 Osborne, F. F., “‘ The Magnetite Occurrences on the West Coast of Vancouver Island; their Contact Metamorphism and Ore Genesis,” University of British Columbia, unpublished thesis, pp. 66-67, 1925. Uglow, W. L., “Genesis of the

Magnetite Deposits near the West Coast of Vancouver Island,” Econ. Geot., 21, P. 357, 1926.

918 Freleigh F. Osborne.

cates and that they are, in some places, surrounded by reaction rims. The evidence against the pneumatolytic origin has been discussed elsewhere, but it is of interest to summarize the reasons why, in the opinion of the writer, such an origin is improbable. The first and probably the most weighty argument is that typical pneumatolytic minerals are wanting. It is true that biotite and hornblende occur, but they are commonly pyrogenetic minerals and in this instance it is believed that they are derived by the interaction of a solid mineral and a magma. The absence of sericite, epidote, scapolite, zoisite, actinolite and other minerals of the same sort is noteworthy and opposes strongly the idea of the formation of the ore by pneumatolytic processes. Commonly, there is preferential replacement accompanying pneumatolytic action. In the experience of the writer, deposits in which magne- tite has been formed by pneumatolytic processes in diorite show replacement of the femic silicates. The ore minerals which com- monly occur as veins in the norite, gabbro, and diorite cut across plagioclase, hypersthene, augite, and hornblende without change of width, indicating the lack of preferential replacement. Fur- thermore some thin sections show that the ore minerals pushed aside the walls of small fractures and that they have been em- placed without replacement.

If the ore were pneumatolytic, then idiomorphic silicates in- closed by iron ore minerals would indicate that some other mate- rial had once formed a matrix for the silicates and had subse- quently been entirely replaced. That such a matrix should be replaced by iron minerals, which have the texture of an igneous rock, with no residuals, seems unlikely, and it seems more probable that the iron ore minerals represent a restmagma originally inter- stitial to the silicates.

The ilmenite-hematite deposits present some evidence that bears on this problem. These deposits bear the same relationships to the country rock as do the magnetite-ilmenite deposits, except that well marked reaction rims are not present. If, then, these ores are of pneumatolytic origin, they are the only result of pneu- matolytic processes observed in the country rock. It seems im-

at int ob ee oe ON

Magmatic Titaniferous Iron Ores. 919

probable that only ilmenite and hematite should form. These ores consist of hematite lamellae in ilmenite. If any one deposit is taken as an example it is found that the ratio between the amount of hematite shown in the prism zone of ilmenite is nearly constant, and this same ratio is found in grains that are isolated from the main mass of the ore body. If the ores were of pneu- matolytic or hydrothermal origin it appears improbable that the solutions should rema-n of constant composition for a sufficiently long time to deposit all the ore with an essentially constant ratio of hematite to ilmenite. Furthermore it is difficult to see how the same proportion should appear in isolated grains in the country rock. That the ratio of ilmenite to hematite is not in the nature of a eutectic is shown by the fact that the ratio varies in different deposits.

In conclusion, the writer believes that the titaniferous iron ore deposits are magmatic, and that although they were accompanied by some volatile constituents there is no more reason for regard- ing the deposits and their accompanying reaction products as pneumatolytic than for regarding biotite, hornblende, and mus- covite in an igneous rock as pneumatolytic. The evidence indicates that the ore was a magma derived as the last product of crystalliza- tion of the parent rock.

Summary And Conclusions.

The deposits of titaniferous iron ore, namely those of ilmenite and magnetite, and of ilmenite alone, in gabbro and anorthosite, raise certain problems in ore genesis, and various hypotheses have been advanced to account for them. The study of these ores was undertaken in an attempt to answer certain questions: (1) Is there such a thing as a central segregation in anorthosite? (2) If so, by what process did it originate? (3) Are there such things as marginal segregations in gabbro? (4) If there are, what was their origin? (5) Are there magmatic segregations of titaniferous ore?

The field work was directed to study certain deposits in New York, Quebec, and Ontario, supposed to belong to one or another

Freleigh F. Osborne.

of the above classes. The origin of the anorthosite, which forms 1 the host rock for some of the deposits, was a subsidiary prob- ( lem. The field study showed that the iron ore deposits in anortho- site are sharply bounded against the host rock, that is, there is no transition from anorthosite to ore. The same is true of the de- 1 posits in gabbro and related rocks. The contacts of the orebodies 1 in both rocks show that the ore was introduced after the country

rock became solid. These observations in so far as the deposits 1 studied are concerned, lead to the following conclusions: (1) 1 Central segregations do not exist, (2) there are no marginal segregations, (3) the ore bodies are intrusive into the host rock.

The laboratory work confirms the conclusions from the field work. The ore minerals in the ore bodies, in the anorthosite, and in most of the gabbro crystallized later than the silicate minerals, and, therefore, the deposits cannot be magmatic segregations, which following the petrologists’ usage are accumulations of the early-formed constituents. Microscopic examination of the anorthosite shows that it was a rather well-defined order of crys- tallization; plagioclase was first, followed by pyroxene, and the iron ore minerals were formed last. The same order holds for the ore bodies and for most of the gabbros except that in some of ( the latter, the periods of crystallization appear to have overlapped 1 somewhat.

Many of the ore minerals show complex intergrowths, which are believed to be due to the unmixing of once-homogeneous solid ( solutions, following a decrease in solubility because of a fall in ( temperature below that which obtained at the time of formation ;

of the ore. The magnetite commonly contains tablets of ilmenite arranged parallel to the octahedral face. Spinel lamellz, parallel to the face of the cube are almost, if not quite, as common as those of ilmenite. In the ilmenite deposits that do not contain magnetite, hematite always occurs as disks parallel to the base of ilmenite. By heating these intergrowths, Ramdohr was able to ( make them go into solution again. The temperature at which ( they dissolve gives an idea of the lower limit of temperature of ore formation. This and other thermometer points suggest a tem- (

Magmatic Titaniferous Iron Ores. 921

perature of ore formation between 800° and 1100°. Evidence derived from the study of reaction rims and ore texture shows that the ore was derived from a magma. A hydrothermal origin for the ore is believed improbable because of the lack of hydro- thermal alteration of the rock minerals and lack of preferential replacement.

The field and laboratory work show that the ores are not mag- matic segregations but are “ magmatic injections.” The follow- ing morphological classification is proposed for such deposits of titaniferous iron:

1. Discordant bodies.

A. In anorthosite. B. In gabbro and related rocks.

2. Concordant bodies.

A. In anorthosite. B. In gabbro and related rocks.

The discordant bodies need no explanation; they are simply those bodies that cut across rock structure in the manner of a dike. The concordant bodies are those in which the ore minerals have crystallized later than the silicates, and follow the primary struc- ture of the host rock.

Inasmuch as the deposits are not segregations, and have not been formed by concentration of early-formed crystals another explanation must be sought to account for them. Filter-pressing, or the pressing out of the last products of crystallization, is ad- vanced as a probable explanation to account for the ore bodies in anorthosite. Since the order of crystallization in the anortho- site is plagioclase, pyroxene, and iron ores, filter-pressing might give a magma consisting of plagioclase, pyroxene, iron ores, or pyroxene and iron ores, or of iron ore alone, depending on the stage of crystallization of the anorthosite from which it was derived. All the products intermediate between anorthosite and ore bodies are found in the anorthosite. If an attempt is made to apply the same hypothesis to the deposits in gabbro, some diffi- culty is encountered: there is not the well defined protoclasic

922 Freleigh F. Osborne.

structure that is such a common feature of the anorthosite; oli- vine, an early product of crystallization, is often formed with the iron ore minerals. The olivine may be explained as a reaction product; however, there is considerable evidence that it is of direct crystallization from the magma.

In addition to the problem of genesis a number of subsidiary problems are raised by the titaniferous ore bodies. For instance, what are the conditions that determine the occurrence of magne- tite and ilmenite or of ilmenite with intergrown hematite? The amount of ferrous iron originally available appears to be the gov- erning factor. The rarity of rutile in the ore deposits of this type shows that the TiO, is usually able to combine with FeO to form ilmenite. However, if a large part of the FeO is required to form ortho- and metasilicates there may not be enough left to unite with the ferric oxide to form magnetite, and hematite results. As might be expected in rocks with a high content of ferrous-bearing silicate minerals, the websterites usually show ilmenite with hema- tite.

LaporaTory OF Economic GEOLoGcy, YALE UNIVERSITY, New Haven, Conn.

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Editorial

Origin Of The Magmatic Sulphide Ores.

THE student of ore deposits may be permitted to register a mild perplexity as to the complete lack of agreement that exists in re- gard to the origin of what are known as the magmatic sulphide ores.

How, he must ask himself, can the widely divergent views be reconciled to which geologists of high repute give expression?

There appears, unfortunately, to be little prospect of the con- troversy being sent to an eternal rest until there is a complete change of tactics and mental attitude on the part of those who write about the deposits of these ores.

The blame lies probably on both sides, but as an ardent sup- porter of the segregation hypothesis the writer may perhaps be pardoned for ascribing the present unhappy situation mainly to its opponents who, of recent years, have undoubtedly allowed minor and often quite irrelevant details to obscure the grander features of the deposits.

A deplorable tendency has also manifested itself deliberately to ignore previous work and conclusions bearing on the subject.

Thus, a distinguished professor, lecturing in London last year, gave as one of his main reasons for opposing this theory that the microscopic study of magmatic nickel ores proves that the sul- phides, instead of being among the first formed minerals, were really the last to consolidate. No one denies this. It was recog- nized by J. H. L. Vogt over thirty years ago. It has been ex- plained repeatedly as due to the freezing point of the sulphides in question being lower than that of the associated silicates. In the circumstances it is difficult to see how on the segregation hypothesis the relation could be otherwise.

Incidentally it may be remarked that no one has as yet come

924 Editorial.

forward to disprove the evidence adduced independently by F. S. Hudson and the writer to show that, whereas the sulphides un- doubtedly crystallized subsequently to the silicates, they began to separate before them.

It should be stated, too, that segregationists have never denied that there has been some corrosive replacement of silicates by sul- phides—this being attributed to shifts in the equilibrium of the complex system—or that the sulphides may have crystallized with the aid of mineralizers such as sulphuretted hydrogen.

The professor, already quoted, uses as a further argument against the segregation theory that some of the ore of the par- ticular deposit that he describes occurs in the hornfels forming its footwall. Had he been familiar with the literature on the subject he would have learned that contact deposits are found in association with some of the best authenticated examples of mag- matic deposits, such as those of Little Namaqualand, Insizwa, and the Bushveld Complex. In any case, to use a purely accidental or environmental relation such as this as an argument against the hypothesis of sulphide segregation is to beg the whole question.

That one-sided studies, however brilliant, do not get us much farther is evidenced by Tolman and Rogers’ admirable paper on “The Magmatic Sulphide Ores” published twelve years ago— probably the most comprehensive and complete study of its kind ever attempted. After establishing a number of fundamental facts in regard to the textural relations between sulphides, primary and secondary silicates in these ores, they reached the rather astonish- ing conclusion that the ores were introduced at a late magmatic stage as a result of mineralizers, presumably by some sort of eruptive after-action. In some deposits of this nature, however, we have proof that there can have been no subsequent introduc- tion of sulphides. I refer here particularly to certain occurrences on or connected with the Merensky Platinum Horizon of the Bushveld Igneous Complex. This, one of the greatest known mineral deposits, has generally the character of a remarkably per- sistent inclined sheet of platinum-bearing sulphidic norite or pyroxenite, over- and underlain by anorthosite or anorthositic

norit relati sulph thosi addit is als pletel by a and as th guish and 1 have It rant pregr concl the s: It wate! becor carrie then relati and i specit origit Fu quiet! tion, anort pletel betwe

Wi

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Editorial. 925

norite to which the ore-bearer clearly stands in a complementary relation. There is evidence to show that the crystallization of the sulphide-bearing norite was posterior to that of the footwall anor- thosite and anterior to that of the hanging wall anorthosite. In addition to the sheet-like development of the platinum-carrier, this is also in places lenticular, and small lenses and patches lie com- pletely isolated in the footwall anorthosite surrounded on all sides by a considerable thickness of that rock. These carry platinum and the same sulphides—pyrrhotite, pentlandite and pyrrhotite— as the normal Merensky “ Reef” from which they are indistin- guishable petrologically. The surrounding anorthosite is fresh and unfractured, and shows no sign of any openings that could have served as channels of access.

It is inconceivable, as has been pointed out elsewhere, that vag- rant solutions could have picked out such isolated lenses and im- pregnated them with sulphides. There is thus no escape from the conclusion that there has been here no later sulphide introduction ; the same obviously applying to the Merensky “ Reef ”’ itself.

It should be stated, however, that in places—evidently where water and mineralizers were abundant—the Merensky “ Reef ” becomes abnormally rich in biotite and hornblende and sometimes carries quartz and primary calcite, with which the sulphides are then intimately associated. In such circumstances the textural relations between sulphides and silicates become very complicated, and it would be practically impossible from a microscopic study of specimens of this nature to draw any definite conclusion as to the origin of the sulphides.

Further, where, instead of the “ Reef” having consolidated quietly, there was considerable movement during consolida- tion, the relation between it and the overlying and underlying anorthositic rocks becomes obscure, so obscure as to have com- pletely misled certain observers in regard to the general relations between them.*

Where such movements merely resulted in the fracturing of the

1N. Jahrb. f. Min., B. B. LVII. 1927, pp. 631-664.

926 Editorial.

ore-bearer, the normally interstitial sulphides have been squeezed into the cracks and fissures thus formed.

Arguing then from a relatively simple case such as that pre- sented by the normal undisturbed Merensky “ Reef” to more complex cases, it is clear that marked differences may be expected in the relation between the ore-bearer and the enclosing rocks and in the textural and spatial relations between sulphides and sili- cates, according as consolidation took place quietly or under dias- trophism, and according to the amount and distribution of the water and mineralizers present. It is easy to see that where the latter were very abundant, and there was at the same time con- siderable movement and fracturing, there might be formed either within or without the limits of the parent rock sulphide deposits differing markedly from the more normal segregations. Perhaps we of the Vogt school have been too tardy in recognizing and admitting this.

There must necessarily, as Graton, Lindgren and De Launay each pointed out long ago, and A. F. Buddington has more re- cently shown,” be transitional facies from strictly magmatic or orthotectic through pneumotectic to hypothermal deposits.

This, however, does not in any way invalidate the correctness of the segregation theory in general. It is the only one competent to explain at once the mineralogical and petrological peculiarities of the deposits and their more striking geological relations, such as the marked differentiation often exhibited by the portions of the parent rock in which the deposits occur; the limitation, where such is the case, of the ore bodies to the differentiated zones; the invariable occurrence of the ore deposits at the lower contact or in the lower part of the parent rock mass, whereas, if the sul- phides had been concentrated by the agency of mineralizers, one would expect to find them at the upper contact; finally the occur- rence, adjacent to all the more important deposits, of great bodies of disseminated ore consisting of small composite sulphide blebs uniformly distributed through unaltered or practically unaltered and unfissured norite or pyroxenite, containing no trace of the

2Econ. Grot., vol. 22, pp. 158-179, 1927.

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by 1 they

Editorial. 927

secondary silicates which, on the assumption that the sulphides have developed by the replacement of the primary constituents of these rocks, must have been liberated in great quantity.

These features are commonly either ignored or slurred over by the advocates of later introduction, for the simple reason that they are quite inexplicable on that hypothesis.

Percy A. WAGNER.

Discussion And Informal Communications

Diffusion In Ore Genesis.

Sir: Allow me to submit a few comments on Mr. Alfred R. Whitman’s interesting and enjoyable paper that appeared in the August number of this journal under the above title. Let me state that I concur with his statement that “ diffusion is a factor in the formation of metalliferous deposits”? (page 482). On this same page, Mr. Whitman refers to the “ novel and swift em- placement of ‘ore-magmas’ as championed by Mr. Spurr.” I wonder if Mr. Whitman realizes that he has unwittingly fur- nished laboratory evidence in support of Mr. Spurr’s theory on the movement of matter, miscalled ‘a magma.’ The sole dif- ference between Mr. Spurr’s theory of emplacement as a viscid mass, and Mr. Whitman’s experimental deduction is, that Mr. Spurr’s theory envisions openings of relatively large dimensions, —created by the matter itselfi—as a path for mobile mass, and gaseous tension—as a resultant of telluric force—as the power to keep the mass in motion until emplacement occurs, whereas Mr. Whitman utilizes crystal contacts and boundary planes as paths for mobile matter, and furnishes water as a carrier agent. In trying to explain the existence of angular fragments of rock in vein filling that were apparently foreign to that place, Mr. Spurr advanced the theory of “ emplacement ” of a mass in a “ viscid” condition, a dough-like substance, ‘‘a magma.” That he did not postulate this condition as applicable at all times, may be inferred from his statements in ‘‘ The Ore Magmas ” where he writes of “ rhythmic precipitation ” and the “ Liesegang rings.” I have no doubt that Mr. Spurr based his hypothesis on some observed phenomenon in his own experience, just as Mr. Whitman accepts

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DISCUSSION AND COMMUNICATIONS. 929 the evidence of “ diffusion” based on his own laboratory experi- ment.

The experiment delineated by Mr. Whitman furnishes a very convenient illustration of what is probably “the mode of em- placement’ of “secondary ore deposits.” It illustrates one “mode” of emplacement, applicable “at times,” while Mr. Spurr’s theory envisions another mode that may be applicable to some cases.

Further on, Mr. Whitman, referring to metasomatism, states that “it is not, strictly speaking, a chemical reaction, because it operates without regard to molecular weights’ (page 484). As a comment on this, let me suggest that in the study of the phe- nomena of metasomatism we should consider atomic instead of molecular behavior.

The statement (pages 484-485) that “ the walls of fissures are under different conditions than the more distant rock,” for “along these surfaces strain has been relieved ” is too ambiguous. A clearer statement would have been more in harmony with the rest of the paper. I may state that some of us at least base our work on the assumption that in metasomatism the metastable (after-stable, unstable) state of the replaced body is one of the operative factors.

Louis H. SMITH. CHICHAGOF, ALASKA.

Reviews

Petroleum and its Products. By WitttaM A. Gruse. Pp. viii -+ 377.

McGraw-Hill Book Co., New York, 1928. Price, $4.50.

In this volume the author has collected all the available information on the chemical composition and physical properties of crude petroleum and its derivatives, and discusses from theoretical and practical viewpoints the processes of distillation, cracking, and refining by chemical and ab- sorption methods, and the technical problems involved in the preparation and in the efficient utilization of gasoline, kerosene, petroleum waxes, fuel oil and asphalts. A final chapter describes miscellaneous petroleum products, such as insulating oils, painter’s naphtha or turpentine substitute, liquid petrolatums and various by-products recovered directly as a result of oil-refining processes and those obtained by subjecting these and the direct products of distillation to chemical treatment.

The author has done his work in a thorough manner and has enriched his discussion by the citation of many references to the literature, among which are to be noted many to German and British authorities.

W. S. BayLey.

Geology. By A. P. BricHam. Revised and expanded by F. A. Burt.

Pp. x +544. Illus. 313. D. Appleton & Co., New York, 1928.

The revised edition of Brigham’s “ Text-Book of Geology” differs from the original edition in that the chapter on pre-Cambrian time has been rewritten and five chapters on mineral products have been added. The main body of the book is as in the first edition, pagination and pic- tures being for the most part identical in both editions. Most of the differences appear in the discussion of historical geology, where a few fossil names have been changed and a few modifications in stratigraphy have been referred to. Schuchert’s paleogeographic maps have replaced Le Conte's maps showing the supposed distribution of land and water at different periods, and a few of the old familiar woodcuts of structural features are replaced by photographs, that are more truthful if less striking.

It is perhaps unfair to expect an elementary text-book to be compre- hensive, but it is fair to ask that it be accurate so far as its statements go, and that these shall not be vague. It certainly will not be clear to the student what ‘metamorphism’ means, when he reads that it is a change

Reviews. 931

of form ‘especially in the direction of hardness and crystalline character,’ but “is not used of the ordinary degrees of pressure and cementation,” etc. It might have been better for the book if the entire section on meta- morphic rocks, because of its vagueness, had been omitted, and a para- graph or two on crystalline schists had been substituted.

A somewhat different criticism may be applicable to the chapter on pre-Cambrian time. This is too specific. It might better have avoided definite statements as to the divisions of the pre-Cambrian formations, since the student, before he advances very far beyond the elementary grade, will soon learn that the stratigraphy of pre-Cambrian time is not as simple as is outlined, and that few geologists will accept the author’s statement as expressing the facts.

The chapter on mineral products deals principally with the uses and places of occurrence of the common mineral materials of economic value. Only rarely are there references to their methods of occurrence and origin. Here too we find a vagueness of statement that is regrettable. Micas, for instance, are said to be “in usable form only in granites and meta- morphic rocks.” The Tennessee Ordovician rock phosphate deposits which are in limestone, are stated to have been concentrated through the weathering of the limestone, whereas Devonian phosphates are harder and are not weathered.

Perhaps these criticisms may appear harsh, since an elementary text- book must present something definite to the beginning student and should not qualify its statements to such an extent as to cause confusion in his mind. The reviewer, however, believes that it is unnecessary to take a definite position on all debatable questions, and that an honest statement of doubt in many cases is of greater educational value to the young stu- dent than the presentation of a questionable opinion as a fact. If the discussion of a subject must be vague it might be better to omit it. Even though only those subjects are discussed upon which there is general agreement, there is enough of interest in geology to attract the attention of any ordinary student.

The book is well printed and is fully illustrated. In the hands of a conscientious teacher it will prove very satisfactory as a text for be- ginners.

W. S. BayLey.

Text-book of Inorganic Chemistry. Edited by J. N. Friend. Chas. Griffin & Co., London, J. B. Lippincott Co., Philadelphia. Vol. VI., Part I., Nitrogen. By E. B. R. Pripeaux and H. Lampourne. Pp. 242, Figs. 25. 1928. Price, $10.00.

This is one of a series of exhaustive textbooks dealing with each branch

of inorganic chemistry, written by outstanding authorities. Volumes I.

932 Reviews.

to IX. have already been published, excepting two parts of Volume VI. and Part IIT. of Volume IX.

Volume VI., Part I., deals comprehensively with the important food producing substance, nitrogen. Its place in the Periodic Table is dis- cussed and its properties and atomic structure are given. All the com- pounds of nitrogen are considered at length from the standpoint of his- tory, properties, constitution, chemical reactions, and preparation, and 25 pages are devoted to its fixation. References are numerous. The volume contains a wealth of factual material and is a real contribution to the series.

Volume IX., Part II. Iron and Its Compounds. By J. Newton

Friend. Pp. 265. Index. 2d impression, 1925. Price, $10.00.

A notable book of the series, giving the history, mineralogy, properties and compounds of iron. Valuable for geologists.

Volume X. Metal-Ammines. By Miss M. J. SuTHERLAND. Pp. 260.

1928. Price, $10.00.

This additional volume of the series treats exhaustively of the metal- ammines of each group in the Periodic Table. Widely scattered informa- tion is here collected in usable form.

Mineralogisches Taschenbuch, 2d Ed. Edited by J. E. Hisscu. Pages x + 186. Cloth 8vo. Julius Springer, Wien, 1928. Price $2.75. The second edition of the “ Mineralogisches Taschenbuch” has just ap-

peared. It is a little volume published in Vienna in which is given a list

in alphabetical order of about 7,500 mineral names, including all those that have been suggested up to the present time.

A second list, also arranged in alphabetical order, gives the composition and physical properties of 2,800 of the most important minerals.

There are also included in the volume directions for the identification of different kinds of gems and of synthetic gem materials.

The volume concludes with a brief description of the mineral products of Austria, arranged according to material and district, a description of the mineral collections in Vienna and a list of the most important dealers in minerals and mineralogical apparatus.

The volume is a handy reference book for all those interested in any way in minerals.

W. S. B. Books Received.

Our Prehistoric Ancestors. By HerpMAan F. CLELAND. Pp. 379, figs. 159, index. Coward-McCann Co., New York, 1928 (Oct.). Price, $5.00.

An interesting story, delightfully written and beautifully illustrated, of early man and his customs and beliefs.

TA Tf

Th

Reviews. 933

Agricultural Geology. By F. E. Emerson. Revised by John E. Smith. Pp. 377, figs. 271, index, tables. John Wiley & Sons, New York, 1928. Price, $3.25.

A well-known text slightly revised. Essentially an unbalanced text of elementary geology, emphasizing those parts pertaining to agriculture. Investigations of Mineral Resources and Mining Industries, 1926. 16

articles, 79 pages. Canada Dept. of Mines, Mines Branch, No. 687.

1928 (Oct.).

Investigations of Fuels and Fuel Testing. 11 articles, 132 pp. Canada Dept. of Mines, Mines Branch, No. 689. 1928 (Oct.).

Diamonds for Industrial Purposes. Anton Smit & Company. Pp. 20. Antwerp, 1928 (Oct.).

Classes of diamonds and their uses.

Reports by the Mining Geologist of Southern Rhodesia, Feb., 1927, to May, 1928. By F. E. Keep. Pp. 79, illus. 18, articles 9. So. Rhodesia Geol. Surv. Bull. 13, Salisbury, 1928.

Geology of the Muddy Mountains, Nevada. By C. R. Lonewett. Pp. 152, ill. 26. U.S. Geol. Surv. Bull. 798, 1928 (Oct.).

General geology, with particular emphasis on the structure of an inter- esting and little known region A contribution to basin range geology. Surface Water Supply of the United States, 1924. The Great Basin.

Pp. 131. U.S. Geol. Surv. Water Supply Paper 590, 1928 (Oct.). Standard and Tentatively Adopted Methods of Testing and Grading

Foundry Sands. American Foundrymen’s Association. Pp. 94, figs.

28. Chicago, 1928. Price, $3.00.

Tests, analyses, classification; apparatus, bibliography.

Mining and Quarry Industries of New York for 1925 and 1926. By D. H. NeEwLanp and C. A. Hartnacer. Pp. 126. N. Y. State Museum Bull. 277, 1928 (Nov.).

Anleitung zur chemischen Gesteinanalyse: Sammlung naturwissensch. Prak., Bd. 15. By J. Jaxon. Pp. 81, figs. 3. Gebriider Borntraeger, Berlin, 1928 (Nov.). Price, 7 R.M.

Geophysical Prospecting: Some Electrical Methods. By A. S. Eve and D. A. Keys. Pp. 41, figs. 33. U. S. Bur. Mines, Tech. Paper 434, 1928 (Nov.).

Results of tests of several electrical methods over a known orebody in Colorado.

Scientific Notes And News

H. G. Ferguson, of the U. S. Geological Survey, has returned to Wash- ington after field work in Nevada and the Alleghany gold district, Cali- fornia. He was a recent visitor in New Haven, Conn.

Wilber Stout has been appointed State Geologist of Ohio, succeeding the late Dr. John A. Bownocker. Mr. Stout has been a member of the Geological Survey of Ohio for seventeen years.

L. C. Graton has returned to Cambridge after a year’s leave of absence spent in studying the deep mines of the world.

Frederick G. Clapp, having left Persia, has located his family in France for the winter, and can be reached at No. 68 Quai d’Auteuil, Paris XVI, or at his usual Church Street address in New York City.

David White and M. R. Campbell, of the U. S. Geological Survey, attended the Bituminous Coal Conference at Pittsburgh beginning No- vember 19.

Joseph Murdock is teaching mineralogy at the University of California, Los Angeles branch.

Frederick B. Plummer has resigned from the Vacuum Oil Company of Houston and is geologist of the Bureau of Economic Geology, University of Texas, Austin, Texas.

Frank Reeves, of the U. S. Geological Survey, has been spending some time in Virginia and West Virginia in continuation of his investigation of warm springs.

Max W. Ball has resigned as president of the Argo Oil Company to engage in consulting practice as petroleum engineer, with offices at the First National Bank Building, Denver, Col., and at the Exchange Na- tional Bank Building, Tulsa, Okla.

J. T. Lonsdale, of the Texas Bureau of Economic Geology, has re- signed his position to be Professor of Geology at the Texas Agricultural and Mechanical College.

Joseph H. Sinclair is re-writing for the New Encyclopedia Britannica the section devoted to Ecuador and the Galapagos Islands.

R. E. Somers and K. C. Heald, of the Gulf Companies, are lecturers at the University of Pittsburgh.

T. O. Bosworth, of Spratton, Northampton, England, is engaged in petroleum geological and engineering work in Ecuador and Colombia, with a temporary address at Santa Elena, Ecuador.

B. Dunstan, chief government geologist of Queensland, Australia, left

Bris!

geop Fr Mine in re Cy Bank TI the I be estab ology geop! Th at W Th Four to 25 Th ciety Jol State since age Ohio the ( Th cago depat

Scientific Notes And News. 935

Brisbane recently for six months in Germany, where he will investigate geophysical methods and modern methods in the treatment of zinc ores.

Frederick W. Horton, as mineral economist of the U. S. Bureau of Mines, is making a study of the mica reserves of the country, especially in relation to War and Navy Department interests.

Cyril W. Knight has opened offices as consulting geologist at the Royal Bank Building, Toronto, Canada.

The Gilliland Oil Company of Tulsa, Okla., has changed its name to the Reserve Petroleum Company.

T. A. Jaggar is proposing an Aleutian Geographic Laboratory, to be established at Dutch Harbor, Alaska, for land and sea mapping, meteor- ology, geology, magnetism, volcanology, seismology, and the study of geophysical, biological, and chemical processes along the Aleutian arc.

The American Mining Congress held its Thirty-First Annual meeting at Washington, December 5 to 8.

The Netherlands Indies Pacific Research Committee is to hold the Fourth Pacific Science Congress at Batavia and Bandoeng, Java, May 16 to 25, 1929.

The Society of Economic Geologists will meet with the Geological So- ciety of America at New York City, December 26 to 29.

John A. Bownocker, chairman of the Department of Geology at Ohio State University since 1916, director of the Geological Survey of Ohio since 1906, died at his home in Columbus, Ohio, on October 20, at the age of sixty-three. He was closely identified with geologic work in Ohio for 35 years, both in teaching at the University and in the work of the Geological Survey.

Thomas Chrowder Chamberlin, professor emeritus of geology at Chi- cago University, died on November 15 at Chicago. He was head of the department of geology and the Walker Museum 1rcm 1892 to 1919.

Index To Volume Xxiii.

[Note.—In this index the titles of principal papers and the headings of departments, as Discussion, are in italics.]

Abrasives, use in polishing, 302 Abt, A., on electrical conductivity of minerals, 778 Acetone, use of 334 Actinolite, analysis, 547 Adams, F. D., on the Morin anortho- site area, 74I Adams, F. D., and Barlow, A. E., on the Pusey mine, Ontario, 901 Adams, G. I., The occurrence and age of certain brown iron ores in. Alabama and adjacent States, on Eocene age of Alabama baux- ites, 90 Adsorption, selective, 773 Agar, W. M., reviews by, 226, 344 Alabama and adjacent States, The occurrence and age of certain brown iron ores in (Adams), 85 Alberta, carbon ratios, 361 coals, 367, 360 Cretaceous, 360 Alberta syncline, 359 Aldrich, H. R., on magnetic survey- ing in Wisconsin, 505, 509 Alkalies, concentration in

magma residuum, 881

Allan, J. A., on Drumheller coal field,

Allanite, 67; analysis, 76

Allen, E. T., and Zies, E. G., on

fumaroles, 877

Allison, I. S., review by, 346

Alluvial prospecting (review), 225

Alps, dolomites, 841

Alston Moor lead and zinc deposits,

Aletiion. Beaverdell district, 440

Guanajuato district, 29

Althans, R., on lead-zinc deposits of Silesia, 845

Alundum, for polishing, 305

Alunite, 252, 260

Ampangabeite, 66; analysis, 74

Amphibole, 4

Amphibolite, analysis, 547

Analyses (see Chemical analyses)

Ancient mining works of Cassandra, Greece (Sagui), 671

Anderson, R. V. V., on artesian pres- sure in the Punjab, 693

Andesite, I1

Andorite, 247

Anorthosite, concordant bodies in,

Anthophyllite, 404, 533; analysis, 547

Apparatus for heavy mineral separa- tion,

Aquifers, artesian, Compression and elasticity of (Meinzer), 263 Aquifers, Dakota artesian basin,

lenticularity, 136, 139 Arbuckle mountains, geology, 46 Argentite, electrical conduction of,

7Oi

Argyrodite, 660

Arsenides and antimonides, tivity, 794

Arsenopyrite, 199

Artesian aquifers, porter tee and elasticity of (Meinzer),

Artesian pressure, origin, 65° recovery, 278 Artesian pressure,

(Russell), 132 Artesian water, effect of earthquakes on, 6096 future supply, 157 loss of head, 146 Artesian wells of North Dakota, rate of discharge, 269 Ashley, G. H., and Robinson, J. F., on carbon ratios and petroleum in Pennsylvania, 356 Aspen district, dolomitization, 827 Atlantic City artesian well, 280 Austin, W. L., on nickel deposits in Oregon, 552 Automatic record of water level, 276 — 68; analysis, 74; occurrence,

conduc-

The

origin of

sec og F. E., on titaniferous iron Raslend i, analysis by, 551

Baeckstrom, H., on electrical con- ductivity of minerals, 778

Baedeker, K., on electrical conduc- tivity of minerals, 779

Bahurin, J., review of paper by, 341

Bain, G. W., on diffusion in Agate Point vitrophyres, 903

Bain, H. F., review of book by, 226

Bain, H. F., Van Hise, C. R., and Adams, G. I., on a in the Mississippi Valley, 824

Bakelite for mounting ore specimens,

Banding, George gold-copper ores,

in igneous rocks, 902 in wood tin nodules, 188

Bardet, on occurrence of germanium,

Barlow, A. E., on Webster nickel de- posits, 529

Barrett, W., and Besterman, T., re- view of book by, 461

Barrio Pasto, Porto Rico, The copper prospect at (Colony and Meyer- hoff), 515

Barton, D. C., reviews by 225, 341, 461, 600, 700

Base exchange, 541

Bastin, E. S., on feldspar depesits of Pennsylvania, 406

Bateman, A. M., on secondary de- formation in sulphide ores, 578

110, Iii,

reviews by, 221, 227, 345, 462, 466, 467, 586 Bateman, A. M., and McLaughlin,

D. H., on dolomite in the Kenne- cott district, Alaska, 833

Bayley, W. S., reviews by, I10, 224, 466, 582, 584, 586, 608, 7o1, 811, 816, 930-932

Bear River greenstones, 193 Beaverdell, B. C., Silver mineraliza- tion at (McKinstry), 434 Beaverdell quartz monzonite, 435 Becker, G. F., on computing diffusion,

Becker, G. F., and Day, A. L., on linear force of growing crystals, Beegerite, 251 Behrend F., and Berg, G., book by, 226 Beijerinck, F., on electrical conduc- tivity of minerals, 779 Beilby, George, on polishing, 299 Benedicks, C., on liquation, 733 Benson, W. N., on intrusion of igne- ous rocks, 904

review of

Index To Volume Xxii.

3erkey, C. P., and Morris, F. view of book by, 467

K., re-

Berry, E. W., on plants from New- castle sandstone, 134

Betafite, 66, 9 analysis, 74; ex- ploitation,

Beyschlag, F., vad lead-zinc deposits of Silesia, 845

Bibliographies, 564

3ibliography, corundum, 432

jitter Creek argillites, 193

Black Hills, northern, Tertiary min- eralization, 337

Blomstrandite, 66, 67

Blowing engine, ancient, 676

3oedlander, G., and Idaszewski, K. S., on electrical conductivity of minerals, 779

Bogojavlensky, L., by, 341

Bolivian copper deposits, A genetic comparison of the Michigan and (Singewald), 55

Boltsburn vein, cross-section, 836

review of paper

Books received, 113, 228, 348, 468, 587, 702, 818, 932 Bornite, 765

Bornite-chalcocite iniergrowths, Ex- periments bearing on (Schwartz).

Botsford, C. W., on the Guanajuato mining district, 8 Bowen, N. L., on differentiation proc- esses, 725, 903 on emery deposits near Peekskill,

on magmatic differentiation, 865 on order of crystallization, 748

on reaction principle in petrogene-

sis, 747 William, review of book by, Boydell, H. C., discussions by, 105,

on colloidal solutions in formation

of eter Sree, 124 Bradley, J. . Jr., review of book by, ar

Braun, F., on electrical conductivity of minerals, 77

Brazil, diamond deposits, 705

Breccia, S4¢ Joao da Chapada, Brazil, 712

Briére, Y., on_uraninite, 73

Brigham, ‘A. P., review of books by, 466, 930

Brock, R. W.. editorial, China and its mineral resources, 209-213

Broderick, T. M., on_ titaniferous

iron ores of Minnesota, 907

938 Index To Volume Xxiii.

Broderick, T. M., and Hohl, C. D., Geophysical methods applied to exploration and geologic map- ping in the Michigan copper dis- trict, 489-514

Bronzite, analysis, 546

Brooks, T. B., on Michigan iron dis- tricts, 501

Brown iron ores in Alabama and adjacent States, The occurrence and age of (Adams), 85; 454

Buckley, E. R., on dolomitization in southwestern Missouri, 825

Buerger, M. J., with Newhouse, W.

., Observations on wood tin nodules, 185-192 Bufa “sandstone,” 10 Burchard, E. F., discussion by, 454 on Dakota sandstone in Nebraska, O5 on iron ores of western Tennessee, 91; of Woodstock district, Ala-

bama, 83

Bushveld lopolith, 897

Butler, B. S., on oxidizing agents in hypogene solutions, 204

Caguana peneplane, Porto Rico, 519 Calamine, germanium in, Calamity Brook iron deposit, 896 Calcite, fractured, Cromwell oil field, Calcite, pressure twinning, 319 Callahan, W. H., on alteration of specularite to magnetite, 204 Campbell, M. R., on transformation of coal, 374 Campo y Cerdan, on germanium, 661 Canada balsam, 324 Canfieldite, 660 Caraga quartzite, 709 Carbon ratio theory, 353 Carbon ratios, Alberta, 361 high, of plains, causes, 373 New South Wales, 810 western Canada, 358 Carbon ratios as an index of oil and ” in western Canada (Jones),

Carborundum for grinding sections,

Cassandra, Greece, The ancient min- ing works of (Sagui), 671

Cassiterite, 245

Cassiterite crystals, 186

Castle Rock sandstone, 45

Cerro Gordo district, California, dolomitization, 831

Cerro Rico de Potosi, views of, 235

Certain magmatic titaniferous iron ores and their origin (Osborne), 724, 895 : pais

Césaro, G., on electrical conductivity of minerals, 779

Chalcocite, 252, 768

electrical conduction of, 781

Chalcocite and bornite intergrowths, 382; photomicrographs, 389, 390

Chalcopyrite, 205, 246, 769

Chamberlin, T. C., and Salisbury, R. D., on magma, 867

Champion iron ore district, Alabama,

Chart for depth of focus, 104 Chatard, T. M., analysis by, 545 Chemical analyses— coals, Alberta, 369, 370 nickel-bearing olivine, 551 nickeliferous vermiculites, Webster,

y ore: olivine from North Carolina, 545 Potosi ore, 241, 250 rocks from North Carolina, 546 serpentine from North Carolina,

talc from Webster, N. C., 5 a ae minerals of Madagascar, uranium minerals of Madagascar, vermiculite, 542 Chemical criteria of peneplanation, Origin of white clays and baux- ite, and (Woolnough), 887 Chemische Geologie (review), 226 Cheney, C. A., on the Cuyuna iron district, 616 Cheney Pond ore body, 808 China and its mineral resources (edi- torial), 209 Chromite, 530; analysis, 547 Church, J. A., on the Guanajuato mining district, 8 City Lake sandstone, 45 Clapp, C. H., on magnetite of Van- couver Island, 917 Clarke, F. W., analysis by, 551 Clarke, F. W., and Washington, H. S., on germanium in earth’s

crust, 660 Classification, Alberta coals, 367 diamond deposits, Brazil, 717 magmatic deposits, 732, 735 Clays, sedimentary, 463 Clays, their occurrence, properties and uses (review), I12 Closed contour, 681 Closed system, 869

Ce

gD aN

i a

an LD eS Aare A Aon mm

an lan an co“.

Index

Coals, Alberta, 367, 360 alteration, chemical causes, 375 alteration by ground water, 376 transformation, 378 western Canada, age, distribution, and quality, 3 Coblentz, W. W., on electrical con- ductivity of minerals, 780 Colloidal chemistry, problems in, 604 Colony, R. J., on albite in hydrother- mal deposits, 429 on non-titaniferous magnetite de- posits, 916 Colony, R. J., and Meyerhoff, H. A., The copper prospect at Barrio Pasto, Porto Rico, 515-527 Colorado, geology and natural sources (review), 466 Columbite, 69 Committee on processes of ore dep- ares Report of the (Lindgren),

re-

Compressibitity and elasticity of ar- tcesian aquifers (Meinzer), 263 Compressibility of aquifers, evidence

of, 266 Compression and expansion of ar- tesian aquifers, quantitative esti- mates, 21 Compression as pressure, 149 Concentration of alkalies in magma residuum, 881 Concordant bodies in gabbro, 901 Concordant ore bodies, 735 Conductivity of minerals, electrical, 778; cause of variation in, 781 Connolly, J. P., review of book by,

cause of artesian

Continental drift, theory of, 582

Copper, native, germanium in, 668

Copper deposits, Bolivia, 55, 583

Michigan, interpretation, 59

Copper deposits, Michigan and Boliv- tan, A genetic comparison of (Singewald), 55

Copper finding by geophysical meth- ods, 491

Copper ores, structures of, 770

Copper prospect at Barrio Pasto, Porto Rico (Colony and Meyer- hoff), 515

Copper veins on Susie Island, Lake Superior (Schwartz), 762

Cordillera Real, Bolivia, 734

Cornec, on occurrence of germanium,

Corocoro copper deposits, 57

Correlation of a well core with out-

crop sandstone (Roth), 45

To Volume Xxiii.

Corundum and albitite bodies, A hy- drothermal origin of (Larsen),

Covellite, 252, 770 Cover glasses, on thin sections, 327 Cracks in ore, interpretation, 164 Crawford, R. D., and Gibson, R., on dolomitization in Red Cliff dis- trict, Colorado, Cretaceous, Alberta, 360 Dakota basin, 684 Guanajuato mining district, 3 Creveling, J. G., with Lindgren, W., The ores of Potosi, Bolivia, 233- Criner Hills, 49 Criteria, petrographic, of structure in the Cromwell oil field, Oklahoma (Somers), 317 Critical temperatures in magmas and solutions, 878 Cromwell oil field, Oklahoma, Petro- graphic criteria of structure in the (Somers), 317 Cromwell sand, 317 Cross-section, Boltsburn vein, 836 Guanajuato mining district, 7 Sirena mine, Guanajuato, 23 Crustification in wood tin, 189 Cryptesthesia, 460 Crystallization order, 747 Cup Coral formation, 50

Curvature, magnification, 178 Cuyuna iron district, Minnesota, Geologic structure of the

(Zapffe), 612

Dakota artesian basin, 132 Dakota sandstone, 686 AD North Dakota, artesian conditions,

Daly, R. A., on the segregation of magmatic iron ore, 728

Dam-site, St. Francis, Geology of (Ransome), 553

Darton, N. H., on the Dakota sand- stone, 684

on underground waters of South

Dakota, 138

Davidson, S. C., with Graton, L. C.. Microscopical interpretation of folded structures, 158-184

Davy and Farnham, on electrical con- ductivity of minerals, 780

Day, A. L., on immiscibility in silicate liquids. 903

Day, A. L., and Allen, volcanic activity, 876

De — A., on diamonds in Brazil,

Bk. On

940 Index To Volume Xxiii.

Dedolomitization, 858 Deese formation, 50 DeGolyer, E. L., editorial, the seduc- tive influence of the closed con- tour, 681-682 review by, 460 De la Sauce, W., review of book by, De Launay, L., on lead and zinc de- posits of northern Italy, 841 Depth, ores of Potosi, 242 relation of Alberta coals to, 372 Depth of focus chart, 100, 104 Derby, O. A., on igneous rocks in Minas Geraes, 711 Desilicated pegmatites, 417 Determination of minerals under the microscope (review), 586 Deweylite, 534 Diagrams— showing drawdown and recovery of head in well, 2 focal relations, 101 ore in Sirena mine, Guanajuato dis- trict, 22 Diamantina conglomerate, 711 Diamantina district, Minas Geracs, Brasil, The upland deposits of (Thompson), 705 Diamond deposits, The upland of the Diamantina district, Minas Geraes, Brazil (Thompson), 705 Differentiation products, Diffraction patterns, bornite-chalco- cite, 305 Diffusion in ore genesis (Whitman), 473; discussion, 928 Diffusion, previous objections to, 482 Dikes, Diamantina district, 711 Directions of progress in economic geology (Ransome), 119 Discordant bodies in gabbro, 753 Discordant ore bodies, 735 Discussion and informal communica- tions— Carbon ratios as an index of oil and gas (Woolnough), Certain brown iron ores in Ala- bama (Burchard), 454 Diffusion in ore genesis (Smith), g2e Effect of earthquakes on artesian waters (Taber), 606 The linear force of growing crys- tals (Taber), 335 Magnesite deposits of Manchuria, Metasomatism and the pressure of growing crystals, 214

Discussion and informal communica- tions—C ontinued An occurrence of schistose galena (Leonard), 57 The opening as a reason for ore (Dougherty), 569 The ores of Potosi, Bolivia (Lind- gren), 459 The origin of artesian pressure (Piper), 683 The origin of corundum aplite (du Toit), A physico-chemical theory of meta- somatism (Boydell), 105 Preparation of sedimentary ma- terials for study (Ross), 334 A simple apparatus for heavy min- eral separation (Fraser), 99 Underground photography—a depth of focus chart (Went- worth, 100) Divining rod (review), 460 Dolomitization, as a guide to ore de- posits, 862 quantitative elements, 850 relation to intrusive rocks, 853 Dolomitization and ore deposition (Hewett), 821 Dolomitized limestone, chemical prop- erties, 849 geologic range, 851 physical properties, 848 Dorsey, G. E., on carbon ratio theory,

Dougherty, E. Y., discussion by, 569 Dowling, D. B., on alteration of coal seams, 372 on induration of rocks of Alberta,

Dowsing, 460

de ean coal field, 372

Dufet, F., on electrical conductivity of minerals, 778

Dunite, analysis, 546

Dunite body, Webster, N. C., 529

Duparc, L., on platinum deposits of the Urals, 72

Duricrust, 893

Dutcher series, 317

Du Toit, A. L., discussion by, 806

Earth and its history (review), 346 Earth and its rhythms (review), 110 Earthquakes, effect on artesian

waters, : Eby, J. B., on carbon ratio in Vir- ginia, 374

on isocarbs in southwest Virginia,

Ec Ec

Eckel, E. C., on iron ores of Ala- bama, 85

Economic geology, Directions of progress in (Ransome), 119

Editorials—

Bibliographies, annotated bibliogra- phies and geological abstracts (Lindgren), 564

China and its mineral resources (Brock), 209

A fault surface (Geijer), 804

The opening as a reason for ore (Locke), 93

Origin of the magmatic sulphide ores (Wagner), 923-927

The seductive influence of the closed contour (DeGolyer), 681

What is the job of the economic geologist ? (Leith), 451-453 Jhat of our future oil supply? (Wrather), 331

Elasticity, Compression and, of ar- tesian aquifers (Meinzer), 263 Elasticity in artesian aquifers, 277 of water, 285 Electrical conductivity and polished mineral surfaces (Harvey), 778 Electrical methods of finding copper ore, 493

Elements of economic geology (re- view),

Elements of ae mineralogy (re- view),

Ellendale ei 281 Emery deposits near Peekskill, New

York, 429 Emmons, E., on Lake Sanford iron

ores, 737 Emmons, W. H., on deformation in

ores, 163

on secondary deformation in sul- phide ores, 578 . on zonal grouping of ore deposits,

Emmons, W. H., and Laney, F. B., on secondary deformation in sul- phide ores, 578

Enrichment, copper ore, Porto Rico,

Entstehung der Bolivianischen Kup- fererzlagerstatten (review), 583

E6étvés torsion balance, 111; (re- view), 225

Equipotential lines, 494

Eruptive rocks (review), 811

Eskola, P., on dedolomitization, 859

Etching technique, 442

Euxenite, 70; analysis, 74; exploita- tion,

Evans, J. W., review of book by, 586

Index To Volume Xxiii. 941

Examination of sandstone, 52 Experiments bearing on bornite- chalcocite intergrowths (Schwartz), 381 Experiments, in diffusion, 474 on bornite and chalcocite, 384 on electrical conductivity of opaque minerals, 782

Fall River formation, 136

Fault surface, 804

Faults, Guanajuato district, 15

Fenner, C. N., on minerals of effus- ive rocks, 748

on volatiles, 870

Fergusonite, 66, 70; analysis, 76

Fiedler, A. G., on the Roswell ar- tesian basin, 280

Field criteria of age of ores, 160

Filtering earths, 774

Filter-pressing process, 901

Filtration differentiation, 750

Findlayson, A. M., on Huaraki gold field, 860

Fisher, James, electrical measurement

Y, 49.

Fiske, L. E., on carbon ratio in eastern Kentucky, 356, 374 Fissure veins, Guanajuato district, 20

Flotation oil, 776

Focus, depth of, chart for, 100

Folded structures, Microscopical in- terpretation of (Graton and Davidson), 158

Folding, close, of schist and lime- stone, 177

nature and intensity, 168 Ford, W. E., on the calcite group,

849 : Foster, S., on filter-pressing process,

gor

Foye, W. G., on Pusey iron ore de- posit, 902

Fractures in ore, interpretation, 164

Frankeite, 660

Fraser, F. J., communication by, 99

Freidheim, Ch., analysis by, 552

French, J. W., on levigating car- borundum, 307

Friend, J. N., review of book by, 931

Fuller, M. E.: on carbon ratios in

Texas, 354 p on compressibility of artesian aqui- fers,

Funds available for research, 593 Fusain, 378

Galena ore, schistose, 578 Gangue minerals, problems, 608 Gas-phase reactions, 876

942 Index To Volume Xxiii.

Geier, Bruno, review of paper by, 583 sie. Per, editorial, A fault surface,

Geikie, A., on magma, 867 Genesis (see also Origin)— 2 concordant bodies in anorthosite,

copper ores of Michigan and Bo- livia, 59 —— bodies in anorthosite,

waees silicates, 538 ore deposits of Beaverdell district, Potosi silver-tin-ores, 257 Pusey iron ore deposit, 902 Genetic comparison of the Michigan and Bolivian copper deposits (Singewald), 55 Genth, F. A., analyses by, 545-548 on tin ore of Mexico, 187 Genthite, 534 Geography of the polar regions (re- view), 586 Geologic history, Guanajuato mining district, 3, 6 Potosi region, 237 Geologic maps (see also Maps)— Arbuckle region, 47 ys Pasto district, Porto Rico, Guanajuato mining district, 7 Iglesias district, Sardinia, 838 Geologic structure of the Cuyuna iron district, Minnesota (Zapffe),

Geologic time, 483 Geology, Arbuckle Mountains, 46 Barrio Pasto, Porto Rico, 517 Beaverdell district, 434 Brazil, eastern, 709 Iglesias district, Sardinia, 838 Madagascar, 62 Piavitza district, 673 Rocky Mountains, 358 Stewart district, B. C., 193 Susie Island, Lake Superior, 762 Tintic district, Utah, 829 Geology (review), 930 Geology of Mongolia (review), 467 Geology of natural gas and petro- leum (review), 813 Geology of St. Francis dam-site (Ransome), 553 Geophysical methods, application to copper finding, 491; to geologic mapping, 500 principles of, 490

Geophysical methods applied to ex- ploration and geologic mapping in the Michigan copper district (Broderick and Hohl), 480

George, R. D., review of book by,

George gold-copper mine, Stewart, B. Mineral association at (Smitheringale), 193 Georgia corundum deposits, 403 Germanite, 661 Germanium, arc spectrographic de- tection, 662 source of supply, 662 spectral lines, 665 Germanium, New occurrences of (Papish), 660 Geyer, G., on Raibl lead-zinc ores,

Gilbert, G., on relation of hardness to sequence of minerals, 654

Gillson, J. L., on granodiorite of Pend Oreille district, 865

Gillson, J. L., Callahan, W. M., and Millar, W. B., on reaction rims,

Glomeroporphyritic texture, 740

Goldman, M. I., on compacting of materials, 267

eo V. M., review of book YY, 403

analyses by, 545-548

Gonyer, F. A., with Ross, C. S., and Shannon, E. V., The origin of nickel silicates at Webster, North Carolina, 528-552

Goodsprings district, Nevada, dolo- mitization, 829

Gorceix, H., on diamonds in Brazil,

Gordon, G. G., on vermiculite-bear- ing veins, 5390

Gould, C. N., on artesian water of Dakota basin, 138

Granite, 11

Granite-porphyry, 12

Graphic intergrowths, 382

Grating ‘structures of chalcocite and bornite, 383

Graton, L. C., and Davidson, S. C., Microscopical interpretation of folded structures, 158-18.

Greece, Laurium, lead-zinc deposits,

Green, C. H., on eutectic patterns in metallic alloys, 383 Gregory, J. W., on magma, 867 on the’ cause of artesian flow, 265 review of book by,

ae ae

bed bed

Greig, J. W., on immiscibility in sili- cate melts, 903

Grinding procedure for thin sections,

Grooves machined on lap, 308

Grout, F. F., on differentiation proc- esses, 725

on primary banding, 902

Grundwasserkunde (review), 701

Gruse, W. A., review of book by, 930

Guanajuato conglomerate, 9

Guanajuato mining district; Mexico (Wandke and Martinez), 1

Guild, F. N., on structures in copper ores, 771

Haalck, H., review of book by, 461

Hachettolite, 70

Hadding, on germanium, 661

Hale, C. W., Meinzer, O. E., and Fuller, M. L., on underground waters of southern Minnesota,

Halloysite, 253

Harder, E. C., on the Cuyuna iron district, 612, 616

Harder, E. C., and Chamberlin, R. T., on geology of Minas Geraes,

Hardness in mineral sequence, 654

Harker, A., on cause of concentration of iron ores, 727

Hartog, V., on diamond-bearing of peridotite of Kimberley, 716

Harvey, R. D., Electrical conduc- tivity and polished mineral sur- faces, 778-803

Hay, Robert, on origin of artesian pressure, 265

Hayes, H. V., on electrical conduc- tivity of minerals, 780

Heat of formation of minerals, 653

Heavy mineral separation, apparatus

or, 90 Hedberg, H. D., on compressibility of rock materials, 265 on gravitational compaction of sedimentary rocks, 153 Hematite, 203, 914 replacement by cassiterite, 187 Hercynite, 905 Hesemann, J., oxide, 910 Hess, F. L., on production of betafite in Madagascar, 79 Hewett, D. F., Dolomitization and ore deposition, 821-863 review by, I Hibsch, J. E., review of book by, 932

on magnetic ferric

INDEX TO VOLUME XXIiill.

Hillebrand and Scherrer, on german- ium, 661

Hogborn, A. G., on Taberg iron de- posit, 761

Hohl, C. D., with Broderick, T. M., Geophysical methods applied to exploration and geologic mapping in the Michigan copper district,

480-514 Seip Holmes, A., on liquation, 733 on segregation, 730 Honess, A. P., on etch figures, 450 review of book by, 345 Hopkins, T. C., on feldspar dikes of Pennsylvania, 406 Hot springs, Guanajuato district, 13 oward, W. V., reviews by, 813, 817 Hoxbar formation, 50 Hubbard, B., on copper in Porto Rico, 515 Hume, G. S., on carbon ratios in western Canada, 357, 363 Hussak, E., on titaniferous magnetite

of Brazil, 908

Hydrothermal contact metamorphic rocks, 417

Hydrothermal! deposits, high tempera- ture,

Hydrothermal minerals, Webster, N.

+, 530 Hydrothermal origin of corundum and albitite bodies (Larsen), 398 Eydrothermal solution, 867 Hydrothermal veins, 416 Hypersthene andesite, 11 Hypogene ore mineral deposition, The — sequence of (Newhouse),

Ichor, 868 Iddings, J. P., on magma, 867

on segregation, 730 Iglesias district, Sardinia, dolomitiza-

tion, 837; geologic map, 838

Ilmenite, 905 Ilmenite and hematite, relations, 911 Ilmenite websterite, Calamity Brook,

Immiscibility in silicate melts, 903

Inglis, Gavin, on tidal fluctuation in artesian wells, 273

Inorganic chemistry, text-book (re- view), 931

Intergrowths, classification, 382

Introduction series of minerals, 655

Improvements in the polishing of ores (Vanderwilt), 292

Intergrowths, bornite-chalcocite, Ex- periments bearing on (Schwartz),

Investigation of iron ores, Technique of the (Osborne), 442

Iron ores, brown, The occurrence and age of certain, in Alabama and adjacent States (Adams), 85

Iron ores, Certain magmatic titanifer- ous, and their origin (Osborne), 724, 895

Iron ores, Technique of the investiga- tion of (Osborne), 442

Iron ores in Alabama, 454

Isocarbs, 354

Isostasy (review), 347

Ivry mine, Quebec, 741

Jackson, C. T., on magnetic anomalies in Michigan, 504

Jamesonite, 251

Jarosite, 253

Joffé, A., on electrical conductivity of minerals, 779

Jones, E. L., review of paper by, 111

Jones, I. W., Carbon ratios as an index of oil and gas in western Canada, 353-380

Jones, L. M., on production of beta- fite in Madagascar, 79

review of paper by, 109

Jones, W. F., on carbon ratio theory,

Kammererite, analysis, 547 Kemp, J. F., on differentiation of iron ore deposits, 727 on origin of Mineville ore, 916 on Sanford iron ores, 737 Kemp, J. F., and Alling, H. L., on re- action rims, 746 Kennecott, Alaska, dolomitization, 833 Kennedy mine, Cuyuna district, 638, 640, 642 Kerr, P. F., on diffraction patterns,

Kerr, P. F., and Cabeen, C. K., on electrical conductivity of min- erals, 780 Kew, W. S. W., on geology of Los Angeles County, 554 Khrushchov, on germanium, 660 King, F. H., on compacting of sand,

on fluctuations in ground water,

King, F. P., on corundum deposits of Georgia, 403 Knopf, A., on Aspen district, Colo- rado, 854 on dolomitization in Cerro Gordo district, California, 831

944 Index To Volume Xxiii.

Knopf, A.—Continued on wood tin of Nevada, 188 Koehne, W., review of book by, 701 Koenigsberger, J., and Reichenheim, ., on electrical conductivity of

minerals, 779

Kootenay formation, 360

Kraus, M., on Raibl lead-zine de- posits, 842

Lacroix, Alfred, on the mineralogy of Madagascar, 62 Lake Ardmore sandstone, 45 Lake Sanford iron ores, 737 La Luz basalts, 8 Lane, A. C., on influence of water on vegetable deposits, 377 Laney, F. B., on intergrowths of chalcopyrite and cubanite, 381 Lap, charging, 310 cleaning, 309 for grinding, 325 for polishing, 307 Larsen, E. S., 4 hydrothermal origin of corundum and albitite bodies,

395-433 ’ l Lattice chalcocite-bornite structures,

Launay, L. de, on lead and zinc de- posits of northern Italy, 841 Laurium, Greece, lead-zinc deposits,

Lead and zinc deposits, Alston Moor and Weardale, 835 dolomitization, 824 Upper Silesia, 844 Leakage of artesian wells, 287 Le Chatelier, Henri, on levigation,

Leiti, C. K., editorial, What is the job of the economic geologist, on compressibility of quartz, on the Cuyuna iron range, Leonard, R. J., informal alien

tion, 578 i Lepsius, on Laurium lead-zinc de- posits, 847 LeVene, C. M., review of book by,

Levigation, 307 Liddle, R. A., review of book by, 584 Liesegang rings in wood tin, 188 Lignites, Saskatchewan, 371 transformation, 378 Lilley, E. R., on carbon ratios and petroleum, 357 review of book by, 813

Linc Lind ]

j

Re

‘

dis on on on on re}

Lind Lind

a Lind Lind Line Liqu Loca Loca Lock Loe Lon Loui Lozz

Lub: Lucl

McC McC Mec McC McC Mac Mck Mck

McL MclL

Lincoln, F. C., on Potosi mining dis- trict, 240, 262 Lindgren, W., editorial, Bibliogra- phies, annotated bibliographies and geological abstracts, 564-568 Report of the committee on proc- esses of ore deposition, 591-611 discussion by, 459 on colloform structure of ores, 771 on magmatic differentiation, 728 on sequence of minerals, 648 on zones of ore deposition, 37 review of book by, 221 Lindgren, W., and Creveling, J. G., The ores of Potosi, Bolivia, 233-

Lindgren, W., and Irving, J. D., on secondary deformation of sul- phide ores, 578

Lindgren, W., and Loughlin, G. F., on dolomite in the Tintic district. Utah, 828

Lindley, H. W., on relations of mag- netite and ilmenite, 908

Linear force of growing crystals, 335

Liquation, use of term, 733

Localization of iron ore bodies, 751

Locating minerals and petroleum (re- view), III

Locke, Augustus, editorial, The open- ing as a reason for ore, 93

Loewy, H., on electrical conductivity of minerals, 798

Longwell, C. R., review by, 347

Louis, Henry, review of book by, 344

Loza, 4

Lubricant for grinding 310

Luchs, A., on lead-zinc deposits of Silesia, 845

McCalley, Henry, on iron ores of Alabama, 91

McCallie, S. W., on iron deposits of Georgia, 91

McCombs, John, and Fiedler, A. G., on leakage in artesian wells, 287

McConnell, R. G., on geology of Stewart district, 193

McCoy, A. W., on carbon ratio theory, 357

Mackay, R. A., on Raibl lead-zinc ores, 842

McKenzie, T. E., on base exchange and the formation of coal, 377

McKinstry, H. E., Silver mineralisa- tion at Beaverdell, B. C., 434-441

McLaughlin, D. H., review by, 337

McLintock, W. F. P., and Phemister,

J., review of papers by, 6990

Index To Volume Xxiii. 945

Madagascar, radioactive minerals, Review of (Turner), 62 Maghemite, 911 Magma, crystallization, 427 signification of term, 866 Magma and hydrothermal solutions, physical differences, 870 Magmatic differentiation and vein formation, Physico-chemical fac- tors controlling (Ross), 864 Magmatic injections, 731 Magmatic ore deposits, 726 Magmatic segregation, definition, 730 Magmatic sulphide ores, origin, 923 Magmatic titaniferous iron ores and their origin, Certain (Osborne), 724, 805 Magnesia, sources, 857 Magnesite, relation to dolomitized limestone, 856 Magnesite deposits of Manchuria, 218 Magnetic surveying, instruments, 503 theory and practice, 503, 512 Magnetite, 203 Magnetite and hematite, relations, 912 Magnetite and specularite, 609 Magnetite, hematite, ilmenite, and spinel, relations, 905 Magnification of curvature, 178 Malacon, 75 Manchuria, magnesite deposits, 214 Manganerzlagerstatte von Tschiaturi im Kaukasus (review), I12 Manganiferous iron ores, 620, 622 Maps (see also Geologic maps)— Alberta coal areas, 362 Cromwell oil field, 318-321 Cuyuna iron district, 614, 628-642 Diamantina district, Brazil, 7 geology of Susie Island, Lake Superior, 763 iron ores of Alabama, 87 Madagascar, 63 Porto Rico, copper locations, 515 Potter County, South Dakota, showing variation in head of artesian water, 143 South Dakota, water areas, 141 Map of the mineral deposits of the world (review), 227 Maroco mine, Cuyuna district, 632 Martinez, Juan, with Wandke, Al- fred, The Guanajuato mining district, Mexico, 1-44 Martonne, E. de, review of book by,

Matildite, 251 Mawdsley, J. B., on foliation of an- orthosite contact, 751

946 Index To Volume .

Mawdsley, J. B.—Continued on St. Urbain area, Quebec, 743 Mead, W. J., on differentiation of igneous rocks, 750 on the geologic role of dilatancy,

Meinzer, O. E., Compressibility and elasticity of artesian aquifers,

Meinzer, O. E., and Hard, H. A., on artesian water supply of Dakota sandstone, 687

on loss of head in artesian water,

Mendeljeff table, 649

Mesabi formations, 623

Metallic sulphides, precipitation, 605

Metals, native, conductivity, 794

Metamorphism in Minas Geraes, 711

Metasomatism, 214

physico-chemical theory of, 105

Methods of applied geophysics for the exploration of oil, ores, and other useful deposits (review),

Meyerhoff, H. A., on physiography of Porto Rico, 519

Meyerhoff, H. A., with Colony, R. J. The copper ‘prospect at Barrio Pasto, Porto Rico, 515-527

Michigan and Bolivian copper de- posits, A genetic comparison of the (Singewald), 55

Michigan copper district, Geophysical methods applied to exploration and geologic mapping in the (Broderick and Hohl), 489

Microphotographs (see Photomicro- graphs)

Microscopic examination of sands, 53

Microscopical interpretation of folded structures (Graton and David- son), 15

Microscopical interpretation of ores,

Miller, B. L., and Singewald, J. T.., Jr., on mines of Potosi, 240, 262

Miller, W. J., on protoclastic texture of anorthosite gabbro, 751

Milling, Guanajuato ores, 44

Millpond iron ore body, 740

Minas Geraes, diamond deposits, 705

Mineral association at the George gold-copper mine, Stewart, B. C. (Smitheringale), 193

Mineral deposition, hypogene ore, The — sequence of (Newhouse),

Mineral deposits (review), 221

Mineral production bt foreign coun- tries (review),

Mineral resources of the South, 816

Mineral sequence, Beaverdell district,

Mineral valuation (review), 344 Mineralization, George gold-copper mine, 195 Mineralizers, 869 in magma, solubility, 879 Mineralogisches Taschenbuch (re- view), 932 Mineralography, titaniferous iron ores, 905 Mineralogy, veins on Susie Island, Minerals accompanying diamonds in Brazil, 720 electrical conductivity, 778 Guanajuato district, 18 order of polishing, 306 Webster, N. C., 533 Miner’s lamp, ancient, 675 Mining, Guanajuato district, 39 Mitchell, G. J., on copper in Porto Rico, 516 Moench, W., on electrical conduc- tivity of minerals, 779 Monazite, yn analysis, 76; exploita- tion Monteponi Hill, Sardinia, dolomitiza- tion, 840 Monzonite, 12 Morey, G. W., on silicate melts, 866 on the rdle of water in magmas,

Morozewicz, J., on crystallization of corundum and spinel, 421, 427 Moulton, G. F., on carbon ratios and

petroleum in Illinois, 356 sh of solutions through rocks,

Miiller, on germanium, 661 Mylius, L. A., review of book by, 817

Nass River argillites, 193

Natal plumasite, 400

Native copper deposits, Michigan and Bolivia, 56

Nature, origin, and interpretation of the etch figures of crystals (re- view), 345

Nettleton, E. S., on artesian pressure,

Neumann, F. R., on origin of white clays of South Carolina, 887 New occurrences of germanium (Papish), 660

Ne

Ni

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oO

© ©6606

OoOaoeso

Newhouse, W. H., The time sequence of hypogene ore mineral deposi- tion, 647-659

Newhouse, W. H., and Buerger, M. bE Observations on wood tin nodules, 185-192

Newhouse, W. H., and Callahan, W. H., on a form of oxidized mag- netite, 910

Newland, D. H., on Sanford iron ores, 737

Nickel silicates at Webster, North Carolina, The origin of (Ross, Shannon, and Gonyer),; 528

Nickel-bearing olivine, analyses, 551

Nickell, F. A., review by, 701

Niggli, Paul, on “leicht-fliichtigen Bestandteile,” 870

review of book by, 700 Nikiforov, P., review of paper by,

Nishihara, H., discussion by, 218 Noche Buena (Christmas) mine, 579 Norbeck, George, on artesian wells in South Dakota, 147 Nordenskjéld, O., and Mecking, L., review of book by, 586 Norma C. Jolliff well No. 1, 45 North Carolina corundum deposits,

North Range, Cuyuna district, 615,

Norton Company, Worcester, Mass.,

Nutting, P. G., Some geological con- sequences of the selective ad- sorption of water and hydrocar- bons by silica and silicates, 773-

Observations on wood tin nodules (Newhouse and Buerger), 185

Occurrence and age of certain brown iron ores in Alabama and ad- jacent States (Adams), 85

Ocean tides, effect on artesian wells,

Ohm’s law, 780

Oil (see also Petroleum)

Oil accumulations, relation to shore lines, 357

Oil and gas development and possi- bilities in east-central Illinois (re- view), 817

Oil supply, future, 331

Olivine, analysis, 545

Olivine rock, analyses, 551

Opaque minerals, electrical conduc-

tivity, 778

Index To Volume Xxiii. 947

Open system, 869

Opening as a reason for ore, 93, 560

Ophitic texture, production of, 900

Optical mineralogy, 816

Orangite, 72

Ore deposition, Dolomitization and (Hewett), 821

Ore deposition, Report of the com- mittee on processes of (Lind- gren), 591

Ore deposition problems, 125

Ore deposits, Stewart district, B. C.,

Ove genesis, Diffusion in (Whitman), Ore genesis, factors in, 486 Ore Mountain ore body, 740 Ore shoots, Guanajuato district, 31 Ore solutions, source, Guanajuato district, 36 Ores, Guanajuato district, 17 microscopical interpretation, 167 Ores. polishing of, ni data in the (Vanderwilt), Ores and gees in ‘te Far East (review), Ores of cee Bolivia (Lindgren and Creveling), 233; 459 Origin (see also Genesis)— albitite and plumasite, 415 artesian pressure, 683 artesian pressure by compression, artesian waters of Dakota basin, Brazilian diamonds, 721 corundum aplite, 806 dolomite, 823 hydraulic gradient by differential uplift, 154 magmatic sulphide ores, 923 ore minerals, Guanajuato district,

titaniferous iron ores, 917 water-rich residual solutions, 877 Origin, A hydrothermal, of corundum and albitite bodies (Larsen), 398 Origin of artesian pressure (Russell),

Origin of nickel silicates at Webster, North Carolina (Ross, Shannon, and Gonyer), 528

Origin of white clays and bauxite, and chemical criteria of pene- planation (Woolnough), 887

Orthite, 67

Osborne, F. F., Certain magmatic titaniferous iron ores and their origin, 724-761, 895-922

948 Index To Volume Xxiii.

Osborne, F. F.—Continued Technique of the investigation of iron ores, 442-450 reviews by, 226, 700 Otterville limestone, 50 Overbrook anticline, 51 Overbrook sandstone, 45 Oxidation, copper ore, Porto Rico, Oxides, conductivity of, 800

Papavasilion, S. A., on corundum in hydrothermal deposits, 429 Papish, Brewer, and Holt, on ger- manium, 662 Papish, Jacob, New occurrences of germanium, 660-670 Paragenesis, Bolivian silver-tin ores, George gold-copper ores, 195, 197 graphical illustration, 198 Madagascar minerals, 78 minerals, Webster dunite area, 533 Parks, W. A., on induration of rocks of Alberta, 366 Parsons, L. M., on dolomite in Lie- cestershire, 834 Pautsch, Erich, review of book by, Payne, H. M., review by, 816 Pegmatites, definition, 416 genetic history, 865 Peneplanation in pluvial climates, 888 in regions of seasonal rainfall, 889 Peneplanation process, 887 Penfield, S. L., on argyrodite, 660 Peridotites of North Carolina, 529 Petrographic analysis, copper ores, Porto Rico, 526 Petrographic criteria of structure in the Cromwell oil field, Oklahoma (Somers), 317 Petrography, Barrio Pasto district, Porto Rico, 522 Calamity Brook ore body, 808 Lake Sanford region, 741 Petroleum (see also Oil)— Petroleum and its products (review), Petroleum investigations in New South Wales, 810 Petroleum possibilities, Alberta and Saskatchewan, 353 Petrowski, A., review of paper by,

Phemister, T. C., on magma, 867 Photomicrographs, bornite and chal- cocite intergrowths, 389, 390 chalcedony and genthite, 540

Photomicrographs—Continued copper ore, Porto Rico, 523-525 copper ores, 766 ilmenite-hematite intergrowths, 447 iron ore, 738; of Calamity Brook, iron ore and chromite, 756 nickeliferous minerals, Webster, N. Potosi ores, 249 prehnite, Porto Rico, 520, 521 Sanford Hill ore, 910 showing differences due to method of polishing, 296, 304 Split Rock mine ore, 907 Physico-chemical factors controlling magmatic differentiation and vein formation (Ross), Physiography, Barrio Pasto district, Porto Rico, 519 Pierce, G. W., on electrical conduc- tivity of minerals, 780 Piezometric surface and hydraulic circulation, 691 Piper, A. M., discussion by, 683 Pirsson, L. V., on tin ore, 187 Pisani, analyses by, 75 Pishel, M. A., on soluble bituminous matter, 376 Pitch of ore shoots, Guanajuato dis- trict, 32 Pleonaste, 905 Plumasite, Transvaal, 401 Plumasite dikes, 413 Pneumatolytic, ‘significance of term,

Polar research, 585 Polarization, spontaneous, 494 Polarized reflected light, 445 Polished mineral surfaces, Electrical conductivity and (Harvey), 778 Polishing, principles of, 294 Polishing of ores, Improvements in the (Vanderwilt), 292 Polishing machine, 312 Polishing sections, procedure, 315 Pontotoc formation, 50 Poor, R. S., review by, 816 Portland Canal, 193 Posepny, F., on lead and zinc ores of Raibl, 842 Potentiometer, 789 Potosi, Bolivia, The ores of (Lind- gren and Creveling), 233 Potosi, general description, 234 origin of name, 233 production figures, 240 Potosi silver-tin ores, genesis, 257

Py

P

Pratt, J. H., and Lewis, J. V., on peridotites of North Carolina,

Pratt, W. E., and Johnson, G. W.., on 7 gen in the Goose Creek oil field, Precipitation a colloidal oxides, 605 of metallic sulphides, 605 Pressure development in a magma residuum, 879 Pressure of growing crystals, 214 Pressure twinning in calcite, 319 Preston, F. W., on polishing, 300, 302 Preszler well, automatic record of water level, 276 Prideaux, E. B. R., and Lambourne, H., review of book by, 931 Primary banding in igneous rocks, Primrose sandstone series, 45 Priorite, 71 Preblems of ore deposition, 595 Problems of polar research (review), Progress in economic geology, Direc- tions of (Ransome), 119 Protoclasis, 751 Pseudomorphs, 610 Pufahl, on germanite, 661 Pusey iron ore deposit, 901 Pyrargyrite, 250 germanium in, 666 Pyrite, 198, 244, 769 secondary, Cromwell oil field, 321 Pyrochlore, 71 Pyrometasomatic deposits, 598

Quartz, 200, 252 critical temperature, 884

Radioactive minerals of Madagascar, Review of (Turner), 62

Raeburn, C., and Milner, H. B., re- view of book by, 224

Raibl Triassic beds, 842

Railroad trains, fluctuations in ar- tesian pressure produced by, 276

Ramdohr, P., on magnetite, ilmenite, and hematite, 905

Ransome, F. L., Directions of prog- ress in economic geology, 119-

Geology of the St. Francis dam- Site, 553-563 Ransome, F. L., and Calkins, F. C., on secondary deformation in sul- phide ores, 578

Index To Volume Xxiii. 949

Rastall, R. H., on magma, 867 on the mechanism of differentia- tion, 729 Raymond, R. W., on liquation, 733 Reaction rims, 745 Reaction zones, 421 Reck, Hugo, on silver mines of Potosi, 240, 262 Recovery of artesian pressure, 278 Red Cliff, Colorado, dolomitization, Reger, D. B., on carbon ratios of coals and petroleum in West Vir- ginia, 356 Reinecke, L., on the Beaverdell dis- trict, 434 Reiter, H. H., on silicate melts, 749 Replacement, Guanajuato district, 26 mechanism of, 488 Replacement series of minerals, 656 Report of the committee on processes of ore deposition (Lindgren),

Reunnig, E., on banding of Bushveld lopolith, 897 Review of the radioactive minerals of Madagascar (Turner), 62 Reviews— Alluvial prospecting, Raeburn and Milner), Bayley, 224 Beitrage zur Frage der Entstehung der Bolivianischen Kupfererz- lagerstatten vom Typus Coro- coro (Geier), Singewald, 583 Beitrage zur Kenntnis der Man- ganerzlagerstatte von Tschiaturi im Kaukasus (de la Sauce), 112 Bulletin of the Institute of Prac- tical Geophysics of the Supreme Council of Public Economy, Bar- ton, 341 Chemische Geologie (Behrend and Berg), Osborne, 226 Clays, their occurrence, properties and uses (Ries), Wickwire, 112 The determination of minerals under the microscope (Evans), Bayley, 5 The divining rod (Barrett and Besterman), DeGolyer, 460 The earth and its history (Brad- ley), Allison, 346 The earth and its rhythms (Schu- chert and LeVene), Bayley, 110 Eine Studie iiber den Braun-Jurass im nordéstlichen Schwaben und seine Eisendolithfléze (Schleh), Nickell, 700

Reviews—Continued

The elements of economic geology (Gregory), Bayley, 698

Elements of optical mineralogy, an introduction to microscopic min- eralogy (Winchell), Bayley, 816

E6tvés torsion balance, Barton,

Eruptive rocks, their genesis, com- position, classification, and their relation to ore deposits (Shand), Bayley, 813

The geography of the polar regions einen and Mecking),

Geology (Brigham), Bayley, 930

Geology and natural resources of Colorado (George), Bayley, 466

Geology of Mongolia (Berkey and Morris), Bateman, 467

The geology of petroleum and nat- ural gas (Lilley), Howard, 813

The geology of Venzuela and Trinidad (Liddle), Bayley, 584

A gravity survey over the Swyn- nerton dyke, Yarnfield, Stafford- shire (McLintock and Phemis- ter), Barton, 700

Grundwasserkunde (Koehne), Bay- ley, 701

Isostasy (Bowie), Longwell, 347 cating minerals and petroleum (Shaw and Jones), Barton, 111

Die magnetische Verfahren der angewandten Geophysik (Haalck), Barton, 461

Map of the mineral deposits of the world, Bateman, 227

Methods of applied geophysics her the exploration of oil, ores, and other useful deposits (Pautsch), Barton, 110

Mineral deposits (Lindgren), Bate- man, 221

Mineral valuation (Louis), Agar,

Mineralogisches Taschenbuch (Hibsch), Bayley, 932

Nature, origin, and interpretation of the etch figures of crystals (Honess), 345

Oil and gas development and pos- sibilities in east-central Illinois (Mylius), 817

Ores and industry in the Far East (Bain), Agar, 226

Petroleum and its products (Gruse), Bayley, 930

Problems of polar research, 585

Index To Volume Xxiii.

Reviews—Continued

A shorter physical geography (Martonne), Bateman, 462

Summary of mineral production in foreign countries (Jones), He- wett, 109

Tabellen zur allgemeinen und spe- ziellen Mineralogie (Niggli), Osborne, 700

The Tertiary mineralization of the northern Black Hills (Connolly) McLaughlin, 337

Textbook of inorganic chemistry (Friend), Nitrogen (Prideaux and Lambourne), Iron (Friend), Metal-Ammines (Sutherland),

Theory of continental drift; a sym- posium (van der Gracht), Bay- ley, 582

Undersgkelser over Lersedimenter (Goldschmidt), Terzaghi, 463

The undeveloped mineral resources of the South (Payne), Poor, 816

The United States of America (Brigham), Bateman, 466

The use of the torsion balance in the investigation of the geologi- cal structure of southwest Persia (McLintock and Phemister), Barton, 699

Ries, H., review of book by, 112 Rison, C. O., and Bunn, J. R., on the

Cromwell oil field, 317

Roberts, J. K., on the Tuscaloosa

formation in Kentucky, 91

Rock alteration, Guanajuato district,

Rocky Mountains, general geology,

Rogers, G. S., on emery deposits near

Peekskill, 429

Ross, C. S., Physico-chemical fac-

tors controlling magmatic dif- ferentiation and vein formation,

discussion by, 334

on preparation of sedimentary ma- terials for study, 325

Ross, C. S., Shannon, E. V., and

Gonyer, F. A., The origin of nickel silicates at Webster, North Carolina, 528-552

Roswell artesian basin, recovery of

head, 279

Roth, Robert, Correlation of a well

core with outcrop sandstone, 45-

]

Rub

Rub Rue

Sag:

SE St.

Sala Sam Sam San

San San: San Sap Sarc Sask Sch:

Sche Schi

Schi

Schi Schy

E. S cies

Rowland, H. A., on germanium in sun’s spectrum, 660

Rubey, W. W., and Bass, N. W., on an of the Dakota basin.

Ruby silver, 250, 438

Ruer, R., and Nakamota, M., on solid solution of magnetite in hema- tite, 450, 913

Rundle, T. F., on faulting in Cuyuna district, 619

Russell, W. L., The origin of artesian pressure, 132-157; cited, 683

on carbon ratios in eastern Ohio,

Russellville iron district, Alabama, 86 Rutile, 914 Sagamore deposit, Cuyuna district, Sagui, C. L. The ancient mining works cf Cassandra, Greece, 671- St. Francis dam-site, (Ransome), 553 St. Urbain anorthosite area, Quebec,

Geology of

Sala, Sweden, lead-zinc deposits, 847

Samarskite, 71; analysis, 74

Samirésite, 66, 71; analysis, 74

Sandberger, F., on diffusion in origin of mineral deposits, 482

Sandstone, examination, 52

Sanford Hill iron deposit, 737

San Francisquito Canyon, 554

Sapphirine, 914

Sardinia, dolomitization, 838

Saskatchewan lignites, 371

Schaller, W. T., on albite drothermal deposits, 429

on pegmatites, 865

Scheumann, K. H., on types of dif- ferentiation, 732

Schistose galena, 578

Schleh, F., review of paper by, 700

Schmidt, A., on dolomite in lead and zinc deposits, 823

Schneider, E. A., analysis by, 546

Schuchert, Charles, review of book by, 110

Schuermann’s solubility series, 652

Schwartz, G. M., Copper veins on Susie Island, Lake Superior, 762-

a0

in hy-

Experiments bearing on bornite- chalcocite intergrowths, 381-397 Scientific notes and news, 116, 230. 349, 470, 588, 704, 819, 933

INDEX TO VOLUME XXIill.

Secrist, M. H., on dolomite in Ten- nessee, 827 Sections (see also Cross sections) albitite bodies, 407 Barrio Pasto district, Porto Rico, Cretaceous strata in Dakota basin,

Piavitza, 673, 674 St. Francis dam-site, 554 showing pressure relations artesian aquifer, 264 showing recovery of head in an artesian well, 278 vein in Brinton’s quarry, Chester, Pa., 410, 412 Sections, polishing, procedure, 315 Sections, thin, Simple methods for making (Weymouth), 323 Sederholm, J. J., on ichor, 868 on reaction rims, 745 Segall, J., on chalcocite-bornite in- tergrowths, 381, 383 Seidle, K., on lead-zinc deposits of Silesia, 845 Selective adsorption, 773 Selenides, conductivity, 794 Semmes, D. R., on copper in Porto Rico, 516, 517 Serpentine, 536 analyses, 548 Serpentinization, Webster, N. C., 531 Shand, S. J., on fugitive constituents, on magmatic segregation, 729 review of book by, 811 Shannon, E. V., analysis by, 247 on diabase of Goose Creek, Vir- ginia, 865 with Ross, C. S., and Gonyer, F. A., The origin of nickel silicates at Webster, North Carolina, 528-

in an

West

Shaw, E. W., on so-called Lafayette deposits, 85

Shaw, H., review of paper by, 111

Shepard, J. H., on artesian waters of South Dakota, 285, 689

Shepherd, E. S., and Merwin, H. E., on the gases of Pelée, 879

Short, M. N., on levigating amounts of abrasives, 307

on sizes of abrasives, 305

Sideronatrite, 253

Sideronitic textures, 757

Siebenthal, C. E., on dolomitization

in the Joplin region, 826

small

952 INDEX TO VOLUME XXIill.

Siebenthal, C. E—Continued on Joplin lead and zinc deposits,

Sierra Gorda, 2

Sierra of Guanajuato, 2

silegs. upper, lead and zinc deposits.

Silver, Ruby, 250 Silver mineralization at Beaverdell, B. C. (McKinstry), 434 Silver mines of Potosi, 240 Silver ore, Guanajuato district, 44 Simple methods for making thin sec- tions (Weymouth), 323 Simpson, E. S., on laterite in Western Australia, 887 Singewald, J. T., Jr., A genetic com- parison of the Michigan and Bolivian copper deposits, 55-61 on mineralizers in ore segregations, on titaniferous iron ores, 896 review by, 584 Sjégren, A., on origin of Taberg ore deposits, 726 Skeats, E. W., on dolomites of south- ern Tyrol, 842, 844 Slides, covering and labeling, 327- Poca vrs ut Smelting practice, ancient, 676 Smith, L. H., discussion by, 928 Smith, S. W., on liquation, 733 Smitheringale, W. V., Mineral as- sociation at the George gold- copper mine, Stewart, B. C., 193-

Snider, L. C., on surface subsidence,

Society of Economic Geologists, 114, 229, 703 Soil, residual, analyses, 550 Webster, N. C., 549 Solubility, problems in, 607 Solubility series, 652 Solution and deposition, cause, 484 Solutions, movements in rocks, 603 ore-bearing, Guanajuato district, 37 Some geological consequences of the selective adsorption of water and hydrocarbons by silica and sili- cates (Nutting), 773 Somers, R. E., Petrographic criteria of structure in the Cromwell oil field, Oklahoma, 317-322 Sorby, H. C., on compressibility of rock materials, 265 Sosman, R. B., and Hostetter, J. C.. on solubility of magnetite in hematite, 912

Souris coal field, 371

Sphalerite, 245

Spinel, 905; analysis, 547

Split Rock mine, 753

Spontaneous polarization, 494

Springer formation, 45, 49

Spurr, J. E., on dolomite in the Aspen district, 823, 827

on gaseous tension, 728 Stannite, 245 germanium in, 666

Stansfield, Edgar, on analyses of coal, Alberta, 364

Stanton, T. W., on the Dakota forma- tion, 685

Stelzner, A. W., on silver-zinc de- posits of Potosi, 240, 262

Stevens, B., on rock segregation and ore deposition, 750

Stewart district, B. C., 193

Stibnite, 251

Stockworks, Guanajuato district, 23

Strahan, A., on dolomite in South Wales, 834

Strain, microscopic evidence, 161

Stratigraphic column, Springer to Arbuckle mountains, 48

Stratigraphy, Dakota artesian basin,

Streintz, F., on electrical conduc- tivity of minerals, 779

Structural features of ore deposits,

Structure, Cuyuna iron district, 618, Dakota artesian basin, 133 Guanajuato district, 14 Structure in the Cromwell oil field, Oklahoma, Petrographic criteria of (Somers), 317 Subgraphic intergrowths, 383 Subsidence in artesian areas, 148 Sulphide melts, Sulphides, conductivity, 798 order of formation, 770 silver content, 855 Sulphides and dolomite, local rela- tions, 852 Superheat of magmas, 872 Susie Island, Lake Superior, Copper veins on (Schwartz), 762 Susie Island ore, 384 Sutherland, M. J., review of book by,

Sylmar, Penn., albitite bodies, 407

Tabellen zur allgemeinen und spe- ziellen Mineralogie (review), 700

Qa

nO rT

qo

Taber, Stephen, discussions by, 335, on the growth of crystals, 335 Table of formations, Guanajuato min-

ing district, 4 Tables— Alberta coals, analyses, 369 Alberta coals, classification, 367 analyses of nickeliferous minerals,

536, 537

analyses of rocks and minerals from North Carolina, 546 analyses of Madagascar thorium

minerals, 76 analyses of Madagascar uranium minerals, 74 Belly River coals, analyses, 370 composition of pegmatite, albitite, and plumasite, 423-425 Cretaceous of Alberta, 360 electrical conductivity of minerals, germanium lines in spectrograms of calamine 669; of pyrargyrite, 668 mineral succession, 651 minerals in ground sandstone, 53 paragenesis of minerals, Webster dunite area, 533 petrographic analysis, Porto Rico, 526 radioactivity of Madagascar min- erals, 77 relation of carbon ratios to oil, silver content of sulphides, 855 spectral lines of germanium at dif- ferent concentrations, 665 succession of elements in deposits of magmatic affiliation, 649 Talc, analysis, 548 Talmadge, S. B., on determination of hardness, 299 Tanton, T. L.. on liquid immiscibility in a silicate magma, 903 Tarr, R. S., on carbon ratio theory,

copper ores,

Teall, J. J. 34, on iron ores, 727 on magma, 867 Technique of the investigation of tron ores (Osborne), 442 Tellurides, conductivity, Temperature, ore Potosi, 258 Terre Rouge, 65 Tertiary, Guanajuato mining district,

concentration of

deposition at

Terzaghi, Charles, experiments, 282 on compressibility of rock ma- terials, 265, 267

review by,

Index To Volume

Tetrahedrite, 246 Theory of continental drift (review),

Thermal contact metamorphic rocks,

Thin sections, Simple methods for making (Weymouth), 323

Thom, W. Jr., and Dobbin, C. EX ‘on i EX contours on Dakota sandstone, 688

Thompson, D. G., on ground water supplies, 282

on tidal fluctuations in

wells, 273

Thompson, L. F., The upland dia- mond deposits of the Diaman- tina district, Minas Geraes, Brasil, 705-723

Thomson, Elihu, on polishing, 300

Thorianite, 72

Thorite, 72; analysis, 76

Thorium minerals, analyses, 76

Time, geologic, 483

Time sequence of hypogene ore min-

eral deposition (Newhouse), 647

nodules, wood, Observations on

(Newhouse and Buerger), 185

Tintic district, Utah, dolomitization,

Titaniferous iron ores, Certain mag- matic and their origin (Osborne),

artesian

Tin

724, 805 4 Titaniferous ores in pegmatites, 904 Todd, J. E., on Graneros shale in

Iowa, 685 Toérnebohm, A. E., on deposit, 727 Tolman, C. F., and Rogers, A. F.. on magmatic sulphide ores, 730 Transvaal corundum deposits, 401 Trechman, C. T., on dolomite in Durham, 834 Triassic, Guanajuato mining district,

Taberg ore

Trinidad, asphalt, 776 geology, 584 Trinity sandstone, 50

Tscheffkinite, 73; analysis, 76

Tscherepennikov, I., review of paper by, 344

Tubandt, C., Eggert, S.. Schibbe, G., on electrical conductivity of min- erals, 779°

Tunneling of ancients,

Turner, H. W., active

Tuscaloosa formation,

Review of the radio- minerals of Madagascar,

Alabama, 88

954 Index To Volume Xxiii.

Uglow, W. L., on secondary deforma- tion of sulphide ores, 578

Underground photography, depth of focus chart, 100

Undeveloped mineral resources of the South (review), 816

United States of America (review),

Upland diamond deposits of the Diamantina district, Minas Geracs, Brazil (Thompson), 705 Uraniferous ores, Madagascar, 65 Uraninite, 73 Uranocircite, 68 Urbain, on germanium, 661 Urbainite, 914

Van der Gracht and others, review of book by, 582

Vanderwilt, J. W., Improvements in the polishing of ores, 292-316

Van Hise, C. R., and Leith, C. K., on the Cuyuna iron district, 618

Van Tuyl, F. M., on the origin of dolomite, 823

Veatch, A. C., on compression and elasticity of artesian aquifers, 266

Vein formation, Physico-chemical factors controlling magmatic dif- ferentiation and (Ross), 864

Vein-forming material, physical con- dition, 876

Veins, Guanajuato district, 16, 19, 25

Potosi, 239, 241 Venezuela, geology, 584 Vermiculite, 408, 534, 539; analysis,

Veta Madre, 15

Vogt, J. H. L., on lead-zine deposits at Sala, 847

on magmation segregation ore

bodies, 727

Volcanic activity, Guanajuato dis- trict, 13

Voltaite, 253

Von Groddeck, B., on origin of Taberg ore deposit, 726

Von Veirmarn’s law, 191

Wagner, P. A., editorial, origin of magmatic sulphide ores, 923 on banding of Bushveld lopolith,

on maghemite, 911 Waldschmidt, W. A., on_ secondary deformation in sulphide ores,

578 : Wall rocks, influence on ore deposi- tion, 38

Wandke, Alfred, on chalcocite-bor-

nite relations, 381, 384 on deformation in ores, 163

Wandke, Alfred, and Martinez, Juan, The Guanajuato mining district, Mexico, 1-44

Ward, L. F., on flora of Dakota beds,

Warren, C. H., on titaniferous iron ores, 906

Warren, C. H., and Johnson, B. L., on Mine Hill, Cumberland, ore,

Wartman, E., on electrical conduc- tivity of minerals, 778

Washington, H. S., on dark con- stituents of basaltic rocks, 749

Water, elasticity, 285

Water in Dakota artesian basin, variability, 140

Water level, automatic record, 276

Waterman, A. T., on electrical con- ductivity of minerals, 780

Waters, A. C., on ophitic texture, 900

Watson, T. L., on corundum in hy- drothermal deposits, 429

on dolomite in the Virginia area,

Watson, T. L., and Taber, S., on nelsonites of Virginia, 761 Weardale lead and zinc deposits, 835 Webster, North Carolina, The origin of nickel silicates at (Ross, Shannon, and Gonyer), 528 Websterite, 529, 530; analysis, 546 Calamity Brook, 897 Well core, Correlation with outcrop sandstone (Roth), 45 Wells, E. H., on dedolomitization,

Wells, R. C., on fractional precipita- tion of ore-forming compounds,

Wells’ solubility series, 652

Wendt, A. F., on geology of Potosi, 238, 262

Wentworth, C. K., communication by, 100

Westgate, L. S., and Knopf, A., on dedolomitization, 859

Westkettle quartz diorite, 435

Weymouth, A. A., Simple methods for making thin sections, 323-

White, David, on carbon ratio theory,

on progressive carbonization of coals, 374

White, W. P., on specific heats of glasses, 875

Whitman, A. R., Diffusion in ore genesis, 473-

Wickwire, G. T., review by, 112

Williams, G. F., on diamond mines of South Africa, 716

Williams, G. H., on websterite, 530

— A. N., review of book by,

Winchell, N. H., on Susie Island, 762 Wolff, J. F., on Cuyuna iron range,

613, 617 Wood tin nodules, Observations on (Newhouse and Buerger), 185 Woodstock iron district, Alabama, 86 Woolacott, D., on dolomite in Dur-

ham, 834 Woolnough, W. G., Origin of white clays and bauxite, and chemical criteria of peneplanation, 887-804 discussion by, Wrather, W. E., editorial, What of our future oil supply? 331

INDEX TO VOLUME XXIiIl.

Wright, F. E., on polarized light, 446

Wright, L. B., on mineralization in the Homestake mine, 339

Wright bi-quartz wedge, 446

X-ray data of bornite-chalcocite in- tergrowths, 395 Xenotime, 75

Young, L. E., and Stoek, H. H., on subsidence produced by mining,

Yttrotantalite, 71

Zapffe, Carl, Geologic structure of the Cuyuna iron district, Min- nesota, 612-646

Zavaritsky, A. N., on classification of magmatic ore deposits, 732

Zinc and lead deposits, dolomitiza- tion, 824

Zircon, 75; analysis, 76

Zoning of ore deposits, 602