The areal geology of the Creede mining district, Colorado
Geology and ore deposits of Creede, Colorado, W. H. Emmons and 3. S. Lars en. Submitted in partial fulfillment of the require ments for the degree of Doctor
Public-domain full text preserved in the Mountain Man Mining Library. Original source: archive.org.
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Aeeal Geology Of 0?Hs Creede District, Colorado.
By
X ' Esebr Larseit.
H B S Is
Submitted in partial fulfillment of the require ments for the degree of Doctor of Philosophy in the University of California. May, 1918.
Approved:
The Areal Geology of
The Greede Mining District,
Esper 3. Larson.
Cokteiit3 .
Chapter I. Introduction 1
Location and topography 1
Field work and acknowledgments
History
Production 10
Dining and treatment of ores 11
General conditions 11
Humphreys mill 13
Amethyst mill 15
Solomon mill 16
Hidge mill 16
Bibliography 18
Chapter II. Preliminary outline of the geology 21
Age of the rocks 21
The Potosi volcanic series 22
Lower division
Upper division
The Creede formation 26
The llacXenzie .'-.fountain quartz latite 26
Intrusive rocks 27
Quaternary deposits 27
Geologic structure 28
Chapter III. Bocks of the lower division of the Potosi volcanic series
General statement
Outlet. 1?unnel quartz latite
General character and occurrence
Petrography
Megascopic
Microscopic
Less coraon rock types
V/gathering and outcrops
willow Creek rhyolite
General character and distribution
Petrography .,.
Megascopic
Microscopic . . .
Chemical .
Structure
",7gathering and alteration
Topography and scenery
Ohapter\IV. Rocks of the uppr devise of the Potosi volcanic series GeneraX statement
Hornblende- quartz latite
General character and Distribution
Thi
Petrography
Megascopic
-.iicroscop
rhyolite.
General character and distribution.
Contacts.
Petrography,
Megascopic.
Microscopic.
feathering and outcrops. Distinction from other rhyolltea. Phoenix Park quarts latte,
General character and distribution. Petrography.
Megascopic.
Microscopic,
Outcrops and Weathering. Equity quarts 1 at its.
General character and occurrence.
Contacts,
Petrography.
Megascopic.
Microscopic.
Chapter IV. Rocks of the upper division of the Potosi volcanic series General statement Hornblende-quartz latite
General character and distribution
Thickness
Petrography
Megascopic Microscopic
Weathering and outcrops Windy gulch rhyolite breccia
General character and distribution Petrography
Megascopic Microscopic
Weathering and outcrops Comparison with Campbell Mountain rhyolite Hazanoth Mountain rhyolite
General character and distribution Character of contacts and thickness Petrography
Megascopic Microscopic
Weathering and outcrops
Comparison with the Campbell Mountain rhyolite Hhyolite tuff
General character and distribution Petrography
Tridyadte latite.
General character and occurrence.
Thickness*
Petrographjr,
Megascopic.
Microscopic.
Chemical.
The rocks n*ar the aouth of Beer Hom Creek* Outcrops, Weathering, and alteration. Andes it e,
General character and distribution.
Petrography,
General.
Microscopic.
Alteration and topography. Quarts latitic tuff,
General character and occurrence.
Thickness,
Petrography.
Massive rock in the tuff.
Weathering and outcropa. Lks quart latte.
General character and distribution.
Thickness*
Petrography*
Megascopic.
Microscopic, Weathering, outcrops, and topography*
Nelson Mountain quartz latite
General character and distribution
Thickness
Petrography
Megascopic Microscopic
Weathering and outcrops Chapter V. Creede Formation
Name, general nature, and occurrence Structure and thickness Subdivision Lower member
General character
Petrography Traverture
Character and distribution
Origin Upper member
General character and distribution
Petrography
Massive rook
Weathering and topography Origin
Position in the section Age
Chapter VI. Latest series of Lava Plows General statement MacKenzie Mountain quartz latite
General character and distrilmtion Character and thictaiess Petrography
Megascopic Microscopic
Weathering and outcrops Chapter VII. Intrusive rocks General statement Rhyolite porphyry
General character and distribution Relation to adjoining rocks Petrography
Megascopic Microscopic
Quarts latite porphyry. Occurrence* Age, Petrography.
Megascopic, Microscopic. Intrusive basalt. Occurrence, Petrography.
Chapter VIII. Quaternary deposits. Introductory atatement. Moraines.
Distribution.
Glacial toographju
Two stages of glaeiation.
Earlier moraine.
Later moraine*
Character of material.
Poet glacial erosion*
Economic considerations. Terrace gravels* Landslides.
denftral statement.
General distribution,
Distribution.
Economic consideration.
Talus.
Alluvial faun 3.3. Alluvium.
Chapter II, Structure, Introduction. Faulting.
Difficulty of detecting faAltg.
Gtoneral haraat<r of the faulting.
Age.
Mineralization along faults. The Amethyst fault eyttenu
/ V
—T — T- re Mine
.iivi'i.iijXi poiti, Equity Fault. )
Fault block of Mammoth Mountain. Bulldog Mountain fault, Ridge fault. Alfaha fault.
Structure and faulting northeast of Sunny side. Minor faults*
t
Geology and ore deposits of Creede, Colorado, W. H. Emmons and 3. S. Lars en.
Introduction,
Location and topography.- The Creede mining district is in Mineral County, southwestern Colorado, near the eastern border of the elevated region which is generally known as the San Juan Mountains. The town of Creede is situated on 77allow Creel-i, a few miles above its junction with the Rio Grande. The area included within the borders of the Creede special map is about 4g- miles east-west and 5J- miles north-south.
3C3Zcceex&I
mi h MM in I 1 1 The lowest portion of this area, about one mile below Creede, is about 8,700 feet above sea level, and the highest is the summit of ITelson Mountain near the north border of the area which is 12,029 feet above sea level. The relief is, therefore, over 3300 feet. Only a few miles to the north is San Luis Peak, which is over fourteen thousand feet
in elevation.
The area may be divided into four topographic units-, the valley of the Rio Grande with its gentle, rolling, grass covered hills; the sharp, rugged canyons of the main streams; the gentle slopes in the upper drainage; and the rounded slopes and mesas of the uplands. The valley of the Rio Grande, which is cut in the soft lake beds, is about two miles wide near Creede which is on the north border.- It is shown on Plate about Creede with the rolling hills of lake bed tuff, and the alluvial
flat of the streams.
The canyons, especially that of Billow Creelc above Creede, present sheer cliffs several hundred feet high, nearly vertical and locally overhanging. Near the Monte Carlo nine or a thousand feet vertically, the cliffs have an
tt
average slope of about sixty degrees. Plate opposite p. , shows these cliffs above Greede.
In the Tapper parts of the streams recent glaciation has modified the valleys and given gentler slopes
Lise most of the mountainous area of the San Juan, the region is timbered and well watered. It is served by a broad guage branch of the Denver and Rio (Jrande railway, which daily carries sleeping cars from Denver and Pueblo. The district is more accessible than many camps of the San Juan and rates for shipping ore are considerably lower.
A3
Bibliography.
On the geology and ore deposits of the San Juan there is an extensive bibliography. Lists of the publications of the survey may be found in United States Zoological Survey Bulletin ITo. 507, by J. M. Hill. Below are listed only those papers treating the geology and ore deposits of the Creede district which is separated from other areas of mineralization in this region by a fairly extensive area that is not Imown to contain mineral deposits. Besides the papers listed there are also numerous brief notes of timely interest in the technical press and mining journals. iliiiiifci Emmons, 7. H. , and Larsen, E. S., A preliminary report on the geology and
ore deposits of Creede, Colo., in Bull. U. S. Geol. Survey no. 530,
1913, pp. 42-65.
Discusses briefly the principal results of the United States G-eological
Survey investigations at Creede, of which the present paper is the
final report.
Emmons, ',7. H., and Larsen, E. S., The hot springs and mineral deposits of tfagon 'Theel (Jap. Econ. Geol., vol. 8, 1913, pp. 235-247. Describes a fissure vein near which hot springs now issue.
Henderson, Charles W. , Colorado. Gold, silver, copper, lead, and zinc in
western states: Min. Res. U. S., 1908, Pt. 1, Metallic products, p. 390, Table of production and notes on development at Creede.
Henderson, Charles ",7. , G-old, silver, copper, lead, and zinc in the western states, Colorado. Min. Res. of the U. 3., 1909, Ft. 1, p. 320.
Tables of production and notes on the mining conditions at Creede. Seven companies produced ore, the total tonnage of Mineral County being 64,941 tons. Over one half of the production was milled, the remainder being shipped to smelters as crude ore.
Henderson, Charles ,7. , Gold, silver, copper, lead, and zinc in the western
states by H. D. MsCasloey: Min. Res. of the U. S., 1910, Pt. 1, Metals, p.425,
The statistics of production in 1910 and mining conditions of the
district are reviewed.
Henderson, Charles, tf. , Colorado, in gold, silver, copper, lead, and zinc in the western states by H. D. SfcCaslcey: Min. &es. of the U. S., 1911, Pt. 1, Metals, p. 551.
Lars en, E. 3., See Smmons and Larsen.
Larsen, E. S., and ",7ells, R. C., Some: minerals from the fluorite-barite vein near 7agon Wheel Gap, Colo., proo. Nat. Acad. So. 2, 360, 1916.
Larsen, E. S., and 'tfherry, E. T., Halloysite from Colorado, Sour. Wash. Acad. Sc. VII, 178-80, 1917.
Lee, Harry K. , Gases in metalliferous mines: Proc. Colo. Scientific Soc. , vol. 7, 1903, pp. 163-188.
Discusses the geology and ore deposits of the Amethyst Lode, and described the gas that issues from the walls into workings in the north
end of the Amethyst Lode. An analysis of the gas is reported.
At"
Lindgren, 'tfaldemar, and others. Gold and silver in Colorado, in Min. Res. of the U. S., 1905, p. 205.
Gives a table showing production of Mineral County and notes on mining conditions at Creede.
ITaramore, Chester, Colorado in gold, silver, copper, lead, and zinc in western states: Min. Res. of the U. S., 1907, Pt. 1, Metallic products, p. 262.
Notes on developments at Creede, with a table showing production.
Parmalee, H. C., Zinc ore dressing in Colorado; the Creede district: Metall. and Chem. Eng. . my> Dec. 1910, pp. 677-678.
Describes the treatment of Creede ores at the concentrating plants of Greede.
Richard, T. A., The development of Colorado's mining industry: Trans. Am. Inst. Llin. Sng. , vol. 16, 1896, pp. 834-848.
Gves an account of the early history of the Creede district and of its early production.
Chapter II, preliminary outline of the geology.
The "bed rock exposed in the area included in the Creede and Vicinity quadrangle and for a number of miles in all directions is made up entirely of Tertiary volcanic rocks and is a part of the great volcanic field of the San Juan Mountains. Lava flows make up the greater part of the material "but tuff and breccia deposits are important and a few small intrusive "bodies are present. Although the rocks show little variety and, with the exception of a single andesite formation, are all classified as rhyolites, rhyolite "breccias, and quarts latites, they "belong to four distinct periods of eruption and are separated by very irregular surfaces of erosion, the rocks of each eruptive series consisting of a number of subdivisions which are themselves commonly separated by irregular surfaces of erosion.
In the course of the general survey of the San Juan Mountains, which has been in progress for some years, under the direction of "hitman Cross, the volcanic rocks have been studied and mapped over nearly the whole of the mountains including the San Cristobal and the Creede quadrangles. The lavas and associated lake beds and other clastic deposits of the Creede area, while in part local in extent and character, are a normal part of the large San Juan sequence and the two lower groups of eruptive rocks, or those underlying the lake beds, correspond to subdivisions of the potosi volcanic series. The lower part of the lower group, including the Outlet Tunnel quartz latite, the Willow Creek rhyolite, and the Campbell Stountain rhyolite, differs somewhat from the normal rocks at this horizon, and as these rocks are not knovm at any great distance from Creede, they probably represent a group of local flows.
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Age of the rocks.
The rocks are all believed to "be of Miocene age as plant remains collected under Bristol Head to the west from a horizon between the lower and upper divisions of the Potosi volcanic series are closely related to those of the Florissant lake beds w.iich are Miocene and probably upper Miocene in age, while plant remains collected from the overlying Creede formation are also closely related to the Florissant flora.
Potosi volcanic series.
Lower division.
The oldest rocks exposed in the quadrangle comprise a succession of rhyolites and quartz latites, chiefly in flows but containing some clastic material, forming the lower division of the Potosi volcanic series. The lowest formation of this series, the Outlet Tunnel quartz latite, is exposed only in two small areas in the bed of East Willow Creek. (See plate II). It is made up chiefly of hornblende quartz latite, but contains some pumiceeus rhyolite; It contains both flows and fragmental material. It is overlain irregular Itt- by the v/allow Greek rhyolite which is mdde up of several flows of purple-drab to gray, fluidal, banded, felsitic rhyolite. Just north of Creede over anthousand feet of this formation are exposed .viyout evidenceof more than a single flow. The Campbell Mountain fhyolite overlies a somewhat irregular surface of the pillow Creek rhyolite and is made up of flows of a dull reddish brown or mottled flow breccia. It attains a thickness of a thousand feet near Creede but becomes much thinner to the northeast a.-.d east. The phoenix Park quartz latite is chiefly over the
Campbell Mountain rhyolite but is in part interbedded with it. The Phoenix Park quartz latite is commonly a light red-brown biotite-hornblende-quartz latite, chiefly in flows but containing some rather coarse breccia. It attains a great thickness at the head of 2ast *7allow Creek but in the Creede and Yicinity quadrangle only a few hundred feet are exposed. The Equity quartz latite is closely related to the phoenix Park quartz latite and probably represents a great flow in that series. It was recognized only in the iiorthern portion of the quadrangle in the drainage of \7est '7allow Creek where it overlies the Campbell Mountain rhyolite rather regularly and has a thickness of about a thousand feet. It is in large part a single flow of a quaker-drab, fluidal, biotite-quartz latite.
Upper division,
The rocks of the upper division of the Potosi volcanic series have at their base a surface of great irregularity and their extrusion was evidently preceded by a considerable period of erosion during which the streams cut canyons in the rock of the lower division comparable in depth and ruggedness to those of the present stream, but in no wise related to the canyons of the present day. The rocks of the upper division filled in these new made canyons aixl probably covered nearly or quite all of the region. The lowest of this series of flows is a hornblende-quartz latite in thin flows and breccia beds and is exposed only in the drainage of Rat and Lliners creeks. Locally it is several hundred feet thick, elseere it is entirely absent. It is overlain by a few hundred feet of light red-brown
rhyolite flow breccia and tuff (the Tindy Gulch rhyolite breccia), characterized by abundant included fragments of pumice. A great flow, locally four hundred feet thick, of tridymite latite overlies rather irregularly this rhyolite breccia. The rock is red-brown in color and is characterized by prominent fluidal banding, by the presence of much tridymite in the more porous bands, and by abundant millimeter-sized crystals of orthoclase, plagioclase, and biotite. The tridymite latite is overlain rather regularly by a series of andesites which vary greatly in thickness but nowhere exceed five hundred feet.
In the eastern part of the quadrangle the two lower formations of the upper division of the Potosi, described in the preceding paragraph, are wanting and their place is taken by a great flow of rhyolite, the Mammoth Mountain rhyolite which is locally a thousand feet thick. This rock is a red-brown flow breccia and is very similar to the Campbell Mountain rhyolite of the lower division. It is overlain by several hundred feet of rhyolite tuff with associated thin flows and this is in turn overlain by the tridymite latite to the east of the quadrangle.
A succession of quartz latites overlies rather irregularly the andesite or tridymite latite and locally rests On other rocks. These quartz latites carry rather abundant millimeter-sized crystals of plagioclase, quartz, orthoclase, biotite, augite, hornblende, and titanite. The lowest formation of this succession consists of several hundred feet of nearly white quartz latite tuff with some interbedded flows. The intermediate formation, the Hat Greet- quartz latite, is made up in large part of flows but contains some tuff. The upper formation, the Nelson Mountain quartz latite, is a persistent mesa-forming flow, about two hundred feet thick.
The Greede formation.
The Oreede formation was deposited in a lake which occupied a valley carved out of the rocks of the potosi volcanic series. This valley was deeper than that of the present valley of the Rio Grande and occupied about the same position from Antelope Park to 'Tagon ".'/heel 0-ap.
The lower part of the Greede is male up chiefly of fine textured, thin bedded rhyolite tuffs with some coarser material, especially near the borders of the old lake. It contains numerous bodies of travertine which indicate the presence of abundant hot springs during this period. The upper part of the Greede formation is somewhat coarser textured and consists chiefly of fairly well bedded breccia and conglomerate with some fine tuff and intercalated thin flows of soda rhyolite.
MacKenzie Mountain quartz latite.
A later series of lava flows overlies the older rocks very irregularly and in the area included in this report is made up of a single great flow, the MacKenzie Mountain quartz latite, which is characterized by abundant large crystals of plagioclase, biotite, and augite. To the north and east this series embraces a great thickness of flows and tuff breccia deposits made up of quartz latites and related rocks which are characterized by abundant large phenocrysts.
Intrusive rocks.
Intrusive rocks are present only as a few comparatively small bodies. Three types have been recognized. The oldest is a rhyolite porphyry which was intruded as irregular or sill-like bodies into the rocks of the lower
division of the Potosi volcanic series. It is a nearly white rock, characterized by large, glassy orthoclase crystals. A single small dike of andesite cuts the rocks of the upper division of the Potosi volcanic series west of Rat Cree. In 'liners Creek a number of dikes of quartz latite porphyry tfut the rocks of the upper division of the Potosi and are probably related to the flows of the MacKenzie Mountain quartz latite. The rock of these dikes is dense and carries rather abundant large crystals of plagioclase and some of biotite, augite, and hornblende.
Quaternary deposits.
Following the volcanic activity, erosion again became the dominant geologic agent and the present mountains and canyons were carved from the great volcanic pile. After the streams had cut down their beds to nearly their present level, the upper parts of the main streams were occupied by glaciers which widened the valleys but did not cut them much deeper. Since the disappearance of the glacial ice the streams have deepened their channels but little.
The terminal moraines of the glaciers of Rat Creekand both forks of ",7allow Creek are within the area included in this report. Large landslides are important geologic features, and talus, terrace gravels, and alluvium
cover small areas.
Geologic structure.
The c;iief structural feature of the region is a rather complex block faulting with some tilting near the faults. The ore deposits are along these faults. The faults are believed to be later than any of the volcanic rocks of the area and they are earlier than the development of the present topography.
In addition to the faulting there is probably a gentle tilting toward the south.
Chapter III. Rocks of the Lower Division of the Potosi Volcanic Series.
G-eneral statement.
The oldest rocks exposed within the area covered by the Creede and Vicinity map are a thick series of rhyolites and quartz latites forming the lower division of the Potosi volcanic series. Only a few miles to the west under Bristol Head they are underlain irregularly by a series of andesitic rocks and still farther up the Bio Grande near the mouth of Lost Trail Greek
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they overlie the San Juan tuff. In other places, as in Upper Ute Creek to the west of Creede and in Cebolla Creek to the north of Creede, they or
Cross, 'Thitman, U. 3. 3eol. Survey Seol. Atlas, Silverton folio (No. 120), p. 7, 1905.
younger volcanic rocks directly overlie pre-Cambrian granites, schists, gneisses, quart zites, and other rocks; in others still, as near Pagosa Springs and south of the Gunnison River, they overlie Paleozoic and Liesozoic sediments.
The top of this division is likewise a surface of marked irregularity, due to the considerable period of erosion which immediately preceded the extrusion of the rocks of the overlying upper division of the Potosi volcanic series. A few miles to the west a great thickness of andesitic rocks lies between these two divisions and beneath the erosional surface. In the Creede Special quadrangle, however, the rocks of the upper division of the Potosi directly overlie those of the lower division.
Te rocks of the lower division as exposed near Creede, are all highyl siliceous; the greater part are Rhyolites, although quartz latites are present
in the northern part of the quadrangle and attain a great development to the north of the -quadrangle in both forks of Willow Creek. The rhyolites and quarts latites alternate and it is probable that they originated from different vents. The group has been subdivided into five formations, two rhyolites and three quartz latites, and most of these consist of several flows or of flows and associated tuffs. The lowest quartz latite, the Outlet Tunnel quartz latite, underlies a part, at least, of the Willow Creek rhyolite. Its upper surface is very irregular and it may represent a much older period of eruption than the overlying flows. Campbell Mountain rhyolite, wherever exposed, immediately overlies a rather irregular surface of the Willow Creek rhyolite. The second quartz latite, the phoenix park quartz latite, in general overlies the Campbell Mountain rhyolite but in places is interbedded with it. The Equity quartz latite, where recognized, immediately overlies the Campbell Mountain rhyolite, and it is believed to be more closely related to the Phoenix Park quartz latite than to the rhyolites.
The Outlet Tunnel quartz latite.
General character and occurrence.
The Outlet Tunnel quartz latite, which is the oldest rock exposed in the area included on the Creede and Vicinity map, is a chaotic aggregate of lava flows and breccia beds. It was found in only two small bodies, in the canyon of East Willow Creek an short distance north of the Ridge Mine, and has not been recognized in the reconnaissance of the adjoining region. In the lower body it gives fair outcrops, although the contacts are everywhere covered so that its extent and. form can be only roughly determined. The
upper body is exposed only in the Outlet Tunnel, from which this rock receives its name. Talus from the overlying rhyolite covers the lower slopes of the hills and the two areas may be connected on the west side of the creek beneath the talus, but it appears more probable that the Willow Creek rhyolite separates the two bodies and this interpretation is shown on the map. Only about 250 to 300 feet of this quartz latite are exposed, but as the base has not been reached it is not importable that the part exposed represents only the top of a body of considerable thickness. It is overlain irregularly by the Willow Creek rhyolite, although the contact has nowhere been seen.
Petrography. Megascopic.- The rocks both of the flows and breccia fragments are chiefly
y
biotite-hornblende-quartz latites of purple-drab to gray color. They carry
I/ Since the color is rather characteristic of many of the rocks of the district and slight differences in color are among the most easily recognized differences between some of the rocks, accurate color descriptions of the rocks are highly desirable. Accurate color names require a standard of color nomenclature and no such standard that is both sufficiently comprehensive and generally accepted exists. Robert Ridgway's "Color Standards and Nomenclature", published in 1912, is the best standard that we have, but to JBK use it properly requires an actual comparison with the plates in Mr. Ridgwayfs book. It therefore seemed best to the authors to use in the text color names that could be properly understood by reference to a ood dictionary and to include the more precise name according to Mr. Ridgway's color standards in foot notes.
According to Ridgway's color standards the rocks of the Outlet Tunnel quartz latite are commonly light purple-drab (lf"fb) to purple-drab (!"" -), less commonly purplish vinaceous b), pallid quaker drab f), or pale
neutral gray (- - d).
phenocrysts from 1 to 2 millimeters across of white plagioclase, commonly altered glassy orthoclase, quartz, biotite, and altered hornblende in an aphanitic ground mass; a few show in addition an occasional crystal of orthoclase or microperthite up to 3 centimeters across. The groundmass equals or exceeds the phenocrysts in
amount. The rocks are mostly rather dense, but some carry visible pores and gas cavities. Fluidal texture is often presnt, but is rarely conspicuous. Small inclusions are abundant in some of the rocks and are chiefly of quartz latite of darker color than the host and carrying fewer phenocrysts; a few are of rhyolites and andesites.
Microscopic*- The microscopic study of the thin sections showed that the following phenocrysts, stated in the order of their abundance as roughly estimated - plagioclase, orthoclase, quartz, biotite, green hornblende, magnetite, apatite, titanite, and zircon - make up about a third of most of these rocks. The plagioclases are much altered to calcite and saricite, in some specimens to kaolinite, while the orthoclase is fresh; the quartz Is greatly embayed from magmatic resorption; the apatite crystals have colorless ends and a faintly pleochroic, smoky, yellow-brown core. The phenocrysts are embedded in a matrix which is indistinctly polarizing in specks and shreds, less commonly in minute, rounded areas. It is clouded and carries numerous minute reddish shreds which are probably hematite.
Less common rock types.- Rare fragments in the breccia show more conspicuous and abundant crystals from 3 to 5 millimeters across of white plagioclase, which have the composition of andesine and of biotite. The groundmass is uneven in Size of grain and much coarser than that in the other rocks; it is granophyric or micrographic in texture.
Within the quartz latite breccia are thin, irregular bodies of rhyolite
y
flow breccia. This rock is pale purplish gray and shows a few crystals of I/ Ridgway's pallid purplish gray (67 f).
quartz, biotite, and glassy crthoclase about a millimeter long in a glassy to
apfcanitic groundraass. it is characteristically porous and carries very abundant and prominent, irregular shaped inclusions of fibrous pumice up to several centimeters across. On weathering the pumice fragments are removed, leaving large ragged cavities. This rock closely resembles the Tindy Gulch rhyolite breccia.
Weathering and outcrops.
The rocks are considerably altered through hydrothermal action. The plagioclase is sericitized, and the hornblende is altered to a crumbly dark red material; secondary calcite and chlorite are abunds, and some sulphides are present. This rock is less resistant to erosion than the overlying rhyolite, and gives inconspicuous outcrops poorly exposed through the talus from the cliffs of the overlying rhyolite.
Willow Creek rhyolite. General character and distribution.
The thick series of flows of fluidal, felsitic rhyolites characteristically exposed above Oreede in the canyons of both forks of Willow Creek is here called the Willow Creek rhyolite. It is also prominent in Dry Gulch, in Miners Creek, and above the Equity mine in upper West Willow Creek. Its distribution is shown on Plate II. From reconnaissance work about Creede it is believed to come out from under the overlying rocks to the north at Bondholder, to the east it outcrops more or less continuously for some miles, but has not been recognized east of Wagon Wheel Gap; to the west it wedges out rapidly, and it is not present to the south of Bristol Head.
On the west slopes of Mammoth Mountain, nearly 2000 feet of this formation are exposed and the base is not seen; in other parts of both forks of Willow
Creek there is nearly as great a thickness. Where its base is exposed in East Willow Creek above the Ridge mine it overlies an irregular surface of the Outlet Tunnel quartz latite and is not over 200 feet in thickness, it therefore decreases in thickness from nearly 2000 feet to about 200 feet in a distance of a mile and a half. It is possible that the Outlet Tunnel quartz latite represents a lens between flows of the billow Creek rhyolite, but this is not believed probable.
Petrography. Megascopic.- The rocks of these flows are commonly light to dark purple
i/
drab, less commonly light drab, buff, or gray. Fluidal structure is always
I/ Ridgway's purple-drab, ranging from pallid to dark (1 fttf f to 1 tf ft i) ; less commonly purple vinaceous (ltff b), light cinnamon-drab (13ftttb), a number of pallid shades of buff, lilac, drab, and gray, and the lighter shades of gull-gray.
present and is commonly prominent and characteristic. The main part of the rock is dense with about the luster of freshly broken porcelain; streaks or lenses are decidedly porous and have a paler color. These streaks, as seen on plates broken along the banding, are up to a few centimeters wide and several decimeters long; they are seldom over a few millimeters thick. They may form as much as 10 per cent of the rock. In addition to these larger streaks and grading into them are closely spaced, narrow bands, giving the rock a beautiful fluidal structure. In places, notably in Rat and Miners creeks, the rock has a more delicate fluidal structure and a somewhat paler shade of color. Near Weaver the rock breaks into thin plates along its well developed and closely spaced fluidal banding; at other places it has an inconspicuous fluidal structure.
In a few places it carries large gas cavities lined with drusy quartz crystals and partly filled with chlorite, rarely it carries inclusions of foreign rock.
The rock carries a very few visible crystals, mostly of orthoclase, but with less white plagioclase, commonly kaolinized, and a very little biotite. Otherwise it is felsitic or aphanitic, that is, its composition can not be determined in the hand specimen, There are believed to be a number of flows present, but their close similarity and the lack of glassy or Other recognizable horizons at the top of base of flows makes it difficult to distinguish between them. Where the greatest thickness is exposed in Willow Creek, if more than one flow is present they are almost identical in character and could not be distinguished. Near Sunnyside several flows, some of them different from the rock of Willow Creek,have been inclusde in this formation.
Microscopic.- The rock is uniformly holocrystalline and contains few phenocrysts chiefly of orthoclase, with some plagioclase, a very little biotite, and accessory apatite, iron ore, and zircon. The plagioclases are albite and albite-oligoclase, and are commonly kaolinized. The orthoclase crystals are commonly broken and in some specimens show on their border between crossed nicols a lacework grading into the groundmass, due to a growth of material from the groundmass on the crystals. The groundmass is beautifully banded. The main part is indistinctly polarizing even with the highest magnification, and is clouded from minute bodies of ferritic material; numerous lenses or streaks are much coarser and are clear. These coarser streaks correspond to the lighter colored bands and lenses seen in the hand specimens and make up a small per cent of the rock; they vary in width from a few millimeters to a small fraction of a millimeter.
The larger lenses show a marked banding and concentration of quartz in
£
their interior, These bans, of which there are commonly three or four in a
lens are not sharply bounded but grade into each other. The outer band, which is not alwqys present, is a few tenths of a millimeter across, and is made of parallel fibers which project from the walls and extend across the band resembling delicate spherulites. Sharply bounded from or grading into this fibrous band or forming the outer band of some of the lenses is a band made up of a comparatively coarse crystallization of quartz and orthoclase. The orthoclase is chiefly in fairly well formed elongated crystals, occasionally greatly elongated or acicular. In part they appear to have grown from the walls and commonly porject out from the walls; in part they are embedded in the quartz crystals poikilitically. They vary greatly in size, and the largest are as much as 0.3 millimeters long. The quartz is interstitial and is occasionally crystallographically continuous with the quart a grains of the interior. As quartz becomes more abundant toward the interior and both it and the orthoclase become more coarsely crystalline, this band grades into the next which is made up of an aggregate of interlocking quartz grains, commonly
a millimeter across, with some gas cavities* Some bands have a core which
r
is largely pore space into which project drusy crystals of quartz and ojctho-
clase which can be easily seen with a pocket lens. Tridymite was not observed. Plates to "$£. are photomicrographs of thijj sections of the Willow Creek rhyolite showing these coarsely crystalline lenses, plat e U£ shows two lenses with the normal groundznass between; Plate V40r shows in detail one of the lenses and the normal groundmass on both sides; Plate VIF- shows between crossed nicols the same area as plate Pate IS sliet/a anpat
-Mae of the lu'oaflsg lemierB Some of the very broad lenses do not$rho¥/an interior quartz band, but its place is taken by irregular areas of quartz grains which are scattered through, these lenses* -b}iaszfc*iHP4fra~-Btr The orthoclase is in general irregularly distributed. The smaller lenses do not show so distinct a banding, and some of the narrower streaks are mere strings of crystals. In some specimens these coarsely crystalline lenses are spherulitic in crystallization or micrographic, with little if any concentration of quartz in the central part. These lenses are less clouded than the main groundmass as the ferritic material is collected in small grains and shreds which are in part black and opaque, in part reddish brown and translucent. In addition there are a few minute, colorless grains with a rather high index of refraction and without perceptible birefringence. The orthoclase is somewhat clouded while the quartz is clear except for scattered streaks and irregular areas which carry abundant minute gas or liquid inclusions 4 no liquid inclusions with gas bubbles were found. Phenocrysta of orthoclase and plagioclase are present in some of the larger lenses.
These lenses, which differ from the main body of the flow in their more, porous character, coarser crystallization, euhedral form of the orthoclase and banded structure evidently represent raagmatio segregations which formed before the magma came to rest. Relief of pressure due to the extrusion of the lava might have caused the mineralizers to concentrate into rounded bodies, perhaps in part as bubbles or aggregates of bubbles sealed in the lava, in part in solution in the magma; as the viscous lava flowed these bodies were drawn out into their present forms. The presence of the mineralizers hfcLd back the crystallization of these streaks until after the main body had solidified;
;
It also caused the coarser crystallization, the concentration of the quartz in the center and the presence of the gas cavities. On account of the coarseness of the crystallization, the euhedral development of the orthoclase, and its concentration on the walls, the authors believe that these streaks were not highly viscous at the time of their crystallization. The crystallization of the lava, and especially of the coarser lenses, is believed to have depended more on the loss of the mineralizeSs than on the cooling of the magma. These mineralizers, relaased during the crystallization of the magma, may have caused the kaolinization of the plagioclase which is almost universal and does not appear to be due to weathering.
These streaks show considerable resemblance to some adularia veins. The concentration of orthoclase on the borders, and the interlocking of the quartz grains i s characteristic of both. However, the form of the orthoclase in the veins is rhomboidal, while in the streaks of the rhyolite it is characteristically prismatic. They also show a resemblance to the small,
irregular pegmatitic bodies which are common in some granites.
Petrography.- 3we chemical analyses of this rock, madebby W. C. Wheeler
in the laboratory of the United States Geological Survey, are as follows:
Chemical analyses of the ~7allow Creek rhyollte.
SiO
A12°3
P62°3
FeO
MgO
CaO
Na2°
K2°
H20 -
H2°
Ti02
™2
?2°5
tr.
S
Cr 0
MnO
none
BaO
I. Typical fluidal rhyolite from So Ionian Adit. II. Typical fluidal rhyolite.
III. Typical fluidal rhyolite from Bachelor Shaft, Nelson Adit, about 30 feet from vein.
The norms of these analyses, calculated in accordance with the quant ita-
U
tive system of Crossm IddAngs, Pirsson, and 7/ashington, are as follows:
I/ Quantitative classification of igneous rocks, Chicago and London, 1903.
Norms of the 77allow Creek rhyolite.
I Ii Iii
Quartz 34.3 34.6 41.5
Corundum £.0 1*0
Orthoclase 52.8 40.0 41.7
Albite 5.2 20.4 11.5
Anorthite 0.6 1.4
Hyper sthene 1.8 .3 Acmite
Na.SiO 2 3
Wollastonite Pyrite Magnetite Ilmenite
H2°* C02' etc*
99.5 99.7 99.5
Analysis 1 falls in Class 1, order 3 near order 4, rang 1, and subrang 1, 3(4).I.I), and is bisbose. Chemically this rock is remarkable for its high silica and potash with low soda and very low lime. Analysis 2 falls in Class 1, order 4 near 3, rang 1, and subrang 2 (1. (3)4.1.2(3) and is an omeose. It differs from analysis 1 chiefly in the greater amount of soda; it is remarkable that the BaO in this rock exceeds the CaO; this may be due to the presence of a small amount of secondary barite. Analysis 3 falls in the madgeburgose (1.3.1.2) and is higher in silica than the other two, but is otherwise intermediate between them. In all these rocks the norm differs from the modCchiefly in the absence of corundum and the presence of biotite
instead of pyroxene.
The -tsafr analyses differ somewhat more than would be expected from
specimens collected from the same flow. Analysis i shows lower Si02 and especially lower soda and higher potash. This may not represent an original difference in the rocks but may indicate that the rock represented by Analysis 1 has had its plagioclase altered to kaolinite. The considerable amount of water shown in the analyses and the presence of corundum in the norm bear out this suggestion. It is possible also that adularia has been added,
Structure.
The strike and dip of the fluidal banding of the rock varies rapidly from place to place an an irregular manner; the dip is not uncommonly steep. So far as could be observed this is not due to tilting subsequent to the extravasation of the lava but to the irregular flow of a viscous magma probably over an uneven surface. A nearly vertical sheeting duCto crushing
or, perhaps, in part to shrinkage on cooling, is commonly present and locally it is close spaced and prominent. Where two or more such sheetings are developed the rock breaks into rude columns on pencils. Plate glllo aad |flll shows the details of a typical outcrop; they picture the cliffs about the Monte Carlo mine on the southeast slope of Campbell Mountain. The broken cliffs above the talus are about a thousand feet high.
Weathering and alteration.
The weathering of this rock has been largely mechanical and due to frost action, and differential expansion caused by temperature changes which are rather extreme in this high altitude. Gravity has also been an important factor and rock slides of various sizes are very common. Te prominent fluidal structure and the sheeting have greatly influenced the breaking down of the rock. Soil is scant and great piles of small rock fragments made up largely of small plates of nearly fresh rock are characteristic at the base of the cliffs. The gentler slopes of the highlands show a scant soil with abundant fragments of nearly fresh rock scattered through it.
Near the mineral veins or other sulphide bodies the rock has been more or less altered by the mineral solutions. In places near the surface along the Amethyst vein there are considerable bodies of white, kaolinized rock, probably due to leaching by acid solutions, in addition mush of the rock while showing no other signs of alteration, has its phenocrysts of plagioclase altered to kaolin. This kaolinization shows no relation to the zone of weathering nor to zones of mineralization and is believed to have taken place immediately after the solidification of the lavas and before complete cooling.
Topography and soenery*
The rock commonly outcrops in jagged and more or less broken cliffs with great talus heaps at their base. Of all the rocks in the area it is exceeded in hardness and resistance to weathering only by the latite of MacKenzie Mountain, and most of the rugged topography and deep canyons about Creede are in this rock* Wherever the streams have cut deeply into it they have sharp ax
t
canyons or gorges with steep rugged walls and brotaen cliffs for a thousand
feet or more. The uniformity of the material does not lead to the development
of benches* Plates, JMfr 1.T, ajftd fflll show characteristic outer-ops of this
rock. Plates XI and Xlla are photographs looking up the creek from a point a few hundred yards below the forks of Willow Creek. The cliffs are all of Willow Greek rhyolite, that in the center between the forks of Willow Creek is about 1500 feet high. The talus slopes at the base of the cliffs are in all cases at about the top of the Willow Creek rfcyolite and the timbered slopes on the right, above the shoulder, show the characteristic topograpny of the Campbell Mountain rhyolite. Platelj Klla abd }flll show5 more detailed views of these cliffs. They were taken from a point about five hundred feet below the Mollie S. Mine and show the cliffs on the west side of East Willow Creek. These cliffs are about 1500 feet high. The Monte Carlo Mine building is indistinctly shown near the top of the cliffs to the right of the center
3Se
of plate VlII. PlatelFshows the canyon of Willow Creek above Creede which
is out in this rock. These rugged outcrops are especially developed in
both forks of Willow Creek and in Dry £ulch. In Miners Creek they are nearly
as characteristic.
'
Campbell Mountain rhyolite,
General character and distribution*
Overlying the Willow Creek rhyolite rather irregularly is a rhyolite flow breccia which is here called the Campbell Mountain rhyolite. In most places no evidence was seen of more than one flow but in East Willow two flows of this type are separated by a few hundred feet of Phoenix Park quartz latite. It is present on both sides of Miners Creek, in lower Rat Creek drainage, and northeast of Monon Hill. It was also found in Windy Gulch Just south of Bachelor. Several isolated outcrops, partly bounded by faults, are present on both sides of Willow Creek, a short distance above Creede. It caps parts of Mammoth Mountain. A narrow band extends f r m the ridge north of Mammoth Mountain in a northwesterly direction and crosses East Willow Creek at Phoenix Park; it continues to the southwest to Campbell Mountain. Just northwest of this it is cut out for a short distance by the Mammoth Mountain rhyolite but comes in again as a thicker member in Nelson Creek and to the northwest. It has been recognized on both sides of the Equity fault. It is not exposed to the west of test Willow Creek except near its mouth, but in the underground workings along the Amethyst vein this rhyolite forms the hanging wall. Ofce details of the distribution in the Creede Special area are shown on Plate II. 0?o the west of the Creede and Vicinity area it is well developed in the drainage of Shallow Creek but is not present south of Bristol Head. It has been recognized as far east as Wagon tfheel Gap and as far north as Bondholder in Spring Creek. It has not been recognized to the south.
About a quarter of a mile a few degrees south of east of the Captive Inca shaft in a great landslide area, at the face of short tunnel, is an exposure, believed to be in place, of a somewhat altered rhyolite flow breccia or tuff which probably belongs to this rhyolite although it has more of the appearance of some of the roc* of the Windy Suloh rhyolite breccia. The latter rock is not known in this part of the quadrangle and if the exposure does not belong to the Campbell Mountain rhyolite it probably belongs to the overlying tuff, in which case it may be a large block of slide rock.
The Campbell Mountain rhyolite varies greatly in thickness and for three reasons: it flowed over a rather uneven surface of the underlying rhyolite; in most places erosion had cut deeply into it before the overlying rocks of the upper division of the potosi were extruded; it thins out and the flows wedge out between flows of the Phoenix Park latite in the northeast corner of the quadrangle. The maximum thickness is south of Nelson Mountain where nearly a thousand feet are present and it was probably thicker since the top is a surface of erosion; less than a mile to the south it is entirely eroded and the rocks of the upper division rest directly on the Willow Creek rhyolite. To the west of the quadrangle in Shallow Creek the thickness is probably even greater; although a few miles farther west, south of Bristol Head, this flow wedges out and the lower division is made up entirely of quartz latite.
Contacts*
The basal contact of the Campbell Mountain rhyolite with the Willow Creek rhyolite where observed, is commonly sharp and shows evidence of some erosion preceding its extrusion. West of Willow Creek, above Creede, at the Equity Mine the contact is well exposed, especially in the mine -¥ workings. It dips steeply to the west. The two rhyolites, each in typical development show a sharp contact and are closely adherent. The overlying Campbell Mountain rhyolite has plucked off numerous blocks from the
underlying Willow Creek rhyolite and carries the latter as angular inclusions up to a foot across. The Campbell Mountain rhyolite is not noticeably different at the contact from the normal rock except for the presence of the numerous, rather large included blocks of the underlying Willow Creek rhyolite and these extend for only a few feet from the contact. At other places the two rocks appear to grade into each other and even with continuous exposures there is up to a hundred feet of rock of intermediate character between the two typical rocks, as it well shows on the East side of East Willow Creek about half a mile above the Ridge Mine. In Miners and Rat creeks also some difficulty was experienced in locating the contact, partly on account of the lack of a sharp contact and distinctive character of theSgo members, and partly on account of poor exposures. The upper contact is everywhere sharp but some of the overlying rocks so closely resemble the Campbell* Mountain rhyolite that the separation was somewhat difficult.
petrography. Megascopic*- In color the Campbell Mountain rhyolite is commonly red-
y
dish brown or drab , has a rather dull luster, a delicate, indistinct
I/ Ridgways livid brown purple-drab light purple drab (lflb)f cinnamon drab (13"f-) or light quake r drab (l"fftb).
fluidal texture and a characteristic mottled or spotted appearance due to inclusions of lighter and darker shade. It is generally porous with pores less than a millimeter across; the larger ones are lined with drusy crystals of quartz and feldspar or are partly filled with a network of minute crystals
arranged in strings like rock candy. The inclusions are in part angular fragments of a somewhat darker colored quartz latite similar to the Outlet Tunnel quartz latite; they are rarely over a centimeter across and are nearly always much altered. Fragments of the underlying Willow Creek
rhyolite are abundant only near the contact with that formation; occasional
fragments of andesites and other ocks are present. Ever more abundant and
characteristic are the rounded to subangular fragments of rhyolite which differ from the host chiefly in their more porous character, slightly paler shade of color, and coarser crystallisation. Soe of them have irregular serrated outlines, some are flattened parallel to the flow structure or rudely lenticular. They are commonly very ragged and as seen in the specimens fray out at their ends. They are believed to represent, in part at least, magmatic segregations not very different from the lighter lenses in the Willow Creek rhyolite, but they may represent fragments of the magma which had already crystallized and were torn from the sides of the vent by the viscous lava. Both kinds of inclusions are commonly bordered by a narrow band of somewhat paler shade than the main rhyolite mass.
A rock of somewhat different appearance is prominent in the body south
of Nelson Mountain and is also present near Sunnyside and in other places.
I/ This is light gray to white and has a delicate flow structure. It carries
I/ Ridgway's gull gray (d(8)) light gull gray (f(9), pale mouse gray
numerous included, flat, splinter- like fragments of a white, rather porous rhyolite, in addition to some of quartz latite and Willow Creek rhyolite;
they tend to have their maximum diameter parallel to the flow structure. It differs from the drab type chiefly in its color, but also has a more prominent fluidal structure and somewhat more conspicuous and abundant included fragments which are in this type commonly porous. It is not believed to be a distinct flow but rather the top or other part of one or more flows of the normal rock. It is everywhere closely associated with the normal rock and south of Nelson Mountain it makes up most of the upper part of the body although it is not confined to any particular horizon. In places rocks transitional between the two were found. Locally leaching of the typical rock has given rise to a rook similar to the gray type and it may have developed much of this type.
Both types show a few one millimeter sized crystals of glassy orthoclase and white plagioclase and rare crystals of biotite and quartz in an aphanitic groundmass. Glassy or highly vesicular layers have not been recognized.
Microscopic." The thin sections show that the rocks differ but little from those of the underlying pillow Creek rhyolite; on the whole they have
somewhat more abundant and more calcic plagioclase, some (fuarts phenocrysts
i F
and a slightly different grAoundpass but individual sections can not be
/ easily distinguished. They carry scattered phenocrysts, chiefly of or-
thoclase with some calcic andesine, embayed quartz, and biotite; and accessory apatite, iron ore, and zircon. The groundmass is delicately fluidal, most of it exceeding finely crystalline to submicroscopio with streaks and irregular patches of coarser spherulitio or granophyric crystallization. The blotches so prominent in the hand specimens are sharply bounded from the host and are somewhat coarser in crystallization
but otherwise similar.
Weathering and outcrops.
This rock breaks up readily under the influence of weathering into fragments, flattened parallel to the flow banding and commonly several inches across; it gives poor outcrops and commonly forms gentle slopes strewn with fragments of the underlying rock. The rock is much less resistant than the underlying Willow Creek rhyolite and to this difference is due much of the topography and scenery on both sides of Willow Creek above Creede. !Phe cliffs are almost entirely of the Willow Creek rhyolite and the soft, gentle slopes above most of the benches, and the flat tops of the ridges are commonly formed from the Campbell Mountain rhyolite. The marked difference between the outcrops of the two formations adds greatly in distinguishing between them, and in most places the contact can be approximately mapped from the change in topography. In plateJSf opposite p. , on the left of the canyon the gentle talus covered slopes above the cliffs of Willow Creek rhyolite are of Campbell Mountain invPlAt* In PlatejH, on the nose
between the forks of Willow Creek, the talus -covered, timbered slope between the two cliffs is formed Campbell Mountain rhyolite overlying the lower cliffs of Willow Creek rhyolite and fagged on the north against the upper cliffs of Willow Creek rhyolite.
Distinction from other rhyolites.
Locally the Campbell Mountain rhyolite is very difficult to distinguish from the Willow Greek rhyolite; it also resembles some parts of the Windy Gulch rhyolite breccia and the Mammoth Mountain rhyolite* The compariosn withv the Willow Creole rhyolite will be taken up in the folio /ing paragraph: compariosn with the Windy Gulch and Mammoth Mountain rhyolites will be left until they have been described
Th© normal drab flow breccia of the Campbell Mountain rhyolite is easily distinguished from any part of the Willow Creek rhyolite. Moreover, the normal fluidal, banded Willow Creek rhyolite, as exposed above Creede, is distinct from any part of the Campbell Mountain rhyolite, yet some less typical kinds of what is believed to be the gray rock of the Campbell Mountain rhyolite can not with certainty be distinguished from some of the finely fluidal parts of the Willow Creek rhyolite. In the drainage of both forks of Willow Creek, except near Phoenix Park, the two rocks are easily distinguished and in places at least the contacts are sharp. However, in some places near Phoenix Park as much as 100 feet of rock of doubtful character waa found between the typical Campbell Mountain rhyolite and the typical Willow Creek rhyolite and there appears to be a dradation between them. In the drainage of Rat and Miners creeks there is considerably more rock whose posit ionls uncertain and the boundary between the two rhyolites of this area could not be followed precisely. It is believed that a flow of delicately fluidal rhyolite carrying a few included fragments lies between the normal coarsely fluidal Willow
Creek rhyolite and the typical Campbell Mountain rhyolite. This has been included in the Willow Creek rhyolite which it most resembles. However,
poor exposures, alteration, and a variability in the two rocks which in
unsome places closely resemble each other have made the mapping/satisfactory
in places. In general the Campbell Mountain rhyolite is a flow breccia with abundant included fragments and a faintly developed flow structure while the Willow Creek rhyolite carries few inclusions and has a prominant and commonly coarse fluidal texture.
Phoenix park quartz latite. Sonera! character and distribution.
The Phoenix Park quartz latite is made up of lava flows and tuff breccias of a fairly uniform rock; it receives its name from its development about Phoenix Park. It is well developed to the north of the Creede special quadrangle where it is made up largely of great lava flows but within the mapped area it is chiefly breccia and tuff with massive flow rock in smaller, less continuous bodies. The breccia is chaoitc, shows little bedding or sorting, and consists mainly of rounded or subangular fragments, many of them several feet across. They are nearly all of quartz latite similar to that of the flows. Fine tuffs are only exceptionally present.
This quartz latite was found only in the upper part of East Willow Creek. The main body overlies a fairly regular surface of the Campbell Mountain rhyolite and is overlain irregularly by the Mammoth Mountain rhyolite. Another body of similar rook occurs as a lens between
flows of the Oaopbell Mountain rhyolite. It is believed that here there was an interbedding of the two types of material. This type of rock makes up the greater part of the upper division for many miles about Creede.
This rock wedges out to the south and thickens rapidly to the north. The maximum thickness within the quadrangle is about five hundred feet, but a few miles to the north it is very much thicker.
Petrography. Megascopic.- In color the rocks are commonly red-brown to pale
y
reddish drab. They are quartz latites and carry rather abundant visible crystals, about a millimeter in cross section, of white, striated
I/ Ridgwayfs purplish or brownish vinaceous (lt>fb to 3ftlb) to light purple or light vinaceous drab (llfttb to S''b), deepening to livid brown (llff-) or dark purple -drab (ltftli) or becoming as light as pallid vinaoeous drab (5tfftg).
plagioclase, and glassy orthoclase in about equal amounts, less of glassy quartz, hexagonal plates of black biotite and prisms of black hornblende and a few of pale yellow, lustrous titanite. These crystals are imbedded in a rather dense to highly porous aphanitic groundmass.
a few feet at the base of some flows the phenocrysts are imbedded in a black glass. Many of the rocks carry inclusions of latites of somewhat different color and texture, and some of rhyolite, andesite, quartz it e, and granular rocks*
In addition to the quartz latites there are a few thin irregular
flows of a pale purplish vinaceous rhyolite flow breccia which are
characterized by abundant inclusions of fibrous pumice* A few crystals of glassy orthoclase and biotite are present in a rather porous aphanitic groundmass. This rhyolite is very similar to that characteristic of the Windy (Julch rhyolite breccia*
Microscopic*- The microscopic study shows that the quartz latites are porphyritic, and that phenocrysts make up nearly half of the rook. Tese phenocrysts, which vary in cross section from 2 millimeters down to a fraction of a millimeter and are somewhat broken, include plagioclase, quartz, orthoclase, biotite, hornblende, and titanite with accessory iron ore, apatite, and zircon. The plagioclase is in well formed, nearly equant crystals; they are zoned and vary from andesine to labradorite, averaging andesine-labradorite. The hornblende is a deep brown variety in some sections, light green in others, and rather dark olive green in still others, although commonly nearly as abundant as biotite it was not found in some of the sections* Titanite is in crystals a millimeter across and is nearly as abundant as the hornblende. The groundmass is very find textured and in part spherulitic, in part indistinctly polarizing and is rhyolitic in character. It is clouded and in addition is dusted with minute incousions, probably of hematite. The rocks are commonly fresh but some are altered, probably by hydrothermal agents and show secondary calcite, sericite, chlorite, and sulphides.
Outcrops and weather ing.
The breccia beds are rather easily broken down by weathering and on the whole the Phoenix Park quartz latite is somewhat less resistant than the overlying and underlying formations. Llany of the outcrops are rather poor and are more or less covered with talus or glacial material. Horth of Phoenix Park flows make up most of the rock and the outcrops are better. T the north of the quadrangle where great flows make up most of this formation the outcrops are much better and the very rugged mountain and canyons about San Luis Peak have bean carved out of these flows* Weathering is due chiefly to mechanical agencies and in many places the rock breaks up into a crumbly, sandy, mass, although the minerals are still comparatively fresh. Much of the area made u p of this, rock has been glaciated.
Equity quartz latite. General character and occurrence.
ThQ Equity quartz latite, which is made up entirely of massive rock and in large part, at least, of a single great flow, is named from its prominent development near the Equity mine. It overlies ratha) regularly the Campbell Mountain rhyolite and therefore occupies aboutk the same position in the section as the Phoenix Park latite from which it differs chiefly in its more massive character but also slightly in its composition whjcis somewhat nearer that of a rhyolite. The two latites have not been found in contact but they are believed to be very closely related and to represent different phases of the same period of eruptive activity.
This rock was found in upper West Willow Creek from TTpper Deerhorn Creek to and above the Equity Mine. It caps the high ridge north of the Equity fault. Just south of the Equity fault it has a thickness of about a thousand feet with an erosion surface at the top.
slide, or |other debris, and their mapping is not accurate. On the crest of the ridge east of the Equity Mine and north of the fault its base is well exposed; here it overlies the Campbell Mountain rhyolite rather irregularly. Just south of the Equity fault the base is mapped only approximately as talus and slide completely cover this area. The lowest exposures, which are about 200 feet above the bed of the creek, are of the Equity quartz latite; the first exposure in the Equity tunnel is of the Campbell Mountain rhyolite; otherwise the mapping of this contact is based on the topography. On the west side of the creek, only a few feet above the creek bed, are good exposures of this latite. North of the Equity Mine the trench of the creek is in latite while the slopes to the east of the flat are in rhyolite, believed to be the /allow Creek rhyfllite; there must be a fault between the two. The western boundary of the mapped body is also believed to be a fault although it is in an area lacking in exposures.
Petrography.
i/
Megascopic,- The fresh rock is commonly light Quaker drab. The
I/ Ridgway*s light quaker drab b) to pale purplish gray less connonly light olive gray (25ttffd), light mouse gray dawn gray (15fftrd), or lighter or darker shades of any of these colors.
hand specimen, shows rather abundant crystals of white plagioclase, and some of quartz, glassy orthoclase, and biotite in a groundmass which is
fairly dense and shows inconspicuous, delicate fluidal texture. In
2/
addition;a few of the rocks show bodies of a pale olive gray color up
2/ Ridgway's pale olive gray (23ftff).
to five millimeters wide and a few centimeters long with a rudely lenticular form but much serrated in detail. They are of coarser crystallization but are not perceptibly porous. The rock is everywhere somewhat altered; the plagioclase and dark minerals are commonly largely replaced by calcite, chlorite, and sulphides. The base of the flow where exposed shows a few feet of dark glass in which are imbedded the usual crystals. On the upper parts of the ridge included on the map in the Equity
quartz latite is a rock which more closely resembles the Phoenix park
3/ quartz latite. In color it is somewhat brighter than the lower part
and represents an overlying flow. They are dense, fresh rocks with
3/ Ridgways light purple-drab (l'"b) to light vinaceous drab (5Tb), rarely cinnamon drab (13tftf-).
abundant two millimeter crystals of white plagioclase and a smaller number of glassy orghoclase, quartz, biotite, green augite, and black
hornblende.
Microscopic.- An examination of thin sections showed that in texture the rock is porphyritic and the phenocrysts nearly equal the groundmass in amount. These phenocrysts are up to three millimeters in diameter and are plagioclase, orthoclase, quartz, and biotite with accessory zircon, iron ore, titanite, and apatite. The plagioclase crystals range from andesine to sodic labradorite, averaging calcic andesine; it is commonly largely altered to calcite, less commonly to sericite. Some of the larger crystals carry inclusions of small biotite plates and of the accessories. The biotite has been much resorbed by the magma leaving grains of iron ore along the border; it is also commonly altered to chlorite. There was probably originally a considerable amount of augite or hornblende present but it is now completely replaced by calcite and chlorite. Secondary calcite, epidote, and chlorite are rather abundant in all the sections.
The large part of the groundmass is very finely crystalline to submicroscopic and is rhyolitic in character; it is clouded from submicroscopic inclusions and in addition is dusted with minute brownish trichites. It has a well-developed, wavy fluidal texture. Very irregular streaks of the groundmass are made up of a coarser aggregate of quartz and orthoclase in grains or patchy micrographic intergrowths. In texture these areas are irregular and they are clear except for scattered trichites of opacite.
The rocks of the upper slopes on both sides of West Willow and Deerhom creeks differ from the above in the greater amount of plagioclase and less of quartz and orthoclase; the original biotite is almost completely resorbed, grains of augite are fairly abundant, and partly resorbed prisms of brown
or green hornblende are present. The groundmass tends to be spherulitic.
Weathering and outcrops.
The rock is probably even more resistant to weathering than the Willow Creek rhyolite but as it occurs only in the upper glaciated parts of the streams, where canyon cutting is not prominent, it forms no cliffs or canyons comparable to those of the Willow Creek rhyolite near Creede,
New the Equity Mine, where a fault separates this quartz latite from the underlying pillow Creek and Campbell Mountain rhyolites, there is a rather striking difference between the outcrops on the two sides of the fault, as shown in Plate XjftT opposite p. The Equity quartz latite on the right shows rather rugged outcrops with jagged, broken cliffs, while the rhyolites form steep but smooth talus covered slopes. Here the v7allow Creek rhyolite is less resistant than to the south and does not differ greatly from the Campbell Mountain rhyolite* It yields a scant soil and at the base of the broken cliffs develops great accumulations of slide and talus. The physical agencies, change in temperature, and gravity have been most important in the breaking down of the rock*
Chapter Rocks of the Upper Division of the Potosi Volcanic Series.
General Statement.
The rocks which are here included in the upper division of the Potosi volcanic series are separated from both the underlying lower division and the overlying Creede formation and Mac-nzie Mountain quartz latite by surfaces of makked irregularlity. Both these surfaces are due to erAsion during periods of comparatively little igneous activity. Che irregular character of these surfaces is not local but has been recognized throughout the eastern part of the San Juan Mountains.
The rocks of this division are chiefly rhyolites and quartz latites but in about the middle of the division is a considerable though variable thickness of andesite. The rocks beneath the andesite are chiefly biotite rhyolites, and latites, those above are closely related to each other and are hornblende-quartz latites. These hornblende-quartz latites are separated from the underlying flows by a somewhat irregular surface of erosion. Locally the base of the tridymite latite is also very irregular. These erosional surfaces do not appear to have been so extensive or so irregular as those at the top and base of the upper division of the Potosi.
The rocks underlying the erosional surface at the base of the quartz latites differ greatly in the western part of the quadrangle from those occupying the same position in the eastern part, and this difference itfl/ the succession of flows and tuff beds for different parts of the area together with the lack of continuity of the members through erosion, cover, or lack of deposition has made the correlations and separations difficult
and in some cases uncertain. In the western part of the quadrangle the lowest formation of this division is a hornblende- quartz latite made up of a chaotic aggregate of thin flows and breccia deposits. It is overlaid by the Willow' Gulch rhyolite breccia, a series of tuff and flow breccia. This is in turn succeeded by a thick flow of tridymite latite which ia in turn succeeded by the andesite. In upper 'tfest Willow Creole are poor exposures of a quartz latite which differs considerably from the tridymite latite but occupies about the same horizon, as it immediately underlies the andesite. In the eastern part of the quadrangle the lowest formation of the upper division of the Potosi is a great flow of the Manmoth Mountain rhyolite, a flow breccia. It is overlain by a rhyolite tuff with associated thin flows of rhyolite and these are, in turn, overlain by the tridymite latite. The andesine is absent.
Ss
T the east of west Willow Creek above the mouth of Deerhorn Creek the
o
section is still somewhat different, although exposures are too poor for a positive determination of the relations existing between the rocks of this area. The tridymite latite appears to be the lowest rock exposed. It is overlain by 50 feet or so of an andesite which is believed to be the same as the andesite which caps Bulldog Mountain. This is overlain by 100 feet or so of thin-bedded rhyolite tuff which probably corresponds to the tuff which overlies the andesite to the west. This, in turn, is succeeded, on the slopes at least , by a thick flow of rhyolite which has not been recognized elsewhere in the quadrangle and which is an unusually thick flow in the quartz latite tuff. Several hundred feet of chaotic tuff breccia overlies this flow.
The thickness of the upper division of the PotosI as might be expected from a series of igneous rooks with irregular surfaces at both the top and bottom and within it, varies greatly from place to place. Within the Creede Special quadrangle it is probably greatest to the east of Rat Creek where it is over 2,000 feet.
Hornblende quartz latite. General character and distribution.
In the western part of the quadrangle the lowest mapped unit included in the upper division is a rather chaotic aggregate of small irregular flows and ciastic. material. The rocks vary considerably in character but are chiefly quartz latites with conspicuous hornblende and the whole mass is therefore called a hornblende-quartz latite, although hornblende andesite is common and rhyolite is present. This rock is present on the small hill just northeast of Monon Hill. A more extensive and continuous but thin layer extends westerly nearly on the contours from a point southwest of Bulldog Mountain to the fault near the Xreutzer mine. West of the fault it is nearly 1,000 feet higher on the slopes and continues to the quadrangle line and farther west into the basin of Shallow Creek, but has not been found farther west and appears to be a very local body.
Thickness.
As this latite overlies the rocks of the lower division of the potosi irregularly and as its top is fairly regular, its thickness varies considerably. 7/thin the Creede Special quadrangle it is probably nowhere over
200 feet, but higher up Miners Creek and in Shallow Creek it is considerably thicker.
Petrography. Megascopic.- In color the rocks vary considerably; the greater part
Tt
are drabs. Most of the fresh rocks are rather dense and nearly all show a
I/ Near Ridgways pale quaker drab (lttffd) or purple-drab less frequently olive-buff (21ffd), or a related color.
more or less prominent fluidal structure. Hornblende, which is the most abundant dark mineral, is conspicuous from its lustrous, black cleavage faces; biotite in the usual black, flexible flakes is commonly nearly as abundant, and green pyroxene can rarely be seen with a pocket lens. White plagioclase crystals are abundant and in some of the rocks many of the crystals are several millimeters across. Colorless orthoclase is considerably less abundant and in many of the rocks it is wanting; large dull, orthoclase crystals several centimeters across are sparsely present in some of the rocks; quartz in visible crystals is rarely present. The groundmass, which is about equal
in amount to the phenocrysta and is holocrystalline, can resolved with a pocket lens, but in most of the rocks it has the appearance of a rh;/olite; less often of an andesite. At the base of some of the flows there are% a few feet of black obsidian carrying the usual phenocrysts. On the whole the rocks are not very different from some of the quartz latites of the lower division, especially those of Outlet Tunnel quartz latite.
Microscopic.- The study of this sedions showed that the greater part of the rocks are quartz latites. The hornblende is commonly brown - less conmonly green; both varieties are present in distinct crystals in some thin
sections. It husually been considerably resorbed by the magma; the biotite,
has teen resorted to a less extent. The plagioolase is andesine or andesinelabradorite with a more sodic border. The accessory minerals are pleochroic apatite, magnetite, and rare titanite. The groundmass varies considerably; in some specimens it is spherulitic, in others microfelsitic, microgranular, micropegmatitic or in part glassy; it is made up chiefly of quartz and alkalic feldspar. Secondary chlorite, calcite, and kaolinite are present.
The andesites, which are not abundant, differ from the quartz latites chiefly in the character of the groundmass which in the former is made up largely of minute laths of oligoclase feldspar with some rods and grains of pyroxene. The phenocrysts are somewhat more abundant and in a few of the rocks those of feldspar are tabular. The rocks, as a whole, are near the border between the quartz latites and the andesitea; those called andesites do not differ greatly from the quartz latites and there is a fairly continuous gradation between the two. In this they differ considerably from most of the other quartz latites of the quadrangle which are considerably nearer the rhyolite.
Locally the rocks are greatly altered by processes other than weathering. The plagioclase is kaolinized, the hornblende altered to chlorite, iron oxide, and other minerals, and the rock is bleached to a dirty lightgray or white.
Weathering and outcrops.
The breccia portion of this formation breaks up readily and offers few outcrops; the massive rock is somewhat more resistant but it too is a comparatively soft rock and rarely gives good outcrops. Within the quadrangle this quartz latite is thin and it has not }iad a great influents on
the development of the topography.
Windy Gulch rhyolite breccia. General character and distribution.
A rhyolite breccia made up of light-colored rhyolite, in part an ordinary tuff, in part a normal flow rock, but chiefly a breccia, probably a flow breccia lies beneath the tridymite latite in the drainage of Windy Gulch and to the west and is here called the Windy Gulch rhyolite breccia. Its porous character and the abundant fragments of pumice which it carries are characteristic.
It overlies a fairly regular surface of the hornblende-quartz latite in the drainage of Rat and Miners creeks but to the east it directly overlies the Campbell Mountain rhyolite. Near its base it commonly carries very abundant inclusions of the underlying rock. It is everywhere overlain rather irregularly by the tridymite latite. To the southeast of Bulldog Mountain it is in contact with the Creede formation which is below it on the slopes.
It forms a layer, broken in places by faulting or by a quaternary covering, from west of MacKenzie Mountain to the Happy Thought Mine. Exposures are very poor and the lower contact in particular can rarely be mapped with accuracy. The body BX west of Mackenzie Mountain is poorly exposed and its lower contact, in particular, is uncertain. On the east side of the Kreutzer Fault the rhyolite breccia is thrown down nearly 1,000 feet; the apparent thickness here is probably due to a disturbance of the beds near the fault but exposures are almost wanting. East of the fault the rock continues to Rat Creek and is
nearly horizontal; beyond Rat Creek it rises rapidly and crosses /indy Gulch just below Bachelor. Beyond Windy (Julch this rhyolite breccia, aside from a small outcrop just south of Bachelor, is mapped only as a narrow band west of the Amethyst fault and between the Last Chance and Happy Thought mines. This is in an area almost completely covered with a mantle of debris from the slopes above and probably also of ancient wash, and is alnost entirely lacking in exposures. Not many of the numerous prospect shafts can be entered, but their dumps furnish sufficient data, although the rocks are commonly much altered, to map this area with a fair degree of accuracy. A~if is probaol$ local in extent as
outside the Creede Special map it has been recognized only to the west, as far as Shallow Creek.
This rhyolite is commonly from 100 to 200 feet thick, although locally it is probably much thicker, as it is believed to have spread over a surface of somex relief. However, it has been faulted and considerably disturbed east of Miners Creek and south of Bulldog Mountain, and the greater apparent thicloiess in both these areas is in part, at least, due to imperfectly understood structure.
,.
Petrology.
u
Megascopic.- In color this rock is commonly pale red-brown; less often
I/ Ridgway's purplish vinaceous (lfttb) to pale purplish vinaceous (lttff), brownish vinaceous (5tftb) to pale brownish vinaceous (5ltff}, pale vinaceous (lftf), pale purple-drab (ltltfd) or some closely related color.
it has one of the lighter shades of gray; the altered rock is white or gray. The luster is always chalky dull. The rock is made up in considerable part of irregular, ragged fragments, rarely over a few centimeters across, of
a
pumice with a fibrous appearance, due to the fin|L elongated pores. These fragments have a somewhat lighter color than the body of the rock; in places they are replaced by a green chloritic material or by drusy quartz. They weather out, leaving very numerous rough cavities of various sizes. In addition there are fewer fragments commonly not over a centimeter across, occasionally over a decimeter, of a variety of foreign rocks; these are generally much altered and are chiefly of the lower rhyolites and the hornblende-quartz latite but are occasionally of other rhyolitic and andesitic rocks. The matrix is always less porous than the pumice fragments and in some of the rock it is fairly dense. It carries a few millimeter crystals of glassy orthoclase, and a very few of biotite. Much of the rock is believed to be a flow breccia, although a part may represent a "mud flow" or tuff. The fragments which make up a large part of the rocfc and are similar to the matrix except for their greater porosity probably represent fragments that were shot into the air and fell back into the molten magma. These flow breccias may also represent material that was
thrown from a crater, settled about the vent while still soft, welded together, and flowed much as any viscous lava.
Microscopic,- The thin sections of the rock show a few crystals of orthoclase and a very few of plagioclase, biotite, magnetite, apatite, and zircon, in a highly porous, glassy groundmass.
Weathering and outcrops.
The Windy Gulch rhyolite breccia is considerably softer than the overlying tridymite latite or even than the Campbell Mountain rhyolite which underlies it in places. It forms smooth slopes, grass-covered or timbered, with rock outcrops only in very favorable places. On exposure to weathering the pumice fragments are removed and the rock breaks up into honeycombed fragments with considerable interstitial fine material. Where erosion is not too rapid a soil is formed in which are few rock fragments and these chiefly of the latitic inclusions. Smooth slopes are characteristic.
Comparison with Campbell Mountain rhyolite.
The typical rocks of the Campbell Mountain rhyolite and of the 7indy Gulch rhyolite breccia can readily be distinguished. They differ considerably in color and much of the latter can be recognized by its porosity and especially by the abundant pumice fragments which it includes. However, the denser, darker-colored varieties of Windy Gulch rhyolite breccia are almost identical with some of the lighter colored, less dense varieties of Campbell Mountain rhyolite. On weathering both lose their inclusions,
become highly cavernous and are still more difficult to distinguish. Areas that caused especial difficulty in the mapping are on the ridge north of the Corsair Mine, on the ridge south of Bulldog Mountain, and northeast of Monon Hill.
Mammoth Mountain rhyolite. General character and distribution.
The Mammoth Mountain rhyolite is a single thick flow of rather uniform character. It is confined to the northeastern part of the Creede area and to the mountains to the east and northeast. The small, triangular body in upper Dry Gulch, east of Mammoth Mountains, is poorly exposed; it is probably bounded by faults. The greatest body is to the north of this and directly overlies the rocks of the lower divisionof the Potosi series. A considerable body is on the ridge between the forks of pillow Creek. This flow has not been recognized to the west, south, nor north of the Creede Special area but it has been followed to the east as far as Bellows Creek.
Character of contacts and thickness.
The base of this flow is very irregular, as may be seen from an examination of Plate II. This is shown to the east of Bast Willow Creek where the lower contact commonly cuts sharply across the contours and several of these contacts were at first thought to be faults. It generally overlies the Phoenix Park quartz Itatite or Campbell Mountain
rhyolite but locally rests on the Willow Creek rhyolite. It occupies about the same position in the section as the hornblende-quartz latite and the Windy Gulch rhyolite breccia to the west but its relation to
these rocks could not be determined. Overlying the Mammoth Mountain rhyolite fairly regularly is the rhyolite tuff or the quartz latite tuff.
East of Bast Willow Creek this flow attains a thickness of a thousand feet "but this diminishes rapidly to the west and the flow wedges out in the upper drainage of Nelson Greek. It is believed that it came from the northeast or east and that it did not extend far west of Campbell Mountain.
Petrography.
If
Llegascopic.- In color this rock is characteristically red-brown.
I/ Ridgway's russet-vinaceous (9tft -)j rarely a darker shade and very rarely a lighter tint; exceptional specimens contain less gray, more oomrnon ones contain more gray as brownish drab (9fftt -), or more orange as fawn-color (13tff
Its luster is dull; its fracture is commonly rough and tends to be hackly. It is a flow breccia and is decidedly mottled, though somewhat less strikingly so than is the Campbell fountain rhyolite which it very closely resembles. It shows a fluddal texture only on close examination,
i,
with very fini, wavy, streaks, discontinuous and irregular. The rock is fairly dense although cavities a millimeter or so across are sparingly present; much larger cavities, in part flattened, are exceptional, but are rather abundant locally; they are probably confined to the base and topoof the flow* They are in part filled with kaolinit ic material. At the base of the flow wherever seen there are a few feet of black glass*
The rock contains rather abundant phenocrysts up to 2 millimeters in cross-section; they make up approximately 10 per cent of the rock. (Jlassy orthoclase is in slight excess over porcelain white plagioclase;
quartz and biotite are less abundant. The groundmass is aphanitic. The mottled appearance is due to the presence of abundant inclusions which are of two kinds, one distinctly foreign, the other differing from the host chiefly in their slightly paler color. The former are commonly much altered and are chiefly of quartz latites similar to those of the underlying series; less common ones are rhyolites similar to those of the older series, or of more coarsely porphyritic rhyolites or of andesites. In some cases they are bounded by narrow bands of lighter or darker color.
Microscopic.- The thin sections show that the plagioclase ±x of the phenocrysts is oligoclase or andesine; zonal growths are inconspicuous. The biotite is slightly resorbed. Minute to submioroscopic particles
of ferritic material give the groundmass a pinkish buff color or a I/ Ridgway's pinkish buff (17fd).
paler tint as seen in a thin section of normal thickness. The groundraass is not markedly fluidal but shows irregular, discontinuous, wormlike streaks of slightly different color and texture. It is largely submicroscopic in crystallization but in part is delicately spherulitic with the minute fibers arranged normal to the fluidal streaks. It has the characteristics of a rhyolite groundmass and is no doubt made up chiefly of quartz and orthoclase. In addition to the streaks mentioned above, and fcuch larger, some several millimeters across, are irregular lenses and streaks of colorless material. These are much more coarsely crystalline than the body of the groundmass and are made up of microscopic intergrowths of quartat and orthoclase; they commonly show a concentration of quartz in their centers. Small rounded areas, rarely a milli-
meter across, are abundant in some of the sections and are largely confined to the coarsely crystalline areas. They are made up of a mineral which has an indea. of refraction of about 1.50 and a very low birefringence, and is probably a zeolite. Some of the original cavities are filled with a fibrous or platy kaolinitic material whictthas an index of refraction of 1.535 and a moderate birefringence; in some specimens this is deposited on the spherulites mentioned above.
In addition to the inclusions of quartz latites there are common inclusions of a rhyolite porphyry with phenocrysts of orthoclase, plagioclase, quartz, and sphene in a fairly coarse granophyric groundmass. Fragments of altered andesite are rare. Other inclusions of rhyolite, differing slightly from the host but sharply bounded and commonly having a more coarsely crystalline groundmass and paler shade of color, are abundant. Many of them are clearly included fragments but some can hardly be distinguished from the areas of coarser crystallization which are believed to have crystallized in their present position.
Weathering and outcrops.
This rhyolite is a rather resistant rock and has weathered largely through mechanical agents. It has no banding or regular direction of weakness but the weathering is influenced by the inclusions and the irregular, streaked structure, causing it to disintegrate into very irregular, hackly fragments commonly about the size of a pea; on the steeper slopes where the loose material is carried away as rapidly as formed the outcropping rock is very rough and hackly. The rock is
uniform in character; structureless for a thousand feet in thickness, and more resistant than either the underlying quartz latite or the overlying tuff. It gives rather prominent outcrops rough in detail, and steep, fairly regular slopes with no prominent cliffs or benches, it is best exposed in First Fork of East 7/allow Creek.
Comparison with the Campbell Mountain rhyolite.
The Mammoth Mountain rhyolite is easily distinguished from all the associated rocks except the Campbell Mountain rhyolite, which it very closely resembles. Both rocks are flow breccias and contain foreign inclusions of quartz latite and other rocks, in addition to those differing from the host chiefly in their paler tone of color; both rocks are rather dense with few small gas cavities and both have a dull luster; both rocks show indistinct and poorly developed flow structure. On the whole they differ slightly in color but the variation in either ono is greater than the difference between the two. The Campbell Mountain
rhyolite is commonly a little duller or more brownish than the Mammoth
I/ Mountain rhyolite. The phenocrysta are about the same in both rocks.
I/ The Campbell Mountain rhyolite is commonly purple-drab or a nearly related color, while the Mammoth Mountain rhyolite is commonly near russet-vinaceous (9ffl-).
The microscopic examination aided but little in their distinction. In brief, the difference between various specimens of the Campbell Mountain rhyolite, specially between the drab and the gray types, is much greater than that between the drab type of the Campbell Mountain rhyolite and the
typical Mammoth Mountain rhyolite and if a number of typical hand specimens of each were mixed together they could not be separated except by one who was thoroughly familiar with both rocks. Only after a carfeul study of both types and of numerous specimens from each has the author been able to distinguish between the two with reasonable assurance.
One of the most constant and characteristic differences between the two rocks is in the weathering. The Campbell Mountain rhyolite weathers into flat flakes with smoother- rounded surfaces often a foot or so across; it breaks with a smooth conchoidal fracture. The Mammoth Mountain rhyolite almost invariably weathers into small, irregular hackly fragments, most of them less than an inch across; its outcrops nearly everywhere show this character of the weathering and specimens broken from even apparently fresh rock show an irregular, hackly fracture; only exceptionally do they show a smooth conchoidal fracture. The difference in color, as stated above, is not in itself conclusive but aids in distinguishing between the two rocks. The general appearance and inclusions of the two rocks are somewhat different. In the Mammoth Mountain rhyolite the fragments of rhyolite similar to the host except in color are less conspicuous, hence the rock is less prominently mottled than is the Campbell Mountain rhyolite. Inclusions of porphyritic rhyolite nearly white in color are locally characteristic of the -ammoth Mountain rock. The phenocrysts, especially those of biotite and plagioclase, are somewhat more abundant in Mammoth Mountain rhyolite, and the whole groundmass has a wavy, chaotic, delicate flow banding. In the thin sections a chaotic, discon-
tinuous wort-like, fluidal banding is rather characteristic of some of
the rocks of the upper division of the Potosi. The black glass at the base of the Manmoth Mountain rhyolite, and the cavernous rock which is locally present in it aid in separating it from the underlying rhyolite. The presence between the two rocks of a small amount of pheonix park quart2 latite, even though not in sufficiently definite bodies for mapping, has aided in separating the two.
After a careful second examination of much of the contact west of Sast Willow Creek the separation has been made with more confidence and accuracy than was at first believed possible. The greatest difficulty was experienced in the area just west of Phoenix -ark where exposures are poor and much of the rock is of doubtful character. The final mapping includes in the Mammoth Mountain rhyolite some outcrops, just south of the small stream which passes through Phoenix park, which from the specimens alone might be included in the Campbell Mountain rhyolite, although they are not typical.
Rhyolite tuff.
General character and distribution.
On the upper slopes to the east of Phoenix Park and in large part beyond the boundary of the quadrangle, a siliceous tuff with associated thin flows of rhyolite breccia rather regularly overlies the Manmoth Mountain rhyolite and is in turn overlain regularly by the tridymite latite. It is about 200 feet in thickness and has one, locally two, thin flows near its base. The main part is a sandy tuff which in places carries scattered pebbles; it is very poorly sorted and bedded.
lot
Petrography.
fI5ie tuff is nearly white to pale drab and has in large part a sandy texture. It carries very abundant glassy crystals of feldspar and black biotite up to several millimeters across and a few small fragments of a rock which carries abundant phenocrysts in a pumiceous matrix. Larger pebbles of similar rock and of a variety of other rocks are locally present.
A part of the material is a quartz latite tuff made up largely of broken crystals of plagioclase with considerable orthoolase, some quartz, biotite, green hornblende, and augite in a very fine glassy matrix, and closely resembles the quartz latite tuff under Nelson Mountain.
The flows are rarely over 25 feet thick KKUL and where two are present they are separated by a small amount of tuff. The rock of the flows is a rather porous, light red-brown rhyolite flow breccia with numerous small fragments of pumice and rhyolite and a few of a fine-
l/ Ridgway's light russet-vinaceous (9tft b)
textured granitic rock. It carries scattered glassy crystals of orthoclase, white andesine, and black biotite with accessory apatite, zircon, and magnetite. The roundmass is largely glass with numerous trichites of red hematite and some ropy streaks which are largely crystalline.
Tridymite latite.
General character and occurrence.
The tridymite latite is the most nearly uniform and distinctly characterized rock of the upper division of the Potosi. The main flow throughout the area studied is a banded, fluidal rock with lenses or bands which are highly porous and are filled with minute drusy crystals of tridymite. Hear Bachelor and elsewhere a denser rock free from tridymite fnrms the base of this formation and probably represents a different flow. The tridymite latite is confined to the area west of the Amethyst fault. The main body forms a band from the west slopes of LlacKenzie Mountain to the Happy Thought Mine; it is commonly nearly horizontal but is displaced by several faults and locally near the faults dips steeply. Small isolated bodies of the typical rock are present about half a mile west of Sunnyside, and others north of Monon Hill; isolated bodies of less typical rock are present in Upper Rat Creek and northwest of the mouth of Deerhorn Creek. The presence of the bodies north of Monon Hill and west of Sunnyside can be explained only on the assumption that the tridymite latite flowed over a surface of greater relief in this area than has been recognized elsewhere. The correlation of these bodies is based entirely on the character of the rocks, without confirmation from the sequence of flows. Such a correlation is rarely beyond question. In the Creede area, however, the tridymite latite is a unique rock and has been found at only one horizon.
7°
TO the west of the area included on the map the tridymite latite is well developed under Bristol Head where its upper surface forms the great flat to the north of Bristol Head; to the east it underlies the great bench called Wason Park. A similar rock which is probably a part of the same flow or a very closely related flow is present south of the Rio (Jrande in the drainage of Trout Creek. Nearly everywhere it immediately underlies the latite tuff but under Bristol Head the two are separated by andesite. The details of the distribution within the Creede Special quadrangle are shown on Plate II.
It is not present on the slopes of Nelson Mountain, although it covers great areas both to the east and west. It was probably locally eroded from this area preceding the deposition of the quartz latite tuff although it may never have covered the area where the Nelson Mountain ridge now is.
Thickness.
The thickness of this latite varies greatly and rapidly from place to place, due chiefly to the irregularity at its base. Its greatest thickness is about 400 feet beneath Bulldog Mountain; from this it decreases to 50 feet west of MacKenzie Mountain.
Petrography. Megascopic.- In color the rock is rather dark red-brown.
I/ Ridgwayfs vinaceous-brown (5ttfi) or a nearly related color; the paler tints, deep brownish vinaceous {5tlf-f and brownish vinaceous (S' are common, as are also the colors with wsLKxjpcxy. more red, livid brown and purplish vinaceous (llflb) and those with more gray, purple drab and dark purple -drab (lfftfb); paler tints and darker shades are exceptional.
It is characteristically banded and platy; the main part is rather dense but irregular streaks and lenses up to a centimeter across, of a paler tint, are decidedly porous. These are rather evenly spaced at a few centimeters apart, and make up a considerable part of the rock. The rock shows phenocrysts of white plagioclase, glassy orthoclase, and black biotite nearly equal in amount to the groundmass and up to two millimeters across. The pores are characteristically lined with minute drusy crystals of tridyraite.
Microscopic.- The microscopic examination showed that the phenocrysts are orthoclase and plagioclase in nearly equal amount, considerable biotite and accessory apatite, iron ore, and zircon. The plagioclase has a core of andesine or andesine-labradorite grading through two or more rather broad, intermediate zones to a narrow border of albite or oligoclase; they average about andesine. They are commonly in part altered to sericite and kaolinite The biotite has been more or less resorbed by the magma with the separation of iron oxide. The groundmass is beautifully fluidal with wavy bands. The main part is so finely crystalline as to be only indistinctly polarizing, it is clouded and reddish-brown in reflected light from numerous microscopic specks of hematite. It is probably made up largely of orthoclase but may have some quartz. There are numerous lenses or streaks of clear material which has a much coarser crystallization and the larger ones are porous. These vary in width from a fraction of a millimeter to several millimeters. They vary greatly in the coarseness of their crystallization and are in part spherulitic or fibrous, in part micrographic or miorogranular. Orthoclase and tridymite are the chief minerals of these streaks. Tridymite is
very abundant and is present as aggregates filling rounded areas or lining the walls of the cavities and is evidently closely associated with the gas cavities. In a few specimens tridymite is absent and its place in the centers of the coarsely crystalline bands is taken by quartz. In one specimen from upper Rat Creek the cavities are lined with botryoidal opal.
The origin of these coarsely crystalline, porous bands is believed to have been much the same as that of the somewhat similar bands in Willow Creek rhyolite, and has been discussed (p. ) in connection with the description of that rock. However, conditions were not identical as in this flow a large part of the silica is in the form of tridymite and the amount of space occupied by the gas cavities is considerably greater.
Chemical.- A specimen from about 100 yards northwest of the schoolhouse at Bachelor was selected for analysis as representing the typical tridymite-rich rock. The material for analysis was taken across the bands so as to represent the average of the flow. The plagioclase is slightly kaolinized but otherwise the rock is fresh. An analysis was made by Gteorge Steiger in the Survey laboratory and is given as follows:
Analysis of Tridymite Latite,
Ti02
A12°3
Zr02
F92°3
W W
none
FeO
? 0
MgO
S
CaO
Na 0
BaO
K2°
SrO
Ho-
f.
H 0
The norm computed according to the quantitative classification is;
Quartz
Hematite
Orthoclase
Ilraenite
Alb it e
Titanite
Anorthite
Apatite
Hypersthene
H 0, etc.
Diopside
The mode differs from the norm chiefly in the presence in the former of tridymite instead of quartz and of Motite instead of pyroxene. The rock is a Roscanose (1.4.2.3).
The flow near the mouth of Deerhorn Creek.
On both sides of Test 77allow Greek just above the mouth of Deerhorn Ore are a few small exposures of a quartz latite, differing somewhat from the no mal tridymite latite. Similar material was found on the dump of the Captiv Inca shaft. Its base is nowhere exposed and the overlying rocks are but poo exposed on the northeast side of the creek. It is immediately overlain by andesite resembling the andesite of Bulldog Mountain and is believed to be closely related to the tridyraite latite and has been mapped as belonging to body.
I/
In color the rock is near quaker drab. The fresh rock shows a rather
I/ Ridgway's quaker drab (lffftf -), light quaker drab (lftftt b), or 1 ight purple-drab ( 1 f ' b ) .
delicate, inconspicuous, wavy fluidal banding which is more conspicuous on the somewhat altered rock. The rock is rather dense and shows scattered crystals approximately 3 millimeters in cross-section of white plagioclase and clear, glassy orthoclase, and a very few of biotite. In much of the rock the plagioclase is altered to kaolinite and sericite.
The thin sections show that the rock contains phenocrysts of orthoclase and plagioclase in about equal amounts with less of biotite and accessory, minute crystals of apatite, zircon, and magnetite. The groundmass is fluid and is in part spherulitic, in part microfelsitic. The rock differs from the typical tridymite latite chiefly in that it lacks the prominent tridymite-rich lenses.
Outcrops, weathering, and alteration.
The tridymite latite is considerably more resistant than the Windy (Julch rhyolite breccia which commonly underlies it , and is somewhat more so than the overlying andesite. It is commonly traversed by a system of vertical joints and these, together with the horizontal fluidal structure, determine its mode of weathering. It breaks into small plates or irregular fragments, with little chemical decomposition. It is a cliff -forming member and as it is underlain by a comparatively weak rock several large landslides, notably the one west of Bulldog Mountain, have broken from its cliffs.
an;l pyominontTtrorcTfij clitf against the snow-covered -
It is bleached and silicified near the amethyst vein, although the orthoclase crystals are still fresh. Nearly everywhere the plagioclases show some alteration to kaolinite or less commonly to sericite, and locally they are completely gone, although the remainder of the rock shows no alteration. This alteration: probably took place after the crystallization but before the cooling of the magma and was probably caused by the gases which occupied the pores in the lava.
Andesite. General character and distribution.
Andesites and related rocks occupy only a very small part of the quadrangle. The only important mass of rock of this character overlies the tridyraite latite and underlies the quartz latite tuff. This mass is made up of a considerable number of thin flows with a somewhat smaller amount of intercalated breccia. The rocks vary considerably and include biotiteaugite-quartz laiites, biotite-hornblende andesites, augite andesites, and olivine andesites. Normal basalts have not been recognized.
The largest area occupied by these rocks is on Bulldog Mountain and the ridge to the north. In upper Rat Creek are two small outcrops and to the west of Rat Creek there is a narrow band of this andesite above the tridymite latite. In west T.7allow Creek just above and below the mouth of Deerhorn Creek are two small outcrops of massive rock which are believed to belong to this mass. Farther east this rock has not been found. To the west of the Creede Special quadrangle, however, on Bristol Head these andesites are locally well developed and occupy their usual position, regularly overlying the tridyraite latite and rather irregularly overlain by the quartz latite tuff.
Throughout the area over which this andesite has been studied it is irregular in thickness and discontinuous; the base is fairly regular and the greater part of the variation in thickness is due to the irregularity at the top. The greatest thickness is north of Bulldog Mountain where there is about 500 feet of this andesite but it becomes rapidly thinner in all directions.
Petrography.
I/ General.- In color the rocks are largely near quaker-drab or mouse gray.
I/ Ridgway's quaker drab -) or deep mouse gray (15'"" i) or a lighter
tone or darker shade of either of these; rarely purple-drab (!"" -)
or olive-gray (23'"" b).
The greater part of the rocks are fine-textured and carry very few crystals that can be seen without a careful inspection with a pocket lens. They are as a rule conspicuously vesicular or amygdaloidal. In many, the vesicules are much flattened by flow, giving the rock a platy, f luidal texture. Megascopically they were thought to be basalts. The lowest flow in the
drainage of 7indy Gulch is dark reddish brown and rather dense. It carries Zj Hidgway s dark purple-drab ( 1 " " i ) .
scattered crystals a few millimeters across of white plagioclase, and brownishblack biotite and hornblende in a felsitic groundraass. On the east bank of
\Vest 7allow Creek just above the first crossing of the road to the Equity
5/
mine is a small exposure of a dense, deep mouse-gray rock which carries
3/ Ridgway's (15" '" i).
abundant tabular crystals of glassy, striated plagioclase up to 5 millimeters long and a little augite in an aphanitic groundmass. A similar rock was
brought from under the white tuff from a shallow shaft a few hundred feet to the south where it is associated with the normal amygdaloidal rock. Another locality ft) r this type is from the bottom of a shallow shaft west of .Vest Willow Creek at an elevation of 10,900 and S. 25° U of the mouth of Deerhorn Creek.
Microscopic.- As seen under the microscope the typical amygdaloidal rock contains rather abundant millimeter tablets of plajioclase, with a little augite and altered olivine which grade into a very fine-textured groundmasa and magnetite with considerable brown, clouded, interstitial glass. A small amount of quartz is present in some of the rocks; it may be secondary, although it appears to be interstitial to the feldspars of the groundmass. -The larger feldspar crystals have cores of labradorite or sodic labradorite and borders filled with glass inclusions, which become; as sodic as albite. The feldspars of the groundmass range from albite to andesine. The feldspars average about andesine. The olivine, which is variable in amount, is completely altered to iddingsite, fibrous strongly birefracting serpentine, carbonates and iron oxide. The augite is commonly reddish-brown from the partial oxidation of the iron, probably before the consolidation of the rock. Some of the sections show areas of opacite which probably represent resorbed crystals of amphibole or other dark minerals. A few of the rocks are holocrystalline and carry considerable interstitial albite and a little orthoclase. Superficially the rocks resemble basalts but differ from normal basalts in the more sodic character of the feldspar and the small amount of dark minerals; it may be called an olivine andesite. It grades into a normal pyroxene andesite. The filling of the vesicules is varied. The most common is a soft, dull, opal-like material with shrinkage cracks and no doubt represents a gelatinous filling now partly crystallized. It is pale yellow-green or
Hidgway's deep sea-foam green (27ft d).
a related color. In part it is amorphous and in part it is made up of fibers projecting from the walls of the cavities. These have a mean index of refraction of about 1.60, a rather strong birefringence and a positive elongation,
It is probably nontronite, oeladonite, or a related mineral.
A very few minute prisms with the following optical properties were found in this material.
Jj 1.62 Birefringence - .01 about and 2V large probably optically - .
On one face it shows an extinction angle (Z A elongation) of 24° , On the other face it shows sensibly parallel extinction and the emergence of X on the edge of the field. It is, therefore, probably monoclinic. It is faintly pleochroic with Z~pale greenish-yellow, Y-pale orange, and X — pale pinkish orange. These properties do not determine the mineral.
Calcite is a rather abundant mineral of the cavities as are also analcite and a number of undetermined zeolites whose optical properties are:
Undetermined zeolite A- This zeolite occurs in spherulites or radiating prisms with the following optical properties:
1.480, Y -1.485, 27- small to medium, optically - , Z elongation about 43 . It is very abundant in spherulites of radiating prisms attached to the wall in the cavities of one of the andesites and is the only mineral in the cavities.
Undetermined zeolite B.- Sparsely present as snow-white, radiating, hair-like fibers. J)~ 1.477, birefringence barely perceptible, elongation - , and extinction sensibly parallel.
Undetermined zeolite G- Aggregate of minute shreds.
1.51, 7f= 1.52, elongation
Undetermined zeolite D.- Colorless to white grains.
1.505, birefringence .01 about, 2V small, optimally-)- , dispersiony< rather strong, extinction sensibly parallel.
The lowest flow in Tlndy 0-ulch is a blot ite-hornblende andesite. It carries rather abundant phenocrysts of plagioclase and less of brown hornblende and biotite. The plagioclases have an average composition of sodic labradorite; the larger crystals carry abundant peripheral inclusions of glass. The hornblende is partly or completely resorbed by the magma, leaving the usual skeleton of magnetite grains and augite. The groundmass carries abundant laths of andesine feldspar, which grade into the larger phenocrysts, in a very fine matrix which contains rather abundant grains of magnetite and augite with indeterminate material which is probably largely quartz and orthoclase. Tridyraite is rather abundant in the small vesicules. The rock is closely related to the quartz latites.
The rock outcropping just below the mouth of Deerhorn Creek on the east approach of the bridge across 7/est Willow Creek and in the two prospects to the west, is a Motite-pyroxene andesite or quartz latite. It carries rather abundant, large phenocrysts of plagioclase, augite, resorbed biotite, and magnetite. The groundmass is made up of plagioclase laths imbedded in a felted to raicropegmatitic intergrowth of quartz and orthoclase, specked with iron oxide. Chlorit ic aggregates were probably derived from small
hfcpersthene prisms. The plagioclase phenocrysts are zoned and range from calcic to sodio andesine, the smaller laths of the groundmass are somewhat more sodic. The rocks are intermediate between the andesites and the quartz latites.
Alteration and Topography.
The alteration of the olivine to iddingsite and the deposition of the minerals in the amygdules are believed to have taken place while the lavas were still hot and decomposition of the minerals of the rook by weathering has not extended to any considerable depth. The breaking up of the rock has been largely physical. These andesites are comparatively soft and are in thin beds but are much more resistant than the overlying tuff. Its upper surface is therefore commonly a bench; its slopes are characterized by a succession of benches, formed by the successive flows.
Quartz latite tuff. General character and occurrence.
At the base of the series of quartz latites which overlies the andesite is a considerable thickness of light-colored tuff fcte in which there are locally thin flows. The greater part of the material in sandy with scattered pebbles which are rarely over a few inches across, a part is very fine grained and a very little is conglomeratic. The coarser material is thick-bedded but some of the finer material has very well-developed, closely spaced laminations.
On the ridge between Deerhorn anl Meat Willow creeks the tuffs carry a larger amount of How rock, the tuff -breccia is more chaotic than elsewhere, and the rocks are more commonly rhyolitic. They probably represent a lower, local horizon deposited in a valley. The base, in this area, is a thinly laminated tuff, not over a hundred feet thick. This is overlain by an irre-ular flow of rhyrlite-latite, locally several hundred feet thick, and this in turn is overlain by a chaotic aggregate of thin flows and
"-
tuff -breccia. On the west slope of the ridge the lower part of this aggregate is a glassy flow about 50 feet 4r more in thickness and with a highly irregular base. The extreme northern part of the body is in part tuff, but contains a considerable amount of massive rock in small irregular bodies. The central part of the body under and north of the point with an elevation of 11,406 is in large part white tuff and latite breccia with subordinate massive roc*. South of this peak and beginning near its top is a fan-shaped body of massive rock with the handle of the fan forming the narrow ridge just below the peak and its broad part on the ridge to the south, where it directly overlies the rhyolite-latite. This fan-shaped body is a steeply dipping flow which overlies successively the tuff and rhyolite-latite. Its outline is indicated on the geologic map (Plate II).
The quartz latite tuff overlies a rather irregular surface of the andesite to the north of Bulldog Mountain. Much of the tuff to the west of ',7est pillow Creek is covered with glacial debris and landslide and even where a continuous deposit of Quaternary material does not cover the bedrock, exposures are almost entirely wanting and the mapping is necessarily unsatisfactory.
Beneath Nelson Mountain the usual tuff overlies in part the rocks of the lower division of the potosi, in part the Mammoth rhyolite. Beyond the quadrangle to the north and west this tuff is very prominent and with the overlying flows of quartz latite forms prominent cliffs and benches for many miles to the north of Bristol Head.
Thickness.
The top of the tuff is fairly regular but the base is locally rather irregular. Its thickness, therefore, varies considerably. Locally, it is less than 300 feet in thickness, while east of Nelson fountain it is over 500 feet, and west of the Park Regent mine and west of Deerhorn Creek it may be even thicker*
Petrography.
The tuff is commonly nearly white with a drab or buff The
finer material is made up of fragments of glass with some broken crystals of quartz, feldspar, and biotite. The sandy part contains a large number of millimeter sized crystals in a finer matrix of glass fragments with some larger fragments of felsitic rock. The crystals are quartz, orthoclase, plagioclase, biotite, hornblende, and the usual accessory minerals. In quantity, size, and mineral character these crystals closely resemble those of the overlying flows with which the tuff as a whole has a close similarity. The larger fragments, which make up the main part of the tuff only on the ridge west of Deerhorn Creek, are chiefly pumice, but fragments of thin platy, fluidal rhyolite ae abundant locally, and those of other rhyolites and quartz latites and of black glass are present.
Andesitic rocks are rare.
Massive rook in the tuff*
Massive flows make up a part of the material designated as quartz latite tuff; their flow and extent are shown approximately on plate II. A prominent flow is on the slopes east of Nelson Mountain and extends from
the eastern landslide to and beyond the quadrangle line. It is about 100
feet thick and is just below the 11,000-foot contour line. The rock is
I/ quaker drab in color. It is dense and shows scattered crystals of
I/ Ridgway's quaker drab (lffttf -).
glassy orthoclase, white andesine, and golden biotite in a felsitio groundmass. Under the microscope the groundmass is indistinctly polarizing and probably contains a considerable amount of glass. Lenses and streaks of the groundmass are somewhat coarser in crystallization and are made up of orthoclase crystals in a matrix with very low index of refraction and indistinct birefringence. It may be secondary opal. A few crystals of quartz, brown hornblende, and augite are present, and apatite, zircon, and magnetite are accessory. It is quartz latite and approaches a rhyolite in composition.
The rock of the flow east of the Amethyst mine has rather abundant kaolinized plagioclase, some glassy orthoclase, considerable pale brown biotite, a little embayed quartz, and accessory apatite, zircon, and magnetite. The groundmass is a glass with incipient crystallization. Rather abundant gas cavities carry drusy crystals of orthoclase, quartz, and
tridymite.
me greatest flow forms the slopes on both sides of Tfest billow Creek above the month of Deerhorn Creek. The rook Is light drab or a nearly related color. It is commonly dense and shs rather abundant crystals
Z Ridgway's light purple-drab t1""' ,, - -
up to 3 millimeters across of white plagioclase, and a few of glassy orthoclase and black biotite in an aphanitic groundmass. A poorly developed streaking or banding is commonly present. The thin sections show that the phenocrysts make up considerably less than half of the rock. Plagioclase, having the composition of andesine, is the most abundant of these; embayed quartz, orthoclase, partly resorted biotite, yellow titanite in large crystals, and the usual accessories are also present. In some specimens the groundmass is very finely crystalline and is largely sphemlitic with perhaps some glass. In another section it is made up chiefly of rather large, irregular, rounded blotches which can be seen only in polarized light and are very fine intergrowths of quartz and orthoclase. They are imbedded in an exceedingly finely crystalline matrix and are packed closely together in parts of the section but are scattered irregularly in other parts. As seen in ordinary light this ground has a rather prominent fluidal streaking and these pass through the polarizing blotches and the finer matrix indiscriminately. The ground is in all cases filled with red trichites of iron oxide. In composition the rock is near the border between the rhyolites and the quartz latites and may proarly be called rhyolite-latite. This rock offers little resistance to weathering aad disintegrates into small hackly fragments. Outcrops are poor and most of these are of the disintegrated rock. The slopes are commonly smooth, rather steep, and show the disintegrated rock in place very near the surface.
The massive rock associated with the tuff which overlies this rhyolite-latite west of Deerhorn Creek is in large part a black or darkgray glass, rarely it is nearly white. It carries rather abundant, inconspicuous crysta s of glassy, striated feldspar up to five millimeters across, and some biotite. Microscopically the rocks show also a few crystals of orthoclase, quartz, and titanite with accessory zircon,
apatite, and magnetite. Many of the rocks have also green hornblende
and augite. The titanite is abundant in millimeter-sized crystals. The plagioclase is zoned andesine. The groundmass of the glassy rocks shows incipient crystallization, chiefly along the cracks. In some specimens abundant red, rounded, botryoidal spherulites, up to several millimeters across are imbedded in a greenish glass* These flows have not been separated from the associated tuff on the geologic map.
The rock of the mapped, fan-shaped flow south of the peak with an
i/
elevation of 11,406 is dark purple-drab and superficially resembles Ridgway's dark purple drab (llttf i).
some of the dense, fine-textured quartz latites of the upper division of the Potosi. It carries abundant, millimeter-sized crystals of andesine with some of quartz, orthoclase, biotite, augite, and titanite, and accessory apatite and magnetite. The groundmass is fluidal in part spheruliti? , and in part very finely crystalline. It is a quartz latite,
feathering and outcrops.
This tuff is but little consolidated and is a relatively soft rock, being much less resistant than the underlying or overlying rocks. The overlying quartz latites in particular are thick, cliff -forming flows and the soft tuff gives way under their load, thttf giving rise to the landslides which so commonly cover the tuff. In general the tuff gives comparatively gentle slopes with few outcrops and there is commonly a more or less well-developed bench near its base. -iriiate-Va,, opposite p. of the tuff are shown to the rear of the mines from the
center of the photograph to the right. On the extreme right the gentle slopes below the cliffs are formed of the tuff; modified by glaciation and landslides. Locally, as on the southeast slopes of Nelson Mountain, they form steep slopes with good outcrops. These white outcrops with their striking castellated forms are locally called the "white elephants". Where flows are present in the tuff they form a succession of benches and cliffs. The white color of the exposures of the body west of the mouth of Deerhorn Creek nd the extreme irregularity of the complex has led the local prospectors to call the mass "The Blowout".
Rat Creek quartz latite. General character and distribution.
Immediately overlying a fairly regular surface of the quartz latite tuff and closely related to it is a series of lava flows with some interbedded tuff, here called the Rat Creek quartz latite from their development in Rat Creek. The flows are mostly thin and rather irregular; the tuff forms a minor part of the material. The several flows and the interbedded tuff are of quartz latites of very similar character; they differ but little from the material making up the underlying tuff and from the overlying Nelson Mountain quartz latite.
This latite underlies the Nelson Mountain quartz latite on the ridge between Rat and West Willow creeks, but owing to the large amount of landslide beneath the cliffs of the Nelson Mountain quartz latite, it is shovra as a number of disconnected outcrops. It is also present on the slopes of Nelson Mountain.
Thickness.
Although there is no marked irregularity either at the base or top of the Rat Creek quartz latite, it varies considerably in thickness. Between Rat and West Willow creeks it is only 150 feet thick at the southern exposures but becomes considerably thicker to the north. On the slopes of Nelson Mountain it is from 400 to 500 feet thick.
Petrogrsp hy.
i/
Megascopic,- In color the rocks are pale drab, They are rather
I/ Ridgway pale purple-drab ( 1 d ) , pale quaker drab (1 f d) f or a nearly related color.
dense and have fairly well-developed flow structure a&own by horizontal planes of fracture but not by prominent banding, Phenocrysts a millimeter or two in cross-section make up about half the rocks. These are chiefly white to moderately clear, striated plagioclase; there is a less amount of clear, glassy quartz and orthoclase and considerable biotite, in the usual black flakes, and hornblende in black cleavable prisms. Light-green augite is abundant in most of the rocks and yellow-brown titanite can be seen in some. The groundmass is aphanitic. Inclusions of quartz latites and rhyolites much like the body of the rock are abundant in some of the flows.
Microscopic.- The microscopic examination shows that the phenocrysts are somewhat variable in size and are commonly broken. Of these, plagioclase is the chief, embayed quartz and orthoclase are never abundant and are not always present, biotite is always present, both hornblende and augite are commonly present, some flows contain only one of them; sphene is abundant and in large crystals in some of the rocks, magnetite, apatite, and zircon are accessory and tridymite was found in a few of the sections. The plagioclases show the usual zonal growths and vary from oligoclase to oligoclase-andesine. The hornblende in nost of the flows is a chestnutbrown variety, but in some a green or olive-green variety. Both it and
the biotite have been greatly resorted by the magma. The groundmass is chiefly spherulitic intergrowths of quartz and orthoclase, in some specimens it is partly glassy. Streaks and irregular areas of coarser crystallization are common in most of the specimens.
Weathering, outcrops, and topography.
Most of the Bat Creole rock is more resistant than the underlying tuff but less so than the overlying flow. Its outcrops are rather poor and it forms rather steep slopes between the gentle slopes of the tuff and the cliff of the overlying flow. Much of it is covered by landslide.
Nelson Mountain quartz latite.
General character and distribution.
The upper flow of the upper division of the Potosi on the Creede Special quadrangle is a regular flow of quartz latite of uniform character. It makes the nearly continuous upper cliff and caps the divide east of Rat Greek in the northeast corner of the quadrangle, it is also the cap-rock onpelson Mountain, whence its name, the Nelson Mountain quartz latite.
A similar flow overlies the tuff and forms the mesa north of Bristol Head. It commonly forms prominent mesas, nearly surrounded b y cliffs.
Thickness.
The top of this flow, no doubt, corresponds approximately to the comparatively flat tops of the ridges. It is fairly uniform in thdckness and between Rat and West Willow creeks it is probably 350 feet thick; on Nelson Mountain it is less than 200 feet thick.
Petrography.
if
Megascopic*- In color the rock is rather uniformly red-brown.
Ridgway's purple-drab (lflfl or a slightly lighter tint on darker shade.
The phenocrysts are about a millimeter in cross-section and in amount about equal the groundmass. They are chiefly of porcelain-white, striated plagioclase, with rather abundant black plates of biotite, prisms of black hornblende, and of pale-green augite; glassy orthoclase and quartz are less conspicuous. The groundmass is aphanitic and rather dense; fluidal structure is present but not conspicuous.
Microscopic.- A detailed microscopic study showed that the plagioclases have an average composition of andesine. The hornblende is of a brown variety and both it and the biotite have been somewhat resorbed by the magma. The quartz phenocrysts are in small amount and are embayed. The accessories are titanite, in rather large and abundant crystals, magnetite, apatite, zircon, and hematite. The latter is in very minute specks and shreds as a pigment to the groundmass. The groundmass is very finely crystalline and is largely spherulitic; it is made up chiefly of orthoclase and quartz. Streaks and bunches are of coarser crystallization; a little tridymite is present in some of these coarser streaks. The rook is not very different from some of the quartz latites of the lower div- i ision of the Potosi.
Weathering and outcrops.
The minerals of this rock show little alteration and the weathering is largely mechanical. It breaks into flat plates along the fluidal
planes and on the tops of the ridges yields a fair amount of soil. As
a it is/very resistant and rather thick flow and overlies softer tuffs and
thin lava flows, it breaks down largely by landslides of various sizes, thus causing a recession of the cliffs of this flow. The flow forms mesas at its upper surface, surrounded by almost continuous cliffs below which are commonly talus heaps and landslide debris. JEke-aasas— oM
the skyline on -the "left
of
gfrmam against the skvllttft on thfr-eactreme right of rlate Ya
ppooito p.
Chapt er V. Creede Formation.
Name, General Character and Occurrence,
A considerable thickness of rhyolitic tuffs and bedded breccia, which contain local bodies of travertinous spring deposits in their lower part, and some intercalated lava flows in their upper part, accumulated in a deep valley or basin which had much the same character and position as the present valley of the Rio Grande from Wagon vtfheel Gap westward to Trout Creer:, a distance of about twenty-five miles. Its width is considerably less. The name Greede formation is here proposed for this deposit from its extensive and characteristic development on the slopes on either side of tfillow CreeK about the town of Creede.
I/
In general the Creede deposits occupy an area in which the Hayden
JL/ U. S. Geol. and Geog. Survey Terr. Colorado Atlas Sheets XV and XVII, Hayden, 1881.
map represented Green P.iver Eocene beds with Carboniferous limestone above
2/
them on the slopes. The Hayden reports contain practically no reference
j, ,
Zj U. S. Geol. and Geog. Survey Terr. Colorado Nlrfkth Ann. Kept. pp. 153-
157, Hayden, 1877.
to these deposits in the Rio Grande Valley and it seems plain that the Creede tuffs in the bottom of the valley were called Green River Eocene, while the interbedded travertine deposits which exhibit massive outcrops in some places were referred to the Carboniferous without any good ground.
ft-
Collections of plant remains from the lower tuff beds of the Creede formation correlate these beds with the Florissant lake beds (Miocene).
The character of the basin in which the Creede formation was deposited is well shown by the way in which the beds commonly lap up on the steep slopes of the older basin of deposition made of the same rocks which form the abrupt slopes of today. This is also shown by the geologic map (Plate II). The Creede beds occupy the floor of the valley of the Rio Grande and are surrounded by steep slopes of the older rocks which commonly rise three thousand feet or more above the valley, in a distance of a few miles. This is not due to folding or faulting but to the fact that the Creede formation was deposited in a basin deeper than that of the present Rio Grande Valley and probably with steeper walls. The irregularity at the base of the Creede formation, resulting from this is well shown on both sides of Creede, to the east the base of the Creede rises nearly two thousand feet in elevation in a distance of less than a mile (Plate II). In PlateJJJ" opposite p. the base of the Creede formation is at the top of the cliffs on the right of the picture over the buildings.
Structure and thickness.
In general the tuff beds dip gently away from the mountains toward the south and southeast, and the dips are commonly steeper near the borders of the body. Dips of ten or even twenty degrees are common near the borders and local dips are much steeper. Away from the borders the dips are small, but accurate estimates of the average dip for any considerable - area are difficult on account of the poor exposures and local undulations.
The general structure in the tuffs of the valley is that of a gentle syncline, It is not known how much of the dip is due to tilting since deposition, as beds deposited rather rapidly in a comparatively small basin surrounded by very steep slopes might have a considerable inclination at the time of deposition and this inclination would probably be steeper near the borders of the basin.
No satisfactory estimate of the maximum thickness of the Creede beds can be made, as the structures are uncertain, the top is nowhere preserved and the base is exposed only on the steep sides of the basin. About five hundred feet of the lower member are exposed on both sides of V7allow Creek below Creede and neither the top nor the base is shown; just east of Creede where the top of the lower member is preserved about eight hundred feet are exposed. A proximately a thousand feet of the upper member are exposed
T)
east of '7indy Gulch. In the middle of the valley the thickness was probably considerably greater as the slopes of the basin of deposition are steep wherever observed.
Subdivisions.
The Creede formation has been subdivided On the map into three lithologic units which, however, have no sharp lines of separation. The lower member is made up entirely of fragmental material, the greater part of which was deposited by water, although some represent talus and similar accumulations. Much of this material is thinly laminated, white,shaly tuff; part is sandy and part is breccia and conglomerate. The material is entirely of igneous origin and a greater part of the fragments was derived from the
rocks of the lower division of the potosi volcanic series.
Interbedded with the tuffs of the lower member are beds of travertine, which was deposited by springs, probably hot springs, at the time the tuff was being laid down and at various horizons* It is in p art a surface deposit; in part a deposit in the channels through which the waters came, and in part a cementation and replacement of the tuffs. It occurs in very irregular bodies whose contacts are not always sharply marked. Only the larger bodies are shown on the map and their boundaries are more or less diagrammatic.
The material of the upper member is somewhat coarser than that of the lower member; it is mainly rather well bedded breccia, conglomerate, and tuff. A considerable part of the fragments is of a rhyolite found only here and in the associated thin, intercalated flows.
The Lower Member.
general character.- The lower member of the Creede formation is made up in large part of fine grained and well bedded material which is commonly creamy or light grayish. In part it is very thin-bedded, shaly, and closely resembles a siliceous shale, although composed almost wholly of volcanic material. Much of this tuff carries plant remains, some of them well preserved Thicker beds of sandy materials and beds of lenses of conglomerate are present. Most of the pebbles are well rounded fragments of igneous rock, though some are subangular or even angular. Near the contacts with the underlying rocks coarse and angular material is much more abundant, and at the base of the original steep slopes of the older rooks the local material of the Creede beds really represents an indurated talus from the cliffs, in the upper part of the member, west of Rat Creel?:, the material is more
thickly bedded and much of it is sandy or conglomeratic*
petrography.- The shaly tuff is rather uniform in character and is made up in large part of fragments of rhyolitic glass with an occasional fragment of feldspar, quartz, and biotite. The sandy beds carry very abundant crystals of orthoclase, plagioclase, quartz, and biotite, rare fragments of other minerals, and a varying number of fragments of felsitic or pumiceous rhyolite.
The pebbles of the conglomeratic part of the tuff consist chiefly of fragments of the underlying rocks, but include also a great variety of other rocks, chiefly rhyolites and quartz latites. The fragments of the conglomerates and breccias about Creede were chiefly derived from the Willow Creek and Campbell Mountain rhyolites with some from rocks similar to the quartz latites of the lower division of the Potosi, Large angular blocks of a tridyrnite latite similar to that of the upper division of the Potosi are present just east of the town of Creede, The nearest exposures of the tridymite latite are much higher on the slopes on Bulldog Mountain or in the high mountains a few miles to the northeast. The fragments may well represent landslide or talus blocks from cliffs of this rock that have since been removed by erosion. A few fragments of a variety of other rhyolites, quartz latites, and andesites that can not be correlated with any of the rocks of- the vicinity are also present and these are more abundant in the thin lenses of conglomerate that are in the thin bedded tuffs at some distance from the jnountains. In other places, as near Wagon Wheel Sap, the fragments in the tuff are chiefly quartz latites similar to those that formed the adjacent slopes at the time of deposition. Where the fragments are angular and especially near the contacts, the greater part is identifiable as belonging to the rocks immediately adjacent, in the Iowa tunnel and elsewhere
near the contact with the lower rocks the material is made up of small angular fragments of the Willow Creek rhyolite somewhat indurated and, no doubt, represents simply an accumulation of talus at the base of the steep slopes of the rhyolites during the Creede epoch.
In general both the tuff and conglomerate are fairly well indurated and are much harder than the somewhat similar tuff of the upper division of the Potosi. Near the travertine bodies it is filled with calcite and much of the coarser material is cemented by reddish brown or yellowish hydrated iron oxide or by a green ferruginous material. Locally, especially in the upper part, the tuff and breccia are cemented by silica.
Interbedded travertine.
Character and distribution.- Within tho tuff of the lower member of the Creede formation are a large number of bodies of travertine. The largest of these is just east of Sunnyside Creek where it forms prominent cliffs; other bodies are mapped between Creede and Sunnyside and between Creede and -Dry Gulch. These travertine bodies are also characteristic accompaniments of the Creede formation beyond the Creede and Vicinity quadrangle. Large bodies are present in lower Lime Creek near the Western extremity of the Creede formation, between Fir and Shallow creeks in the drainage of Farmers Creek, near Wagon Wheel Gap, and in the drainage of Deep Creek. They are rather more abundant near the borders of the Creede formation than in the c nter. The bodies are very irregular in form and their contacts are often indefinite; consequently the mapping is much generalized and is intended to show only the approximate form and location of the larger bodies. A great part of the travertine is a light grayish deposit of very fine-grained calcite, in some places very dense, in others rather porous or highly
cellular. There is commonly a considerable amount of limonite especially in the more porous variety and some of the material is highly ferruginous. Locally, as in parts of the body north of the hill with an elevation of 920o feet to the west of Willow Creek, there is a light, porous deposit of gypsum, stained and coated with limonitic material. Part of the travertine encloses fragments of rock. Locally, it carries much chalcedony with some quartz, filling original cavities or as ve inlets, and in places it contains deposits of siliceous sinter.
Origin.- The travertine and less common siliceous sinter were deposited about the openings of springs, probably hot springs, which must have been very abundant about the borders of the old basin. The travertine is usually mixed with or interbedded with the fine-grained tuff, and in some places it appears to act as a cement for the tuff; in part it forms lenses or less regular bodies in the tuff. Some of the travertine was probably deposited in the conduits of the springs; the strip through the middle of the body east of Sunnyside, which forms the pronounced outcrop of this body, is believed to have been so deposited. It forms a dike-like outcrop striking a little north of east above the 9100 foot contour with very steep walls, over a hundred feet high in places on the southern side, and a bench or low depression on its northern side. It is almost entirely travertine while the rest of this area, included on the map in the travertine, carries much tuff intimately mixed with the travertine.
Travertine deposits, somewhat similar to those in the Creede formation, have been formed elsewhere on a large scale about the borders of interior, undrained lakes, from the precipitation of calcium carbonate from the water of the lake. The abundance of silica and iron in the travertine of the
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Creede formation and the presence of bodies of cellular gypsum and limonite, together with the small size, irregular form, and distribution of the bodies make it seem unlikely that this travertine was so deposited.
The Upper member*
general character and distribution.- The upper member of the Creede formation is made up of considerably coarser material than is the lower member and the material is better sorted, better bedded, and more rounded than is the talus-like material which makes up most of the coarse part of the lower member. In addition to fragments of the older rocks from the nearby slopes it carries abundant fragments of rhyolites found only in this member and some of the conglomerates are made up entirely of such rhyolites; thin flows of similar rhyolites are interbedded with these conglomerates.
general character and distribution.- The two members are not sharply distinct and the contact has been drawn more or less arbitrarily. It was intended to include in the upper member all the pebble beds which are made up of rooks similar to those of the intercalated flows.
This member is best exposed in the drainage of Windy Gulch and on the slopes west and south of the Commodore Mine. The contacts of this body are fairly well mapped except where it is in contact with the rocks of the upper division of the potosi for there exposures are very poor and the contacts and relations are uncertain. The flat about and to the northwest of Bachelor are included in this member although much of this may be Quaternary wash. It has also been mapped on the south slope of Manmoth Mountain but this area is almost entirely lacking in exposures and its contacts are very greatly generalized. The typical rock was seen only in a few prospects at an elevation of about 10,550 feet just west of the isolated
cliffs of Billow Creek rhyolite.
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Petrography.- 3ie clastic rock varies greatly in tho Degree of sorting and rounding of the fragments, the coarseness of the material, the petrographic character of the fragments, and the amount of induration. On the whole it is considerably coarser than the lower member of the Creede formation, is somewhat more indurated, and carries abundant fragments of rock similar to the associated flows. A small part of the material is rather thin bedded and of fine shaly texture, somewhat more is sandy, while the greater part is conglomerate or breccia.
A large part of the material is rather well "bedded and sorted and consists of fragments up to several inches across, seldom well rounded, in a large amount of matrix. The fragments are chiefly of the IJamnoth Mountain rhyolite with some of the "7allow Creek rhyolite, but locally, west of Windy Quick, they are almost entirely of rocks similar to the associated rhyolite flows, especially of the third, banded type, of massive rock (see p. ). Rare specimens of rocks resembling the ttidymite latite and more or less similar to the rhyolite breccia of the lower division of the potosi are present; other rocks are rare. The matrix is dark red or drab, rarely white, and is very fine, hard, and dense, except for occasional cavities, and the rock superficially resembles a flow breccia. The cementing material is chiefly quartz and chalcedony with considerable iron. Barite and jarosite were found in a number of places, chiefly in the cavities; they do not appear to be confined to the vicinity of the veins as they are common in 7indy Gulch. A large part of the breccia has apparent ly been cemented by ferfuginous quartz with some sulphates.
The fine textured layers are in large part indurated to hard, flinty rocks, in most places stained red or drab by iron. They carry a few poorly preserved fragments of fossil plants. The sandy layers in the main consist either of broken crystals of feldspar and biotite or of fragments of felsitic rhyolite and have a ferrugionous quartz cement.
The coarser beds of conglomerate which are interbedded with the lava flows in .Vjndy Oulch and near the Commodore nine, and are also present in lenses in the overlying breccia, are made up of well rounded pebbles up to a foot or more across with a moderate amount of sandy matrix. They are usually cemented by silica and iron compounds to a hard rock. The fragments are nearly all of rock like that of the associated flows; in some places they are of a rock with more prominent phenocrysts. Pebbles of the latter type are always much altered.
Massive rocks.- In the lower part of the section in the drainage of 'tfindy Gulch and to the northeast as far as the Bachelor Mine a number of thin flows are present and alternate with beds of conglomerate made up of fragments of rocks similar to that of the flows. They have not been separat* onthe geologic map as they are local, thin, poorly exposed, and are closely associated with the conglomerates which, in turn, grade into the normal brec<
The greater part of the flows is made up of a rather porous, drab-
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colored rock which carries phenocrysts from one to two millimeters in cross- l/ Ridgway fs quaker drab (1 f f ) to purple drab ( ' f -) .
section, of glassy orthoclase, white, porous feldspar, and dark brown plates of biotite. These phenocrysts make up about half the rock and are imbedded
in a porous, aphanitic inaterial. The microscope reveals a few quartz grains, and shows that the white feldspar is a peculiar microperthite in porous, skeleton crystals and is partly altered to sericite. This gives an imperfect wavy extinction as the intergrowthe are almost submicroscopic. Orthoclase predominates. The biotite is partly resorbed. Accessory apatite, magnetite, and zircon are sparsely present. The groundmass shows beautiful wavy, fluidal bands of lighter tone and coarser crystallization. In part the crystallization is submicroscopic but irregular streaks and bands are made up of closely packed spherulites, here minute, there large* The rocks are fresh except for the presence of a little secondary calcite and sericite. The albite may have been derived from a more calcic feldspar. In one of the sections the phenocrysts have very ragged borders due to their influence in orienting the adjoining groundnass. The groundmass is granophyric and rather coarsely though very irregularly crystalline. Just south of the Comodore Mine one of the flows shows white blotches which can be recognized with a pocket lens as areas of radiating, fibrous spherulites.
Another type of massive rock found only west and south of the Commodore Mine carries rather numerous rough cavities up to an inch in largest dimension. It has fewer phenocrysts than the first type and a much finer groundmass, but is otherwise similar.
S0uth of Bulldog Mountain a third type is exposed partly as a massive rock and makes up much of the breccias. It is a pale purple-drab rock with rather broad bands and lenses of lighter tone. It carries rather abundant crystals of glassy orthoclase and a few of white feldspar and dark brown biotite. The microscope shows that the plagioclase is near albite and is
somewhat altered to sericite. The orthoclase crystals include well formed albite crystals. The groundraass of the broad, dark colored bands is very
finely crystalline, that of the light bands is rather coarsely crystalline with the quartz and orthoclase intergrown. The fibers or prisms of feldspar tend to grow out from the walls and the quartz fills in between them. All the flows are of rhyolites rather rich in soda.
feathering and Topography.
The beds of the Creede formation are by far the least resistant of any of the large bodies of rock within the Creede Special quadrangle, or in its vicinity, and they have had a marked influence on the development of the topography. The broad valley of the Rio Grande, with its comparatively gentle contiguous slopes from Trout Creek to Wagon TCheel Gap, a distance of about 25 miles, corresponds to the area occupied by these tuffs and is in marked contrast with the steep rugged slopes surrounding the valley, and with the canyon of the Rio Grande both above and below. Equally marked is the contrast between the rugged rock canyon of Willow Creek above Creede where it is cut in the billow Creek rhyolite and the broad valley and the rolling hills where it is eroded in the Creede formation, plate jf. 'opposite page shows the low, rolling, grass covered hills of tuff about Creede and the marked change in topography above Creede where the \7allow Creek rhyolite comes out from under the tuffs. This is also shown in plate 5F opposite p.
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Origin.
The greater part of the clastic material of the Creede formation was clearly deposited by water. The conglomerate was probably deposited in part along the shore of the lake, in part by contributory streams during freshets. Near Creede conglomerate is much more abundant in the upper part and makes up the greater part of the clastic material in the upper member; but this may not have been true in other parts of the basin. Indeed the upper member near Creede is probably largely fluviatile material. Ancient talus and similar subaerial accumulations make up a small part of the formation. The fine shaly tuff must have been deposited in comparatively still water and its considerable though local distribution, togethir with its great thickness and the form of the basin of deposition are rather satisfactory evidence that it accumulated in a local, intervolcanic lake. The form of the old basin beyond the present Rio Grande valley is not known, as younger rocks hide it both above Trout Creek and beyond ',7agon 'Sheel (Jap.
The material making up this formation was derived from four sources. Much of the fine tuff represents volcanic ash which was thrown out from some unknown, but nearby, volcanic vent and fell in a lake or on sides of the basin surrounding it and was deposited, after sorting, in a quiet water of the lake. The greater part of the coarse material was derived by the ordinary process of erosion from the mountains surrounding the basin and was brought into the lake by the torrential streams which fed it. This portion is made up almost entirely of the underlying rocks. The travertine, as has already been stated, was deposited by hot springs contemporaneously with the deposition of the fine tuff. Finally, a number
of thin lava flows are present in the upper part of the formation.
Position in the Section.
The Creede formation clearly overlies the rocks of the lower division of the Potosi as the contacts and relations are well exposed on both sides of Creede and in other places and a large part of the fragments from the coarse material in the Creede formation was derived from the two rhyolites of the lower division of the Potosi* it also overlies the rocks of the upper division of the Potosit but although the two formations are in contact near Sunnyside, in the drainage of 7indy Gulch, and near Bachelor the exposures are nowhere sufficient to establish the relation between the two. The two formations are also in contact to the east of the area included in this report, but here too the relations are obscure. Although the Creede formation everywhere occupies the slope below the upper division of the Potosi, the form of the contacts indicates that the Creede was deposited in a basin with steep slopes made up of the rocks of the Potosi volcanic series; this is shown more clearly to the east of the area included in this report where the soft tuffs and breccias of the Creede formation abut against steep slopes of flows of the Potosi volcanic series.
On the whole the Creede breccias contain fewer fragments of rocks a that can be identified as belonging to the upper division of the Potosi than would be expected from the relations, but just east of Creede fragments of the tridymite latite were found in the lower member of the Creede formation and in 7indy Gulch in the upper member. Moreover, two of the formations of the upper division of the Potosi that should have contributed largely to the Creede beds, the tfindy Gulch rhyolite breccia and the Mammoth Mountain rhyolite, weather into small fragments and these could hardly be distinguished from fragments of the older Campbell Mountain
rhyolite. T0 the east in the drainage of Farmers Creek, where the Creede beds lap against large masses of rocks belonging to the upper division of the Potosi fragments of those rocks are more abundant in the Creede formation.
In the area included in this report none of the rocks have been found overlying the Creede formation. However, at Tagon \Vheel Sap, only eight miles to the southeast, a thick flow of a hornblende-quartz latite, which is without doubt related to the Mac Kenzie Mountain quartz latite, overlies the tuff. The position of the Creede formation is 9 therefore, between the MacKenzie Mountain quartz latite and the Potosi volcanic series.
Age.
The thin bedded tuffs of the Creede formation have furnished the only determinable fossils found in this vicinity. These tuffs commonly carry a moderate number of plant remains, some of which are well preserved. Collections were made from three localities in the Creede quadrangle and submitted to P. H. Knowlton for identification.
Following are the lists of forms reported by him from each locality: 2 (2951). Creede quadrangle, Colorado. Ridge N. of Pierce Creek, near ",7agon 'Theel (Jap Hot Springs Hotel, at 9000 feet elevation.
Minute fragments of bark, coniferous leaves, etc., but nothing determinable.
24 (5952). South bank Rio Grande 150 yds. above wagon bridge over Rio Grande,
3|- miles below Creede, Colorado.
Fontinalis pristina? Lesq.
Cercocarpus antiiuus? Lesq. A very narrow leaf.
93 (5953). Tuff, west bank: of Rio Grande -J- mile north of Seven Mile bridge,
seven miles above Creede.
Pinus '.Theeleri Cockerell. Leaves in 5*s.
Sequoia sp. ?
Thuya sp.
Dicotyledon, narrow, wedge-shaped at base; no nervation.
uercus pyrifolia? Lesq.
Fontinalis pristina Lesq.
_1. This has been shown to be a feather and not a moss.
536 (6198). Same locality as 93.
Adiantites? sp. Itfew?
planera myricaefolia (Lesq.) Cockerell.
Pinus ,/heeleri Cockerell.
Pinus sp. cone, like P. florissanti but smaller.
Carduus florissantensis Cockerell.
Vitis florissantella Cockerell.
Conifer, probably Picea or Abies, and undoubtedly new.
LJahonia? sp. Trifoliolate and new.
Abies, two species, new.
Juglans, new?
Celtis mocoshii Lesq.
Lomatia hakaefolia Lesq.
Ribis protomelaeum Cockerell.
Insect, beetle?
Feather, like those of Florissant.
Dr. Khowlton makes the following comments on this flora:
"These ppecies are all or nearly all found in and are highly characteristic of the Florissant flora. The question then arises as to the age of the Florissant deposits. For many years they were supposed to be in the approximate position of the Green River formation, but on the determination of the fossil insects the beds have of late years been referred to the Oligocene. Still more recently extensive explorations have been made in the Florissant plant beds with the result of bringing to light a great number of new and on the whole of very modern appearing forms, and the conviction has been growing that these beds had been placed too low in the scale. Extensive Tertiary floras from this country, as well as from other parts of the world, have lately become available for comparison, and it now seems probable that the Florissant beds are Upper Miocene in age. In any event I feel perfectly justified in stating that the plants here submitted from the vicinity of Creede, Colorado, are of the same age as the plant beds of Florissant, Colorado, and that this age is .certainly Miocene, and in all probability upper Miocene." As to the original reference of these beds to the Green River, I can only say that none of the plants submitted belong to the Green River flora, while all but one a£ or two that have been identified do occur in the Florissant beds."
Chapter 6.
Latest series of lava flows.
General statement.
In the Creede and Vicinity quadrangle a still later volcanic series is represented about Mackenzie Mountain by a great lava flow here named the Macnzie Mountain quartz latite. Beyond the quadrangle in nearly all directions, this latest volcanic series is extensively developed and attains a great thickness on the divides both to the north and south of the Rio Grande and tongues and patches of it extend toward the Rio Grande Valley and locally cross the valley. It is made up of a succession of lava flows and pyroclastic deposits. some of the flows are several hundred feet thick and persits for long distances; others are irregular and local. The pyroclastic part is largely very chaotic and contains rather abundant irregular bodies of flow rock. The rocks of this series are quartz latites and rhyolites and are nearly all characterized by larger and more conspicuous phenocrysts than are the other rock of the area and they can usually be recognized with little difficulty.
The relation of this volcanic Series to the potosi volcanic series is well shown in the vicinity of MacKenzie Mountain where the great lava
flow, here named Mackenzie Mountain quartz latite, overlies the various
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members of the Potosi volcanic series in an irregular manner and clearly
flowed over an erosion surface of considerable relief and filled in the valleys and other irregularities. Beyond the limits of this quadrangle
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in Upper Bast billow Creek, and elsewhere, the irregularity at the base of this series is still more marked and is persistent wherever this series has been found.
The relation of this volcanic series to the Creede format ionic not shovm on the area covered by this report as the two are nowhere in contact. However, about 8 miles to the east, at Wagon Thoel Gap, a great flow belonging to this volcanic series rests successively on the Greede formation and rocks of the Potosi volcanic series and has a lower contact which cuts across the contours very irregularly. The hard rock which forms the Gap is of this formation. There was clearly a considerable interval of erosion succeeding the deposition of the Creede lake beds and preceding the extravasation of the rocks of this upper volcanic series.
MacKenzie Mountain quartz latite. General character and distribution.
On Macnzie Mountain and the ridge to the north and south is a prominent development of a quartz latite characterized by abundant green augite crystals and large and prominent phenocrysts of feldspar and biotite. Within the area mapped it probably represents a single flow, although a part may represent material which solidified within the vent through which the material of the flow was extruded. As shown on Plate II, this rock occupied the large area including Mackenzie Mountain and two smaller areas on the same ridge to the south of the Xreutzer fault. In addition, there is a small outcrop of this rock surrounded by glavial drift in upper Rat Creek, another just east of Hat Creek, and a third west of Rat Creek and north of east of MacKenzie Mountain. This latter body, which caps the hill with 10,800 contour
is poorly exposed and may represent slide rock from the cliffs above, although it is believed to be in place. Similar rocks, which no doubt represent the same eruptive series although not the same flow, are extensively developed on the higher slopes both to the north and south of the Rio Grande,
Character and thickness.
'Vherever observed this flow carae o'ver a surface of relief comparable with that of the present surface and within the Greede Special quadrangle the extreme irregularity of its base is well shown. On the east side of the saddle which is just north of MacKenzie Mountain the base of this flow is well exposed at an elevation of 11,150 feet; from here to the southeast the base of the cliff, which is practically at the base of the flow, as is indicated by the glassy rock at this horizon, cuts sharply across the contour and within a distance of an eighth of a mile crosses the 10,850 contour, a fall of 300 feet. Just north of this the steep slope of the base is even more striking as it is also on all sides of the isolated body on the west side of Rat Greek. This latter body evidently rests on very steep slopes of the older rocks and it was at first believed to be intrusive but the identity of the rock with that of the flow across the creek, and the glassy horizon at its base, together with the details of the contacts, /here exposed, show that the body is a flow resting on the rocks of the upper division of the Potosi series.
Owing to the extreme irregularity at the base of this flow its thickness is variable. Moreover, the top is nowhere certainly exposed, although the small plateau to the northwest of MacKenzie Mountain is believed to represe t nearly the top of this flow. It is commonly several hundred feet thick and locally as much as 500 feet.
Petrography.
Megascopic.- The rock is fairly uniform in appearance, and varies chiefly in color, porosity of the groundraass and the amount of glass in the grounc mass. The color is purple-drab (lflf) in thfee denser rock to light quaker
I/ i/
drab (lf?fffb) or light mouse gray (15ttf"b) in the more porous rocks.
" I/ The glassy horizon at the base is deep olive-gray (23 r -).
I/ Ridgway's names and symbols.
Except for a glassy layer at the base, all the rocks show a few small gas pores and some of the rocks are decidedly porous. The flow is characterized by the member and especially the size of its phenocrysts which about equal the groundmass in amount and are commonly from 2 to 4 millimeters across, occasionally a centimeter or more. They are chiefly of porcelain white plagioclase with rather prominent black flakes of biotite and prisms of pale green augite. The groundmass is aphanitic.
Microscopic*- In addition to the minerals recognized with a pocket lene the microscope shows a few crystals of zircon, apatite, and magnetite. The groundmass is largely submicroscopic in crystallization and is rhyolitic in character. The plagioclase is andesine-labradorite in composition and carries central inclusions of the other constituents. The biotite is considerably resorbed and the augite somewhat alteredtto chloritic material.
7/gathering and outcrops*
roclc is little altered; it is very hard and resistant to weathering and nearly everywhere forms prominent cliffs. It is underlain by softer rocks and landslides have been important factors in breaking up the flow. The slopes beneath its cliffs are commonly covered with a heavy mantle of landslide material from this cliffforming quartz latite.
Chapter VII. Intrusive rocks. General statement.
In the Greede and Vicinity quadrangle, intrusive rocks are neither great in extent, numerous in individual bodies, nor varied in character. Three types have been recognized, rhyolite porphyry, quartz latite porphyry* and basalt. The rhyolite porphyry is fairly uniform in character and was probably all intruded at about one time, during the latter part of the period represented by the lower division of the Potosi. It is found south of Bulldog Mountain and east of billow Creek in irregular or sill-like intrusions, none of which are of large extent. The quartz latite porphyry is very uniform in character and no doubt the intrusions were formed at about the same time. It is confined to the southwest corner of the quadrangle and occurs in dikes up to a few hundred feet across, striking west of north. It is probably of about the same age as the upper quartz latite and may be the intrusive equivalent of that or a related flow. The basalt was found in only one narrow dike in Lower Rat Creek, and cuts the lower part of the upper division of the Potosi.
Rhyolite porphyry. General character and distribution.
A nearly white rhyolite porphyry, characterized by large, glassy crystals of orthoclase is present in several considerable sized bodies to the south of Bulldog Mountain and in some smaller bodies on the slopes west of 7/allow Creek just above Creede. Small bodies of this rock were found in some of the underground workings and a body of somewhat similar rock is exposed on both sides of West pillow Creek just below '.yeaver. The details of the distribution are shown on Plate II.
This rhyolite porphyry is very soft and gives few good outcrops. T0 the south of Bulldog Mountain, in particular, outcrops are almost entirely wanting and the boundaries were mapped largely from the soil and talus. The character of contacts was rarely observed and the mapping is therefore generalized.
Relation to adjoining rocks.
Wherever exposures are sufficient to offer any data as to the relation of this rhyolite porphyry to the adjoining rocks an intrusive origin is strongly suggested. It nowhere appears to occupy a definite horizon in the section, although it is confined to the rocks of the lower division of the Potosi and it is commonly between the two rhyolites of that division, or near their contact. The two small mapped bodies on the east side of the ridge south of Bulldog Mountain are
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clearly cross cutting as shown by their contacts where exposed in several places. The other bodies on this ridge are so poorly exposed that their relations could not be determined. They probably represent in part gently dipping dikes or sills, perhaps in part irregular intrusives. The small masses of this rock just north of Creede are somewhat better exposed and are in part, at least, crosscut ting bodies. Dikelike bodies of this rock are crossed by the Kelson Tunnel. The body south of Weaver has the form of a sill*
The porous character of the rock suggests a surface rock, while the texture rather suggests an intrusive. The intrusion probably took place very near the surface and, indeed, a considerable part of the rock may have reached the surface.
Petrography.
Megascopic.- In luster the rocks are dull and chalky, in color they are nearly pure white with a very slight vinaceous cast and a few grade to quaker drab. They are commonly somewhat porous and some carry abundant very fine pores. Much of the rock shows an imperfect fluidal banding. It carries a few thick tablets of glassy orthoclase nearly a centimeter across in a very fine-textured to aphanitio groundmass. Dark minerals are almost entirely wanting. The rock just worth of leaver is much altered and differersoaewhat from the -est of the rock. In addition to the usual large orthoclase it carries a few crystals of quartz, biotite, and of a completely altered feldspar, probably a plagioclase. It has a somewhat brecciated appearance.
Micro scopic*- The tnin sections show that, in addition to the large phenocrysts of orthoclase, there are a very few partially resorbed crystals of biotite and the usual accessories - apatite, zircon, and magnetite. Quartz phenocrysts and kaolinized feldspar, probably plagioclase, were found only in the specimens from the body south of ',7ever. The groundmass is irregular in texture. It carries very abundant grains of quartz up to a few tenths of a millimeter in cross section, and a few of orthoclase; these are imbedded in and are not sharply distinct from a very fine matrix of quartz and orthoclase. The quartz of the groundmass ad.jace.nt to the quartz grains has oriented itself with respect to these grains and is crystallographically continuous with then.
Quartz latite porphyry. Occurrence.
In the drainage of Miners Greek and to the west there are a number of dikes, some of them several hundred feet across, of a quartz latite porphyry of rather uniform character. Most of these dikes are nearly vertical and strike a little west of north. To the west of the boundary of the quadrangle are several more dikes of the same character, and striking in about the same direction with some tendency to radiate from the hill north of MacKenzie Mountain.
The difee on the west boundary of the quadrangle is the largest of those shown, although some to the west are still larger. This dike is over 200 feet wide in places and has been followed for over a mile. The dike Just west of Miners Greek is somewhat narrower and is about threequarters of a mile long. The bodies near the fault to the east of '.liners
Creek probably represent three distinct dikes, two of them about half a mile and the other only a few hundred feet in length. The southern of these dikes was broken by the fault. Four small, poorly exposed bodies are mapped in the southwest part of Seo. 34, and poor exposures of another small body were found on both sides of Rat Creek in the southern part of Sec. 27. The body on the quadrangle line west of Macnzie Mountain is a part of a larger body most of which is beyond the limits of the quadrangle. The dikes are rather resistant to weathering and commonly give prominent outcrops. 3/here they cut soft rocks they stand out as broken walls.
Age*
These dikes clearly cut the rhyolites of the lower division of the potosi and to the west they cut the rocks of the upper division of the Potosi reaching as high in the section as the andesite. They are older than the last movement along the Xreutzer fault but may possibly be younger than the main movement, as will be shown in the chapter on structure. This rock resembles the upper quartz latite of Mackenzie Mountain both in composition and habit, and in places to the north of Mackenzie Mountain there is some indication that the two grade into each other. They are, therefore, of about the same age as the upper quartz latite and probably represent the channels through which this or very similar and closely related flows were extruded.
Petrography. Megascopic*- In color the rocks vary somewhat but are with few except io
It
gray. They are dense and show phenocrysts, commonly several centimeters
I/ Near Ridgway's light olive gray (23f*M) or light mouse gray (15fft d
across, nearly equal in amount to the aphanitic groundmass. The phenocrysts are chiefly of white plagioclase in thick plates with rounded to hexagonal outline. Biotite is rather abundant and in some of the material augite and less commonly hornblende are present. Phenocrysts of orthoclase and quartz are rare.
Microscopic.- The microscopic study shows that the plagioclase phenocrysts are not greatly zoned and are andesine-labradorite in average composition. The biotite is commonly resorbed. The accessories are apatite, zircon, and magnetite. Less than half of the rocks show remnants of augite, largely altered to calcite and chlorite; nearly all show areas of calcite and chlorite which probably represent altered augite; a few show in addition a little green hornblende. The groundmass is usually a rather coarse micrographic intergrowth of quartz and orthoclase with some small laths of plagioclase. The rocks are comnonly somewhat altered with the development of calcite and sericite from the plagioclase and of oalcite and chlorite from the augite.
Intrusive basalt.
Occurrence.
A single small dike of dark colored rock was found on the quadrangle. It cuts the rocks of the upper division of the Potosi as high in the section as the V7indy Grulch rhyolite breccia on the slopes west of Rat Greek in the southern part of Sec. 22. It has a width of about 10 feet and strikes nearly north and south. It is rather poorly exposed.
Petrography.
!/
The fresh rock is iron gray , but on exposure it bleaches to deep
I/ Hidgway's (23 K).
£/
olive gray. The rock is dense and shows to the naked eye a very few
2/ Ridgwayf8 deep olive-gray (23 -).
glassy crystals of plagioclase. With a pocket lens numerous minute laths of plagioclase can be recognized by their bright cleavage faces.
Thin sections of the rock show that it is made up in large part of small, thin laths of plagioclase, arranged nearly parallel from flow and imbedded in a smaller amount of glass clouded with opacite and with incipient crystallization. Augite grains and prisms are rather abundant and olivine was originally about equally abundant but is now altered to a rather strongly birefracting fibrous serpentine. The plagioclase crystals are labradorite. The usual apatite and black iron ore are present. The rock is fairly fresh except for the Berpentinization of the olivine and the deposition of a little secondary calcite.
Chapter VIII.
Quaternary deposits. Int r oduct ory s tat ement
A. comprehe naive Imowledge of the topography and of the Quaternary deposits is important to the geologist as it enables him to interpret more or less imperfectly the geologically recent history of the area, such as the extent and character of the glaciation and the carving out of the present mountains and valleys. Such a Imowledge should be of considerable value to the miners and prospectors as it aids in an understanding of enrichment, of the disappearance of some veins along the strike, such as the Amethyst vein to the north, and of other important features of the ore deposits. In the early days of the Creede camp it would have enabled the prospectors and operators to avoid the useless driving of tunnels and shafts into terminal moraines or great landslides.
As may be seen on the geologic map (Plate II), of Creede and vicinity, Quaternary deposits cover considerable areas in this district; on the basis of their origin and relative age they have been divided on the map into a number of units*.
The earliest of these deposits with the possible exception of some gravel deposits near Bachelor, are represented by the terminal moraines of the earlier glaciers and their outwash deposits which are probably represented by the upper terrace deposits. Following this were formed the moraines of the later glaciers and their outwash benches. Nearly or quite all of the landslide* were developed later than the glaciation. The alluvium fans and alluvium are in large part recent deposits and have been enlarged and modified up to the present time.
Moraines.
Distribution.
Thousands of years ago, but geologically only one short period ago, the high mountains and divide to the north of Creede were covered by a great thickness of perennial snow. These great snow fields were the gathering grounds for many glaciers which occupied the upper valleys of all the larger streams, such as Miners Creek, Rat Greek, ani both forks of Willow Creek. The glaciers in these streams did not reach the valley of the Rio Grande but ended at an elevation of about ten thousand feet. The glacier that occupied the upper basin of Rat Creek and those of the two forks of Willow Creek came into the area included in the Creede and Vicinity map but that of Miners Creek ended a mile or so northwest of that area.
Glacial topography.
In going up any of the main streams, such as T7est Willow Creek, one is struck by the marked change in the character of the valleys in passing from the lower unglaciated to the upper glaciated part. Below the glaciation the valleys are all strikingly V-shaped canyons with steep rock walls coming to a sharp angle in the creek bed. In the glaciated part the valleys open up and have almost continuous, nearly flat parks and meadows near the streams. They are U-shape in cross section in contrast to the
Two stages of glaciation.
I/ Three stages of glaciation in the Quaternary have been recognizedT
I/ Atwood, ,7. 7., and Mather, K. P., The evidence of three distinct epochs in the pleistocene history of the San Juan Mountains, Colorado, Jour. Geol., vol, 20, pp. 385-409, 1912.
in the San Jtian Mountains. Each of these stages covered a long period of years and in each the glaciers crept down the vail ley and then gradually receded. After a long interval the glaciers again increased in size and the next stage began. Only two of these stages have been recognized near Creede and their deposits are not easily distinguished from each other.
Earlier Moraine.
The earlier glacier of Rat Creek probably ended northeast of MacXenzie Mountain and the gently sloping, hummocky area just north of the hill with the 10,800-foot contour represents its terminal moraine. The numerous large landslides of this area have masked the glacial material to a considerable extent. In 'Test Billow Creek the earlier glacier reached nearly to the mouth of Nelson Creek and may have crossed the ridge into Nelson Creek. Its terminal moraine has the usual hummocky, imperfectly drained surface but it is not so well preserved as is that of the later glacier. The separation of the two moraines is only approximate. The earlier glacier reached nearly or quite to Phoenix Park in East Willow Greet and its morainal material extends for only a short distance up the creek before being covered fcy the later moraine.
Later moraine.
In the Creede area the later glaciers reached to within a mile or so of the maximum extension of the earlier glaciers. Their terminal moraines are somewhat better preserved than are those of the earlier stage and in general the surfaces of their moraines are but little modified and have the hummocky surface, the lack of drainage, the numerous ponds and lakes
characteristic of glacial moraines. In rat Creek much of the area of the moraine is on steep hillsides but in both forks of Willow Creek considerable areas have little relief*
Character of material.
Gravels make up nearly all of the morainal deposits. The pebbles are of the volcanic rock from the upper part of the drainage basins, are well worn, and are in large part rather fresh. Many are flat and soraa show typical glacial striation although on the whole they are not of such a character as to preserve the markings well.
Post Glacial Erosion.
Snce the disappearance of the glacial ice the agents of erosion have been active but have as yet modified the canyons but little. In East pillow Creek, about half a mile southeast of the northeast corner of the area included in the map the creek is in a canyon about 100 feet deep with a nearly vertical wall of quartz latite on the east and steep slopes in the glacial debris on the west. This is exceptionally deep for the post glacial canyon and is very local and cut in comparatively soft rocks. In most places the post-glacial erosion has been slight.
EC onoraic cons iderat ions .
Much prospecting has been done in the glacial material especially in East 7allow Creek. Pew of the prospects have reached bedrock as the glacial cover has a considerable thickness even when deposited against steep hillsides. Anyone prospecting in t.iis material should understand that the
mineral veins do not extend into the glacial material, that the glacial debris varies greatly in thickness and it is in most places more than a thin mantle; and that it was deposited in a valley comparable in form to the present valley but somewhat deeper. Tunnels, especially in the flatter parts of the surface will generally have to be run for considerable distances before bedrock is reached.
Terrace gravels.
In the valley of the Rio Grande to the south and in the lower parts of the smaller valleys are two rather prominent gravel-covered terraces respectively about 50 and 100 feet above the streams. These terraces are remnants of older valleys, somewhat higher than the present valleys and considerably broader. The upper terrace was probably formed during the earlier glacial stage by the streams hich emerged from the lower ends of the glaciers and resulted from the melting of the ice. The lower terrace was probably formed during the later glacial stage. In addition to the two rather prominent and nearly continuous terraces are remnants of other higher, gravel-covered
terraces.
These terrace gravels were found only in the lower parts of the streams and are in large part cut in the softer rocks. They extend up the valley of Rat Creek for some distance. The details of their distribution are shown on Plate II. As topographic features they are rather prominent especiall; in the valley of the Rio Grande just south of the area included in the map. They form rather gently dipping benches on both sides of the river and have been dissected by the larger gullies. The benches are covered with gravels made up of well-rounded bowlders with considerable finer material, and probably represent material carried down from the glaciers.
Landslides* **eneral statement.
In the Creede area, as in other parts of the San Juan Mountains, landslides are abundant and some are of considerable size. Favorable conditions for their development are a steep slope cut in rather thick lava flows which overlie soft tuffs; thick flows such as those of the lower rhyolites are not favorable. In general the slopes of the canyons of the four main streams are very steep and the glaciated parts are especially favorable for landslides.
Landslides were no doubt formed before the glaciation of the region, but the greater part at least of those shown on the map are post-glacial, and their formation has continued to the present time.
Description.
In the mapping, those bodies have been considered landslides which v/ere formed by large masses of rock moving down the slopes more or less as units. In some cases a great mass of rock broke from the top of a mountain and moved rapidly down the slopes; in many the movement took place in stages. Some of the areas mapped as landslide represent aggregates of a number of small slides. Near the mouth of the Nelson Tunnel the material is merely talus. In their surface forms landslides, like glacial moraines, are characterized by hummocks, lakelets, and imperfect drainage. However, the sags and ridges of landslides are lateral to the valleys, while those of moraines are transverse; moreover, landslides commonly show jagged cliffs, pinnacles, and similar forms.
The material making up a landslide is derived from the slopes above, and commonly from a few or even a single rock body - the harder rock from near the crest of the mountain. The fragments are characteristically angular and some are hundreds of feet across. There is no sorting and the arrangement is prominently chaotic. 3 me of the great blocks resemble cliff outcrops of rock in place except for their inconsistent structures and relations and general "jumbled up" arrangement.
Distribution.
The distribution of landslides is shown on Plate II and only a few special features will be discussed here. No attempt has been made to map any but the larger bodies. The large slide northwest of Bulldog Mountain was formed by the slumping of great blocks of the thick, resistant flow of tridymite latite, which is here underlain by soft beds of tuff. It is made up of exceptionally large blocks, some of which give nearly vertical cliffs of the latite but little broken, and as much as a hundred feet in height. However, the chaotic arrangemant and the undrained, hupnocky topography are characteristically those of a landslide area.
The landslide southwest of Nelson Mountain is made up in large part of the ii niuiu ntniiLl4J.it JOTl quartz latite tuff and has some of the characteristics of a mud flow. The material moved down the slopes while soft and acted in some measure as a fluid mixture of rock and water.
The long body southwest of Mammoth Mountain might be called a rock stream. It came from the great cliffs of the Willow Creek rhyolite. The rock mass that broke from the cliffs readily crumbled into small fragments and flowed down the small gulley, much as a large mass of crushed rock or coal flows down a chute of gentle incline, forming a crumpled, humraocky
surface.
Economic c ons iderat ion.
Considerable prospecting has been done in the landslides, notably east of Deerhorn Creek. Evidently any bodies of ore found in a landslide will be small and discontinuous and the ledges in place from which they came will be on the slopes above. If the object of the prospecting is to uncover a vein in the bedrock beneath the slide rock, an estimate of the thickness of the mantle of slide rock is important. Over nearly all the area indicated as landslide on the geologic map (Plate II) the tMckness is probably to be measured in tens of feet and over much of it in hundreds of feet.
Talus .
Accumulations of rock debris at the base of steep slopes from the weathering of the rock above cover considerable areas and obscure the bed rock geology in many places. No attempt has been made to map these consistently, as they are by nature indefinite bodies. The large talus accumulations above the Helson Tunnel prevented a satisfactory interpretation of the bed rock geology and is indicated on the geologic map (Plate II) in the same pattern as the landslides.
Alluvial fans.
.There the smaller streams and gulches reach the valleys of the larger streams at torrential stages they deposit their loads of sand and rock, which accumulate as a more or less conical or fan-shaped body called an alluvial fan. These fans which have been separated in a general way from the main alluvium, are found chiefly in the area of gentler topography near the valley of the Rio Srande.
Alluvium.
The alluvium here includes the recent deposits of the present streams i excepting the alluvial fans. Most of it is ordinary stream wash of sands and gravels and is but little above the present stream channel. It is confined almost entirely to the lower courses of the streams and the upper glaciated portions, as the canyons are too sharp and the stream gradients too steep for its extensive accumulation in the unglaciated portions of the canyons.
The body in Rat Creek, about a mile northwest of Bulldog Mountain represents the filling of a small lake formed by the large landslide from the east.
/3o
Chapter IX.
Structure. Introduction.
The mountains for many miles on all sides of Creede are made up of volcanic rocks of much the same character and belonging to the same general volcanic era. Their structure is simple on the whole, and it is characterized by gentle tilting toward the Rio Grande, and by a few zones or areas of block faulting. The Creede mining district embraces part of such an area,
In a number of places the rocks near the faults or between two faults show considerable dips and have evidently been tilted during the faulting. In others, as at the north end of the Alpha fault, the faults appear to merge into zones of steep dip, probably associated with more or less minor faulting. Bxcept in the immediate vicinity of the faults the rocks lie nearly flat and probably dip gently toward the south.
Faulting.
Difficulties in detecting faults.
Considerable difficulty was experienced in locating and interpreting the faulting of the district, chiefly on account of the great and varying thickness of nearly all the rock bodies, the lack of regular bedding planes and horizon markers, the frequent pinching out in a very short distance of one or more of the geologic units, the general lack of a regular and persistent succession of rocks, the marked irregularity at the base of a large number of the rock bodies, the cover of quaternary material on critical areas, and in places the difficulty of distinguishing between some of the rocks. The irregular character of the faulting has added to
/3f
the difficulty. A glance at the geologic map (Plate II) will amply illustra most of these points. However, a satisfactory interpretation of nearly all the important faulting is made possible by the great relief, the comparative! good natural exposures, and the considerable amount of prospecting. Although some points, such as the northern extension of the Amethyst fault, have not been settled beyond doubt, the mapping as shown on Plate II is believed to be essentially correct.
General character of faulting.
Most of the major faults are normal faults and strike a little west of north. The Amethyst fault and the Solomon-Ridge fault dip steeply west, the Bulldog Mountain and the Alpha faults dip east. The block northeast of the Corsair Mine is bounded by a nearly vertical northwest fault and by a poorly exposed southeast fault. South of the Commodore Mine the Amethyst fault breaks up into a number of faults with varying strike, but their general course is considerably more to the east than is that of the main fault. The great Equity fault strikes nearly least and west and dips steeply to the north; it is the only reversed fault that has been recognized in the area mapped.
In general, brecciation of the walls adjacent to the faults is not extensive and commonly the walls are clear cut and slickensided. Not uncommonly, however, as in the Amethyst fault at the Commodore Mine and in the Ride fault near the Rid-e and Solomon Mine, the faults branch forming large horses, or bodies of rock inclosed between the two branches of faults.
One of the most striking characteristics of the faults is the manner in which they die out along the strike. The great Amethyst fault, southeast of the Commodore Mine, breaks up into a number of faults, several of
which have throws of over a thousand feet and none of which appears to far to the south or east. To the north of the park Regent Mine the throw of the Amethyst fault is believed to rapidly decrease and is probably not great south of the Equity Mine, but here the great Equity fault joins it and beyond the junction it again has a great throw for some miles; farther north it has not been recognized.
The Alpha and the Ridge faults pass into folds at their north efctremitiei probably with considerable fracturing, and are lost. S0rae faults, as the Equity at its western extremity, end abruptly against other faults. Sharp turns are not uncommon as in the Amethyst fault at the Commodore Mine and the Alpha fault near the Xreutzer Mine,
Age*
Nearly or quite all the faulting is believed to have taken place at about the same period in the history of the region and it is probable that the chief displacement occurred during a rather brief geologic period. This period preceded metalization. However, minor movements have taken place since the metalization as is evidenced by some crushing and slickensiding of the vein material in nearly all the veins. The age of all the faults can not be positively determined owing to the limited distribution of the later volcanic rocks. But the Alpha fault cuts both the MacKenzie Mountain quartz latite and the intrusive quartz latite porphyry and no fault is interrupted by later igneous rocks. The Amethyst fault is known to cut the Greede formation. It is, therefore, probable that at least the major faulting took place after the extrusion of the youngest volcanic rocks of the area.
However, in the surrounding country a considerable thickness of volcanic rocks, younger than any in the area here described, are known. The MacKenzie Mountain quartz latite represents only the base of a considerable thickness of related flows and breccia beds and at least one later group of rocks is known. It is not necessary to assume that the faulting is later than these younger volcanic rocks and it may have occurred in one of the more recent periods of volcanism whose products are not preserved, if they ever occurred, in the Creede Special area.
The faulting took place long before the glaciation. Indeed since the faulting, erosion has removed a thickness of rock measured in thousands of feet. This is evident from a consideration of the geologic imp (Plate II). Faulting that takes place rapidly and reaches the earth's surface must develop along the outcrop an escarpment in which the surface of the up thrown side of the fault is raised relative to that of the downthrown side by an amount equal to the throw of the fault. Although the Alpha, the Equity, the Amethyst, and the three faults east of North Oreede all have throws of over a thousand feet they have no escarpments and they affect the topography only in so far as they bring together rocks of different hardness. For example, East 7/allow Creek crosses the extension of the Amethyst fault at a point where the throw is about 1500 feet, yet there is no change in the gradient of the stream. To accomplish this there must have been removed by erosion 1500 feetore of rock on the north side of the faulting than on the south side. At the time of the faulting the topography, therefore, must have been very different from the present topography and the faulting must have taken place at the time before the present drainage system came into existence or at least when it was very youthful. In this connection
it must be remembered that a considerable thickness of volcanic rocks younger than any found about Creede, is present in surrounding areas and was probably once present in this area.
A glance at the geologic map (Plate II) shows that many of the faults parallel the main stream, and suggests a close relation between the two. However, the faults strike a little east of south and the natural course of the streams is directly toward the Rio Grande valley, which is southwest. There is no evidence that the faults have diverted the streams from their normal courses.
Comparatively small areas, much broken up by block faulting, chiefly of the normal type but with occasional reverse faults, surrounded for considerable distances by nearly flat rocks in which important faulting is almost entirely absent, are common in great volcanic regions and geologic relations often indicate that the faulting was more or less closely related to the igneous or volcanic activity of the region. The faulting about Creede is of this character and is believed to have taken place in late in the period of igneous activity and before the existing mountains and canyons of the region were developed.
Mineralization along faults.
Nearly all the faults of the region show some mineralization and nearly all the ore produced has come from veins along faults. The chief production has come from the Amethyst vein, which is along the major fault of the area. The ore produced at Sunnyside Camp came from the vein along the Alpha fault ; the S0lemon, Ridge, and Holy Moses mines are along the Solomon-Ridge fault,
and the Equity Mine is along the great Equity fault. Of the five major faults or fault systems the Bulldog Mountain fault is the only one that has produced no ore.
The chief production of the region has come from the Amethyst fault, between the south end line of the Bachelor and the north end line of the Park Regent claims. North of the Park Regent the vein has not been located and in the network of faults into which the Amethyst breaks southeast of the Commodore Mine, no large ore bodies have as yet been developed.
The Amethyst fault system.
The Amethyst fault system is the most prominent structural feature of the region and it gains additional importance from the fact that the productive Amethyst vein is along it. It is here, somewhat arbitrarily, taken to embrace the main fault between the Bachelor and Park Regent mines, the network of faults extending south and southeast of the Bachelor Mine, to and beyond Lianmoth Mountain, and the poorly exposed system of faulting from the Captive Inca shaft to and beyond the Equity Mine. Strictly the Equity fault might be considered a part of this system but it is discussed separately herein.
North of the Commodore Lline the Amethyst fault is a clean cut, nearly straight fault with a strike of about N. 23° 'V. and a dip of about 50° to 70° y. The western or hanging wall at the surface is made of the upper member of the Creede formation in the southern part and of 7/indy Gulch rhyolite breccia in the northern part; only a few hundred feet below the surface in the underground workings the hanging wall is the Campbell Mountain rhyolite and this continues to the Nelson Tunnel level. The
throw must, therefore, be at least 1400 feet, in that part of the fault which is north of the branch north of the Commodore Mine there are no sharp changes in direction of the fault plane, and no branching faults of any considerable size have been recognized, with the exception of a po sible fault of unknown throw in the hanging wall just south of the Last Chance Mine. In the Last Chance and New Yrk mines there is a great amount of fissuring and crushing of the hanging wall rocks. Throughout this distance exposures are fairly good. Ii4I2U-¥aM8polJtejx,
North of the south end line of the Commodore claim the fault branches toward the south. The details of this faulting are shown on Plate n nrnd
UST. The main branch is on the west and has a throw
of at least 700 feet, while the eastern branch has a throw of at least 100 feet, making the combined throw at least 800 feet. It is probably much more,
The two branches come together again near the south end of the Commodore claim. To the southeast exposures are poor, but there must be a great fault, striking nearly southeast and well exposed at only one small outcrop about half way between the Nelson Tunnel and the portal of the adit which on the map is designated as Commodore Mine. Beyond this there are no surface IX exposures west of \7est Willow Creek. Farther to the south and east there is a network of related faults and no one of them alone can be considered the continuation of the Amethyst fault.
A great fault with a throw of about 1500 feet crosses the nose of the ridge between the forks of 71 How Creek. It strikes about s. 60° B. and has a steep dip. It is indicated chiefly by the manner in which the Campbell Mountain rhyolite abuts against the Willow Creek rhyolite, but it is poorly exposed in one tunnel - on the east side of the ridge. This fault continues with about the same course and crosses the ridge south of ffiammoth Mountain. In some of the tunnels where it is best exposed it shows considerable brecciation in the hanging wall and some mineralization. Where it separates the Campbell Mountain and Willow Creek rhyolites its position is appro ximately shown by the abrupt change from the cliffs of the latter rhyolite to gentle talus-covered slopes of the Campbell Mountain rhyolite. S0uthwest of Mammoth Mountain, however, it joins in an acute angle with a fault which lies south of it and which has a large throw in
the opposite direction. To the east of this junction the fault continues
with to andbeyond the east boundary of the area included in the map but /a throw
that is probably not more than two hundred feet.
The southernmost of these two faults is nowhere well exposed, but the rocks are well exposed on both sides of the fault and its position is indicated by a change form the cliff-like outcrops of the Willow Creek rhyolite to the south, to the gentler talus-covered slopes of the Campbell Mountain rhyolite to the north. It strikes about S. 70° E and has a throw estimated at several hundred feet with the downthrow on the north side. Its dip is not known but it is probably nearly vertical. This and the fault to the north leave a narrow sharp-pointed wedge of the Campbell Mountain rhyolite surrounded by Willow Creek rhyolite. This triangular block is bounded on the west by a third great fault with downthrow on the west side.
The latter fault has a strike a little east of south, it is nowhere well exposed but its presence is shown by the Campbell fountain rhyolite abutting against the pillow Greek rhyolite east of East '.Villow Greek and by the presence of Campbell Mountain rhyolite forming gentle slopes at the base of great cliffs of 'Pillow Creek rhyolite. Its position could not be accurately located especially south of Bast Billow Creek as talus covers much of this area. VThere it crosses the creek it has a throw of about a thousand feet and beyond the point where the nearly east -west fault joins it the throw is increased to about 1500 feet. To the south, after being joined by a number of small cross faults, it is covered by landslide and talus before reaching the Creede formation. It is believed that this fault rapidly decreases in throw to the south, partly by tilting of the fault blocks, partly b y the cross faulting, but it is not likely that it dies out before reaching the Creede formation, although it was not located in that formation.
To the west of this north-south fault four cross faults with throws up to 250 feet or more are shown on the geologic map (Plate II). These were located largely by the displacement of the base of the Campbell Mountain rhyolite.
The three faults on the opposite side of Willow Creek are all of small throw, That just north of Windy Gulch forming the south boundary of the Campbell Mountain rhyolite is shown in a few prospect pits. It has a throw of less than a hundred feet. The other two faults are indicated chiefly by slight displacement of the intrusive rhyolite.
Just west of West Willow Creek, from the main Amethyst fault to the south there is probably a fault of considerable throw. This fault is indicated by the fact that the base of the Campbell Mountain rhyolite on the
A? 7
east side of West Willow Creek is nearly flat and at an elevation of about 9050 feet while on the west side of the Greek, less than a quarter of a mile away, the base of the Campbell Mountain rhyolite is at an elevation of 9750 feet. Moreover, a few hundred feet in the Nelson Mrafea* tunnel the Campbell Mountain rhyolite forms the hanging wall of the vein although on the hill to the west its base is about 400 feet higher.
The northern extension of the Amethyst fault beyond the Park Resent Mine can not be so positively determined owing to the lack of exposures at critical places over much of the area and to some uncertainties in the interpretation
of the bed rock geology west of Deerhorn Creek, prom the Happy Thought shaft
to Deerhorn Creek the glacial moraine covers the bed rock nearly everywhere and for half a mile beyond this, landslide covers the line of the fault. Beyond the Park Regent Mine there are few prospect shafts that show bedrock and the interpretation of this area is based on data secured from these and from a simple outcrop.
A short distance north of the Park Regent shaft numerous surface prospects and the underground workings show that the fault turns rather sharply to the east and has a nearly north-south strike. This strike probably does not continue far because there is no place on Nelson Mountain or to the west as far as the main branch of Deerhorn Creek, where a fault of any considerable throw could pass. Nelson Mountain itself shows good exposures and is made up of rather regular alternating flows and tuff beds; a fault of even small throw could easily be recognized. T0 the west of Nelson Mountain, at an elevation of 11500 feet and near the north border of the area included in the map, is an abrupt change in the bedrock, and a large area of the Equity quartz latite is exposed. This is not found
to the southeast. This quartz latite is separated from the other rocks of the lower division of the Potosi to the southeast by a considerable landslide, and its relations here are not entirely clear. However, it is a part of a great flow, or perhaps of several great flows, which extend northward to the Equity Mine and here clearly overlie the Campbell Mountain rhyolite. T" the north and northwest of Nelson Mountain this quartz latite clearly underlies the tuffs and flows that form the upper slopes of Nelson Mountain and if the two are separated by a fault on the west slope of Nelson .Mountain the fault can have only a small throw. The down throw side, moreover, would be on the east instead of on the west as in the Amethyst fault. A fault might pass into this thick quartz latite series but no evidence of such a fault was found and a fault with a throw comparable to that of the Amethyst fault should bring down some of the higher rocks to the west or otherwise indicate its presence.
There is some evidence as to the position of the Amethyst fault in this moraine-covered area. In the bed of Test Willow Creek, just west of the point where the road crosses the creek is a fair outcrop of the andesite and a few hundred yards to the south are some prospects that bring up the tuff and the andesite. This area is, therefore, similar to the isolated area of good exposures just west of the mouth of Deerhorn Creek and belong to the upper division of the Potosi. The shallow shaft shown on the map about two hundred yards southeast of this road crossing brings up only altered rhyolite of a type that can be identified with reasonable certainty as belonging to the mi low Creek rhyolite. The main fault can, therefore, be placed with reasonable assurance between the road and this shaft.
Farther north, beyond Deerhorn Greek, the only probable position for a fault is between the rocks of the upper and lower divisions of the potosi volcanic series. The contact is nowhere well shown in this area, but more or less alteration and mineralization along this line indicate a fault as does also the- nearly straight contact with the rocks of the upper division of the Potosi series below steep slopes of rocks of the lower division. The latter is not in itself conclusive evidence of a fault as similar relations elsewhere are due to the very rugged surface over which the rocks of the upper division of the Potosi were extruded. However, a fault of considerable throw is believed to separate these two divisions of the Potosi and is so shown on the geologic map (Plate II).
This fault probably forks at about the crest of the ridge; one branch separates the rocks of the upper and lower divisions of the Potosi while the other passes into the Equity latite. This latter fault is indicated largely from the relations of the rocks near the bed of tfest Willow Creek just below the Equity Mine. The rocks of this area are all of the lower division of the Potosi. In the creek bed there are good exposures of the Equity latite, while on the steep slopes to the east the base of this formation is about a hundred feet above the creek bed. This continues to the Equity fault which ends against the north-south fault. North of the Equity fault the throw of the north-south fault has greatly increased. It brings Willow Creek rhyolite against Equity latite. It continues Just east of the creek bed for a mile or more to the north but it probably dies out rapidly in that direction as it could not be located on the divide at the head of the creek. Its position is shown by an inconspicuous series of
flats and sags along its line which Are no doubt erosional features and are due to the soft nature of the rhyolite to the east as compared with the latite to the west.
The displacement of the Amethyst fault, or of the fault zone of Test pillow Greek near Deerhorn Greek is probably not great since the base of the Nelson Mountain quartz latite is at an elevation of about 11600 feet on the wes side of the valley and on Nelson Mountain; only a mile and three quarters away it is at 11900 feet, a difference of only three hundred feet. The base of this flow is fairly level and regular on both sides of the creek.
Equity fault.
The Equity fault is one of the few structural features that stand out clearly enough to be correctly interpreted from a hurried examination. South of the fault the Equity latite gives the rugged outcrops and broken cliffs of a hard resistant rock. Along the fault line which runs straight up the hill over very steep slopes for over a thousand feet, this type of outcrop gives place abruptly to steep grass-covered or talus-covered slopes of the '.?allow Creek and Mammoth Mountain rhyolites. A zone of white
and iron-stained rock follows the fault plane. The fault is shown in
/v Plate which is a photograph taken from a point on the road opposite
the Equity tunnel. The fault strikes a little south of east, dips steeply to the north and is reversed, in the area included in this report it is entirely in rocks of the lower division of the Potosi. The south or foot wall is of Campbell Mountain rhyolite near the creek bed but in great part it is of Equity latite. The north wall is of billow Creek rhyolite in the lower slopes and Campbell Mountain rhyolite on the upper slopes. A quarter of a mile north of the place here the fault crosses the ridge
between fest Willow and Deerhorn creeks and only a hundred feet or so higher in elevation, the Equity latite overlies the Campbell Mountain rhyolite. The throw of the fault is about 1200 feet as measured by the displacement of the base of the Equity latite.
The Equity fault ends on the west against the Amethyst fault, just e ast of West Willow Greek. To the east it crosses Deerhorn Creek ani passes into the drainage area of East 7/allow Creek but with a much diminished throw.
There is considerable brecciation and mineralization along this fault and in the Equity Mine, from which the fault is named, a vein along it is worked.
Fault block of Maramoth Mountain.
On Mammoth Mountain are three faults that may be closely related to the Amethyst system. They form a part of the boundary of the body of Campbell Mountain rhyolite. (See Plate II). The East-West fault on the north is probably the largest of these faults and has a throw estimated at about five hundred feet. It is well exposed only in the cliffs on the west slope of the mountain where it is nearly vertical. It separates the Willow Creek rhyolite from the Campbell Mountain rhyolite and to the east from the Llammoth Mountain rhyolite.
The fault on the west of the Canpbell Mountain rhyolite is well exposed in the cliffs at its northern part. It separates the Willow Creek rhyolite on the west from the Campbell Mountain rhyolite. Its throw is at least a hundred feet and is probably not much greater.
The third fault of this group separates the rhyolites of the lower division of the Potosi from the Mammoth Mountain rhyolite and runs into the east-west fault. This fault is somewhat uncertain as the only evidence of its presence is the form of the contact and this may be accounted for as due to the great irregularity at the base of the upper division of the Potosi. The throw of this fault is uncertain but it is probably considerable.
Bulldog Mountain fault.
",7est of Windy G-ulch is a fault which strikes a little west of north and dips about 50 degrees to the east. On the east slope of Bulldog Mountain it displaces the base of the andesite about two hundred feet. To the south it plays out or is lost in the poorly exposed indy (lunch rhyolite breccia, while to the north it can be followed more or less continuously to the Rat Creek road. Its throw decreases to the north and it disappears in the tuff.
There has been some prospecting along this fault but it shows less mineralization than many of the other faults of the region.
Solomon-Ridge fault.
The S0lemon-Ridge fault is west of East Willow Creek. It has a small throw but is important because along it are the veins of the Solomon, Ridge, and Holy Moses mines. It strikes a few degrees west of north and dips steeply to the west. The downthrow side is on the west and the displacement is probably only a few hundred feet. To the south it has been recognized only a few hundred yards south of the Solomon tunnel. Near the Solomon tunnel it branches but the two branches come together again north of the Ridge Mine It was followed northward to a point nearly a mile north of the Holy Moses
! Mine and is lost on the slopes west of Phoenix Park in an area where exposures are very poor and interpretation of the geology somewhat uncertain, in this area there is some fracturing and mineralization which is probably related to the Ridge fault.
Alpha Fault.
The Alpha Fault east of Miners Creek is a characteristically crooked fault. Along it are the Alpha, Corsair, and Kreutzer mines, and north of the sharp turn at the Kreutzer Iline it has been prospected nearly to the crest of MacXenzie Mountain,
In its southern part between the Alpha and Kreutzer mines its average atrike is about N. 31° 7. and it dips at from 54° to 65 degrees to the east. The hanging wall is commonly much fractured. The fault throws down the Campbell Mountain rhyolite on the east against Willow Creek rhyolite. The displacement along the fault in this southern part is not known but it is believed not to be great. A narrow, much fractured dike of quartz latite porphyry forms the southwest wall in parts of the Alpha Mine; a little to the north a similar dike forms the northeast wall and it is probably the same dike indicating a considerable movement of the east wall to the north, A little farther north another dike of the same rock forms the east wall of the fault for a considerable distance. The relation of these dikes to the faulting is not entirely clear but some, possibly all, of the faulting is later than the dikes.
A few hundred yards south of the Kreutzer Mine a very poorly exposed cross fault joins the main fault from the east. Its exact relations are uncertain as it has not precept ibly displaced the quartz latite dike although it is believed to have crossed it. If the quartz latite porphyry dike formed a nearly continuous tain layer on the hanging wall of the main fault, as it appears to do on the surface, a moderate displacement by a cross fault might not be perceptible in a poorly exposed area. The throw of this fault is not known but it is believed that the north side has been dropped several hundred feet near the Alpha fault and that this displacement rapidly decreases to the east. It therefore gives the Alpha fault an increase throw to the north.
At the Kreutzer Mine the Alpha fault makes a sharp turn to the north and beyond it has an average course of a few degress west of north; it dips steeply to the east.
It is a remarkable fact that Just north of the Kreutzer Mine the top of the Campbell Mountain rhyolite is dropped over a thousand feet by this fault, yet but a mile to the north, near the crest of the MacKenzie Mountain Ridge, the andesite is dropped but little. This great change in the amount of displacement along the fault is explained by the dip of the rocks lying to the east of the fault. Few of the rocks of this area are of such character as to afford opportunity for even rough estimates of dips and strikes but the relations are brought out clearly by mapping the various rock bodies (see Plate II). It is evident that the flows to the west of the fault are nearly flat or dip gently to the south, while those to the east, on the ridge of MacXenzie Mountain, dip at a considerable angle to the southeast but in
going south their dip swings to the south and in the wedge included in the angle of the main fault, and north of the cross fault (see Plate II) they apparently dip to the southwest.
North of MacXenzie Mountain the fault passes into an area covered by Quaternary rocks and is lost. For a mile beyond, however, the rocks show considerable tilting which is no doubt related to the faulting. This structure is best seen in the tridymite latite in which the good flow lines are normally nearly or quite flat. In this area these flow lines are especially well shown in the isolated body of tridymite latite which is west of the earlier glacial moraine (see plate II). This entire body dips to the east at from 35 to 40 degrees on the upper slopes and at somewhat smaller angles on the lower slopes. Indeed this slope is nearly a dip slope and is probably at about the top of the tridymite latite since the andesite appears on the northwest corner of this outcrop with its base at an elevation of 11250 feet while less than half a mile to the east it is at an elevation of 10450 feet. The MacKenzie Mountain quartz latite is probably also affected by these structures. Its base is nearly everywhere covered by landslide but over nearly the whole of this area the top of the landslide laps but little upon the latite as is indicated by the exposure of the glassy base of the latite at very numerous places. No faulting was observed in this area of steep dip but minor faulting is probable; indeed, without fracturing, it is difficult to conceive how these sharp changes in structure could take place in brittle rock which was under only a moderate loa
This zone of steep dips has not been recognized as far north as the later glacial moraines. It is confined to a narrow strip, less than half a mile wide and about two miibes long.
7/5-
About a mile or less to the east of this on the east side of Hat Creek is an area of rocks which probably have considerable dips to the west although the structures are not clearly shown and are indicated chiefly by the form of contacts that are nowhere regular, and by the great vertical extent covered by the 7indy Gulch rhyolite breccia. These two belts would make a canoe-shaped syncline in Hat Greek basin, with the point of the canoe at about the south end of the upper glacial moraine. The continuation of this zone of complex structure to the north may account for the irregularities in the relations of the andesite and tridymite latite in Hat Greek, just south of the northwest corner of the area included on the map. Exposures are poor in this area and no satisfactory interpretation could be made. TO the south this structure probably extends to the faulting east of Corsair Mine. This small structural basin is bounded to a considerable extent by faults of large throw and is probably associated with much minor faulting*
Structure and faulting northeast of Sunnyside.
Northeast of Sunnyside, as shown by the geologic map (Plate II), there is an area of block faulting, with tilted blocks. Poor exposures, the irregular erosional surfaces separating each of the members, the decomposition of the rocks, and their lack of development in their most characteristic forms, introduce some uncertainty into the mapping and interpretation of the structure.
The rocks of this area are the '.Tillow Creek and Campbell Mountain rhyolites, the intrusive rhyolite, the hornblende-quartz latite, the Tindy Oulch rhyolite breccia, the tridyraite latite, and the various members of the Creede formation. Here the Campbell Mountain rhyolite resembles both the underlying '/Villow Creek rhyolite and the T7ndy Gulch rhyolite breccia anl separation is difficult. The hornblende-quartz latite is poorly exposed and is not altogether characteristic; the tridymite latite is fairly characteristic, and the rocks of the Creede formation, except for subordinate tuff in the hornblende-quartz latite, are easily distinguished.
A nearly vertical fault, whichstrikes about N. 40° E. forms the northwest boundary of this area. On the west side of Rat Creek it is poorly exposed and is somewhat uncertain, but from Rat Creek to the northeast it is easily followed. Just east of Rat Creek for several hundred yards its line is marked by low cliffs of the Billow Creek rhyolite facing to the southeast. (Plate X'Efc). The slopes below only a few feet northeast of these cliffs, show fair exposures of the tridymite latite which is here soft and weathers into low pinnacles. The flow banding of this is cut off sharply by the cliff of rhyolite and prospects along the fault expose the actual contact at a number of places. There is little mineralization. The throw of the fault can not be estimated, but it is considerable near Rat Creek, and it is believed to decrease rapidly to the northeast as the latite dips to the southwest, ffo trace of this fault was seen aouth of the Corsair Mine and it probably goes under the alluvium and is lost in the alluvium and tuff. It may join the Alpha fault in which case it would increase the throw of that fault to the south. To the northeast it can be followed nearly to the crest of the ride and it must quickly die out
or join a fault running southeast.
The southeast contact of this narrow strip of tridymite latite may also be a fault. East of Rat Creek the nearly straight contact cutting across the topography suggests a fault although normal contacts at the base of the lake beds are of this character in other places. The contact is indicated as normal on the map as the facts can be interpreted as well without assuming a fault to be present.
Five other faults all of small throw have been mapped in this area. They have throws of from fifty to a few hundred feet and probably continue for no great distance along their strikes. The east-west fault on the north boundary of the tridymite latite strip is nowhere exposed but its presence is reasonably certain from the form of this contact. The way in which the Creede formation is displaced by the east extension of this fault indicates that the north block moved to the west. The northeast boundary of the tridymite latite body also is believed to be a fault on account of the form of this contact. This fault probably continues to the northwest and joins the main fault of this block. The two small faults that cut the hornblende-quartz latite are well exposed to prospect pits. The supposed fault bounding this latite on the northeast is nowhere exposed and this may be a normal contact.
Minor faults.
in addition to the faults already described are a large number of faults and slips of uncertain but probably small throw, In some the throw may be a hundred feet or even more and they could be consistently mapped in an area of comparatively regularly bedded rock, but not without great difficulty and uncertainty in this area. Only a few of these are indica ted on the geologic
map,
Illustrations.
PLATE I. Topographic map of the Creede Special area Pocket
II* Geologic map of the ureede Special area . . . . Pocket
III. Creede
IV. Photomicrographs of Willow Creek rhyolite . . .
V. Photomicrographs of Willow Creek rhyolite
VI. Photomicrographs of Willow Creek rhyolite
VII. Willow Creek Canyon above Creede
VIII. Hugged cliffs of Willow Creek rhyolite
IX. Equity fault
X. Fault east of Sunnyside
T La. ire.
,
III. Creede
Frontispiece
Photomicrographs of pillow Creek rhyolite. Plane polarized light with inclined illumination to bring out relief. Show two of coarse "bands. Magnified 10 diameters.
Photomicrographs of v/iilow Greek rhyolite. Plane polarized light with inclined illumination to bring out relief. Across one of the coarse bands, ilagnified 40 diameters.
Photomicrograph of 'Villow reek rhyolite. Plane polarized light. Inclined illumination to bring out relief. A part of one of the broad bands, ilagnified 40 diameters.
\7allow Creel: Canyon above Creede. Shows cliffs of Villow Creek rhyolite with overlying gentler slopes of Campbell Mountain rhyolite.
fill. Rugged cliffs of 7/allow Creek rhyolite. Above forks of '.'/allow Creek.
Equity fault, from road across from Equity. mine.
X. Fault east of Sunny-side, from Sunnyside<