DESCRIPTION OF THE OTJRAY QUADRANGLE.

By Whitman Cross, Ernest Ho we, and J. D. Irviiig;.

GEOGRAPHY AND GENERAL GEOLOGY OF THE QUADRANGLE.

By Whitman Cross and Ernest Howe.

INTRODUCTION. Miguel Range the Mount Wilson group of the tion -the Algonkian. The Algonkian rocks have Animas, Dolores, San Miguel, and Uncompahgre Telluride quadrangle are geologically a portion suffered great metamorphism and stand on edge or valleys. LOCATION AND AKEA OF THE QUADRANGLE. of the San Juan, cut off by erosion. are greatly disturbed, and the relations of the In the Rico Mountains a fauna that is related to The Ouray quadrangle is situated in the south­ Though the San Juan Mountains are surrounded isolated exposures to contemporaneous formations the Permo-Pennsylvanian of the Mississippi Valley western part of Colorado and is bounded by paral­ by an arid plain country the annual snowfall and elsewhere are quite unknown. On the Hayden has been found in the lower portion of the reddish lels 38° and 38° 15' and meridians 107° 30' and rainfall upon them is heavy, especially on the map these quartzites were called "Metamorphic series, and this fossiliferous zone is called the Rico 107° 45'; its area is 234.87 square miles. Parts western portion. The higher peaks and basins Paleozoic." In this folio they are called the formation. It has also been identified in the Ani­ of Ouray, Hinsdale, and Gunnison counties are are seldom entirely free from snow. This abun­ Uncompahgre formation. mas drainage, but its presence has not been recog­ included in the quadrangle. The town of Ouray, dant moisture supports a heavy forest growth in Great continental movements of uplift, folding, nized in the Uncompahgre Valley. : a local mining center, is near the southwest corner, many places on the western and northern sides. and faulting, followed by enormous erosion, inter­ The nonfossiliferous portion of the reddish beds, at the head of the Uncompahgre Valley; it is con­ Spruce and aspen cover the higher slopes, yield­ vened between the deposition of the Algonkian which rests upon the Rico formation, and which nected by a branch line with the Denver and Rio ing to white pine, scrub oak, pinon, and cedar on sediments and the oldest Paleozoic formation. In is limited above by unconformable Triassic strata, Grande Railroad at Montrose. The relation of the the flanks, as the streams sink into canyons cut in the Animas Valley the Paleozoic sectioii begins with has been called the Cutler formation. The name Ouray to other quadrangles of the San Juan region the lower plains of sedimentary rocks. Timber a thin quartzite, less than 200 feet in thickness, Dolores has been given to the Triassic portion, is shown by the index map on the title-page of the line lies in the zone between 11,500 and 12,000 which has been called the Ignacio quartzite and which is distinguished from underlying beds not folio cover. feet above the sea, and large areas in the interior which, on scanty fossil evidence, has been referred only by its fossils but also by an unconformity. are thus barren of tree growth and support only a to the Cambrian. Following this quartzite with Above the red Triassic beds come other forma­ OUTLINE OF THE GEOGRAPHY AND GEOLOGY OF low alpine flora on favored surfaces. apparent conformity come thin limestones and tions that are correlated in general with the fresh­ THE SAN JUAN PROVINCE. Valuable deposits of the precious metals have shaly strata, the latter characterized by casts of water Jurassic of other parts of Colorado, and The term San Juan region, or simply " the been found in many parts of the San Juan region. cubical salt crystals, and the whole having a thick­ above these comes the Upper Cretaceous, from the San Juan," used with variable meaning by early Coal beds of great extent and fine quality occur ness of less than 100 feet. These beds are distin­ Dakota to the uppermost coal-bearing member, the explorers, and naturally with indefinite limitation along the southern base, and agricultural lands guished as the Elbert formation and are assigned Laramie. Below Durango the post-Laramie forma­ during the period of settlement, is now generally form the valley bottoms and certain of the lower to the upper part of the Devonian, on the evidence tion made up of eruptive rock debris and known applied to a large tract of mountainous country in slopes adjacent to the snow-fed streams from of characteristic fish remains. Above them occurs, as the "Animas beds" rests upon the Laramie, southwestern Colorado, together with an undefined the mountains. With the development of these in conformity, a series of limestones with thin and is in turn overlain by the Puerco and higher zone of lower country bordering it on the north, resources several important towns have been estab­ interstratified quartzites, 200 to 400 feet thick, Eocene deposits. west, and south. The Continental Divide traverses lished in sheltered valleys on all sides. Railroads known as the Ouray limestone, the lower and Structurally, the most striking feature in the this area in a great bow. The principal part of the encircle the group and xperietrate\to sonle^of the middle portions of which are characterized by a present attitude of the formations described, from district is a deeply sco/ed volcanic plateau, more mining centers of the interior. Creejle, Silverton, definite upper Devonian invertebrate fauna. the base of the Paleozoic upward, is their general than 3000 square miles in extent, drained on the Telluride, Ouray, and Lake City, all situated in Although these thin formations have been fol­ southerly, westerly, or northerly dip away from a north by tributaries of Gunnison River, on the mountain valleys, are thus conrieeted^with the lowed for many miles up the Animas Valley, no point in the west-central part of the San Juan west by tributaries of Dolores and San Miguel rivers, main lines of traffic. indication of Silurian or Ordovician strata has Mountains. As seen in the section of the Animas on the south by numerous branches of the San The geologic history of the San Juan region is been found there, nor any evidence of the strati- Valley, all of these formations appear to be con­ Juan, and on the east by the Rio Grande. All but too complex and as yet too imperfectly known to graphic gap which must exist if there are no beds formable. None of the various unconformities by the Rio Grande drainage finds its way to the Gulf justify even an outline statement of satisfactory of those ages in the section. In the Uncompahgre overlap represented on the Hayden map as occur­ of California through Colorado River. accuracy. The pre-Tertiary surface of the entire Valley near Ouray the Ignacio formation is lack­ ring in the area between Animas and San Miguel The San Juan Mountains, as that term is now region was completely buried by the volcanic for­ ing and the Elbert formation is not everywhere rivers exist within that territory. But two great understood, embrace the area bounded on the north mations which now cover the main area, and, while present, and at certain localities in the Silverton orogenic disturbances, not indicated in the Animas by the generally abrupt descent to the sloping mesas erosion has again exposed some of the older rocks quadrangle the Ignacio is absent. These facts section, are clearly shown on the northern slopes extending for 25 miles to the canyon of the Gun­ on all sides of the volcanic complex and even in show that the seeming conformity in the section of the San Juan, and possibly one of them also on nison, on the west by the great plateau of Colorado some of the interior valleys, the reconnaissance seen on the slopes of the Needle Mountains is mis­ . the southern side, not far east of Animas JRiver. and Utah, on the south by the more gradual descent observations of the Hay den and other early sur­ leading and that there is a stratigraphic break of A pronounced unconformity exists between ther to the rolling plateaus of New Mexico, and on the veys were far too meager, and the present resurvey great importance between the Ignacio and Elbert Dolores and older beds in the Uncompahgre Val­ east by the broad and level San Luis Park. From has thus far covered too small an area to afford formations. ley, and the Dolores itself and all older' sediments this main area a broad spur leads off to the south­ solutions to many of the problems concerning the The extreme upper part of the Ouray limestone are wanting in the plateau traversed by the Gun­ east, losing its mountainous character near the earlier geologic development of this most interesting contains Mississippian (Lower Carboniferous) fossils nison and its southern tributaries east of Uncom­ Colorado-New Mexico line. The San Juan Moun­ field. /' at a few points observed in the Animas Valley. pahgre River, and the granites and gneisses are tains thus have a length of nearly 80 miles east In view of this condition, no attempt will be Above this limestone occurs a reddish cafcareous overlain by the probable equivalent of the La Plata and west and a width of 25 to 40 miles north and made at this time to present a thorough review shale, the Molas formation, containing many chert sandstone, of assumed Jurassic age. A similar con­ south, and their summits form a great group rather of San Juan geology, but in order that the sig­ pebbles, and some of these cherts also carry a Mis­ dition exists east of the Animas, in the drainage than a range. nificance of the observations made in the J Ouray sissippian fauna. Thin limestone layers in -the of Los Pinos and Piedra rivers, according to the In the western part of the San Juan Mountains quadrangle may be more fully appreciated, an out­ shale contain Pennsylvanian (Upper Carboniferous) Hayden map, but reconnaissance observations by the topography is very rugged. There are hun­ line sketch of the geologic development of the fossils, so that there is evidence of the stratigraphic A. C. Spencer in the Piedra Valley show that pre- dreds of summits exceeding 13,000 feet in eleva­ region will be given. This outline is particularly break following the Ouray. The Molas shales have volcanic faulting and erosion may be, in part at tion, and several which reach more than 14,000 feet applicable to the western part of the San Juan, for been found everywhere lying on the Ouray lime­ least, the causes of the irregular relations in this above sea level. Here, too, the bounding scarps of it is in the valleys of this portion, near the moun­ stone with but slight unconformity. There is no district. the group are in many places very precipitous, and tain front, that the best exposures of the older rocks known indication of the extent of the Mississippian Other periods of uplift, erosion, or subsidence, some of the valleys in the heart of the mountains may be found. formations that may have overlain the Ouray lime­ in Paleozoic or Mesozoic time, are indicated by the have been cut down to 9000 feet or less above the The Animas Valley, between Silverton and the stone, or of their character, except that the basal apparent absence, or slight development, of Cam­ sea. To the east the configuration is less rugged, vicinity of Durango, shows apparently a complete portion must have been a cherty limestone very brian, Ordovician, and Silurian sediments, the and high table-lands, of varying extent, represent exposure of all formations of the San Juan, from similar to the Ouray and grading into it. presence of remnants of the Mississippian beds, in a measure old plateau surfaces. the Archean to the Puerco formation of the Eocene, The red shales of the Molas constitute the oldest the local development of the fossiliferous Triassic, Within the bordering zone of lower country, inclusive. Much of this section has now been stud­ sediments of a great complex of sandstones and and the absence of the marine Jurassic and of having a general elevation of 6000 to 9000 feet, are ied in detail, but definite correlations can not yet be fossiliferous shales and limestones, belonging to the recognized equivalents of the great Lower Creta­ situated several small groups of high peaks bearing made between the older formations here seen and Pennsylvanian. Above the Molas these beds reach ceous section of Texas. special names. The Needle Mountains on the the isolated exposures reported from some other a total thickness of 2000 feet and have been named The geologic structure and constitution of the south, the La Plata and Rico groups to the south­ parts of the San Juan Mountains. the Hermosa formation. They form the great scarp San Juan Mountains of to-day are mainly the west, and the San Miguel Mountains on the west Ancient granites, gneisses, and schists are known on the west side of the Animas Valley and other result of dynamic forces which were intensely are the most important of these outliers. in the Animas Valley on the south and in the prominent topographic features on both southern active during three great epochs of Tertiary time. The Needle Mountains are almost continuous Uncompahgre Plateau on -the north. These rocks and northern sides of the mountains. With the first of these epochs the long cycle of with the San Juan proper, but the local name is have usually been considered Archean, but the Above the Hermosa strata appears an important Upper Cretaceous sedimentation was terminated by amply justified by the character of the group, granites are probably younger than the great series series of reddish conglomerates, sandstones, marls, a continental uplift which was of unknown extent, which is a result of its geologic structure. The La of quartzites exhibited in the Needle Mountains and thin limestones, in the upper part of which but which may have been very great. To what Plata Mountains form an isolated group, and the and seen beneath the volcanics in the canyon of Triassic fossils occur. These rocks occupy a much extent the Cretaceous sea covered the San Juan Rico Mountains also are not connected with the San the Uncompahgre above Ouray, which have been larger area than the Hermosa in the zone adjacent Mountain area is not yet known, but the sedi­ Juan in origin. The eastern summits of the San referred to the pre-Cambrian age of sedimenta­ to the mountains, and are conspicuous in the ments of that sea are now exposed, dipping at generally low angles away from the mountains, on activity and of orogenic disturbance, the Tertiary Potosi volcanic series, are present in good but not bottom land more than a mile wide, which is lim­ the northern, western, and southern sides. The history of this region may be summarized as a con­ extensive development. ited on the north by«/ a Ogreat moraine that crosses Ouray, Telluride, and Silverton quadrangles con­ flict between volcanic forces, which built a plateau The Pleistocene and Recent history of the region the valley near the town of Ridgway, just beyond tain evidence that the erosion of the epoch under by stupendous emissions of lava, and agencies of is also indicated in the Ouray quadrangle by espe­ the boundaries of the quadrangle. For a distance discussion produced a plain of moderate relief erosion, which removed the igneous material and cially well preserved and definite evidence in the of little more than a mile the river flows outside the across the oblique edges of the entire series of carved deep canyons to the very base of the vast shape of ancient terrace gravels, moraines, and quadrangle, but reenters at the point where it cuts Mesozoic and Paleozoic formations. This plain lava plateau. The volcanic forces were most effect­ landslide debris. through the moraine and thence flows northward, seems to have bordered a higher land mass in ive in the earlier part of the period, when nearly leaving the quadrangle again on the west within a the heart of the San Juan Mountain area, and to the entire thickness of 5000 feet of volcanic rocks TOPOGRAPHY. mile and a half of the northern boundary. have extended a considerable distance how far found in the western San Juan was accumulated, RELIEF. must ever remain a matter of hypothesis to the but the agents of degradation became prominent CLIMATIC CONDITIONS. north, west, and south. Upon this nearly plane in the later part of the Tertiary and are still actively The area included within the Ouray quadrangle The variation in climatic conditions in different surface, in the region where it is now exposed, the at work on the higher mountain masses. has a considerable variety in its relief. The north­ parts of the quadrangle is in direct relation to the sandstones and conglomerates of the Telluride for­ Quantitatively, the work performed by the geo­ western quarter consists of open valleys between topography and relief. The higher mountain region- mation were deposited and had already attained a logic agencies acting in this region in Cenozoic which are rounded hills or low, flat-topped mesas, has an abundant rainfall and in consequence sup­ thickness of several hundred feet when the great time has been very great, but the estimation of the few7 of them more than 500 feet above the level of ports a vigorous forest growth of spruce and aspen. epoch of volcanic activity began, producing the post-Cretaceous disturbance, as well as the general the valleys. On the east the land rises from Cow The lowland of the northwest corner of the quad­ complex of rocks out of which the present San deciphering of all earlier geological history, has Creek for nearly 3000 feet to the foot of the high rangle gets little rain in comparison with the Juan Mountains have been sculptured. From been rendered very difficult by the mantle of vol­ cliffs that form the face of Cimarron Ridge. This mountain areas and really belongs to the region exposures in the Telluride quadrangle it was canic rocks; and the original extent of this covering ridge is one of several that extend northward from of semiarid plateaus stretching off to the west and inferred that the earliest volcanic tuffs were water- is left to speculation on account of the more recent the high mountains of this part of the San Juan northwest. Although the land is extremely fertile laid deposits in the Telluride lake, but from the erosion. The detailed examinations now in progress group. Two such ridges, separating forks of when irrigated, sagebrush, pinon, and cedar rep­ relations seen in the Silverton quadrangle it appears have thrown much Tight on these great problems. Cimarron Creek and lying wholly or partly in the resent the normal vegetation. Between these probable that the Telluride conglomerate is, in part, Thus, the Telluride conglomerate becomes of first Ouray quadrangle, have been named Turret and extremes of wet .and dry is a zone of moderate at least, a flu via tile formation and that there was, importance in their solution, since its base presents Pinnacle ridges, from their characteristic topo­ rainfall characterized by scrub oak, which clothes locally, valley erosion below the plane of that con­ the best evidence as to the post-Cretaceous erosion graphic detail. These narrow and rugged divides the middle slopes in an almost impenetrable tangle. glomerate before the volcanic cycle opened. and its top forms the surface upon which the vol- are due to the deep dissection of a lava-capped pla­ . The volcanoes and fissure conduits of the San canics rested in the western part of the district. teau. A remnant of the plateau form is seen in DESCRIPTIVE GEOLOGY. Juan, assisted, perhaps, by vents in adjacent The present elevation of this entire region above Park Mesa, in the extreme northeast corner of the SEDIMENTARY FORMATIONS. regions, emitted an enormous amount of volcanic sea level is to be regarded as the result of numerous quadrangle. material, partly in fragmental form and partly in oscillatory movements of uplift or subsidence which In the western part of the area there is an abrupt ALGONKIAN SYSTEM. lava flows, and this material covers an area of cer­ have taken place since the close of the Cretaceous, rise from the lowlands to the summit called Baldy XEEDLE MOUXTAIXS GROUP. tainly not less than 15,000 square miles to a depth affecting greater or smaller areas. A slight tilting Peak, which forms the northwest extremity of a UNCOMPAHGRE FORMATION. of many thousand feet in the central portion. of the Telluride formation in an easterly direction ridge separating the waters of Uncompahgre River The lowest member of the volcanic complex thus may be connected with the uplift of the extreme and Cow Creek. A corresponding upland with a Name. The name Uncompahgre formation was far discovered is a bedded deposit of andesitic rocks western San Juan region, leading to the great ero­ surface of moderate relief lies between the Uncom­ proposed in the Silverton folio for a great succes­ that reaches 2500 feet in observed thickness, called sion which has caused such an abrupt face to the pahgre and Coal Creek. sion of supposed Algonkian quartzites, slates, and the San Juan tuff breccia or conglomerate. Its mountains in and about the Telluride quadrangle. The highest mountains of the quadrangle are in subordinate conglomerates represented in the can­ source is unknown. From the relations exposed The complex fault system existing in the Silverton its southern and southeastern parts, in an elevated yon of the Uncompahgre by several thousand feet in the Silverton quadrangle it is certain that the quadrangle, which traverses all volcanic rocks in its region the greater part of which is above timber of upturned strata. The exposures of these beds San Juan beds were in some places entirely removed course, is further evidence of Tertiary movements. line and which is scored by deep canyons. In the begin near Ouray and extend for a few miles by erosion before the eruptions of the next great heart of this mountainous area the town of Ouray southward in the Silverton quadrangle, but neither volcanic epoch. RELATIONS OF THE OURAY QFAl>RANGLE. is situated, in a park formed at the junction of sev­ top nor bottom of the larger conformable series to Succeeding the San Juan epoch came a time in The situation of the Ouray quadrangle is-stich eral tributaries of Uncompahgre River. Many of which the visible section belongs is there shown. which rhyolitic and audesitic magmas, together that it presents in epitome many of the principal the higher summits are over 13,000 feet in eleva­ The name was proposed for the entire assemblage with others of intermediate composition, alternated, features of topography and geology characterizing tion. Wetterhorn Peak, near the eastern boundary of pre-Cambrian conformable quartzites and slates and built up the Silverton volcanic series of flows the broad area of the San Juan. It exhibits a of the quadrangle, is 14,000 feet high, and east, of which extensive exposures exist also in the and tuffs to an aggregate thickness of 4000 feet or rather typical section from the high and rugged south, and west of the quadrangle many of the Needle Mountains. During the survey of that more. Above them lies the Potosi volcanic series, mountains in its southern zone, a number of which highest peaks of the San Juan Mountains are area it was found that the main quartzites and consisting predominantly of rhyolitic material, and rise above 13,000 feet, to the lowlands of the clustered. slates of the Needle Mountains were like those of observed to have a thickness of more than 1000 northwestern portion, about 6000 feet below. The the Uncompahgre section, but from structural com­ DRAINAGE. feet. During the Silverton epoch, and after it, common character of the intermediate slopes is plexities the thickness represented was less, and, as there were intervals of great erosion. illustrated, and the ridges bordering Cimarron Creek All the streams which drain the Ouray quadran­ in the Uncompahgre Canyon, the upper and lower The three series of lavas mentioned make up the are remnants of a high plateau which, for many gle belong to the Gunnison River system and their parts were not seen. At the southeastern base of greater part of the western San Juan Mountains. miles eastward, is more or less prominent between waters eventually find their way through Colorado the Needle Mountains, in Vallecito Canyon, how­ In the central San Juan the two earlier series the mountain front and-the Gunnison Canyon. River to the Gulf of California. In the quadran­ ever, a heavy conglomerate was discovered and become greatly diminished in thickness, and disap­ The canyon and lower valley of the Uncompahgre gle itself there are four drainage systems. North traced eastward, to a point where it was found in pear under the Potosi and other lavas of various are typical of the larger streams rising in the heart Fork of Henson Creek, in the extreme southeast such relations with the Uncompahgre quartzites kinds not yet fully investigated, but known to of the San Juan. corner, joins Henson Creek a few miles to the east, and slates that it appeared to be unquestionably include rhyolite, andesite, and basalt. The deep dissection by Uncompahgre River and and the main stream enters Lake Fork of Gunnison the lower part of the group. To this conglomerate Most of the San Juan lavas came from sources its tributaries has disclosed the character of the River at Lake City. The three other systems, the formation name Vallecito was given and to the that are unknown, but those of the Silverton series prevolcanic formations to a degree exceeded only which drain more than nine-tenths of the whole group which includes it and the Uncompahgre, were in part poured out through fissure channels in the Animas and other valleys traversing the area of the quadrangle, are those of Uncompahgre together with higher members that may hereafter now identifiable on account of the dike masses Needle Mountains on the south side of the San River, Cow Creek, and Cimarron Creek. The two be recognized, the name Needle Mountains group which fill them. The bedded series are penetrated Juan. Archean gneiss and schists are not revealed forks of the Cimarron flow almost due north along was applied. by several massive bodies of rocks that are in many by the Uncompahgre, but in its canyon is exposed the eastern border of the quadrangle from the two The total thickness of the Needle Mountains places coarsely granular, such as gabbro, diorite, an excellent section of the pre-Cambrian quartzites sides of Coxcomb Peak and join the main stream group is unknown. Two thousand feet or more and monzonite, and it now seems probable that the and slates named after this river. in a beautiful open valley in the extreme northeast. of the basal conglomerate member occur a few intrusive bodies of diorite porphyry and the allied From the Uncompahgre (Algonkian) beds north­ Cow Creek, the longest individual stream, rises miles east of Vallecito Creek, in the vicinity of varieties found in the sedimentary beds adjacent to ward almost the entire Paleozoic and Mesozoic in the high open country known as the American Emerald Lake, on Lake Fork of Pine River, and the San Juan Mountains on the west are also of section is well exhibited within the Ouray quad­ Flat and flows diagonally across the entire quad­ 8500 feet must be allowed for the section of quartz­ later date than many of the surface lavas. rangle. The columnar section sheet gives a sum­ rangle from southeast to northwest. In its upper ites and slates exposed in the Uncompahgre Canyon. The volcanic eruptions in the San Juan area prob­ mary statement of the character and relations of portion the main stream and its chief branches, The complicated folding and shearing within the ably continued at intervals until late in Tertiary the formations. Difficulty and Wildhorse creeks, flow through unu­ formation indicated in the Needle Mountains area time, although only the products of the earlier out­ The broad doinal structure, truncated by the sually wild canyons bordered by bare rock walls and the complete unconformity with overlying bursts are well known. Thus the volcanic period surface upon which the Telluride conglomerate that make the gorges inaccessible except at a very beds, indicating a long period of deformation and of building up was in part synchronous with the and the succeeding San Juan tuff rest, is well few points. Cow Creek canyon widens between erosion preceding the deposition of the earliest fos- third great period already referred to that of sculp­ shown in the valley section. Of special interest is Oban Creek and Ramshorn Ridge, and beyond the siliferous rocks, make it impossible to estimate the turing by erosion by which the mountains now the unconformity below the Dolores (Triassic) for­ ridge the stream flows in a gradually opening original thickness of the Needle Mountains group existing have been produced. Within the volcanic mation, more clearly exhibited in the cliffs over­ valley, being in its lower portion bordered by fer­ or to recognize an upper limit of the section. area little evidence has been discovered by which looking Ouray than at any other known locality. tile ranches. The creek joins Uncompahgre River Description. The rocks of the Uncompahgre the sequence of events can be correlated with the The Tertiary geology of the San Juan, recorded less than half a mile beyond the northwest corner formation exposed in the Ouray and adjoining Sil­ established divisi6ns of Tertiary time. Deposits of in volcanic formations and their relations, is in of the quadrangle. verton quadrangles consist principally of quartzite, Eocene age are known in the zone bordering the large part well illustrated in the Ouray quadrangle. Uncompahgre River rises in the high mountains with bands, in many places hundreds of feet in volcanic area, but they have not been found in The San Juan tuff, the earliest and most extensive of the Silverton quadrangle, and after passing thickness, of argillaceous rocks in most of which a direct contact with the lavas. Certain calcareous of the Tertiary formations, is represented in its through a narrow canyon, the lower portion of secondary slaty cleavage has been developed. Both tuffs of the Silverton and Ouray quadrangles con­ maximum development. Only the upper portions which is in the Ouray quadrangle, is joined by kinds of rock are well bedded, the quartzites, from tain scanty plant and invertebrate remains, indicat­ of the Silverton volcanic series are to be found, but Canyon Creek. Below the town of Ouray the their greater hardness, preserving original struc­ ing that they were formed in Oligocene or early their relations to the San Juan, as seen in this area, valley is narrow and hemmed in by high and steep tures better than the slates. Ripple-marked sur­ Miocene time. Although it may safely be assumed furnish particularly clear evidence of the great ero­ cliffs almost to the mouth of Dexter Creek. From faces are common and have been the means of that, the closer study of the San Juan will result in sion interval which followed the deposition of the that point downstream the sides of the valley recede, determining the relative position of the upper and the recognition of different epochs of eruptive San Juan tuff. The uppermost lavas, those of the leaving, north of Cutler Creek, a broad stretch of lower parts of the formation, which is not at first evident on account of the nearly vertical attitude DEVONIAN SYSTEM. found in Colorado. For a more complete discus­ obtained from the Ouray limestone are the of the beds. sion the reader is referred to the Needle Mountains following: INTRODUCTION. The quartzites are very thoroughly indurated folio and to special papers. Schizophoria striatula. Camarotcechia endlichi. and consist of nearly pure silica in the lighter The presence of Devonian sediments in the San Distribution. The Elbert occurs at only one Schuchertella chemung- Camarotcechia contracta. ensis. Paracyclas sp. colored varieties, with very small amounts of seri- Juan region was established in 1874 through a locality in the Ouray quadrangle, being well Productella semiglobosa. Naticopsis gigantea. cite or chlorite. Darker varieties show a little collection of fossils made by F. M. Endlich, of the exposed at a number of points below the cliff of Productella subalata ? Naticopsis 1 (Isonema) Ouray limestone that overlooks the Uncompahgre Athyris coloradoensis. humilis. finely divided magnetite or hematite and rarely Hayden Survey. The formation containing the Spirifer coniculus. Straparollus clymeni- minute flakes of biotite. The colors of the quartz- invertebrate fauna discovered by Endlich has been Canyon south of Ouray. To the north it is lim­ Spirifer disjunctus var. oides ? ite vary from pure milk white through blues, named the Ouray limestone, and the beds imme­ ited at the box canyon of Canyon Creek by an animasensis. Orthoceras sp. grays, and pinks of delicate shades to dull browns diately below it, containing an upper Devonian east-west fault, and to the south it thins out and Concerning the general relations of this fauna or blacks of the less pure kinds. Microscopical fish fauna, have been distinguished as the Elbert within a short distance disappears, leaving the Girty states that "In general the Devonian fauna examination of thin sections has shown well-defined formation. Ouray formation resting upon the Algonkian sedi­ of the Ouray belongs to Upper Devonian time. It S}'stems of parallel cleavages in the grains of quartz ments. It is probably present between the Ouray is but distantly related to the Devonian faunas of ELBKRT FORMATION. constituting the rock, the cleavage cracks being and Ignacio in the Cow Creek locality already upper New York, and its relation with those of filled with sericite. Name and definition. The name Elbert forma­ mentioned, but the rocks were not recognized the Mississippi Valley, or even with other known The argillaceous members have been much more tion has been applied to a succession of calcareous among the materials removed from the shaft. western Devonian faunas, is not close." affected by the dynamic metamorphism of the shales, thin limestones, and local quartzite beds, The Carboniferous portion of the Ouray has OURAY LIMESTONE. region, and most of them are now slates or aggregating less than 100 feet in thickness, as a nowhere been found to be distinguishable lith­ schists. Some are extremely dense, fine-grained rule, and characterized by scanty remains of ganoid Name and age. Since the Ouray limestone has ologically from the Devonian. Its existence on the argillites which under the microscope appear to be fishes. On the southern side of the San Juan been described in the Silverton and Needle Moun­ southern slope of the San Juan was detected through made up largely of quartz, with sericite in minute Mountains these strata generally rest upon the tains folios and in special papers it is unnecessary the presence of lower Carboniferous invertebrate needles all having practically the same orientation. Ignacio quartzite, of middle or upper Cambrian to enter here into many details of age and correla­ fossils in the chert pebbles of the succeeding for­ Others are coarser and less completely metamor­ age, although a great stratigraphic break comes tion that are discussed at length elsewhere. The mation. During the investigations as to the origin phosed and contain much "earthy" impurity that between them; above they are covered conformably name was first given to the formation from its typ­ of these pebbles G. H. Girty found that some of remains dark between crossed nicols. A few con­ by the Ouray limestone. The name is derived from ical occurrence south of the town of Ouray, and the fossils of the chert pebbles occurred also.in the tain andalusite or staurolite in impure crystals. Elbert Creek, a western tributary of Animas River was applied to the Devonian limestone member of uppermost strata of the massive limestone hitherto The prevailing colors are dark greens, browns, that drains the Ignacio Lakes and flows for some the pre-Carboniferous Paleozoic. The subsequent supposed to belong wholly to the Devonian. and purples, although some soft, shaly bands miles upon a broad bench where all the lower Pale­ discovery of Mississippian fossils in the upper por­ Among the fossils from the upper part of the may be almost jet black. Local crumpling is ozoic formations are well exposed. tion of this limestone necessitated a change in Ouray are the following: not uncommon in portions of the thin-bedded Lithologic character. The formation is composed definition. Since it is impossible to draw a line Rhodocrinus sp. Spirifer centronatus. argillites, but in general the beds are parallel mainly of calcareous shales and thin, sandy lime­ between the two portions, the Ouray becomes pre­ Platycrinus sp. Spirifer peculiaris ? Rhipidomella pulchra. Spiriferina solidirostris ? with the quartzites and dip at a high angle to the stones varying in details of development from place eminently a lithologic unit transgressing the faunal Schuchertella insequalis. Eumetria marcyi ? north. to place. The beds are drab, buff, or yellowish in boundary between the Devonian and Carboniferous Productus semireticu- Camarotoechia metallica. Distribution. The quartzites and slates of the color. The shaly layers break up readily on expo­ systems. latus var. Myalina keokuk. Productus Igevicosta. Phillipsia peroccidens. Uncompahgre formation are exposed on both sides sure, so that outcrops are obscured by the resulting Lithologic character. The Ouray limestone .as of Uncompahgre River for about a mile and a half scales and thin plates. Quartzites are present but are known at present has a thickness varying from 100 The localities at which the Carboniferous forms north of the southern boundary of the quadrangle. everywhere very thin and subordinate to the cal­ to 250 feet. The upper and major part of the for­ have been found in the limestone are few, mainly They are limited on the north by a fault which careous or shaly portion. The shaly beds are in mation is massive limestone, either in one bed or near Cascade Creek, in the Engineer Mountain brings them into contact with Paleozoic sediments; many places characterized by pseudomorphous casts with such thin intercalated shales that the tendency quadrangle. Chert pebbles have been found in on the east they are covered by the San Juan after salt crystals. The crystals were in general of the limestone to cause mesas, benches, and place in these upper strata but rarely, and this tuff and on the west by the oldest Paleozoic sedi­ more or less clearly skeleton cubes with sunken prominent cliffs as characteristic topographic forms fact, together with their abundance in the suc­ ments. The formation is exposed farther south, faces, ranging in size up to an inch or more in is everywhere notable. Below the more massive ceeding Molas conglomerate, shows that an inter­ in the Silverton quadrangle, for nearly 2 miles. diameter. So abundant are these casts that they portion a third or less of the section is made up of val of erosion followed the formation of the The hard quartzites and the slates protected by may be found at nearly every outcrop, but they are well-bedded limestone with distinct shaly layers limestone. In the area thus far studied this them form the precipitous walls of a canyon more perfect and of larger size in some localities than and, rarely, thin quartzites between them. Some erosion effected the removal of nearly the whole of unusual grandeur, through which Uncom­ in others. of the lower layers have a wavy bedding; some of the chert-bearing, Carboniferous portion and an pahgre River flows to the outskirts of the town As may be seen from the accompanying section, are arenaceous or earthy; and large chert concre­ unknown amount of the subjacent massive rock, of Ouray. the occurrence at Ouray is not characteristic. The tions, free from fossils, are common at a horizon but nowhere cut away enough of the Ouray to total thickness does not exceed 40 feet; the beds near the base. The lowest stratum is characterized make angular unconformity noticeable except in CAMBRIAN SYSTEM. are more sandy and contain a greater number of in most places by crinoid stems and rarely a cup small detail. At no known point has the erosion IGNACIO QUARTZITE. thin quartzite layers than usual; moreover, the coral. penetrated to the horizon of Devonian fossils in the The lowest lithologic division of the Paleozoic casts of salt crystals, so typical of the shaly mem­ The greater part of the formation is dense, com­ massive limestone. Inasmuch as neither this horizon section known in the San Juan Mountains is made bers in the Animas Valley, are rarely seen in them. pact limestone, but portions of the upper ledge are nor that of the Mississippian forms is recognizable in up of quartzites, and varies in thickness, in the coarsely crystalline. In general, the rock is nearly many sections the question as to how much, if any, Section of Elbert formation on west side of Uncompahgre Animas Valley, where its best development has Canyon, 1 mile southeast of Canyon Creek. white, straw yellow, or buff, with local pinkish of the Ouray should, in such cases, be referred to thus far been found, from a few feet up to 200 Ft. tones. Some of the lower beds are strongly yellow the Carboniferous is a matter of uncertainty. feet. This formation is fully described in the Sil­ Top. and these are commonly more or less sandy. The Distribution. The Ouray limestone occurs at 11. Sandstone, buff, in general fine grained, verton and Needle Mountains folios and was named but grading within a few feet into a gritty contrast with the dark-gray, dense limestones of the two localities in the quadrangle, directly south of the Ignacio quartzite from its typical occurrence quartzite; bands' are not uniform in thick­ Hermosa is marked, layers of such character occur­ Ouray and in Cow Creek north of Oban Creek. ness nor continuous for considerable dis­ near Ignacio Lakes, in the Animas Valley. From tances ; lenses of calcareous pink shale or ring only near the base of the Ouray. No change The exposures near the town are of the upper, layers near the middle of these quartzites a single gnarly limestone (6 to 18 inches thick) in character of the limestone occurs toward the top massive limestone and have a steep dip to the generically determinable shell has been found, iden­ occur near the top under a fine-grained, unless a prevalence of coarse-grained, rotten beds, north, which carries them beneath the alluvial fan massive, buff sandstone______.__ 15 tified by Charles D. Walcottas Obolus sp.?, related 10. Shale, calcareous, and thin-bedded lime­ observed locally and belonging to the Carboniferous of Portland Creek and the landslide debris of The to forms found in the middle or upper Cambrian stone, buff and pink; much of the lime­ portion, in some places at least, be regarded as Amphitheater. On the south they are limited by stone in lenses and as a rule not more than at various localities in the western United States. 3 or 4 inches thick except near the top, characteristic. The conditions of sedimentation a fault contact with the Algonkian quartzites. Fragments of shells, belonging, it is believed by where it becomes nodular and gnarly seem to have remained the same and deposition The best exposures of the whole formation are Walcott, to the same genus, were found in the and more lenticular ______12 0 seems to have been continuous from Devonian into found west of the Uncompahgre, extending south­ Quartzite, bluish gray, gritty ______8 only known occurrence of the formation in the Shale and sandstone, thin bedded, buff ____ 0 Carboniferous time. eastward from the box canyon of Canyon Creek. Ouray quadrangle. Quartzite, gray, conglomeratic and gritty. 0 Fauna. The Devonian invertebrate fauna of The beds rise from the east-west fault at the box The Ignacio quartzite is not present in the Pale­ Shale and very thin bedded sandstone, buff 6 the Ouray limestone occurs at horizons ranging canyon for nearly a thousand feet to the broad, Sandstone, pink to green______10 ozoic section south of Ouray, where the lowest Sandstone, buff to greenish, fine grained, from one near the base of the formation to one flat bench on the west side of the Uncompahgre known beds are the Elbert shales, resting directly shaly near top.______which in many places is not far below the top of Canyon, at the edge of which they form, with the Shale, red, sandy______-___--.____- upon the Algonkian quartzites. Its absence in Limestone, siliceous and not sharply sepa­ the upper massive ledge. The greater number of Elbert beneath them, an abrupt cliff 200 feet or this region may be due to nondeposition or to rated from quartzite below; shaly partings species occur in this upper portion, but many more in height. Half a mile south of the bound­ (red) above; general color greenish; red­ erosion preceding the Elbert. The only locality der and sandy near top______of them range to a horizon within a few feet of ary of the quadrangle the Elbert shales disappear at which the Ignacio is known to occur in the Quartzite, gritty and conglomeratic, green­ the base. Fossils have been obtained at Ouray and the Ouray rests directly upon the upturned Ouray quadrangle is on Cow Creek, a little north ish.______and at several localities in the Needle Mountains, edges of the Algonkian. Upturned edges of Uncompahgre slates. of Oban Creek, where there is a small area occu­ Engineer Mountain, and Durango quadrangles, as The occurrence in Cow Creek is peculiar and pied by Ouray limestone which is bounded by 38 0 well as at the point where Endlich first found a of greater structural than stratigraphic importance. faults on all sides. Through this limestone a num­ Age. Meager remains of ganoid fishes found in few characteristic species a short distance south of The exposed part lies almost wholly on the east ber of prospecting shafts have been sunk, and in association with the salt-cast shales in the Animas the Needle Mountains quadrangle. side of the stream, and belongs to the upper, mass­ one at least Ignacio quartzite was encountered. Valley region have been characterized by C. R. The invertebrate fauna of the Devonian portion ive member of the formation. It is entirely sur­ The shaft is now closed by rubbish, but material Eastman as belonging to a distinct upper Devonian of the Ouray limestone has been fully described rounded by landslide debris and its relation to the lithologically like the Ignacio quartzite was found fauna, allied to that of the Catskill and Chemung by G. H. Girty and compared with similar faunas other rocks is obscure. This locality is discussed on the dump, apparently obtained from the bot­ groups of New York, and a correlation, on fossil hitherto collected in Colorado, notably in the Elk in detail under the heading "Structural geology." tom of the shaft. That this material indicates evidence, has been suggested between the Elbert Mountains, at Glenwood Springs on Grand River, the presence of the Ignacio is considered certain formation and the "Parting quartzite" of the Aspen near the head of White River, and on East Mon­ CARBONIFEROUS SYSTEM. from the characteristic trail markings and frag­ district. Bothriolepis and Holoptychius are the arch Mountain, Chaffee County. Full correlations MISSISSIPPIAN SERIES. ments of the distinctive shell Obolus. The sig­ generic forms most abundant. 1 No other beds of the sections in these localities with that of the UPPER PORTION OF OURAY LIMESTONE. nificance of this occurrence, both structural and that are lithologically like the Elbert have been San Juan region can not be made, however, until As has been shown in describing the Ouray further examinations have been carried out. limestone, it is known that a Mississippian (Lower stratigraphic, is discussed under the the heading 1 Cross, W., and Eastman, C. R., Am. Jour. Sci., 4th ser., "Structure." vol. 18, October, 1904. Among the most typical Devonian species Carboniferous) fauna occurs in the upper part of Ouray. that formation in certain localities, separated from Section of Molas formation on north side of Canyon Creelt, ern flanks of the San Juan Mountains. As orig­ Marginifera wabashensis Aviculipinna ? peracuta. the uppermost Devonian fossiliferous stratum by 100 feet above junction with Uncompalrgre River. inally defined, its lower and upper limits were var. Allerisma terminate. Bpii'ifer roekymontanus. Eclmondia subtruncata,, about f50 to 75 feet of massive limestone in which Top. ' < Feet. marked by the Molas and Rico formations, respec­ Spirifer cameratus. Euomphalns catilloides. no fossils have thus far been discovered. The 14. Conglomerate, red and filled with chert frag­ tively. In the Ouray district no beds carrying a Squamularia perplexa. Belleroplion crassus. ments, becoming finer grained above until it Spiriferina campestris. Patellostium montfort- erosion in the interval following; Mississippian sed­ resembles a gritty red quartzite.----- ____--__ 4 typical Rico (Permo-Pennsylvanian) fauna have Coinposita subtilita. ianum. imentation removed those strata completely over 13. Sandstone and conglomerate with shaly part­ been found, and the Hermosa is here apparently Acanthopecten carbon- Euphemus nodocarinatus. large areas. As neither the upper Devonian nor ings, red.______.______3 followed by the Cutler red beds, of supposed Per­ iferus. Phillipsia major. 12. Shale, red ______10 Myalina subquadrata. the basal Carboniferous is everywhere fossiliferous, 11. Conglomerate, chert fragments in red cement _ 3 mian age. The name was derived from Hermosa it is impossible in many places to decide whether 10. Shale, red______2 Creek, a tributary of Animas River, which trav­ The fauna of the Hermosa formation occurs alao in Conglomerate, red, pebbles smaller than at the Weber limestone and lower Maroon formation of the the upper portion of the Ouray limestone is Car­ lower horizons and cement more abundant.. 1 erses a large area of these rocks in the Engineer Crested Butte district, and in the Weber formation of boniferous or not. Shale, red, with a few limestone and chert peb­ Mountain quadrangle. the Tenmile and Leadville districts. From this fact From the great quantity of Carboniferous chert bles; near the middle, mottled light yellow__ 5 Lithologic character. In the Animas Valley, Conglomerate, dark red, containing chert and and the similarity in stratigraphic occurrence a correla­ nodules in the succeeding formation it must be quartzite pebbles in a reddish calcareous near the mouth of Hermosa Creek, the lowest third tion of these formations appears to be justified. The assumed that above the known horizons of the cement.______1 to 1| of the complex is made up of green sandstones and Hermosa fauna represents early Pennsylvanian sedimen­ Shale, red, sandy, and with no pebbles except Ouray there once existed in this region a consid­ shales with some gypsiferous shales, and the rest of tation, and it is probably older than the '' Upper Coal for a thin layer near the base______---_-__ 3 Measures " faunas of the Kansas and Nebraska sections. erable thickness of chert-bearing limestone of Mis- Conglomerate, light yellow or straw color, with the formation shows limestone layers distributed sissipian age. It seems not unlikely that somewhere black cherts embedded in a calcareous cement, throughout. Along the great scarp facing the Distribution. The immediate vicinity of Ouray less conglomeratic near top ____.____-._-__- 2 on the slopes of the San Juan Mountains notable Shale, red and calcareous, containing many Animas, which extends for 10 miles in the is the only place in the quadrangle where the Her­ remnants of these beds may be found. chert and some limestone pebbles------5 Engineer Mountain quadrangle, the limestones mosa occurs. In a distance of less than a mile the Conglomerate, largely made up of chert pebbles In the Ouray quadrangle a short distance north­ with a varying amount of reddish cement­ become more and more prominent and certain formation rises from below the level of the Uncom­ west of the Mineral Farm mine, near the upper ing material somewhat resembling the shale beds are very thick. pahgre Valley to a height of more than a thousand road which leads up Canyon Creek, there is a below; in part more like a fine red grit______4 In the Rico district the massive limestones are feet and forms the greater part of the precipitous Shale, gnarly, reddish._____--______--_--- 3 marked local unconformity between the Ouray Conglomerate, siliceous, dark red or purple, prominent only in the middle portion of the for­ cliffs west and northeast of the town of Ouray. limestone and the Molas formation. The locality containing numerous chert fragments ______2 mation, the upper part consisting mainly of black East of the river the Hermosa is not exposed south is very near the base of the Hermosa formation and gray shales alternating with green grits and of the north wall of The Amphitheater except for and the Ouray appears to rise in a sharp ridge sandstones and with a few limestone layers. one small patch between the Molas and the San through the Molas, which rests with angular The base rests upon Ouray limestone, massive At Ouray the character of the formation is again Juan tuff south of Portland Creek. West of the unconformity against the flanks of the ridge. and somewhat sandy, seeming to belong to a higher different; the lower 300 feet consist of relatively Uncompahgre the Hermosa beds, where not obscured The limestone contains many concretionary chert horizon than that commonly seen. It is in places thin, alternating beds of sandstone, shale, and a by landslide debris, may be traced southward along nodules similar to those that occur as bowlders or traversed by many crevices in which reddish shaly few gnarly fossiliferous limestones. The prevail­ Canyon Creek, and above the Ouray limestone pebbles in the Molas, and a poorly preserved Spir- material has been deposited. ing colors are dark greens, grays, and buffs, with bench west of the Uncompahgre Canyon to the ifer (believed to be a Mississipian form) was found. Age and stratigraphic relations. Many of the here and there reddish shales and pinkish grits. southern boundary of the quadrangle. chert fragments in the lower part of the formation The limestone appears more sandy or siliceous than In the Animas Valley section the actual base of PERMIAN (?) SERIES. the familiar Ouray, near the top, generally recog­ carry a Mississippian invertebrate fauna, the deri­ the formation is marked by a massive and per­ CUTLER FORMATION. nized in this region. It is believed that this occur­ vation of which from the Ouray limestone has been sistent limestone, but at Ouray this is not present rence represents a remnant of the Mississippian discussed. In the uppermost part of the formation, and the section begins with a rather coarse quartz­ -Name and definition. The name Cutler forma­ portion of the Ouray, although sufficient fossil at a single locality in the Needle Mountains quad­ ite and chert conglomerate, the pebbles of which tion was proposed in the Silverton folio for the evidence to prove it is lacking. rangle, some thin limestone beds are intercalated lie in a gray gritty cement. The line between the 2000 feet or more of red shales, sandstones, grits, between sandstones, and these contain a Pennsylva­ PE'N'NSYL.VANIAN' SERIES. Molas and Hermosa is not sharp and has been and coarse conglomerates forming a part of the nian fauna which, according to Girty, is "related drawn on purely lithologic grounds, the Hermosa assemblage familiarly known in many parts of MOLAS FORMATION. to that of the Hermosa formation but contains no beginning where the red marls and shales of the the West as the "Red Beds." At Ouray, where Name and definition. The name Molas forma­ species in common with the Ouray limestone. Some Molas end. The upper and greater part of the the necessity of distinguishing the formation was tion was proposed in the Silverton folio for the points of individuality distinguish the Molas fauna Hermosa consists of pink massive grits and sand­ first apparent, it follows the Hermosa with apparent lowest Pennsylvania!! formation distinguished in from that of the Hermosa, but it can not be con­ stones, red sandy shales, and thin, gnarly fossilif­ conformity, but on the southern and western slopes the Animas Valley and adjacent regions. The jectured how far this would be borne out by full erous limestones. The massive sandstones, which of the San Juan the Rico formation, of Permo- name is derived from Molas Lake, which lies on collections." The species obtained from the Molas, are coarse and gritty, vary from 50 to 75 feet in Pennsylvanian age, intervenes between the Cutler the bench west of the Animas Canyon and just as determined by Girty, are the following: thickness and are separated from one another by the and the Hermosa. The full original thickness of south of the Silverton quadrangle, at an altitude Archseocidaris triplex. Spirifer boonensis ? red shales and thin-bedded sandstones or calcareous the formation is perhaps unknown as yet, since it of 10,500 feet. The lake basin is partly excavated Rhombopora lepidoden- Coinposita subtilita. layers. The heavy limestone members so charac­ is evident that the stratigraphic break between the in the Molas beds and they are well exposed for droides. Myalina perniformis. teristic of the formation in the southwestern San Cutler and Dolores formations, marked at Ouray by Rhipidomella pecosi. some distance south of the lake. Juan are here altogether lacking. Individual sharp angular unconformity, may be present else­ The Molas formation is distinguished as a carto­ Wherever the Molas formation has been observed strata change laterally both in character and in where, and that an indefinite upper portion of the graphic unit on lithologic grounds, being a well- it rests upon a surface due to the erosion by which thickness and under these conditions it has been Cutler may have been removed over a large area. characterized element in the Carboniferous section, the Mississippian portion of the Ouray limestone found impracticable to subdivide the series into In the earlier reports on the San Juan region the and because it records in certain peculiarities of its was almost completely removed. Although it has smaller lithologic units for purposes of mapping. present Cutler formation was provisionally con­ sediments important events of the preceding inter­ not been observed to lie upon any formation other The Rico formation, which succeeds the Hermosa sidered as a lower unfossiliferous part of the Dolores val of erosion, including the almost total destruc­ than the Ouray limestone, the nature of the case on the southwestern side of the San Juan Moun­ and was supposed to be of Triassic age. The tion of a Mississippian formation. There may permits the supposition that somewhere in the tains, has not been identified in the Ouray section. discovery at Ouray, however, of the unconform­ also be a further reason for separating the Molas region adjacent to the San Juan Mountains it Lithologically the uppermost limestone of the Her­ ity at the base of the lowest known fossiliferous and Hermosa formations in certain observed faunal must transgress the lower Paleozoic formations mosa resembles a bed in the Rico formation at the Triassic showed that this grouping was incorrect, differences, which are, however, not yet suffi­ and rest upon Algonkian quartzites, granite, or type locality in the Rico Mountains, but the scanty and the Cutler, in the entire absence of fossil evi­ ciently established to warrant laying much stress schist. fauna which this limestone contains has been pro­ dence, is now regarded as of probable Permian age, on them. The Molas and Hermosa seem to be thoroughly nounced by Girty to be of the Hermosa type succeeding, as it does without stratigraphic break, The Molas formation was defined as a thin series conformable and probably the former is the product and the highest fossiliferous strata are immediately the Permo-Pennsylvanian Rico formation in the of reddish calcareous shales and sandstones, with of the first epoch within the long period of Penn­ followed by characteristic red beds of the Cutler, Animas-Valley and Rico Mountains sections. For many pebbles of chert and some of limestorie, quartz- sylvanian sedimentation. The red color of its which contain no fossils. a fuller discussion the reader is referred to a recently ite, and other rocks, and with thin foBsiliferous sediments and the other peculiarities mentioned Fauna and correlation. Girty has summarized published paper. 1 The name of the formation is limestone lenses, the fossils showing intimate rela­ characterize it. his views as to the fauna and correlation of the derived from Cutler Creek, a tributary of Uncom­ tions to those of the lower Hermosa limestone. It Occurrence. The Molas formation is present formation as follows: pahgre River about 4 miles north of the town of occurs immediately below the Hermosa formation south of the town of Ouray at a number of locali­ Ouray. In this region the Cutler formation is well and rests upon a surface of erosion. It represents ties, although from its soft, friable nature little of it The fauna of the Hermosa formation is distinctly exposed and very characteristic in development. Pennsylvanian (Upper Carboniferous) in age. the earliest sediments of Pennsylvanian Carbonif­ is well preserved. Remnants may be seen clinging Description. The Cutler formation, so far as it If the formation be divided into three portions, espe­ erous in this region. to the steeply dipping surface of the Ouray at the cially as the division was carried out in the Rico region, has been observed in the San Juan Mountains, is Lithologic character. The Molas formation is edge of the town by the wagon road leading to the fauna of each is to a certain extent characteristic. composed of essentially shallow-water or fluviatile especially characterized by its deep-red, friable, Uncompahgre Canyon. The best exposures are That of the lower division consists almost altogether of deposits whose materials were not transported far sandy strata, which are variably calcareous and in just below the box canyon of Canyon Creek, and brachiopods. In the middle division a number of gas- from their sources. A very great majority of the many places shaly. As a rule they are not very portions of the lower chert conglomerates may be teropods are introduced, and in the .upper, in addition beds are arenaceous; a few are coarsely conglom­ distinctly bedded, and they disintegrate so rapidly seen in the vicinity of the Mineral Farm mine. to brachiopods and gasteropods surviving from the mid­ eratic, and a fair proportion of the whole consists of dle division, a considerable force of pelecypods appears. on weathering that good exposures are rare. In The broad bench formed by the Ouray limestone The brachiopods remain nearly constant in number, but sandy and in places calcareous shales. At no point the lower part of the formation dark chert nodules between Uncompahgre Canyon and the steeper form a diminishing proportion of the entire fauna. The in the section do beds of one sort greatly predomi­ abound, forming at numerous points a large part of slopes of the Hermosa indicates the former pres­ brachiopodous representation remains fairly uniform, nate, an alternation of sandstones and shales being flat lentils or discontinuous laj^ers. A complete ence of these soft beds. They occur also on the though some changes occur in species and in abun­ the rule. Ripple-marked surfaces and rain-drop section of the formation is exposed on the north south side of The Amphitheater, and the charac­ dance. The lower bed especially is characterized in impressions are often seen; cross-bedding is a fea­ side of Canyon Creek near its junction with teristic red stain of clay filling cracks in the lime­ many places by Productus gallatinensis, Productus inflatus, ture of many of the sandstone members, and abrupt and a large variety of Spirifer of the roclcymontanus type. Uncompahgre River, and, although the formation stone may be seen at nearly all places where the The general characteristic of the Hermosa fauna may changes in the character of the beds are more com­ is not so thick here as it is on the southern slopes old erosion surface of the Ouray is exposed. be illustrated by the following partial list of species: mon than persistency for even short distances. of the San Juan Mountains, its development is The sandstones are in few places more than 20 characteristic. This section is limited above by a HERMOSA FORMATIOST. Triticites secalicus. Meekella striaticostata. feet in thickness and generally less. Some of the Chsetetes milleporaceus. Chonetes mesolobus. coarse grit or conglomerate that is assumed to be Name and definition. The Hermosa formation Rhombopora lepidoden- Productus semireticula- coarser grits are pink and purplish, but most of the basal stratum of the Hermosa, and at the includes the series of alternating limestones, shales, droides. tus var. hermosanus. them are a clear brick-red or indian red. The and sandstones, having a maximum thickness of Prismopora serrata. Productus cora. bottom the Molas is sharply in contact with an Chainodictyon laxum. Productus punctatus. 1 Cross, W., and Howe, E , The Red Beds of southwestern erosion surface of the Ouray limestone. 2000 feet, which occurs on the northern and south­ Orthothetes crassus. Productus nebraskensis. Colorado: Bull. Greol. Soc. America, vol. 16, 1905, pp. 447-498. greatest variety of textures can be found, the rocks Juan Mountains and in near-by regions that ber reaches a thickness of 30 feet in the La Platas, divisible in this region or in other parts of the San ranging from those of extremely fine grain, hardly have been examined the Dolores rests with appar­ but in general is less than 15 feet thick. The base Juan where it has been observed, as the individual to be distinguished from the shales, through coarser ent conformity upon the Cutler. The Dolores is of the formation is the well-known plane of uncon­ beds are too thin to be mapped separately. and coarser sandstones and grits to some of the finer succeeded above by the La Plata sandstone, but is formity by which the lower sandstone overlaps all Description. The McElmo formation consists of conglomerates. Cross-bedding in some of the sand­ separated from it by another stratigraphic break older sedimentary rocks to the Archean, as shown an alternating series of sandstones and shales, which stones is very striking on account of the alternation that is not everywhere evident. north of the San Juan Mountains and elsewhere. varies considerably both in total thickness and in of lighter and darker layers. Description. In the Ouray quadrangle the rem­ This angular unconformity is evident in Cow Creek, lithologic character. The sandstone beds are much In the Ouray region several very coarse, massive nant of the Dolores formation does not exceed 100 where the La Plata rests upon the Cutler red beds, like those of the La Plata and Dakota. They are conglomerates occur in the upper portion of the feet in thickness. Several limestone conglomerates the Dolores and probably not a little of the upper fine grained, quartzose, friable, and gray in color; formation. They are purplish and of a darker color occur near the base, separated by fine-grained sand­ part of the Cutler being absent. The upper limit the lateral variation in thickness is marked, as also than the other beds and are at once remarkable on stones or shales. The rest of the formation consists of the La Plata is placed at the base of a marked the transitions to shaly sandstones by increase of account of their coarse texture. In many localities of marls, sandy shales, and fine-grained sandstones, clay shale of green or reddish color which begins argillaceous matter. A characteristic of many sand­ the relative amounts of bowlders and matrix vary all of a bright vermilion color. The limestone con­ the alternating series of shales and sandstones stones is the abundance in them of green shale greatly in short distances, but ordinarily their pro­ glomerates, of which the lowest one is taken as the grouped in the McElmo formation. No determi­ flakes or scales, a feature which is of value in dis­ portions are about equal, although an abrupt base of the formation, may be from 1 to 10 feet in nable fossils have been "found in the La Plata for­ tinguishing the McElmo from the Dakota or the increase of the coarse materials may be accompanied thickness and are of a somewhat lighter color than mation adjacent to the San Juan Mountains. La Plata where exposures are poor. Few of the by a reduction of the cementing material to a mini­ the other beds. They are composed almost entirely Description. In the Ouray quadrangle the La sandstones exceed 20 feet in thickness. mum. The pebbles and bowlders are well rounded of small limestone pebbles or fragments held in a Plata sandstone does not as a rule exceed 100 feet The shales are of various colors. Greens pre­ and in general not more than six inches in diameter; calcareous earthy cement and are extremely persist­ in thickness and in some localities is even less. dominate near the top, and, in fact, are found they represent most of the older rocks of the San ent over wide areas, forming many notable escarp­ The lower sandstone member comprises fully three- everywhere in the section, but from the middle Juan, the greater number being pre-Cambrian ments. In the Ouray region the conglomerates are fourths of the whole formation, ranging from 60 of the section downward, reds and browns become granites, schists, greenstones, or quartzites. uniformly fine grained, the pebbles or fragments to 80 feet in thickness. The calcareous member very prominent; some bands are nearly pure white The shales, like the sandstones, are bright red. not exceeding 1 or 2 inches in diameter, and as a varies from 8 to 15 feet, and the upper sandstone or cream colored. Many of the shales are extremely They are in places calcareous and almost invariably rule being still smaller. When the pebbles are has an average thickness of about 10 feet. The dense and fine grained in the Ouray district and sandy and, as has been said, it is not everywhere very small and uniform the rock resembles piso­ lower sandstone has a cream or light-buff color, have a distinct porcelain-like texture. As a rule, possible to differentiate them from the fine-grained, lite. In places the limestone is in small, angular becoming grayish near the top. In one fairly however, they are not free from sand, and a grada­ argillaceous sandstones. Some of the shales are fragments and the rock may have somewhat the characteristic section examined in the Uncom­ tion to sandstone is more common than sharp lines fairly compact and well bedded and where protected appearance of a recemented friction breccia or pahgre Valley the rock was soft and friable and of division. by massive sandstones above do not weather so intraformational conglomerate. The limestone is of a fine, even grain. Near the top it was impreg­ The induration of the McElmo beds varies from easily as the shales of the Hermosa; more commonly, dense and hard and of a buff or bluish color. nated with some soluble salt and where this had place to place and also within the section at a given however, they are soft and friable and occur in beds The cementing material, as a rule less in amount been removed the rock crumbled to a fine soft locality as illustrated in the accompanying section. a few feet thick between the sandstones, or as thin than the pebbles, is dull red and calcareous, and sand. In the region between Dexter Creek and In the areas of metalliferous deposits the sandstones partings in the sandstones themselves. For this contains considerable earthy impurity and locally a Canyon Creek, however, both the lower and upper are commonly changed to quartzite. reason, and notwithstanding their abundance in the very little quartz sand. sandstones have been silicified and hardened, in The average thickness of the McElmo in the section, the sandstone escarpments are not obscured From these conglomerates vertebrate and inverte­ many places to quartzites, by some processes prob­ Ouray quadrangle is 700 feet, the maximum being and the formation as a whole is a resistant one in brate fossils have been obtained, by means of which ably related to the intrusions of monzonite por­ somewhat over 800 and the least observed about which cliff outcrops are common. it has been possible to determine the Triassic age of phyry. The occurrences in Cow Creek, where 500 feet. The most striking feature of the Cutler forma­ the formation. No fossils of determinative value they are also quartzitic, are almost the only ones The highest and lowest beds assigned to the tion is the bright vermilion or brick-red color which have been found in the Ouray quadrangle, although in which the cliff-making quality, so typical farther McElmo are shales that are stronglyO J marked, and characterizes all of its beds. poorly preserved fragments of bone and teeth are south, has been developed. The upper sandstone highly colored, contrasting with the Dakota and Distribution. From the canyon of the Uncom- abundant. Elsewhere teeth and bone fragments of is essentially the same as the lower member, but La Plata sandstones. pahgre north of Ouray the red Cutler beds extend a crocodile (Belodon) and of a megalosauroid thinner and apt to be more crumbling. Section. The best outcrops of the McElrno for­ down the valley for 6 or 7 miles to a point near dinosaur (Palceoctonus) have been found and are The local development of the calcareous member mation near Ouray are those on the east side of the mouth of Coal Creek, where the prevailing regarded by F. A. Lucas, who determined them, as is probably the most striking feature of the La the river, above the Pony Express mine, south of northwesterly dip carries them below the valley of distinctly Triassic types. Plata in the Ouray quadrangle. Although vary­ Dexter Creek. At this point the following section floor. Except for a few places on the west side of Besides these vertebrates the Dolores beds have ing somewhat from place to 'place, two portions can extends downward from the base of the Dakota the river, where considerable glacial drift covers yielded a poorly preserved gasteropod, similar to generally be recognized. The lower part consists formation: the hillsides, the formation is well exposed. The Viviparus, and a Unio. One determinable plant, of black laminated shale and limestone, few of the Section of McElmo formation south of Dexter Creek. sharp fold that crosses the Uncompahgre in the Pachyphyllum munsteri, has been found. For fur­ massive layers being more than 4 or 5 inches thick. Feet. vicinity of Bridalveil Creek (fig. 9, illustration ther details concerning the formation reference may Some folding or brecciation is noticeable in these 26. Shales, slaty, black, alternating with shaly bituminous sandstone; individual layers less sheet) lifts the Cutler beds so high on the east side be made to the Telluride folio, to the report on the beds, especially near the top. This part may be than 2 feet thick______.______1___ 16 of the river that they are cut off by the unconform- Rico Mountains, 1 and to the paper already cited, altogether absent and when present does not exceed 25. Sandstones, yellowish or greenish, with shaly able Dolores formation. On the west side of the 5 feet in thickness. Above it are from 8 to 10 feet layers.______14 on the "Red Beds." 24. Quartzite, dense, gray ______6 river south of the fold the beds of the Cutler are Distribution. The Dolores occurs in the Uncom­ of a breccia composed of small fragments of black 23. Shales, sandy, black; and fine-grained sand­ preserved and, gradually assuming apparently con­ pahgre Valley, directly beneath the La Plata sand­ limestone and shale, none exceeding 2 inches in stones, largely quartzitic, thin bedded____ 23 22. Quartzite, hard,-white.______-___ 20 formable relations with the Dolores, may be traced stone, extending from The Amphitheater on the length and most of them much smaller; they are 21. Sandstone, friable, white, containing clay_____ 10 past Oak Creek almost to the southern boundary east and the Canyon Creek laccolith on the west sharply angular and seem identical in character 20. Porcelain shales and thin argillaceous sand­ of the quadrangle. On Cow Creek the Cutler stones ______21 northward to the mouth of Coal Creek. Its uncon- with the limestone beds below. Although strati­ 19. Shales, fine grained near top, dense, porcelain- occurs, more or less obscured by landslide debris, formable relation to the Dolores and Hermosa is fication' is at many places visible throughout the like below, with sandy layers.______42 between Oban Creek and the porphyry of Rams- particularly well shown at the end of the ridge bed the fragments are not everywhere arranged 18. Quartzite, massive, white, more friable below, with thin clay layers near base.______5 horn Ridge. The best exposures of this region are between Cascade Creek and The Amphitheater with their longer diameters parallel. This breccia 17. Sandstone, coarse, white, lower portion indu­ in the canyon of Red Creek, which has been exca­ 1200 feet above the town of Ouray, as illustrated almost invariably forms a ledge or cliff outcrop rated and containing a 2-foot shale layer. __ 13 vated for nearly 2 miles in the "Red Beds." Here in fig. 7 (illustration sheet). North of Dexter and and it is very persistent throughout the Ouray 16. Shale, green, with some purple and gray layers, very fine grained, much of it hard like porce­ the La Plata sandstone rests upon the 'Cutler, some Corbett creeks the unconformity is no longer clearly region. It has been the means of identifying the lain ; some sandy layers, more numerous near of which, together with the Dolores, it has trans­ evident and the formation, high up on the valley presence of the La Plata where the sandstone base; dark red; rests upon 5 feet of very white and massive porcelain shales.._____.. 50 gressed, through the unconformity at its base. sides, can not be distinguished at a distance from members have been obscured. 15. Shale, red, sandy, and shaly sandstone__._ 16 the underlying Cutler beds. Distribution. The La Plata sandstone is exposed 14. Green and red porcelain shales and sandstones TRIASSIC SYSTEM. In Cow Creek, although the Cutler is present, on both sides of the Uncompahgre Valley from the with a 2-foot pink, fine-grained limestone at the top _.__.__.______'.. 78 DOLORES FORMATION. the Dolores does not occur, the La Plata, through vicinity of Ouray to a point near the mouth of 13. Sandstone, massive, green above, white below. 9 Name and definition. The Dolores formation the unconformity at its base, resting upon the Per­ Coal Creek, where the northwesterly dip carries 12. Sandy shales and shaly sandstones, green, white, and red______69 was originally defined in the Telluride folio as mian "Red Beds." the beds below the level of the valley. To the 11. Sandstone, white, saccharoidal______20 embracing the Triassic strata of southwestern Colo­ south, on the east side of the river, the beds are 10. Sandstones and sandy shales, with some porce­ JURASSIC SYSTEM. rado and adjacent territory, the name being derived cut off by the unconformity at the base of the lain layers, red and green. ______26 LA PLATA SANDSTONE. 9. Sandstone, massive, fine grained, gray, white._ 10 from Dolores River, in the valley of which the for­ San Juan tuff, but disappear beneath the land­ 8. Sandy shale and sandstone, alternating green mation occurs. Triassic fossils were found only in Name and definition. The La Plata sandstone slide debris of The Amphitheater; on the west side and red.______... 20 7. Sandstone, fine, greenish white, becoming red the upper part of the section, but a great thickness was first described in the Telluride folio, its name they cross Oak Creek and continue up the north­ and shaly below ______17 of underlying "Red Beds," now known as the being suggested by its widespread occurrence in the west side of Canyon Creek to the point where they 6. Sandstone, extremely massive, white; red stains Cutler formation, was included as a part of the La Plata Mountains. It was defined to include a are cut off by the laccolith of monzonite porphyry. from shales above; quartzite in lower part and a thin green shaly layer near base______70 Dolores formation. At Ouray an unconformity marked lithologic unit consisting principally of two The La Plata also occurs in Cow Creek southward 5. Sandstones, with thin limestones and calca­ occurs at the base of the lowest fossiliferous Triassic massive sandstones with a variable calcareous mem­ from the vicinity of Ramshorn Ridge to the mouth reous shaly layers; some reds and pinks, pre­ vailing colors buffs and yellows. ______120 stratum, and on account of this the older "Red Beds" ber between them lying at the base of the fresh-water of Oban Creek. The beds are not present on the 4. Sandstones, heavy bedded, saccharoidal___. 11 have been separated from the Dolores and described complex assigned to the Jurassic in Colorado. The east side of Cow Creek south of Red Creek, having 3. Shale and thinly bedded buff sandstone.____ 10 as the Cutler formation. The name Dolores will thickness of the formation is at a maximum 500 disappeared through the unconformity at the base 2. Quartzite, light colored, with bluish stains____ 18 1. Sandstones and shales, red ______65 be applied, as heretofore, to the Triassic strata embrac­ feet in the La Plata Mountains, decreasing north­ of the Telluride. La Plata sandstone. ~~ 779 ing the fossiliferous conglomerates and overlying ward to 100 feet in the Telluride and Ouray areas. beds. The lower limit of the formation is marked The sandstones are ordinarily milk-white, quartzose, M ELMO FORMATION. Distribution. The McElmo has essentially the by a stratigraphic break, but as yet this has been and very massive; they vary somewhat in relative Name. -The McElmo formation was named in same distribution in the quadrangle as the La Plata, observed only at Ouray, where a sharp angular thickness and to the north and west have a deep the Telluride folio from an important tributary of being present on both sides of the Uncompahgre unconformity causes the Dolores to cross the yellow, orange, or red color. The calcareous mem- San Juan River heading on the Dolores Plateau. Valley from Canyon Creek and The Amphitheater inclined and beveled strata of the entire Cutler In the main McElmo Valley and in its side canyons northwestward beyond the mouth of Coal Creek, formation and to rest upon similarly inclined 1 Cross, W., and Spencer, A. C., Geology of Rico Mountains: the strata of the formation are well exposed below and again in Cow Creek near Ramshorn Ridge. Twenty-first Ann. Kept. U. S. Greol. Survey, pt. 2, 1900, pp. beds of the Hermosa. Elsewhere in the San 67-73. the Dakota. The formation does not appear to be In addition to these localities its upper members Ouray. occur near Dallas and farther north, their reappear­ A little over a mile south of Dallas the Dakota Distribution. The Mancos shale occupies nearly lack of exposures little can be said of the local ance .being due to the east-west fault which crosses reappears along a fault, which can be traced for one-third of the area of the Ouray quadrangle, and character of the formation. The boundary also Uncompahgre River a mile south of Dallas. The many miles westward. A number of northeast- except for a few isolated occurrences above the between the Mancos and Mesaverde is purely hypo­ formation, with the exception of the lowest beds, southwest faults of no great displacement cause Dakota in the vicinity of Dexter, Cutler, and Coal thetical and has been drawn on the map with ref­ may be seen to best advantage in this last area. breaks in the continuity of the Dakota east of the creeks, it lies wholly to the northwest of the moun­ erence to the outcrops of coal seams, as shown by Notwithstanding its thickness and prominent posi­ river and within a short distance an easterly dip tain front. The shales are, as a rule, poorly old workings. tion in the section of the .Uncompahgre Valley carries the sandstones beneath the soft Mancos exposed, but their presence is easily recognized Age. From invertebrate fossils found at various farther south, it is, as a rule, poorly exposed, shales. West of the river an edge of a mesa, which by the stiff clay soils, which have a characteristic horizons in the La Plata quadrangle and adjacent partly on account of its shaly character and partly is part of the great Uncompahgre Plateau, enters yellowish-drab or gray color. The only region districts the Mesaverde has been referred by T. W. because of the abundance of glacial drift that covers the quadrangle. Here the Dakota assumes its most where even partial sections of the Mancos are well Stanton to the Pierre portion of the Montana group. much of the valley sides. characteristic physiographic expression as the cap­ exposed is that lying between Dry Creek and There is no ground for assigning the formation to Age and correlation. The McElmo formation ping formation, with a bold escarpment bounding Uncompahgre River, especially in the dry ravine the Fox Hills, as was done by the Hayden Survey. appears on stratigraphic and lithologic grounds to the mesa on all sides. In this region and especially between the two large moraines shown on the map. Among the more important species recognized are occupy the .place of Lower Cretaceous sediments; about the mouth of Cow Creek and on the west As has been already said, thin remnants of the the following: but its age is assumed to be Jurassic, from the side of the mesa between that creek and Uncom­ Mancos occur at a number of places along the Inoceramus crippsii var. bai'abini Morton. .opinion prevalent among paleontologists concern­ pahgre River thin coal seams have been locally upper Uncompahgre Valley; they are covered Cardium bellulum Meek. ing the vertebrate fauna long known from the prospected. The deposits are too thin and discon­ either by the Telluride conglomerate or by the San Cardium speciosum M. and H. Callista deweyi M. and H. Morrison formation on the eastern flanks of the tinuous and the coal is of too poor a quality to be Juan tuff, and have served, in not a few localities, Anchura newberryi Meek. Front Range and in the equivalent Como beds of of any commercial value. They occur near the top as convenient horizons, for the intrusion of the Actseon intercalaris Meek. Baculites compressus Say. Wyoming. Morosaurus and other representatives of the Dakota in a series of thin sandstones and monzonite porphyry of the region. Between Cor- Baculites anceps var. obtusus Meek. of this fauna have recently been found by E. S. bituminous shales beneath the uppermost massive bett Creek and Coal Creek there may be a consid- Placenticeras intercalare M. and H.

Riggs in McElmo beds in the Grand River Valley sandstones or quartzites. able thickness of the shales, but they are obscured TERTIARY SYSTEM. at the north end of the Uncompahgre Plateau. Cow Creek in the vicinity of Ramshorn Ridge by Pleistocene and Recent landslides or glacial EOCEXE (?) SETIIES. The McElmo represents the upper part of the Gun- is the only other locality in the quadrangle where gravels. nison group. That the McElmo, Morrison, and the Dakota occurs. On the west side of the creek TELLURIDE CONGLOMERATE. MESAVERDE FOBMATJON. Como formations embrace certain equivalent strata it dips steeply to the north beneath the monzonite- Name and definition. The Telluride conglom­ is a conclusion scarcely open to further question. porphyry laccolith. It occurs again on the east Name and description. The Mesaverde forma­ erate is a well-marked formation which underlies That the three formations are coextensive and thus side of the stream, but here it lies above the main tion was characterized in the La Plata folio as a the San Juan tuff and was formed in the period fully equivalent cannot be considered as demon­ porphyry intrusion, a considerable thickness of the succession of alternating sandstones and shales, immediately preceding the beginning of the long strated by present knowledge. McElmo shales intervening between it and the por­ with a few marls or thin limestones, and a number succession of volcanic eruptions in this region. It phyry. Its southern extension is here limited by of coal seams. The name was first applied by includes the sediments first deposited after the tre­ CRETACEOUS SYSTEM. the unconformable Telluride conglomerate. W. H. Holmes and was suggested by the very mendous erosion that prepared the peneplain upon DAKOTA SANDSTONE. characteristic occurrence of the formation in the which it rests. It was first identified in the Tellu­ Description. The lowest member of the Creta­ MANGOS SHALE. Mesa Verde, southwest of the San Juan Mountains. ride quadrangle, where it is well developed and ceous, succeeding the McElmo formation with Name. The Mancos formation was first named The formation has a maximum thickness in the revealed for examination, and was renamed after apparent conformity, is the Dakota sandstone. It in the Telluride folio from its characteristic devel­ Mesa Verde district of 1200 feet; it is lithologically that area in the Silverton folio, as it was found that is here, as commonly in Colorado, a series of opment in the valley of Mancos River, an impor­ distinct from the soft black shales of the Mancos the term San Miguel, under which it was originally extremely variable gray or brown quartzose sand­ tant tributary of San Juan River rising on the formation upon which it rests, and from the heavy described, had been previously given to a Creta­ stones, in many places cross-bedded, with several western slopes of the La Plata Mountains. It Lewis shale succeeding it. ceous formation in Texas. shale layers at different horizons. Its thickness consists there of about 1200 feet of soft black or In his original description Holmes recognized Description. The Telluride formation varies in the Ouray quadrangle is in general 100 feet and dark-gray carbonaceous shales, containing some three divisions of the formation the "Lower greatly in texture and thickness from a thin, coarse nowhere exceeds 150 feet. The Dakota is com­ very thin bands of impure fossiliferous limestone, Escarpment sandstone," the "Middle Coal group," conglomerate in the Silverton and Ouray quadran­ monly indurated to a greater extent than the La which are too few and discontinuous to serve as and the "Upper Escarpment sandstone." Con­ gles to a complex of fine-grained conglomerates, Plata sandstone, and the term quartzite is locally practical guides in subdivisions of the great shale siderable variation is found, ^however, in the sandstones, and shales about 1000 feet in thickness, appropriate to designate it, especially in the vicin­ series in which they occur. The formation is development of the different members, in both as developed in Mount Wilson, on the western ity of metalliferous deposits. sharply limited below by the massive sandstone thickness and hardness, and distinct scarps are not border of the Telluride area. It is made up of The basal conglomerate, carrying small chert of the Dakota and above by the lowest sandstone everywhere present. The "Middle Coal group," detritus of schists, granite, Algonkian quartzite pebbles of white, dark-gray, or reddish color, member of the Mesaverde formation of sandstones which in the Mesa Verde section occurs about 400 and slate, and lesser amounts of the harder sedi­ which is so persistent over large areas elsewhere in and shales. As was pointed out in the La Plata feet above the base of the formation, consists of ments of the Paleozoic formation in particular of the Rocky Mountains, is here practically lacking, folio, the Mancos shale is a lithologic unit which about 600 feet of grayish shales and sandstones, limestone. although a few pebbles occur at places in the lower embraces the Colorado group and a part of the with coal seams abundant throughout the section. In the Ouray area a granite porphyry that meg- beds. Pierre division of the Montana group. It is in most places, owing to its soft, friable nature, ascopically resembles a coarse andesite is abun­ The shale members are well developed near the Description. The Mancos shale is very similar poorly exposed, except in such semiarid regions as dantly represented in the pebbles and bowlders at middle and also near the top of the series. They to the most typical clay shales found commonly in the Mesa Verde. Below the coal group are nearly almost all the localities where the Telluride has are dark and strongly carbonaceous, with abundant the Colorado group elsewhere in the State. It is 400 feet of alternating sandstones and shales with been studied; greenstones also are common. indistinct plant remains, and north of Dallas and in few places as highly bituminous as the Benton the massive "Lower Escarpment" sandstone at the On Cow Creek and in various places farther north near the point where Cow Creek joins Uncom­ shale along the foothills of the Front Range, but top varying from 100 to 150 feet in thickness. an impure red limestone occurs within a few feet pahgre River there is at one or both of the shale it has much less sand mixed with it than is com­ Above the coal horizons is the "Upper Escarp­ of the base of the formation. It is about 10 feet horizons a thin coal seam, sufficient to induce pros­ monly found in the Pierre -shales of the Denver ment" sandstone, which may reach a thickness of thick and grades upward into a coarse pink and pecting on its outcrops, but nowhere of much eco­ region. Where fresh the colors are dark leaden 200 feet in the vicinity of San Juan River, but brown grit which in many places becomes coarsely nomic importance. As elsewhere in the San Juan grays, in places with a greenish tinge; on weather­ near the La Platas does not exceed 25 feet. conglomeratic. Some of the bowlders of the con­ Mountains the variability of the sandstones in ing the shales become yellow or buff. Distribution and occurrence. The Mesaverde has glomerate exceed 1 foot in diameter, but they thickness and in purity makes close correlations of The calcareous and locally sandy layers con­ a considerable areal distribution to the south and are generally only a few inches. Many are well sections from different places difficult. In the taining fossils occur most abundantly near the base west of the San Juan Mountains and is very rounded, but subangular fragments are common, Ouray quadrangle the sandstones are all much of the formation or else well up toward the top; important economically on account of the excellent especially of the local material that has not been indurated and almost everywhere occur as very yet some fossils are found nearer the middle of the coal that has been found in it at a number of local­ transported far. hard massive quartzites, a facies that is not uncom­ section. The lower fossil-bearing beds have long ities. The strata referred to the Mesaverde in the Stratigraphic relations. There is a great strati- mon to the Dakota in other regions. It therefore been known. In the course of mapping the Tel­ Ouray quadrangle are very much obscured by gla­ graphic break below the Telluride conglomerate, as resists erosion and becomes very prominent in the luride, Rico, La Plata, Durango, and Ouray quad­ cial and landslide debris, so that neither extent nor is evident from the character of its pebbles and valley sides and in scarps facing the canyons that rangles, and in reconnaissance work in adjoining thickness can be accurately determined. At two bowlders, which were derived from many older cut below it in the plateau country. It forms the areas, the various fossiliferous horizons have been localities on the headwaters of Lou Greek, a tribu­ formations. The magnitude of the break thus floor of the Dolores and Uncompahgre plateaus seen and persistent faunas have been found. tary of Cow Creek, coal seams have been opened indicated is confirmed by angular unconformity, over hundreds of square miles, thin remnants of At some localities in the San Juan region Gry- and coal of an inferior quality has been taken out best exhibited on the southern side of the San the Mancos shale resting upon it and producing phcva newberryi Stanton is abundant in the lower in small quantities. A mile or so north of the Juan Mountains in the Silverton and Telluride minor undulations. No determinable fossils have 100 or 150 feet of shales, and a somewhat higher quadrangle, in the Deer Creek and Burro Creek quadrangles. In these areas the conglomerate vis­ been discovered in the Dakota in this region, but zone is characterized by Prionocyclus macombi drainages, more extensive prospecting has been ibly transgresses the edges of upturned and eroded indistinct plant remains and a few leaves occur in Meek, Inoceramus dimidius White, Ostrea lugu- done and one of the two seams is now being sedimentary beds from the pre-Cambrian to a hori­ the shale layers. bris Conrad, and other typical Benton species. worked for local consumption. The deposits are zon well up in the Mancos shale. For a discus­ Distribution. Except in a few places where it is The Pierre species from the upper part of the from 12 to 20 feet thick, and, except at the Burro sion of the peneplanation resulting in the surface covered by landslide debris, the Dakota is almost Mancos shale include Inoceramus crippsii var. bara- Creek openings, where heavy, crumbling, light- upon which the conglomerate rests, and for other continuously exposed on both sides of the Uncom­ bini Morton, Syncyctonema rigida (H. and M.), yellow sandstones occur above and below the lower stratigraphic details, the reader is referred to the pahgre Valley from The Amphitheater and a point Baculites compressus Say?, and species of Mactra, coal seam, the rocks with which the coals are asso­ Silverton folio. a little south of Oak Creek nearly to the great Scaphites, etc. ciated are nowhere exposed. This is due, in part, In the Ouray quadrangle the Telluride conglom­ moraine which crosses the valley just above the In the Ouray quadrangle the Mancos shale is not to the extremely soft, friable nature of the beds, erate is not continuous and hence the angular junction with Dallas Creek. It forms a prominent protected above by any massive formations, and but more especially to the heavy mantle of land­ unconformity at its base is not so prominent as ledge outcrop high up on the valley sides, and only areas occupied by it have only moderate relief. slide debris and old glacial drift which completely elsewhere; but the map shows that the conglomerate at its northern limits does an increased northerly None of the rocks are resistant enough to form covers the western slopes below Cimarron Ridge. rests upon various formations from the Hermosa dip carry it down nearly to the level of the valley. cliffs or escarpments and the sides of such isolated The position of the coal deposits with reference (Carboniferous) to the Mesaverde (Cretaceous), and The presence of the moraine obscures the actual dis­ hills or mesas as are formed of them, though at to the Mancos formation and the presence of the small patches are known between the San Juan appearance of the formation due to the dip, but just many points steep, are smooth and covered with heavy sandstones observed in association with them tuff and Algonkian beds in the Uncompahgre west of the quadrangle, near the town of Ridgway, the clay soil that results from the weathering of north of the quadrangle are sufficient grounds for Canyon less than a mile south of the quadrangle the descent of the beds is very clearly shown. the shale. recognizing the Mesaverde formation, but from the line. The Telluride conglomerate is in most places prise the alluvial or flood-plain deposits of the races, so that correlations are impossible. Rem­ than 2 miles long, with an average width of 1 mile. apparently conformable with the overlying San larger valleys, with associated fans of detritus at nants of this lower terrace are preserved as benches It has relatively steep upstream faces and gentler

Jiian tuff,/ but there was much erosion,' locallyt/ at the mouths of tributary streams, moraines, terraces or isolated mesas on both sides of Cow Creek and slopes downstream with a very uneven, hummocked least, between the two epochs of deposition, as is of glacial gravels, and also accumulations, covering Uncompahgre River and are the most striking top­ surface containing numerous small depressions or best shown in the Silverton quadrangle. It is pos­ considerable areas, of disordered land waste which ographic features of the northwestern part of the kettle holes, one of which, near the western bound­ sible that the discontinuity of the conglomerate in have come to their present positions as landslides quadrangle. The terraces bordering directly on ary of the quadrangle, is large enough to be repre­ the Ouray area is due to that erosion, although from the heights above. In addition to these sur­ the Cow Creek drainage are covered by gravels sented on the topographic map. The moraine is proof of such action has not been obtained. face deposits there are in certain of the high amphi­ composed almost entirely of volcanic material, the cut through about in the middle by Uncompahgre Though volcanic material appears to be absent theaters or cirques great talus heaps and "rock pebbles of the lower level being well rounded and River, and exposures along the stream show the from the Telluride conglomerate proper, non- streams" such as are characteristic of many parts waterworn. Both terraces are covered by a coating typical morainal character of the materials unstrat- igneous pebbles occur locally in the San Juan tuff of the San Juan Mountains. of fine red soil. These terraces are shown in fig. ified subangular striated bowlders, gravel, and fine near its base and. in a few places a transition zone In the Ouray quadrangle the first direct evidence 10 of the illustration sheet; the inclined surface at sand, representing all the rocks known to occur in of mixed conglomerate has been observed. has been found of more than one stage of glacia- the extreme right and the higher of the two small the upper Uncompahgre Valley and its tributaries. Distribution. The Telluride conglomerate occurs tion in the mountains of southwestern Colorado. mesas seen in the middle distance belong to the The distance of this moraine from the head of at a number of widely separated localities in the Records of the latest stage are abundant through­ upper level; the lower of the small mesas represents Uncompahgre River and Canyon Creek is about Ouray quadrangle. A thin but characteristic band out the San Juan and from time to time indications the lower terrace level. 20 miles. extends on the west side of Canyon Creek from a of an earlier glaciation have been observed, but The terrace gravels of the Uncompahgre drain­ Another large moraine lies on the slopes north point near Oak Creek to the laccolith of inonzonite the evidence bearing directly on this point is age belonging to the lower level accord closely of Baldy Peak. The long ridge extending north­ porphyry that cuts across it and upward into the extremely meager. In this place only the charac­ with those of Cow Creek in elevation, but the westward from the summit, although its form is San Juan tuff. The unconformity at the base of ter and distribution of the deposits are described, materials are somewhat different, comprising, in not entirely due to morainal deposits, since it the Telluride is well shown here, the conglomerate their origin and age being discussed under the addition to volcanic rocks, porphyries, possibly extends beyond them, is thickly covered with resting upon the Dakota near Oak Creek and upon heading "Historical geology." A special descrip­ derived from the Telluride conglomerate, and gravels and huge bowlders of San Juan tuff, the McElrno farther south. The only other occur­ tion of the glacial phenomena of the San Juan fragments of nearly all the sedimentary rocks of massive Potosi agglomerate, Algonkian quartz­ rences west of the Uncompahgre are near the head Mountains has been published. 1 the Uncompahgre Valley, including Algonkian ites, some Dakota sandstone, and considerable of Coal Creek and just north of Corbett Creek. quartzites. monzonite porphyry. The greater part of this These two exposures are believed to be connected EARLIER DEPOSITS OF GLACIAL ORIGIN. The origin and the age of these terraces and material was clearly derived from the Uncom­ with one another, but the enormous accumulation Moraines of the earlier stage. In the area their gravel deposits are discussed in the section on pahgre Glacier, but the numerous large bowlders of landslide debris on the broad ridge between bounded by Lou Creek, Cow Creek, Cimarron historical geology, and the whole subject is treated of massive Potosi latite can have come only from Coal Creek and Uncompahgre River has com­ Ridge, and the northern border of the quadrangle in greater detail in the recently published paper on the Cow Creek drainage, and it is believed that the pletely covered the intervening space. much of the ground is covered more or less thickly glacial phenomena which has been cited. It is drift covering the ridge, as well as that extending From the end of the ridge north of Cutler Creek by gravels and bowlders; near Cimarron Ridge the sufficient to state at this point that all the deposits northeastward nearly to Cow Creek, represents a continuous exposures of the Telluride, covered at deposits almost completely cover the shales and thus far described are older than the last stage of lateral moraine of the Uncompahgre Glacier, first by the San Juan and later by the Potosi tuffs, sandstones of the Mancos and Mesaverde forma­ glaciation. blending with the terminal moraines of the Cow may be traced around the western and northern tions. The debris lies in disordered heaps, modi­ Creek and Courthouse Creek glaciers. In addition flanks of Baldy Peak, past Cobbs Gulch to Cow fied somewhat by erosion, but in many places PLEISTOCENE LANDSLIDES. to the more pronounced lateral moraines of the Creek, on both sides of which it is exposed to a retaining the form of kettle moraines. The mate­ In the Ouray quadrangle, as in other parts of the Uncompahgre Valley, most of which are on the point a third of a mile south of Oban Creek. rial consists of bowlders and pebbles derived from San Juan Mountains, landslides have taken place western side, drift may be found on all the slopes At its southernmost exposures, except in a few the San Juan agglomerates of the ridge, and, in from time to time, and the great heaps of debris that are not too steep, and at higher and higher places where it is obscured by landslide debris, it addition, crumbling blocks of the agglomerate itself that have resulted from them are striking features elevations to the south, the highest observed being rests upon the La Plata on the west side and upon from 10 to 30 feet in diameter. Near the ridge of the topography, especially near Ouray. Else­ at 9500 feet on the north side of Bridalveil Creek, the Cutler red beds on the east side. Northward these deposits, which are not thick, are covered by where in the San Juan the landslides seem to have just south of the American Nettie mine. Notable the La Plata, McElmo, and Dakota successively debris of recent landslides from the cliffs of the been of Recent age that is, they occurred after the accumulations of glacial gravels lie along Canyon appear, and north of Courthouse Creek the Tellu­ ridge. Except where obscured by landslides the disappearance of the last glacial ice. The majority Creek, just above its junction with the Uncom­ ride rests upon the Mancos shale. From Court­ slopes westward from Cimarron Ridge are the even- of the Ouray landslides undoubtedly belong to pahgre, but the greater part has been removed by house Creek northward exposures are rare and graded ones of low angles characteristic of a mature this time, since their detritus in many places rests erosion and mud} is obscured by landslide debris. north of Nate Creek possible outcrops are covered topography, but they have been materially modi­ upon moraine or drift. In the Cimarron drainage basin areas greater than by landslide debris as far as North Fork of Lou fied by a renewal of the activity of the streams The region lying between Coal Creek and in other valleys are covered by drift, some of'which, Creek, where the conglomerate rests upon the Mesa- that drain them, and gravels are preserved in their Uncompahgre River, as well as more extensive along Middle and West forks, has the characteristic verde formation. original positions only on the divides and ridges areas west of the quadrangle, is unique in being form of lateral moraines; in general, however, most Age mid correlation. The Telluride conglom­ between the incised streams; south of Lou Creek covered with landslide debris of a much older of the slopes are covered with a thick mantle of erate has as yet yielded no fossils by which its age similar materials were undoubtedly present but period, believed to have been nearly contempora­ glacial de'bris without distinctive forni, extending may be determined. The reference to the Eocene practically all have been removed. The gravels neous with the earlier stage of glaciation. The upward for nearly a thousand feet above the main in this place is in accord with the procedure in the were evidently laid down before the surface was accumulations cover the broad ridges between stream. North of the point where West Fork joins Telluride folio, no additional data bearing on the dissected and they are therefore believed to have streams and rest upon what appears to have been the Cimarron, kames and kettle holes are common question having been discovered. The conglom­ been deposited by glacial or neVe ice at a period or a continuous surface extending northward with a but no well-defined, continuous ridges of lateral erate is later than the great orogenic movement periods much earlier than the one during which very gentle slope from the foot of the mountains. moraines are preserved. In this region no indi­ which affected the entire older sedimentary section the great moraines of the Uncompahgre Valley There can be no doubt as to the landslide origin of cations of the earlier period recognized farther west of this region, and also later than the enormous were formed. the detritus, which consists of a chaotic mingling have been found, although some of the higher drift erosion which produced the peneplain upon which Terrace gravels earlier stage. Gravel-covered of San Juan, and Potosi materials and blocks of under Cimarron Ridge and on the opposite side of it rests. c i/ terraces occur at two well-marked levels below that Telluride conglomerate, angular and unstriated. the valley may belong to a stage earlier than that The relations mentioned suggest a correlation of the old drift. The upper terrace is merely an The relatively great thickness of the masses near of the kettle moraine of the valley bottom. The of the Telluride conglomerate with the Arapahoe extension of the old graded surface upon which the the foot of the mountains and their gradual thin­ low divide at the head of Owl Creek is covered formation of the Denver region, and such a com­ drift rests, and is covered by stratified gravels sep­ ning out at a distance from their sources are features with drift of the west Cimarron Glacier and the parison implies the further correlation of the arated by a rather indefinite line from the strictly characteristic of similar accumulations whose land­ detritus may be traced for nearly a mile down Owl San Juan tuffs with the Denver beds, which are morainal material. The terrace form and gravel slide origin is undoubted. The dissection of these Creek, showing that at the time of maximum gla­ composed largely of andesitic debris. Both the cover are preserved only as remnants between the deposits, as well as of the surface upon which they ciation of the last stage a distributary of the west Arapahoe and Denver formations are fossiliferous, streams tributary to Cow Creek and in a few isolated rest, corresponds in time and magnitude to the ero­ Cimarron ice stream crossed the divide and entered and paleontological opinion seems to be that they hilltops and mesas, one of which, shown in fig. 10 sion that preceded the last stage of glaciation, as the Cow Creek drainage. This drift is distinct should be considered Cretaceous, although sep­ of the sheet of illustrations, lies west of Cow Creek pointed out in the section on Pleistocene history. from that of the older period near the divide, but arated from the Laramie proper by an interval of and nearly opposite Deer Greek. Their occurrence on both sides of Coal Creek near on its outer edges the two can not be clearly great orogenic disturbance and subsequent erosion. The gravels consist entirely of material derived its mouth with exposures of v*the Dakota and separated. The Denver beds have, in fact, an equivalent, indi­ from the Potosi volcanic series and vary from an McElmo beneath them proves conclusively that Later gravel-covered terraces. Distinct terraces, cated by plant remains, in the "Animas beds" of inch or less to a foot in diameter. The deposits they could not have been laid down after the dis­ gravel covered and similar in form to the older and the section well shown a short distance below are of variable thickness and are mostly thin near section of the upland. higher ones previously described, occur on both Durango, but there is no conglomerate comparable the ,old morainal material; the maximum observed The configuration of this landslide area is in sides of Uncompahgre River below the great with the Telluride in that section and the corre­ thickness is 50 feet at the edges of terraces over­ itself an indication of its age, since its surface is moraine, and also on Cow Creek as far upstream spondence of the San Juan tuffs with the "Animas looking Cow Creek and on the mesa opposite Deer rolling but no longer possesses the hummocked as the mouth of Owl Creek. The level of these beds" is purely lithological, so far as known. Creek the only points where the gravels are well appearance characteristic of recently slidden mate­ terraces is about 50 feet above that of the present Under these circumstances it is thought best to exposed, as they are elsewhere covered by 2 or 3 rial, and the enormous blocks of which it is com­ flood plains of the streams. The gravels are water- await further investigation in the San Juan region feet of fine reddish soil, possibly of eolian origin. posed, seen in exposures due to recent slips, have worn, stratified, and of only moderate coarseness, before making any positive correlations of the Tel­ Earlier alluvium. A second gravel-covered ter­ lost their angular outlines from long weathering. few individual bowlders reaching 1 foot in diam­ luride and San Juan formations. For a discussion race occurs below the first one, the vertical distance eter; their lithologic character is appropriate to the of this question the reader is referred to the Tellu­ between the two decreasing downward from the LATER DEPOSITS OF GLACIAL ORIGIN. drainage in which they occur, those of Cow Creek ride folio and to the monograph (Mon. U. S. Geol. upper portions of the streams tributary to Cow Later moraines and drift. Typical lateral and being largely of Potosi materials, and those of the Survey, vol. 27) on the geology of the Denver Creek. At the mesa opposite Deer Creek their terminal moraines, whose forms have been modified Uncompahgre consisting of sedimentary and vol­ Basin. respective surfaces are a little over 200 feet apart, but little since their deposition by the ice, occur in canic rocks mingled. but on the sides of Deer Creek a difference of more the three principal drainage systems of the quad­ The relation of the lower Uncompahgre terrace QUATERNARY SYSTEM. than 400 feet exists. Intermediate terraces occur rangle. The largest and most perfectly preserved to the great moraine shows clearly that the gravels INTRODUCTION. at a number of levels but they are poorly preserved are those of the Uncompahgre Valley. The great represent the out wash deposits laid down by waters Surficial deposits of Pleistocene and Recent age and merge into one another, or into the main ter- terminal moraine south of Dallas is over 400 feet escaping from the foot of the glacier and are essen­ characteristic of glaciated mountain regions are well high, and, including the portion lying west of the tially contemporaneous with the moraine itself. 1 Howe, E., and Cross, W., Hull. Geol. Soc. America, vol. developed in the Ouray quadrangle. They com- 17, 1906, pp. 251-274. quadrangle, near the town of Ridgway, is more The gravels of the lowest Cow Creek terrace are Ouray. 8 of the same age and doubtless of similar origin, glaciers. The surfaces are as a rule ridged or fur­ the intermediate phases in which angular, suban- ored, presenting the shades and tones more nearly although their relation to a terminal moraine is rowed, the lines strongly suggesting slow move­ gular, or, less commonly, well-rounded fragments normal for fresh aggregates of volcanic rocks. This not evident. ment of the whole mass. The origin and character are inclosed in a tuffaceous matrix, are most abun­ general decomposition is thus greatest in the areas of the rock streams is discussed under the heading dant. The term tuff-agglomerate best expresses its of extensive ore deposition, as might be naturally ALLUVIUM. "Historical geology," and for further details the average mechanical character. By far the greater expected. Alluvial sands and gravels, or flood-plain depos­ reader is referred to the Silverton folio, where these part of the formation consists of a tuff whose Origin. The actual source or sources of the its, occupy all the larger valleys, the most notable forms were first described. particles range from microscopic size to a few volcanic materials of the San Juan formation are being the broad and fertile flood plain of the Uncom- IGNEOUS FORMATIONS. tenths of an inch in diameter, holding larger sub- unknown. The rock fragments do not seem to be pahgre between Portland and the great moraine angular fragments that reach a maximum diameter of the nature of bombs or lapilli ejected in a par­ SURFACE VOLCANIC ROCKS. which, from its unusual width, causes the others of of 10 feet or more. The coarser beds are generally tially molten condition from an explosive volcano. the region to seem almost insignificant. Similar INTRODUCTORY STATEMENT. rather thick and the finer grained layers thin, and They appear rather to represent pre-San Juan lava though less extensive deposits of alluvium" occur The surface lavas and pyroclastics of the western there is much variation in the relative amounts of flows or perhaps pyroclastic formations which along the upper and lower reaches of the Uncom- San Juan hav?e been referred to three principal fine- and coarse-grained beds in different parts of the have been broken up and their detritus transported pahgre, on Cow Creek, and on the two forks of the divisions called the San Juan tuff and the section. There is, however, no regular progressive from their original sites of consolidation or deposi­ Cimarron that lie in this quadrangle. Silverton and Potosi volcanic series. This group­ change from bottom to top of the formation. The tion. The great amount of volcanic debris and its Fan-shaped deposits of detritus occur at the ing serves to bring out three great and clearly dis­ coarse agglomerates are in many places extremely multifarious constitution make it clear that a huge mouths of many of the smaller streams; they con­ tinct epochs of eruption, and, further, to distinguish chaotic, and exhibit bedding only when viewed volcanic pile or several piles must have contrib­ sist of bowlders and gravels that have been carried the lavas of these time divisions, which possess cer­ from a distance. Such deposits have a thickness uted to this San Juan detrital accumulation. Noth­ down the gulches at times of unusual activity on tain fairly well marked petrographic differences. of over 2000 feet near the forks of Cimarron Creek ing yet observed locates clearly the situation of the the part of the streams and deposited on the flood The San Juan tuff contains only fragmental and, being poorly consolidated, weather in countless older complex, but it seems almost certain that it plains or broad bottoms of the large valleys. Some material, the detritus of the earliest eruptions; the fantastic "hoodoo" forms, many rising as nearly must have been in the eastern or central San Juan of these alluvial fans, as that at the mouth of Silverton and Potosi series consist chiefly of lavas vertical pinnacles or columns for over 200 feet, Mountains, and that it was very largely destroyed Portland Creek upon which the town of Ouray is and contemporaneous tuffs. In general terms the protected at the summit by a remnant of a more in the process of forming the San Juan tuff and built, may cover considerable areas. For the most San Juan consists mainly of andesitic lavas, the resistant layer or by a single large bowlder. Lenses agglomerate. Its remains may be entirely hidden part their outer portions are merged with the flood Silverton of latite and rhyolite alternating with of finer grained tuffs, many of them porcelain-like by the lavas and tuffs of the long succession of later plains of the main streams and are not distinguished andesite, and the Potosi chiefly of latite and in texture, are found at numerous places in the Tertiary eruptions. from them on the map. rhyolite. midst of coarser material. The San Juan formation does not represent, in The full significance of the main eruptive epochs The fragments of the San Juan tuff are of andes­ the area thus far studied, a subaerial accumulation RECENT LANDSLIDES. and of the intervals between them is perhaps best ite or latite, so far as known. But, though some about an explosive vent, for its beds exhibit no In addition to the ancient landslides already shown in the Silverton quadrangle, but the rela­ of them contain much orthoclase in the ground- tendency to dip away from a center. The detritus described, others of more recent age have occurred at tions exhibited in the Ouray district throw light mass, all are characterized by plagioclase pheno- of the formation has, therefore, been transported, numerous places in the Ouray quadrangle. Their on some of the obscure points. crysts, and microscopical or chemical examination and fluviatile agencies appear to be the only ones origin is discussed under the heading "Historical It is known that above the Potosi lavas later is required to determine the latites. Labradorite is capable of performing the gigantic work which has geology." Noteworthy slides are to be found volcanic rocks occur, but the districts where these the common lime-soda feldspar occurring in pheno- clearly been done. Certain portions of the San in Coal Creek and on the east side of the ridge appear have not yet been studied in detail. crysts. Since andesite is apparently much more Juan tuff in the Telluride quadrangle may possi­ between that stream and Uncompahgre River, abundant than latite, and the latite possesses the bly be lake-bed deposits, as stated in the Telluride SAN JUAN TUFF. where slips have taken place involving in part the andesitic habit, it is sufficiently accurate for all folio, but it is plain that most of the formation now loosely consolidated materials of the Pleistocene Name and definition. The earliest deposits of general purposes to describe the materials of the known must have been a land or continental deposit. slide deposits. Other slips have occurred about the volcanic origin thus far observed in the San Juan San Juan as andesitic in petrographic character. Correlation and age. The observed extension of head of Corbett Creek and on the north side of Mountains consist of tuff, breccia, and agglomerate None of the latites is of the facies to be described the San Juan tuff northward along the narrow Cascade Mountain, where the accumulations are of without intercalated lava flows. They represent as prominent in the Silverton or Potosi series. ridges between the branches of Cimarron Creek to sufficient thickness and extent to obscure completely the transported products of an earlier epoch, accu­ As to their dark silicate constituents the observed a point within a few mi'es of Gunnison Canyon mulated during a time in which no lavas were rocks range from pyroxene andesite to hornblende- makes the conclusion unavoidable that the San the solid-rock ogeology O*/ over a considerable area. The Amphitheater, directly east of Ouray, is erupted, at least in the western San Juan. The biotite andesite or latite, with many intermediate Juan was once continuous with the "West Elk filled to a depth of several hundred feet with land­ simple term tuff will be used frequently in speak­ varieties containing augite, hypersthene, horn­ volcanic breccia," consisting of andesitic rocks, slide debris that fell from the cliffs of the San Juan ing of the whole formation, since fine-grained mate­ blende, and biotite, one or more, in many degrees described in the Anthracite-Crested Butte folio. formation in which the cirque has been eroded, and, rial is an important part of all the beds and is of relative and absolute abundance. The most Although the Hayden map of the Gunnison Valley although the deposits are covered for the most generally dominant. basic rocks are the pyroxene andesites, the most sili­ is incorrect in many important respects, it expresses part with vegetation, Portland Creek and its side The San Juan tuff overlies the Telluride con­ ceous are quartz-bearing hornblende-biotite latites. well the broad fact, recently confirmed, that two streams have carved deep trenches, in whose sides, glomerate conformably through the greater part of In texture the rocks naturally exhibit much volcanic formations of the West Elk and San Juan which are at many places more than 200 feet high, the area in which the two have been found in con­ variety. Most of them are rather obscurely por- mountains formerly extended across the valley. the chaotic nature of the accumulations is well tact, but that a time of important erosion intervened phyritic, some are clearly of that texture, and These are the San Juan tuff and the Potosi vol­ displayed. is shown in the Silverton quadrangle. Where the others are dense aphanites. Nearly all the types canic series. Further investigation will be neces­ No better example of the peculiar topography Telluride conglomerate is absent the San Juan tuff possess the microscopic textures and other features sary to determine whether the San Juan and West resulting from landslide debris coming to rest on rests upon pre-Tertiary formations, in some places of surface lavas rather than of intrusive rocks, but Elk pyroclastic formations derived their materials moderately steep slopes can be found than that of with pronounced unconformity. It is to be pre­ highly vesicular forms are extremely rare, and those from the same source or not, but it is at least true the region southwest of Ouray lying between Can­ sumed that somewhere the San Juan overlaps the with a glassy base are by no means common. that they are contemporaneous deposits of the same yon Creek and Uncompahgre River. The whole borders of earlier lavas representing a portion of The San Juan tuff of the Ouray quadrangle general structural and lithologic character. northwestern slope of the spur between these two the complex which has yielded the detritus for the exhibits a greater variety in the petrographic char­ It was suggested in the Telluride and Silverton streams is covered with great blocks of San Juan clastic formation, but that relation has not been acter of its component rocks than was observed in folios that the San Juan tuff might be in part breccia, Telluride conglomerate, Hermosa sandstone, found and may be nowhere exposed. the Silverton or Telluride quadrangles, and this equivalent to the "Animas beds" near Durango, and fine detritus, which lie in disconnected but The accumulation of the San Juan deposits was variety becomes still more striking in the Lake City which are known through their fossil flora to be nearly parallel ridges with deep trenches between. succeeded by great erosion before the earliest lavas area, lying to the east. This change in detailed equivalents of the Denver beds that is to say, Similar accumulations occur on Cow Creek near of the Silverton epoch were poured out, and in constitution consists principally in the increasing post-Laramie. In view of the magnitude and com­ Red Creek and at the head of the main stream, on many places the evidence of this erosion is seen in abundance from west to east of very light colored plexity of the San Juan tuff it is now considered North Fork of Henson Creek, and on Middle and striking unconformities. The name San Juan has hornblende-biotite or augite-biotite andesites and more probable that the "Animas beds" belong to West forks of Cimarron Creek.' Particularly exten­ been used in several folios for this pyroclastic for­ latites, of kinds not seen in the Telluride and Sil­ one of the pre-San Juan epochs of eruption. sive slides have taken place all along the western mation, which occurs normally between the Tel­ verton districts. The significance of this observed No new evidence as to the age of the San Juan face of Cimarron Ridge, and the peculiar topogra­ luride conglomerate and the lowest lavas of the fact will be referred to in discussing the origin of has been obtained in the Ouray quadrangle. It is phy resulting from them can be recognized readily Silverton volcanic series. The formation has a the formation. manifestly much older than the calcareous tuffs of from a distance, although the surface is now cov­ maximum observed thickness of nearly 3000 feet A small amount of pre-Tertiary, material, espe­ the Silverton series, which scanty plant and inver­ ered with a thick mantle of vegetation. in the walls of The Amphitheater, east of the town cially of granite and schist, was observed in the tebrate fossils indicate to be of Oligocene or early In the extreme southeast corner of the quadran­ of Ouray. lower 200 feet of the San Juan tuff in the Tellu­ Miocene age. As has been already stated, the for­ gle, south of North Fork of Henson Creek, is a Lithologic character. Although composed almost ride region, but this is not so noticeable in the mation must be younger than the enormous com­ large and instructive landslide area, extending east­ exclusively of volcanic rock fragments, the San Ouray quadrangle, except in the lower zone, a few plex of lava flows whose destruction resulted in its ward into the Lake City quadrangle. The ridge Juan presents much variety in aspect, due to differ­ feet or yards in thickness, where there is locally a accumulation, and nowhere in Colorado is there south of the stream, consisting of igneous rocks of ences in texture, the prominence or obscurity of its mingling of volcanic materials with the materials evidence of volcanic 'eruptions in the Cretaceous the Silverton series, has been greatly shattered and bedding, the variations in composition, and the of the Telluride conglomerate. period until after the close of the Laramie proper. nearly the whole slope for 2 or 3 miles is made up degree of alteration it has undergone, affecting par­ The general appearance of the San Juan pyro­ It is therefore plausible to refer the deposition of of landslide debris. Near the crest are some blocks ticularly its color. As the formation is a notable clastic beds is that most common for such materials. the San Juan formation to the Eocene. several hundred feet in diameter, while on the cliff maker, there are many opportunities for The less altered and less indurated parts are at many Distribution. The San Juan tuff occupies a lower slopes the detritus is much broken up and in observing its general character, as well as for points colored pinkish, but many cliff exposures are large part of the quadrangle, as a glance at the part covered by vegetation. studying its composition in detail. dull green, reddish, or purplish in color, dependent geological map will show. Its exposures along the The San Juan has a distinct parallel bedding largely on the kind and degree of alteration to southern border are continuous with those of large ROCK STREAMS. that is plain in all large exposures, but may be which the tuffs and agglomerates have been sub­ areas in the Silverton and Telluride quadrangles. Certain peculiar masses of detritus known as obscure within some of the coarser and more mass­ jected. The alteration is greatest in the southwest­ On the east it is prominent in several valleys and rock streams occur in a number of high basins in ive beds. The bedding is brought out by the ern part of the Ouray quadrangle, the soft greenish- ridges of the Lake City quadrangle, but disappears the southeastern part of the quadrangle. They alternation of layers of various textures and is gray or darker reddish tones of the cliffs in The gradually through the descent of the Potosi lavas. consist of large and small angular fragments of much more pronounced in some places than in Amphitheater or along Canyon Creek being very Northward the San Juan and Potosi both extend rock, derived mainly from the heads of the cirques others. much like those in the adjacent Silverton and within a few miles of the Gunnison Canyon, fol­ in which they lie, and possess the sharp outline The textural varieties range from fine sandy tuffs Telluride quadrangles. On branches of Cimarron lowing the ridges between several streams, and the and in many places simulate the forms of small to coarse breccia, agglomerate, or conglomerate, but Creek, however, the formation is less strongly col­ very similar clastic beds of the West Elk Moun- 9

tains, called the "West Elk breccia," seem to have the Burns tuff is well exposed to a thickness of characterized by the reddish-brown color of the or subangular fragments of pyroxene andesite, rhyo­ been once continuous with the San Juan, whether about 75 feet. They are here well-stratified green­ massive rock, or by the nodules of agate, chalced­ lite, or quartz latite. The andesite is similar to the derived from the same immediate source or not. ish-gray sandstones in which few grains exceed ony, and other siliceous deposits in the vesicular rock of the underlying flows, and the rhyolite and 3 mm. in diameter. The particles are much zone, many of which have weathered out and are SILVERTOjXT VOLCANIC SERIES. latite are distinctly of the facies of flows of the decomposed, representing latitic or andesitic lavas, strewn over the surface. Potosi series and unlike any known earlier lavas. Name, Scope, and. General Character. mainly. Many layers are crumbling shales. A Distribution. The pyroxene andesites of the Such fragments reach a diameter of 1 foot or more The San Juan tuff was succeeded in the western calcareous cement is more or less abundant, and Silverton series appear only in the southeastern and they have been found locally of such abun­ San Juan region by a complex of lavas, tuffs, and thin gray or black limestone layers, from mere portions of the Ouray quadrangle. The principal dance as to make a volcanic breccia with subordinate agglomerates, named after the Silverton quadran­ films to a few inches in thickness, are present at exposures are in the southeast corner of the area tuff matrix. This phase of the Henson is well gle, in which they have their greatest development. several horizons. Many of the thin shaly layers at the head of North Fork of Henson Creek. exposed on the divide between North Fork of Between the deposition of the San Juan formation contain carbonaceous matter, chiefly as broken They rise from the bed of the stream for more Henson Creek and Cow Creek, on and near the and the outpouring of these lavas there was exten­ stems of undeterminable character. A few remains than 2000 feet to the crest of the ridge on the trail. The tuffs on the south slope of Wildhorse sive erosion of the former. For the reasons that have been specifically identified, as is fully set forth Silverton quadrangle line, possessing in general Peak also contain many fragments of the rocks the Silverton lavas appear to have been in large in the next section. the northerly dip recorded for the underlying named. The occurrence of such rocks in the tuff degree erupted within an erosional valley or basin, These tuffs dip northeastward at about 10°. This Burns tuffs. In the other occurrences of this of the latter locality was observed during the sur­ and that their outlying portions were eroded away structure, in which the overlying andesite flows par­ vicinity they appear beneath the Henson tuffs vey of the Silverton quadrangle and was one of the before the next epoch of eruption, the rocks of ticipate, is required to bring the Burns tuff to the through their erosion at the heads of Cow, Wild- facts which led to a reference of these tuffs to the this series now occupy comparatively small areas. much higher levels which it occupies in the Silver- horse, and Bear creeks, and are limited on the Potosi series. They are the most important element among the ton quadrangle, 3 or 4 miles to the south. north by abutting against San Juan tuff. No plant or other fossil remains have been found rocks of the Silverton quadrangle, reaching an The Burns tuff reappears on the south side of In the southwest corner of the quadrangle the in the Henson tuff, in spite of its resemblance to observed maximum thickness of about 3000 feet. the creek just west of the eastern border of the pyroxene andesite appears in Whitehouse Moun­ the Burns tuff. To the west they thin out and within the Telluride quadrangle, where it has been penetrated by the tain as a nearly aphanitic flow 60 feet in thickness Relations. The Henson tuff rests upon pyroxene quadrangle they disappear. In the Telluride folio Deadwood tunnel. It is also well shown in resting on San Juan tuff and overlain by tuff of andesite wherever the base is exposed in the Ouray they were called the "Intermediate series." a northern tributary of North Fork of Henson the Potosi series. In Potosi Peak, about 1^ miles quadrangle. A few miles farther east, in the Lake The Silverton rocks do not now extend south of Creek less than half a mile within the Lake City to the south, the Silverton series is represented by City area, the thinning out of the andesite flows the Telluride quadrangle. Eastward their devel­ quadrangle, where it contains a few fossil leaves. 300 or 400 feet of flows and tuffs, and southward causes the Henson and Burns tuffs to come in opment is subordinate and in the Ouray quadran­ The southward rise of the tuffs may be traced from from that point the section rapidly thickens. contact. gle, on the north, they occupy but a small area, as the Deadwood tunnel around the east end of the That pyroxene andesite and Henson tuff abut on Henson Tuff. shown by the map, yet the relations of the Silver- ridge between North Fork and the main Henson the north against a San Juan surface of general ton tuffs and lavas exhibited in this district are Creek, although landslide debris conceals them in Name, definition, and distribution. It is here southerly dip is amply shown by the limitation of of much importance to an understanding of the many places. .proposed to apply the name Henson tuff to a pyro- the formations expressed by the map. Owing to history of the series. Deposition and age. The character of the upper clastic formation consisting chiefly of well-bedded, landslide debris and intrusive bodies the actual con­ In the Silverton folio the Silverton volcanic Burns tuffs shows them to be waterlaid deposits, fine-grained, greenish or brownish-gray, sandy tuffs, tact with the San Juan is not well exposed in most series was divided into the following members, probably in lakes. The fossil plants from the cal­ composed of andesitic material and forming the localities; but on the west side of Wildhorse Creek which were distinguished on the geological map careous shales of North Fork of Henson Creek uppermost division of the Silverton series. They and on the northern branch of North Fork of of that folio: have been determined by F. H. Knowlton as lie upon the pyroxene andesites and are succeeded Henson Creek the relations are very clear. 1. Picayune andesite, the earliest lava of the Cratcegus Jiolmesiif Lesq. and a new species of by the Potosi volcanic series. Their varied thick­ The appearance of Potosi types of rhyolite and series, of very local development, and not exposed Pinus. The former occurs also at several places nesses and present restricted occurrence point to latite as fragments in the Henson tuff must indicate in the Ouray quadrangle. in the Silverton quadrangle. The type was found erosion of importance in the period immediately that the effusion of Potosi lavas had begun in some 2. Eureka rhyolite, a flow breccia with subor­ in a local rhyolitic tuff of the Rosita Hills, near preceding the effusion of the lowest Potosi lavas of adjacent district before the accumulation of the dinate tuffs. This rock occurs in heavy flows and Silver Cliff, Colo. In the Burns tuff of the Sil­ this district. Henson tuffs had ceased in the area now drained was one of the most widely distributed of the Sil­ verton quadrangle are several species of Pinus, in The name is derived from Henson Creek, on the by Heuson Creek. In all probability, therefore, verton lavas. It does not outcrop within the Ouray cones or leaves, and one of them is regarded by headwaters of which the tuffs have their best known the Henson tuff is really the last formation of the quadrangle, but extends down Henson Creek as far Knowlton as Pinus Jlorissanti? Lesq., the type of development. They are especially well exposed Silverton series, although the restricted occurrence as Lake City, appearing within a mile of the south­ which was found in the Oligocene lake beds at and reach their maximum observed thickness of and varying thickness of the tuffs suggest an erosion east corner of this quadrangle. Florissant, Colo. 600 feet on the divide between North Fork of interval before the earliest Potosi lavas of the west­ 3. Burns latite, a group of highly siliceous horn- A limestone layer in the tuff near the Deadwood Henson Creek and Cow Creek in the Ouray quad­ ern San Juan Mountains, during which rocks not blendic lavas and tuffs consisting of several rocks tunnel was observed b}T Benjamin Guionneau to be rangle. They extend southward across American now represented at any locality may have been of local importance, with two notable thin-bedded full of minute shells, which are said by R. S. Bass- Flat and as far south as Engineer Mountain in the removed. ' tuffs which serve as horizon markers. The upper ler to belong to the genera Cypris and Bairdia, of Silverton quadrangle, where they were seen during tuff is the only portion of the Burns latite exposed the Ostracoda. In the Silverton quadrangle some the survey of that area. But the relations of the POTOSI VOLCANIC SERIES. in the Ouray quadrangle. It is particularly impor­ gasteropod shells occurring with the plants referred tuffs were not understood at the time the Silverton Name. The uppermost division of the great tant because of fossil remains which, though scanty, to are stated by T. W. Stanton to belong to two folio was published, mainly because the body of volcanic complex developed in the western San \ give the only known direct evidence as to the age of species of Limncea very closely resembling L. meekii quartz-mica latite shown on the Ouray map as an Juan Mountains was described in the Silverton any of the volcanic rocks of the San Juan region. Evans and Shumard and L. shumardi Meek, which intrusive in the Henson tuff was regarded as the folio junder the name "Potosi volcanic series." The Burns latite was named from Burns Gulch, occur in the White River beds, usually assigned to lowest flow of the Potosi series. The tuffs below The same rocks had previously been called the in the Silverton quadrangle. The rocks embraced the Miocene but considered by some paleontolo­ that flow were therefore classed with the pyroxene "Potosi rhyolite series" in the Telluride folio, the under this head are principally massive flows of gists as Oligocene. According to the concurrent andesite and those above it with the Potosi. The change in name following a study of the more latite, a rock intermediate between trachyte and evidence of plants and shells the upper Burns tuffs intrusive character of the quartz-mica latite is placed extensive complex in the Silverton quadrangle. andesite. These latites are as a group character­ may be regarded as Oligocene or early Miocene in beyond question by occurrences within the Lake In the original description of the series, as devel­ ized by hornblende. There are many different age. City quadrangle. oped at the type locality of Potosi Peak, the rocks, flows in the Silverton quadrangle, which overlap The cartographic distinction of the'Henson tuff consisting of massive flows with a few thin agglom­ Pyroxene Andesite. on their borders. The massive rocks are in some is warranted by its lithologic character and its repre­ erate and tuff beds, were referred to as rhyolites, places separated by tuffs, but the principal develop­ The Burns latite was succeeded by a series of sentation assists materially in expressing the impor­ although it was pointed out that the single avail­ ment of such beds is at the base and at the top of flows of pyroxene andesite or latite with inter­ tance of the long Silverton epoch, between the San able analysis showed a certain glassy facies to the group. calated tuffs. These rocks reach their greatest Juan and Potosi, of which there is no suggestion in belong to the highly siliceous lavas, containing 4. Pyroxene andesite, the latest known massive development in the central part of the Silverton the volcanic rocks of a large part of the Ouray and alkali and lime-soda feldspars (plagioclase) in lava of the Silverton epoch. This member is well quadrangle, where they exceed 3000 feet in adjacent quadrangles. nearly equal amount, that are properly designated developed in the Ouray quadrangle. thickness. Description. The Henson tuff is very much as quartz latite. Analyses of rocks from a number A fifth, uppermost member of the Silverton Description. The pyroxene andesites are dark, like the Burns tuff in texture and composition and of different localities in the Silverton quadrangle series, the Henson tuff, is distinguished in this variably porphyritic lavas of dense texture in the hence also in appearance. There are, however, no showed that the greater number were latites or folio for the first time. It rests upon the pyrox­ greater part of each flow but vesicular in upper and calcareous shales or limestone layers in the Henson quartz latites and that true rhyolites there form ene andesite and through its structural relations lower zones. As developed in the Ouray quadran­ tuff at the points where that formation has been probably a subordinate part of the complex. To becomes of much value as a cartographic unit. gle the vesicles are commonly filled by chalcedony, examined. Much of the tuff consists of rather state the petrographic character of the series both The reader is referred to the Silverton folio for opal, quartz, and calcite. Scoriaceous zones occur rounded sand grains, evidently derived in large rocks must be named. details concerning the earlier members of the Sil­ between many flows. The individual flows vary degree from pyroxene andesite. Some grains exhibit Occurrence and distribution. The rocks of the verton series; those occurring in the Ouray quad­ in thickness, but few of them exceed 25 feet in the parts of crystals of labradorite, augite, or hyper­ Potosi series, which once doubtless covered the rangle will now be described more fully. Ouray quadrangle. sthene embedded in dark ferritic or microlitic entire area of the Ouray quadrangle, are now groundmass, but many grains are without large found only in high mountains of the southwest­ Burns Tuff. Petrographically these rocks may be described as containing phenocrysts of labradorite, augite, and crystals and probably represent particles of the ern and southeastern portions. Elsewhere erosion Occurrence and description. The upper tuffs of hypersthene in a groundmass rich in labradorite, andesitic groundmass. The tuff grains are in gen­ has removed them and exposed the older formations. the Burns latite occur in the bed of North Fork orthoclase, and magnetite. Some of the flows in eral only 2 or 3 mm. in diameter, and some beds The remnant of the Potosi lavas in the south­ of Henson Creek near the eastern line of the the Silverton quadrangle are so rich in orthoclase consist of very fine particles. Bedding is nearly west corner of the quadrangle composes the upper Ouray quadrangle. They have been exposed as to belong to the latite group, but the rocks of perfect, but is not conspicuous in some exposures 1000 feet of Whitehouse Mountain and extends through the deep erosion of this stream, which the Ouray district are more distinctly andesites. owing to the uniform texture and friable condition from that point along the sharp ridge connecting has cut through the pyroxene-andesite flows, and Hypersthene is commonly decomposed, yielding of the strata. it with Potosi Peak, which lies just across the the underlying tuffs would now be seen in a narrow chlorite and serpentine. A brown glassy base is The colors of the tuffs are either brownish or boundary in the Silverton quadrangle. In the strip following the creek across the border into the present in the contact zones of most flows. For a greenish as a rule, generally of dull hues, but in southeastern portion of the quadrangle there are Lake City quadrangle were it not for the obscuring full description of the petrographic character of some places a vivid green, due to abundant chlorite, several isolated areas of Potosi rocks, as shown landslide debris from the southern slope. these rocks the reader is referred to the Silverton is strongly developed. The color of some such beds by the map. These exposures are but a few miles On the south side of North Fork of Henson Creek, folio. is visible for miles. west of the large area of Potosi lavas occurring in 1 mile above the southern line of the quadrangle, In the field the pyroxene-andesite exposures are The Henson tuff at some localities carries angular the Lake City quadrangle, the thickest section Ouray. 10 being that of Uncompahgre Peak. Similar small are everywhere the principal constituents of' the only slight textural differences exist between the rock North Fork of Bear Creek occur several dikes or areas of these lavas occur in the adjacent portion of groundmass. of dikes and that of larger bodies. In none of the small irregular masses of a facies of quartz mon­ the Silverton quadrangle. Although quartz latite, rich in andesine or lab- occurrences has strictly fresh rock been found and zonite porphyry characterized by rather large The Potosi rocks of the southeastern part of the radorite, is more common than true rhyolite, banded no chemical analysis has been made, but the com­ phenocrysts. These bodies are undoubtedly of the Ouray quadrangle represent the lower flows of felsitic masses of the latter rock, almost free from plete crystallinity of the rocks and their relatively same rock which forms a mountain summit between the series, with some fragmental beds. They have plagioclase, occur sparingly. In Coxcomb Peak simple texture have made satisfactory microscopical the forks of Bear Creek a short distance within the been intruded by magmas which also belong in some of the rhyolite is notably spherulitic, nodules determination possible in a majority of cases. Silverton quadrangle. all probability to the Potosi series, and, as many of of this character several inches in diameter having One of the principal bodies of quartz monzonite Description. The typical quartz monzonite por­ such intrusions have the texture of lava flows, with been found. porphyry is in Dexter Creek, where a large cross- phyry is a moderately coarse grained rock, light glassy or vesicular contact zones, some of them were The rock of the Wetterhorn, which may be an cutting mass rises from the stream for more than gray in color, with a greenish tinge due to the not recognized during the Ouray field work as intrusive, as already noted, is a dark-gray porphyry 1000 feet. It has been intruded partly in the presence of secondary chlorite. Phenocrysts of intrusions. They were not distinguished on the with many phenocrysts of feldspar arid biotite, few Mancos shale and partly in the San Juan forma­ feldspar are very abundant and altered ferro- map unless their crosscutting relations were plain. of which exceed 3 or 4 mm. in diameter. The tion. A thick sheet extends to the west and south magnesian minerals, hornblende and biotite, are It is now known that lavas of indistinguishable groundmass is about equal to the phenocrysts in as an offshoot from the main mass; it lies between generally present but subordinate in amount. The character occur in the upper part of the Potosi amount. Quartz and augite occur in phenocrysts, the Dakota and the base of the San Juan and seems to occurrence of quartz is somewhat variable; it may section of Uncompahgre Peak and that numerous but are not prominent. A mottled appearance is have followed the horizon of the very thin remnants be present in megascopically visible grains, although dikes, sheets, and more irregular intrusions of the given to the rock by the presence of many small of Mancos shale which were preserved on the Dakota as a rule the microscope is necessary for its deter­ same magmas occur in the upper San Juan forma­ aphanitic inclusions, most of them less than 1 inch surface at the time of the San Juan accumulation, mination. The groundmass of the rock is invari­ tion in the adjacent portion of the Lake City in diameter. These inclusions appear to have the for patches of the black shales are-seen at many ably holocrystalline and consists of a microgranular quadrangle. same mineral composition as the surrounding rock, places above or below the porphyry or included in aggregate of orthoclase and quartz with a very little The principal area where intrusive masses have but are much finer grained. The analysis of the it. This body with its sheetlike apophysis is plagioclase and minute flakes of biotite. The feld­ been mapped with the Potosi flows is about Cox­ Wetterhorn rock presented in the table of analyses typical of the others of this area. spar phenocrysts consist of a soda-lime plagioclase, comb Peak and in the high ridges southwest of (p. 12) shows it to be chemically near other rocks West of the Uncompahgre, between Oak and too much altered in most of the rock to permit an Wetterhorn Peak. The massive rock which forms of the region, some of which are intrusive, while Corbett creeks, is a still larger body; its boundaries exact determination, but probably oligoclase or the upper 800 feet of the Wetterhorn and the sharp others are Potosi lavas. are not well exposed on account of the landslide andesine. Orthoclase may also occur as pheno­ ridge north from it, and which also extends north­ The quartz latite of Wetterhorn Peak has a debris which partially covers it, but its form seems crysts, but is less abundant than plagioclase. eastward to the Matterhorn, a short distance within black vitrophyric lower contact zone on the ridge to be nearly that of a laccolith intruded in the soft Quartz appears in irregular corroded grains and the Lake City quadrangle, is perhaps an intrusive, southeast of the peak. Such glassy zones are com­ Mancos shales near the base of the San Juan. is locally very abundant, especially in the "blow­ but the occurrence in the Matterhorn of much mon in the intrusive masses of quartz latite and Local crosscutting conditions exist, especially with out" mass. breccia composed mainly of the same type throws are also exhibited in certain masses supposed to be respect to the San Juan in Oak Creek. The por­ In the majority of the rocks secondary minerals some doubt on such a conclusion. This type is not intrusive, but mapped as Potosi. One of the best phyry body of Corbett Gulch and other isolated now far exceed the original constituents. The known,elsewhere, a fact supporting the idea of its localities to study this development is on the spur patches shown by the map are undoubtedly con­ alteration of the feldspars has resulted in the devel­ intrusive character. leading eastward from the summit with an elevation nected with this mass. opment of kaolin, sericite or muscovite, and cal- Description. The lowest portion of the Potosi of 12,980 feet, just north of the trail divide between The laccolithic body of Canyon Creek, which cite. Chlorite, calcite, and rarely epidote have in the Whitehouse Mountain area consists of about North Fork of Henson Creek and Cow Creek, at extends into the Silverton quadrangle, has, like­ followed the decomposition of hornblende or bio­ 50 feet of alternating thin flows and beds of coarse the level represented as the base of the Potosi series. wise, been intruded at the base of the San Juan, tite. These changes are so widespread and so uni­ gravel-like tuff. The flows are characterized by but among strata of lower horizons than were the form in character that they can not be attributed INTRUSIVE ROCKS. marked fluidal texture on weathered surfaces and others so far described, lying for the most part solely to surface weathering, and it is believed that in thin sections. They contain many included INTRODUCTORY STATEMENT. below the La Plata sandstone. The base rests upon they are due to the action of mineralizing agents fragments of the same kind of rock, in some places Within the Ouray quadrangle occur a great many the Hermosa at the point where the body crosses that closely followed the period of intrusion and in sufficient abundance to become flow breccias. dikes, sills, laccoliths, and irregular bodies of igne­ the quadrangle boundary, but rises farther north were associated with the formation of many of the The rock is commonly pinkish in color, containing ous rocks, appearing at all horizons from that of to a horizon a little above the base of the Cutler ore deposits of the region. These matters are prominent phenocrysts of feldspar and a bronze the oldest sedimentary beds to that of the youngest formation; the upper contact is a crosscutting one discussed in greater detail in the section headed mica in a felsitic or glassy groundmass. The feld­ surface volcanics now remaining. These rocks and passes from the La Plata to and into the San "Economic geology." spars are found to be andesine and orthoclase in belong to a number of petrographic types, nearly Juan. Two sheets extend northward from the The rock of the quartz monzonite porphyry nearly equal amounts. A little quartz is also pres­ all of which correspond in chemical and mineral Canyon Creek body and after crossing Oak Creek dikes in Porphyry Basin and on each side of ent, both in phenocrysts and more abundantly in composition to certain of the effusive rocks, or lavas. become parts of the complicated system of sheets Middle Fork of Cimarron Creek is slightly dif­ association with orthoclase in the groundmass As nearly all the intrusives are known to penetrate and dikes that are exposed in the cliffs northwest ferent from that of the laccolithic bodies and sheets wherever that is crystalline. The tuffs are poorly the lavas at some point, it is concluded that in gen­ of Ouray, which can not be represented in detail near Ouray, being richer in hornblende and con­ consolidated and consist of more or less rounded eral they represent the same magmas as the surface on the present map. taining, as a rule, very little quartz. Phases occur, fragments, 1 to 6 inches in diameter, of latite and rocks, and that in many places the effusive and The body of porphyry in the gulch between however, in which the rock* differs in no way from some andesite, held in a finer matrix of the same intrusive materials of the same composition are Bridalveil and Cascade creeks known locally as that of the large Dexter Creek body; in others the materials. Individual layers range from 1 to 5 feet contemporaneous in origin. "the blowout" is so altered, as are also the sedi­ plagioclase is more calcic and augite is present. in thickness. In support of this conclusion, it is observable mentary rocks near by, that the exact character of An analysis of the rock from the main body above Above these tuffs and thin flows are a few very that the higher the horizon of intrusion the more the intrusion is somewhat uncertain. The main Porphyry Basin, which may be considered as char­ massive flows of quartz latite, having a total thick­ do the textures of the intrusive rocks become like mass appears to be crosscutting, and, though a acteristic of most of the rock, is given in the table ness of about 500 feet. The rock^is pink or purple those of the corresponding effusive varieties. This majority of its many apophyses are dikes, a few of analyses. and has abundant phenocrysts of both orthoclase similarity becomes so striking in the rocks of the penetrate the adjoining sedimentary rocks as sheets. The small bodies of quartz monzonite porphyry and plagioclase. Nearly black or dark-bronze higher horizons that certain of the intrusive masses The porphyry bodies exposed on both sides of on the divide between Cow and Bear Creeks have biotite, although not as abundant as either of the were considered to be lava flows in the field work Cow Creek in the vicinity of Ramshorn Ridge are, an unusually prominent porphyritic texture, due to feldspars, is very conspicuous megascopically on of the first season, because crosscutting relations like the others to the southwest, laccolithic, but the size of the phenocrysts. Glassy sanidine occurs account of its dark color and brilliant luster. were not clearly defined. Not all mistakes of this extremely irregular and with many crosscutting variably in crystals 1 or 2 cm. long. Biotite is con­ Above the massive flows are 400 feet of thinner kind have been corrected. These textural differ­ relations. The intrusions occurred at a, place where spicuous, but augite, which was primarily of equal flows, with tuff layers here and there and at least ences among otherwise identical rocks lead to all the pre-Telluride formations are sharply folded importance, is greatly decomposed and scarcely two thin bands of dark glassy latite near the base. regrettable inconsistencies in nomenclature which in a monocline that gives the strata a steep northerly noticeable to the naked eye. The groundmass is Quartz latite occurs in the upper and lower parts can not well be avoided under the prevailing sys­ dip and causes the Dakota and older beds to disap­ commonly very evenly granular. of the section, but the massive flows near the middle tem of naming igneous rocks. Thus, the intrusive pear beneath the Mancos shale. West of Cow LATITE. are less siliceous. A lava from the summit of porphyries of lower horizons commonly have the Creek the porphyry rests upon the inclined surface Whitehouse Mountain is one of the most distinct microgranular groundmass texture characteristic of of the Dakota and seems to have invaded a wedge- Introduction. rhyolites of the series, as it contains only a very most deep-seated laccolithic rocks, and are described shaped mass of Mancos shale which lay between The rocks called latite contain both potash little plagioclase, but is rich in orthoclase and as monzonite porphyry, whereas rocks of the same the Dakota and the Telluride conglomerate. In feldspar (orthoclase) and the soda-lime series quartz with a little biotite. In many cases the magmas, at higher levels and with glassy contact the process the shales, which are highly contorted (plagioclase) as quantitatively the most important groundmass is full of dark trichites which mark zones and microlitic, perhaps fluidal, groundmasses, and indurated near the contacts, appear to have constituents, with variable amounts of quartz and the fluidal texture. Spherulitic crystallization is are termed latite, corresponding to the surface lavas been forced aside and the Telluride elevated V»,nearly ' ' one or more of the dark silicates biotite, horn­ not uncommon. they so closely resemble. 400 feet. At about the point where Cow Creek blende, augite, and a rhombic pyroxene. They The lavas of the southeastern section, belonging The principal groups of intrusive rocks will be now flows the porphyry cut across the Dakota and also exhibit, especially in the groundmass, the undoubtedly to the Potosi series, are rather fine­ described under the headings used on the geological east of the stream lies in the McElmo formation. textures of surface lavas rather than of intrusives grained pink or gray quartz latites with a variable map. A second body, smaller than the first, is exposed at considerable depth, such as the monzonite por­ development of biotite, hornblende, augite, and a short distance to the north. It has been intruded phyry already described. Among the many intru­ QUARTZ MOSTZONITE PORPHYRY. hypersthene, the biotite being most common. The entirely in the Mancos shale and its contacts are sive masses in the San Juan tuff-agglomerate and dark constituents are everywhere subordinate. The Occurrence and distribution. The rocks called nearly parallel to those of the larger body. more recent surface volcanics the great majority feldspar phenocrysts, among which plagioclase is quartz monzonite porphyry occur for the most part Although the two have not been observed to join, have the composition and texture of quartz latite, far more abundant than orthoclase, are as a rule either in the sedimentary formations below the vol­ the smaller body is undoubtedly connected with but the textural distinction between these rocks greatly broken. Quartz is not uniformly developed canic rocks or at the base of the volcanics. A few the larger mass at a short distance below the sur­ and quartz monzonite porphyry can not be sharply in large crystals. The groundmass is in the main are dikes of higher levels in the San Juan forma­ face and serves to emphasize the very irregular drawn in all cases, both because there are many cryptocrystalline or partly glassy, with obscure tion. Nearly all the numerous dikes and sheets form of the intrusion as a whole. bodies of intermediate position, and because the spherulitic growth as a common feature. Much of near Ouray and the larger bodies of Canyon Creek, A crosscutting body, of quartz monzonite por­ texture is not exclusively dependent on the hori­ it is clouded by ferritic specks and trichites, and Dexter Creek, and "the blowout," together with phyry occurs on the eastern border of the quadrangle zon of intrusion. Hence some of the rocks called generally it exhibits fluidal texture. From the the laccolithic body of Cow Creek near Rams- in Porphyry Basin, a tributary of Middle Fork of latite might in themselves be as well called mon­ analyses of a few Potosi rocks and the composition horn Ridge, consist of quartz monzonite porphyry. Cimarron Creek. The greater part of this body zonite porphyry, but from association and details determinable in the more coarsely crystalline Remarkably little variation in the composition of the lies farther east in the Lake City quadrangle. of composition they belong in one of the latite specimens, it is plain that quartz and orthoclase rocks from different localities has been noted and On the high divide between Cow Creek and the groups and are so included. 11

The microscopical study of the various latites Cimarroii Creek Latite. spar phenocrysts are mainly mere specks in the Residual grains of quartz phenocrysts surrounded shows several groups particularly characterized by The name Cimarron Creek latite is applied to a predominant felsitic groundmass. Numerous small by an aureole of oriented groundmass quartz are their dark constituents and in many places by per­ variety of intrusive rock especially common in the rounded and embayed quartz crystals are visible on characteristic. The groundmass is mainly a patchy sistent textural features. As these groups repre­ drainage of Cimarron Creek in the Ouray and Lake close scrutiny. micropoikilitic intergrowth of quartz and orthoclase. sent beyond much question contemporaneous City quadrangles. The rock is petrographically a Under the microscope it is apparent that horn­ Latite at Lake Lenore. A peculiar rock occurs injections of a given magma their distinction quartz-pyroxene latite. blende was once present, but has been resorbed or in two very irregular crosscutting bodies in the on the map seems desirable. They probably all Occurrence and distribution. The Cimarron decomposed, only its outlines, indicated by a few sedimentary beds near the mouth of Dexter^Creek. correspond to lavas of the Potosi volcanic series. Creek latite occurs in many' dikes, sills, and irreg­ opaque grains, remaining. Plagioclase, principally One, directly west of Lake Lenore, is nearly cir­ In order to avoid the long terms necessary to ular intrusive masses, for the most part in the San oligoclase, is abundant in broken crystals, much cular in form and is not more than 100 yards in express the distinctive mineral characters of these Juan tuff-agglomerate, but is also present in the exceeding the orthoclase in amount. The ground- diameter. North of the lake is a smaller mass, groups they are designated in the legend and in Potosi volcanic series, in which it is not everywhere mass is a cryptocrystalline or fine microcrystalline, which, although no actual connection has been the descriptions by local names, emphasizing also easily distinguishable from flows. Some of the irregularly granular mixture of orthoclase and found, is believed to be a horizontal or sheetlike the localities where they are best developed. sheets or sills are traceable for several miles at quartz. The amount of plagioclase is less than apophysis of the crosscutting body. The contacts approximately the same horizon, and there are in in the other intrusive latite and the rock is of the main mass with the sandstones of the Cutler Difficulty Creek Latite. some localities a number of bodies, now seemingly unquestionably identical with or very close to formation, which are exposed at several points, are Occurrence. One of the most clearly defined disconnected, that undoubtedly belong to the same the lower Potosi flows designated quartz latite. It very irregular and nearly vertical. The rock is varieties of porphyritic latite occurs at the upper intrusion. The masses vary from but a few feet to is believed that this intrusive type is contempo­ dark and dense, and has a fluidal banding that is forks of Cow Creek, where the main stream is hundreds of feet in vertical dimensions. They are raneous with some of the Potosi lavas not far above everywhere parallel to the contact line. Inclu­ joined by Wildhorse Creek and Difficulty Creek. most numerous in the Cimarron Creek drainage, the base of that series. sions of sedimentary rocks and of minute grains The largest mass is cut by the wild canyon of Dif­ but they also occur on the headwaters of Cow derived from them are numerous. Another larger Other Varieties of Latite. ficulty Creek and the variety may be called after Creek, and on the divides toward North Fork of body of this rock is intruded in the McElmo for­ that locality. This mass has crosscutting relations Henson Creek. Among the numerous intrusive latites of the mation on the north side of Dexter Creek at an to the south, yet its base is conformable, approx­ Description. The most common phase of the southeastern section of the quadrangle are several elevation of about 8500 feet and about 300 feet imately, with the bedding of the San Juan tuff, Cimarron Creek latite is a dark-gray, almost varieties notably different from the three already above the stream; this body is in the form of an giving the mass the form of an asymmetric lacco­ aphanitic rock, with many minute soda-lime feld­ described, but not deemed worthy of distinction on irregular branching dike. lith. Its lateral extension is small and the visible spar (plagioclase) phenocrysts, 2 mm. or less in the map. They are allied rocks, presumably equiv­ The rock composing these crosscutting masses is disturbance of the bedded volcanic rocks which it diameter. Labradorite and andesine are the prev­ alent to some Potosi flows. The principal varieties oft too dense a texture and too greatly decomposed intrudes does not correspond in degree to the size alent kinds of plagioclase. The ferromagnesian are the following: to permit an altogether satisfactory microscopical of the mass. In the cliffs west of Difficulty Creek silicates are megascopically hardly visible except Hornblende latite. This rock, readily distin­ determination. It has been grouped, for purposes the Potosi tuffs are clearly distorted by the intru­ where biotite or hornblende is added to the actually guishable under the microscope, forms the notable of mapping, with the latites, but the relative sion. more abundant pyroxenes, augite and hypersthene. sheet in the east wall of The Amphitheater near amounts of quartz, potash, and soda-lime feld­ Another mass occurs on the point of the ridge The hypersthene is very commonly decomposed to Ouray, reappears at the same level (about 11,500 spar present are very -difficult to determine. No north of Wetterhorn Creek at a level in the San chlorite or serpentine; the augite may be entirely feet) in the valley of Wildhorse Creek, and near fresh ferromagnesian minerals remain. Juan tuff-agglomerate about 2000 feet above the fresh, but where decomposed yields calcite as its the head of Cow Creek east of Wildhorse Peak. base of the main Difficulty Creek body. The beds chief product. A few small bodies occur on either side of Cow INTRUSIVE RHYOLITE. which once covered it have been removed by ero­ The groundmass of these rocks is either glassy, Creek and one north of North Fork of Henson Occurrence. An irregular crosscutting body of sion, so that it is not possible to see the disturbance at least in part, or cryptocrystalline. Where most Creek. These masses occur so nearly at one gen­ rhyolite porphyry of considerable size and having which took place as a result of the intrusion. The clearly resolvable into its constituents it is found eral level in the San Juan formation, and in one a still greater extension farther east in the Lake lower surface of the body is very uneven and rests to consist of orthoclase and quartz with a small east-west zone, that they may be plausibly regarded City quadrangle occurs on the north side of North upon extremely coarse and poorly consolidated amount of augite, magnetite, and apatite. The as parts of one main intrusion with a few minor Fork of Henson Creek. It has been intruded agglomerates. A pronounced columnar structure dark color of the rock is due to the fine magnetite offshoots. partly in the Silverton andesites and tuffs and with the joint planes uniformly at right angles to dust of the groundmass, but the total of this min­ The rock is a dark-gray or reddish, very fine partly in the Potosi volcanic series and sends out a the contact is a striking feature of this body. It eral is less than 5 per cent, most of it being in grained porphyry with abundant plagioclase and large number of arms that branch and intrude the is connected with the lower mass by a dike. m icrophenocry sts. hornblende phenocrysts in a dense groundmass. older rocks in an extremely complicated manner. To the west of the Difficulty Creek mass a body The plagioclase of the rock is apparently all The abundant hornblende is in most places wholly Description. The rock is nearly white and very of the same rock caps the mountain whose summit developed in phenocrysts, but none of the ortho­ altered by resorption, its place being occupied by fine grained, containing numerous large quartz is at 12,445 feet. Its base lies at about 12,000 feet. clase has that development. The habit of the rock the well-known mixture of ferritic particles, quartz, phenocrysts and small flakes of biotite. The As dikes of the latite cut this body it is probably is thus strongly andesitic, but as the chemical orthoclase, etc. But in some specimens the horn­ groundmass consists of finely granular quartz and to be regarded as an earlier intrusion of the same analysis (given in the table, p. 12) of a typical blende is very fresh. The groundmass is rich in orthoclase, with a very little plagioclase. The magma. rock shows, there is nearly or quite half as much orthoclase and quartz. phenocrysts are of orthoclase and quartz, with Still another mass of this latite occurs on the potash feldspar as plagioclase, with 10 or 15 per Biotite latite. The variety of latite which forms small amounts of biotite and magnetite. The ridge just south of the principal body. Its lower cent of free silica, so that the rock is a quartz latite the summit of Wildhorse Peak and occurs also in rock is intermediate in texture between a rhyolite contact is not accurately mapped, as it occurs in rather than a quartz andesite or dacite. In the several other masses is a quartz-biotite latite of dif­ and a very fine grained granite porphyry. inaccessible cliffs. larger bodies the lower contact zone is generally ferent habit from the otherwise similar rock of As shown by the map a considerable number of in part a glassy flow breccia with dark vitrophyre Difficulty Creek. The principal body cuts the INTRUSIVE ANDESITE. dikes of the Difficulty Creek latite occur in the fragments in a yellowish or reddish matrix of prac­ older rocks almost on the line between the San A number of dikes and small intrusive bodies immediate vicinity of the main mass and radiate tically the same material. In these vitreous parts Juan tuff-agglomerate and the Henson tuff. It have been distinguished on the map as andesite. more or less markedly from that center. These the hypersthene is uniformly fresh and nearly exhibits a steeply northward-dipping fluidal band­ Some of them are not far in composition and tex­ are not all exactly of the type of the larger masses equal to augite in amount. The development of ing, which makes it resemble a tilted series of lava ture from rocks that have been called latite, but as but are very closely related. biotite and hornblende is extremely variable, but flows. The other prominent bodies form two east- a group they are characterized by a notable decrease Description. There is very little textural or nearly all masses have a small amount of reddish- west ridges of jagged crest line situated northeast in the amount of orthoclase and quartz. With mineral variation to be observed in the rock of brown biotite. of Wildhorse Peak. They rise from a mass of this decrease plagioclase and augite enter into the different parts of the large bodies, or of the dikes. landslide debris which covers the contacts effectually. groundmass. Several somewhat different varieties American Flat Latite. The color is very light gray or nearly white, in This rock is reddish, pink, or dark gray in color, might be given special description, but their sub­ many places with a pink or light-purplish tinge. Occurrence. A variety of latite which it has with prominent phenocrysts of plagioclase and bio­ ordinate development does not warrant full treat­ Phenocrysts of feldspar and biotite lie in a strongly been thought best to distinguish from the others tite, that nearly equal in amount a very dense ment in this place. predominant felsitic groundmass. The feldspar on the map occurs as an intrusion in the Henson groundmass. The groundmass is extremely fine Occurrence and distribution. The largest mass phenocrysts consist almost entirely of a sodic plagi- tuff, its largest known body being that of Ameri­ grained and apparently granular in much of the included here occurs as an intrusive sheet in Man- oclase, most commonly oligoclase, and a very little can Flat, on the Silverton-Ouray quadrangle line. rock, but the contact zone is in places a black vit­ cos shale in Nate Creek. It forms bluffs on the orthoclase. Biotite and quartz are the only other This rock so closely resembles some of the lower rophyre. The plagioclase is mainly andesine or north side of the stream. A smaller body occurs phenocrysts universally present. Hornblende occurs flo^s of the Potosi volcanic series that it was taken labradorite and its crystals are perfect in shape, few on the north side of Nate Creek at the mouth ^f with biotite in the rock of the 12,445-foot summit as the basal flow of that series in mapping the Sil- of them being broken as is common in some other Devils Canyon. This body probably intrudes San west of the Cow Creek forks, and is common in verton area, and is so represented in the Silverton intrusives of the region, notably in the adjacent Juan tuff-agglomerate, but the thick mantle of land­ several of the dikes of this region. folio. Its intrusive character is now evident, being American Flat latite. slide debris all along the base of Cimarron Ridge The groundmass of the rock from the large body particularly distinct in the Lake City quadrangle Although no analysis has been made of the obscures the relations except in the canyon. is a very evenly microgranular aggregate of ortho­ near Uncompahgre Peak. The. rock is confined to Wildhorse Peak rock, there can be no doubt that Andesitic dikes are particularly numerous in two clase and quartz containing a small amount of plagi- the zone of the Henson tuff and fragments of nearly the groundmass is as rich in quartz and orthoclase localities. In Dike Ridge, the divide between the oclase and magnetite dust. The smaller masses and identical rock are contained in that clastic formation, as the more highly crystalline rocks. heads of Middle and West forks of Cimarron Creek, a part of the large one have an equally fine grained as has been stated. Augite latite. A rock of distinctive character north of Coxcomb Peak, they rib the abrupt slopes groundmass, but the texture is trachytic and the Only three areas of this rock are represented on forms an extensive intrusive sheet which occurs in of San Juan and Potosi rocks by projecting as high rock contains, in some places, a little glass. Here the map. Of these the small tongue at the extreme the valley of Wildhorse Creek at the northwest walls visible for miles. Fig. 8 of the illustration and there orthoclase and quartz occur in micro- head of Bear Creek is directly connected with the base of Wildhorse Peak and extends westward into sheet shows some of the dikes on the eastern face graphic intergrowth. The varietal character of the sheet of American Flat. Another small body Difficulty Creek and eastward into the head of Cow of Dike Ridge. rock is expressed by the term quartz-biotite latite. appears in contact with Henson tuff and rhyolite Creek. It comes into contact with both the Diffi­ On the ridge south of Porphyry Basin several The analysis of this rock given in the table on porphyry close to the eastern boundary of the culty Creek latite and the hornblende latite. It small dikes appear with trends nearly parallel to page 12 was made from material characteristic of quadrangle, north of North Fork of Henson was not possible to follow this mass connectedly some of those in Dike Ridge, but their continuity nearly all the observed occurrences. Banded struc­ Creek. from one gulch to another in the inaccessible cliffs, was not established. ture is common in contact zones. In some dikes Description. The American Flat latite is a but it seemed from favorable view points to have Another locality of notable andesite dikes is the the grain of the groundmass is comparable with light to dark gray porphyritic rock notable for the extent represented by the map. ridge west of Difficulty Creek. A radiate arrange­ that of the monzonite porphyries already described, its glistening biotite leaves, which by their subpar- This rock is a pink or gray felsitic porphyry ment is suggested by the map, but as by no means but it seems best to call all the dikes connected allel arrangement express a fluidal texture that very rich in minute phenocrysts of plagioclase and all dikes are represented this may be deceptive. with this center of eruption by the same name. appears also in the fracture of the rock. The feld­ augite, with rather sporadic hornblende and biotite. Some of these dikes extend for miles and in many Oimiy, 12 places project above the crumbling San Juan as Chemical analyses of rocks of the Ouray quadrangle. have been disturbed but little since the time of monocline begins, and thence northward the only high walls. Fig. 5 (illustration sheet) shows one their deposition. In the Telluride and Silverton beds of the Paleozoic section exposed are those of Constituent. l. 2. 3. 4. of these walls on the divide at the head of Dexter quadrangles the conglomerate has a slight but the Cutler formation. Just above the point where Creek. Si0 2 _-__ . ______68.81 61.36 60.69 59.83 definite inclination to the east. Although not the valley begins to widen near the mouth of In other parts of the quadrangle single dikes of A1S 03 ______._....___ 15.54 16.36 15.90 15.86 easily recognized on account of the discontinuity Corbett Creek the Cutler beds return to a very andesite occur, but many of these are not mapped, Fe,O3 . 1.78 3.59 4.52 4.07 of the beds, the same tilting is shown in the Ouray moderate northerly or northwesterly dip and FeO . 1.45 1.72 2.12 the time required to trace them being more than .80 quadrangle, the inclination here being to the north­ assume nearly conformable relations with the MgO - ____ - .52 1.75 1.93 '2.73 could be given to it. CaO ______2.43 3.59 5.23 4.34 east, overlying Dolores. % East of the river precisely Description. The andesites here discussed vary Na»O 4.24 4.04 3.55 3.00 the same structure (section B-B) may be made STRUCTURE OF THE MESOZOIC FORMATIONS. like the latites in the development of their dark K 3 O ------_ _ 4.07 3.64 3.22 3.55 out, except that in places in the region between silicates augite, hypersthene, hornblende, and H.O -______-___- .50 1.34 .93 1.09 Generally throughout the southern and western Cascade and Bridalveil creeks it is much pbscured biotite. On the whole, the andesites are richer H B Q+ ,___ __-_ .78 1.56 .96 2.04 San Juan Mountains all the Mesozoic beds appear by the metamorphism of the rocks; local upturn­ TiO,'-. --_ _. ------.28 .51 .73 .70 in these ferromagnesian minerals than the latites, to be conformable from the base of the Dolores ing has also taken place in the vicinity of the ZrO s Trace. Trace. Trace. yet none of them is a strongly basic rock. All C0 2 ______.48 .64 .27 .59 upward. These relations are well shown in the large body of intrusive porphyry south of Bridal- carry some orthoclase in the groundmass and P O .13 .36 .31 .31 Uncompahgre Valley, where the Cutler beds as veil Creek. From the point of Cascade Mountain quartz is its common companion, but in relatively MJ.O-.- _ __ _ -- .12 .07 .13 .06 well as the Mesozoic formations have a low north­ southeastward to The Amphitheater the influence small amount. BaO__ --_-- - .13 .12 .10 .08 westerly dip. Just north of Cutler Creek they are of the Canyon Creek monocline may be observed The Nate Creek sheet is a brown or yellowish- SrO .04 .12 .03 .02 affected by a gentle monoclinal fold which carries in the 10°-15° NW. dip of the Hermosa beds. gray hornblende andesite with a little biotite. 100. 65 100. 50 100. 22 100. 39 all but the Mancos successively below the level of This structure accounts for a slightly greater ele­ Most of the Dike Ridge rocks are hornblende- the Uncompahgre flood plain; this structure is vation of certain horizons of the Hermosa on the biotite andesites, but one at least is an augite 1. Quartz-biotite latite (Difficulty Creek latite). Northern shown in section A-A. To the east; in Cow Creek, east of Uncompahgre River just north of Cascade part of intrusive mass between Difficulty and Cow creeks. andesite. Both augite and hypersthene occur in Fine-grained light-gray porphyry. Has many small pheno- where the sediments are again exposed beneath the Creek. some of the dikes west of Difficulty Creek. crysts of oligoclase and andesine, with some biotite and large volcanics, the same monocline, slightly sharper, Summarized briefly, the Paleozoic rocks near magnetite grains, in a groundmass of orthoclase and quartz. may be observed. Elsewhere in the quadrangle a Ouray have a gentle northwesterly inclination DIABASE DIKES. A little calcite of secondary origin. Traces of apatite. 2. Quartz monzonite porphyry. Large dike on quadrangle moderate northwesterly dip characterizes the Meso­ that is complicated by two monoclinal folds Occurrence. Dikes of diabase occur at two dif­ border, east side of Porphyry Basin. Grray or slightly pinkish zoic rocks except in the extreme north, where the whose axes lie at an angle of 45° to one another. porphyry. Contains as phenocrysts many small white pla- ferent localities in the Ouray quadrangle. One gioclase crystals (oligoclase and andesine), biotite, a small Dakota has a northeasterly inclination. Another instance of the structure resulting from cuts quartzites of the Uncompahgre formation, but amount of green hornblende, and a little quartz, all in a A discordance at the base of the La Plata is the pre-Triassic movements is found in the occur­ granular groundmass of quartz and orthoclase. Magnetite no younger rocks; the other visibly intrudes the and apatite as accessories. A little secondary calcite. known to exist to the northeast and actual evidence rence of Ouray limestone on Cow Creek. Although Cutler, Dolores, and La Plata formations, and may 3. Quartz-pyroxene latite (Cimarron Creek latite). Arm of of it is to be seen in this quadrangle along Cow completely surrounded by a thick cover of land­ have reached still higher horizons, but evidence for intrusive sheet in San Juan formation on east side of Pinnacle Creek, where the La Plata rests, not upon the slide debris, the relations of this exposure to Ridge below remnant of Potosi volcanic series. A fine-grained this has been lost through erosion. The dike in dark-gray rock of andesitic habit, with numerous small meg­ Dolores, but upon the Cutler formation. Although near-by outcrops of the Cutler and La Plata for­ the Algonkian area has been traced from the fault ascopic crystals of andesine and labradorite. Dark silicates, this unconformity is an indication of deformation mations seem to show that it is part of a block mainly augite (fresh) and hypersthene (altered), with a lit­ at the box canyon of Canyon Creek, where it dis­ tle hornblende and biotite. Groundmass cryptocrystalline, preceding the La Plata and following the Dolores, faulted upward during the pre-Triassic movements. appears beneath the Paleozoic sediments, in a direc­ brown from disseminated ferritic specks, and in most dis­ the movements, so far as they are recorded within the Simple folding can not account for the position of tion south of east for about a mile and a half, not tinctly crystalline spots clearly made up chiefly of orthoclase area of the Ouray quadrangle, were relatively slight the Ouray, because the beds of the Cutler forma­ and quartz. Magnetite in fine dust. quite to the point where the quartzites are covered 4. Quartz-biotite latite (Potosi volcanic series). Point of as compared with those which affected all the Mes­ tion, exposed less than 200 feet above the Ouray by the San Juan tuff. The thickness of the dike elevation 13,400 feet on divide l£ miles north of Wetterhorn ozoic formations, and consisted in a very moderate to the east, are nearly horizontal, whereas the Peak. Dark-gray medium-grained porphyry containing many averages 50 feet, but is variable from place to place, white andesine or labradorite crystals and biotite tablets, westward tilting, starting at some point between Ouray strata dip 45° SW. To the west and 200 and near the town reservoir it is fully 150 feet. with a few crystals of orthoclase and quartz in an aphanitic Cow Creek and the Uncompahgre and now obscured feet above the Ouray the La Plata is exposed with The dike cutting the Mesozoic rocks is exposed on groundmass. Augite is also an important constituent where by the volcanics. Section C-C brings out these a southwest dip of 30°. The faults supposed to the rock is fresh. The strongly fluidal groundmass varies the east side of the Uncompahgre Valley nearly from a coffee-brown glass in contact zones to a microcrystal- relations and also shows the unconformity at the bound this mass must be regarded as older than opposite the mouth of Coal Creek. It has a nearly line aggregate of quartz and orthoclase stained by ferritic base of the Telluride. the La Plata and probably antedated the Dolores, north-south direction, is from 6 to 8 feet wide, and particles. although the absence of this formation here, due to STRUCTURE OF THE PALEOZOIC FORMATIONS. is exposed for a little over a quarter of a mile from The classification of these rocks by the quantita­ the La Plata unconformity, makes it impossible to 100 to 300 feet above the bottom of the valley. tive system is represented concisely in the table The region of most pronounced deformation in assert this definitely. No movements comparable Description. The diabase from the Algonkian below: the Ouray quadrangle is that bordering Canyon in magnitude to this faulting have been recognized Creek and Uncompahgre River south of Corbett in the La Plata in other parts of this region, and area is dark greenish brown, massive, and of a Quantitative classification of Ouray rocks. medium texture. Long, thin crystals of labrador- Creek. Northward from the exposures of the it seems justifiable to correlate the Cow Creek ite are abundant and easily recognized megascop- Rang. Ouray and Hermosa that lie west of the Uncom­ faulting with the period at which the folding 4.OrderQuardo- SubrangSodi-3. ically and with them augite is associated. In potassic. pahgre at the southern border of the quadrangle, occurred near Ouray. felic. "3 "3 . _o the Paleozoic rocks have a general northerly or The movements recorded by the Mesozoic rocks addition to these minerals the microscope shows a 5 is o OJ .* 8 rjj 2« that magnetite and considerable accessory apatite _e3 GM 3 northwesterly dip that is evident from a number of also affected the Paleozoic formations, but they can are present. O si 05 points of view near Ouray. This structure is prin­ not be recognized in the region of more complex The rock from the dike on the east side of the 1. Quartz- biotite latite (in­ cipally of pre-Triassic i. e., pre-Dolores age, but deformation, although evident in other places trusive), Difficulty Uncompahgre Valley is black and has a dense, Creek=toscanose _____ I ( x X X was accentuated by later uplifts of the same gen­ where apparently conformable relations exist even texture in which individual minerals can not 2. Quartz monzonite por­ eral character. Minor structures of a less simple between the Paleozoic and Mesozoic rocks. phyry =toscanose _ ____ I X X be distinguished. Microscopically this rock is seen X nature, also pre-Triassic, occur at two different 3. Quartz-pyroxene latite ALGONKIAN STRUCTURE. to possess the characteristic ophitic texture of a dia­ (intrusive) = harzose localities and call for special description. The near amiatose __ _ II ( x ; X base and the usual minerals labradorite, augite, first to be noted is that best displayed by the So far as the exposures within the Ouray quad­ 4. Quartz-biotite latite= and magnetite, the last occurring in irregular grains harzose near amiatose_ II X X X older Paleozoic rocks south of Oak Creek. When rangle are concerned the quartzites and slates of the and skeleton crystals. The augite has suffered the Ouray limestone is followed northward on the Uncompahgre formation have a simple structure. alteration to chlorite and calcite. STEUCTUEE. bench above the Uncompahgre, it is found to have From the southern boundary of the quadrangle a moderate inclination slightly north of west, until, northward to the fault .near the mouth of Canyon CHEMICAL COMPOSITION OF THE IGNEOUS ROCKS. INTRODUCTION. beyond the small east-west fault about one-half Creek, where the exposures end against Paleozoic The descriptions of the igneous rocks, both The structure of the sedimentary and bedded mile north of the quadrangle boundary the dip rocks, the beds have a dip of 45° to 65° in a direc­ intrusive and effusive, show them to be closely volcanic rocks of the Ouray quadrangle is relatively becomes nearly northwest and increases to 25° or tion west of north on the west side of Uncompahgre related in chemical composition. .To illustrate this simple throughout the greater part of the area, but 30°, carrying the Ouray limestone to the level of River, and north or slightly east of north east of fact more fully, and for purposes of comparison in the vicinity of the town of Ouray more complex Canyon Creek and representing a drop of nearly the river. If we assume, as seems probable, that with rocks from other parts of the San Juan, a few structures are found, and the movements that they 1000 feet in a mile. To disregard, for the moment, the upper limits of the formation are to the north, of the freshest and most important types have been represent have an important bearing on the stra­ the northwest-southeast fault crossing Canyon Creek the structure of the beds is that of an anticline analyzed by George Steiger in the chemical labora­ tigraphy of the whole San Juan region. Five a short distance above its mouth, the northwest pitching steeply to the north. Further evidence tory of the Geological Survey, with the results periods may be recognized during which deforma­ inclination of the beds continues, with slight vari­ for this is found in the southward extension of the given in the following table. tion took place, affecting all the beds to a greater ations in the angle of dip, to Oak Creek, beyond beds in the Silverton quadrangle, where their atti­ The analyses demonstrate the close relationship or less degree, and the resulting structures may be which the dips swing more to the north and are tude appears to be that of an elongated domal uplift. between these rocks that has been brought out in described best by beginning with the youngest and of a lower angle. Southwest of Oak Creek the Minor irregularities or plications may occur, espe­ the descriptions. They show with special clear­ simplest. Hermosa beds have a northwest dip of decreasing cially in the slates, but great uniformity prevails. ness the contact presence of a large amount of The broad structural features of the San Juan angle to the southern boundary of the quadrangle. Section B-B shows the general attitude of the beds orthoclase and more or less quartz in the extremely Mountains have been outlined in the introduction The structure north of Ouray is admirably and the relation of younger formations to them. fine grained groundmass and justify the reference of this folio, and the features analyzed in greater shown in the rocky walls that border the Uncom­ The northerly dip of the Paleozoic rocks, due to of the rocks to the latite rather than to the andesite detail at this point are the elements which taken pahgre almost as far as the mouth of Corbett Creek structures previously described, shows that a part group. together make up the composite domal structure of (fig. 9). West of the river the Hermosa formation at least of the inclination of the Uncompahgre On comparison of the analyses with those pub­ the San Juan region. and a thin remnant of the Cutler beds preserved strata belongs to the later period of deformation, lished in the Silverton folio it is evident that all In this section the structure only is described, beneath the unconformable Dolores have a dip of and still younger movements, recorded elsewhere the Ouray rocks are intermediate between certain its stratigraphical significance being discussed 10° or 15° N., which continues for about one-half in the region, have undoubtedly exerted some more highly siliceous and alkalic rocks, the rhyo- under the heading "Historical geology." mile northward, to a point where the beds, influ­ influence on the Algonkian, but the results are lites of the Potosi volcanic series and of some enced by a sharp monoclinal fold, with an axis obscured by the greater complexity of the older STRUCTURE OF THE TELLURIDE CONGLOMERATE known intrusions, and the pyroxene andesites of slightly north of west, dip to the north at an angle structures. AND THE BEDDED VOLCANIC ROCKS. the Silverton volcanic series. For a more detailed of more than 35°. (See structure section A-A.) As has been pointed out in describing the forma­ discussion of the chemical relations of the magmas Except where affected by faults, which are This dip is sufficient to carry the Hermosa beds tion, the metamorphism of the Uncompahgre strata of the San Juan region the reader is referred to the described in a later paragraph, the Telluride con­ below the level of the valley withjn a quarter of a is to be ascribed to regional dynamic processes and Silverton folio. glomerate and the bedded volcanic rocks above it mile of the point where the south limb of the not to contact relations with granitic masses. 13

exposed, are modified by numerous dikes that FAULTS. are well suited to purposes o"f description. The phosed, as indicated by the brilliant red and yellow upper Uncompahgre Valley and the region of low coloring. stand out as walls in many localities and by the In only two areas in the quadrangle has faulting relief in the northwestern portion of the quadran­ In the southwestern portion of the quadrangle ledges or other projections resulting where intrusive occurred to any marked degree. One of these is gle constitute two of these divisions and in them the San Juan formation has its greatest known sheets or irregular masses of latite cut the San in the vicinity of Ouray and the other in the the sedimentary formations are those which call development in thickness, more than 3000 feet Juan. Landslides and rock streams also modify northwest corner of the quadrangle, examples of for special consideration. The two other areas are being present in the walls of The Amphitheater. the valley walls, but in the opposite sense, tending both being shown in section A-A. The faulting the upper part of Cow Creek valley and the region The main mass of Whitehouse Mountain is also to lessen the angularity and produce gentler slopes. in the vicinity of Ouray is younger than the vol­ along the eastern border of the quadrangle, includ­ composed of the San Juan, the cliffs of the upper The extreme heads of Cow and Wildhorse creeks canic rocks. The most pronounced fault of this ing North Fork of Henson Creek and Cimarron 1000 alone being due to the massive flows of the have cut back across the boundary of the San Juan region is the one which extends from the southern Ridge. These last two areas are occupied almost Potosi volcanic series with a thin band of Silver- into the soft Henson tuff and have there excavated side of The Amphitheater northwestward to Oak exclusively by volcanic rocks. ton andesite between them and the San Juan. basins of gentle relief contrasting with the gorges Creek. The downthrow is to the north and at the A description of the geologic features of the below. The divide at the head of Cow Creek con­ point where it crosses Canyon Creek amounts to UPPER UNCOMPAHGRE VALLEY. southwestern region would be incomplete without tains remnants of the Potosi volcanic series. Wet­ about. 300 feet. As shown by the geological map The geology in the vicinity of Ouray is more a reference to the accumulations of landslide debris terhorn Peak, the highest summit of the quadrangle its eastern extension is lost in the San Juan tuff. complex than that of any other part of the quad­ that cover about one-fifth of this area. The huge (14,020 feet), is composed of an unusually massive To the west the fault is represented as dying out in rangle, and although the structural and strati- piles of debris resting upon the floor of The rock referred to that series. Beyond the divide, the McElmo shales on the south side of Oak Creek; graphic features are the most important, nearly all Amphitheater are particularly impressive, possibly in the extreme southeast corner of the quadrangle, it is clearly seen to dislocate the La Plata formation the various elements characteristic of the geology because the houses of the town of Ouray, lying at is the head of North Fork of Henson Creek, which and sheets of porphyry, but the Dakota is appar­ of the other areas are present. the foot of the slide, supply a scale with which the is carved deep in the lavas and tuffs of the Silver- ently unaffected. A northwest fault, branching to Uncompahgre River enters the quadrangle mass of debris may be compared. Other note­ ton series, revealing details of geology quite unlike the southeast, occurs on the north side of Oak Creek. through a canyon whose walls are composed of worthy landslide areas are in Canyon Creek and those of the remainder of the quadrangle. It is of apparently the same age as the longer fault the quartzites and slates of the Uncompahgre for­ Corbett Creek, aiid the - greater part of the ridge Between Oban Creek and Ramshorn Ridge, to the south, since it also causes a break in the por­ mation. The quartzites extend upward for nearly between Coal Creek and Uncompahgre River is Cow Creek flows through a canyon whose lower phyry sheets. The older Paleozoic rocks lying on 2000 feet above the river and their upturned edges covered with landslide materials. walls are composed of the bright-red beds of the the bench west of Uncompahgre River have been are covered unconformably by 2000 feet or more Cutler formation. On the west side of the canyon NORTHWESTERN AREA. dropped 150 feet to the north by a fault having a of the San Juan formation. West of the canyon the Cutler is covered unconformably by the La direction slightly north of east, which disappears the oldest Paleozoic rocks occupy a bench about The northwestern part of the quadrangle, bounded Plata formation, the Dolores and an unknown within a short distance on passing into the Hermosa 1000 feet above the river; the San Juan and a thin on the east and south by Cimarron Ridge, Rams- portion of the Cutler having been, removed by to the west and the Uncompahgre slates to the east. band of Telluride conglomerate here rest upon the horn Ridge, and Baldy Peak, is a region of com­ pre-Jurassic erosion. To the east the La Plata Although these are the only faults in this region of Hermosa formation. A northwesterly dip carries paratively low relief in which the soft black or itself is cut off by the unconformable Telluride sufficient magnitude to be represented on the map, the Molas, Ouray, and Elbert formations down to gray shales of the Mancos formation predominate. conglomerate. Between Red Creek and Oban a very large number of minor dislocations are found the level of Canyon Creek, where a fault farther The smooth, rounded slopes of the hills and the Creek landslide debris occupies the lower slopes in the neighborhood of Ouray. These are discussed lowers them, until, at Ouray, the continuing dip heavy adobe soil are characteristic of areas occu­ and bottom of the canyon except at a point about by Mr. Irving in connection with the ore deposits carries them below the floor of the valley. pied by the easily eroded clays of the Mancos one-half mile north of Oban Creek, where there in the section on economic geology. East and west of Ouray rise abrupt cliffs that formation and actual outcrops are rare. is an isolated outcrop of south westward-dipping The faults of the northwest area are of a more for 1200 feet are composed of beds of the Hermosa Through a nearly east-west fault, which clearly Ouray limestone. This is completely surrounded pronounced character. They belong to a definite formation having a northerly dip. These are manifests itself about 1 mile south of Dallas, the by landslide debris, and, although clearly a faulted system having a general northeasterly direction, sharply truncated by the unconformable Dolores Dakota and the upper part of the McElmo forma­ block, its actual contacts with the younger sedi­ and in each the strata have been elevated on the formation, as may be seen from many points of tion reappear on its northern side and are well ments are unknown and the significance of its northwest. The greatest of these faults is the one view to the west. The northerly dip of the Car­ exposed in a number of the low hills and mesas on structure can not be made out. crossing the Uncompahgre just north of the great boniferous rocks, which near the mouth of Bridal- the east bank of the Uncompahgre, although sev­ EASTERN AREA. moraine; it was noted by Peale and shown on the veil Creek develops into a sharp monoclinal fold, eral small northeasterly faults cause breaks in the Hayden map, though not given the usual special causes the Hermosa beds to disappear at a point a continuity of the beds; not far to the west a mod­ The eastern zone of the quadrangle is charac­ representation. As a result of this fault the Dakota little more than a mile north of Ouray. The erate dip carries them below the Mancos shale. terized by the long north-south ridges separating formation reappears and is exposed in the hills east Cutler formation, consisting of bright-red strata, West of Uncompahgre River the Dakota forms the the Cimarron and Uncompahgre waters and the of Dallas. Within a shorl^-distance the local east­ which, on account of the Dolores unconformity, top of a plateau which extends westward for many three parallel forks of Cimarron Creek. These erly dip carries the Dakota below the surface and disappear or are reduced to a few thin layers in the miles with a gently rolling surface. A short tongue ridges are almost wholly made up of the San Juan it is impossible to trace the fault in the soft Mancos vicinity of Cascade and Oak creeks, assumes appar­ of this plateau just enters the quadrangle at the deposits and exhibit in most typical development shale, but it is believed to be responsible for the ently conformable relations with the Dolores north western border. At a few points impure seams of the topographic detail characteristic of that forma­ difference of nearly 1000 feet that exists between of Corbett Creek and forms the lower slopes of the coal have been opened in the sides of the mesa. tion where it is comparatively little altered and the elevation of the Telluride conglomerate at the Uncompahgre for more than 4, miles. They occur about the middle of the Dakota for­ indurated. West and Middle forks of Cimarron head of Nate Creek and that of exposures of the North of Dexter Creek, on the east side of the mation and are of no commercial value. Creek flow in U-shaped glacial valleys and their same formation at the head of North Fork of Lou river, the younger sedimentary rocks appear to Somewhat more promising coal deposits of higher steep walls have been carved into a wonderful' Creek. In this region solid-rock formations are advantage, the nearly white La Plata sandstone horizon occur near the head of North Fork of Lou labyrinth of turreted and pinnacled forms, capped much obscured by landslide debris and drift and standing out in sharp contrast above the red beds Creek and extensive openings have been made in at many points by huge blocks of the agglomerate. the exact location of this fault can not be deter­ of the Triassic and Permian, and the Dakota quartz­ them about the head of Burro Creek, 2 miles north To these features are due the names applied to the mined ; its representation on the map is purely hypo­ ites forming a nearly continuous escarpment above of the quadrangle. These deposits occur near the ridges. Even the topographic map is able to thetical. West of the Ouray quadrangle the fault the gentler slopes of the McElmo shales. North base of the Mesaverde formation. express something of the character, owing to the has been traced for more than 10 miles, its general of Coal Creek an increase in the northerly dip A sheet of andesite porphyry, intrusive in the many towers represented by closed contours. position being well marked by the McElmo and carries the Dakota lower and lower until the soft Mancos shale, forms, sharp cliffs on the north side Toward the heads of the streams the number Dakota escarpment on the north side of the valley shales of the Mancos formation cover it and mark of Nate Creek and also in the Owl Creek drainage, of intrusive masses increases, modifying the detail of Dallas Creek. There is no means of accurately the beginning of the northwestern area. where it lies beneath the Telluride conglomerate. as in Cow Creek. The view shown in fig. 8 (illus­ measuring the displacement due to this fault, since The western side of the Uncompahgre Valley A body of similar rock outcrops at a few places in tration sheet) shows Dike Ridge with its precip­ the depth at which the top of the Dakota lies has comparatively few exposures of bare rock, the midst of landslide debris in the vicinity of itous cliffs ribbed by dike walls. beneath the surface on the south side of the fault much more of the surface than is represented on Devils Canyon. In the northeast corner of the quadrangle is a is unknown. The Dakota in the top of the mesa the map being covered with either landslide debris Pleistocene deposits are abundant in all parts of small area representative of the plateau country north of Dallas Creek is nearly 1000 feet above or drift. this area, the most striking being the great ter­ that is more prominent farther east in the Lake the valley, which is composed of Mancos shale, and The region bounded roughly by Corbett and minal moraine which crosses the Uncompahgre City quadrangle. The gently undulating surface a minimum displacement of 1000 feet must be Dexter creeks on the north and Oak and Cascade south of Dallas, fully one-half of it lying beyond is common where the surface flows of the Potosi allowed; this corresponds with that indicated by creeks on the south is noteworthy because of the the boundary of the quadrangle. The slopes formation have any considerable lateral extent. the Telluride conglomerate to the east. number of bodies of monzonite porphyry which as northeast of Cow Creek in the Deer Creek drain­ In this case the San Juan forms Park Mesa, but The smaller faults to the north have throws of dikes, sheets, or laccoliths, have been intruded at age are covered thickly with old drift, and west the horizon of the Potosi is just above it. The 100 or 200 feet, but are evident only where the different horizons of the sedimentary rocks and of Cow Creek near its mouth Pleistocene stream shallow drainage courses of Park Mesa are stretches Dakota sandstone is exposed. The most northerly the San Juan tuffs. The two largest bodies are gravels cap the mesas and low hills. of beautiful meadow fringed by dense growths of one crosses Uncompahgre River and is shown in those in Dexter Creek and on the end of the ridge spruce and fir. UPPER VALLEY OF COW CREEK. the cliffs on the west side; the others are lost under between Oak Creek and Corbett Creek. The The lower parts of Middle and West forks and the valley alluvium or in the Mancos shale. prominent cliff of porphyry above the American The laccolith of quartz monzonite porphyry, a Cimarron Creek itself are covered with a thick The relation between the faults and folds of the Nettie mine belongs to the Dexter Creek body. part of which forms Ramshoru Ridge, marks the mantle of drift, landslide debris, and vegetation, Ouray quadrangle is not clearly determinable. It Another body of laccolithic character is the one lower end of the canyon of Cow Creek. Above this which effectually conceal the Mancos or Mesaverde is assumed, however, that the faults of the Ouray northwest of Canyon Creek at the southern bound­ point Cow Creek and its tributaries flow in wild rocks that might reasonably be expected to appear area, like most of those in the broader San Juan ary of the quadrangle. Two sheets extending gorges, difficult of access, carved in the agglomerates beneath the volcanics near the northern border of region, are Tertiary and are not related to the folds northward from this mass cross Oak Creek and and tuffs of the San Juan formation, which weather the quadrangle. of the pre-Tertiary sediments which have been dis­ appear in the cliffs west of Ouray, where other in fantastic columns and pinnacles. Between Wild- cussed. This statement is based on observation in thin sheets and dikes are numerous. horse Creek and the west fork of Cow Creek, now HISTORICAL GEOLOGY. many localities that faults cut both volcanics and One of the most striking spots near Ouray is called Difficulty Creek, is a large body of intrusive older sediments, whereas faults clearly connected the region of highly colored and decomposed rocks, porphyry from which a number of dikes radiate; Although certain events in the geological history with the folding of pre-Tertiary strata are very rare. locally known as "the blowout," in the gulch another body of similar rock caps the ridge north of the San Juan region, outlined in the introduc­ directly south of Bridalveil Creek, where a large of Wetterhorn Creek. tion to this folio, are not recorded in the Ouray A REAL GEOLOGY. body of porphyry with numerous apophyses has The headwaters of Cow Creek and its principal quadrangle, others and especially those which INTRODUCTION. cut through various horizons of the sedimentary tributaries are in rugged basins above timber line. occurred during Mesozoic and Quaternary time, The leading geological features of the Ouray rocks from the Carboniferous to the Dakota (Cre­ The steep slopes with hoodoo forms, prevailing are illustrated better than in any other district. quadrangle belong to four distinct areas, which taceous) and all have been extensively metamor­ in most places where the San Juan formation is The earliest records preserved in the Ouray region Ouray. are concerned with late Algonkian time, and in able as a contact phenomenon of the granitic slope of the San Juan Mountains once had the Uncompahgre, and from this more elevated region order to make their meaning clear the events which intrusion. This mineral is developed in notable development they now exhibit in the Animas Val­ many streams flowed out over the lowlands to the preceded them, as shown in near-by regions, are degree at few places except in the Needle Moun­ ley, on the south side, where they aggregate nearly west and north, depositing the coarser part of their briefly summarized. In chronicling the closing tains. 12,000 feet in thickness. With the close of the loads of land waste first in the form of broad flood events of Mesozoic time recourse must be had also The erosion by which the basin later occupied Laramie proper, this region was involved in a great plains, which gradually merged into one another to the more complete history of the sedimentary by the Ignacio sea was produced can be safely orogenic movement, and the erosion began which as grades became less and distances from the high­ formations as presented in the Durango quadran­ called enormous in its amount, and yet we have no produced the pre-Telluride peneplain and the lands increased. Ultimately the streams probably gle and adjoining territory. The volcanic history means of determining its extent. It produced a adjacent area of more rugged topography. emptied into a shallow lake where fine sands and of the region differs in no essential feature from surface, now seen at the base of the Ignacio quartz­ silts were deposited. TERTIARY HISTORY. that of the Telluride and Silverton quadrangles, ite, of but very gentle undulations and unevenness Post-Telluride erosion. From the apparent con- although the influence of local conditions is appar­ as far as it has been studied. For this region the PREVOLCANIC INTERVAL. formability of the Telluride and San Juan forma­ ent in both the character and extent of the products erosive interval was closed with the subsidence that Between the cessation of Laramie sedimentation tions in the Telluride quadrangle it would appear of different eruptions. Much information relating formed the body of water in which these earliest and the deposition of the San Juan tuffs, which are that the deposition of the breccias, tuffs, etc., fol­ to Pleistocene and Recent history has been obtained known Paleozoic strata were deposited. made up of debris from the earliest lavas of the lowed closely that of the conglomerates without in the course of surveying the Ouray quadrangle, volcanic series, there was continental uplift of the any interval of erosion. It was found in the Sil­ and its bearing on the recent physiographic devel­ EVENTS OF PALEOZOIC AND MESOZOIQ TIME. whole region, permitting erosion of the entire sec­ verton quadrangle, however, that the San Juan opment of the San Juan region is briefly discussed. The history of the Ouray district during the tion of sediments. As there is no evidence of there rests upon an extremely rugged surface, Paleozoic and Mesozoic eras was in almost all further aqueous deposition on a large scale in this lying in many places at elevations much lower PKE-PALEOZOIC HISTORY. respects that of the San Juan region generally, and region, it may be said that the erosion period has than the Telluride conglomerate, indicating that Archean schists. The oldest rocks that are the record is found mainly in the sedimentary rocks. extended to the present time. In this immediate here at least an interval of active erosion occurred known to occur in the San Juan Mountains are Concerning this history little can here be said in area, however, the only product of that erosion, between the Telluride and San Juan. It is prob­ schists and gneisses exposed in the Animas Can­ addition to the statements made in the introductory aside from recent detritus, is the conglomerate able that this erosion began while the Telluride yon and elsewhere in the Silverton and Needle sketch except in regard to the Dolores formation which underlies the lowest tuffs, the Telluride for­ was still being deposited and it developed a topog­ Mountains quadrangles. They are believed to be and to the unconformity at its base. mation. raphy, comparable in its relief to that of the pres­ of Archean age, but little is known of their origin Paleozoic history. The fact that the Ignacio Post-Laramie movement. The uplift that ter­ ent day, before the appearance of the first volcanic or of the events which followed their formation is absent in the Uncompahgre Canyon section, minated Laramie sedimentation in the San Juan rocks. No direct evidence for this is found in the more than that they underwent great dynamic although known to be present on Cow Creek, may region was undoubtedly the broad continental Ouray quadrangle, since the absence of the Tellu­ metamorphism and were extensively eroded before indicate either nondeposition or an interval of ero­ uplift that embraced the whole Rocky Mountain ride beneath the San Juan at many points east of Algonkian time. sion between the Ignacio and the Elbert, but of province, perhaps with local structural centers. the Uncompahgre may well be explained by lack Algonkian rocks. The destruction of the this there is no definite knowledge. Elsewhere the One of these was in the San Juan Mountains and of deposition. The general elevation at which the Archean rocks supplied material for the great Ignacio is followed with apparent conformity by its main result appears to have been a broad anti­ San Juan occurs east of the Uncompahgre Canyon, thickness of probable Algonkian sediments repre­ the Elbert, but the absence of rocks of ages inter­ clinal fold with an east-west axis, part of the north­ where it rests upon a rugged Algonkian surface, is sented in the Needle Mountains and in part in the mediate between the Ignacio (Cambrian) and the ern limb of which lies in the Ouray quadrangle. above that of the Telluride to the west and directly canyon of Uncompahgre River near Ouray. Sedi­ Elbert (Devonian) must represent a definite inter­ Post-Laramie erosion and the resultant surface. opposite. This surface is believed to be a remnant mentation was not continuous, and from time to val of which no records are preserved. Beginning It is to be assumed that erosion began with the of the Telluride uplands modified, perhaps, by post- time intrusions of igneous rocks occurred, resulting with the Elbert the geological history of the region retreat of the Laramie sea and continued after the Telluride erosion from the east. in a complex of greenstones, conglomerates, quartz- becomes more clear. Sedimentation continued in elevation of the land ceased. It is not known to ites, and slates having a maximum observed thick­ quiet waters until Mississippian time, when the what sea the detritus of this erosion was carried to VOLCANIC ACTIVITY. ness in the Needle Mountains of more than 12,000 upper part of the Ouray was deposited; then a form new sediments; only the result of the long A complete record of the volcanic history of the feet. The older greenstones and conglomerates are change in conditions caused a more or less com­ period of degradation can now be seen, and that San Juan region is not preserved in any of the not now exposed in the Ouray region, but evidence plete destruction of the upper part of the Ouray over a limited area. The surface on which the quadrangles surveyed thus far. Each new area of their former existence is found in the abundance and the redeposition of some of the detritus derived Telluride conglomerate rests affords the measure of examined throws some light on particular parts of of pebbles and bowlders, derived in all probability from it as the Molas formation, the earliest sedi­ the post-Laramie erosion. the problem and the relations of the volcanic rocks from these rocks, which occur in conglomerates of ment of the Pennsylvanian epoch. The Molas At the southern boundary of the Ouray quad­ in the southern part of the Ouray quadrangle are the Cutler formation. marks the beginning of a very long epoch of con­ rangle the erosion amounted to the removal of all especially significant. Granitic and other intrusions. Many of the tinuous deposition, during the earlier part of which the Cretaceous and Jurassic sediments and a small Earliest eruptions. The San Juan formation is large bodies of granular rock occurring in the limestones were formed, but in decreasing amounts amount of older strata, remaining after the pre- made up of the debris from a huge accumulation Needle Mountains and elsewhere are now known as time went on, giving way to clays and sands. Jurassic denudation. To the east it was more; to of earlier lavas. None of these products of the first to be younger than the Uncompahgre formation No marked change from the sandstones of the the west, less. period of volcanic activity is known except through but older than the first Paleozoic sediments; they Hermosa to those of the Cutler can be noted except The surface resulting from this enormous erosion its fragments in the San Juan tuff-agglomerate. are unchanged in their texture, except in compar­ in the degree of coarseness of the Cutler rocks. was a gently undulating plain a peneplain -from The rocks of the San Juan formation so far atively minor degree,- and can not have undergone The abundance of coarse conglomerates, grits, and the line of Animas and Uncompahgre rivers west­ examined are andesites and latites. The diversity such complex folding and faulting as that to which cross-bedded sandstones and the lack of continuity ward. It is known to have had this character of rock types in this formation is probably no the Uncompahgre series had been subjected. No in individual horizons indicate that the Cutler in southwest of the Silverton quadrangle, and similar greater than that exhibited by the lavas of the granite occurs in the Ouray quadrangle, but a small the Ouray region was of fluviatile origin or at most conditions- prevailed from Cutler and Oban creeks Potosi series, and it is worthy of note that many body, intrusive in the quartzites, is exposed a short laid down in shallow, turbulent waters. It is not northward, beyond the boundary of the Ouray of the San Juan fragments are very similar to distance to the south, in the Silverton quadrangle, known how long Paleozoic sedimentation continued quadrangle, to the locality where the conglomerates certain lavas of the Potosi. and granite pebbles are common in the conglomer­ after the time of which records are preserved in the of the Telluride rest upon the Mesaverde formation. No direct evidence as to the location of this ates of the Cutler formation. Ouray region. The pre-Telluride erosion produced a planation earliest center of eruption is known, and yet it can Post-Algonkian interval. A very long period The Dolores unconformity. The interval of of the domal or anticlinal uplift previously discussed, not be assumed that its lavas were wholly destroyed elapsed after the Uncompahgre formation was deformation and erqsion which immediately pre­ and at the resulting surface the various sedimentary in the formation of the San Juan. The vast bulk deposited before the next sediments now known, ceded the Triassic and which is so strikingly indi­ formations must, therefore, have outcropped in more of the latter deposits indicates that the volcanibx the Ignacio quartzites and shales, were laid down. cated by the unconformable relations of the Dolores or less concentric zones that were rendered irregular pile from which the materials were derived must Apparently the region was a land area until the to the older beds in the cliff exposures near Ouray in detail by minor structures of which little evi­ have been of huge dimensions. The texture and Ignacio epoch, which is tentatively thought to be is an event heretofore unrecognized in the history dence is now visible. A central region of consid­ structure of the San Juan do not point to any par­ Saratogan (Upper Cambrian). The events of that of the San Juan region and its significance is at erable elevation and bold relief, where erosion was ticular location for the source of its materials. long period of which we have evidence may be present only partly understood. At Ouray the still active through the time that the Telluride con­ Variations in texture, in the size of fragments, or in grouped as orogenic movement, metamorphism, Cutler is the youngest of the pre-Dolores forma­ glomerate was being deposited, lay to the east of the regularity of bedding are greater between the beds intrusion of granite and other rocks, and erosion. tions preserved, but its apparent conformity with pre-Telluride surface as now recognized, and this of adjacent localities than between those of widely The orogenic movements by which the gneisses the Triassic everywhere noted, except in the vicin­ region, consisting of the older Paleozoic sediments separated districts. and quartzites were brought into approximately ity of Ouray, is far from offering conclusive proof and Algonkian and Archean rocks, supplied the^ Deposition and erosion of the San Juan. The their present relations were certainly of great mag­ that still higher horizons did not exist before the material for the Telluride conglomerate. The east­ source of the San Juan materials being unknown, nitude. So much area is covered by the volcanics time of pre-Dolores erosion. ward limit of the Telluride^ conglomerate on the the method of their transportation and deposition that little detail of this structure is visible. The Further discussion of this problem, however, as northern flanks of the San Juan Mountains is not remains a problem. That they owe their present main result was a steep upturning by faulting or well as of those connected with the later formations, known, and the elevated area may have been at distribution in some measure to eruptions of great folding, so that the subsequent erosion separated is not justified here on account of the incomplete­ least partially surrounded by a lower region in explosive violence is probable, and that they were the two quartzite areas of the Uncompahgre Can­ ness of the sedimentary section. The sedimentary which deposition took place. in part laid down beneath a lake more or less yon and the Needle Mountains by a schist zone, a sequence above the Dolores was completed in this Formation of the Telluride conglomerate. The coextensive with the Telluride lake is undoubtedly part of which is seen south of Silverton. This region, as elsewhere in the San Juan, by the depo­ coarse texture of the Telluride deposits in the true, but a very large part, probably including all general effect would be produced by an east-west sition of the Jurassic and Cretaceous formations Ouray quadrangle and the apparent lack of con­ of the formation lying in the Ouray quadrangle, anticline passing through the Silverton quadrangle, following another interval of uplift and erosion. tinuity indicate that they lie close to their source was more likely of direct subaerial deposition, the crest being eroded. That this was a simple In this interval, which occurred just before the of supply and that they were laid down by streams. possibly influenced by contemporaneous fluviatile fold is not probable, since the Uncompahgre sedi­ deposition of the La Plata, the Dolores and lower In the southwestern part of the Telluride quadran­ action. The reason for this opinion lies in the ments of the Needle Mountains exhibit a complex formations were removed on the east by erosion, gle the formation is 1000 feet thick and consists of fact that the Telluride of the Ouray quadrangle synclinorium largely bounded by faults. The so that when the La Plata was at last laid down, much finer materials, favoring the opinion expressed is in thin, discontinuous deposits most plausibly domal folding in the Uncompahgre Canyon is its base rested to the west upon the Dolores and to in the Telluride folio that the deposits were laid of fluviatile origin, and that the lake if present at probably a comparatively local feature. the east upon pre-Cambrian schists. down in a shallow lake. It now seems clear that all, was represented only by shallow estuaries. The induration and general metamorphism pro­ A realization of the thickness of the sedimentary the formation is to be regarded as in part of lacus­ As to the original thickness and distribution of ducing quartzites and slates out of the Algonkian section at the close of the cycle of Upper Cretaceous trine and in part of fluviatile origin. The land the San Juan little is known, since it has been sub­ sediments is plausibly to be connected with the deposition is necessary to a comprehension of the area, still containing remnants of the older Paleo­ jected to erosion from its first deposition more or period of folding and faulting, although the pro­ events next to be considered. It is reasonable to zoic rocks, must have extended from the Needle less continuously to: the present time. The 3000 duction of chiastolite in the slates is readily explain­ assume that the formations present on the north Mountains as far north as the canyon of the feet preserved east of The Amphitheater near 15

Ouray represents the greatest observed thickness, Potosi lavas and were intruded when a greater leys and from them streams of ice descended to the must be attributed the U-shaped modification of but at this point Potosi flows rest directly upon it, thickness of the Potosi rocks existed. They prob­ lower country. The topography did not favor the cross section of the valley of Canyon Creek. and it must be assumed that a considerable amount ably belong to the close of the Potosi period. glaciers of great length and on leaving the nar­ Between Ouray and the terminus of the glacier of the San Juan had been removed during the The laccolithic masses and sheets of Canyon rower valleys they deployed and deposited over lateral morainal deposits occur on the valley sides Silverton volcanic epoch. Creek and Uncompahgre River near Ouray may the lower valleys a mantle of drift which extended at elevations gradually decreasing toward the north. There is no evidence of extensive erosion during have been intruded at any time after the San Juan well up toward the divides. Some of this drift, The highest point at which such deposits have been the period that the San Juan tuffs were forming, epoch, so far as their relations to the inclosing especially that still preserved north of Lou Creek, observed is at 9500 feet, on the east side of the but between the close of the San Juan eruptions rocks give evidence, but their similarity in compo­ was transported but short distances, probably by Uncompahgre just north of Bridalveil Creek, indi­ and the beginning of the Silverton epoch the San sition to the dikes of Porphyry Basin and to the neve ice on a valley side rather than by true cating a thickness of about 2000 feet for the ice at Juan was vigorously attacked by streams from the Potosi lavas themselves favors the belief that they glaciers. The streams issuing from the ice-filled this point. east, the formation farther east being apparently belong in general to the Potosi epoch. valleys were greatly increased in volume and car­ Outwash gravels, contemporaneous with the unaffected and covered conformably by later erup- ried with them enormous quantities of detritus moraines, were carried far down the valleys tives. The best evidence for this is seen in the POST-POTOSI TILTING AND FAULTING. supplied by the glaciers, which was deposited as beyond the limits of the ice, as in the earlier vicinity of North Fork of Henson Creek and near At some period later than the eruptions of the outwash gravels or valley trains. Gravel deposits, stage. Such gravels are well preserved in Cow the head of the north branch of Bear Creek, where volcanic rocks the whole region was affected by believed to be of this origin, have been observed Creek and north of the terminal moraine of the andesite flows from the uppermost tuffs of the Sil­ slight differential tiltings accompanied by faulting. far down the Gunnison Valley, and pre-Cambrian Uncompahgre. verton series abut against very steep walls of the The tilting was in general to the east and is only quartzites and schists derived from the Needle San Juan. This seems to show that the basin vaguely recorded in the Ouray quadrangle by vari­ Mountains have been found near San Juan River, RECENT HISTORY. occupied by the Silverton lavas was bounded on ations in elevation in the Telluride conglomerate. 100 miles from their source on the south side of Landslides. Since the final disappearance of the the north by a mountain wall of the San Juan. In the Silverton and Telluride quadrangles the the mountains. ice, changes of a minor character have been taking The basin was several thousand feet deep and this inclination of the conglomerate is clearly shown. The only true moraine that has been recognized place. The most noteworthy events and those is an approximate measure of the erosion to which The faulting, which was very marked in the Sil­ as belonging to this early stage of glaciation occurs which have brought about the most pronounced the San Juan was subject before the first lavas of the verton region, is restricted to two areas in the west of the quadrangle at Horsefly Peak. Part of modifications in the topography were landslides. Silverton series were poured out. In the Silverton Ouray quadrangle in the vicinity of the town of this eminence, as well as of the surrounding hills, The topography that resulted from the revival of quadrangle more definite records are preserved of Ouray and near the northern border of the quad­ consists of Mancos shale, but it is covered with a the streams in interglacial times was of a youthful this erosion in areas where deep valleys were carved, rangle. Except the few large faults near Ouray thick mantle of gravel and bowlders having the type, the valleys being deep and their sides precipi­ to be later filled by the flows and tuffs of the Sil­ no great displacements occurred, but small frac­ hummocked surface of a kettle moraine. tous. The effect the ice may have had was further verton series. tures were abundant and were closely associ­ Pleistocene landslides. Toward the close or im­ to steepen the valley sides and cirque walls, leaving Eruptions of the Silverton series. Records of the ated with the formation of the ore deposits of the mediately after the retreat of the ice, landslides of them, on its retreat, in a more or less unstable con­ Silverton volcanic epoch are incomplete in the Ouray region, as pointed out in the section on economic enormous proportions took place from the abrupt dition, depending from place to place on the charac­ quadrangle; but, as has been shown in the Silver- geology. mountain faces which then, as now, extended west­ ter of the rocks composing them. In the Silverton ton folio, a succession of lavas and tuffs more than ward from the Uncompahgre Valley. The debris folio the landslides of that region were discussed in 3000 feet in thickness was accumulated in the lower PLEISTOCENE HISTORY. of these slides was spread far out on the lowland detail and it was suggested that many of the great country within a few miles south of the Ouray Erosion. At the close of the long period of at the foot of the mountains. A remnant of this rock falls were caused by earthquake shocks. Such quadrangle. Some idea of the sequence of Silver- volcanic activity and following slight oscillatory accumulation is preserved on the present upland an explanation seems entirely reasonable and almost ton lavas has been given in this folio, but the three movements accompanied by minor faulting, a cycle west of the Uncompahgre, drained by Coal Creek, necessary to account for the large number of land­ formations of the series occurring in the valley of of erosion began that is still in progress. The prin­ and at many other places farther west. slides, the debris of which is to be found in nearly North Fork of Henson Creek are the uppermost. cipal drainage systems were early determined and a Interglacial erosion. On the disappearance of the all the higher valleys of the Ouray quadrangle. In The fossiliferous calcareous shales interbedded with vast amount of erosion appears to have been accom­ ice the streams entered on a period of very active Coal Creek and on the west side of the Uncom­ the Burns tuffs indicate a period of quiescence dur­ plished before the beginning of the glacial epoch, erosion, during which the canyons of Uncom­ pahgre comparatively recent slips have occurred ing which animal and'plant life existed. developing a topography which did not differ in pahgre River and Cow Creek were developed. from the sides of the Pleistocene landslide deposits Erosion preceding the Potosi. Although erosion its broader features from that existing to-day, and The reason for this renewal of activity on the exposed by interglacial erosion. took place more or less continuously throughout many elements of which are still preserved in the part of the streams is not fully understood, but it Rock streams. In many of the cirques or basins the Silverton volcanic epoch, as is strikingly shown region north and west of Ouray. The mountain­ seems necessary to recognize two elements as taking in the higher mountains occur accumulations of in the Silverton quadrangle and in the regions to ous area was bordered by a region of very mod­ part in their revival. One of these was doubtless debris resembling ordinary talus in character; these the east, its results were obscured by repeated erup­ erate relief occupied by broad, open valleys with a marked increase in volume of the streams inci­ were first described in the Silverton folio and there tions or outpourings of lava, and it was not until low divides between them; active erosion in this dent to the melting of the ice, but this in itself named rock streams. They consist of rock frag­ the cessation of volcanic activity of that epoch that lower region had ceased, but it was still in progress can not account for such extensive erosion as took ments or blocks, which cover the bottoms of cir­ the streams began to make headway in the destruc­ in the mountains, where narrower and deeper val­ place near the head of Cow Creek, resulting in the ques to a depth in many places of 100 feet or more, tion of the volcanic rocks. The greatest work leys existed, with sharp ridges between them sur­ cutting of a canyon at least 1000 feet below the and extend far out from the bases of the cliffs from seems to have been accomplished by the streams mounted at many places by crests of bare rock. level of the former valley. The other element and which the material was derived, in forms similar to situated some distance south of the Ouray quad­ Near the northern border of the quadrangle the the one which, from our present knowledge, seems those of small corrie or hillside glaciers. The origin rangle, in the present Rio Grande drainage area. valley of the Uncompahgre extended from what is to have been the more potent, was an uplift of the of these rock streams is discussed in detail in the Sil­ The configuration of the base of the Potosi lavas now Cimarron Ridge to the region of Horsefly whole region, probably of a differential character verton folio and is here briefly summarized. The in the southeastern part of the Ouray quadran­ Peak, about 15 miles west of the quadrangle. and following the lines of the post-Laramie dom­ deposits are believed to be due to landslides of gle shows that at the time of their outpouring From the mountains long, low spurs extended ing of the San Juan district. In any event, the essentially the same character as those of the lower that region was a rolling country with shallow northward, separating the waters of Cow Creek, streams carved deep canyons near their heads valleys that have been described, but it has seemed valleys. Uncompahgre River, and Dallas Creek, which joined and lower down incised themselves in their old necessary to explain their form and configuration Potosi volcanic eruptions. The eruptions of the at nearly the same point as now. Remnants of valley floors, developing new and broad flood as the result of having fallen upon the surfaces of Potosi lavas in the Ouray region were very exten­ these old valley slopes are shown in the panoramic plains bounded by sharp terraces on which old the basin glaciers or snow fields which were the sive and were possibly of more varied character view of the Uncompahgre and Cow Creek valleys stream gravels were preserved. last to disappear at the close of the glacial epoch. than those of which records are preserved in the (fig. 10, illustration sheet). Later glaciation. After the interval of active In the Ouray quadrangle they are neither as abun­ Telluride and Silverton quadrangles. Lava flows These surfaces may be traced southward into the erosion glacial conditions again prevailed and of dant nor as large as in the Silverton region, though predominated, with minor eruptions of fragmental mountains, where in both the Uncompahgre and this stage abundant records are preserved. The almost the best examples observed occupy basins material, and, although the majority of the lavas Cow Creek valleys benches are preserved high glaciers extended only to the edge of the lowlands tributary to Canyon Creek a short distance south were latites, andesites appeared from time to time. above the present valley bottoms, marking the at the foot of the mountains, where the Uncom­ of the Ouray boundary. From the greater development of the Potosi rocks position and configuration of the old valleys. pahgre Glacier deposited a large terminal moraine. Postglacial erosion. A small but definite amount in the central San Juan Mountains, and the widely Cow Creek had an even grade from its source The small catchment area at the head of Cow Creek of erosion has been accomplished by the streams distributed outliers, it is plain that the lava floods to its junction with the Uncompahgre, the old produced only a moderate supply of ice and the since the glacial epoch. This is most clearly covered thousands of square miles and that the surface being still practically preserved in the glacier in this valley was not large. To judge shown in the lower parts of Cow Creek and Uncom­ epoch was of long duration. It remains for studies vicinity of American Flat. Between the point from the large quantity of drift that occurs in the pahgre River, where the outwash gravels of the later in the central and eastern San Juan Mountains to where the stream now leaves the shallow basin lower parts of the two western forks of Cimarron stage of glaciation occupy terraces from 25 to 75 feet determine whether the Potosi was succeeded by at its head to plunge into the present canyon Creek, heavily laden glaciers occupied both .of these above the level of the present flood plains. That still other volcanic epochs capable of distinction. and Oban Creek, traces of the old valley slopes valleys and also that of the main stream to the this erosion is something more than a mere incision No information has been obtained which would are preserved on the crests of spurs between trib­ east. So full was West Fork that a part of the of the streams in the outwash-gravel deposits is suggest that the lavas were derived from a restricted utary streams, but they are very obscure. North ice flowed over the low divide at the head of Owl shown at many points along Cow Creek, where volcanic center; on the contrary, their different char­ of Oban Creek the gentler outlines of the old val­ Creek and descended some little distance down this Mancos shale is well exposed beneath a compar­ acter and distribution, as well as the presence of ley can be clearly recognized above the present stream. atively thin gravel cover. This erosion is compar­ basic flows, indicate more than one center of erup­ canyon walls to a point north of Lou Creek, where The most conspicuous erosional features resulting able to that on a much grander scale which followed tion. It is probable that many of the andesite and they merge with those of the Uncompahgre. from the ice are the cirques at the heads of the main the first stage of glaciation, but in this instance it latite dikes or small crosscutting bodies of the Similar relations between the older and younger valleys and tributary streams. The development is believed to be largely due to the increased cutting Ouray quadrangle occupy conduits through which levels may be recognized in the upper portion of of some of these basins, examples of which can be power of the streams resulting from their overloaded some of the later lavas may have reached the the Uncompahgre Valley, although less clearly found in the eastern tributaries of Cow Creek, was condition during the melting of the ice. surface. preserved on account of recent landslides. undoubtedly begun during the earlier stage of gla­ Although the larger streams have to-day reached Summarized briefly, postvolcanic erosion had ciation, but the process continued and the forms a condition of grade and are depositing broad flood INTBUSIOKS. developed a mature topography before the advent were perfected in the later stage. The Amphi­ plains in the lower country, the smaller streams Probably during the time that the Potosi lavas of the glacial epoch. theater, east of Ouray, the floor of which beneath are active in the higher mountains and the fans of were being erupted the laccolithic and crosscutting Earlier glaciation. With a change in climatic its landslide cover lies at about the present level of detritus at the mouths of many of them bear witness bodies that occur at various places in the quadran­ conditions, the mountains were covered with the Uncompahgre, must owe its very perfect cirque to the erosion and transportation of land waste that gle were intruded. Certain of these bodies cut snow. Snow fields occupied also the higher val­ form to the later stage of glaciation, to which also is still in progress. Onray. ECONOMIC GEOLOGY OF THE QUADRANGLE.

By J. D. Irving* and Whitman Cross.

METALLIFEROUS DEPOSITS. by impervious shales, lateral enrichments have andesite-breccia cap and comprising the following Again, where the vein passes through limestone, By J. D. IRVING. been developed, many of them of great extent. beds, named from above downward: flat shoots of ore are developed, running parallel The Bachelor mine is an example of the simple Section of country rock containing silver-bearing veins. to the main fissure and many of them extending The metalliferous deposits that have been devel­ fissure in which replacement has operated only in laterally for distances of 25 to 30 feet or more 1. Mancos black shale. oped within the limits of the Ouray quadrangle a small degree. No flat ore bodies can be observed 2. Dakota quartzite and sandstone, alternating with black from the main body. These shoots are formed by are all in the near vicinity of the town of Ouray in the mine, because the workings have not yet shale. replacement of the limestone by ore and gangue and occupy mainly the precipitous country on penetrated sufficiently deep to reach beds of lime­ 3. A highly varied series of clay shales, sandstones, sandy minerals and are clearly derived from the main shales, calcareous shales, and limestones, belonging both sides of Uncompahgre River. No one of stone. The Newsboy mine is an instance of a vein to the McElmo formation of the Jurassic. vein. Such replacement bodies constitute a class the important mines is at a greater distance from of twofold character, where a simple fissure in 4. Red sandstones and conglomerates, with some reddish of deposits so distinct from the fissure veins in shales. Ouray than 3^ miles. impervious rocks gives rise to large lateral shoots their general character that they will be further Although these ore deposits include only a few on passing through beds of limestone. The Bright These sediments have as a whole a gradual described under the head of "Replacement deposits highly productive mines, they form a series of very Diamond and Mineral Farm mines are examples downward inclination in a direction north of east, in limestone." unusual scientific interest. There is a close rela­ of immense flat bodies of ore in which the fissures which eventually brings them down across the bed The silver veins of the Dexter Creek and tion between the geology of the region described in are so small that many of them are difficult to of Uncompahgre River along its northward course. Uncompahgre district do not pass upward into the foregoing portion of this text and the character detect. The dip in undisturbed localities is about 10°. the andesite breccia of the San Juan formation. of the ore bodies. The striking features of each Those in The Amphitheater, on the contrary,, do ore deposit are due in large measure to the litho- extend upward into the San Juan. The reason Scale of feet Quartz monzonite porphyry] logic or stratigraphic nature of the rock in which aoo 400 600 800 Black shale >Mancos for this difference is that a heavy black shale Horizontal and vertical scale the same American Nettie mine.Quartzite it occurs. Dakota CRETACEOUS (of the Mancos formation) lies above the ore in Shales and quartzite the Dexter Creek country, but no such imper­ GENERAL FEATURES. Greenish altered shales and shaly sandstones vious rock is present in The Amphitheater. Black Diamond mine { Q Considered as a whole, the ore deposits of the The veins, being of but slight displacement, seem

Ouray quadrangle form the northern extension of Reddish and greenish shalesand sandstones to have been lost in these shales, particularly at a group of metalliferous deposits that is widely the Bachelor and Wedge mines, where the ore McElmo developed in the Silverton quadrangle, which Sandstone and vein terminate abruptly and bluntly at the JURASSIC adjoins this quadrangle on the south. Although Sandy shalesand clay shales shale horizon. they are thus in a sense closely related to the Sil­ with thin limestone beds _ Quartzite Clay shalex-rj FAULTING BEFORE FILLING. verton deposits, they form a group which possesses Quartzitic sandstone^Sg marked individual features. Sandy shales and <=.anH<^tnnp ^' BlackGirl^PonyExpress mines.Limestone £ Many of the fissures have been affected by bed­ A few of the larger ore bodies are situated in Massive gray or white cross-bedde^|||g||||| ding faults, which invariably follow the shale beds. sandstone La Plata localities where the rocks are .much broken and Gold Hill mine. Conglomerate Dolores Such faults have in many places, as in the Black disturbed, and near intrusive dikes and sheets of Red sandstone Cutler Girl and Newsboy mines, shifted the upper portion porphyry, but most of them are found in formations of the vein as much as 30 feet away from its down­ which have been only slightly disturbed. The ward continuation. This faulting seems to have FIG. 1. Geological section of Grold Hill, 2 miles north of Ouray, Colo. veins of the Black Girl and Newsboy mines, on occurred before mineralization, as the ore is in gen­ the east side of Uncompahgre River, occur in strata eral unbroken and follows the shales in a thin so regular and so free from intrusions that their The accompanying section of Gold Hill (fig. 1), Local disturbances have increased or reversed the band between the two separated portions of the mineralization occasions no little surprise. very much generalized on account of the thinness dip, as in the Bachelor mine, where the strata dip fissure. of the beds, will give a somewhat inadequate idea at an angle of about 10° SE. Steeply inclined CLASSIFICATION. of the varied rocks through which the fissures pass beds occur in only a few of the mines. The dip The ore deposits are difficult to classify on account and will also serve to illustrate roughly the hori­ at the Calliope and Iowa Chief mines is about 25° of the insensible gradations between even the most zons in which a lateral extension of the veins most SE., but this inclination holds good for only a diverse types. A single deposit may display very commonly takes place. The influence of the wall small area. In some places the ore is contained different characters in different rocks, so that it rock on the form of the ore deposit can be further within two fairly well defined walls; in others the will conform to one type of deposit in one part of understood by reference to the detailed diagram­ fissure is divided into branches that separate from its course and to another type in another part. matic drawings (figs. 2 and 3). and unite with one another repeatedly. Cross For convenience of description the classification The ore deposits will be briefly considered in the fractures further unite the divided branches so below is given. It will serve to bring into the order of the classification already given. that in many places the resulting network passes | same group the types of ore bodies most readily into a more or less brecciated structure. i FISSTJEE VEINS. discussed together. RELATION OF FISSURES TO WALL ROCK. 1. Fissure veins. SILVER-BEARING VEINS. LOCATION AND GENERAL FEATURES. 2. Replacement deposits in quartzite. The relation of the ore in the fissures to the wall Clay shal« 3. Replacement deposits in limestone. The fissure veins are readily divisible into two rock is peculiar. The ore is much more abundant and of very much higher grade where it is included These three main classes of ore bodies owe their distinct groups silver-bearing veins and gold- FIG. 3. Type of silver-bearing vein unmodified. existence to the presence of fissures, in general bearing veins. between walls of quartzite, and it is either absent nearly vertical, in the country rock through which The silver-bearing veins are developed largely or of a low grade where the fissure passes into ORE AND GANGUE MINERALS. ' the mineralizing waters have circulated. The form along Dexter Creek, where the chief interest has shales. In some mines, such as the Iowa Chief, The ore minerals found in these silver veins are of the ore body deposited is dependent on two centered about the Bachelor, Wedge, and Calliope on Dexter Creek, this is due to the narrowness of of three classes original sulphides, oxidation prod­ factors the amount of open sjiace in the fissures mines, and on the east side of the Uncompahgre the vein in the clay shales. It is so narrow7 as to ucts, and secondary sulphide-enrichment minerals. and the kind of rock through which the fissures Valley about 4 miles north of Ouray, the principal be in most places a mere fracture in the sediments, The original sulphides occur in largest amount pass. mines here being the Newsboy and Black Girl. A with no appreciable open space. On passing into in the deeper portions of the mines, but in many Where the fissures have been open the resulting few minor developments, as yet only of prospective the quartzite, a rock more capable of supporting places, as in the Bachelor mine, extend upward to vein exhibits in many deposits a roughly parallel value, occur on the uplands lying west of the river; the top of the vein beneath the black shales. They alignment of minerals and little replacement of the among these the Gem and Teller mines are the consist of argentiferous galena and an antimonial wall rock is noticeable. Where, on the other hand, most important. sulphide of copper, locally called gray copper, the fissures have been narrow and more or less dis­ The veins are fissures in the country rock filled Quartzite which carries very high values in silver and is continuous the maximum replacement is to be with high-grade ores of silver and the accompany­ probably related to freibergite. Chalcopyrite and observed along soluble beds. The narrow fissures ing gangue minerals. The strike of the fissures is pyrite occur in many veins intermingled with the are apt to be developed in considerable number and in general fairly uniform, approximating an east- ^=-=- -~3frL " .'~; ~ Clay shale other sulphides, and sphalerite is present in some of large, flat masses or shoots conformable to the west direction. Thus in the Bachelor and Calliope the mines but is generally subordinate in amount. bedding are formed. mines the strike is N. 83° E.; in the Black Girl it Limestone Extensive oxidation has taken place near the sur­ Of far more wide-reaching importance is the is N. 85° Ei. The dip of the fissures varies from "j"-=!^-L-g^^^-'^g^gs^^ Black shala face and much native silver has been developed. nature of the rocks in which the fissures occur. a vertical position to about 60°. Dips of 45° are Its position shows that it has uniformly been Most of the veins pass through an immensely varied known, but are zather uncommon. The fissures Gray sandstone reduced during the process of oxidation. A notable series of rocks and many of them can be traced as a rule show a slight displacement, rarely reach­ example of such development is afforded by the upward from the red beds of the Hermosa forma­ ing as much as 7 feet and generally so small as to PlG. 2. Type of silver-bearing vein modified by replacement Calliope mine, where a deep gulch cuts directly tion through greatly differing sedimentary deposits be distinguishable only by close observation. The and bedding fault, Ouray, Colo. across the strike of the vein. The zone that sep­ into the andesite breccias of the San Juan, which width varies from a few inches to as much as 8 feet, large cavities, the vein widens and rises so much arates the native silver from the unaltered sulphide form the massive gray cap rock of all the higher a fair average being about 3 feet. Most of the in value as to furnish very considerable profits. ore closely parallels the surface slopes, passing hills and mountains. The ore bodies vary as veins are ra'ther uniform, retaining their width for In other places, however, the vein suffers no appre­ down beneath the gulch at a depth slightly less widely as the rocks that contain them. Where a the larger portion of their course without marked ciable diminution in size on reaching the shales, than the distance between it and the tops of the fissure penetrates only impervious rocks little variation, although pinches and swells are notice­ but the ore minerals seem to have been deposited hills on either side. Thus in a profile of the vein replacement occurs and no lateral shoots are able in some of the mines. only between the layers of quartzite, the portion of the outcrop would show a deep U-shaped depres­ developed. Where there are replaceable lime­ The country rock in which the veins occur is the vein in the shales being occupied by barren sion where intersected by the gulch, and the zone stones, or in some places even quartzites capped the varied series of sediments lying below the gangue material and clay. between the two kinds of ore would present the 17 same profile, slightly less accentuated. Similar opened to the east for more than 1000 feet, to from the numerous black-shale beds just above the flush with the canyon walls. The veins occur relations occur also in the Black Girl vein. the point where it becomes barren, and to the west La Plata sandstone. either wholly within'the dikes or closely associated Bodies of ruby silver occur here and there in the through the workings of the Wedge and Neodesha The high-grade ore is shipped to the smelters with them. They comprise a group of highly veins, but are rarely found at great depths below mines into Uncompahgre Canyon. At the Wedge and gives no trouble other than that involved in a inclined sheeted zones, made up of a series of the surface. They are thought to be the result of shaft, about midway of its course, it divides, both careful sorting. For the treatment of the lean ore irregular and closely spaced fissures, mostly of secondary sulphide enrichment of the gray copper branches passing across Gold Hill and emerging a small mill has been erected at the mine. It is small throw and in many places aggregating 5 or ore, between the zone of oxidation and the zone of in the cliffs which form the east bank of Uncom­ fitted with jigs, Bartlett tables, slime-settling tanks, 6 feet in width. The ore consists of gold-bearing unaltered sulphides. pahgre River. The two branches are separated by and canvas slime tables. Great difficulty has been pyrite and chalcopyrite in a gangue of country rock The gangue minerals are quartz, barite, second­ an interval of about 200 feet in this outcrop. The experienced here, as in the other silver veins, in and clay, the clay having been derived partly from ary silica, a pinkish carbonate, probably containing northern branch is worked by the Neodesha mine saving the silver values contained in the gray intercalated shale beds and partly from porphyry. magnesium and manganese, and country rock. The and the southern presumably by the Pony Express, copper. This mineral is converted into slime even The filling of open spaces seems to have been sub­ secondary silica is as a rule light grayish in color although no connection has yet been established. with the most careful crushing, and floats on the ordinate to the replacement of the wall rock, which and clearly secondary when examined microscop­ The eastern portion of the vein is vertical, as is the top of the water so that a large part of it is lost. has occurred to a considerable extent. The ore lies ically. The barite is very abundant and occurs in portion operated by the Neodesha mine, but the The canvas tables save a little and are a valuable in small shoots in the vein and is so irregular in places in amounts so large as to fill the vein almost branch that extends into the Pony Express mine feature of this mill, but even by their aid a con­ its distribution that the veins are mined with some completely. is reported to have a slight dip to the south. The siderable loss of silver can not be avoided. difficulty. trend of the vein east of its division is N. 83° E., The veins are later than the porphyry, as they THE BACHELOR MINE. or very nearly east and west. In this respect VEINS IN THE AMPHITHEATER. can be seen distinctly faulting that rock. Both Among the silver veins the Bachelor vein is of it follows very closely all the other fissures which A group of silver-bearing fissure veins of a very the dikes and the associated veins strike about N. the greatest economic importance, having been the outcrop in the region about the junction of Uncom­ different character is found at the head of The 83° E., parallel to the majority of silver-bearing chief silver producer in the district as well as the pahgre River and Dexter Creek. Amphitheater, in the almost inaccessible cliffs that fissures. The dip is in some places vertical, in most productive of the fissure veins. It possesses The fissure, as well as the ore deposited in it, surround the glaciated valley. They exhibit many others as low as 50°. so many features that can not readily be presented was formed subsequent to the solidification of the different strikes and dips and pass from the lime­ Nearly all the mine workings are situated high in a general discussion and is in some respects so breccia or so-called "dike," as it breaks across from stones and shales into the andesite breccia of the above the bottom of the canyon, in the steep faces peculiar that an account of the silver veins would one side of the "dike" to the other or is here and San Juan formation. of the cliffs, and the ore was brought down by hardly be complete without a detailed description there entirely included within it. A faulting by These veins contain silver and gold in about means of wire-rope tramways, all of antiquated of this mine. 1 which the south side of the vein has been lowered equal proportions. The silver preponderates in model and now in disuse. Owing to their promi­ The Bachelor vein has been opened in three about 7 feet may be observed along certain por­ most places, but here and there the reverse is true. nent position these mines were among the first to places at the Bachelor tunnel, in Dexter Creek; tions of the vein. It must have been followed by The gangue is in the main white quartz and rho- be operated in the district. They are now either at the Wedge shaft, on Gold Hill; and in the other lateral movements before the deposition of dochrosite, or some carbonate intermediate between idle or operated under lease. They were never Neodesha mine, at the base of the cliff in Uncom- the ore, as the striations on the harder rocks which that and calcite, but in many of the veins the very productive or profitable, presumably because pahgre Canyon. The main openings are those in form the walls are invariably horizontal. Subse­ gangue is entirely quartz. The veins vary in width the irregularity of the veins and their small size Dexter Creek. A tunnel here driven southward quent to this movement occurred bedding faults from a few inches to 4 feet and are apt to pinch rendered their working uncertain, and the methods into the hill intersects the vein at a distance of 720 which have shifted the upper portion of the breccia out within short distances. In the sediments below of treatment were not adapted to the ore. feet. The country rocks through which the vein to the north. These took place before the vein the andesite breccia they form numerous lateral The values are largely in gold, although a little passes are sediments of the Man cos, Dakota, and formation, as there is no evidence that the ore was enrichments in the limestone, but none of those silver is generally present. Exact and reliable data McElmo formations. (See section forming fig. 1.) broken. The ore follows the bedding planes of noted were extensive. Ore minerals occur in very are difficult to obtain, but the gold values, where From the highest level of the mine downward they the strata along the faults, connecting with that in small quantities in the gangue. Those observed present, are undoubtedly high. The average yield consist of (1) a series of very fine black shales, in both portions of the dislocated fissure. are galena and gray copper, but stephanite and is probably between 2 and 3 ounces of gold per ton, places highly bituminous and containing many thin The ore occupies mainly what must have been other rich silver minerals have been reported from making a high-grade ore of $50. In some mines, beds of blackish sandstone; (2) layers of quartzite open spaces, as much of it shows parallel banding. these veins. The ore deposits of this district are as the Grand View, a little of the ore runs up to 8 ranging from 2 to 30 feet in thickness, separated It also enters the breccia along cracks and fissures as yet of merely prospective value. ounces, and as much as 30 ounces has been reported. from one another by varying intervals of black and and seems to have replaced this material in some REPLACEMENT DEPOSITS IN QUAHTZITE. clay shales. The overlying black shales belong to measure. COMMERCIAL CONSIDERATIONS. the Mancos, the upper quartzites and the black The ore is a high-grade silver ore carrying from The ore from the silver mines ranges in value CHARACTER AND GENESIS. shale layers in them to the Dakota, and the clay $20 to $75 per ton. Pockets of ruby silver (said from $30 to $800 per ton. Very high values have The ore deposits that have been grouped under shales and other interbedded rocks to the McElmo to be pyrargyrite) which are reported to have con­ been obtained from the oxidized portions, where the this heading contain gold with very subordinate formation. The clay shales are mainly greenish tained 15,000 ounces per ton in silver were discov­ native silver is in large amount, and the bonanzas silver and occur in irregular bodies in strata of in color, in places nearly white, but are so much ered shortly after the writer's visit. Gold is either of ruby silver occasionally found have also yielded massive quartzite. They are termed "replacement altered by the action of near-by eruptive rocks absent or present in very small quantities. exceptionally high returns. In the majority of the ores" because they have been formed, not by the that they break with conchoidal fracture into The ore minerals are argentiferous galena and mines, however, values of 60- to 200 ounces per ton filling of open spaces existing previous to their large angular masses. Weathering has locally an argentiferous antimonial sulphide of copper may be regarded as the average for shipping ore. deposition, but by a chemical interchange of ore disintegrated them so that they do not show locally called gray copper, but probably related The rich ore is usually shipped by rail to smelting material for original county rock. The miner­ much difference from ordinary greenish shale closely to freibergite. The copper sulphide is centers and there smelted, and where lean ore is alizing waters by which they were produced have unaffected by metamorphism, but in many places either scattered irregularly through the galena or encountered, as in the Bachelor mine, the concen­ been both the solvents of the country rock and the they retain their porcelain-like character even occurs alone in irregular patches in a pinkish-white trates are shipped in like manner. A local smelter agents which have effected the ore deposition. In after prolonged exposure. The black shales that carbonate which presumably carries a considerable at Ouray was in operation at the time the district some places the solvent action ,of these waters occur in the upper levels of the mine are amount of manganese. was visited, and treated the lower grade material, has been greater than their depositing action, so repeated at various depths but in thinner layers, The gangue minerals are quartz, barite, and but it has not yet received from the mine owners that cavities exactly similar to solution caves in and in the lower levels give way entirely to the country rock, angular fragments of which are fre­ the encouragement necessary for profitable opera­ limestone have been produced, and yet no ore has greenish variety. quently found in the eastern portion of the mine. tion. been deposited. Prior to the formation of the ore body the fis­ The amount of barite in the vein increases from GOL,r>-BEARLNG VEINS. COUNTRY ROCKS. sures were filled with a peculiar material, now the tunnel westward toward the Wedge mine, where consolidated and locally termed a "dike." It some parts of the vein are no more than a solid The gold-bearing veins are far subordinate to These ores are found in a number of the Meso- consists of a dense, homogeneous rock containing tabular mass of massive white barite about 3 feet the silver-bearing veins in economic importance. zoic quartzites altered sandstones which outcrop an immense number of angular fragments of black in width. The manganese-bearing carbonate occurs They are developed along the course of Uncom­ on both sides of the Uncompahgre Canyon. The shale and other country rock in a fine-grained in small amount, generally associated with the gray pahgre River between the mouth of Dexter Creek uppermost of these rocks, in which the majority of greenish groundmass closely resembling an erup­ copper. It is not a prominent gangue mineral. and the town of Ouray, outcropping in the steep ore bodies have been found, is the highly indu­ tive rock. This is a friction breccia formed in the The Bachelor mine alone, exclusive of the Wedge, cliffs on either side of the canyon. The country rated Dakota sandstone of the Cretaceous, which fissure. A later opening of the fissure furnished has produced nearly $2,000,000, although unfavor­ rocks which form these cliffs are, from below will be referred to as a quartzite. The others lie the cavities in which the ore was deposited. able conditions have materially lowered the profits upward, the red sandstones, conglomerates, and at varying intervals below this rock, and consti­ This breccia is as a rule 3 or 4 feet wide but derived from its exploitation. If the Wedge, Neo­ sandy shales of the Hermosa and Cutler formations tute a portion of the McElmo formation of the varies greatly in width and in many places makes desha, and Pony Express were included in the esti­ (Carboniferous); the red shales and sandstones of Jurassic, but there are many similar beds and they out in minute stringers into the wall rock, locally mate, the total production would be very much the Dolores formation (Triassic), lying unconform- vary so much from place to place that they can showing compact stringers only one-sixteenth of greater. Exact figures on these properties have ably-above at a slightly different angle; the white not be correlated with one another. It is a pre­ an inch in width that are yet perfectly solid and not yet been obtained. and gray sandstone of the La Plata (Jurassic); the vailing impression in the district that there are two exhibit all the clastic characters of the wider por­ The vein does not outcrop at the surface; owing alternating shales, sandstones, and limestones of the ore-bearing quartzites, an "upper" and a "lower," tions. It is formed so largely of £he clayey and to the very slight faulting involved in the forma­ McElmo (Jurassic); and the Dakota sandstone and which are readily traceable for great distances. aluminous material resulting from the comminu­ tion of the fissure it is lost in a mere distortion of black Mancos shale (Cretaceous). The inner and This impression is founded on error. The Dakota tion of the shales and limestones which make up the black shales which form its cap. These shales steeper walls of the canyon are formed of rocks of quartzite alone is a prominent and constant stratum. the larger part of the McElrno formation that it show an irregular twisting in the near neighbor­ the lower part of this column that is, the red beds Below this are many other quartzite beds, and of may originally have had the correct composition hood of the vein, but no fissure is observable and below the La Plata sandstone. the ore bodies in them it can not be proved that for a natural hydraulic cement. The seepage of not even the dike itself can be traced upward into Numerous dikes and irregular intrusions of any two occur in the same bed. This may readily hot mineralizing waters into the fissure would them. They have acted essentially as a plastic quartz-bearing monzonite porphyry cut diagonally be understood from the geological section (fig. 1), have supplied the necessary moisture, so that the medium, and even in the often repeated differential across the sedimentary rocks, some of them vertical, although even here it has not been possible to consolidation was perhaps accomplished more by movements the formation of the breccia and the others with a marked dip toward the south. On represent all the thinner quartzite beds. virtue of the chemical composition of the material later lateral, vertical, and horizontal movements the west side of the river irregular sheets extend The Dakota quartzite is a dense white rock than by a high degree of lateral compression as they have not been sensibly ruptured. The black- outward from these dikes and pass between the varying from 25 to 100 feet in thickness and suggested by Ransome l . shale fragments in the clastic dike are generally sedimentary beds. The small dikes weather so containing intercalated beds of shale, mainly at From the Bachelor tunnel the vein has been arranged with their flat surfaces parallel to the much more readily than the sandstones that they the center, but not constant in either position or walls of the vein. They were probably derived in can be found only at the bottom of deep clefts in amount. In the upper portions it is finer grained, 1 For a previously published description by Dr. F. L. Ran­ some see Trans. Am. Inst. Min. Eng., vol. 30,1901, pp. 227-236. part from the overlying black shale and in part the cliffs, but many of the larger dikes lie nearly the bedding planes are closer together, fine shaly Ouvay. 18 partings are observable, and it is slightly blackish, ore emanates from this fault breccia and makes out gradually changes to pyrite. This mineral is vertical fissures. The most important are in the owing to included carbonaceous material. Above from it into the country rock, but an examination either massive, with cavities here and there into Newsboy, the Pony Express, and the Mineral Farm it is a series of black shales that are extremely thin of the mine and a glance at the mine map, as well which complete crystals of pyrite project, or is com­ mines. In the Newsboy and Pony Express a fis­ bedded and so highly charged with coaly matter as a comparison with a large number of precisely posed of crystalline grains of pyrite embedded in a sure elsewhere productive as a vein is connected as to resemble strongly the shales commonly asso­ similar occurrences in quartzite, where no fault matrix of gray and white secondary quartz. Large with the deposits; in the Mineral Farm no impor­ ciated with coal. The Dakota is underlain by the breccia and no porphyry dikes are present, show admixtures of chalcopyrite and galena with other tant vertical fissure veins are found, and it is only greenish shales of the McElmo, and owing to the that there is no connection between the two. sulphides are found, among which are sphalerite, by close observation that the knife-edge fissures comparative ease with which these shales are disin­ The quartzite is seamed by irregular branching telluride of gold and silver (perhaps hessite, found may be noticed. tegrated and worn away it forms in most places an fissures, in general nearly vertical in position, some most extensively on the Jonathan claim), molyb­ At the Newsboy mine the limestone which car­ abrupt cliff. It outcrops on either side of the of them extremely narrow and others as much as denite, and gray copper (probably argentiferous ries the ore is bluish black in color, and is overlain Uncompahgre Canyon about 2000 feet above the 2 or even 6 inches in width. The displacement tetrahedrite). Barite is also of very abundant by clay shale and sandstone and underlain by thin- bottom of the valley and follows the indentations along the fissures is very slight, the maximum occurrence in the sulphide ore, and fibrous bands bedded black shales and a heavy gray sandstone. made by the steep gulches on either side. observed being 10 inches. They are filled with of gypsum were seen between the ore and the over­ It is about 5 to 7 feet in total thickness. The white crystalline quartz and almost invariably show lying black shales. The sulphide ore is more shoots of ore here are flat masses and as a rule fill DISTRIBUTION. a perfect comb structure. The wider portions show common directly beneath the black shales, as are the entire space between roof and floor. Laterally The ores are widely distributed along the out­ open vugs lined with quartz crystals. The fissures also the more extensive ore bodies. the valuable portions of the ore material extend crop of the formation. Among the mines and intersect one another in two prevailing directions, The contact between the ore and the quartzite from 5 to 30 feet from the fissure, although the prospects are the American Nettie, Valley View, though many local variations and irregularities is in most places very sharp and shows the irreg­ silicification goes much farther. The shoots run Ceutar, Rock of Ages, Samoa, and Stenographer. occur. Most of them pass upward into the shales, ular and undulating surface so often seen in replace­ nearly east and west, parallel to the direction of In form, mode of occurrence, and contained values where they are generally lost, but those on Cascade ment deposits. the fissure, for distances of 300 feet or more in all these deposits are similar, differing only in Creek pass into the porphyry sheet, which rests The value of the ores is almost entirely in gold. places. Most of the shoots lie north of the vein. unimportant details. In but one mine, however, directly upon the quartzite. They are therefore Silver is subordinate, although as much as 120 The ore-bearing limestone bed is at the base of the the American Nettie, have the ore bodies yet later than the porphyry. A few of the fissures are ounces is found in some places, especially where McElmo formation, immediately above the La Plata proved sufficiently extensive to be of commercial wide enough to be termed "fissure veins," and carry gray copper and galena occur mingled with pyrite. sandstone. Its position may be seen in the Gold value. The general description of this class of ore minerals in sufficient quantity to be mined as The gold values are unusually high, some of the Hill section shown in fig. 1. ores will therefore best be accomplished by a such, but these are uncommon. ore yielding as much as 30 ounces. At the time The same limestone bed extends along the canyon description of this mine. Along the course of the fissures ore bodies or the mine was visited the ore would average when to the Pony Express mine, where it is about 30 feet shoots extend outward for as much as 20 feet. sorted about $60 per ton. Owing to high freight and in thickness, and is overlain by shales. The shoots AMERICAN NETTIE MINE. The most typical of these shoots are irregular smelter charges $35 per ton is considered the lower in this mine are much larger and thicker than usual. The openings of the American Nettie mine are bodies, ranging from a few inches to 15 feet in limit for sulphides. In the early days of mining They extend into the hill for 1000 feet along the located near the lower portion of the steep cliff thickness, longer in the direction of the fissure than the values were uniformly higher than at present. course of the vein, and are in many places 100 feet which forms the top of the canyon wall on the east across it, and with their larger dimensions in a hor­ Between 1889 and January, 1905, the mine wide. The stopes now form great chambers, sep­ side of the Uncompahgre, 1800 feet above the bot­ izontal position. Many of them are exceedingly produced 23,641,316 pounds of ore, valued at arated by irregular pillars of limestone or ore, and tom of the valley. The buildings that have been irregular, wandering through the quartzite in all $1,464,923.35. This would give an average value are very suggestive of a coal mine. The ore in directions, but generally they are concentrated along of $123.12 per ton, or, roughly, 6 ounces of gold places occupies the entire 30 feet of rock, but is a definite bed, as a rule just under a fine shale per ton of sorted rock. This agrees very closely more commonly 8 to 10 feet thick. It is richer band, which is locally so thin that it can be detected with the average values obtained from the Bright immediately beneath the shale roof, but is not only where the rock is broken parallel to it. In Diamond quartzite ores and the most important of everywhere in contact with it. The limestone Quartz monzonite the vicinity of the supplying fissures some of the the other similar though smaller deposits found where un mineralized is much brecciated and full porphyry 500' shoots are pear shaped, with their longer axes par­ along the outcrop of the Dakota quartzite. of cavities, which have probably been more impor­ allel to the planes of stratification of the quartzite, Much of the ore is shipped direct. It is trans­ tant in determining the mineralization of the rock the ore solutions having seemingly passed laterally ported by a wire-rope tramway from the mouth of than the overlying shales. In the Mineral Farm ' N , - . ' ' - - \ , \ ~ N -V ' - '^ along the bedding. The largest shoots lie imme­ the mine across the Uncompahgre Canyon to the mine the limestone belongs to the Ouray forma­

Black carbon­ diately beneath the black shale at the top of the ore bins along the side of the railroad. A mill tion. The ore shoots are here from 8 to 10 feet aceous shale 40'i quartzite. In places several tiers of shoots occur adjoining the ore bins has been used to concentrate thick and 6 to 50 feet wide. The workings follow along a single fissure. the leaner sulphides. It is not in operation at the one shoot for a distance of perhaps 600 feet. This The ore in these masses is located chiefly near present time. The tramway is a striking feature of shoot has a strike of N. 63° E. The narrow fis­ the supplying fissures, where it forms a solid mass, the mine. It has a total length of 4100 feet and a sures with which the ores are associated may be but outward along the strata it only partially fills span across the canyon of 1800 feet. The highest seen at many places in the roof of the ore, but are open cavities, merely lining the interior in many point of this span is 900 feet above the bottom of not noticeable without careful examination. of them. On the extreme outer limit of the shoots the valley. The ores of the limestone replacement bodies are only empty cavities lined with quartz crystals are bluish gray or buff, according to the color of the OTHER MINES. to be seen. These contain no ore whatever, and original limestone. They consist of fine-grained many of them extend for 30 feet beyond the ore Except in point of production the other deposits silica heavily charged with crystalline barite and shoot. It is a saying among the miners that as in quartzite closely resemble those of the Ameri­ containing argentiferous gray copper (freibergite), soon as one finds crystals in the quartzite there can Nettie mine. From a lower quartzite in the galena, and in some places chalcopyrite. The FIG. 4. East-west section of the rocks and ore bodies in the American Nettie mine, Ouray, Colo. will be no more ore. These empty cavities have Bright Diamond mine high-grade ore to the value metallic minerals are either disseminated in small all the characteristics of cavities produced by solu­ of $32,000 was obtained from small shoots. It is grains through the silica and barite especially the erected near the entrance are about 100 feet above tion in limestone and can in no wise be distin­ probable that further exploration may reveal other gray copper or concentrated in bunches of irreg­ the base of the perpendicular bluff, and so little guished from them except for the lithologic important bodies of ore similar to those of the ular form and character that are distributed at support does the rock face afford that they are kept character of the containing rock. There can be American Nettie. uncertain intervals through the siliceous gangue in place partly by wooden brackets, and except for no question that they were produced by solution. rock. their modern construction might well pass for cliff The ore was first discovered on the face of the REPLACEMENT DEPOSITS IN LIMESTONE. Irregular cavities lined with druses of quartz dwellings. The position occupied by the country quartzite cliff and occurred in irregular open cav­ A series of broad, flat ore bodies are found in crystals form a prominent feature of the ore. In rocks may most readily be understood from the ities, which were exposed by the breaking away of beds of limestone either adjoining fissure veins or the unoxidized portions silver chloride and second­ little sketch forming fig. 4. Above the mine is the outer portions of the rock. Here, and for a associated with numerous small vertical or nearly ary black copper sulphide can be observed at many an immense sheet of quartz monzonite porphyry long distance into the side of the hill, it consisted vertical fissures that intersect the stratified rocks. places. The silica extends to great distances beyond aggregating about 500 feet in thickness, underlain of a brownish, iron-stained material resembling These deposits have been formed by the removal of the metallic replacements and continues locally for by 40 to 50 feet of fine black shales, very heavily limonite and containing free gold, either too fine limestone and the simultaneous substitution of ore the entire visible outcrop of the limestone bed. charged with carbonaceous matter and containing for observation or in the form of wire gold. It minerals for it by mineralizing waters, which here In the Pony Express mine, however, the siliceous a few thin beds of black sandstone. Conformably did not entirely fill the cavities, but was concen­ gained access to the more soluble rock by means of replacement is slight, and the gangue of the ore below these shales is the ore-bearing quartzite, a trated in their lower portions. The gold was crys­ the fissures. They are more regular than the consists to a great extent of immense masses of massive white rock, fine grained and blackish at talline and was almost invariably richer at the replacements of quartzite just described, and much barite in otherwise unaltered limestone. the top and coarser grained and white below. It bottom of the mass. The present foreman reports more extensive, a few of them being fully 300 feet In the massive sulphide bodies the gray copper is separated into two portions somewhat below the only wire gold, but it is stated that large nuggets wide. Gold predominates in some of them, silver can be readily seen, but in the usual run of ore it middle by a thin parting of light-colored shales. were found, some of them of great size. The in others, but the ores which they furnish are uni­ is apt to be disseminated through the gangue in Below this quartzite lie the variegated shaly rocks brown material is undoubtedly limonite in great formly of lower grade than those in quartzite. The grains so small as to be hardly perceptible to the of the McElmo formation. These beds dip into part, but in all probability further examination silver-bearing deposits are associated with the silver- the hill at a low angle, from 5° to 10°, slightly will show that it contains a large percentage of bearing fissure veins already described, and are The larger structures of the country rock, such to the east of north. Locally, however, the dip ferric sulphate. Mixed with the oxidized ore are found only where these veins penetrate strata of as brecciation, bedding planes, layers of shale, etc., is much steeper. All the rocks are cut by a num­ siderite, barite, kaolin, and sulphur. soluble limestone. extend uninterruptedly into the ore, except where, ber of dikes of dark-colored fine-grained porphyry, In many cavities honeycombed quartz is observ­ These replacement deposits may be most conven­ as in the bodies of sulphides, the mineralization has probably connected with the monzonite sill above; able, as much as 4 or 5 inches in thickness, either iently classified according to the prevailing gangue obliterated them. some of the ore is found beside them, but the larger lining the cavity or completely filling it. Some of minerals present, as baritic siliceous ores (silver The values are mainly in silver. The gold part is widely separated. the cell-like spaces in this quartz are irregular, but bearing) and magnetite-pyrite ores (gold bearing). content rises in a little of the ore to $4 or $5 per A heavy fault fissure filled with a consolidated many have the form of pyrite crystals, showing All the ores of both groups differ a good deal from ton, but is generally so low as to be negligible. In breccia similar to that observed in the Bachelor clearly that they are caused by the oxidation and one another, so that collective description will give the largest masses, such as those in the Pony mine runs eastward and is followed by the main removal of former masses and crystals of iron sul­ only a most general idea of their character. Express and Mineral Farm mines, the values in tunnel of the mine. Locally it is simply a fault phide. Formerly much of the oxidized ore was the ore are extremely irregular and uniformly low. fissure with no filling material in it. The breccia shoveled directly from the cavities into ore sacks BARITIC SILICEOUS ORES. The ore from the Pony Express is reported to is termed a "dike," but is not correctly so consid­ and shipped, but careful sorting is now necessary. The baritic siliceous ores are lateral enrichments average as a whole about $30 per ton, although ered. It is the impression of the miners that the On penetrating farther into the hill the ore of silver veins or flat masses associated with small much higher values are encountered. In the Min- 19

era! Farm mine the average is so low and the richer Cobbs Gulch and Cow Creek. It has been pros­ be found. At and near the tunnel on Middle Fork the chief value of these waters is not for uses within portions of the ore are so irregularly distributed pected for gold, and an experimental stamp mill of Lou Creek there is almost nothing to be seen of the quadrangle, but rather for the irrigation of the that the deposit has never yet paid for exploita­ has been erected near the mouth of Cobbs Gulch the strata above or below the coal. This tunnel lowlands of the Uncompahgre and Cimarron valleys tion. In the Newsboy the values are uniformly to test its value. The possible occurrence of placer exposes coal to a thickness of 15 feet or more, the to the north. higher, and many carload lots will carry as much gold in this rock forms a problem of unusual scien­ base not being seen, and at about 30 feet from its Irrigation. The irrigable lands of the .Ouray as 100 ounces of silver. It is difficult to form a tific interest on account of its bearing on the geo­ mouth reveals the shaly roof, which has a strike of quadrangle comprise chiefly the flood plains of correct estimate of the production of this class of logic period during which the ore deposits of the N. 78° W. and a dip of 25° NW. The structure Uncompahgre River and Cow Creek and the ores, as they have generally been worked intermit­ region were formed. thus indicated is probably that prevailing in this gravel benches adjacent to those streams. By tently under lease. If the conglomerates carry placer gold, there vicinity, though perhaps with considerable varia­ far the largest area is the bottom land of the must have been ore bodies containing free gold in tion in both strike and dip. If the northwesterly Uncompahgre Valley, extending from Portland MAGNETITE-PYRITE ORES. existence before their deposition. It can be proved dip is prevalent, it is probable that the other coal to Bidgway, a distance of 6 miles, and having an The magnetite-pyrite ores are of greater scien­ that the known ore bodies were formed at or later opening, situated a mile to the southeast and about average width of 1 mile. This land is practically all tific than commercial interest. They have been than the close of the Eocene epoch. The conglom­ 300 feet higher than that first mentioned, is in the under irrigation by water taken from the river and found at only one locality in the Bright Diamond erates, however, were formed in the early Eocene same coal bed. At this latter point the workings several tributary streams, the principal ditches being and Iron Clad mines, on the east wall of the and were clearly in existence before any of the have caved in, but it is clear that the coal bed is shown on the topographic sheet. Narrow bands of Uncompahgre Canyon, about 600 feet below the known gold ores were deposited. The source of comparable to that of Middle Fork of Lou Creek bottom land along the Uncompahgre below Bidg­ American Nettie mine. The ore occurs in a fine­ the placer gold therefore in case it is proved to in thickness, and is of the same general character. way and on Cow Creek are also cultivated. grained dark-blue limestone, apparently quite pure be present must be sought in deposits belonging Blocks of creamy-yellow, friable sandstone are Next in importance to the alluvial lands are the and, where unmineralized, without any evidence of to an earlier geologic period deposits of whose found about this opening, but no distinct outcrops lower gravel benches bordering Cow Creek and the alteration. This stratum is about 10 or 15 feet existence there is as yet no evidence. occur. Uncompahgre below Dallas, the extent of which thick and is overlain by green shale that has been Gold may, of course, have been deposited in To the north from the coal banks of Lou Creek may be seen from the geological map. The soils altered by the monzonite-porphyry intrusions into these gravels long after they were consolidated scanty evidence was found showing that the meas­ of these benches are perhaps in part eolian, but are a green porcelain-like argillite heavily charged with into rock, but in that case the character of the ures descend with gentle dip, and on Deer Creek, largely derived from the low alluvial fans of the epidote and other metamorphic minerals, and of an gold present would show clearly that it was not about 1 mile north of the Ouray quadrangle line, side streams. The principal ranch lands of this exceedingly dense, impervious character. The ore deposited by placer action. are two openings. The lower of these is at an ele­ origin are on the east side of Cow Creek and they occurs in broad, flat shoots, conformable to the With these facts in view it seemed necessary to vation of about 8950 feet, and is presumably in are mainly irrigated by the waters of Owl, Nate, stratification and having a uniform thickness of determine two points (1) whether the gravels con­ the same coal bed shown on Lou Creek, or in an Lou, and Deer creeks. Some water for these about 6 feet. It lies in close contact with the shale tain appreciable quantities of gold; (2) the physical adjacent one. The workings are caved in, but it benches is also obtained from West Fork of Cimar­ roof. The strata are nearly horizontal, but show condition of the gold, if present. For this purpose may be seen that the coal is about 15 feet thick and ron Creek by a ditch crossing the low divide north local dips, and are intersected by a series of widely a series of nine carefully selected samples were taken that massive but crumbling saccharoidal sandstones of Chimney Peak and discharging into Owl Creek. spaced fractures which run in a nearly east-west from portions of the rock most favorable for the occur, both above and below, within a few feet, the Between the Uncompahgre and Cow Creek, in direction and fault the strata in many places as occurrence of gold, and from widely separated details of the section being obscured. The lower the basins of the intermittent Dry and Alkali creeks, much as 10 feet, with downthrows here to the north localities. Some samples were collected from places sandstone has a northeasterly clip of 3° to 5°. there is some land cultivated by the aid of water and there to the south. The ore outcrops for a recommended as especially rich; others at points Not far from the above-mentioned opening is from Cow Creek. The soil is here largely adobe, long distance in the cliff near the mines. In the where no prospecting had been done. The sam­ another, at an elevation of about 9400 feet. A derived from the underlying Mancos shale, and Bright Diamond mine a porphyry dike 10 to 30 ples were very carefully assayed and four of them tunnel has been driven into the coal, but neither several ranches of this area have been abandoned. feet in width occurs north of the ore, and the heavy panned, the concentrates being tested microscop­ top nor bottom is- exposed. Fragments of a buff Small patches of arable land are under cultiva­ monzonite-porphyry sheet above the American Net­ ically. The results showed a uniform gold content sandstone with indistinct plant remains are scat­ tion at higher levels than those mentioned, adjacent tie mine lies about 700 feet higher. The ore is of 0.005 ounce, or 10 cents, per ton, the panning tered on the surface near the tunnel but gravel to Coal Creek and other minor streams. No attempt not in contact with the dike, but is some distance tests giving small grains of magnetite sand, but no conceals the details of the section. So completely has been made to cultivate the meadows of Cimar­ south of it. colors. is the coal-bearing formation covered by superficial ron Creek within this quadrangle, the elevation All the rocks except the unmineralized portions While these determinations are by no means the materials that it can not be determined whether being too great. The water of Cimarron Creek of the blue limestones have been profoundly altered result of an exhaustive sampling, they clearly show these two openings on Deer Creek belong to the reaches Gunnison River above the tunnel now by the intrusions, so that they are now very porce­ that if gold is present in this rock in appreciable same horizon or not. If they do there must be a under construction for the diversion of the river lain-like in their character. The shoots where quantities it must be contained in those portions of fault of several hundred feet between them. into the lower Uncompahgre Valley. explored are about 300 feet wide, and have been the deposit from which no samples were taken. A Coal is mined at the upper tunnel for local use The crops of the Ouray ranches are principally followed into the hill in one place for 400 feet. content of 10 cents per ton in gold is so small that of ranchmen. It is reported to have been tested hay, largely used for the winter feed of cattle, with Wherever the workings leave the ore they pass into it can hardly be determined even by the most care­ for steam-producing purposes at Ouray, but found some grain, potatoes, and other vegetables for the unaltered blue limestone singularly free from the ful methods of assay, and since almost any rock in to be less satisfactory than the coal of the market. local market. The range of products is limited by contact rnetamorphism that has altered the more a mineralized region in the neighborhood of ore No tests fully determining the character of these the shortness of the season, due to the altitude, the argillaceous rocks of the series. deposits may contain values equaling or slightly in coals have been made. lowest point on the Uncompahgre within the quad­ The ore lies in close contact with the shale roof. excess of this amount, the inference seems justifi­ The Mesaverde strata of the Ouray quadrangle rangle having an elevation of 6700-1- feet. It is an intimate mixture of a dense, granular able that no gold-bearing lodes existed prior to the are at the southern extremity of an isolated area of Underground waters. The abundance of surface magnetite and pyrite, with a little chalcopyrite, deposition of the conglomerate. The purely neg­ this formation which extends northward along water and the small area of arable land have made interwoven in the most complicated manner with ative results shown by these samples seem also to Cimarron Ridge for about 12 miles, with a width it unnecessary to develop the underground-water epidote, actinolite, garnet, quartz, and calcite. In afford to those interested in mining in this region of 3 to 5 miles. The edges of the nearly horizon­ resources of this district. There is probably a con­ the Bright Diamond mine a large fault-fissure vein a strong reason for the exercise of extreme caution tal strata are exposed at few places in this district, siderable underflow in the Uncompahgre Valley, lies on the north side of the ore, between it and in the exploitation of these Eocene gravels. the landslide and talus debris from the overlying but it is not needed under existing conditions. No the monzonite-porphyry dike> and it was in the MISCELLANEOUS MIKEKAL RESOURCES. San Juan beds or uppermost Mesaverde sandstone great flow of this character can be present elsewhere prospecting work on this fissure that the ores were By WHITMAN CROSS. concealing them effectually. The coal has been unless it is in the Cimarron Valley. first discovered. opened at the north end of the field, a few miles It may be seen by a glance at the geological map The ore carries $10 to $14 in gold and is of COAL. from the Denver and Bio Grande Bailroad, but that the area within which all possible water-bearing nearly uniform value, but is concentrated with Coal occurs within the Ouray quadrangle in the either the character of the coal or other controlling strata outcrop is very small, and that the outcrops difficulty and has not yet proved profitable. It is Dakota and Mesaverde formations and has been conditions have prevented important development. are not far above the valley, hence the supply of not improbable that magnetic separation may be of developed to some extent at both horizons. artesian water is probably inconsiderable and under material assistance in its exploitation. In the Dakota formation. Coal is present com­ BUILDING STONE. low pressure. Furthermore, the fault crossing the monly within one or both of the zones of carbona­ There is no lack of fairly good building stone in Uncompahgre between Dallas and Ridgway must GEOLOGIC AGE OP THE OEE DEPOSITS. ceous shale characterizing the Dakota formation, the Ouray quadrangle, but the local demand is interrupt the circulation of water derived from the Wherever the geologic age of the ore deposits as described on page 6. These zones occur near naturally small and the abundance of good stone mountains. These facts make it probable that can be ascertained with measurable accuracy the the middle and top of the formation, and the coal, in the field which might afford a market has dis­ artesian wells of much importance can not be evidence points to a single period of mineralization which is a striking feature of many outcrops, has couraged the development of quarries for export. secured for the ranch lands of the Ouray quadran­ for all the deposits discussed. They are of very been opened and worked to some extent at several Some of the light-red sandstones of the Hermosa gle. recent formation; all of them are later than the localities, especially in the northwestern part of the formation have been used at Ouray. The darker Thermal springs. Several thermal springs occur latest of the igneous rocks, as many of the frac­ quadrangle, where it is easily accessible. This red sandstones of the Cutler and the gray rock of within the Ouray quadrangle. One of these issues tures in which they have been formed extend into Dakota coal is subbituminous in character and few the La Plata might be easily obtained from acces­ from the lower part of the Hermosa formation, on the eruptives and some of the veins lie wholly or none of the beds are more than 1 foot thick. sible outcrops. the eastern border of the town of Ouray, and is within the dikes. These eruptive rocks were It has never been mined except for local use and used in two bath houses. According to the pro­ LIMESTONE. formed later than the andesites of the San Juan lost even its limited importance with the introduc­ prietor of this spring, Mr. John McLeod, the water formation, as in many places they cut the andesite tion of better coal from other formations on the Inexhaustible supplies of limestone fol* all pur­ has a temperature of 125° F., and fills a 3|-inch and spread out beneath it in the form of horizontal advent of the railroad. poses are furnished in the Ouray and Hermosa pipe, with a pressure of 25 pounds to the square sills. The eruptives are therefore later than the In the Mesaverde formation. Coal measures formations, adjacent to the railroad, and they have inch. No analysis of this spring water has been andesite breccia, which is of Eocene age. We can much more important than those of the Dakota been used to some extent for lime and as a flux in made. then conclude that the ore was formed at the close occur in the Mesaverde formation at the head of smelting. Analysis of water from hot spring at Ouray. of or later than the Eocene epoch. Lou Creek, on the western slope of Cimarron Grams per Bidge. The coal has been opened by short tun­ WATER RESOURCES. liter. SUPPOSED GOLD-BEARING CONGLOMERATES. SiO s _____--____-____-____--____-___--___-__--___ 0.3760 nels at the only two points where outcrops are dis­ As in all districts in the border zone between 8O 3 .______.__-_-____.______.___-___.. ___ .6263 Underlying the San Juan andesite breccia, which tinct. The locations of these openings and the mountains and a semiarid plateau country, the 01 ______. _._._ .. _ _ - .0361 CO 2 ______.0323 forms the cap rock of the hills in the Ouray quad- roads leading to them are shown on the maps of water resources of the Ouray quadrangle are of OaO ____-__-____-___-_____-_-___-____--___.--_ .4966 rangle, and rests unconformably upon the stratified this folio. much importance. The heavy snow and rainfall MgO___ .0173 Na,O _ _ .0979 rocks below, is a formation known as the Telluride Landslide debris and glacial gravels conceal not of the mountains within or adjacent to the area FeO .___.___.______...__._____..__.______._._ .0202 conglomerate. This is not everywhere present, but only the coal but other strata of the Mesaverde for­ feed several perennial streams, which are used for 1. 7027 occurs in large development in the vicinity of mation so extensively that but few outcrops are to irrigation and development of water power. But Total residue on evaporation 1.880 grams. Ouray. 20

Other springs within the town limits issue from water of this spring yielded the result shown in ginous tufa covering several acres. According to no doubt derives its iron content. Peale refers to the Ouray limestone near the fault line which the foregoing table. the owner. Mr. L. F. Orvis, jr., the water has a these springs in the Hay den Survey report for crosses both Canyon Creek and Uncompahgre Another spring, or group of springs, occurs on temperature of about 110° F. It issues under 1875 as follows: "A short distance above the River, a short distance above their junction. One the eastern side of the alluvial flat of Uncompahgre slight pressure and the discharge is sufficient to mouth of Dallas Fork are the springs which give of these, situated on the river about 50 feet above Park about 2 miles southeast of Ridgway. The irrigate 15 acres of land. No analysis has been the name of Uncompahgre red-water spring to the mouth of the creek, has a temperature of 68° C. water is heavily charged with iron and there has made. The water presumably comes from the the river and park." According to Mr. G. E. Kedzie an analysis of the been deposited from it a thin sheet of red ferru­ red strata of the Cutler formation, from which it June, 1906.

OLPMMJy nrio

GENERALIZED SECTION FOR THE OURAY QUADRANGLE. SCALE: 1 INCH = 1000 FEET.

COLUMNAR THICKNESS SYSTEM. FORMATION NAME. SYMBOL. SECTION. IN FEET, CHARACTER OF ROCKS.

<% ixgogggg*

An alternation of rhyolite, quartz latite, and pyroxene andesite flows and tuffs, the flows greatly predominating. Potosi volcanic series. TP 2000 g=rg:"T^ :^L"'. Some of the thin flows in Whitehouse Mountain are glassy.

aV^^v^V r^^^=S~^r^' -=OL >--»"__ ,^ ^ ^V^*' * ;% ? / 75 Fine-grained greenish tuff, of andesitic and latitic material, well stratified and nonfossiliferous. Greatest known (n Henson tuff. T\ development is in the Lake City quadrangle. IU E toUJ \-__^~~-~- - "---jg.ga. o *- . z < . A succession of andesite flows and tuffs, characterized by both augite and hypersthene. Greatest known develop­ > j^ Pyroxene andesite. Tpa 60 2500 ment is in the central part of the Silverton quadrangle. In the Ouray quadrangle only flows are known to occur. DC o > L.....?,.,jiffrzr...-.:.....B»3g,jhj.a-.il,.. ._ , s^r. Tl 'iJljr'jgir z =*===> 1 . -jf... -_» ^ < o J_-_!1_SL. » M N* _g_'--j H ,_____^ ...I.JJ.I > -. J-.^l.;-;^^ £ Ul h- ^ - J- ' ~ ~~~~_ -~~~I.~~"i_ _^- -- '~"7i _1 UNCONFORMITY DC Fine-grained dark-green latite tuff containing lenses of impure limestone. Scanty fossil leaves and shells in calca­ w Burns tuff. n/ *:^;iv*.*H*'->---*v< \ 100 reous layers. UJ . a-. '«' .* ? ' ' h- ' * t» o 'J? * ".* '.^. +* %vV^ .""-.&" >'*-:x >:;. :

; * ', .* '» ' ' ** ' -1 ' . '. .» . " > * . Almost exclusively andesitic debris. Near the base is a well-stratified tuff which becomes coarser and less distinctly bedded in its upper portion. Fossils are not known to occur. The beds vary greatly in thickness, owing to San Juan tuff. T=j 3000 irregularity of deposition and to erosion preceding the eruption of the Silverton series. The maximum thick­ ''. :t-'-v.<''^v'-Vr ness observed is in the vicinity of Portland Creek. ^'. 's^.-..'a- .« ' .' .* .V, ",-' ' -t-k^,-:-;.-^.--'.;-.^:-:^;^;^^'i'^ .i-^-^^'-'--'-:^- «-1- . v"-: .""A -'t^ r:-; :"'-v £ * ' k» ; V^."i.>- '?':; : v i-'^Vi*';H%%V 'cJS-^t>'-^^V/'-';/i"^ '^.'.ft'iaj^:.^'*' ''» "«' 5.- Coarse conglomerate containing pebbles and bowlders of schist, granite, quartzite, limestone, and other rocks of Telluride conglomerate. T«\ r^ f^7 A?.i' .V -.>>* ._« - - * ^eZ /35-100 Paleozoic age, with fine sandy limestone locally. UNCONFORMITY tt. '-* *..;. ;:^.- ::^^~, A series of gray or yellowish quartzose sandstones and sandy shales, massive and cross-bedded in the lower portion Mesaverde formation. Kmv 300+ and containing a number of productive coal seams in the upper portion. Invertebrate fossils numerous.

m~^ '. " * '^^' . r===

CRETACEOUS Soft dark-gray or almost black carbonaceous clay shale containing thin lenses of impure limestone. Embraces the Mancos shale. Kmc 1200 Colorado group and a portion of the Pierre division of the Montana group. Fossils more or less abundant.

-^r 1^^-"""""-""-^ / /100-200 Gray or rusty-brown quartzose sandstone with a variable conglomerate containing small chert pebbles at or near Dakota sandstone. Kd\ / the base. Carbonaceous shale and coal of poor quality locally present. Indistinct fossil leaves occur sparingly. yfe;?^^^!^^ o A series of alternating, friable, fine-grained, yellowish or grayish sandstones, shales, and thin limestones. The sand­ c/y McElmo formation. Jme 500-800 stones often include flakes of grayish clay -or shale. The shales are chiefly green in color but may be pink, dark <:CO red, or chocolate-brown. No fossils found. cc ra Two massive, friable, white, cross-bedded, quartzose sandstones with a narrow band of dark limestone or calcareous La Plata sandstone. Jlp\ X 100 shale between. The limestone is locally brecciated. No determinable fossils found. UNCONFORMITY TRI- sx^ 100 Sandy marls and fine-grained sandstones and shales of bright-red color with fine limestone conglomerate near the "fid / base in which are found teeth and gasteropod shells indicating Triassic age. UNCONFORMITY

^^P^L.^.^.^.^k

>'

- »o *-'OCioO*oOoo O "» *?A^ t?.°A -o9 *OV S?- .' Hv.:t^Tv.: :;v::v:

0 00 0 0 CJ0 '0*C 0 »0 »'OT

'-A--, '. . ; '..' .'. :

A series of grits, sandstones, shales, and limestones of various distribution and development. Grits and sandstones Hermosa formation. Ch 1200-2000 in massive beds predominate in the middle and upper parts of the section, the lower portion consisting of thinner bedded sandstone, shale, and limestone. Numerous invertebrate fossils occur in the shale and limestone......

~ \ /0-50 ing Pennsylvania!! fossils. *' MISS. .* UNCONFORMITY i i i i r " Ouray limestone. DCo Limestone, lower third shaly with thin quartzites. Devonian fossils in lower two-thirds and Mississippian fossils in upper part. DEVO­ ' -^^t*J- i i i T3'" 175 NIAN Elbert formation. DeX ^^P^iV-'./T!^ \0-50 Thin limestone, sandstone, and calcareous shale containing Devonian fish remains. fWk ' 4 ' ' m'-v"^ ALGONKIAN Massive white or smoky quartzite and dark slate, with a few zones in which the rocks alternate in thinner layers. Uncompahgre formation. . Au 8000+ No fossils found.

''%''' A'"'* '^^ ^^^i^^^^s ^r