DESCRIPTION OF NEEDLE MOUNTAINS QUADRANGLE

By Whitman Cross, Ernest Howe, J. D. Irviiig, and W. H. Emmoiis.

TOPOGRAPHY AND GENERAL GEOLOGY.

By Whitman Cross and Ernest Howe.

INTRODUCTION. tude and in the boldness of their forms. Some of volcanic or other rocks. South of Needle and region points to their own greater antiquity, for the more prominent ones are shown in figs. 4, 8, Johnson creeks and west of the Vallecito, however, they are either overlain or intruded by the others, Geographic position. The Needle Mountains and 9 of the illustration sheets. They are com­ this surface has been preserved and reexposed by except the Irving greenstones, from which they quadrangle is situated in parts of San Juan and posed of granite, schist, or ancient quartzites that the removal of the sedimentary rocks which cov­ are separated by the great mass of the Eolus La Plata counties in southwestern Colorado. It form-the core of a dome-like uplift, the influence ered it. To-day it can be recognized in the broad, granite. The oldest rocks in contact with them lies between 37° 30' and 37 J 45' north latitude of which may be noted in the outlying rocks for southward-inclined table-land, which has been more are those of the Needle Mountains group, of Algon- and 107° 30' and 107° 45' west longitude and 15 to 20 miles in all directions. This central mass or less deeply dissected by modern streams between kian age, and they contain in a few places charac­ contains about 236 square miles. Animas River, has been deeply dissected by streams tributary to which lie isolated mesas. Figs. 1 and 2 well illus­ teristic pebbles of the schists, conclusive evidence an important tributary of San Juan River, sepa­ the Animas or the Vallecito, and the various rocks trate these features. Fig. 2 shows, in addition to of the relative youthfulness of the Uncompahgre. rates the northwestern portion from the rest of the are well exposed and their relations to one another the north end of Stag Mesa, resting upon the Eolus For these reasons the gneisses and schists have quadrangle by its deep canyon, while Vallecito clearly shown. granite, the escarpment of Paleozoic rocks on the been considered to be of Archean age and were Creek, Florida River, and Bear Creek (one of the Canyons. Almost as striking a topographic fea­ west side of the upper Animas Valley and in the so described in the Silverton folio. forks of the headwaters of the Rio Grande) have ture as the central massif is the canyon of Animas far distance the La Plata Mountain group. Fig. Distribution. These rocks follow the canyon of their sources in the Needle Mountains. River (fig. 6), which crosses the northwest corner 1 shows Lime Mesa in the middle distance, seen the Animas from a point to the west in the Engi­ Relation to adjacent mountains and plateaus. of the quadrangle in a somewhat irregular course, nearly from the north. In the far distance to the neer Mountain quadrangle, in a band 3 or more The Needle Mountains form the southwest but­ in a gorge nearly 15 miles long and from 3000 to right and left, respectively, are the valleys of miles wide, northward to the Algonkian quartzites tress of the San Juan Mountain group, a region 4000 feet deep. At a few places the river flows Animas and Florida rivers. near Elk Park, with an arm extending northeast­ of considerable elevation, composed largely of late between rock walls and on a rock bottom, and at The comparatively low relief is in strong con­ ward from the mouth of Ruby Creek to Balsam volcanic rocks, that extends westward from the such points the roadbed of the railway has been trast to the sharp peaks of the Needles, and for Lake. They are interrupted at several places by San Luis Valley on the east nearly to Dolores blasted from the solid rock; but for the most part this reason the mesas form one of the pronounced intrusive bodies of granite. North of the zone of River and is limited on the north by Gunnison narrow strips of alluvium are to be found on both physical divisions of the area. * Algonkian rocks the schists again appear at the River. On the south the mountains are bordered sides of the stream at the base of the canyon walls, Culture. The difficulty of access and the rela­ mouth of Whitehead Creek and extend continu­ by foothills of sedimentary rocks that are drained and at Elk Park (fig. 6) and Needleton these widen tive unimportance of their mineral resources, ously along the northern border of the quadrangle by San Juan River and its eastern tributaries. out and form open "parks." The canyon itself is together with the rugged, forbidding character of from near Molas Lake to Bear town. These expo­ The Needles occupy an intermediate position an almost impassable barrier between the east and the Needles, have not encouraged permanent set­ sures continue for a few miles northward in the between the foothills and the volcanic areas. At west Needles. A few. trails extend up into the tlement. A few prospectors spend the summer Silverton quadrangle until covered by sedimentary the northeast corner of the quadrangle are a few mountains to the east, following the larger streams, months in the vicinity of Needle Creek and or volcanic rocks. outlying patches of the volcanics, and to the south but the western wall is almost without a break Chicago Basin, or at the head of Bear Creek. General character. The complex varies in pet- appear the gently upturned sedimentary rocks of and can be surmounted only on foot after long Elsewhere human habitations are rarely «seen, the rographic detail from place to place, but it possesses the foothills, that merge, toward San Juan River, and arduous climbing. log cabins of prospectors being now deserted and no clearly marked divisions which it is practical with the nearly horizontal beds of the plateaus of Needle Creek, which is the largest stream that falling into ruin, while the few trails are little or desirable to recognize as cartographic units. New Mexico. enters the Animas within the quadrangle, has traveled and bad. The most important signs of Although certain zones are lighter or darker in carved for itself a narrow valley that offers the man's occupancy are the Denver and Rio Grande color than others, the schists as a rule are a GEOGRAPHY AND TOPOGRAPHY OF THE QUADRANGLE. only means of reaching by a trail a pass to the Railroad, which follows the Animas Canyon uniform gray, but tricks of weathering may cause Physical features. Crossing the central portion Vallecito or the mesas to the south. It sharply upstream to Silverton, a few miles to the north them to have a distinctly banded appearance, even of the quadrangle from east to west is a range defines the southwest limits of the central moun­ of the quadrangle, and a storage reservoir which when seen from a distance. This banding is due of extremely rugged mountains of granite and tain area. Part of this valley is shown in fig. 4, has recently been constructed in Upper Park, near to alternations of nearly black amphibolites with schist, from which rise sharp peaks, the summits which presents a view looking across the lower part the head of Florida River, and which is Connected rocks richer in quartz and the feldspars, the various of four of which are over 14,000 feet in eleva­ of the valley from Mountain View Crest toward by a good trail with the ranches to the south. bands being only a few feet or yards in thickness. -, tion, while nearly 6000 feet below Animas River Pigeon and Turret peaks. Names. During the present survey names have The general character of the complex is made rushes through a canyon which separates the The Vallecito, which heads at the Continental been given to a considerable number of peaks and still more varied by the presence of granite bodies West Needles from the main group. Adjoining Divide, in the extreme northeast corner of the other prominent features in the quadrangle which which have intruded the schists and which have these peaks on the north is the Grenadier Range, quadrangle, can hardly »be said to occupy a can­ had previously possessed no such means of identi­ sent out irregular arms and dikes, in some places whose serrated crest of quartzite extends from Ani­ yon in its upper portion, but rather a deep valley fication. Endlich Mesa, between Florida River and coinciding with the direction of schistosity, else­ mas River eastward to Vallecito Creek. These, hemmed in by lofty mountain walls of quartzite or Vallecito Creek, was named after F. M. Endlich, where crosscutting. The most striking , example with the main mass of the Needles and a few high granite (fig. 7). Below its junction with Johnson of the Hayden Survey, who first made there a of this Js the Twilight granite, which, for 2. miles peaks of quartzite, form a mountain group that Creek the walls come closer together and canyon collection of Devonian fossils from the Ouray, before Xnimas River leaves the quadrangle, ribs is the most important physical feature of the conditions prevail, culminating in a mere rift in limestone. Mountain View Crest and Overlook the canyon walls of black amphibolite ; with its quadrangle. the rocks 1000 feet deep where the stream leaves Point, as well as many of the names of the nearly white parallel dikes. So numerous are South of the mountains, and separated from them the quadrangle. Florida River also drops sud­ Grenadier Range, have been suggested by pecu­ these dikes that it has been impossible to represent by the deep valley of Needle Creek, is a plateau denly into a very bare, narrow canyon a short liar topographic features. Irving Peak, after Prof. them, even diagrammatically, on the map. An dissected by Florida River and tributaries of the distance below Lower Park. Not the least strik­ R. D. Irving, seems an especially appropriate name area has therefore been outlined in which the Animas and having a slight inclination to the ing of the canyons is that of Lime Creek, a part of for the first and most prominent peak of Algon­ Twilight intrusive occurs either exclusively or south and southwest. It is composed of the same which lies just inside the western boundary of the kian rocks seen on ascending the Vallecito. in excess of the Archean schists; but within its granite that forms the main mass of the Needles, quadrangle. This canyon limits the West Needles Emerson, Amherst, and Greylock mountains all limits considerable schist may often be found. and on this rest isolated remnants of sedimentary in that direction. possess some memorial significance in their names. The character of these intrusions and their rela­ rocks of early Paleozoic age. West Needles. This group of peaks belongs The peafe of quartzite extending in a bow from tion to the schists will be discussed later. The The Continental Divide crosses the extreme structurally with the main mass of the Needles, the Animas Canyon to the Vallecito have been schists on the north side of Whitehead Creek and northeast corner of the quadrangle from north­ but topographically it is more closely related to named the Grenadier Range, from their charac­ east of Molas Lake are cut by a complicated west to southeast. This is a region of moderate Mountain View Crest and the mesas south of it. teristic profile. network of dikes and veins that are doubtless relief, when compared with the neighboring Gren­ That is, the summits of the West Needles form the offshoots from the larger granite masses in the adier Range, though its elevation is almost every­ outer rim of the core of the dome-like uplift and DESCRIPTIVE GEOLOGY. Silverton quadrangle. where above 12,500 feet. On one side are the from them the surface of the older rocks upon ROCK FORMATIONS. Petrography. When examined in detail the sources of the Rio Grande and on the other rise which the younger sedimentaries rest slopes gently ANCIENT METAMORPHIC ROCKS. schists show great diversity both in composition Vallecito Creek, an important tributary of Pine away as it does south of Needle Creek. The West and texture. The commonest variety is a moder­ River, and Elk Creek, a branch of the Animas. Needle Mountains extend from Snowdon Peak SCHISTS AND GNEISSES. ately fine-grained, well-laminated rock of a dark- Central granite peaks and the Grenadier Range. south westward to the junction of Cascade Creek Age. A considerable area within the quad­ gray color which on weathered surfaces becomes a The central portion of the Needle Mountains quad­ with the Animas, and to them rightfully belongs rangle is occupied by gneisses and schists which lighter shade of reddish brown. Quartz and feld­ rangle is occupied by massive peaks of granite or the outlying mountain called Potato Hill, in the have every appearance of being of very ancient spar can be distinguished in fine grains and with quartzite that have, on account of their wonder­ Engineer Mountain quadrangle to the west. origin. They have been profoundly metamor­ them is hornblende or a dark mica. Bands of fully sharp pinnacles and abrupt faces, suggested Mesas. Sloping away gently in all directions phosed by compression, folding, and mashing, so such rock may be found at almost any point in the name which has long been applied to the from the central mass of the Needles are the old that the original character of the rocks which now the canyon, while in a number of places it pre­ mountain region as a whole. From Needle Creek land surfaces upon which the Paleozoic sediments make up the complex has been completely obliter­ dominates. Distinct from this are rocks which are on the south to Elk Creek on the north and from were deposited. To the north and east the pre- ated, but their composition indicates an igneous coarse textured and irregularly foliated rather than the Vallecito to the Animas they form a group Paleozoic land surface lies beyond the boundaries rather than a sedimentary origin. Their relation banded. They are as a rule very light in color of peaks almost unequaled in this country in alti­ of the quadrangle, or is almost entirely covered by to the other rocks of determinable age in the and contain greater amounts of quartz and the

.^jiKiSMiit?^^ feldspars in proportion to the dark minerals than even a superficial examination. The innumerable erate, which are also indicated on the map as in under the three general heads of quartzites, green­ do the finer-grained schists which have been bands or layers which succeed one another like the fault contact, may, so far as can be seen in the area stones, and granitic rocks. described. Rocks like these occur on the east strata of sedimentary formations have dips of high examined, be in original contact with one another, Quartzites. This general term includes a number of quartz­ side of the Animas in the neighborhood of angles, which vary from one side to the other of modified by infolding of the two terranes and ites and certain other siliceous rocks whose original character Needleton, and are supposed to be granites the vertical. The general strike of the schistosity, accompanied in places by minor shearing. The is not all clear. Beginning with the massive quartzites, a suite of rocks may be chosen which shows a gradual transi­ which have had their original structure destroyed which is the same as that of the banding, swings, presence of the Irving elsewhere in the quadrangle tion to fine grained schists containing feldspars, dark mica, by compression or mashing. Such rocks are not with local variations, from a few degrees east of or in the region immediately surrounding it has and even hornblende in appreciable amounts. Microscopi­ common in the region and are to be clearly distin­ north where Animas River leaves the quadrangle not been observed. cally, rocks at this end of the series are hardly to be sepa­ rated from the more siliceous mashed eruptives; the structures guished from the other intrusive granites. Nearly to a nearly east-west direction just below Elk Park. General description. The rocks which make up are the same and the considerable quantity of feldspar makes black amphibolites are very common in almost all Although the prevailing strikes in the strip along the Irving complex possess a great variety of it extremely doubtful whether the rocks in question could be classed with the quartzites. It is entirely reasonable to sup­ parts of the complex. As their name implies, they the northern border are east and west, a difference textures, but a composition that is very uniform pose that certain of the more feldspathic of the quartzose are composed very largely of dark amphiboles or of 90° within a mile has been observed in a few in the different groups. The majority may be rocks may have been originally deposited as arkose sand­ hornblende and the majority are finely laminated places. As in the area to the south, the dips are broadly described as greenstones; some resemble stones, especially when it is remembered that the rocks from which these sediments were probably derived belonged to the and cleave well along the lines of schistosity. all nearly vertical. massive granular rocks, others are porphyritic Archean, in which no very pure siliceous rocks are now known Some of these amphibolites appear to have been Greater complexity of structure doubtless exists with a dense groundmass, while still others are to occur. These matters are of more theoretical than practi­ cal importance, since the occurrence of the quartzites is not intruded as dikes into the other rocks at a very and there may be repetitions of various bands finely laminated schists. Hornblende, chlorite, prominent and they can not be differentiated from the rest of remote period. From their present composition, through faulting or close folding, but no evidence epidote, and rarely biotite can be recognized meg- the Irving complex for purposes of mapping. they are assumed to have been diabases or other has been found which could warrant the definite ascopically in nearly all, and usually these dark Greenstones. This field term is a useful one to apply to the rocks of rather varied texture but similar composition that equally basic rocks, but the regional metamor­ assumption that such conditions do or do not minerals appear to be in excess of the lighter sili­ make up the greater part of the Irving series. Some are very phism has obliterated all of their original char­ prevail. cates. What feldspar there is has evidently been dense and fine grained, others are coarsely granular or por­ phyritic, and many are now well-laminated schists. The field acteristics and brought about a complete recrys­ much altered. The greenstones are dark greenish IKVING GREENSTONE. relations are often obscure, and practically no information as tallization of their constituents. For the purposes brown or black and give a somber tone to the to their origin can be obtained there, except that they were of description the various rocks composing the Name and definition. The name Irving green­ hillsides and canyon walls where they are exposed. probably intruded into the older sedimentary rocks. The microscope shows that at present the rocks consist very Archean complex of the Needle Mountains may stone is here proposed for a complicated series of The most significant features of the rocks in dif­ largely of pleochroic green hornblende, considerable feld­ be grouped under three general heads gneiss, schists, greenstones, and subordinate quartzites, a ferent parts of the complex are the variations in spar, and generally biotite and quartz, with accessory ilmen­ ite or magnetite. All have undoubtedly been derived from biotite-schist, and hornblende-sohist portion of which is prominently exposed in the texture and structure, which are conspicuous in the same magma and crystallized as gabbro or diabase; sub­ southeastern part of the quadrangle and of which the field and still more so when the rocks are sequently they were profoundly metamorphosed through Gneiss. Rocks of this sort are in almost all cases light Irving Peak is composed. In many respects similar examined microscopically. These variations are dynamic action, and at present it is only in the more massive colored and moderately coarse textured. Some are strongly examples that any trace of the original structure or compo­ banded or laminated, while others have but an imperfect foli­ to the Archean schists of the Animas Canyon, the due partly to original textural differences in the sition can be found. The processes by which a complete ation, and this variation may occur even in different parts of Irving greenstone must, nevertheless, be distin­ rocks themselves and partly to the dynamic meta­ change in the rnineralogical composition of gabbro or dia­ the same mass. A 11 are feldspathic and many of them contain base may be brought about have been discussed in detail by feldspar in excess of the other minerals. A dark mica is uni­ guished from them, on account of the distinctive morphism that has produced in some instances G. H. Williams in his description of the similar greenstone formly present, the parallel arrangement of whose blades helps character of certain of the more massive members finely lanimated schists, in which all traces of rocks of the Lake Superior region. Briefly, the original to bring out the banded or foliated structure. A microscopical of the series and the presence of sedimentary rocks. original structure have been obliterated, and in pyroxene alters to a fibrous green hornblende (uralite) and examination shows them to have about the same mineralogi- the labradorite to zoisite and albite; quartz, orthoclase, chlo­ cal composition as granites. With orthoclase are associated The actual base of the Irving has nowhere been others breccias, in which the fragments have been rite, muscovite, and biotite may also be developed. In some microcline and plagioclase, to be recognized, however, only seen, for on the west it has been found only in welded together by pressure, with a rude schis­ cases the original granular or ophitic structure may still be in the more imperfectly banded or less mashed varieties. In made out, while extreme alteration brings about a complete those rocks which have undergone great compression it is contact with the Eolus granite, while to the east tosity, not well marked as a prevalent structure, recrystallization and develops a finely laminated schist which almost impossible to estimate the relative amounts of quartz it appears to. be in many cases in fault or shear but sufficient to modify the massive appearance. betrays no sign of its former character. Such has probably and feldspar, so completely have the original crystal grains contact with the Vallecito conglomerate or the With the greenstones are a few lighter-colored been the history of the Irving greenstone-schists. In no spec­ been broken down and granulated, while diagnostic features imen has it been possible to find a pyroxene in the stage of of the feldspars have been completely obliterated. Next to Uncompahgre quartzite. The portion of the for­ rocks, generally much mashed, which appear to passing over into uralitic hornblende, but there can be little feldspar, quartz is the most abundant mineral, and these two mation observed belongs within the area repre­ have been granitic intrusions; there are also more doubt as to the secondary nature of the mineral. It occurs make up the greater part of the rock. Biotite is conspicuous commonly in large, irregular patches, the different parts of on account of its dark color and in places is a very abundant sented as "metamorphic Paleozoic" on the Hay- recent granite-porphyries and pegmatites. Prob­ which seldom extinguish at the same time; the borders are constituent. It occurs in small scales or blades arranged in den map. It has been described by Howe in the ably the least numerous are bands of extremely uneven and ragged, and high powers of the microscope show more or less parallel positions. " Certain accessory minerals, Journal of Geology (vol.' 12, 1904, pp. 501-509). siliceous rocks which were formerly sandstones or that they are fringes of fine green needles or fibrous aggre­ which are characteristic of granites, are also found in many gates of uralite. In the more massive rocks may still be made of the gneisses. These include garnet, magnetite, and apa­ Thickness and structure. The thickness of the quartzites. At only two places is their sedimen­ out lath-shaped, striated feldspars whose general appearance tite. Muscovite, except as an alteration product of the feld­ Irving greenstones and schists is not known. The tary origin beyond doubt, namely, at a point suggests that they were basic plagioclase, though they are spar, is rare. never fresh enough to make satisfactory measurements of So far as one may judge from their petrographical charac­ formation consists very' .largely of unstratified south of the quadrangle line near the granite con­ their extinction angles; they are now largely altered to zoisite, ter, the gneisses seem to be metamorphosed intrusive granites. eruptive rocks, into which other igneous masses tact and at another just west of the saddle north of albite, and calcite. In many cases the arrangement of the Biotite-schist. As has been said, the commonest rocks of have been intruded. All have been extensively Irving Peak. The rocks at these two localities are feldspar is that typical of diabase. Later alteration pro.d- the Archean complex are biotite-schists. Many of them are ucts, such as chlorite, are common and in some cases have but little different in composition and appearance from the affected by dynamic metamorphism, which has so simple massive quartzites like many that belong to completely replaced the hornblende. Ilmenite, s'urroujided gneisses, while others are with difficulty distinguished in the often obliterated the original structure that it is the Uncompahgre formation. At other localities in many instances by a border of secondary titanite, is'to be field from the hornblende-schists. Most of them are much found in most of the greenstones. finer grained and more perfectly schistose than the gneisses. impossible to say whether or not the series, now quartz-schists have been found which may have Partly on account of the ophitic or granular structure of This schistosity and the excellent cleavage which accompanies consisting largely of rocks with a nearly vertical been massive quartzites like the others, but have the unsheared specimens and partly by comparison with the it are doubtless emphasized by the large quantity of biotite schistosity, exposed between the Vallecito con­ yielded to the influence of dynamic metamorphism rocks of other regions, in which all the stages of alteration present, which also gives the rock a much darker color. The are shown, the greenstones of the Irving series are considered rocks are compact and well laminated, with alternating dark glomerate and the Eolus granite represents a and have had developed in them secondary schis­ to have been derived from gabbro or diabase and to have and light narrow bands which combine to give a general dark simple section. If there has been no repetition tose structures and new minerals. The alterna­ acquired their present character through dynamic meta­ greenish-gray color to the whole. According to the amounts morphism. Rocks which possess the diabase habit are by of the light silicates present they weather to rusty pinks or through folding or thrust faulting, more than tion of harder and softer layers has in many cases far the commonest. dark browns. The microscope shows that they consist of 10,000 feet must be allowed as the minimum caused a ribbed surface when the rocks have been Granitic rocks. Rocks having a composition like that of quartz and orthoclase in nearly equal proportions and each thickness now exposed. What are supposed to subjected to long weathering. Tlie relation of the granite occur as intrusives of different periods in the Irving but slightly in excess of dark-brown or greenish biotite. In complex. The youngest are simply aplites and pegmatites rare cases a little muscovite and accessory apatite and titan- be the lower portions are limited by a fault con­ quartzites to the greenstones is obscure; they do or granite-porphyries and appear to be closely related to the ite occur. Magnetite is not common. There are no crystal tact with the Eolus granite. The Vallecito forma­ not appear to have been formed as contempo­ Eolus granite, if, indeed, they are not actual offshoots from boundaries, but all the minerals occur in elongate grains or the great mass to the west. They are, with the exception of blades arranged with the greater diameters parallel. It is tion, a conglomerate made up largely of greenstone raneous deposits, but rather to have been torn the pegmatites, dense, finely granular pink or red rocks coin- not possible to determine with any certainty the original pebbles, has been infolded in a complicated man­ away from their original positions as members in posed of orthoclase, microcline, and quartz, with small quan­ character of these rocks; they may well have been either ner with the Irving schists in the area to the east, a larger series of quartzites, by the intrusion of the tities of biotite and the usual accessories of granite. They sedimentary or igneous. cut the greenstones and quartzites without regard for schis­ Hornblende-schist. Next to the biotite-schists, hornblendic and both have acquired a schistosity younger than basic magma of the greenstones, and held as inclu­ tosity or bedding, and rarely show signs of a mashing which ones are the most numerous. They are nearly black and that common to the Irving formation as a whole. sions in it. The appearance of quartzite here and was probably connected with the movements in which the have a fine fibrous texture, which is lustrous on fresh sur­ Uncompahgre was involved. Evidently older than these are faces. A few of them are banded like the biotite-schists, but This makes it extremely difficult to distinguish there in the Irving section is of interest as indica­ light-gray gneisses and schists which at some points lie as the greater part are densely schistose. In rare instances they between the two formations, particularly in the ting the source from which the quartzite bowlders sheets between altered diabase or quartzite and elsewhere are less dense and compact and consist of coarser aggregates region northeast of the Quartz Mill. Wherever of the Vallecito conglomerate were derived. In appear as if they were crosscutting bodies. Some are coarse of hornblende. The lighter-colored minerals are as a rule gneisses, in which biotite occurs in disconnected lenticular inconspicuous. When examined microscopically, they are it has been possible to discover evidence of orig­ this connection should be mentioned the occur­ patches or lamellae; others are fine-grained, evenly banded found to be made up very largely of a peculiar bluish-green inal bedding in the sedimentary- rocks, the dips rence of a 15-foot band of more or less siliceous biotite-schists whose schistosity is often more linear than in hornblende in long needles, all nearly parallel, generally planes. The microscopic structure of all indicates mashing. some biotite, and interstitial feldspar and quartz. The feld­ or strikes have been found to be more or less magnetic iron on the hillside west of Irving Peak. In some only the edges of the quartz and feldspar grains have spar is fine grained or granulated and difficult to determine discordant in different areas. A secondary schis­ It is interstratified with siliceous schists or mashed been granulated, in others the original structure has been in all cases, but it appears to be generally orthoclase with tose structure, often well developed, is difficult to quartzites, and itself bears evidence of sedimentary completely broken down and probably reerystallization has variable amounts of plagioclase. Accessory titanite, ilmenite taken place. They consist of orthoclase, microcline, a little or magnetite, and apatite are uniformly present, and in some distinguish from a possible bedding, but it is origin. Parts are composed of nearly pure mag­ plagioclase, abundant quartz, and variable amounts of bio­ cases actinolite is found. The parallel arrangement of the not a constant feature of the Irving formation, netite, while others are of coarse yellowish quartz, tite; in addition to their principal constituents accessory mag­ mineral particles and the marked schistosity which results netite and apatite are prominent in many specimens. Garnet from this is one of the most characteristic features of the certain portions, both sedimentary and igneous, intermediate portions showing an almost schist-like is not uncommon and a little green hornblende has also been rock. As a rule those amphibole-schists which are supposed being massive and unmashed. In this massive banding due to the alternation of the two minerals. found. Alterations of feldspar to muscovite and kaolin and to represent mashed basic dikes are richer in hornblende character they differ much from the Archean This single observed occurrence is an example of of biotite to chlorite occurred frequently, and the resulting than the others. dull-green color in some cases caused the rocks to be mistaken Garnet-schist. At only one locality in the quadrangle, rocks. many such beds which must have once existed and for greenstones in the field. A number of specimens, sup­ although found elsewhere in the region, do garnet-schists Distribution. The Irving greenstones are ex­ which supplied the magnetite, hematite, and jasper posed in the field to be quartzites, have been found, on micro­ occur. They are exposed at the south end of a wider portion scopical examination, to be more probably of eruptive origin, of the canyon about 2 miles north of Needleton. The rock is posed for a little over 7 miles on one side or the pebbles so characteristic of the Vallecito conglom­ on account of the prominence of feldspar, biotite, and the very coarsely laminated and has the earthy, greenish color other of Vallecito Creek from the southern bound­ erate and the lower horizons of the Uncompahgre accessory minerals, though quartz may be the most abundant characteristic of decomposed amphiboles. Certain bands are ary of the quadrangle northward. On the west formation. The structure of the Irving greenstone single mineral. There can be little doubt that these rocks composed almost entirely of more or less crushed red garnets formerly intruded the greenstones and quartzites as granite as large as small peas or even hickory nuts. Garnet is also they are sharply bounded by probable fault con­ and the character of the complex as a whole is dikes or irregular crosscutting bodies, and that they were present in the amphibolitic portions as an important acces­ tact with the Eolus granite, and to the north, east, more fully discussed in later sections. afterwards involved in the dynamic metamorphism of the sory. The rock as a whole consists of nearly equal amounts region. of amphibole, probably actinolite, orthoclase and plagio­ and south, where not covered by the Paleozoic Petrographic details. A microscopical examina­ clase feldspar, and quartz with accessory garnet, magnetite, formations, they are seen in contact with the tion of the rocks is necessary to an understanding Age and correlation. It is impossible to tell and apatite. Secondary muscovite, chlorite, and hematite Needle Mountains group, their eastern limit being of the origin of the various members of the Irving what the relations may have been between the are common. not more than a mile beyond the border of the series and of their probable age relations to one Archean rocks of the Animas Canyon and the Irv­ Structure. -The only structure that can now be quadrangle. The contact with the Uncompahgre another. In the following paragraphs some of the ing greenstone, because the space between them is made out in the Archean complex is a compara­ quartzite, north of Irving Peak, is clearly a fault, more important details will be given. For pur­ occupied by the great mass of Eolus granite. tively simple one, and its nature is evident from but the greenstones and the Vallecito conglom­ poses of description the rocks will be grouped That they are petrographically distinct there can be no question, and the presence of sedimentary neighborhood of the Quartz Mill the rock is a very minerals, such as chiastolite, garnet, chlorite, seri- able shell has been found, identified by Charles rocks in the Irving formation is a good reason for compact conglomerate. The well-rounded, water- cite, etc., are frequently seen, and original struc­ D. Walcott as Obolus sp.?, related to forms found believing that it is of later age. Little can be worn pebbles and bowlders range from one-half ture and bedding are in many places completely in the middle or upper Cambrian at various local­ learned from a study of the Irving-Uncompahgre an inch in diameter to 12 inches in rare instances obliterated by readjustments necessitated by the ities in the western United States. Minute frag­ and Irving-Vallecito contacts, except that where and are composed of vein quartz, purplish quartzite, folding, which caused movements in the rock anal­ ments of other undeterminable shells were found the formations are not sharply separated by faults and many varieties of schist, mostly dark amphibo- ogous to flowage. Schistosity also is developed in in association with the fossil named. From this they are intricately infolded and the original con­ lites and rocks rich in chlorite, clearly derived from these rocks, both parallel to the bedding and cut­ evidence it is assumed that in this region, as else­ tacts of sedimentation have been obscured. The the Irving series of greenstones. A few massive ting it at various angles. The quartzites, consist­ where in Colorado, the Cambrian system is repre­ lower portions of the Vallecito conglomerate, how­ granular fragments have also been observed. The ing essentially of silica, are naturally less favorable sented only by a thin series of quartzites belonging ever, are largely made up of greenstone pebbles cement is a fine-grained, dark-gray quartzose mate­ to the production of new minerals than are the probably to its upper division, and for those the and bowlders, with many of quartzite, and this rial, at some points in excess of the pebbles and slates or clay rocks, which contain various ele­ name Ignacio formation or quartzite was proposed is sufficient to prove the greater age of the Irving bowlders which it incloses and at others quite sub­ ments necessary for their formation, and hence in the Silverton folio, from the lakes in the Ani­ and indicate the former abundance of quartzite in ordinate. Generally the rock has the appearance the quartzites as a rule show simple induration mas Valley above Rockwood, near which the forma­ it. For these reasons the rocks of the Irving for­ of having been compressed or sheared. Many of through the deposition in them of silica. The tion is very well exposed. It is at present believed mation are considered to be most probably .of the pebbles are fractured and the cracks are filled entire series of rocks is traversed by fracture that all the Paleozoic quartzites beneath the "Salt Algonkian age. with vein quartz; others are bent or welded into planes, now filled with secondary quartz, which shales," or Elbert formation, belong to the Ignacio, No other rocks of similar character are known their neighbors, while the cement looks as if it had have their best development in the brittle quartz­ except a. thin band locally developed at the base of to occur in the San Juan Mountains, but similar flowed about the pebbles. ites. These joints run through the quartzites in the Elbert, which contains Elbert fossils; and, schists have been found by Cross near Salida, in Higher up in the section the conglomeratic different directions and cause the rocks to break off although the top of the Ignacio may be followed the Sangre de Cristo Kange, concerning the occur­ character becomes less pronounced. Coarse grits in large slabs or blocks, forming high, precipitous by a stratigraphic break, as shown by its complete rence and distribution of which there is very little or finer-grained, cross-bedded sandstones and cliffs. absence in the country directly east of Vallecito information. Petrographically these rocks and arkoses begin to appear and form an important The lithologic character of the rocks and the Creek, there is ordinarily no angular unconform­ those of the Irving formation appear to be prac­ part of the series, but in no case, so far as observed, deformation to which they have been subjected ity and no recognized evidence of the exact plane tically identical. In many respects the Needle to the exclusion of conglomerate beds. In the have had a material influence on the topography. representing this break in the local sequence of Mountain rocks closely resemble the greenstones of upper portions greenstone pebbles are much less The Grenadier Range (figs. 8, 9, and the extreme rocks. the Marquette and Menominee regions, described numerous, most of the materials being quartzites left of 5) owes the ruggedness of its jagged peaks Generalized section of Paleozoic rocks. by G. H. Williams. The Menominee greenstone- with a considerable amount of jasper, gray hema­ and their 2000-foot precipices to a massive quartz­ Feet. Feet. schists have been reexamined and described by tite, and magnetite. ite, dipping steeply to the north, which extends in Bright-red sandstone, lighter- red or pinkish grits and con­ W. S. Bay ley, who has named them the Quinnesec The change from conglomerate to fine-grained a great bow from the Animas Canyon across the glomerates, alternating with schists. They are believed to be of Archean age, quartzite is not seen in the quadrangle, but above Vallecito to the region about Mount Oso, at the sandy shales and earthy or o sandy limestones; nonfossilif- on account of their lithological and structural fea­ Emerald Lake (east of the quadrangle, in the head of Pine River. The slates, on the other erous...... 1500 tures, although it is stated that there is no positive Pine River drainage) the dark, massive conglomer­ hand, generally occupy gentle slopes and saddles, 18. Dark reddish-brown sandstones proof that they are older than the Huronian. ate is succeeded, with no structural break, by nearly or have influenced streams in the carving of their and pink grits, with interca­ lated greenish or reddish shales SEDIMENTARY ROCKS. pure quartzite-conglomerate or quartzite similar to valleys. and sandy fossiliferous lime­ rocks formerly considered as basal members of Distribution. The Uncompahgre formation stones...... 300 Algonkian System. the Uncompahgre in the West Needles and the crosses the quadrangle in its northern portion. f!7. Limestones, grits, sandstones, NEEDLE MOUNTAINS GROUP. and shales of variable distribu­ Grenadier Range. These quartzites are terminated Its exposure is less than a mile wide in the neigh­ tion and development. Lime­ Introductory statement. The name Uncom- by a fault against the Irving greenstones after only borhood of Lime Creek, but from this point east­ stone, in thick, massive beds, predominates in the middle pahgre formation was proposed in the Silverton a few hundred feet of exposure, but the close rela­ ward the rocks exposed cover a wider and wider and upper parts of the section, folio for a great succession of supposed Algonkian tionship of these upper quartzites and those of the zone, and at the eastern margin of the quadrangle the lower portion being mainly H sandstones and shales, with a quartzites, slates, and subordinate conglomerates, Grenadier Range is hardly to be doubted. there are continuous outcrops for more than 8 few limestone layers. Numer­ represented in the Uncompahgre Canyon by At several places on the north side of Tenmile miles. The rocks occurring in this area consist o ous invertebrate fossils occur several thousand feet of upturned strata. The Creek, in contact with the Eolus granite, a more or of the massive quartzites and slates only; in the in the shales and limestones... 2000 ri6. Shale, red arid calcareous, con­ exposures of these beds begin in the Silverton less mashed quartzite-conglomerate occurs, nowhere southeast corner of the quadrangle the Vallecito taining numerous nodules of quadrangle and extend for a few miles northward more than 20 feet thick, which grades upward into conglomerate is found on both sides of the canyon chert and saccharoidal lime­ stone which are fossiliferous; almost to the town of Ouray, but neither top nor the fine-grained massive quartzites. It is litholog- of the Vallecito. The only other occurrence of becomes more sandy and shaly bottom of the larger conformable series to which ically very similar to the upper part of the con­ the Uncompahgre is just south of Mountain View upward; different strata can the visible section belongs is here shown. The glomerate east of the Vallecito and may represent Crest, where there is a small patch of quartzite rest­ not be distinguished. Rests on a weathered and broken name was proposed for the entire assemblage of a small band that escaped the faulting or shearing ing on the Eolus granite and intruded by small surface of the uppermost conformable quartzites and slates, of which it was which separated the rest of the conglomerate from dikes of the same. Ouray limestone...... 75 f 15. known that extensive exposures existed in the the overlying beds. Limestone, dull rusty brown, PALEOZOIC SECTION. earthy, laminated, and wavy; Needle Mountains. fossiliferous. Yellowish, with During the survey of the Needle Mountains UNCOMPAHGKRE FORMATION. The section of Paleozoic rocks which occurs in large rounded cherts near top. 25 3900 14. Limestone, massive, blue, not ' quadrangle it was found that the main quartzites Name and definitions. The rocks of Algonkian the Needle Mountains quadrangle and adjoining always continuous ...... 5. and slates were like those of the Uncompahgre age which constitute the main part of the Grenadier regions is developed in two unequal parts. Prom­ 13. Limestones, similar to 15, with­ out cherts...... '... ',20 section, but from structural complexities the por­ Range (figs. 5, 8, and 9, on the Illustration sheet) inent in the upper portion are the massive lime­ 12. Limestone, thin bedded, dense, tion represented was less, and, as in the Uncom­ and which are characteristically exposed in the stones, sandstones, and shales of the Hermosa bluish gray; certain layers pahgre Canyon, the upper and lower parts were canyon of the Animas (fig. 6) and in the upper formation, which has an average thickness of over extremely fossiliferous ...... 25 11. Quartzite and sandy limestones; not seen. In the Vallecito Canyon, however, at portion of the Vallecito (fig. 5, to the right) are 2000 feet. This formation is known, from the evi­ no fossils...... 4 the southern margin of the quadrangle, a heavy lithologically the same as those which occur in the dence of fossils, to be of Pennsylvanian (Upper 10. Sandy limestone containing numerous crinoid stems and conglomerate was discovered and traced to the Uncompahgre Canyon to the north and to which Carboniferous) age. Above the Hermosa are about a few cup corals...... 25 east, where it was found in such relations with the the name "Uncompahgre" was given in the Sil­ 200 feet of shales and sandstones, of peculiar greens Shale, fine red clay, not forming Uncompahgre quartzites and slates as to appear verton folio. and browns, which have been described as the Rico distinct outcrops...... 10 Limestone, dense, gray, con- unquestionably as the lower part of the group. Description. The lower 1500 feet are massive formation, of late Pennsylvanian age. Following choidal fracture in several To this conglomerate the name Vallecito is given quartzites of light colors, gray and white, and with the Rico conformably are about 1500 feet of red beds, very variable in devel­ opment and not always pres­ and to the group which includes it and the the exception of occasional pebbly layers they are sandstones and shales of the Cutler formation that ent ; not fossiliferous...... 0-35 Uncompahgre, together with higher formations uniformly fine grained and compact. Above these are of probable Permian age. At the base of the Sandstone or quartzite, variable that may hereafter be recognized, the name Needle Hermosa is a thin horizon of reddish shales and in character; extremely fossil­ massive rocks occurs a great series of alternating iferous in places...... 1 Mountains is applied. quartzites and slates or shales. The quartzites are chert nodules which has been named the Molas Thin limestones or calcareous The total thickness of the Needle Mountains often thin bedded, and even in more massive hori­ formation and whose age is also Pennsylvanian. shales containing pseudo- morphs or salt crystals...... 30 group is unknown. Two thousand feet or more zons individual layers are separated by shaly part­ Below these Pennsylvanian rocks are from 300 to Dense gray limestones with thin of the basal conglomeratic member occur about 2 ings. The quartzites show but little variation in 500 feet of sediments of rather varied character shale parting; in lower part miles east of the quadrangle, in the vicinity of which have been separated, on lithologic grounds fish scales have been found texture. Some are less purely quartzose than locally; forms ledges...... 30 Emerald Lake, on the Lake Fork of Pine River, others, but the admixture of argillaceous material and by means of the fossils preserved in them, Limestones, sandy and impure, while more than 5000 feet must be allowed for the is nowhere great and the only marked differences into three distinct formations, ranging in age from with thin fossiliferous quartz­ ites and shale layers; reds and overlying quartzites and slates of the Grenadier which they show among themselves are those of Cambrian to Mississippian (Lower Carboniferous). yellows...... 25 210 Range. The complicated folding and shearing color, the range being from the purest white These have elsewhere been named, beginning with f r 3. Quartzite, massive, fine grained, within the group and the complete unconformity through pinks, reds, purples, and grays to black. the lowest, the Ignacio, Elbert, and Ouray forma­ beds 2-5 feet thick...... 25 2. Quartzite, thin bedded, fine with overlying beds, indicating a long period of Rocks which in the field were referred to as tions. grained, often with reddish deformation and erosion preceding the deposition "slates," of a composition distinctly argillaceous, The accompanying somewhat generalized section i § -i shaly parting or sandy layers. 20 1 Eil i. Quartzite conglomerate, fine of the earliest fossiliferous rocks, make it impossi­ occur in several well-marked horizons and also of the Paleozoic rocks will serve as a basis for the 1 ^ o grained, in places giving way ble to estimate the original thickness of the Needle associated with thin-bedded quartzites. In all discussion of various divisions, especially of kthe to red crumbling shales and Mountains group or to recognize an upper limit of cases they seem to have been clay rocks which lower ones. sandstones, resting unconform- ably on pre-Paleozoic rocks... 5-50 95 the section. have undergone varying amounts of dynamic CAMBEIAN SYSTEM. Total. 4205 metamorphism, resulting in the development of VALLECITO CONGLOMERATE. secondary minerals and structures, and of rocks IGNACIO QUARTZITK. Lithologic character. The Ignacio consists of The basal formation of the Needle Mountains whose outward appearances are often very differ­ Name and definition. The lowest lithologic nearly pure siliceous strata, with some feldspar group, the Vallecito conglomerate, is exposed on ent. On.the one hand are soft black shales, while division of the Paleozoic section in the Animas locally in the lowest beds. The greater part is both sides of Vallecito Creek for about 1 mile the other extreme is represented by compact massive Valley is made up of quartzites, and varies in fine grained, white, gray, or pinkish, and highly northward from the southern boundary of the argillites and andalusite-schists. In the deforma­ thickness, in the area thus far examined, from a indurated. The lower portion is commonly a quadrangle. This, however, is a less complete tion that has affected the whole Uncompahgre series few feet to 200 feet. From layers near the middle massive quartzite of prevalent pink or reddish section than that exposed on Pine River. In the the clay rocks have suffered the most. Secondary of these quartzites a single generically determin- color, while the succeeding strata are nearly Needle Mountains. white. Distinct bedding is common, as is an irreg­ Point, is where Endlich obtained the first Devonian that the whole limestone complex in question must ated in his report, and others have since been ular jointing. Above this the strata are often invertebrates from the Ouray limestone and now be of Devonian age, but at a number of localities obtained. Nearly all classes of organic life are wavy in bedding, with shale partings, in which becomes of further interest as furnishing fossils a distinctive Mississippian (Lower Carboniferous) represented in the fauna, but brachiopods and mud cracks, trails, and other markings are common. determining the age of the Elbert formation. fauna has been observed in the upper part of the mollusks are most abundant. Some of the more These layers are friable sandstones in places. More As noted in the section given above, a thin cal­ most prominent limestone ledge of the formation. important species are massive quartzite layers generally succeed this series, careous sandstone at the base of the Elbert at Devon Since it is impossible to draw a line between the Schizophoria striatula. Schuchertella chemungensis. but not in a thickness equal to those below. Point carries a great many fragments of fish scales, two portions, the Ouray becomes preeminently a Productella semiglobosa. Productella subalata ? At the bottom of the formation a true basal con­ plates, and bones, and similar remains are abun­ lithologic unit transgressing the faunal boundary Athyris coloradoensis. Athyris vittata var. Spirifer coniculus. Spirifer disjunctus var. ani- glomerate is often found in hollows of the granite, dant in a thin, discontinuous quartzite stratum between the Devonian and Carboniferous systems. Camarotcechia endlichi. inasensis. schist, or Algonkian quartzite floor. The hard below the red shale at the top of the formation. Lithologic character. The Ouray formation as Paracyclas sp. Naticopsis gigantea. quartzites of the last named are most abundant Fish remains have also been found in the course at present known has a thickness varying from Naticopsis ? (Isomena) humilis. Straparollus clymenioides. Orthoceras (several species). among the pebbles, which range in size up to a of the present work on a slab of quartzite in a 100 to 250 feet. The upper and major part of rare diameter of 1 foot. talus pile near the limestone quarries south of the formation is massive limestone, either in one In addition to the invertebrate fauna a single Rockwood. The talus occurs below a bench where bed or with very thin intercalated shales, so that shark's tooth was found, which has been made by DEVONIAN SYSTEM. the Elbert formation is in place, but the stratum mesas, benches, and prominent cliffs of the lime­ O. P. Hay the type of a new species, Gladodus The existence of Devonian sediments in the San from which the fossil-bearing slab came could not stone are characteristic topographic forms. Below formosus. It is said to be most closely related Juan region was established in 1874 through a be identified. A further locality carrying similar the more massive portion a third or less of the among known species to certain Pennsylvania!! collection of fossils made by F. M. Endlich, of fossils is less than a mile south of Columbine Lake, section is made up of beds of limestone, with dis­ forms, such as C. springeri of Russia. The tooth the Hayden Survey. The formation containing in the Engineer Mountain quadrangle. tinctly shaly layers and, rarely, thin quartzites was found on Lime Mesa under conditions making the invertebrate fauna discovered by Endlich has All the vertebrate remains obtained have been between them. Some of the lower layers have a it probable that it came from the Devonian portion been named the Ouray limestone, and the beds studied by Dr. C. E. Eastman, who has described wavy bedding, some are arenaceous or earthy, of the Ouray, although the limestone fragment immediately below it, containing an upper Devo­ them in the American Journal of Science, 4th and large chert concretions, free from fossils, containing it was not in place. nian fish fauna, have been distinguished as the ser., vol. 18, 1904, pp. 253-260. The slab from are common at a horizon near the base. The Correlation. The Devonian fauna of the Ouray Elbert formation. near Rockwood exhibits three specimens of a plac- lowest stratum is characterized usually by crinoid limestone has been shown by Dr. Girty to be rep­ oderm ganoid, Bothriolepis, a well-known genus of stems and rarely by a cup coral. resented more or less fully in older collections from ELBERT FORMATION. the upper Devonian, the species being apparently The greater part of the formation is dense, com­ various parts of Colorado, notably in the Elk Moun­ Name and definition. The name Elbert forma­ new. The same form is abundantly represented in pact limestone, but portions of the upper ledge are tains; at Glen wood Springs, on Grand River; near tion is applied to a succession of calcareous shales, the collection from Devon Point, where it is asso­ coarsely crystalline. In general the rock is nearly the head of White River; and on East Monarch thin limestones, and occasional quartzite beds, ciated with other ganoid fishes of Devonian types, white, straw-yellow, or buff, with local pinkish Mountain, Chaffee Countv. Full correlations of ' c/ aggregating less than 100 feet in thickness as a Holoptychius being the most prominent. From tones. Some of the lower beds are strongly yel­ the sections in these localities with that in the San rule, and characterized by scanty remains of ganoid near Columbine Lake a scale apparently belonging low and these are commonly more or less sandy. Juan region will not be possible, however, until fishes. These strata occur below and conformable to the genus Coccosteus was obtained. The contrast with the dark-gray, dense limestones further examinations have been made. Dr. Girty with the Ouray limestone, the main portion of These fish remains are characterized by Dr. of the Hermosa is marked, layers of such color says: which is of Devonian age, and are usually found Eastman as belonging to a distinct upper Devo­ and character occurring only near the base of the In general the Devonian fauna of the Ouray belongs resting upon the Ignacio quartzite, although a great nian fauna, allied particularly, so far as known, to Ouray. No change in character of the limestone to upper Devonian time. It is but distantly related to stratigraphic break comes between these. The that of the Catskill and Chemung groups of New occurs toward the top, unless a prevalence of the Devonian faunas of New York, and its relation to name is derived from Elbert Creek, a western trib­ York and Pennsylvania and also to that of certain coarse-grained, rotten beds, observed locally, and those of the Mississippi Valley, or even to other utary of the Animas, which drains the Ignacio European formations. belonging to the Carboniferous portion, in some known western Devonian faunas, is not close. It Lakes and flows for some miles upon a broad bench Correlation. No beds having the lithologic places at least, be found to be characteristic. The shows many points of approximation to the Athabas­ where all the lower Paleozoic formations are well character of the Elbert formation have been conditions of sedimentation seem to have remained can fauna described by Whiteaves, and is somewhat strikingly similar to the Devonian of Eussia. exposed. The formation was first described and found in other parts of Colorado, but according the same and deposition seems to have been con­ named by Cross in 1904 (Am. Jour. Sci., 4th ser., to Eastman the scanty fish remains obtained by tinuous from Devonian into Carboniferous time. CAKBONIFEROUS SYSTEM. vol. 18, pp. 245-252). Spurr from the "Parting Quartzite" of the Aspen The Carboniferous portion of the Ouray is at MISSISSIPPIAN SERIES. Lithologic character. The formation is mainly district, Elk Mountains, indicate a close correla­ present indistinguishable lithologically from the UPPER PART OF OURAY LIMESTONE. composed of calcareous shales and thin, sandy lime­ tion between the two formations. This is ren­ Devonian. Its existence was detected through As has been shown in describing the Ouray stones, varying in details of development from place dered all the more plausible by the presence of the presence of Mississippian invertebrate fossils limestone, it is known that a Mississippian fauna to place. The beds are drab, buff, or yellowish in the Ouray invertebrate fauna in the Leadville in the chert pebbles of the succeeding forma­ occurs in the upper part of that formation, in cer­ color. The shaly layers break up readily on expo­ limestone, which overlies the "Parting Quartzite" tion. During the investigation as to the origin tain localities, separated from the uppermost Devo­ sure, so that outcrops are obscured by the resulting throughout central Colorado. It is thus rendered of these pebbles Dr. George H. Girty found that nian fossiliferous horizon by about 50 or 75 feet of scales and thin plates. Quartzites are present, but probable that strata corresponding to the Elbert some of the fossils contained in them occurred also massive limestone in which no fossils have thus far are always very thin and subordinate to the calcare­ formation in age, however differing in composi­ in the uppermost strata of the massive limestone been discovered. The erosion in the interval fol­ ous and shaly portion. tion, are present below the Ouray or Leadville hitherto supposed to belong wholly to the Devo­ lowing Mississippian sedimentation removed those The shaly beds are in many places characterized limestone in many districts. nian. The localities at which the Carboniferous strata completely over large areas. As neither the by pseudomorphous casts after salt crystals. They It is also possible that an equivalent of the forms have been found in the limestone are few upper Devonian nor the basal Carboniferous hori­ occur, as noted by Endlich, on the under side of Elbert exists in the lower Kanab Canyon, adjacent mainly near Cascade Creek, in the Engineer zon is everywhere fossiliferous, it is impossible to slabs, as is natural, considering their origin. The to the Grand Canyon of the Colorado, in Arizona, Mountain quadrangle. Chert pebbles have been , decide in many places whether the upper portion of crystals of salt must be supposed to have formed on where Walcott found "placoganoid fishes of a but rarely found in place in these upper horizons, the Ouray limestone ledge is Carboniferous or not. or near the surface of the mud of a desiccating body Devonian type" in a formation consisting of "but and this fact, together with their abundance in From the great quantity of Carboniferous chert of salt water, and to have been dissolved by the 100 feet of purple and cream-colored limestone the succeeding Molas conglomerate, shows that an nodules in the succeeding formation it must be influx of fresh water and the crystal space filled by and sandstone, passing into gray calciferous sand­ interval of erosion followed the formation of the assumed that above the known horizons of the the sand or mud of a new deposit. The crystals stone above." limestone. In the area thus far studied this ero­ Ouray there once existed in this region a consider­ are usually more or less clearly skeleton cubes with Distribution. The Elbert formation is particu­ sion effected the removal of nearly the whole of able thickness of chert-bearing limestone of Missis­ sunken faces. They range in size up to an inch or larly well shown on the south slopes of the the chert-bearing portion of the Mississippian and sippian age. It seems not unlikely that somewhere more in diameter. So abundant are these casts that Needle Mountains in connection with the other an unknown amount of the"4 subjacent massive on the slopes of the San Juan Mountains notable they may be found at nearly every outcrop, but lower Paleozoic formations. It caps several of rock, but nowhere cut away enough of the Ouray remnants of these beds may be found. they are much more perfect and of larger size in the summits of Mountain View Crest and in such to make angular unconformity noticeable except in From a comprehensive study by GK H. Gfirty of all Carbon­ some localities than in others. places the ground is covered by plates of thin small detail. At no known point has the erosion iferous invertebrate fossils thus far collected in Colorado, and An important variation in the lithologic character limestone coated with well-defined casts of salt penetrated to the horizon of Devonian fossils in available to him, the following brief summary concerning the Mississippian of the San Juan region may be made. Thirty- of the Elbert formation is in the appearance of dense, crystals. the massive limestone. Inasmuch as neither that two determinable species have thus far been obtained from earthy, nonfossiliferous limestone, of conchoidal About Lime and Stag mesas and at other simi­ horizon nor that of the Carboniferous forms is the chert pebbles of the Molas formation, which were derived fracture, in several beds in its upper portion; each lar localities the Elbert shales cause a bench or recognizable in many sections the question as to from the eroded beds, and from two localities in the upper zone of the Ouray limestone. This fauna is closely similar to bed may be several feet in thickness. Thin shales gentle slope between the ledge- or cliff-making how much, if any, of the Ouray should, in such that of the Wasatch limestone of Utah and the Madison lime­ usually separate the massive beds. This develop­ Ouray and Ignacio formations. cases, be referred to the Carboniferous is a matter stone of the Yellowstone National Park. It is also similar to the small faunas obtained in Colorado from the Leadville ment was specially noted at Bluebird Park, south The Elbert occurs beneath the Ouray east of the of doubt. limestone in the Mosquito Range, at Aspen, Crested Butte, of Molas Lake. Vallecito Valley, but its beds are very much Fauna. The Devonian invertebrate fauna of the and near Salida, and that from the Millsap limestone of the The reddish clay or shale stratum at the top of diminished in thickness, if present at all, under Ouray occurs from near the base to a horizon which Front Range near Canyon and in Gfarden and Perry parks. Dr. Gfirty concludes that the Leadville limestone, which was the formation in the section already given seems to the Ouray patches of the Continental Divide and in many places is not far below the top of the upper, originally assigned to the Mississippian because of fossils be a persistent feature. on the west slope of the West Needle Mountains. massive ledge. The greater number of species occur found it its upper part, and the Ouray limestone, which has been considered Devonian on account of the fauna in its lower Fossils. The presence of fish plates, scaleSj and They are believed to be absent at these localities, in this upper horizon, but many of them range to and middle portions, are equivalent. He thinks that careful bone fragments in beds closely associated with the as indicated on the map. within a few feet of the base. Fossils have been search would in many cases show both faunas where only one salt-cast shales was observed by Endlich, although obtained at Ouray and at several localities in is now known. OURAY LIMESTONE. The most diagnostic forms are he did not collect specimens for study. The local­ the Needle Mountains, Engineer Mountain, and Zaphrentis tantilla. Spiriferina solidorostris ? ity at which the remains were found is on Endlich Name and definition. In 1900 A. C. Spencer, Durango quadrangles, as well as at the point Menophyllum ulrichanum. Eumetria marcyi ? Mesa, about 1 mile south of the Needle Moun­ after studies in connection with the United States where Endlich first found a few characteristic Schuchertella ingequalis. Camarotcechia metallica. tains quadrangle line. During the present resurvey Geological Survey work, proposed the name Ouray Chonetes illinoisensis. Myalina arkansasensis ? species, not far south of the Needle Mountains Productella concentrica. Myalina keokuk. . strata carrying fish remains were found at several limestone, from the town of Ouray, on the south­ quadrangle. Productus parviformis. Straparollus utahensis. points, the most productive locality being on the ern border of which is a prominent outcrop, for The invertebrate fauna of the Devonian portion Productus Isevicosta. Phillipsia peroccidens. Spirifer centronatus. east edge of Endlich Mesa, below and just north of the Devonian limestone member of the pre-Car- of the Ouray has been fully described by George a triangulation station of the Hayden Survey, situ­ boniferous Paleozoic, excluding the quartzites and H. Girty and compared with similar faunas hith­ PENNSYLVANIAN SERIES. ated on the Ouray limestone at its present northern shales here called the Ignacio and Elbert forma­ erto collected in Colorado, but not separated from MOLAS FORMATION. limit on this border of the mesa. This station, tions, although they were then thought to be pos­ Carboniferous. Name and definition. The name Molas forma­ which deserves a name and may be known as Devon sibly of Devonian age. It was supposed by Spencer Twenty-eight species or varieties are discrimin­ tion was proposed in the Silverton folio for the lowest of the three formations of the Pennsylva- Ouray limestone, the chert pebbles form, over In the Animas Valley, near the mouth of Her­ mosan fauna represents early Pennsylvanian sedimenta­ nian series distinguished in the Animas Valley and large areas, almost continuous coatings of resid­ mosa Creek, the lower third of the complex is tion, and it is probably older than the upper Coal adjacent regions. The name is derived from Molas ual material, mingled with some of the reddish made up of green sandstones and shales with some Measure faunas of the Kansas and Nebraska sections. Lake, which lies on the bench at 10,500 feet, west sandy shale. Many small isolated remnants of gypsiferous shales, while the rest of the formation RICO FORMATION. of the Animas Canyon, in the extreme northern the Molas also occur on these mesas. shows limestone layers distributed throughout. The formation succeeding the Hermosa in the part of the Needle Mountains quadrangle. The The cherts are, as a rule, well-rounded pebbles Along the great scarp facing the Animas for 10 normal section of the San Juan region was first lake basin is partly excavated in the Molas beds up to 6 inches in diameter. They are dark gray, miles in the Engineer Mountain quadrangle the identified in the Rico Mountains and Avas named and they are well exposed for some distance to green, red, white, or nearly black. Some are limestones become more and more prominent and and described in 1900 by A. C. Spencer in the the south. banded, some homogeneous. In only a portion certain bands are very thick. report on that area. It is a thin formation, not The Molas formation is distinguished as a carto­ of them can fossils be found. In the southern part of the Needle Mountains easily identified in regions of poor exposures, and graphic unit on lithologic grounds, being a well- Fauna. On Stag Mesa some thin, discontinu­ quadrangle the formation has essentially the same though its presence in the northwest corner of the characterized element in the Carboniferous section, ous limestone beds rich in fossils were found inter­ characteristics as on the western side of the Ani­ Needle Mountains quadrangle and in the adjacent and because it records, in certain peculiarities of calated iu red and highly ferruginous sands of the mas Valley above Rockwood. In the northwest parts of the neighboring quadrangles has been its sediments, important events of the preceding Molas and only a few feet below the characteristic corner of the quadrangle the limestone members demonstrated by Mr. Spencer through the finding interval of erosion, including the almost total limestone at the base of the Hermosa. At no are thinner, but distributed with great regularity of a few fossils, it was not possible to trace it or to destruction of a Mississippian formation. There other point have the fossils of the Molas yet been in the section. define its thickness. It is assumed from the exist­ may be a further reason for separating the Molas discovered. The limestones are usually massive, bluish gray ence of fossils that the formation is present in a and Hermosa formations in certain observed faunal The fossils mentioned have been studied by in color, and commonly contain bituminous mat­ narrow band between the Hermosa and Dolores differences, which are, however, as yet not suffi­ George H. Girty, who has made the following ter, causing a distinct odor when struck by the formations, as shown by the map. ciently established to warrant laying much stress summary statement concerning them: hammer. Many layers are rich in fossils, while The Rico formation, in the mountains from upon them. others appear to be destitute of them. which it derives its name, is a series of sandstones The fauna of the Molas formation, though at present The Molas beds were not included in the origi­ imperfectly known, comprises representatives of the The sandstones are generally fine grained, gray and conglomerates with intercalated shales and nal definition of the Hermosa formation by A. C. echinoids in Archceocidaris triplex?, of the bryozoa in or greenish, consisting largely of quartz but in sandy fossiliferous limestones, about 300 feet in Spencer, though the Hermosa was said to rest on Ehombopora lepidodendroides, of the brachiopods in many cases rich in feldspar. They are massive or total thickness. Lithologically it resembles the the Ouray limestone. In the localities examined Rhipidomella pecosi, Spirifer boonensis?, and Seminula thin bedded with shale partings. The cement is red-bed complex above it more than the Hermosa before that definition was formulated there was, in subtilita, and of the pelecypods in Myalina pernifor- calcareous as a rule and the green color is due to below, but its fossils are clearly of Carboniferous fact, a hiatus in the observations, the thin Molas mis?. The Bhombopora, Spirifer, and Seminula occur an amorphous substance, the character of which type, and related to those of the Hermosa. The also in the Hermosa formation, and in the case of the beds being nowhere well exposed. latter with the same varietal modifications and with has not been determined. general characteristics of the formation near Rico The Molas formation was defined in the Silver- equal abundance. Thus the Molas fauna is seen to be The shales vary from those which are greenish are, first, its calcareous nature, in which it resem­ ton folio as a thin series of reddish calcareous related to that of the Hermosa formation, but contains and sandy in character to dark, calcareous, bitumi­ bles the strata above and below; second, the arkose shales and sandstones, with many chert pebbles- no species in common with the Ouray limestone. Some nous varieties. character and the coarseness of its sandstones, in and some of limestone, quartzite, and other rocks, points of individuality distinguish the Molas fauna from With all this variety in constitution the basal which respect it differs from the Hermosa and that of the Hermosa, but it can not be conjectured how and with thin fossiliferous limestone lenses, the stratum of the Hermosa over the entire area resembles the Dolores; and, third, its chocolate or far this distinction would be borne out by full collections. fossils showing intimate relations to those of the examined is constant. It is a highly fossiliferous dark-maroon color, which contrasts sharply with lower Hermosa limestones. It occurs immediately Stratigraphic relations and distribution. In the limestone somewhat less than 15 feet in thick­ the gray or green of the Hermosa and is more or below the Hermosa formation and rests upon a Silverton, Needle Mountains, Engineer Mountain, ness. The uppermost strata are not so well less distinct from the bright vermilion of the surface of erosion. It represents the earliest sedi­ and Durango quadrangles the Molas formation known, but in the Rico Mountains they con­ Dolores. The color distinction was found to be ments of the Pennsylvania!! in this region. rests upon a surface due to the erosion by which sist of fine-grained micaceous, greenish, sandy insufficient for mapping purposes, even in the Lithologic character. The Molas formation is the Mississippian rocks were almost completely shales beneath which is a thin but persistent restricted area of the Rico Mountains. specially characterized by its deep-red, friable, removed. Although it has not been observed dark limestone filled with the minute shells of PERMIAN (?) SERIES. ' sandy strata, which are variably calcareous and overlying any other formation than the Ouray Triticites secalicus, formerly called Fusulina cylin- often shaly. They are seldom very distinctly limestone, the nature of the case permits the sup­ drica. CUTLER FORMATION. bedded and disintegrate so rapidly on weathering position that somewhere in the region adjacent to Few members of the Hermosa complex reach a Name and definition. The sandstones, grits, that good exposures are rare. In the lower part of the San Juan Mountains it must transgress the thickness of 100 feet. Some limestones and shales conglomerates, shales, and calcareous beds, mainly the formation dark chert nodules abound, in many lower Paleozoic formations and rest upon Algon- attain that figure, but thicknesses ranging from 10 of reddish color, occurring between the Rico beds cases making a large part of flat lentils or discon­ kian quartzites, granite, or schist. to 50 feet are much more common. Individual and the Triassic Dolores strata, form the greater tinuous layers. Many of these cherts carry a Mis­ From the nature of the Molas beds they waste strata change laterally, both in character and in part of the well-known "Red Beds" section of sissippian invertebrate fauna, the origin of which away rapidly on exposure. At the base of the thickness. Under these conditions it has been the region and have been hitherto provisionally has already been discussed. In the uppermost high Hermosa escarpment on the west side of the found impracticable to subdivide the complex included in the Dolores formation. In the Tellu- part of the formation, on Stag Mesa, some thin Animas is a bench underlain by the Ouray lime­ into smaller lithologic units for purposes of map­ ride and La Plata folios and in other publications limestones contain the fauna described in a suc­ stone. The surface of this bench represents the ping, As will appear «from the statements con­ the Dolores has been consistently defined as includ­ ceeding paragraph, which is believed to characterize Molas horizon. Red sandy outcrops occur here cerning the fauna, no division lines on that basis ing the Triassic strata of southwestern Colorado, the Molas beds. and there and chert nodules are scattered about can be suggested. with the statement that fossils demanding a refer­ The best section of the formation thus far in many places, but talus and other debris conceal Fauna and correlation. Dr. Girty has sum­ ence to the Triassic have been found only in the observed is situated on the southwest slope of the contact with the Hermosa. marized his views as to the fauna and correlation upper part of the "Red Beds" section. On the the Needle Mountains, on the south side of Tank On the southern slopes of the Needle Moun­ of the formation as follows: western and southern slopes of the San Juan Creek. At that locality there is a continuous tains are several dipping mesas caused by the Mountains no stratigraphic break in the "Red The fauna of the Hermosa formation is distinctly section, about 75 feet in thickness, representing Ouray limestone, and in patches scattered over Upper Carboniferous, or Pennsylvanian, in age. It con­ Beds" has been found, but the field work of 1904 the whole formation, as there developed, between the surface are remnants of the Molas, represented tains such characteristic species as in the Ouray quadrangle, on the northern side of the Ouray limestone and the Hermosa complex. mainly by rounde'd, chert pebbles. the mountains, revealed a notable angular uncon­ Triticites secalicus. Spirifer boonensis? There is no distinct stratification in the section. The Molas and Hermosa seem to be thoroughly Lophophyllum profundum. Spirifer rockyniontanus. formity occurring immediately below the most At the base is a zone of gradation into the Ouray conformable, and probably the former is the prod­ Campophyllum torquium. Spirifer cameratus. commonly fossiliferous bed of the Dolores forma­ limestone, for the upper zone of the latter is much uct of the first epoch within the long period of Chsetetes milleporaceus. Squaumlaria perplexa. tion. Through this unconformity the Dolores Archseocidaris ornata. Spiriferina campestris. broken up, with the red calcareous mud of the Pennsylvania!! sedimentation.' The red color of Mstulipora carbonaria, Spiriferina kentuckyensis. strata may be seen to transgress more'' than 1000 Molas filling the interstices. This relation of the its sediments and the other peculiarities mentioned Rhombopora lepidodendroi- Ambocoelia planiconvexa. feet of older "Red Beds" and several hundred feet des. Seminula subtilita. formation is very common. For some feet above characterize it. Prismopora serrata. Lirnipecten occidentalis. of the Hermosa formation, the full extent of the this transition zone there is a chaotic mixture of Chainodictyon laxum. Acanthopecten carboniferus. unconformity being obscured by erosion. HERMOSA FORMATION". chert and limestone 'fragments, with not a few of Stenopora carbonaria. Edmondia subtruncata. In view of these facts, the strata between the Orthotetes crassus. Allerisma terminale. bluish or white quartzite, but none of granite or Name and definition. The Hermosa formation Meekella striaticostata. Chsenomya leavenworthensis. Rico beds and the base of the Triassic are dis­ schist. In all the lower part of the section much was named and defined by A. C. Spencer in 1900. Chonetes mesolobus. Naticopsis altonensis. tinguished as a formation and named from Cutler Productus gallatinensis. Euomphalus catilloides. of the material is an impure limestone, reddish in It includes the series of alternating limestones, Productus cora. Bellerophon crassus. Creek, which enters Uncompahgre River from the color, but with the saccharoidal texture of the shales, and sandstones, having a maximum thick­ Productus nebraskensis. P*atellostiuin bellum. east about A miles north of Ouray and along which Ouray. It is probably a calcareous sandstone. ness of 2000 feet, which occurs on the northern Productus punctatus. Phillipsia major. a considerable part of the section is well exposed. Marginifera wabashensis var. The matrix in which the fragments of chert, and southern flanks of the San Juan Mountains. As the Cutler beds are conformable with the under­ quartzite, and limestone are held is a red marl- It lies between the Molas and Rico formations. If the formation be divided into^three portions, espe­ lying Hermosa and Rico strata they are provision­ like material. There are in places indications An extensive invertebrate fauna, especially char­ cially as the division was carried out in the Eico region, ally referred to the Permian series. It is recognized, the fauna of each is to a certain extent characteristic. that a calcareous mud was broken up before con­ acterized by brachiopods, is found in the forma­ however, that the Cutler formation may belong, That of the lower division consists almost altogether of solidation and was worked over with the fragments tion from bottom to top. This fauna is distinctly brachiopods. In the middle division a number of gas- wholly or in part, to the Pennsylvania!! series, the of foreign rocks. From the base upward the sec­ Pennsylvanian in character. The name is derived teropods are introduced, while in the upper, in addition stratigraphic break above them being in that case tion becomes more and more sandy, but chert from Hermosa Creek, a tributary of Animas River to brachiopods and gasteropods surviving from the mid­ much greater than the observed field relations indi­ fragments were found almost up to the fossiliferous which traverses a large area of these rocks in the dle division, a considerable force of pelecypods appears. cate. No fossils have been found in the Cutler limestone of the Hermosa. Engineer Mountain quadrangle. The brachiopods remain nearly constant in number, but beds. While the Molas beds exhibit much variability Description. The Hermosa presents a variable form a diminishing proportion of the entire fauna. The The name Dolores will continue to be applied to brachiopodous representation remains fairly uniform, in texture, the section described is in most respects development of limestone, sandstone, and shale in though some changes occur in species and abundance. the Triassic strata as originally defined, embracing characteristic. different districts. Thus at Rico there is a fairly The lower bed especially is often characterized by Pro­ the fossiliferous conglomerates and overlying beds. The chert pebbles which are one of the most well-defined division into three sections, the lower ductus gallatinensis, Productus inflatus, and a large variety Description. The Cutler formation is a complex notable features of the formation are so abun­ consisting chiefly of sandstones and shales with of Spirifer of the rockymontanus type. of sandstones, grits, and conglomerates, alternating dant locally as to form dark conglomerate layers. but little limestone, the middle portion being rich The fauna of the Hermosa formation occurs in also the with sandy shales and earthy or sandy limestones. Weber limestone and lower Maroon formation of the These are in many places not continuous. The in massive limestone beds, the upper containing The greater part of the formation has a bright Crested Butte district, and in the Weber formation of the chert is distributed through the whole formation, mainly black and gray shales alternating with Tenmile and Leadville districts. From this fact and vermilion-red color, but there are some pale-red and on the inclined mesas south of the Needle green grits and sandstones and with a few lime­ the similarity in stratigraphic occurrence a correlation or pinkish grits and conglomerates. A calcareous Mountains, most of which have scarps of the stone layers. of these formations appears to be justified. The Her- cement characterizes nearly all the strata. Indi- Needle Mountains. 6 victual beds of uniform "character are in few cases siderable material derived from the Molas, and at is sharply defined through a sudden change in The most extensive moraines in the region are more than 25 feet thick, and the formation shows a few points Molas in place. The loose material materials. The impression is that the mechanical those which are found at various points on the great lateral variation in composition and thickness. consists of residual chert nodules and red clays. process of deposition was practically continuous, plateau south of Needle Creek. Near the head Distribution. The Cutler formation occupies the The most extensive exposures of the Molas are on but that there was a sudden change in the nature of the Virginia Gulch drainage there are two that extreme northwest corner of the quadrangle. Its Stag Mesa, where the development is less shaly of the debris after the outburst which began the possess distinct boundaries and are of considerable southwesterly dip brings it against the Telltiride than conglomeratic, the bowlders and pebbles con­ great volcanic period. size and very typical form. Probably more exten­ conglomerate with angular unconformity a few sisting of chert and limestone. This phase of the The only occurrences of the Telluride conglom­ sive accumulations occur on the hillside between miles to the north, in the Silverton quadrangle, and Molas also occurs on the south side of Canyon erate in the Needle Mountains quadrangle are a Tank and Canyon creeks, but later landslides have the formation is but partially represented in the Creek, near the western boundary of the quadran­ few exposures near the heads of Whitehead and Elk covered or otherwise obscured them, so that a long Needle Mountains quadrangle. To the southwest, gle, and fossils were obtained at this locality. creeks and at the divide between the two. The lateral moraine on the northwest side of Canyon in the Engineer Mountain quadrangle, the "Red In this area good exposures of the Hermosa are relation of the conglomerate to the older rocks Creek and a corresponding one on the southeast Beds" of the Cutler are very prominent and they almost entirely lacking, and the most noteworthy indicates that in general it occupies the beds of side are the only ones that can now be recognized. also occupy large areas in the Rico, La Plata, feature of the region occupied by the formation is watercourses that existed when the conglomerate Less than a mile west of the boundary of the quad­ Durango, and other quadrangles on the southern the number of landslides in which its rocks have was being formed. The materials consist of coarse rangle, both north and south of Canyon Creek, slope of the San Juan Mountains, while in the val­ been involved, both southeast and northwest of gravels or bowlders, as a rule unstratified and in accumulations of glacial gravels occur whose posi­ ley of Uncompahgre River, in the Ouray quadran­ Canyon Creek. Landslide or glacial debris covers many cases only partly rounded or waterworn, tion and materials indicate that they are lateral gle to the north, their unconformable relation to much of the region occupied by these rocks and indicating a near-by source. The bowlders and moraines deposited by the glacier which occupied the Dolores is best shown. the characteristic cliff exposures presented on the the coarse matrix in which they rest are almost the Animas valley and canyon. It is possible that west side of the Animas Valley are here almost entirely composed of quartzites from the Algonkiaii much of the morainal material which is found gen­ DISTRIBUTION OF THE PALEOZOIC EOCKS. entirely absent; the gentle lower slopes are soil area to the south. erally over the gentle slopes below Stag Mesa and The most conspicuous occurrences of the Pale­ covered and the steeper hillsides south of Pole Age and correlation. The Telluride conglomer­ north of Canyon Creek may also have been derived ozoic sedimentary rocks are at the northwest and and Canyon creeks support a dense forest growth. ate has yielded as yet no fossils by which its age from this source. southwest corners of the quadrangle. The more Although of small size, the occurrence at Devon may be determined. The reference to the Eocene Small terminal moraines occur near the mouths extensive section is that to the northwest, though it Point, on Endlich Mesa, of the three oldest Paleo­ here is in accord with the assignment in the Tellu­ of many of the larger streams that enter the Ani­ probably does not represent the maximum thickness zoic formations is one of the most important. It ride and Silverton folios, no additional data bear­ mas. Some of them, as in Needle Creek, have of the formations nor their most characteristic devel­ was here that Endlich discovered the Devonian ing upon the question having been discovered. been almost completely destroyed by subsequent opment ; that on the south is a part of the complete invertebrates of the Ouray limestone and noted The conglomerate is later than the great orogenic stream action. In the northwest section, east of section traversed by the Animas Valley, in which the existence of fish remains in the underlying movement which affected the entire older sedimen­ Lime Creek, near the laccolith of granite-por­ all formations to the Wasatch (Eocene) are exhib­ shales now assigned to the Elbert formation. tary section in this region, and also later than the phyry, is a moraine of some size; three small ited. On the map the section to the northwest is Besides these occurrences Paleozoic rocks are enormous erosion which produced the peneplain ones east of the canyon of Lime Creek were prob­ made to include the Rico formation, for, although it preserved in the southeast corner of the quad­ on which the conglomerate rests. The observed ably laid down by a small glacier heading in the has not been positively identified here, it has been rangle, east of the Vallecito, and are also repre­ angular unconformity at its base transgresses the region south of Snowdon Peak. recognized in the immediately adjacent parts of sented on the Continental Divide, in the northwest section from the Algonkiaii to the Mancos shales Extent and character of glaciation. Only partial the Silverton and Engineer Mountain quadrangles, corner, by two small patches of Ouray limestone, (Cretaceous). evidence is to be found in the Needle Mountains and from these known occurrences its position in one of which, near the trail to Bear Creek, is The relations mentioned suggest a correlation of quadrangle as to the extent of the glaciation to the present quadrangle was determined. The small nearly covered by San Juan tuff. In both of the Telluride conglomerate with the Arapahoe for­ which the region was subjected. Judging from patch of the Cutler formation represented in the these localities the Ignacio formation is signifi­ mation of the Denver region, and such a compari­ the morainal material alone one would be led to extreme corner above the Rico is also very poorly cantly absent, and on the Continental Divide the son carries with it the further correlation of the suppose that the ice was restricted to the smaller exposed, and it has been described from better Elbert as well has not been found. East of the San Juan tuffs with the Denver beds, which are valleys and that no great glaciers filled the large occurrences outside the quadrangle. Between the Vallecito the presence of the Hermosa is known composed largely of andesitic debris. Both the valleys of the Animas, Florida, and Vallecito. Rico and the older Paleozoic rocks the Hermosa is only from a few outcrops, as it is covered by a Arapahoe and Denver formations are fossiliferous, In other words, the conditions prevailing in the exposed on both sides of the east fork of Lime thick mantle of soil and vegetation. and according to current paleontological opinion Needle Mountains during that part of the Glacial Creek in a fire-swept region almost bare of green they should be considered Cretaceous, though sep­ period of which the best records are left appear to TERTIARY SYSTEM. timber. The alternation of hard limestones and arated from the Laramie proper by an interval of have been nearly the same as those which exist at sandstones with soft shales has helped to develop a EOCENE (?) SERIES. great orogenic disturbance and subsequent erosion. the present time in the Alps and similar moun­ bench- or step-like topography, and individual hori­ TELLURIDE CONGLOMERATE. The Denver beds have, in fact, an equivalent, indi­ tain regions. It is known, however, from the zons may be followed by the eye for several miles Name and definition. The Telluride conglom­ cated by plant remains, in the Animas beds of the existence of well-defined terminal and lateral in and out among the hills. With the exception erate is a well-marked formation which underlies section well shown a short distance below Durango, moraines in the Animas Valley, that during the of the Molas the older Paleozoics are poorly exposed the San Juan tuff and was laid down in the period but there is no conglomerate comparable with the last recognized period of maximum glaciation an and do not have here their usual development. immediately preceding the beginning of the long Telluride in that section and the correspondence of ice stream descended the valley as far as what is The Ignacio is altogether absent except at two succession of volcanic eruptions in this region. It the San Juan tuffs and the Animas beds is purely now the site of the town of Durango,, some 25 places, south and east of Molas Lake, where it is includes the sediments first deposited after the tre­ lithological, so far as known. Under these circum­ miles southwest of the Needle Mpuntains, and that in the form of a coarse conglomerate 50 feet thick, mendous erosion that .prepared the peneplain on stances it is thought best to await further investiga­ glaciers of corresponding magnitude 'occupied the indicating that coastal conditions probably pre­ which it rests. It was first identified in the Tel­ tion in the San Juan region before making any Vallecito and the valley of the Florida. The vailed there at the time of the Ignacio sea. South­ luride quadrangle, where it is finely developed and positive correlations of the Telluride and San Juan rock debris found in the terminal moraines con­ west of Snowdon Peak a considerable patch of exposed, and in the Silverton folio the formation formations with similar formations in other dis­ sists very largely of material deriA^ed from the Ouray limestone rests directly upon the Archean was renamed after that area, since the term San tricts. For a discussion of this question the reader Needle Mountains. The grooved and polished rocks and upon it lies a small remnant of the Miguel, under which it was originally described, is referred to the Telluride folio and to the mono­ surfaces of bare rock that are to be seen every­ Molas. was preoccupied for a Cretaceous formation in graph on the "Geology of the Denver Basin" where throughout the Needles, except on the The most characteristic sections of the older for­ Texas. (Mon. U. S. Geol. Survey, vol. 27, 1896). sharper peaks and ridges, together with the fore­ mations, from the Ignacio to the Molas, are found Description. The Telluride formation varies going facts, indicate that while glaciation was at in the southern third of the quadrangle. These greatly in texture and thickness, ranging from a QUATERNARY SYSTEM. its height the greater part of the Needle Moun­ occurrences, of which Lime and Stag mesas may thin, coarse conglomerate in the Silverton and Deposits of subaerial origin of Pleistocene and tains area was buried beneath a thick mantle of be taken as examples (figs. 1 and 2 on illustration Needle Mountains quadrangles to a complex of Recent age in the Needle Mountains quadrangle ice and snow, from which only the higher sum­ sheet), present strata resting on the granite floor fine-grained conglomerates, sandstones, and shales fall under three heads: Glacial debris," valley mits projected. So far as known, no strictly with a gentle southerly or southwesterly dip arid about 1000 feet in thickness as developed in Mount alluvium, and rock streams. The landslides will limiting moraines belonging to this maximum are directly connected with the section of Paleozoic Wilson, on the western border of the Telluride also be described here, though of exceptional charac­ period are to be found within the area of the and later sedimentary rocks exposed on both sides area. Nearly all stages of this change can be ter and not properly to be regarded as sedimentary. present quadrangle, but the greater part of the of the Animas Valley. Each of the formations is studied in the distance of 17 miles between the drift which lies on the slopes south of the Needles exhibited in very typical exposures. The Ignacio eastern ledges of Grand Turk, north of Molas GLACIAL DEBRIS. and west of Snowdon Peak is to be regarded as commonly forms broad benches covered by loose Lake, and the exposures on Mount Wilson. The Glacial deposits occur throughout the region, the material deposited during the recession of the blocks of quartzite split from the solid rock by formation is described in detail in the Telluride but in only a comparatively few places have they ice. frost. This is particularly true of the isolated folio. It is made up of detritus of schists, granite, been indicated on the map. Elsewhere they may The relatively small accumulations of glacial patches capping the higher points of Mountain Algonkiaii quartzite, and slate, and lesser amounts often be recognized, but they are not of sufficient materials which possess the distinct forms of View Crest, West Silver Mesa, and the two occur­ of the harder sediments of the Paleozoic formations, importance to represent, since by so doing the rela­ terminal or lateral moraines are believed to have rences east of Sheridan and Emerson mountains. in particular of limestone. The bowlders of the tions of the massive rocks which underlie them been deposited, as has been said, by the small Buff-colored, rounded hilltops or slopes character­ conglomerate often exceed 1 foot in diameter, but would be obscured. In many cases in the larger hillside or valley glaciers that lingered for a con­ ize the few occurrences of the Elbert shale where it are generally only a few inches. Many are well valleys glacial gravels have been recognized which siderable time after the region as a whole became is unprotected by the Ouray, as at Overlook Point rounded, but subangular fragments are common, do not appear to have been deposited directly by free from ice. and at the head of Virginia Gulch. Elsewhere the especially of the local material, which has not the ice, but seem to have been worked over and Elbert is at many points obscured by its own debris been transported far. The isolated exposures of redistributed by later stream action; some undoubt­ VALLEY ALLUVIUM ABTD DETRITAL CONES. or that from the Ouray above it and may not be the conglomerate found east of Animas River, edly represent contemporaneous "outwash" gravels. Notwithstanding the youthfulness of the Needle easily detected. about the head of Whitehead Gulch, are natur­ They have been left on many narrow, steep ridges Mountains nearly all the larger valleys contain The Ouray limestone forms the prominent ally made up of local materials. with outcrops of rock in place in the ravines on alluvial deposits. Undoubtedly much of the escarpment of Lime Mesa (shown in fig. 1), as Stratigraphic relations and distribution. As either side or as knobs projecting through them, as material is of glacial origin, but it has been well as those of West Silver Mesa and Stag Mesa stated above, the Telluride formation conform­ in the stream entering the Animas at the lower end very generally worked over by the streams in (fig. 2). The first two mesas are almost entirely ably underlies the San Juan tuffs. While no of Elk Park and heading to the north of Snowdon the main valleys or cut through by the lateral covered by the Ouray, in which there are a con­ volcanic matter appears in the Telluride proper, Peak. Evidently the gravels were once thick near streams and mixed with their debris. This redis­ siderable number of sink holes and other erosional there is a zone of transition in some places, the the mouths of the high basins where the descent is tribution has been more complete in some places features, such as the ribbed surfaces, or "karren- pebbles of quartzite or schist being scattered rapid to the main valley. These deposits probably than in others and makes it difficult to draw sharp felder," common in limestone regions. through a few feet of the tuff and sporadically represent the morainal material from the basin lines between moraines and stream gravels. Well- On the surface of many of these mesas lies .con­ for a hundred feet or more. In general the line glaciers and belong to the later periods of glaciation. stratified and, locally, terraced gravels occur nearly continuously in that portion of the Animas Can­ partly because the accumulations are less thick Mountains region is the Twilight granite. It (fig. 11). In places where the brecciated schist yon which is included in the Needle Mountains and partly because in a great many cases the loose occurs as an intrusive in the Archean schists in predominates and the intrusive is more in the quadrangle. The materials are of local origin and rock and soil have slipped down over glacial both the Needle Mountains and Engineer Moun­ nature of a cement, and also in the small off­ have been supplied principally by the branch gravels or moraines, causing a mingling of the tain quadrangles, the best and most typical expo­ shoots from the dikes, the body of the intrusive streams that have built large fan-like deposits of two sorts of material. This is particularly the sures being about the mouth of Cascade Creek, rock has a distinctly massive texture and is often detritus at their mouths. case in the Florida or Spruce Gulch area. from 2 to 3 miles west of the boundary between pegmatitic. The change from gneissose to granular More complex accumulations are found in Valle- On the broad floor of Elk Creek, about 3 miles the quadrangles. As shown on the map, a large rock is abrupt, but there is not the slightest evi­ cito Creek from the base of The Guardian to a mile from its mouth, a great heap of rock debris three- area is occupied by this granite along this bound­ dence that the granular or pegmatitic offshoots or more below the mouth of Johnson Creek. The fourths of a mile in length extends nearly from ary, extending from a little south of the point occur as later intrusions. In all cases their origin large area in this valley represented on the Hayden one side of the valley to the other. Its surface is where the Animas leaves the Needle Mountains seems to have been contemporaneous with that of map as moraine is a mixture of probably true hummocked and crevassed and a steep slope nearly quadrangle to the Uncompahgre quartzites on Lime the larger gneissoid dikes. In some of the larger moraine, river gravels, and torrential fans, the last 75 feet high bounds it on all sides. It possesses Creek ; the body extends far enough eastward to offshoots from the main dikes there is a good being the most extensive at the surface and the gla­ many of the characteristics of a rock stream, though include the peaks of the West Needles. As will be banding at right angles to that of the dikes them­ cial deposits beneath. In the more southerly por­ its position in the middle of a long valley is unu­ explained later, the lines representing the bound­ selves; in offshoots of intermediate size there is tion the deposits are fluviatile, covering a broad, sual. It seems probable, however, that the mate­ aries of the Twilight granite are in most cases quite apt to be a suggestion of gneissic structure near flat area nearly at the present stream level and rial has been supplied by landslides from the arbitrary and do not indicate any contacts that can the contacts, while the mass of the interior is traversed by old stream channels. From near quartzite cliffs to the south. The rocks here are be actually found or followed in the field. granular. The textural relations show clearly that bench mark 9577 northward the conditions are much jointed and the well-developed planes of Character of the intrusions. The Twilight gran­ the gneissoid banding of the intrusive is a primary quite different. The stream flows for a large part cleavage lying at high angles have caused enor­ ite is essentially gneissose in texture, but varies flow texture and not one induced by pressure or of this distance in a narrow channel 50 feet or more mous masses of rock to slip from the cliff face to from a purely granular rock to a distinctly foliated mashing in the solid rock. deep and often with rock walls and a rock bottom. the valley below. gneiss, which in the canyon is very light colored, Petrographic description. The rock is light gray or almost Above the stream, on both sides, is a broad bench in places almost white, while the schists which it white in color except in the region between the West Needle BOCK STREAMS. covered with detritus and often swampy. North intrudes are very dark amphibolites. In the can­ Mountains and Potato Hill, where it is more dense and pink or red. It is of a medium grain and is usually foliated or of the mouth of the creek which enters the Valle- Closely connected with landslides in their origin yon area the granite intrudes the schists in the gneissic in texture. The dark silicates are nowhere in excess, cito from the east opposite The Guardian this detri­ are large accumulations of angular rock fragments form of dikes from a few inches to 10 or 20 feet in but are always found in sufficient quantities and so arranged tus is in the form of hummocks and mounds which and sand or soil that occur in some of the high width, the plane of intrusion corresponding to the as to emphasize the characteristic foliation to the rocks. The principal constituents are feldspar, quartz, biotite, and horn­ suggest glacial origin. These facts seem to indicate mountain basins or cirques. They were first schistosity of the older rocks. The dikes recur blende. Their relative proportions are extremely variable. that at one time extensive moraines existed here, described in the Silverton folio, where they were with wonderful regularity and the intervening In most instances feldspar is in excess of quartz and both more than the ferromagnesian silicates, but it is not uncom­ but that since their formation the main stream has given the name "rock streams." There are bands of schist are about as wide as the dikes mon to find quartz in excess of the feldspars. Biotite may be fed upon them and the two or three large trib­ numerous occurrences of such deposits in the themselves. The zone of most regular banding entirely lacking and hornblende the only dark silicate pres­ utaries entering in this vicinity have nearly Silverton quadrangle, and the reader is referred is in the Engineer Mountain quadrangle and lies ent, or the reverse may be true. The plagioclase feldspars are almost uniformly present and as a rule are quite prom­ obliterated them, either by the removal of old to that folio for a full discussion of their nature between Cascade and Little Cascade creeks, extend­ inent, in some cases exceeding orthoclase in amount. The material or the addition of new in the form of and origin. It is sufficient to state here that they ing about three-fourths of a mile across the strike rock, therefore, has a monzonitic character in such portions. The plagioclase ranges from albite to andesine in composi­ broad alluvial fans. The result has been that appear to have originated as landslides falling of the schists. To the south the granite shows a tion, oligoclase being the most abundant. Biotite is found little, if any, of the accumulations can now be upon the surface of the glaciers which at the greater tendency toward irregular intrusion and at in irregular patches arranged so as to give the rock a foliated distinguished as of true morainal form, and it has time covered the floors of the cirques, and with many points cuts across at right angles or diagonal appearance. Near the western boundary of the quadrangle, in the Animas Canyon, it occurs in thick masses from which been necessary to class the whole with the fluvia­ the gradual melting of the ice the material settled to the schistosity; in some instances the schist has cleavage flakes may be easily removed by the point of a tile deposits. The alluvium of Needle Creek is to the position it now occupies. The nature of the been brecciated by the force of the intrusion and knife; elsewhere it is in thin, disconnected blades. The accessories most often found are magnetite and garnet; apa­ undoubtedly of this character, while that of the material itself closely resembles that of ordinary fragments of all sizes are included in the granite. tite, titanite, and zircon occur sparingly, and rarely musco- smaller streams represents the overloading by talus accumulation at the base of cliffs, but the low Transitions from one form of intrusion to the other vite is present as an original mineral. At first sight the detritus from the fans of the lateral ravines and angle at which it rests and the distance of the are found in the field; the different stages are shown texture in the dikes seems to be due to the same cause as in the amphibolites that is, some secondary structure intro­ gulches. outer extremities from the cliffs which have sup­ in figs. 10 and 11. Offshoots from the dikes, cut­ duced by great pressure after the consolidation of the intru­ plied the debris at once distinguish it from simple ting across the schistosity, occur very generally sives but if the different conditions of occurrence and the LANDSLIDES. variations in structure are taken into account, as well as the talus. and range from the merest threads to bodies 12 to fact that under the microscope none of the gneissose granites Evidence is found in the southwest corner of Rock streams occur in the basin south of Snow^ 18 inches in width (fig. 10). As the region of show any evidence of having been crushed or subjected to the quadrangle that landslides have materially don Peak, in a cirque southwest of Balsam Lake, irregular intrusives is approached the dikes become unusual strains since their consolidation, it becomes evident that the banding of the granite must be an original texture changed the character of the post-Glacial topog­ and under Aztec Mountain, in the southern part smaller and less prominent, while the offshoots are which was assumed during consolidation. raphy. They have been confined entirely to the of Chicago Basin. Another occurrence is west of more and more numerous and variable in their All the principal constituents show a tendency to form elongate or rudely lenticular grains parallel to the general areas occupied by the Herrnosa formation, where Storm King Peak, at the head of Tenmile Creek. habit, until finally dikes near together are con­ foliation of the rock, but none are more striking in this respect the alternation of shale with massive limestone or This does not possess the stream-like character of nected by innumerable bands and what schist thantthe plagioclase feldspars. sandstone leads to favorable conditions for land­ the others, but is to be classed with the smaller there is appears as included masses in the granite In ordinary eruptive granites not having a foliated or banded structure anhedra of plagioclase are very rare and slips. The principal region in which such accu­ occurrences in the Silverton quadrangle. These nearly idiomorphic individuals are the rule. In the Twilight mulations are found is that immediately adjoining were supposed to^ave been formed by snow banks The granite of the West Needles and south of .granite, however, parallel elongation of the plagioclase grains has been brought about apparently regardless of any crystal- Canyon Creek and its tributaries, Camp Creek which permitted rocks from the exposed cliffs above Potato Hill, in the Engineer Mountain quadran­ lographic directions in the minerals themselves. Sometimes, and Pole Creek. They may be recognized by to roll down and out beyond the zone of ordinary gle, appears to be the central mass of which the it is true, stubby laths are found in which the longer diame­ the uneven surface, due to great blocks of rock talus accumulation. The resulting forms are in dikes just described are apophyses. In only a few ters are parallel to the general foliation, but alongside or at a short distance behind or ahead of them other grains of pla­ tilted at various angles, with finer materials partly many instances like broad terraces with steep sides places have schists been found in that area, and gioclase occur in which the elongation has taken place at filling the spaces between them. No regular 50 feet or more high and not infrequently sepa­ their field relations indicate that they are inclu­ right angles to the twin lamellae, or in many cases at an oblique angle. The occurrence of elongate automorphic and drainage system exists in such areas, but stagnant rated from the cliff base by a depression or trench. sions in the granite. The gradual decrease in vol­ anhedral grains side by side, as well as the absence of any pools, morasses, or deep trenches are common. IGNEOUS ROCKS. ume of the granite away from the central mass is evidence of crushing or strain subsequent to their crystalli­ The slides are of considerable antiquity, and the well shown in the Animas Canyon from the west­ zation, would seem to indicate that this structure is a primary PRE-TEETIARY INTKUSIVE ROCKS. one, the result of original crystallization from a magma, and disordered heaps of rock and soil have yielded ern boundary of the Needle Mountains quadrangle evidence found in the field fully corroborates this view. somewhat to the influence of the weather and The most prominent rocks of igneous origin in eastward for about a mile and a half. Here there TENMII/E GRANITE. have long been covered by forest or grass. Even the Needle Mountains are granites which intrude are a number of bands of granite 200 to 300 yards the scars on the hillsides at the points from which the Archean schists and in some cases the Algon- in thickness, separated fi'.om each ovther by nar­ Occurrence. The Tenmile granite is exposed the slides took place have as a rule been healed kian quartzites and greenstones. They display rower areas in which schists predominate, but On both sides of the Animas Canyon from a short and in many cases the places where solid rock not a little textural and mineralogical variation which also contain the Twilight granite. Finally, distance below Elk Park to the mouth of Ruby occurs may be recognized only by steeper slopes and in certain individual masses various periods the outermost zone is essentially like that described Creek, its highest outcrops being 2500 feet or and more even surfaces. of intrusion may be recognized. The different near Cascade Creek, where the granite and schist more above the river. 'Its greatest width is nearly In nearly all cases the slides seem to have conre occurrences or bodies have been given local names alternate in narrow bands until the granite disap­ 3 miles in an east-west direction. West of the from the sides of the steep ridges separating the for purposes of description and mapping. They pears altogether. river the contact of the granite with the Archean smaller valleys and ravines or at the heads of the include (1) the Twilight gneissose granite, which From this description it will be seen how diffi­ schists is poorly exposed at the north and south streams. This seems to be clearly the case on both makes up a greater part of the West Needles, cult a matter it is to draw boundaries for the Twi­ ends of the body. To the east conditions are sides of Canyon Creek, while to the northwest, on including their culminating point Twilight Peak, light granite, since the relatively small scale of the more favorable for observation. An attempt has the lower slopes southwest of Stag Mesa, the pres­ and is particularly well exposed where Cascade map makes it quite impossible to represent on it been made to show on the map in a generalized ent moderate relief and the low angle of the slopes Creek joins Animas River, in the Engineer Moun­ any of the numerous small irregularities that have manner the great irregularity of the contact between can apparently be adequately explained in no tain quadrangle, 5 miles below Needleton; (2) the been described. The line as drawn includes that the granite and the schist, but it is impossible to other way. High, narrow ridges must have Whitehead granite, in the northern part of the area within which the Twilight intrusive predom­ represent the actual details. Unlike the White- existed in this area, and the slipping of their quadrangle; (3) the Tenmile granite, named from inates; schist may be found in considerable quan­ head body there is no network of intersecting materials into the drains not only lowered the a stream that enters the Animas southeast of Snow- tity within the generalized boundary, while many dikes surrounding the central mass, but rather a ridges, but filled the drains and resulted in the don Peak; (4) the Eolus granite, a great body that small dikes of the granite may occur outside of the series of elongate bodies, nearly parallel to one present irregular topography, of low relief. The occurs in Mount Eolus and other high peaks and line, but do not extend far beyond it. another and to the general boundary of the gran­ upper limits of these slides are pretty clearly about the headwaters of Florida River; and (5) Texture. The texture of the Twilight rock ite, which decrease in size outward from the main shown and may be traced about the ends of the the Trimble granite, which intrudes the Eolus in varies with its mode of occurrence. In the main body. At the north and south ends many arms ridges from one drain to another. the neighborhood of Trimble Pass. In addition to dikes the rock is a distinct gneissose granite, with extend into the schists, in places for nearly half a The only other locality where landslide condi­ these, dikes of aplite and pegmatite cut both schist the banding parallel to the walls of the dike and mile, the outlying members being hardly recognized tions prevail is in the Florida drainage, east of and granite, and a single occurrence of granodiorite hence generally to the schistosity of the amphibo­ as belonging to the Tenmile granite, because of the divide at the head of Camp and Pole creeks, has been observed. lites. Larger irregular masses have the gneissic their fineness of grain and the subordinate char­ the characteristics being the same as those of the texture where the intrusive is in excess of frag­ acter of the dark silicates. In almost all cases the region to the west. While the upper limits of the TWILIGHT GRANITE. ments of included schist; the laminations are not apophyses, of which there are a countless number, landslides are almost everywhere well defined, the Relation to the schists. One of the most note­ parallel, however, but fold or flow about the inclu­ are parallel to the schistosity of the Archean com­ lower or outer edges are much harder to map, worthy of the older igneous rocks of the Needle sions in whatever position they may happen to lie plex. While the irregularities of contact are great, Needle Mountains. the actual contacts of the apophyses and the schists granite. It Occupies about a third of the whole Petrographic description. With the exception of the con­ are prominent. The microscopic structure is hoiocrystalline quadrangle and extends southward for several tact facies, the Trimble granite has an even texture and is of and in general granular, with a rather common tendency are sharply defined. a moderately fine grain; the color is light gray with a pink­ toward a trachytic development of the feldspar. In many PetrograpJiic description. The Teumile rock as a whole is miles until it is covered by the sedimentary for­ ish tinge. There is a very uniform sprinkling of fine specks specimens the feldspar is quite without crystallographic form a rather coarse light-pink or gray granite, but there is an mations; it is also found in the Durango and of a dark mica and lath-shaped crystals of feldspar 2 to 3 mm. and appears as a groundmass in which small flakes of biotite long. The rock consists of microcline with relatively subor­ rest as phenocrysts. Occasionally it is found as a poikilitic intricate mingling of granite of other textures, shades, and Engineer Mountain quadrangles. Many of the colors, ranging from coarse pink pegmatites to fine dark-gray dinate amounts of orthoclase and plagioclase, quartz, biotite, intergrowth with large phenocrysts of hornblende or biotite. gneissoid rocks rich in biotite, that resemble some mica- highest peaks of the region four of them over and locally a little muscovite, with magnetite or apatite as The phenocrysts, when present, are of biotite and horn­ diorites. The association of these rocks is most intimate, but 14,000 feet in elevation are composed of this accessories. The structure is very characteristically hypidio- blende or actinolite, the mica largely predominating and notwithstanding the diversity of their megascopic characters, rnorphic-granular. The contact phases differ from the ordi­ generally unaltered. Hornblende, on the other hand, is not their mineralogical compositions show no marked differences. granite. Its distribution is so clearly shown on nary rock only in the presence of phenocrysts of feldspar, fresh as a rule, the alteration being to chlorite. The ground- The lighter-colored rock closely resembles the Whitehead the map that it needs no further discussion here many of them 1 inch long, and in the color, which is slightly mass, which is of the same composition as the mass of the granite, both megascopically and microscopically. The pinker. unporphyritic rocks, is composed largely of feldspar with except in regard to the occurrence in Tenmile abundant flakes and patches of biotite and usually a little feldspars are orthoclase, microcline, and a very little plagio- GRANODIORITE. clase; quartz is abundant, while biotite is nowhere promi­ Creek and that at the head of the gulch north quartz. The accessory minerals apatite, magnetite, and nent, and there is generally a little muscovite. Accessory of Leviathan Peak. The rock of these two local­ Just west of the junction of Virginia Gulch with rutile occur in all the specimens, while in some they assume magnetite, apatite, and titanite are found in nearly all the unusually prominent rdles, especially apatite. Rutile occurs specimens. The darker rock, which appears to be quite dis­ ities, although isolated from the main body, is of Florida River a body of dark rock cuts the Eolus very typically as fine needle-like inclusions in biotite. tinct in the field, is more variable in texture, ranging from precisely the same petrographical character, and granite. It is about three-fourths of a mile long The alkali feldspars predominate in the majority of the coarsely granular to finely banded gneissic, and contains a because of the nearly horizontal contacts with the and half a mile wide, and apparently has no off­ rocks, but with them a little acid plagioclase is almost always greater percentage of the dark silicate biotite. In many to be found. In only one instance, that of a small dike out­ places unmistakable transitions of one rock into the other overlying rocks the exposures in question are shoots or apophyses, nor has any similar rock been cropping along the trail east of the Quartz Mill, has pla­ have been found and a comparison of thin sections has thought to belong to thick sheet-like extensions found elsewhere in the quadrangle. The rock is gioclase been found to the almost complete exclusion of proved that they are essentially the same. The principal orthoclase. In this case the rock, a porphyry, contained differences seem to be the greater amount of biotite in the of the main. mass. With the exception of the moderately coarse grained, even textured, and of well-formed, fresh phenocrysts of labradorite (Ab 2 Ans ), with dark rock, together Avith a slight increase in the quantity great fault which bounds the body on its eastern a clear-gray color; a noteworthy feature is the a groundmass composed of labradorite laths and blades of and basicity of the plagioclase. Hornblende also is occasion­ side, most of the contacts of the Eolus granite with large quantity of the darker silicates present. The biotite and accessory magnetite a mineralogical composition ally found in the darker rock. corresponding to that of kersantite. All the other rocks Relations of the dark and light varieties. That there has the older rocks are primary. This is uniformly microscope shows that plagioclase, ranging from would fall under the head of quartz-bearing mica syenite, or been more than one period of eruption is evident, and the true in the case of the Archean schists. In the oligoclase to andesine, is more abundant than minette. None are strictly fresh, the abundant alteration differences between the lighter and darker rocks are to be products being calcite, epidote, muscovite, chlorite, and attributed to the lack of uniformity in composition of a Leviathan Peak and Tenmile Creek areas and on orthoclase or microcline, and that hornblende, hydrous oxides of iron. single magma. A more acid portion, possibly contempo­ Mountain View Crest the Eolus is seen to intrude biotite, and rarely a little augite nearly equal raneous with the Whitehead intrusion and derived from the the feldspars; a very little quartz may also be DIABASE DIKES. same source, was erupted first and before its complete consol­ the Uncompahgre quartzites, although elsewhere idation the slightly more basic portion was forced into the faults generally separate them. The Tenmile present. The accessories are magnetite, apatite, Dikes of diabase are not at all numerous in the first intrusion. In places fissures formed in the older granite granite is also cut by apophyses from the Eolus and titanite. The rock is to be regarded as a Needle Mountains quadrangle, although common and these were filled by the younger rock as dikes; in other places the lighter-colored rock was fractured and coarsely where they are in contact southwest of Mount granodiorite containing a rather unusual amount just to the north in the Silverton region. Those brecciated, the fragments being included in the darker gran­ Garfield. The Eolus granite is older than the of the dark silicates. that do occur in this area are connected directly ite, while at some points the older rock was still so nearly Paleozoic sedimentary rocks, for the Ignacio with the Silverton dikes and are found intruding liquid that no sharp boundaries are seen and the two rocks APLITE AND PEGMATITE. seem to blend into one another. The banded or gneissic tex­ quartzite, of late Cambrian age, is found to rest the schists along the northern border of the quad­ ture characteristic of the darker rock and occasionally found unconformably on the granite. For these reasons Occurrenee. Throughout the whole region of rangle. Some are remarkably fresh and possess in the lighter granite shows no evidence of having been induced secondarily through pressure after the consolidation the Eolus is considered to be of early Cambrian the Needle Mountains dikes and veins of aplite the usual texture and mineral composition of dia­ of the rock, but rather is primary, produced while the magma and pegmatite are very common. As a rule the base; others show signs of alteration and dynamic was still in a viscous condition, and probably corresponds It is not unnatural that a body of granite as pegmatites are in ore intimately associated with the to flow texture like that of the Twilight gneissose granite. metamorphism and, in extreme cases, are now Inclusions of schist are not uncommon in the Tenmile gran­ large as this should show some variation in texture larger masses of granite, while the aplites are seen totally different from the original rock. These ite. As a rule the included masses are large, 20 to 50 feet in and mineral composition in different parts of the everywhere in the granites and far away in the differences have been observed in the same dike, diameter, with sharp contacts with the granite, and at many points cut by pegmatite veins or dikes which enter from the mass, but marked differences are, as a rule, local, schists. The dikes of pegmatite are nowhere very one portion of which may be a normal diabase, granite. There is no evidence of fusion and assimilation of and the general character of the rock throughout is large or of great prominence; they are in few cases while another is an amphibolite with schistosity schist material by the granite magma. extremely uniform. more than 3 or 4 feet in width and may be but a often well developed. The fresh and unaltered WHITEHEAD GRANITE. few inches. They are particularly well developed diabase is a medium-grained rock, brown-black in Petrographic description. In its typical occurrence the Occurrence. As shown on the map, the occur­ Eolus granite is coarsely granular and is composed of pink in association with the Whitehead and Tenmile color, and possessing the usual ophitic texture. rence of the Whitehead granite is restricted to the feldspar, bluish quartz, and black biotite and hornblende, granites. : Laths of plagioclase and a pyroxene may be made region about the mouth of Whitehead Gulch, to the last two minerals being grouped together and occupying a very prominent position in the rock. Near contacts a pri­ Petrographic description. Though nowhere very coarse, out megascopically and an ophitic structure is the southeast of Molas Lake, and near the Conti­ mary gneissic texture is commonly developed, though locally the pegmatites are extremely variable in texture; many peg­ pronounced microscopically. The metamorphism nental Divide, at the head of the north fork of the rock may be merely of much 'finer grain and contain a matite dikes change their habit and become, within a few which has affected these rocks is precisely the larger portion of the dark silicates. About the head of feet, fine-grained aplite. The principal constituents, feldspar Elk Creek. It probably has a much wider distri­ Needle Creek the granite becomes coarsely porphyritic, with and quartz, are most irregularly intergrown, few of the indi­ same as that noted in the greenstones of the bution than this, being associated with the Archean large Carlsbad twins of pink feldspar held in a groundmass vidual minerals measuring more than an inch in diameter Irving series, and needs no further description, schists along the whole northern side of White- of but little finer grain than that of the average rock. Still and none exhibiting perfect crystal forms. The feldspars another variation is found at the divide south, of Emerald are orthoclase and microcline, no plagioclase being present here, as it reaches its maximum only within the head Gulch in the form of innumerable dikes and Lake, where biotite and hornblende are of such abundance in the sections examined. Besides the quartz occasional Silverton area. There is no means of closely fix­ irregular bodies, but on account of the small size that the rock is extremely dark and is hardly recognized as large blades and plates of biotite and, more rarely, muscovite ing the age of these intrusives, but it seems prob­ belonging to the Eolus. The granite weathers readily to a occur. The color is usually pink to bright red, but white and the confused relations of individual occurrences coarse sand composed of broken feldspar crystals, horn­ dikes or veins are not uncommon. able that they belong to the time that the Irving it has been impossible to represent the granite in blende, and quartz. The aplitic dikes may attain greater width than the largest intrusions took place. this area. West of Animas River the granite Under the microscope orthoclase is found to be the pre­ pegmatites, but as a rule they are not wider than from 8 to dominating feldspar; microcline is almost uniformly present 15 inches. They have sharp contacts with the rocks they are GRANITE-PORPHYRY. amounts to little more than a network of dikes and in some cases in amounts equal to the orthoclase. In cutting, whether it be granite or schist, and pass from one to except at two points near the Archean-Uncom- the porphyritic Needle Creek rock the phenocrysts are micro­ the other with no apparent change in structure or in the char­ cline, which also occurs in excess of the other feldspars in the acter of their mineral constituents. They are fine grained Certain rocks, found both as dikes and in larger pahgre contact, where it has a more massive devel­ coarse groundmass. The lime-soda feldspars are much more and even textured, in many cases saccha,roidal. The color intrusive masses in various parts of the quadrangle opment. A small patch directly east of Molas prominent than in the Whitehead or Tenmile granite, espe­ varies from white to bright pink or purplish. They are very and probably of different ages, may be grouped Lake, which is part of a larger body extending cially near contacts, where andesine has sometimes been nearly of the same age as the associated pegmatites. In about found in excess of orthoclase. In the Needle Creek occur­ an equal proportion of the occurrences they cut the pegma­ conveniently as granite-porphyries. Their miner­ northward in the Silverton quadrangle and which rence they are much more subordinate. Biotite occurs abun­ tites or are themselves cut by the coarser rock. The constit­ alogical composition is about the same in all cases, was described in that folio as the Molas granite, dantly in large patches without crystal boundaries; it is jet uents are usually only feldspar and quartz, though sometimes black in, hand specimens and in thin sections either brown or a little biotite or muscovite is found. The accessories are and chemically they would probably differ but may, for convenience, be mentioned here with the green. It contains numerous inclusions of magnetite and nowhere prominent. Garnet is fairly abundant locally. little from one another. The dikes are usually Whitehead granite. East of the river and directly apatite. Quartz is less abundant in rocks of this type than Magnetite occurs regularly, but sparingly, and hematite is found in the immediate vicinity of the larger north of the mouth of Whitehead Gulch there is in the biotite-granite to the north, but megascopically it is often found as a pigment. Of the feldspars, orthoclase is very characteristic, for it is everywhere of a bright-blue color. uniformly in excess of microcline and plagioclase. Micro- granite bodies and are particularly numerous in a rather large body for which no very sharp bound­ Hornblende is uniformly present, though in varying amounts, pegmatite is strikingly developed in places, and quartz is the region of the Irving greenstones and less fre­ ary can be drawn, as it sends out into the schists at some points equaling the biotite; its color is green to yel­ everywhere very prominent, in many instances equaling or lowish and it is strongly pleochroic. Pale-green augite has surpassing the feldspars. quently observed intruding the Uncompahgre a multitude of dikes and arms which branch and been found in specimens collected near contacts, and at one formation. The majority differ only in texture rebranch. until they gradually disappear. The locality it was only slightly less in amount than biotite and LAMPROPHYRIC DIKES. from the coarser granite-porphyries which were occurrence near the Continental Divide is essen­ more than hornblende. It was at this same locality that pla­ gioclase exceeded orthoclase, but both occurrences are evi­ Occurrence. Dikes of mica-syenite or minette described in connection with the Eolus and Trim­ tially the same as that at the mouth of Whitehead dently quite local and of comparatively little importance. are found in considerable abundance in the north­ ble granites, and undoubtedly belong to the same Gulch. The contacts of the individual dikes with ern third of the quadrangle, cutting the schists, series of eruptions. They have not been repre­ TRIMBLE GRANITE. the schists are in all cases sharp and well defined. granites, and quartzites, the greater number being sented on the map, because of their relative Although lying so close to the Uncompahgre for­ Occurrence. In the region northeast of Missouri found in the Uncompahgre formation. A few unimportance and insignificant dimensions. Two mation, the Whitehead granite is nowhere seen to Gulch and including Vallecito Basin a fine-grained occur as far south as Emerson Mountain and the occurrences of granite-porphyry in large masses intrude the quartzites and it is assumed to be older gray granite is found intruded in the Eolus. The Quartz Mill. They are commonly found intrud­ are shown on the map one in Chicago Basin, than the Algonkian. contacts between the two rocks are very sharp and, ing the softer shales and slates of the Uncom­ near the head of Needle Creek, with a large dike Petrographic description. The Whitehead granite is a except in the southwestern part, there are almost pahgre, and in their field occurrence are mainly of similar rock near the head of Virginia Gulch, rather coarse-textured rock, pink or light red in color, in no apophyses. Offshoots from this (the Trimble) remarkable for their variability in thickness, which and the other in the extreme northwest corner which the dark silicate biotite is not prominent. Near con­ tacts with schist the granite is much finer grained, locally granite which intrude the Eolus have in many in few cases exceeds 10 feet, and for their lack of the quadrangle, at the forks of Lime Creek. somewhat gneissic in structure, and darker colored on account cases the form of nearly horizontal sheets whose of continuity. The occurrences in the region of The body in Chicago Basin intrudes the Eolus of the greater abundance of biotite; in the Molas body the roak is often coarsely porphyritic. Microscopically the rock actual relations are difficult to represent on the the Continental Divide and in the neighborhood granite, from which it is distinguished with diffi­ is found to be a very typical biotite-granite with the usual map. For this reason the contacts in the south­ of Elk Park are the most noteworthy. Only a culty on account of the extensive metamorphism granular texture. The essential minerals are the feldspars western portion have been somewhat generalized, few of the more important dikes have been shown which has affected both rocks. At several locali­ (orthoclase, microcline, and a little oligoclase), quartz, and biotite; the last-named mineral, though uniformly present, and the irregularities are greater than the map on the map. ties the porphyry contains inclusions of a quartzite becomes in some cases of almost secondary importance and shows. The rock is uniform in texture, except conglomerate or of quartzite pebbles held in a occurs as minute flakes sparingly disseminated through the near contacts with the Eolus, where a faint band­ Petrographic description. These rocks are of a rusty- rock. In addition to these principal constituents muscovite brown color on weathered surfaces, but much darker on cement of porphyry. The nearest point at which is occasionally found as an original mineral. The usual acces­ ing may be made out. At these places, also, a fresh fractures and in the finer-grained varieties suggest dia­ quartzites or conglomerates are exposed is more sories are rather lacking, magnetite and apatite being the porphyritic appearance is not uncommon, due to base or other equally basic rocks, the coarser ones being than 3 miles away and they are separated from the commonest, with a little titanite and zircon. lighter and with a pinkish tinge. The texture is variable, a slightly finer texture of the rock as a whole and ranging from aphanitic to coarsely granular. Many are por­ granite-porphyry by an unbroken mass of Eolus EOLUS GRANITE. the development of large phenocrysts of feldspar phyritic, the phenocrysts being few and of small size and in granite. From the nearly horizontal, sheet-like Occurrence. The most conspicuous granitic which are in many instances arranged with their nearly every case composed of the dark silicates. In the more coarsely granular varieties a lath-like development of the character of the granite intrusion to the north, intrusive of the Needle Mountains is the Eolus greater diameters parallel to the contacts. feldspars may be noted and flakes of lustrous black biotite described elsewhere, it seems not unlikely that 9 the basal conglomerates of the Uncompahgre for­ The San Juan tuff is a well-stratified formation appearing as a black band in the pink tuff of several group and the deformation of the Uncompahgre mation may be buried beneath the granite at in nearly all localities. Its beds vary in texture, cliff exposures. strata, are among the most interesting and striking this point and that the granite-porphyry, in forc­ but are predominantly rather fine grained, with Andesite flows. As represented by the map, the features of the Needle Mountains region. The ing its way upward into the Eolus granite, also few fragments exceeding 6 inches in diameter, ridge opposite the head of Bear Creek exhibits an present distribution and structure of the Needle penetrated these conglomerates and carried up though coarse breccia or agglomerate is locally andesite flow in the Potosi series. While this is Mountains group were largely determined by two fragments of them as inclusions in its mass. present. Fragments are rounded or subangular unusual, a similar thin flow was observed between or more periods of great deformation which occurred The granite-porphyry at the forks of Lime and the matrix is a fine gravel or sand. The quartz-latite masses in the Telluride quadrangle. before the deposition of the earliest Paleozoic sedi­ Creek intrudes the Herrnosa formation and is stratification is mainly due to water and in some As far as known, the andesite flows of the Potosi mentary rocks and by a later faulting which fol­ therefore Carboniferous or younger. The form of places there may have been lakes, but much of the epoch of eruption are uniformly local and of rela­ lowed, in the main, old lines of weakness. The this body is that of an irregular laccolith, whose arrangement is more plausibly assigned to fluviatile tively small importance. The one under discus­ broader problems touching the character and boundaries in many places cut across the bedding agencies. sion is clearly of quite local development. The extent of the forces which have combined to planes of the sediments. Near its borders the The prevailing color of the tuff is dark gray or andesite is a very dark aphanitic rock, with but bring about the present structure can not be limestones have been hardened or converted into purplish, with greenish tints strongly developed in few minute crystals discernible to the naked eye. fully discussed until the less complicated east­ crystalline marble impregnated with pyrite or places, as on Deep Creek in the Silverton quad­ Under the microscope the rock is found to be an ern extension of the Needle Mountains forms traversed by a network of fine quartz veins and rangle. It is in few cases highly indurated and in augite-andesite of predominant microlitic ground- beyond the Vallecito has been examined. druses filled with crystals of fluorite. consequence the formation causes gently rounded mass, with much magnetite in very minute grains, slopes as a rule. causing the dark color. IRVING GREENSTONE. Granite-porphyry dikes. The granite-porphyry dikes are The fragments making up the greater part of Within the Irving greenstone practically nothing from 5 to 20 feet in thickness, but show almost no variation INTRUSIVE RHYOLITE. in texture in different parts of the body, though different the tuffs are chiefly andesite, hornblendic and can be made out beyond the fact that the rocks dikes may be more or less strikingly porphyritic. The pre­ augitic types prevailing. Granite, schist, and An intrusive rock which clearly belongs to the have suffered much from orogenic disturbances. vailing color is pink, a few being nearly white, while others are a clear brick-red. As a rule the texture is fine grained other rocks may be very prominent in the lowest Tertiary volcanics of the San Juan is represented The great preponderance of igneous rocks of essen­ and not strongly porphyritic to the unaided eye. The phe- horizons at some localities, but only volcanic rocks in the extreme northeast corner of the quadrangle tially the same composition and appearance does nocrysts are not numerous and are small, few of them appear­ ing larger than 3 mm. in diameter. The groundmass is appear in the greater part of the formation. by a stock-like body of rhyolite-porphyry, cutting not favor the solution of the grander structures that megascopically felsitic and very dense or almost aphanitic, Erosion removed most of the San Juan tuff the Potosi lavas and underlying schists. The mass may have resulted from these movements, and the with here and there a sprinkling of specks of biotite. The adjacent to the Needle Mountains before or after has irregular boundaries which can not be traced effects of dynamic metamorphism are the only microscope shows that the phenocrysts are orthoclase or inicrocline and quartz and that the groundmass is very finely the eruptions of the Silverton volcanic epoch, and accurately in the forest on the lower slopes, but it recognizable records of the forces which first granular, in some cases micropegmatitic, and is composed of recent erosion has so nearly completed the destruc­ surely does not extend as far as the bed of Bear acted upon these rocks. The presence of pebbles feldspar and quartz, a few flakes of brown biotite, and a little magnetite, the feldspars being clouded with a fine dust, prob­ tion that only a few patches now appear. The Creek and probably the larger part of the body is of greenstone-schist in the Vallecito conglomerate ably hematite, to which the color of the rock is due. Most of map shows the tuff to be present on the ridge within the quadrangle. is conclusive evidence that deformation occurred the rocks are remarkably fresh, the only secondary minerals north of Elk Creek and on or near the Conti­ The prevalent habit assumed by the rock is that prior to the beginning of the Needle Mountains developed being a very little muscovite and iron oxides derived from the biotite and magnetite. nental Divide. At several points it has protected of a strongly marked gray porphyry, with many deposition, but it is practically impossible now to Granite-porphyry in larger masses. The porphyry of the the Telluride conglomerate from complete removal. distinct crystals of sanidine, quartz, and biotite in distinguish between the results of these movements Chicago Basin and Lime Creek bodies differs from that of the dikes more in hand specimens than in thin sections. This is The uneven character of the surface at the begin­ a very fine-grained cryptocrystalline groundmass. and of those due to later deformation, by which the due to a slightly different texture of the groundmass and ning of the San Juan epoch is well shown by the The crystals vary in size, some of the sanidines Vallecito conglomerate was also affected. very largely to the greater alteration to which the larger basal line of the tuff as it runs in irregular course being an inch in diameter, but the average is much Irving-Vallecito contact. The structural rela­ masses have been subjected. The rock of the Lime Creek laccolith is of a light-yellow or cream color with a faint pink­ over or around hills and ridges. smaller. tions between the Irving greenstone and the Val­ ish tinge. Its groundmass is of an extremely fine texture and Near the contacts, especially in apophyses at the lecito conglomerate are best shown on the broad largely feldspathic; it contains hardly distinguishable phe­ POTOSI VOLCANIC SERIES. nocrysts of quartz and somewhat larger ones of an altered southwest extremity, the rock is a very nearly ridge or mesa which lies just east of the quadran­ feldspar. Biotite is not at all abundant and is invariably Relations and distribution. In the northeast cor­ white felsitic porphyry with small phenocrysts gle and separates Vallecito Creek from Lake Fork decomposed. The groundmass far exceeds the phenocrysts ner of the quadrangle, above the San Juan tuff, and predominant groundmass. The border zone, a of Pine River. In a general way, the Valle­ in amount. The Chicago Basin porphyry and that of the dike to the southwest have larger and more numerous phe­ occur remnants of certain other tuffs and lava few inches wide in places, is vitrophyre, the glass cito conglomerate rests as a thin remnant on the nocrysts, but less in amount, than the groundmass. Most of flows belonging to the Potosi volcanic series. The base being dark gray, reddish, or yellowish, and at Irving greenstone, with a gentle dip to the south­ the specimens indicate that the rock has been subjected to stresses which have not been of sufficient force to cause schis- dominant rocks of the succession are pinkish many points exhibiting perlitic sundering. A flu- east, but from place to place sharp and abrupt tosity, but which have produced an almost microscopic sys­ quartz-latite and pink or gray banded rhyolite. idal texture is developed here and there in the glassy changes in dip occur, and the contact surface, tem of joints that give the rock a texture on weathered sur­ The rocks of this series have been fully described or felsitic groundmass of the border zone. which follows these changes, shows that the two faces much like that of the Eolus granite. Near contacts the porphyry becomes very dense, almost without phenocrysts, in the Telluride and Silverton folios. They con­ This rock is clearly a rhyolite, of unknown rela­ formations have been intricately infolded. This and well banded. The whole mass seems to have suffered stitute one of the principal divisions thus far tions to the Potosi lavas of this character. Other folding was undoubtedly accompanied by differ­ alteration and to have been sparingly but quite generally impregnated with pyrite. recognized in the volcanic section of the San crosscutting bodies of the same rock occur on the ential movements of one formation upon the other, A microscopical examination of rocks from the two local­ Juan region, coming in time after a succession ridge southwest of Bear Creek, and there is a dike for many of the conglomerates have a certain schis- ities shows that they are essentially of the same mineralogical of flows and tuffs of andesite and of dark latite, of similar porphyry in the Silverton quadrangle tosity, shown by elongated pebbles and laminated composition and hardly to be distinguished from the rock of the granite-porphyry dikes. The main difference seems to together known as the Silverton volcanic series. between Cunningham Gulch and Deer Park. matrix, while the greenstones, even for some dis­ be in the groundmass, which in the case of the dike rock is The latter rocks possess a thickness of several These dikes are all characterized by a large num­ tance away from the contact, have been fractured, finely but distinctly granular, the individual grains being thousand feet, all told, in the central part of the ber of sharply formed quartz phenocrysts, while brecciated, and sheared in a manner that in places sharply bounded as in a granite. The rocks of the larger bodies, on the other hand, have groundmasses which are Silverton quadrangle. Erosion' preceding the erup­ the rhyolitic lavas of the Silverton and Potosi vol­ strongly suggests flow breccia or tuffs. essentially felsitic, with interlocking grains and micrographic tion of the Potosi lavas entirely removed the Sil­ canic series in few cases exhibit quartz crystals. or micropoikilitic intergrowths of quartz with the feldspar. NEEDLE MOUNTAINS GROUP. None of the phenocrysts of feldspar, which are more numer­ verton rocks and much of the San Juan tuff from STRUCTURE. ous than those of quartz, are fresh, but their appearance the northern slopes of the Needle Mountains, so Unlike the .Irving formation, the Needle Moun­ indicates that a number should be considered as microperth- that the lowest Potosi lavas rest on the schists in GENERAL STATEMENT. tains group consists of well-stratified sedimentary ite, a few are of an acid plagioclase, and the rest orthoclase. Magnetite and zircon are the usual accessory minerals, while some places, as in the head of Haynes Fork, in The structural features of the Needle Mountains rocks, mainly quartzites with some argillaceous secondary muscovite, kaolin, and oxides of iron now make up the Needle Mountains quadrangle. The Silverton are due to several periods of deformation. The layers; this clearly defined stratification permits a large part of the rock. Although grouped with the granite- porphyries, these rocks are to be considered more strictly as volcanic series is represented by a few hundred feet greatest complexity is to be found in the pre- the complicated folds to be worked out in a fairly granite:felsophyres, approaching rhyolite-porphyry in struc­ of lavas and tuffs in Sheep Mountain, only 4 miles Cambrian rocks, which were folded, dislocated, satisfactory manner. The total lack of fossils and ture and composition. north of Beartown. and compressed more than once before the begin­ the absence of horizons possessing peculiar litho- TERTIARY VOLCANIC ROCKS. The remnants of Potosi lavas occurring in the ning of Paleozoic history. Later movements also logic features have been serious disadvantages at a Needle Mountains quadrangle belong, of course, to affected the pre-Cambrian terranes, but their best number of critical points and have prevented an The Needle Mountains are situated directly on the lower part of the complex, the principal area expression is to be found in the structure of the altogether satisfactory solution of the structural the southern border of the large area of volcanic being that on the ridge northwest of Bear Creek. Paleozoic rocks. problemsj The presence of argillaceous beds has rocks which are the most prominent element in the The rocks of this locality dip northeasterly down It will be seen from the distribution of the Pale­ been of great assistance in unraveling obscure San Juan Mountains. Apparently the high peaks this ridge to its end opposite the junction of Bear ozoic sediments on the map and from» the struc­ structural relations, but it has not been possible of granite or quartzite rose above the levels of vol­ Creek with the Rio Grande. Below this point ture sections (A-A and C-C) that they dip away actually to correlate the different horizons in dis­ canic accumulation to the north during a large the Potosi lavas become one of the most important from the central portion of the Needle Mountains tant localities, largely because of the variability of part, at least, of the volcanic cycle. It is not formations of the Rio Grande Valley. They also in all directions. There can be little doubt that their lithologic character from place to place, due known whether the summits of the Needle Moun­ approach the Needle Mountains from the east and the different formations of which these are the to differences in dynamic metamorphism and the tains area were ever entirely covered with volcanic the extreme western tongues now remaining are remnants were once connected and, together with influence that the granite intrusions have had rocks, but lavas and tuffs of several epochs cer­ shown by the map, north of Mount Nebo. thousands of feet of younger terranes, covered the upon them. In order to represent the structural tainly extended far up the northern slopes and Quartz-latite and rhyolite. The Potosi series pre-Cambrian rocks that now form the highest relations more fully, the quartzite and argillaceous two members of the complex are still represented consists principally of grayish or pinkish lavas summits of the Needles. At some period, prob­ members of the Uncompahgre formation have been by remnants which have been mapped. These characterized by glassy sanidine or plagioclase ably at the close of the Cretaceous, this region was differentiated on the map, a study of which, in con­ rocks are much more important in the Silverton crystals and glistening biotite leaves in a pre­ locally affected by earth movements, which resulted nection with the cross sections, will give a fair idea quadrangle and have received such full treatment dominant felsitic groundmass. At many points in a quaquaversal folding or doming of the strata of the general attitude of the beds, which is that in that folio that the present description will be the rocks have a wavy banding or flow texture. about a center not far from the present Pigeon of a broad synclinal flexure of the earth's crust brief. It has been found that the most common lavas Peak. The attitude of the earlier Paleozoic sedi­ involving lesser plications in other words, a syn- are quartz-latite, while true rhyolite is also pres­ ments, as well as that of younger formations, pre­ clinoriuin, with an axis describing a semicircle SAN JUAN TUFF. ent. In Sheep Mountain the summit is made up served beyond the boundaries of this quadrangle, about a center not far from Overlook Point. The The earliest member of the San Juan volcanic of rhyolite flows, succeeding several hundred feet is to be attributed to this domal uplift, and is inde­ synclinorium is much compressed and in part hid­ succession at present known is a tuff or agglomer­ of quartz-latite. pendent of the broader uplift of the whole San Juan den by overlying Paleozoic strata to the west and ate composed almost exclusively of andesitic mate­ The latter rocks are the principal element in the region. its minor folds are obscured by faults. To the east rials, which was called the San Juan tuff in the ridge adjacent to Bear Creek, the lowest portion it becomes more open and a series of southeasterly Telluride folio. It is a widespread formation all being of tuffaceous character. North of Mount ALGONKIAN ROCKS. pitching anticlines and synclines are to be made through the western San Juan region, reaching its Nebo there is some black, glassy, porphyritic The structures presented by the rocks of the out. Section B-B, which passes through White maximum observed thickness of nearly 2500 feet quartz-latite belonging to a thin flow in the tuff. Algonkian system, including the relations between Dome in a direction normal to the axis of the syn­ adjacent to Canyon Creek, near Ouray. This material has some extension to the northeast, the Irving formation and the Needle Mountains clinorium, represents the general attitude of the Needle Mountains. 10 beds in the Vallecito region. This same section where not Overlain by Paleozoic strata, the Valle­ still the change from southwesterly to southeasterly magmas in dike form took place; these in turn is shown in fig. 5 on the illustration sheet. Just cito conglomerate being nowhere observed in con­ dips along the southern border of the quadrangle yielded to dynamic metamorphism and became south of the zone of faulting that characterizes the tact with the younger Algonkian formation. A and the complementary northerly and northwest­ finely laminated amphibolites. Before the close northern boundary of the Uncompahgre is the zone of shearing extends along the whole northern erly inclination of the strata to the north, illustrated of this period the Twilight gneissose granite may open anticline of White Dome, slightly over­ border of the Uncompahgre, two or more prom­ in sections A-A and C-C, are sufficient to bring have been intruded also, as portions of it, at least, turned to the south; a syncline, the trough of inent faults being recognized wherever exposed. out the quaquaversal relations. appear to have suffered strains which led to a which is occupied by slates, flanks it on the north, Between these faults the quartzites are crushed Both in the Canyon Creek drainage and in the development of schistose or gneissic textures. and to the south for a distance of more than a and brecciated and traversed by many minor vicinity of Lime Creek the sedimentary rocks have After these movements, which probably resulted mile the strata are crumpled and broken by a fractures, and the bedding, when it can be dis­ been dislocated by faults or locally folded. In in mountain building, came a long period of ero­ number of faults, one of which is shown in the tinguished, is found in general to be nearly ver­ the Lime Creek area the more complicated struc­ sion accompanied by deposition. Of the resulting section. From this zone of crumpling, which ends tical; in the Animas Canyon opposite Whitehead ture, involving the Hermosa, lies mainly in the rocks almost none now remain, and it is only from at the west fork of the Vallecito with a closely Gulch it is slightly overturned and dips to the Engineer Mountain quadrangle, but considerable indirect evidence that it is possible to arrive at any compressed overturned anticline, southward to the north. The region of greatest crushing extends deformation, independent of the broader structures conclusions regarding the former extent and thick­ Eolus granite, the structure is simple and the folds westward from Bear Creek for about 4 miles. of the Needle Mountains dome, occurs between the ness of the deposits. From the abundance of are open. In the broad syncline of which Grena­ West of the Animas the contact with the Archean Algonkian and the Lime Creek laccolith to the quartzite bowlders in the conglomerates of the next dier Range forms the southern limb the shale mem­ is covered by Paleozoic rocks. north. The deformation has resulted in open succeeding formation, the Vallecito, it is reasonable bers are conspicuously traceable, and in the center The southern contact is of a different character folds with east-west axes, further complicated by a to assume that strata of a considerable thickness of of the flat anticline to the south the quartzites are and for the most part is in the nature of a thrust number of minor faults. such rocks must once have existed and that from cut through to the underlying schists and intrusive fault. This is best shown south of Snowdon Peak, The principal fault of the region is that lying their destruction the materials of the Vallecito granite. illustrated in section A-A, where the mountain between the Algonkian and Paleozoic rocks and conglomerate were in part supplied. Before this East of Vallecito Creek, from Bear Creek to a crest is capped by a thin remnant of quartzite, part extending from the edge of the Animas Can­ second great period of erosion began, these rocks, point just south of Mount Nebo, lies a region of a mass which was overthrust upon the schist. yon westward for more than 5 miles. The Her­ which may be referred to as the sedimentary mem­ of considerable structural complexity. The folds To the east the same relations may be recognized, mosa is more or less sharply upturned against the bers of the Irving formation, were intruded by are more closely compressed, some are overturned, especially south of Balsam Lake, where a small north side of this fault throughout its entire length, basic igneous rocks, in the form both of dikes and and local divergence of the axes of the minor overthrust mass of quartzite is entirely isolated while at two points, one of which is near Lime of larger and more irregular bodies. As a result folds from the general direction of the axis of the from the main area, but for a considerable dis­ Creek and the other midway between Lime Creek of these intrusions and of the movements which synclinorium is not uncommon. Directly north of tance along the southern limit the Uncompahgre and the Animas Canyon, remnants of the Elbert probably accompanied them, the sedimentary rocks Mount Nebo is an unusually large a,rea of slates in is intruded by the Eolus granite. It is probable and Ouray sediments occur on the south side of were compressed and indurated and were changed which practically no structure can be.made out, on that the intrusion followed closely the thrust plane the fault, resting on the rocks of the Uncompahgre from sandstones to quartzites. Masses of them account of the crumpling and fracturing that these between the Algonkian and the Archean, which formation. were caught up and held as inclusions in the soft rocks have undergone incident to their defor­ everywhere is marked by a zone of brecciation and In addition to a few small faults which affect the larger bodies of igneous rock, and the few occur­ mation. South of Mount Nebo simple structures evident weakness. These relations are represented continuity of their boundaries, the sediments of the rences of massive quartzite in the Irving green­ prevail, which correspond to those south of the west in section B-B. The small patch of Uncompahgre southwest section are traversed in a southeast- stone that may be seen to-day are probably of this fork of Vallecito Creek along section B-B. quartzite which rests upon the Archean and is northwest direction by a fault nearly 6 miles character. To the west the folds of the Vallecito and Bear intruded by arms of the Eolus granite on the long which has dropped the strata to the south­ After the period of intrusions had ended, earth Creek sections become more compressed and axial southern slope of Mountain View Crest, 5 miles west 500 or 600 feet, On both sides of Stag Mesa movements took place, accompanied by compres­ faults are developed by which many of the nearly from the main mass of the quartzite, is possibly an and southwest of Lime Mesa the location of this sion and possibly folding that led to a metamor­ vertical beds are cut out. There is also an east- indication of the extent to which the quartzites fault is clearly shown in the field by bold escarp­ phism of the igneous rocks into greenstones west system of faults oblique to the axial faults, by were thrust over the Archean. ments of the sediments and in Canyon Creek by and in extreme cases into schists. These move­ means of which many more beds disappear to the The absence of the Vallecito conglomerate at the an abrupt cliff of granite. The position of the ments may have affected the contact between the west and further complexity is added to the com­ base of the Uncompahgre, where it would natur­ fault is further emphasized by several streams Irving greenstone and the Archean, but of this pressed flexures. Most of these faults converge ally be expected, is difficult to explain. It is prob­ which have taken advantage of this line of weak­ nothing is known. They did, however, elevate to a narrow zone which crosses Animas River at able that it shared with the Uncompahgre in the ness to carve deep ravines. the region sufficiently to bring about its wide­ the lower end of Elk Park. To the north the deformation which initiated the synclinal structure spread degradation. The detritus resulting from strata still preserve some of the simpler structures of the region, and that later movements accom­ HISTORICAL GEOLOGY. this erosion, both quartzite and greenstone, as well that are common to the east. panied by faulting sheared out the conglomerate as waste from the Archean areas, supplied the GENERAL STATEMENT. In the Animas section the strata at the northern near the present margins and at the same time materials out of which the Vallecito and Uncom­ boundary of the Uncompahgre are in fault contact thrust the quartzites over the Archean. It is also A connected history of the events which have pahgre formations of the Needle Mountains group with the Archean schists, massive quartzites, over­ believed that the great fault which separates the combined to bring about the present geologic and were built up. At first the Vallecito conglomerate turned and dipping steeply to the north, resting Eolus granite from the Irving greenstone may have physiographic features of the Needle Mountains was laid down, but as erosion became less intense against the fault. The dips soon become vertical developed as a fracture at this time and directed may be traced from the evidence which has finer materials were deposited, and for a very long and then southerly until the beds are terminated the intrusion of the granite. Later movements been set forth in the descriptions of the various period sedimentation continued with only slight by a fault, on the other side of which are slates. followed along this general line oft weakness and rocks. It involves the geologic history of the oscillations of the land surface and sea bottom, These slates, at first sharply upturned, quickly for some distance determined the contact between whole San Juan region, and, though a record of that brought about an alternation of sands and assume dips of lower angles, and then for a dis­ the .granite and the Uncompahgre. nearly all its stages is to be found within the clays. After a considerably greater thickness than tance of half a mile or more are closely folded present quadrangle, some reference to observa­ now remains had been laid down, orogenic move­ and plicated with associated thin-bedded quartzites, PALEOZOIC ROCKS. tions in neighboring districts is necessary for a ments took place which resulted in a complicated and are eventually carried beneath the valley floor All the structures so far considered were devel­ complete understanding of the sequence of events. folding and faulting of these beds over a wide by the prevailing southerly dips. Beyond this oped before the beginning of Paleozoic sedimenta­ Since crystalline and early sedimentary rocks area, those which are now preserved in the Needle point a simple southerly dip continues, becoming tion, with the exception of the later movements are the most conspicuous ones in the Needle Mountains probably representing but a very small somewhat steeper, to the mouth of. Elk Creek. along the great fault between the Eolus granite Mountains, greater prominence is given to them part of the whole formation. The present struc­ The continuity of the beds is broken at several and the Irving greenstone. Here a small patch of here than to the younger terranes. For a more ture of the Needle Mountains group in the Needle places by minor faults, but being entirely within Ignacio quartzite resting on the Irving north of detailed account of the later sedimentary and vol­ Mountains region is that of a synclinorium, which the quartzites the faults can not be traced far nor Sheep Draw is faulted against the granite. In the canic history of the San Juan the reader is referred was probably connected by a corresponding anti­ represented on the map. Opposite the mouth of northwestern portion of the quadrangle the Paleo­ to the Telluride and Silverton folios. clinal structure with the same rocks in the canyon Elk Creek is the trough of a close syncline, the zoic rocks have been dislocated at a number of of Uncompahgre River, subsequent erosion having thin-bedded quartzites being much crumpled for places by faults which appear to be along prede­ PRE-CAMBRIAN HISTORY. removed the intervening beds. The structural a distance of several hundred feet until they grad­ termined lines of weakness or fracture in the The earliest rocks of which there is any record connections between the members of the Needle ually assume a high dip to the north. From this Uncompahgre. The probable progressive char­ are the crystalline schists and gneisses that are Mountains group found at Rico and those of the point to the southern limit of the Uncompahgre acter of this faulting is well shown in the region conspicuously exposed in the canyon of the Ani­ Needles are not known; there is nothing at either the quartzites and slates appear to have been very just east of the quadrangle, above Lake Fork of mas and at the northern border of the quadrangle. point to indicate any similarity to the relations closely folded, but as this is within the zone of Pine River and not far from Irving Peak. Here So completely have they been metamorphosed that between the rocks of the Needles and those of the pronounced faulting practically nothing remains the Paleozoic sediments rest in one place on both with but few exceptions it is practically impossible Uncompahgre Canyon. The beds are probably to indicate the initial character of the plications. the Vallecito conglomerate and the Irving green­ to state with any certainty what their origin has continuous and deeply buried by Paleozoic or Three principal faults are represented on the stone, which are sharply separated from each other been. Some are clearly igneous, such as the later sedimentary rocks. During the time that map, but it is probable that more actually occur by a fault, of unknown but probably great dis­ schists occurring near Rockwood in the Durango the forces which produced these structures were and that all the rocks are traversed by numerous placement. Within a distance of 50 feet another and Engineer Mountain quadrangles, in which a acting, much faulting and shearing took place, slipping planes and are very generally crushed. fault, parallel to the first, dislocates the Paleozoic porphyritic texture is apparent. Others, from the not only within the Needle Mountains group The beds are nearly all in a vertical position, as strata a small but definite amount. There can be abundance of their ferromagnesian silicates, would itself, but also along the contacts between the are those which rest against the fault that separates little doubt that this second fault is due entirely to seem to be of igneous origin also. Many more, Algonkian and the Archean rocks; in addition, them from the Archean on the south. later movements along the earlier and well-defined however, give no clew, either from their texture a complicated infolding occurred between the The extremes of compression and shearing are line of weakness. or composition, as to whether they were derived Irving greenstone and the Vallecito conglomerate. found in the section exposed on Lime Creek, where The quaquaversal structure of the Paleozoic from igneous or sedimentary rocks. In the At the close of the movements that deformed the beds are nearly vertical and are traversed by rocks has already been mentioned. The preser­ absence of more definite information they are the Algonkian, intrusions of granular rocks, many faults against which one horizon after another vation of the tilted floor upon which they rest, grouped with similar rocks in other parts of largely in the Archean, took place. The great­ is sheared out. Section A-A of the structure-section even where the sediments themselves have been Colorado which have been regarded as of Archean est of these bodies, the Eolus granite, cuts not sheet passes through a point intermediate between removed, helps to make this structure easily recog­ age. They have from time to time been involved only the Archean but the Needle Mountains rocks, the Lime Creek and Animas localities and com­ nized both in the field and when represented on in movements of the earth's crust, and the accom­ and dikes of it are numerous in the Irving green­ bines some of the features of both. The peculiar the map. Although to comprehend properly the panying stresses have nearly obliterated primary stones. Other granite bodies, which appear only relations at Snowdon Peak are discussed below. relation of the encircling sediments to the exposed structural relations and have produced a schistose as intrusive in the Archean, are also believed to As shown by the map the Uncompahgre strata pre-Cambrian core of tiie dome, areas to the east texture in all the rocks. After schistosity had belong to this general period of eruption, mainly are bounded by granite or the Archean schists, and west of the quadrangle should be considered, been induced to some extent, intrusions of basic because they show no evidence of having been 11 subjected to severe dynamic metamorphism, which stormy period, the Molas formation, were of no between the conglomerate and the San Juan tuff, attacked the elevated central region and, in con­ they must have endured had they existed at the great thickness and consisted of muds and rolled and continuous accumulation appears to have taken nection with the Vallecito and Lime Creek sys­ time of the deformation of the Uncompahgre. and rounded fragments of the older beds. The place. These conditions may well have existed at tems, developed an extremely rugged topography. Although granites are the most numerous of the Ouray is entirely absent from its appropriate place the same time and the erosion may be accounted for The less active Florida drainage succeeded in granular intrusives, more basic rocks have been in the sedimentary section exposed in Silver Creek, by the capture of certain streams belonging to the removing only a part of the sedimentary cover found in the region. Of these the granodiorite near Rico, and it is possible that the greatest ero­ Telluride system by the headwaters of the eastern within its province and entrenched itself on the near the junction of Virginia Creek with Florida sion took place west of the San Juan, in what is drainage, causing sudden and active erosion, while old peneplain surface upon which the Paleozoic River is the only one occurring in the Needle now the plateau country. quiet deposition continued to the west. rested. Such tributaries of the Animas as Tank Mountains quadrangle; but a large mass of gab- After the Molas the quieter conditions of deep and Canyon creeks have accomplished still less, bro lies just beyond the western boundary, in the water returned and continued, with minor oscilla­ VOLCANIC ERUPTIONS. since they have left Lime and Stag mesas capped southeast corner of the Engineer Mountain quad­ tions, for a very long time, during which the sands, Toward the close of this period of sedimenta­ by sedimentary rocks and have only slightly dis­ rangle. clays, and limestones of the Hermosa were depos­ tion, but after the rugged topography to the east sected the Paleozoic floor where it is exposed ited to a depth of 2000 feet or more. had been developed, volcanic eruptions began and (figs. 1 and 2). PENEPLANATION AND PALEOZOIC SEDIMENTATION. Slight changes in the conditions of sedimenta­ continued with explosive violence for some time. After the intrusions of igneous rock, which may tion began as the Rico formation was laid down and Gradually the more active eruptions gave away GLACIATION. be regarded as occurring either at the close of pre- became more and more important as time went on. to quieter outpourings of lava, accompanied by In all its broader features the present drainage Cambrian time or at the beginning of the Paleo­ The Rico marks the beginning of a long period occasional explosive outbursts similar to the first, system of the region had been developed before zoic, great orogenic movements took place which during which limestones were not prominent at but of less magnitude. In this way the vast climatic conditions changed and glaciers took up resulted in the uplift, erosion, and gradual pene- first and later gave way almost entirely to the red deposits of the San Juan tuff and breccia were the work of rivers. Geologists who are familiar planation of an area of continental proportions. sandstones and shales of the Cutler formation. formed, and were covered by the lavas and tuffs with the Pleistocene history of North America In the beveling process all the rocks of the Needle The Rico is the last formation of which the com­ of the Silverton and Potosi volcanic series. The are of opinion that the glaciation which may now Mountains region were exposed, in one place or plete history is shown in the Needle Mountains record of these events is far from complete within be recognized in the mountains of Colorado belongs another, at the surface of a nearly level plain and quadrangle, but sedimentation continued until the the boundaries of the Needle Mountains quad­ to the last main stage of the Glacial epoch the in positions suggestive of the vicissitudes through close of the Cutler. rangle, and for a more detailed history of this Wisconsin. In other Rockv*j Mountain regions,o 7 which they had passed. period reference should be made to the Silverton especially in the Wasatch and Uinta mountains At the close of the long period of base-leveling POST-PALEOZOIC MOVEMENTS. folio. of Utah, traces have been found of earlier stages, the land areas subsided and were covered by ocean Just what happened in the interval between While the greater part of the material involved and it is thought that an interval of great ero­ waters. The subsidence, however, was not general, Paleozoic and Mesozoic time is not apparent in the in these volcanic activities reached the surface sion preceded the Wisconsin. If this is so, it for certain parts of the land were uplifted, and it sedimentary records preserved in the southern and either as ash or lava, some was intruded in the is possible that the extensive denudation which was from their erosion that the materials were sup­ western parts of the San Juan Mountains, but to form of dikes or laccoliths into the sedimentary preceded the known glaciation of the San Juan plied for the deposition, in the neighboring shal­ the north evidence has been found that deforma­ formations and even into the older volcanic rocks. Mountains may have been contemporaneous with low seas, of the first of the Paleozoic formations, tion and elevation occurred and that the Cutler It is probable that the Lime Creek laccolith belongs or followed earlier stages of glaciation, but of the Ignacio. One of these uplands doubtless and part, at least, of the Hermosa were eroded to this period, while the intrusive rhyolite of Bear this no distinct evidence has been observed in the existed to the north of the present Needle Moun­ before subsidence permitted the beginning of Mes­ Creek forced its way into the tuffs and flows at a region. tains area, as shown by the shore-line character of ozoic sedimentation. Breaks are recognized in the late stage in the volcanic history. So far as the Needle Mountains are concerned the basal conglomerates in the vicinity of Molas sedimentary column at various stages in the Meso­ Throughout this period of vulcanism active the glaciation which can be observed to-day may Lake; there must have been another to the east, zoic, but none of these concerns the present history. erosion continued, wearing away the recently be regarded as having effected merely a modifica­ for bowlder beds are found underlying the finer- It is sufficient to state here that in all probability erupted tuffs in places, only to have the valleys tion of a youthful or at most a submature topog­ grained Ignacio in hollows in the granite floor the whole sedimentary section of the San Juan refilled by fresh ejecta. At the close of the erup­ raphy. It has recorded itself mainly in a slight near Rockwood, in the Durango quadrangle, to covered what is to-day the site of the Needle tions an enormous amount of material had been rounding of the cross sections of the valleys, in the southwest. The land area to the north may Mountains to a depth of 12,000 feet or more. thrown out and deposited over a very large area, the development of cirques at the heads of many well have represented the partially dissected anti­ and a much greater thickness existed then than of the streams, and in the laying down of morainal clinal connection between the younger Algonkian POST-CRETACEOUS UPLIFT AND EROSION. remains to-day. It is impossible to say how thick gravels upon the hillsides and valley floors. Prob­ rocks of the Needle Mountains and those of the At the close of this long period of sedimenta­ the deposits were, or to what extent they buried ably the most interesting features are due to the Uncompahgre Canyon. The lithologic character tion widespread orogenic movements began which the mountainous region of the Needles. That glaciers near the heads of the smaller streams, of the Ignacio formation indicates that tranquil, affected the whole Rocky Mountain region. The they covered the mountain slopes on the north which continued to exist for some time after the shallow-water conditions prevailed which favored effects have been recognized in many localities far to some depth is shown by the remnants of San disappearance of the ice from the main valleys. the deposition of sands and rarely clays. distant from each other, and the resulting general Juan tuff and later lavas occurring north of Elk Slopes and streams with northern exposure col­ No records have been found in this region to elevation of the land is commonly referred to as Creek and in the vicinity of the Continental Divide. lected and retained more snow and ice than the indicate that rocks of periods intermediate between the post-Laramie or post-Cretaceous uplift. In the south-facing hillsides opposite them, and glaciers the Ignacio (middle or upper Cambrian) and the San Juan these movements,manifested themselves DEVELOPMENT OF THE PRESENT PHYSIOGRAPHIC in such localities remained, probably, long after Elbert (late Devonian) were deposited, yet where in a broad doming or quaquaversal folding of all FEATURES. the main ice stream had vanished. The familiar the two formations are in contact no evidence of the sedimentary rocks, accompanied in places by After the last great eruptions had died away, process of headward flattening of valley floors, unconformity has been observed, and it is difficult faulting and closer folding. The erosion incident active erosion began its task of removing the initiated during the period of maximum glaciation, to conceive that land conditions existed during the to this uplift in time reduced wide areas to low accumulations of volcanic debris and, in its early continued for a long time in these protected basins, time that intervened between the deposition of the relief, but probably during all this period that stages, initiated in general the existing drainage so that, when the ice finally disappeared, it left Ignacio and that of the Elbert; nor does it seem region which now centers about the Needle Moun­ systems. In the Needle Mountains province two not simple lateral valleys with grades that increased likely that great subsidence of the sea bottom took tains continued to rise slowly as a local dome, and, conditions appear to have prevailed in different gradually upward, but short, broad valleys which place. The presence of Silurian beds elsewhere in on account of the more active erosion, older and parts. On the southern slopes of the domal had low grades disproportionate to their length Colorado, as in the Elk Mountains, may mean that older formations were uncovered until the pre- uplift erosion had been progressing without seri­ and from which the streams descended with a the gap between the Ignacio and the Elbert is one Cambrian rocks appeared. ous interruption since early Telluride time, and marked increase in grade to the master streams. of deposition and subsequent erosion which locally it is improbable that in this region volcanic mate­ The valleys of Needle and Tenmile creeks have no left a plain so level that the Elbert seems conform­ TELLUBIDE CONGLOMERATE. rials accumulated sufficiently to influence the drain­ flood plains; in fact, with minor exceptions, the able with the Ignacio. After peneplanation a more rapid uplifting of age; it is even possible that they may never have streams flow on bed rock nearly all the way to the The seas in which the Elbert and Ouray forma­ the land of which the Needle Mountains are now appeared here, the southern slopes being protected torrential fans at their mouths, and their discord­ tions were deposited covered wider areas than those a part seems to have taken place. Many torren­ by the higher central summits from the ejecta of ant southern laterals can not be explained by the of Ignacio time and at the beginning were shallow tial rivers flowed from this region over nearly level the northern volcanoes. On the flanks of the dome theory of the greater erosive power of the main and undisturbed by strong currents. Land areas country to the northwest and possibly emptied into a simple consequent drainage developed which, ice stream, nor do they represent laterals "pro­ were of low relief and small islands may have been a shallow lake or sea. The waste that these streams after reaching maturity, was revived, possibly more tected" by residual glaciers and left "hung up" scattered about the seas, with many flats on which carried down was deposited as the Telluride con­ than once, by uplifts of the whole region, and by the rapid erosion of the main streams whose pools were left by the retreating tides. The evap­ glomerate. In the border land of the mountains eventually became superposed on the pre-Cam- glaciers had disappeared, for the main streams oration of the saline waters caused the formation it occupied gullies or the bottoms of small valleys; brian rocks. The northern slopes, on the other have accomplished but very little erosion since of salt crystals, which on the return of the tides farther away the materials were spread out over hand, had had their initial drainage obliterated Glacial time. The local basins must be attributed were covered by fine silts and subsequently dis­ the plains by numerous streams and became more in its early stages by volcanic outpourings, and solely to the continuance of headward ice quarry­ solved, leaving their casts in the sand. At the continuous, thicker, and of finer texture. The under this protection, frequently renewed, suf­ ing long after glacial action had ceased to be of close of the Elbert the seas gradually deepened and formation, consisting of fine conglomerate, sand­ fered comparatively little from the attack of regional importance, and they represent the only cleared. Corals appeared for a time and then gave stone, and shale, is a thousand feet thick in the streams until the close of the volcanic period. In marked topographic changes that the ice accom­ way to an abundant molluscan fauna when deep- Mount Wilson group in the Telluride and Rico the meantime streams from the south had pre­ plished. sea conditions prevailed and the limestones so quadrangles, but the occurrences in the Needle sumably carved deeply into the dome. One of The lateral cirques of Needle and Tenmile characteristic of the Ouray were deposited. Mountains quadrangle are in the form of coarsely them, the Animas, passed completely through and creeks are merely examples, possibly more perfect, After the Ouray epoch, which had continued conglomeratic valley fillings of torrential streams. obtained control of the drainage beginning to of many others that occur throughout the Needles, without a break from Devonian to Carboniferous There are good reasons for believing, as shown in develop on the volcanic rocks to the north, prob­ in addition to the amphitheaters at the heads of time, an uplift took place which raised the sedi­ the Silverton folio, that an interval of great erosion ably capturing a number of the heads of the youth­ nearly all of the larger streams (figs. 3 and 4). ments to the surface of the sea. This resulted, in followed the deposition of the Telluride conglom­ ful Rio Grande. Thus the northern flanks of The lakelets which are found in many of these the Needle Mountains region, in the destruction of erate. In the country lying east of the Animas the Needles came to be drained by a stream orig­ cirques fill rock basins that owe their origin the upper parts of the Ouray, laid down in early and Uncompahgre rivers there is evidence that the inally consequent on the southern slopes, the area directly to the ice. It is to the glaciers that occu­ Carboniferous time. The erosion, probably in a headwaters of a drainage system, corresponding in left to the control of the Rio Grande being com­ pied these cirques that the greater part of the large degree submarine and due to strong currents, magnitude and position to that of the Rio Grande paratively small. The upper tributaries of the morainal material found in the region is to be both tidal and from revived streams from the raised of to-day, attacked not only the newly deposited Animas, at first resting on the volcanics, soon cut attributed, especially that which still covers many lands, resulted in a churning of the sea bottom and Telluride conglomerate but also the older rocks, and through them and became superposed on the older of the higher hillsides. a redeposition of these materials with others derived developed a rugged topography. In the Telluride rocks which composed the former northern slopes On the final disappearance of the ice the streams from the land. The deposits resulting from this quadrangle, to the west, no break was recognized of the Needle Mountains dome. Other laterals resumed their activity, but up to the present time Needle Mountains. 12 have accomplished little more than the removal or Contemporaneous with these latest events were of the Needle Mountains, frost is undoubtedly the teristic of the regions of volcanic rocks to the redistribution of some of the morainal materials the landslides which took place in the region most active, on account of the high altitude of north, is comparatively insignificant. The Needle and minor erosion near the heads of the smaller of Paleozoic rocks in the southwestern section of the region, which causes the temperature to fall Mountains exhibit a striking instance of a youth­ laterals. Both of these processes have resulted in the quadrangle; these, together with the closely below the freezing point nearly every night in the ful topography retained long after surrounding the laying down of gravels as flood-plain deposits related rock streams, have been discussed in detail year. Owing to the compact nature of the older regions had reached early maturity, largely on on the floors of .the larger valleys and the forma­ at their appropriate place under the heading rocks which compose the higher summits and account of the greater hardness of the rocks. tion of detrital cones at the mouths of the smaller "Descriptive geology," on page 7. steeper cliffs of the Needles, remarkably little streams. Of the agents now at work in the degradation weathering has occurred and talus, so charac­ June, 1904. ECONOMIC GEOLOGY.

By J. D. Irviiig and W. H. Emmons.

The economic interest of the area included within one fissure, but fades out gradually beyond the briefly described above, they are zones of min­ The metamorphic rocks intersected by the veins the Needle Mountains quadrangle centers in the limits of the sheeted zone, to the barren and unal­ eralization formed along a closely spaced series occur along Needle Creek below Chicago Basin. gold and silver veins, which are found in con­ tered country rock. of fractures. Open spaces are of small extent They comprise very prominently banded gneisses, siderable numbers at several localities. The ore and mineralization has proceeded more by means showing in many cases a wavy lamination and deposits have not so far proved of great commer­ UNMODIFIED FISSURE VEINS. of replacement than by the filling of cavities. composed of biotite, quartz, and orthoclase, the cial importance, nor does the present development Unmodified fissure veins are the more important Such sheeted zones usually intersect others at latter usually much crushed and locally altered hold large promise for the future, but the mines of the two classes, as it is to them that the region various angles and in some places, as in the by dynamic stress into secondary minerals. The have produced some ore and the fact that the entire owes whatever production it has so far attained. Bluff and Waterfall mine, in the southern side other members of the metamorphic series are area has not yet been fully developed leaves a These fissure veins are in many instances charac­ of Needle Creek, the zones intersect at such close interbedded bands of hornblende-schist and bio- reasonable hope that further mining may lead to terized by a remarkably prominent banding and intervals as to form almost a stockwork vein, tite-mica-schist, apparently derived from the earlier important discoveries. show the feature termed "crustification," usually no single fissure extending for any appreciable basic eruptives and showing, so far as observed, no Mining development has been confined to two observed in typical fissure veins, to a greater degree distance without being lost in its intersection traces of sedimentary origin. areas within the quadrangle. One of these is of perfection than the writers have before observed with another vein trending at a widely different Relation of schistosity to fissures. The veins located in the northeast corner, in the country at any locality. These veins may best be described angle. These sheeted zones in rare cases exceed where they intersect the metamorphic rocks are immediately surrounding and to the southeast of by considering, first, the fracture into which the 5 feet in width and vary from that down to an in all cases independent of the lamination. They Beartown. To this may be given the name ore material has been introduced, and second, the extremely narrow veinlet which would not be cut across the schistosity most commonly at right " northeastern area." The other and more impor­ filling material. apparent but for the silicification which has pro­ angles, or less regularly at an angle only slightly tant of the two includes those deposits which occur The fissures. The fissures are usually short, few ceeded outward from the initial fracture. The athwart this structure. along Needle Creek from Needleton to Chicago of them exceeding 300 yards in length, and ter­ zones also have many branches, this feature being The laminated structure of the country rock Basin, a large number of ore bodies in the Chi­ minate at their extremities by a gradual pinching more common in the hanging wall than in the foot has, however, exercised a much greater effect on cago district, and others in Columbine Pass, Aztec out into the country rock. In some cases, as in wall. Where branches are numerous the original the material forming the vein, which in many Mountain, Bullion Mountain, East Silver Mesa, the Aztec mine, a fissure has been followed for a vein terminates within a very short distance beyond instances extends for some distance along the and Vallecito Basin. These may be termed col­ distance of 1500 feet and may be traced on the sur­ a branch and a new fissure takes its place, only to planes of schistosity from the original supplying lectively the "central area." face for as much as a mile. This is not, however, be itself lost in a new branch not far away. fissures, thus forming ragged and irregular hang­ Prospecting has been carried on with consider­ a very common occurrence, as the majority of frac­ These sheeted zones or lode fissures occur most ing and foot walls. able activity throughout the entire extent of the tures seem to have been more in the nature of commonly in the metamorphic gneisses and schists. The granites constitute the country rock hi quadrangle, and numerous isolated occurrences of lenticular spaces in the granite, without great In their strike they invariably cross the lamina­ which the fissures are more commonly found. mineralization have been noted, but none of them lateral or vertical extent. Little work has been tion of these rocks, in no instance showing a They are in most cases normal biotite-quartz- have thus far proved to be of sufficient importance done at any great depth, so that it is difficult to trend parallel to the prevailing schistosity. They orthoclase granites containing a little plagioclase to justify a detailed study. determine the downward limit of the fractures, but are much less numerous than the modified type first and considerable microcline. In some instances it seems very probable that they do not extend described. they are porphyritic and show evidences of dif­ CENTRAL AREA. far below the surface, because' of their limited Relationship of fissures. As in most regions ferent ages of intrusion; in others they are aplitic. The gold and silver veins which characterize the outcrop. The fissures in this region are probably where fissure veins are developed, the fissures As seen on the surface they are invariably red central area are of a very simple character, being merely the roots of larger fractures, the upper por­ where they intersect the surface trend somewhat in color, owing to the oxidation of a portion of the in all cases fissure veins, usually without complica­ tions of which have been worn away by the exten­ regularly in certain prevailing directions. To fis­ contained iron, but at some depth a uniform gray tions of structure. A few are extensively modified sive erosion to which the region has been subjected. sures which follow with but slight divergence tint is to be observed and may be said to charac­ by later distortion and mineralization, but these are In detail the fissures are extremely irregular, a single direction the term "fissure system" is terize the main type of wall rock. of secondary importance. The veins are in most and while they continue in general in one prevail­ usually applied. If more than one fissure system The veins in the granite have more regular walls cases very similar, both in detailed structure and in ing direction they bend from side to side at very is present in a given locality intersections of the than those in the metamorphic rocks, show less . the manner of occurrence, but for convenience of short intervals along their course, so that a direc­ several systems occur and form a network which is replacement, and are much more uniform in dip description they may be divided into two classes tion determined at any one point will rarely give in many cases rather complicated. and general direction. (1) unmodified fissure veins and (2) modified fissure an accurate idea of the trend of the vein. The In the central area above described two striking Where more than one variety of granite is inter­ veins. width of some of the fissures is upward of 10 feet, instances of such fissure systems occur and the sected by a single fissure the difference in the wall Unmodified fissure veins are those which have the open spaces which originally existed in the directions followed by them show so little vari­ rock has exerted no appreciable effect on either the been formed by the filling of open fissures in the vein being almost completely filled with ore and ation that it is often impossible to detect even character of the fissure or the vein material which country rock with gangue and ore minerals, the gangue mineral. The fissures vary in width from the slightest divergence. The stronger system, fills it, gangue minerals being in far greater amount in this as an outside estimate to mere knife-edge i. e., that which includes the greater number Vein filling. The material which fills the fis­ the vein. In this type of vein the ore formation fractures, hardly discernible in the comparatively and the longer veins, trends N. 10° W. To this sures possesses unusual interest as regards its min­ has been due entirely to the deposition of minerals little-altered surrounding country rock. It is system belong the Aztec, St. Paul, and Apache eralogy, structure, and genesis. It may be divided in empty cavities, and the inclosing rock is rarely if very probable that many of the pinches and veins, besides many others of less importance. into two general classes ore minerals, or metallic ever found to yield appreciable values. A certain swells which have caused these variations are The veins of this system usually dip about 60°, minerals yielding the values sought for in mining, amount of mineral material, notably silica and due to movement on a curving fracture which but in some instances are nearly perpendicular. and gangue minerals, or nonmetallic minerals of pyrite, has, however, penetrated the country rock has brought convex or concave surfaces on either The other system trends N. 80° E., exactly at right comparatively worthless character, which make up in the immediate vicinity of the fissures, producing side opposite to one another. Such movement angles to the first. the balance of the vein filling. a high degree of alteration, which fades away gradu­ has occurred both laterally and vertically. The These two systems intersect in Chicago and Val­ The ore minerals in the deep-seated portions of ally until unaltered rock is reached. fissures are rarely single, but send out many curv­ lecito basins and the neighboring country and the veins, where neither oxidation nor secondary The modified fissures are of less frequent occur­ ing arms which return within a short distance to form a rectangular network which is the more sulphide enrichment has exerted any influence, rence and of less importance than the type just the vein from which they branched. Auxiliary striking owing to the bare and uncovered nature are, in the order of their abundance and develop­ described. They are sheeted zones in the country fractures cross the included masses of country of the rock surface. At some intersections the ment, pyrite, chalcopyrite, galena, and here and rock where the filling of open spaces has been rock between the various arms, so that the result veins cross one another without displacement on there a little sphalerite; the gangue minerals are slight as compared with the alteration and replace­ of the fissuring has been to produce a long zone of either fissure; at others N. 10° W. fissures termi­ quartz, rhodochrosite, fluorite, chalcedonic silica, ment of the rock intervening between the fractures. reticulated character. In some cases this branching nate abruptly against N. 80° E. fissures, while the and locally a little barite and calcite. To this type of fissure Ransome, in his study of has been so extensive that the vein can be described reverse conditions prevail at no great distance. The minerals, in the manner of their arrange­ the Silverton mining region, has given the name as little more than a heavily brecciated zone. Relative age of fissures. From these observa­ ment in the fissures, possess to a remarkable degree "lode fissure," a term which may well be used to In dip the fissures usually diverge quite widely tions it seems probable that all the fissures belong the structure called by Posepny crustification describe these deposits. Such lode fissures are from the vertical, the average inclination being in to a single period of formation. To account for that is, they are arranged in successive layers formed by the occurrence of a large number of the neighborhood of 60° to 70°. Parallel fissures their occurrence is a matter of some difficulty, on the walls, each layer on the one side corres­ minute and generally parallel fractures, in many generally dip in the same direction. In some but it seems not unlikely that the compressive ponding to its prototype on the other until the cases very close together and nowhere exhibiting a cases, as in the Aztec mine, the dip varies from strains to which the San Juan region has been center is reached. If the fissure is narrow the large amount of open space into which mineral nearly vertical in Chicago Basin to about 65° on repeatedly subjected may be the cause. space is entirely filled; if wide a cavity or vug matter might be introduced. The rock between the south side of Aztec Mountain, where the vein The wall rocks. The fissures are confined to is left into which the crystals of the last-formed these fractures has been altered very extensively has been Opened in the Republic tunnel. two general types of wall rock the metamorphic mineral project. In only a few cases, however, is and much of it replaced by minerals having a gneisses and schists of Algonkian age and the later this structure perfect and undisturbed, because character wholly different from that of the origi­ MODIFIED FISSUEE VEINS. intrusive granites, the latter rocks showing some the veins have been filled, reopened, their filling" nal constituents of the rock. Such alteration and Modified fissure veins occur chiefly in the local variation, but in general being readily recog­ material shattered, and the veins filled again as mineralization does not terminate abruptly at any gneisses and schists along Needle Creek. As nizable as a single group. many as three separate times. 13

The original gangue filling of the veins seems to oped. Thus in the upper workings of the Aztec miles of this distance is over a wagon road by way gray copper. These are very intimately asso­ have been a fine-grained dark- to light-gray silica, mine brittle and native silver were found, but soon of Howardsville, up Cunningham Gulch, and the ciated. What appeared to be a single mineral cryptocrystalline in character and microscopically gave place to low-grade ore as depth was attained. remainder is over a good pack trail. The district from the Good Hope mine gave tests for tellu­ showing a considerable admixture of water of crys­ Some of the ore is reported to have yielded $1800 may also be reached by wagon road from Creede, a rium, copper, arsenic, antimony, and sulphur/ tallization. Disseminated through it and also scat­ per ton, largely in silver. distance of about 40 miles, and in the summer of This mineral had the physical properties of gray tered in large, irregular masses is a fine-grained, In like manner in the Mastodon mine irregular 1905 a trail was being constructed from Elk Park, copper. The tellurium mineral probably occurs velvet-like pyrite containing considerable copper crystals and masses of argentite, intermingled with on the Denver and Rio Grande Railroad, about 9 also in a much purer state, since roasted speci­ and in some cases grading into chalcopyrite. In clay and other surface materials, were found in the miles away. The country is extremely rugged and mens which have been taken from the Good Hope this sulphide gold and silver are also present, the lower portion of the oxidized zone. In the Bluff much of it is above timber line. The region was mine showed light-yellow globules of considerable latter metal being in large excess and as a rule con­ and Waterfall groups native wire silver and wire prospected as early as 1878. The Gold Bug mine size sticking to the white-quartz gangue. stituting a slightly greater portion of the actual gold were found within reach of the oxidizing sur­ was among the first properties located and was sold White iron pyrite is found in relative abun­ value than the gold. face waters, but gave way in depth to unaltered to Kansas City parties, who, it is said, took out dance, especially at the Sylvanite and Kankakee After a shattering of these earlier deposited sulphides. about $50,000 worth of very rich ore. The Good properties. Its occurrence is similar to that of the materials had occurred, in the secondary fissures There is little question that these high-grade Hope and Sylvanite have also produced consider­ ore minerals, and it has been mistaken for a tellu­ so formed and around the angular fragments of minerals are produced by the action of unaltered able amounts and shipments have been made from ride. It may be distinguished from the tellurium gray silica and pyrite a filling of later gangue sulphides on descending surface waters which have a number of other properties. The entire district ores by the darker color of its powder, its greater minerals was introduced. In most cases these leached the values from the upper portions of the has produced about $200,000, chiefly in high-grade hardness, and its tendency to assume botryoidal or minerals simply filled open cavities, but in some veins, and prospectors would do well to exercise tellurium ores. Nearly all of this was packed out roundish forms. instances a replacement by them of the earlier caution in regarding such rich croppings as evi­ by mule trains. Pyrite and chalcopyrite (copper pyrites) occur as silica may be detected. The later minerals are, dence of the continued downward extent of payable When the camp was visited in August, 1905, the small masses and crystals in the gangue, and to a in the order of their deposition, rhodochrosite, ore. most largely developed mines had been idle for certain extent in the country rock. Assays of white quartz, fluorite, white quartz, fluorite. Age of the veins. There is no available evidence years, and the portals of several of their tunnels these minerals show that they carry A^ery low The layers of white quartz and fluorite are gen­ which will serve to determine the age of the fissures were filled with snow and ice, so that entrance to values in gold and silver. The copper, lead, and erally repeated several times, at some points the and of the introduction of minerals into them, but all the workings was impossible. A number of zinc minerals have not been found in sufficient fluorite filling being the last formed and at others the fact that they are of limited extent both verti­ smaller properties were being developed, and data quantities for profitable exploitation. the white quartz. cally as well as horizontally indicates that a large gained from the examination of these and from the Ore deposits.- Some of the veins are well defined Galena is sometimes found in large bunches, but amount of erosion has taken place, and that the surface workings of the larger properties appear to and their decomposed outcrops, marked by yellow its relative age is difficult to determine. Barite also veins in their present condition are but the rem­ be in full accord with those reported for the work­ iron stains, may be followed along the surface for occurs locally. nants of much more extensive fissures whose ings then inaccessible. considerable distances. Their usual trend is nearly The final product of the mineral deposition is a upper portions have been carried away by erosive Geology. The Bear Creek deposits are located north and south. Those of an important group vein which, though low in commercial value, is action. That such extensive denudation should in an area of pre-Cambrian schists, slates, and run about S. 83° W. The veins are usually verti­ of singular beauty. Dark-gray silica in angular, have occurred within a comparatively limited time quartzites, partly covered by San Juan andesite cal or are inclined less than 10° from that position. broken fragments, bright-red rhodochrosite, yellow seems improbable, though such criteria are of no tuffs and later flows. The Archean schists, which The fissures do not appear to be closely related to pyrite, and chalcopyrite, with large druses of milk- very positive value in determining geologic age. are the oldest rocks, have a laminated, streaked the larger structural features of the country, but white quartz, alternating with layers of brilliant Origin of veins. The veins present features appearance and are composed mainly of mica, cut the faults, the axes of the folds, and the fissil- green crystalline fluorite, combine to make a vein which are in many ways unlike those of other quartz, and feldspar. Seams or veins of quartz, ity of the rocks at a high angle. The fissures cut strikingly varied in color and unusually fascinating areas in the San Juan region. They carry a many of them 3 or 4 inches wide, are quite all the rocks, but are most clearly defined in the to mineral collectors. higher percentage of fluorite than has been 'common. These are known to prospectors as quartzite or in the siliceous portion of the slate. The values in the vein where unoxidized and observed in most veins of the region, and are "bull quartz" or "barren quartz." The schists These rocks are strong and brittle and are better unenriched are uniformly low. If the whole face more nearly related to ore deposits usually found are described more fully under "Descriptive adapted to forming arid holding open the cavities is broken and no sorting attempted an average of in granitic areas. For this reason it seems possi­ geology." in which the quartz and ore may be deposited than $19 is a fair outside estimate of the grade of ore. ble that they may have been produced by pneu- The slates and the quartzites, though of pre- are the slates. Along the outcrops of the Gold Of this $4 to $6 is gold, $5 to $9 silver, and the matolitic emanations from some one of the varieties Cambrian age, are younger than the Archean Bug lead there are a number of shallow surface balance copper. Selected samples may run much of intrusive granites, several of which are present schists, upon which they rest unconformably, workings. Some of them are said to have pro­ higher.o Where oxidized and secondarily*/ enriched in the region. If such be the case, the veins would and are the products of metamorphism of shale duced several hundred dollars' worth of ore. the veins carry values, as will be later explained, resemble in large part a type of pegmatite veins and sandstone. They belong to the Uncompahgre They are all in hard, rather siliceous phases of in many cases phenomenally high. and would present analogies to deposits of tin.in formation. Certain portions of the shale were the slate, and it is reported that ill the develop­ The pay values, except where the irregular granitic regions. It seems not improbable that sandy, and after metamorphism became a hard, ment of the Gold Bug mine the profitable ore bunches of silver-bearing galena occur, are the prevalence of fluorite and quartz may support dense, and easily fractured rock, much like the pockets were found in siliceous walls. undoubtedly contained in the pyrite as a result this view, but in the absence of any other definite quartzite. The veins vary in width from the thinness of a of the first period of mineralization. This has data the writer hesitates to venture an opinion The pre-Cambrian rocks are rather closely knife blade to 2 or 3 feet, and the Gold Bug vein been demonstrated in a number of cases by a sepa­ on the question. It is one which is hardly folded, the axes of the folds trending generally is said to reach a maximum of 6 feet. While they ration of the various constituents of the veins. likely to be solved with any degree of satisfac­ a few degrees south of east. They are also faulted, usually have well-defined limits, there is often For example, in the Bluff and Waterfall group, tion from the limited number of occurrences now the faults running nearly east and west, approxi­ a considerable amount of the country rock con­ in Needle Creek, the sulphide ore without concen­ to be observed. mately with the strike of the beds. The andesite tained in the vein matter, so that in certain tration carried about $18 in gold and silver. The tuffs and flows are of volcanic origin and rest upon cases where the country rock is cut by quartz separated pyrite, on the other hand, carried several FUTURE OF THE DISTRICT. a very irregular eroded surface of the pre-Cambrian stringers it is impossible to definitely locate the ounces in gold and even a larger proportion of sil­ In most districts where veins of gold and silver rock. They have suffered little change since their wall. For considerable distances the veins may be ver, while the gangue minerals left were practi­ occur 'the probability of striking other veins of deposition. entirely barren of the precious metals, while at the cally worthless. similar character is so strong as to afford much A long, narrow fault block of the Uncompahgre end of this barren streak a rich pocket may occur Alteration. The oxidation of the ore bodies has encouragement to the prospector, for where ore quartzite enters the area just south of Beartown in which are large and profitable quantities of high- produced very marked effects on the surface crop- bodies of a certain type are found others of sim­ and extends nearly due west to the head of Elk grade ores. When this pocket has been removed pings of the vein. The rhodochrosite, of which ilar nature in almost all cases occur in the near Creek. On the north it is in fault contact with another barren streak is.'1 apt to be encountered. there is usually a large amount in the mines, has vicinity. In the Needle Mountains region, how­ the Archean schist, and on the south with other This erratic distribution of the ore has had a tend­ altered to a jet-black, soot-like mass of manganese ever, the veins already discovered are very numer­ Uncompahgre schist and quartzite. It is partially ency to discourage continued effort at exploitation, oxides, probably pyrolusite or wad, while the iron ous and the outcrop pings are so prominent in the covered by the Tertiary volcanics. Where it is the and while a considerable amount of development present, though in places in sufficient amount to uncovered granite of the barren summits that it surface rock, it stands up conspicuously above the work has been done the principal mines are at give a rusty stain to the gossan, is as a rule seems improbable that any should have escaped surrounding country and is very generally mis­ present idle. obscured by the preponderating black of the the diligent eye of the prospector. The uniformly taken for a dike, which form is strongly suggested PROSPECTS AT THE HEAD OF THE NORTH FORK manganese. low grade of the sulphide ores, moreover, leads one by its long, narrow, regular outline. A number OF ELK CREEK. This black material is mingled with the highly to suppose that new discoveries would not be more of claims allocated along this fault block, among resistant white quartz, which has not become easily promising than those already made. That any which are the Good Hope, the New York and At the head of the north fork of Elk Creek, near stained, so that the croppings of the vein are in of the veins so far opened should constitute the Brooklyn, and the Silverton. The lodes cross the the northern border of the quadrangle, there are a many instances readily seen and of an unusually basis of extensive commercial enterprises seems block nearly at right angles to the fault; there number of small openings in the Archean schist prominent nature. This is especially true on the very improbable, but worked on a small scale with seems to be no explanation for such a distribution and granite. The Elkora group of claims, which south side of Aztec Mountain. To the north the proper methods of concentration the ores may yet of developments except that the veins are more lies north of the head of the gulch, was located and iron present in the veins has been larger in amount be made profitable. The high grade of the concen­ clearly defined in the quartzite than in the sur­ patented in the early nineties, but has been deserted and has produced the more usual type of iron- trates as compared with the untreated ore would rounding slate and schists. for eight or ten years. At the Eldorado claim, stained quartz gossan so familiar to the student of seem to indicate that concentration on a small scale Minerals of the ore deposits. The ore deposits which is about a mile southeast of the Elkora vein phenomena. might prove successful. A consolidation of inter­ are narrow fissure veins composed chiefly of white group, a tunnel is being driven to strike a lead Oxidation has not usually proceeded very far est in the many small and irregular veins of the quartz, in which are scattered small masses or pock­ which, from surface outcrops, is supposed to run below the surface. On steep hillsides it rarely district, coupled with suitable means of treatment ets of gray ore minerals. While quartz is by far S. 46° E. extends more than a few feet underground, and on a modest scale,- might produce results beneficial the most important gangue mineral, calcite, barite, The country rock at the Eldorado is a plicated indeed in some cases the sulphides crop almost at alike to the investor and the district. and a white mud, probably kaolinite, are also micaceous chloride schist containing small quartz- the surface. Under the crests of the hills and on NORTHEASTERN AREA. present. The metallic minerals are a telluride of itic areas. At the surface a decomposed outcrop of slopes of low gradient it has, however, reached gold and silver (probably petzite), tetrahedrite (or the vein is composed of quartz, calcite, barite, BEAR CREEK DEPOSITS. much greater depths, often 300 or 400 feet below gray copper), iron pyrite, marcasite (or white iron limonite, magnetite, pyrite, chalcopyrite, mala­ the outcrop. The Bear Creek mining camp is located in the pyrite), copper pyrites, bornite (or peacock ore), chite, azurite, galena, and sphalerite. No tetrahe­ Effect of secondary alteration on values. Between northeast comer of the quadrangle, near the head galena, sphalerite, arsenopyrite, limonite, hematite, drite or tellurium minerals have been discovered. the outcrop and unaltered sulphides a zone of vary­ of Bear Creek. It includes also a group of claims malachite, and azurite. None of these minerals The lode has not yet been exposed in depth. ing vertical range occurs in which rich minerals, at the head of Elk Creek, just over the Continental have been discovered in sufficient quantities to be often deceptive to the prospector, have been devel- Divide. It is about 17 miles from Silverton; 9 of economic importance except the telluride and June, 1904. Needle Mountains.