DESCRIPTION OF THE RICO QUADRANGLE

By Whitman Cross land F. Lu Baiisome.

GEOGRAPHY AND GENERAL GEOLOGY OF THE QUADRANGLE.

By Whitman Cross.

INTRODUCTION. Features, of the Rico Mountains. The small the quadrangle the mesa floor is affected in pre­ F. M. Endlich examined the district to the east, group of mountains in the northeastern section of cisely similar manner by the uplift of those moun­ the one hundred and eighth meridian, passing The Rico quadrangle is situated in southwestern the quadrangle is in large degree a local center of tains, the steeper slopes of which begin a mile or through Telescope Mountain, being apparently the Colorado, about 50 miles west of the Continental uplift which is apparently independent of igneous two south of the quadrangle line. Between the general western boundary of his field of work. In Divide, in the zone bordering the San Juan Moun­ intrusion; but it is also to an important extent Rico and La Plata mountains the mesa is cut ,off 1876 W. H. Holmes made a rapid reconnaissance tains, almost at the head of the Dolores River. It characterized by many injected laccolithic masses. by the Dolores Valley and does not reappear on over the plateau country to the west. The compli­ is bounded by meridians 108° and 108° 15' west The intrusive rocks are of kinds common in the the eastern side because of the upturning of all cated geology of the Rico uplift, coming on the longitude and parallels 37° 30' and 37° 45' north so-called .laccolithic mountain groups of the pla­ formations on this general line, under the influence border zone between the fields of different men latitude, embracing about 236 square miles. teau country, embracing the La Plata, El Late, of the broad San Juan structure. working in different seasons, did not receive ade­ Carriso, Abajo, La Sal, and Henry mountains, Alnxost ,the entire surface of these mesa or pla­ quate attention, and the Hayden map of this area GENERAL, RELATIONS OF THE QUADRANGLE. most of which are plainly visible from the Rico teau remnants is covered by a forest growth in is, therefore, quite unsatisfactory. Relations io the plateau country. The Rico summits. This character of the Rico group was which white pine and aspen are the chief elements. J. B. Farish and T. A. Richard.- The only quadrangle lies in the north-south zone that not recognized during the Hayden Survey. The mesa border southwest of Bear Creek is espe­ geological explorations of the quadrangle since the marks the eastern border of a very notable pla­ The sedimentary section. In- general the sec­ cially characterized by a magnificent growth of time of the Hayden Survey have been connected teau surface which covers the greater part of the tion of sedimentary formations exposed in the stately aspens. At lower levels pin on, white pine, with mining developments in the Rico Mountains. area between the Colorado River in Utah and the valley of the Dolores River is that normal to cedar,, and scrub oak become more and more In the course of descriptions of some of the min­ San Juan Mountains of Colorado. Below the the zone about the San Juan Mountains. It is, prominent. ing properties near Rico there have been brief .dis­ gently undulating surface of this plateau many for example, like that shown in the adjoining The Rico Mountains. The summits of this com­ cussions of the geology of the mountain group. canyons have been carved by streams, one of the Telluride quadrangle by the erosion of the San pact and rather isolated group lie within an,oval These discussions were for the most part founded principal gorges being that of the Dolores River. Miguel River, and extends from the Mancos area about 7 miles in diameter from east to west on observations near and in the mines of New- Entering its canyon valley within the Rico quad­ (Cretaceous) shales down into the Carboniferous and 5 miles from north to south. The peaks are man Hill. In 1892 John B. Farish read a pap.er rangle this stream flows with irregular course for red beds. But in consequence of the Rico uplift nearly all included, within the northeast section of before the Colorado Scientific Society entitled "On about 18 miles in a southwesterly direction and and its bisection by the Dolores the lower Paleo­ the Rico quadrangle, but a few lie east of the one the Ore Deposits of Newman Hill, near Rico, Col­ then swings to a general north-northwest trend, zoic formations are shown locally, and even cer­ hundred and eighth meridian, in the Engineer orado" (Proc. Colorado Sci. Soc., vol. 4, pp. 1 SI- which it maintains for over 100 miles to the tain quartzites of the Algonkian. The formations Mountain quadrangle. ,,. 164). The description of the ore deposits was Grand River. thus revealed in the Dolores Valley have the gen­ The topographic map of the quadrangle shows preceded by some general remarks on the geology. The larger part of the plateau surface, lying eral character of the complete section more per­ the general character of the mountains as compared The structure of the mountains was recognized by between the Dolores and Colorado rivers is called fectly exposed in the Animas Valley, about 12 with the plateau area and the long lateral ridges of Earish as a domal uplift. the Great Sage Plain, while its direct continuation miles to the east. The Mesozoic formations are the Dolores Valley. The special sheet exhibits A detailed description of the Enterprise mine eastward and toward the head of the Dolores is the same that characterize the canyons of the the finer details of form and includes the peaks was published in 1896 by T. A. Rickard, then named the Dplores Plateau. plateau country to the west, :but it is known situated east of the quadrangle line. . superintendent of the mine (Trans. Am. List. This broad plain surface is due chiefly to a that most of those formations exhibit .progres­ From these maps it may be seen that the Rico Min. Eng., vol. 26, pp. 906-980). In this paper heavy sandstone, the Dakota (Cretaceous), and its sive changes as distance from the Colorado moun­ Mountains consist of a circle of high and rugged there are but few statements concerning the gen­ undulations are in part structural, in harmony tain area increases. These changes have not yet peaks, divided into two crescent-shaped halves by eral geology. The strata about Rico are said to with the slightly varying dips of the sandstone, been examined in detail. the Dolores Valley. There are twelve peaks, each be fossiliferous and to belong to the lower Car­ and in part owing to remnants of the soft, thick exceeding 12,000 feet in elevation above sea level, boniferous, and the common igneous rock is called GEOGRAPHY AND TOPOGRAPHY OF THE QUAD­ shale formation normally overlying the sandstone. and the narrow crest connecting them rarely sinks porphyrite, and is concisely described by R. C. RANGLE. The Great Sage Plain of Utah has a general eleva­ below 11,500 feet on either side of the river. In Hills. Rickard refers to "a large dike of porphy­ tion of 6000 to 7000 feet above the sea. Eastward The Rico quadrangle presents three especially passing through the group the Dolores receives rite" crossing the valley north of Rico, "making the Dolores Plateau gradually rises with the dip of prominent types of topographic forms, each dom­ several important tributaries on each side, which a fault which breaks the continuity of the coun­ the sandstone until, on the western border of the inating a considerable part of the area. These not­ expose the internal structure of the group in many try on either side." It would appear that this Jlico quadrangle, it has an altitude of over 9000 able features are (1) the Dolores Plateau, (2) the important respects. These lateral gulches are all reference must be to the mass of schists with small feet. , Beyond that line it rises more rapidly as the Rico Mountains,, and (3) the Dolores Valley, with deep, with steep sides, and their streams are still dikes of hornblendic porphyry; but the position and Dakota sandstone and other formations take part its many lateral branches. . actively engaged in the work of erosion. importance of the fault are not further indicated. in the local structures of the Rico and La Plata The Dolores Plateau. -The western .half of the The characteristic forms of peaks and gulches U. S. Geological Survey. In the course of the Mountains, to be described in detail. Rico quadrangle belongs to the Dolores Plateau. are illustrated in the photographs reproduced in present resurvey of. the Rico quadrangle the geo­ Relations to the. San Juan Mountains. The A glance at ,the topographic map shows that this folio. Fig. 1 in particular shows the details logic, complications in the Rico Mountains were southwestern front of the volcanic San Juan between the Dolores River and Stoner Creek, there of form .commonly present in the higher summits found to be,;so great that a detailed topographic Mountains lies 6 to 8 miles northeast of the Rico is a gently inclined mesa crossed by the western on the eastern side of the river. map and a.,. special report on its geology and quadrangle. The intervening space is character­ meridian of the quadrangle at an elevation of Timber line in the Rico Mountains lies between mineral resources were found necessary. This ized by irregular foothill topography, with features about 9400 feet. The flat crest of the narrow 11,500 and 12,000 feet, and its course may be report appeared in the Twenty-first Annual due in part to the upturning and erosion of various ridge between Stoner Creek and the West Dolores traced in several of the illustrations of the folio. Report of the Geological Survey under the title, sedimentary formations about the ancient San Juan is clearly a remnant of the same plateau level and The Dolores Valley. The Dolores River has "Geology of the Rico Mountains, Colorado," by center of uplift and in part to large masses of intru­ on the northern line of the quadrangle it appears carved its valley through the heart of the Rico Whitman Cross and Arthur Cpe Spencer. As the sive igneous rocks. These intrusions are similar again. Mountains, and near the western boundary of the Rico Mountains are the most important and most in. character to those of the Rico Mountains. South of the Dolores the same notable mesa quadrangle it enters a canyon, cut far below the cqmplex part of the quadrangle the text of this No surface volcanic rocks of the San Juan suc­ feature may be recognized. The actual extent of plateau level, in which it flows, to its junction .with folio is in large degree descriptive of the phe­ cession -occur in the Rico quadrangle. It is prob­ the mesa surface in the quadrangle may be most the Grand River. The branches of the main nomena exhibited in the,mountains, But as only able, however, that the San Juan volcanics once clearly appreciated by an examination of the geo­ stream within the area are all short, except Bear the broader features of the geology can be treated extended over this area and have been removed by logical map, where its outline is shown by means Creek, which heads a-few miles.to;the south in the in this place the reader will often be referred for erosion. In support of this idea may be mentioned of the mapping of the distribution of the Dakota La Plata Mountains. , The West Dolores Valley is details to the publication just cited, which will be called in general terms "the Rico report." The 1the - fact I that..-'*/; only *. a_ \ few miles.... north. of . the' ; Rico. . sandstone, its floor. The mesa remnants are nearly as large as the main fork, but lies wholly area, on the south slopes of the San Miguel Moun­ bounded by distinct scarps formed by the sandstone. within the plateau region. The extreme head of special map of that report is republished in this tains, a line of high peaks which are geologically The plateau feature gradually disappears as its the Dolores is at the northeast base of the Rico folio as the economic sheet. as well as topographically western outliers of the sandstone floor comes under the influence of the Mountains. A report on the ore deposits of the Rico Moun­ San Juan Mountains, remnants, of the horizontal local domal uplifts of the Rico and La Plata The canyons of the Dolores River, Lost Canyon, tains, by Frederick Leslie Ransome, appeared in surface lavas of that district, as well as great masses mountains. The contours of the map clearly Stoner Creek, and the West Dolores are. character­ the Twenty-second Annual Report of the Geologi­ of intrusive rocks, are fduildl The base of the sur­ express the changing dip of the Dakota sandstone, istic of the drainage channels of the plateau coun­ cal Survey, Part II, pp. 229-397. A summary of face volcanics in the San Miguel peaks stands at and with it the changing slope of the mesa sur­ try.. The sides are steep and are modified by that report constitutes the section of this folio on about 12,000 feet, which is higher than any por­ face itself as those mountains are approached. many minor scarps representing resistant sand­ "Economic geology." tion of the Rico quadrangle except certain points West of the Rico Mountains the dip slope of the stone strata. , ; Folios presenting the geology of the Telluride in the local area of uplift in the Rico Mountains. mesa reaches an elevation of 11,500 feet on the quadrangle on the northeast and of the La Plata GEOLOGICAL INVESTIGATION OF THE REGION^ The Rico area is geologically related to the San ridge west of Eagle Peak. This corresponds quadrangle on the south have been issued. Those Juan region chiefly in regard to pre-Tertiary for­ closely to the level attained by the similar plane The Hayden Survey. The country adjacent to of the Engineer Mountain and Durango quad­ mations and structure and tile Quaternary ero­ on the east side of Bear Creek, north of the La Rico was visited by geologists of the .Hayden rangles, respectively east and southeast of the sion of streams heading sOa the San Juan flank. Plata Mountains, for across the southern part of Survey in 1874 and 1876. In the former year Rico, are in preparation. elevations of 9200 and 9500 feet, showing a con­ mation deserving recognition occurs between the of Devonian age. It was supposed by Spencer CULTURE. tinuous exposure at one place to a thickness of quartzites and the Devonian limestone. This that the whole limestone complex in question must The agricultural development within the Rico 350 feet, though from the structure it is probable intermediate formation consists, as known in the be of Devonian age, but as will be shown, it has quadrangle is limited to small areas of bottom that a greater thickness is present. The strike and Animas Valley, of thin-bedded limestones and been proved that an indefinite but subordinate part land, principally in the valley of the West Dolores dip may be determined in this region and, while calcareous shales with varying amounts of thin of the most prominent limestone ledge of the and to a less extent in that of the main river. The both are variable, the former is generally about quartzites, the whole less than 100 feet in thick­ Ouray is Mississippian. Since it is impossible to level expanses of the plateau are not available N. 1CT-300 E. and the latter is steeply toward the ness. Fragments of fish scales and bones have been draw a line between the two portions, the Ouray for cultivation, because of the lack of water. They south of east. On the north, south, and west the found in these beds and although but a few specifi­ becomes a lithologic unit transgressing the faunal afford excellent grazing land in many places. boundaries of this mass of quartzite are not known, cally determinable forms have yet been obtained, it boundary between the Devonian and Carbonifer­ Metalliferous deposits in the Rico Mountains since they are covered by surface debris; but from is considered probable by Dr. C. R. Eastman, who ous systems. have led to extensive mining operations and the the adjacent occurrences of porphyry belonging to has studied them, that these fossils are identical or General lithologic character. The Ouray for­ foundation of the town of Rico, situated on the the thick sill of Newman Hill it is almost certain closely related to fish remains occurring in the Cats- mation as at present known has a thickness vary­ river in the heart of the mountain group. The that the quartzite is limited on the south and kill formation of the upper Devonian, in Penn­ ing from 100 to 300 feet. The upper and major Rio Grande Southern Railroad crosses the quad­ west by faults, in the manner indicated on the sylvania. In the Silverton folio the fish-bearing part of the formation is massive limestone, either rangle, following the valley of the Dolores River. map, while on the north it may connect under­ series of beds was named the Elbert formation. in one bed or with such thin intercalated shales neath the valley wash with the quartzite on the The observations made at Rico do not indicate the that the ability of the limestone to resist erosion, DESCRIPTIVE GEOLOGY. north side of Silver Creek. presence of the Elbert beds at that locality, but it and thus to cause mesas, benches, and prominent Within the area just mentioned the rocks are very is possible that the limited exposures and the more cliffs as characteristic topographic forms, is always THE ROCK FORMATIONS. imperfectly exposed, except in local patches, but or less metamorphosed condition of the rocks may notable. Below the more massive portion a third SEDIMENTARY AND METAMOEPHIC KOCKS. from these and from the data derived from tunnels have hindered recognition of the characteristic or less of the section is made of well-bedded lime­ and prospects it is definitely known that the north­ features of this formation. stone with distinct shaly layers and, rarely, thin ALGOJSTKIAN SYSTEM. ern limit is along the Last Chance fault, which has The lowest lithologic division of the Paleozoic quartzites, between them. Some of the lower Introductory statement. The rocks which are a nearly east-west course. The highest exposures section in the Animas Valley is made up of quartz­ layers have a wavy bedding, some are arenaceous described as Algonkian occupy a small area in the are near this fault, at about 9400 feet, and the ites, and varies in thickness from a few feet up or earthy, and large chert concretions, free from center of the Rico Mountains, where they have quartzite can not extend much beyond this point, to 200 feet. A single fossil shell, determined by fossils, are common at a horizon near the base. been exposed by the carving of the Dolores Valley since green shales and sandstones are exposed at Charles D. Walcott as Obolus sp.? and resembling The lowest stratum is characterized usually by through the heart of the uplift. They comprise about the same elevation in the draw below the certain Upper Cambrian species, has been found in crinoid stems and rarely a cup coral. qtiartzites and quartzitic schists and are similar to Alma Mater mine. these quartzites, and therefore it seems at present The greater part of the formation is dense, com­ the series of rocks exposed in the Uncompahgre best to refer the formation to the upper or Sara­ pact limestone, but portions of the upper ledge are SCHIST. Canyon on the north side of the San Juan Moun­ togan series of the Cambrian. In the Silverton coarsely crystalline. In general, the rock is nearly tains and in the Needle Mountains on the south Character. The remaining rocks of probable folio this was named the Ignacio formation, from white, straw yellow, or buff, with local pinkish side of the San Juan. In the latter region they Algonkian age may be termed schists, since they its occurrence near the Ignacio Lakes in the Engi­ tones. Some of the lower beds are strongly yel­ were represented on the Hayden map as "meta- have a more or less distinct foliated structure, not neer Mountain quadrangle. low and these are commonly more or less sandy. morphic Paleozoic." due to original bedding, but superinduced by met- The Igtiacio beds at Rico. The quartzites here The contrast with the dark-gray, dense limestones The quartzites of the Animas Canyon section amorphism under stress. In these schists the provisionally referred to the Ignacio formation of the Hermosa is marked, layers of such character through the Needle Mountains have been examined stratification may be made out in some cases by may be seen in the bed of the Dolores River just occurring only near the base of the Ouray. by Ennnons and Van Hise, who have assigned differences in the character of adjacent bands, and above Rico and along the west bank of the stream. The Carboniferous portion of the Ouray is litho- them to the Algonkian system. The correctness to this structure the foliation is generally, though These strata dip at an angle of a few degrees south­ logically indistinguishable from the Devonian. of this assignment is confirmed by recent work of not always, parallel. The direction of foliation ward, passing under the mineralized limestone of Faunas and correlation. The Devonian inver­ the Geological Survey in the Needle Mountains does not vary greatly from east and west, and its the Atlantic Cable claim. They were encountered tebrate fauna of the Ouray occurs from near the and the discovery of Cambrian fossils in the lowest position is nearly vertical wherever observed. beneath that limestone in the bore hole sunk on base to a horizon which in many places is not far Paleozoic formation of that area, which rests uneon- The schists are dense bluish-gray rocks, the foli­ the claim mentioned. It is probable that the below the top of the upper, massive ledge. The formably on the quartzites and other pre-Cambrian ation being caused by the arrangement of very quartzites reach a thickness of at least 200 feet. greater number of species occur in this upper rocks. In the Silverton folio the quartzites, slates, minute particles of biotite and actinolite, not recog­ This basal quartzite is a massive rock, very horizon, but many of them range to within a and conglomerates of this ancient complex were nizable to the unaided eye. A delicate luster is dense and highly indurated. Its colors are dull few feet of the base. called the Uncompahgre formation. The Uncom­ visible on the planes of easier fracture, but the yellow-white with red and brown stains. There is The Mississippian fauna has been found at sev­ pahgre quartzites and slates are underlain in the schistosity is never very highly developed and the a slight variation in grain, the mass of the formation eral localities in the Animas Valley in coarsely Needle Mountains by a thick conglomerate called rocks often break readily across the structure with being fairly homogeneous. The stratification is crystalline beds near the top of the formation. the Vallecito formation. The Vallecito and the almost conchoidal fracture. sometimes discernible, though usually obscured Fossils have not been found at Rico, but have Uncompahgre together constitute the Needle Moun­ In a few places the rock has quite clearly the by jointing and rifting. The formation is not been obtained at Ouray and at several localities tains group, according to the nomenclature pro­ character of a mashed product, apparently derived clearly distinguishable from the Algonkian quartz­ on the southern slope of the San Juan, including posed in the Needle Mountains folio. from a porphyry in which there were phenocrysts ite except by its more regular bedding and by the that where Endlich first found a few character­ of quartz and feldspar. There is a slight develop­ conformable attitude which it bears to the over­ istic Devonian species. UNCOMPAHGRE FORMATION. ment of tourmaline in such rocks. lying Paleozoic rocks. The invertebrate fauna of the Devonian portion Character. The Algonkian rocks, very imper­ Intruded into these schists, in general parallel to Occurrence. The most clearly defined quartz­ of the Ouray has been fully described by G. H. fectly exposed at Rico, consist of quartzites and the structure, but sometimes crosscutting, are many ites of the Ignacio formation occur in the valley of Girty, and compared with similar faunas hitherto quartzitic schists bearing small amounts of mica. thin dikes of a dark porphyritic rock. These are the Dolores River, just north of Rico and south of collected in Colorado, but not recognized as dis­ The quartzites are found only in the valley of Sil­ prominent on both sides of the river, but have not the Smelter fault. Certain other quartzites, which tinct from the forms of the Mississippian. It is ver Creek, in small upthrust fault blocks, and are been found in the Algonkian quartzites nor in any are associated with Algonkian schists on both sides represented more or less fully in older collections not distinguishable in character from other massive other rock than the schists; hence they are sup­ of the river near the Last Chance fault, have also from the Elk Mountains, at Glen wood Springs on quartzites, to be described later, which are supposed posed to be very old intrusions, independent of the been referred to this formation. Still other quartz­ Grand River, near the head of White River, and to be of Cambrian age; but the visible thickness other eruptions of the region. This idea is sub­ ites, mapped as Devonian on the special map accom­ on East Monarch Mountain, Chaffee County. Full and the structural attitude of the Algonkian rocks stantiated by the mashing of some of the dikes. panying the Rico report, occur in the valley of correlations of the sections in these localities with make it impossible to refer them to the thin Cam­ Stout prisms of hornblende are the only promi­ Silver Creek. These quartzites seem to occur in that of the San Juan region can not be made, brian formation of this region. They are white nent crystals of the rock. There is also much conformity with the Carboniferous rocks of New­ however, until further examinations have been or tinged with brown, with occasional red or rusty secondary hornblende and epidote revealed by the man Hill. If they are Cambrian, however, the carried out. Concerning the fauna Mr. Girty bands. They are composed almost entirely of microscope. The former subordinate feldspathic absence of the Devonian limestone above them writes: quartz, occurring usually in small, even-grained constituent is so much crushed and altered that the must be explained. In the Rico report, to which la general the Devonian fauna of the Ouray belongs particles, but sometimes in the form of pebbles original character can not be determined. Plagio- the reader must be referred for further discussion to late middle or, more probably, to upper Devonian less than an inch in diameter. The rock is com­ clase was probably predominant over orthoclase. of this question, it was assumed that the Devonian time. It is but distantly related to the Devonian pletely indurated by the interstitial deposition of Occurrence. The Algonkian schists occur only limestone had been removed by erosion at this faunas of New York, and its relation with those of the quartz, so that it is now glassy quartzite, very in the Dolores Valley just above Rico in small point before the deposition of the Hermosa (Car­ Mississippi Valley, or even with other known western resistant to erosion. Distinct partings between the upthrust fault blocks, and the structure about them boniferous). Devonian faunas, is not close. It shows many points of beds of quartzite are nowhere observable in present is so complicated, as shown by the special sheet, approximation to the Athabascan fauna described by DEVONO-CARBONIFEROUS ROCKS. Whiteaves, and is somewhat strikingly similar to the exposures. However, the bedding or stratification that the relations of the schists to the Algonkian Devonian of Russia. planes may frequently be made out from a study quartzites and of the latter to small areas of Paleo­ OURAY LIMESTONE. ctf the massive quartzites, where differences of grain zoic quartzites have not been satisfactorily demon­ Name and definition. The presence of Devo­ The following named species are particularly are found or where cross-bedding is observable. strated in all cases. nian strata in southwestern Colorado was first characteristic of the Devonian portion of the Ripple-marked surfaces are also occasionally seen. recognized in 1874, through collections of fossils Ouray fauna: CAMBRIAN (?) SYSTEM. Occurrence. There are six separate areas of made by F.-M. Endlieh, of the Hayden Survey, on IGSTACIO QUARTZITE. Schuchertella Chemungensis. Camarotoechia Endlichi. quartzite in the valley of Silver Creek, and of the southern slopes of the Needle Mountains. Productella semiglobosa. Camarotoechia contracta ? these one, that below Allyn Gulch, is certainly Introductory statement. The lowest member of The name Ouray limestone was proposed by A, C. Athyris Coloradoensis. Naticopsis? humilis. Algonkian, as must be inferred from its great the Paleozoic section displayed in the Rico Moun­ Spencer, in 1900, after the strata had been reex- Spirifer cqniculus. Orthoceras sp. Spirifer disjunctus var. Animasensis. mass; another, on the opposite side of Silver tains is a quartzite which was grouped with the amined in connection with the U. S. Geological Creek, is probably of that age; while the others overlying limestone in the Rico report, both being Survey work, from the town of Ouray, on the As to the Mississippian fauna of the Ouray have been assigned to the Paleozoic. In the place referred to the Devonian, though with a reserva­ southern border of which is a prominent outcrop limestone Mr. Girty makes the following state­ first mentioned the quartzites have their greatest tion as to the quartzite, since it was recognized of the limestone. ment: development. They are bounded on the east by a that that formation might be much older than the The name was proposed by Spencer for the The fauna which at one time occupied the higher beds Devonian limestone member of the pre-Carbon- well-marked fault, shown in the Laxy mine; thence limestone. Recent investigations in the quadran­ of the Ouray limestone is very different from the assem­ toward the southwest they may be traced for a gles lying east of the Rico have shown not only iferous Paleozoic, excluding the quartzites and blage of Devonian types which occurs below, and belongs quarter of a mile along the hillside, on the slope of that the quartzites are probably of Saratogan shales here called the Ignacio and Elbert forma­ to a phase of Carboniferous life which was widely dis­ which their outcrops are to be seen between the (Upper Cambrian) age, but that another thin for­ tions, although they were thought to be possibly tributed over the continental sea. It is found in the The coarser sandstones are usually cross-bedded sas section, as worked out by Prosser, it is found maximum thickness of 40 feet. Their alternation established it was recognized that future discoveries and occur in massive beds from 2 or 3 to 25 feet to correspond more nearly with that of the Neo- with finer-grained and softer beds causes a succes­ might show that the major part of the "Red Beds" in thickness. Some of the coarse sandstones are sho and Chase formations, the " Permo-Carbon- sion of benches and ledges on numerous lateral may belong to the Carboniferous system, although of very much lighter color than the mass of the iferous" of Meek and other writers, than with the ridges of the Dolores Valley. The grits are usu­ the fossil content of the beds had already demon­ formation. When fine grained the sandstones are Marion formation, which is regarded as answering ally rich in pink feldspar and white quartz and are strated the correctness of the common view that usually somewhat laminated and pass into sandy more strictly to the true Permian. either gray in color or have a much lighter pink the upper portion, at least, was Triassic. shales. The shales, aside from the sandy varieties, A full list of fossils from the Rico formation tone than the average of the formation. Pebbles Deducting the Cutler beds, which were provi­ are of two kinds the fine-grained, unlaminated, may be found in the report by Cross and Spencer. are scattered sparingly through the grit layers, sionally assigned to the Dolores in the Telluride red, marly beds, similar to those of the Cutler, and Mr. Girty supplies the following partial list of causing transitions to conglomerate, and the gravel and La Plata folios and in the Rico report, there the equally fine-grained, laminated clay shales of a characteristic forms: matrix is abundant in the latter strata. remain to the Dolores formation in its normal The pebbles of the conglomerate average but a development several hundred feet of typical "Red green color. Productus cora. Allerisma terminate. Intercalated with the sandstones and shales, Seminula subtilita. Schlzodus pandatus? few inches in diameter, but are occasionally more Beds," consisting chiefly of vermilion sandstones which are for the most part very calcareous Limipecten occidentalis. Pleurophorus subeostatus. than a foot. The clastic materials of the Cutler and calcareous sandy shales. At the base is a Myalina Wyomingensis. Eduiondia gibbosa. throughout, there are several beds of impure Myalina subquadrata? Loxonema plicatum. beds are derived chiefly from granites, schists, variable section of shale, sandstone, and limestone limestone, some as earthy, gray, sometimes nod­ Myalina perattenuata? Naticopsis monilifera. and the ancient quartzitic sediments of the San conglomerate, scattered through which are fossil Pseudomonotis Hawni. Strophostylus remex. Juan region. remains. This section is not always of reddish ular bands associated with the marly shales, and Pseudoinonotis equistriata. Bulimorpha chrysalis. others as sandy limestone of a red color, in strata Pseudomonotis Kansasensis. Euphemus nodicarinatus. The coarser strata are often rich in feldspar. color, as it presents gray or greenish hues in from 6 inches to 2 feet in thickness. The latter, Aviculipinna Nebraskensis. Limestone pebbles characterize certain conglom­ many places. These variably fossiliferous strata and a 6-inch layer of limestone which was taken Section of the Rico formation on Scotch Creek. erates, but are comparatively rare. Certain much- always occur at the base of the formation so far as the upper limit of the formation in Scotch Feet. decomposed porphyritic rocks have been noted in as known. In the San Juan region the formation 22 Two fossiliferous limestones, each 6 inches the Cutler conglomerates of the Dolores Valley, is delimited both above and below by stratigraphic Creek, are very fossiliferous. The sandy fossil- to 1 foot thick, separated by about 3 iferous bands have a characteristic appearance feet of green shale...... 4 but such materials are much more abundant in breaks, which are, however, not demonstrable as wherever they are found, since the fossils are 21 Poorly exposed slope, containing several the conglomerates of the Uncompahgre Valley. such at many places. The upper limit of the thin beds of light-colored sandstone preserved in white calcite, in sharp contrast to and in the upper part red sandy shale.. 20 A calcareous cement is common in all the Cutler Dolores, adjacent to the San Juan Mountains, is the red matrix of calcareous sand. They are 20, Arko?e sandstone, rather coarse, contain­ sediments. Not infrequently certain marly strata probably everywhere the plane of pre-La Plata ing some pebbles up to H inches in found in the lower third of the formation, and diameter; color, pink...... 7 grade into sandy limestones which have a peculiar (Jurassic?) erosion. That erosion was greatest to while some of them are of local development and Shale and thin-bedded sandstone...... 7 conchoidal fracture. There are local occurrences the north of the San Juan, and it is possible that may be seen to grade both vertically and horizon­ Earthy limestone, unfossiliferous...... 1 of thin reddish or gray limestones, which appear in the plateau country of Utah the Triassic beds Shales, forming a slope with a few thin tally into the sand rock with which they are bands of fine-grained sandstone; color, to be homogeneous on freshly fractured planes, but may preserve their full thickness. usually associated, at least one band is known gray, green, and red...... 18 look rather like conglomerate on weathered faces. The name was chosen because of the typical 16, Massive arkose sandstone of ared color, In some cases there is an apparent gradation from to be persistent in the Rico region, and its equiv­ quite conglomeratic in the middle, rest­ exposures of the upper "Red Beds" in the Dolores alent has been recognized in those parts of the ing upon a pink arkose conglomerate 2 massive limestone to a rock with many rounded Valley in the Rico quadrangle. Animas Valley where its horizon has been studied. feet thick...... 22 fragments in a scanty matrix of sand. These Description. Where the Dolores formation is 15. Crumbling shale, containing nodules of This fossiliferous band thus becomes diagnostic gray limestone; color, red...... 20 appear to be intraformational conglomerates. best developed it exhibits a bipartite character. of the Rico formation, and is especially valuable in 14. Sandy fossiliferous limestone; red...... 2 Nearly all the fine-grained strata are deep red in* Its lower portion consists of an alternation of 13. Series of variable sandstones; in the color and in such cases contain a ferritic pigment rather thin-bedded sandstones, sandy shales, and the study of the stratigraphy of the region, since it upper part the sandstone is thin occurs within a few feet of shales which contain bedded and alternates with shale; in in minute particles. The color is never so bright limestone conglomerates, in the aggregate some Hermosa fossils. At Rico its position varies little the lower part the sandstones are of red in the Cutler beds as in the overlying Dolores 250 feet in thickness. This series of beds is coarser grain; one layer showed prob­ from 30 feet above the Fusulina limestone of the able worm borings; color, dark red, formation. The grits and conglomerates are gray­ characterized by the limestone conglomerates and Hermosa formation, from which it is separated by except for a few gray streaks...... 27 ish or pinkish in color, owing chiefly to the abun­ by the fact that the strata are more often green­ Grnarly limestone, earthy in the upper green micaceous shales carrying shell fragments part...... 4 dance of pink grains of feldspar. ish or gray than distinctly red in color. All the and crinoid stems, and is thus a reliable guide in Friable sandstone and thin shale layers; A characteristic of the shaly strata is the abun­ known fossils of the formation also occur in these defining the two formations. The formation is color, dark red...... 15 dance of mica, commonly muscovite, but in some beds. Thin-bedded sandstone, passing down­ without any definite limit at the top, since the ward into massive arkose sandstone cases biotite. Such strata are notably fissile as The limestone of the conglomerate is usually rocks which follow immediately above the highest and conglomerate; color, reddish...... 16 compared with the more calcareous layers. very fine grained and seldom resembles the lime­ Crumbling shales of a dark-red color, con­ Distribution. The Cutler formation has a wide stone of the Carboniferous. No fossil-bearing peb­ known fossil-bearing beds are similar in every taining band of gnarly limestone in the respect to the strata of the lower series; nor is it middle part...... 10 distribution throughout the quadrangle, as will be bles have been observed. Commonly the pebbles possible to apply the change in color as a criterion, Arkose sandstone and conglomerate, pink evident on a glance at the map. In the main are very small, and they are often so minute and or white, 2 to 3 feet...... 3 except in a very general way; so that it has been Micaceous calcareous sandstone and shin­ Dolores Valley the terraces or benches caused by so uniform in size as to suggest a pisolitic charac­ found necessary to assume the thickness of the ing shales; color, dark red...... 3 the more massive grit conglomerate are very strik­ ter, an impression which is, however, not confirmed Fossiliferous limestone, 1 foot to 15 inches 1 ing. The formation appears also in the valley of by microscopical examination. The diameter of the formation as equal to the greatest known thick­ Micaceous sandstone, showing poikilitic ness between the base and the uppermost fossil­ structure, and containing some fossils; the West Dolores River, but is there by no means pebbles reaches several inches in some cases, but iferous stratum. In Scotch Creek the thickness the upper part is rather massive; the so prominent as in the main valley. The forma­ averages only a small fraction of an inch. A few lower part rather thin bedded and shaly 9 on this basis would be 237 feet; on the north side Fossiliferous limestone, 18 inches to 2 feet 2 tion occurs chiefly in the region east of the Rico pebbles of quartzite, granite, and other rocks occur of Silver Creek, near Uncle Ned Draw, it would Heavy micaceous sandstone of alternating quadrangle, in the Animas Valley and its various locally in the conglomerate, and the matrix is vari­ red and gray colors, (sometimes varie­ branches. ably sandy, with a calcareous cement. .' be about the same; but on the south slope of Nig­ gated; this contains a 12-inch layer of ger Baby Hill it is more than 300 feet. In draw­ red, sandy, fossiliferous sandstone about Age and correlation. The Cutler formation is These conglomerates characterize several bands ing this upper boundary on the map the formation 8 feet from the base...... 20 provisionally assigned to the Permian series on in the lower 200 feet or more of the formation, Micaceous sandy shale of a dark-red color 20 has been represented as about 325 feet in thickness. Sandy limestone, very fossiliferous, grad­ account of its stratigraphic position. It succeeds and apparently they may appear at any horizon The measured section taken in the lower part of ing downward into gray flags...... 6 the Rico beds, whose fauna shows some affinities within this part of the section. In some places a Scotch Creek, which is given in the next column, 237 with that of the Permian of the Mississippi Valley, ledge 20 feet in thickness may consist chiefly of illustrates the features brought out in the forego­ PERMIAN (?) SERIES. but is perhaps more closely allied to the fauna -of conglomerate with numerous sandy partings, while ing description. beds underlying the Permian of that area. Above only a few yards away the same beds may consist Immediately below the lowest fossiliferous lime­ CUTLER FORMATION. the Cutler beds occurs theu Stratigraphic break dis­ chiefly of sandstone with numerous thin layers of stone there are 25 feet of micaceous calcareous Name and definition. This formation is defined covered at Ouray below the Dolores (Triassic) con­ conglomerate, some of them less than an inch in shales, carrying a few shell fragments and repre­ as including the Carboniferous portion of the "Red glomerate. If the Carboniferous section is complete thickness. Other bands of conglomerate are more senting the topmost beds of the Hermosa formation. Beds" lying above the Rico formation where that is in the San Juan region the Permian portion is surely persistent, but it seems probable that no single Correlation. Permian fossils, consisting for the present, or, where that can not be distinguished, as represented in the Cutler formation. stratum of conglomerate is continuous for any most part of plant remains, were reported from succeeding the Hermosa formation. It is of course possible that the pre-Dolores uplift great distance. various parts of Colorado by members of the Owing to the existence of a stratigraphic break and erosion may have occurred in Permian time, These peculiar conglomerates are most common Hayden Geological Survey (Report on the geol­ above the Cutler formation on the northern slope and in that case the Cutler beds are referable either in association with a series of thin-bedded gray ogy of the Grand River division, by A. C. of the San Juan it can not at present be asserted to the early Permian or to the late Pennsylvania!! sandstones and greenish shales aggregating 50 to Peale: U. S. Geol. and Geog. Surv. Terr, for 1875, that the full thickness of the Cutler is shown at epoch. 75 feet in thickness. Carbonized plant stems are p. 74), but the collections and the systematic study any point. But in the Rico quadrangle and adja­ A reference of the Cutler beds to the Permian common in these beds, but determinable leaves of the formations were not sufficiently complete cent territory the unconformity referred to has not seems natural at the present time, because of the have not been obtained. to establish the presence of rocks of this age in been detected and the Cutler strata are overlain by current assignment to the Permian of somewhat Limestone conglomerate always occurs at the such development that they could be separated the Dolores (Triassic) beds. The Cutler formation similar formations in the Zufii Plateau of New Mex­ base of the formation, or is, at least, present in from the Triassic above or from the Pennsyl- embraces an observed maximum of nearly 1600 ico, in the Grand Canyon region, in Wyoming, the lowest beds in the variable manner described. vanian below. Investigations in the Rico dis­ feet of alternating sandstones, sandy and calca­ Kansas, Oklahoma, and other districts. In all Cross-bedding is common in the sandy conglomer­ trict have been the first to reveal the occurrence reous shales, grits, and conglomerates, with occa­ these cases invertebrate faunas have been obtained atic strata. Wavy ripple marks, trail markings, of strata containing a fauna with Permian affini­ sional impure limestones or marls. which are, to say the least, much younger in aspect and mud cracks are occasionally found in the ties as a definitely separable formation within the The name is derived from Cutler Creek, a trib­ than the Rico fauna. The Cutler beds may for the finer-grained sandy shales. limits of Colorado. utary of the Uncompahgre River a few miles north present be considered as corresponding more or less The upper portion of the formation is usually The fossils of the Rico formation consist almost of Ouray, and it was first used as a formation name closely to these upper portions of the Paleozoic sec­ a succession of fine- and even-grained quartzose entirely of marine invertebrate types. The fauna in the Silverton folio. tion now commonly called Permian. sandstones and sandy shales, in many places pre­ is a mixed one, containing forms of both Permian Deseriptio7i. rlihe principal features of the Cut­ senting no distinct subdivisions. It varies from TRIASSIC (?) SYSTEM. and Pennsylvania!! affinities. For this reason it ler formation are, on the one hand, the heavy grits place to place, being massive and resistant to ero­ is regarded as transitional and since no distinctive and conglomerates and, on the other, rather crum­ DOLORES FORMATION. sion or friable and crumbling. The color is gen­ and unequivocal Permian fauna has come to light bling, calcareous sandstones free from pebbles and Name and definition. The name Dolores was erally bright vermilion and these strata thus stand from the Rico beds the term Permo-Pennsylvanian grading into sandy marls or impure limestones of originally proposed in the Telluride folio (1899) in marked contrast to the La Plata sandstone above, is retained for them. conchoidal or irregular fracture. The grits and for the Triassic strata of southwestern Colorado which throughout the San Juan district is nearly In comparing this fauna with those of the Kan­ conglomerates occur in massive beds reaching a and adjacent territory. When the formation was white. 5

The thickness of the Dolores formation varies base of the marked clay shale of greenish or red­ tion. Shale and sandstone alternate in variable blance to the Mesaverde formation but differing in greatly throughout the Rico and adjacent quadran­ dish color which signalizes the beginning of the proportions. The shales are usually apple-green, detail. This series has not yet been fully exam­ gles. This is chiefly due to the pre-La Plata ero­ alternating shales and sandstones grouped in the but are sometimes of a deep Indian-red color, with ined. It may contain equivalents of the Fox sion. About the La Plata Mountains the forma­ McElmo formation. No determinable fossils have occasional variegated bands of red and green. The Hills and Laramie divisions, as ordinarily recog­ tion reaches nearly 800 feet in thickness. Thence been found in the La Plata formation adjacent shales are fine grained or sandy and occur in homo­ nized. northward it becomes much thinner, being about to the San Juan Mountains. geneous bands, often several feet in thickness, with (6) Animas beds. A complex of tuffs or con­ 400 feet near the northern line of the Rico quad­ Description. The total thickness of the La but little or no distinct lamination. The sand­ glomerates, mainly of andesitic debris. The fossil rangle, locally but 30 feet on the San Miguel River, Plata ranges from 250 to 500 feet in the Rico stones are also even grained and friable in tex­ plants obtained from the tuff layers indicate a cor­ near Ouray about 50 feet, and still farther north quadrangle. It decreases to a minimum of 100 ture, those of the lower portion resembling the relation with the Denver and other post-Laramie disappearing entirely. feet in the Telluride quadrangle, and increases La Plata sandstones, while at least one of the formations referred by paleontologists to the Meso- Distribution. As the geological map clearly to an unknown maximum toward the west. The upper beds is very similar to the Dakota sand­ zoic, although they are stratigraphically known to shows, the Dolores is one of the most widely sandstones are very white, massive, fine and even stone. The arenaceous layers are white or yel­ be later than the great revolution which termi­ distributed and prominent formations of the grained in texture, and consist chiefly of quartz. lowish and are often found to grade horizontally nated the conformable succession of Cretaceous quadrangle. The brilliant cliffs of the upper They present in some places bands 75 feet or into sandy shale and thence into clay shale. In sediments. massive sandstones are striking features of both more in thickness, within which the stratification the upper portion of the section there is a fine­ Of these formations the Dakota and Mancos the main Dolores Valley and its western branch. is hardly discernible except in large exposures. grained conglomerate which is practically iden­ only are present in the Rico quadrangle. In the As the strata rise toward the Rico Mountains they Such beds often form steep cliffs or smooth and tical in character with the lowest conglomerate of La Plata folio may be found descriptions of the are less prominent, but many striking cliff expo­ rounded faces of bare rock. The sandstone is the Dakota. The frequence of crumbling beds in formations from the Dakota to the Lewis shale, sures exhibiting the characteristic vermilion color normally very friable and in some of the more the formation causes numerous gaps in all dis­ inclusive. may be seen. massive layers a marked cross-bedding appears covered exposures, and no detailed section can be DAKOTA SANDSTONE. The lower division, with its fossiliferous lime­ which is sometimes brought out by lines of given. stone conglomerates, is well exposed in many shading rather than by change in character of Distribution. The distribution of the McElmo Description. The Dakota formation of the Rico localities and its general gray-green color often the sandy particles. Intricate and delicate vein- formation within the Rico quadrangle is limited quadrangle has the general character common to causes it to be conspicuous. The railroad cut­ ing of secondary white quartz occasionally appears to a narrow band between the La Plata and it in Colorado. It is composed of variable gray tings from the mouth of Bear Creek westward in the more massive layers. the Dakota sandstones. It occurs immediately or brownish quartzose sandstones, often cross for several miles exhibit this section of the for­ The lower sandstone, while usually of white beneath the plateau scarp caused by the Dakota bedded, with a peculiar conglomerate at or near mation very well. color, is in some places brilliantly colored in and is usually marked by a debris-covered slope the base and several shale layers at different hori­ Age and correlation. The Dolores formation is varying shades of orange and yellow. This with numerous benches representing the sandstone zons. Its thickness in the Rico quadrangle ranges regarded by paleontologists as of Triassic age, coloration ordinarily extends quite irregularly layers. On account of the generally soft and fri­ from 100 to 250 feet. The basal conglomerate this determination being based on the scanty, yet from the base of the formation upward and able nature of the strata, outcrops of notable extent carrying small chert pebbles of white, dark-gray, widely distributed vertebrate, invertebrate, and is usually very distinct from the vermilion of are much less common than in the more uniform or reddish colors, which is so persistent over large plant remains obtained from it. The limestone the underlying Dolores sandstone. The inter­ sandstone formations both above and below. areas adjacent to the Rocky Mountains, is here conglomerates usually contain fragmentary bones mediate calcareous member of the formation is Age and correlation. As mentioned above, the very variable in development. Conglomerate of and fairly well-preserved teeth of crocodiles and less homogeneous than the sandstones between McElmo formation appears on stratigraphic and this character is not, moreover, strictly confined to dinosaurs of Triassic types. According to F. A. which it lies. Within the Rico quadrangle it lithologic grounds to occupy the place of Lower the base of the section. The sandstones occur in Lucas, the former belong to the genus Belodon or varies in thickness from 75 to 260 feet. It con­ Cretaceous sediments. But the age is provision­ beds reaching 30 to 40 feet in thickness. They allied forms and the latter to the megalosauroid sists of an alternation of sandstones, some of which ally assumed to be Jurassic from the opinion are separated by clay shales which are usually genus Palceoctonus. The limestone conglomerates are red and some white, with sandy and calcareous prevalent among paleontologists concerning the carbonaceous and as a rule carry thin seams of are so often characterized by these remains that shales, which in many places grade into thin lime­ vertebrate fauna long known from the Morrison coal. These shaly members are strongly devel­ careful search of a good exposure seldom fails to stones. One of the bands of red sandstone in this formation on the eastern flanks of the Front oped near the middle and again near the top of reveal bone or tooth fragments. complex is characterized by little rosettes or angular Range and in the equivalent Como beds of the formation. They contain abundant indistinct A small gasteropod shell, determined by T. W, grains of carnelian. Wyoming. Representatives of this fauna have plant remains. Stanton as belonging to Viviparus or some closely Distribution. The La Plata sandstones* form a recently been found by E. S. Riggs in McElmo The coal from the shaly layers of the Dakota related genus, has been found in the limestone con­ very conspicuous member of the sedimentary series, beds in the Grand River Valley at -the north end has been mined on the west bank of the Dolores glomerate in the Rico and La Plata quadrangles. commonly appearing at a certain distance below the of the Uncompahgre Plateau. The McElmo rep­ River, just north of the Rico quadrangle. It is A single determinable plant, stated by David Dakota sandstone which forms the scarp of the resents the upper part of the Gunnison group. known to occur at several places in the western White to resemble the Triassic species Pachyphyl- Dolores Plateau. The white sandstones contrast That the McElmo, Morrison, and Como forma­ part of the quadrangle and also in the vicinity of lum munsteri, was found in a railroad cut near the markedly with the bright red of the Dolores for­ tions embrace certain equivalent strata is a con­ Lost Canyon, and it seems probable that one or northern border of the quadrangle. mation below and the duller tones of the McElmo clusion scarcely open to further question. That more thin coal seams are present wherever the Correlation of the Dolores formation with strata above. Such massive sandstones naturally produce the three formations are coextensive and thus fully formation is represented on the map. This coal of supposed or determined Triassic age in other cliffs and the prominent ledge outcrops can often equivalent can not be considered as demonstrated is not comparable in quality with the excellent parts of Colorado, or in the adjacent portions of be seen for long distances. As the map shows, by present knowledge. seams of the Mesaverde formation and can not Utah, Arizona, and New Mexico, is at present there is a tendency to the formation of projecting be of much economic value except in the plateau CEETACEOUS SYSTEM. complicated by the fact that in most places the shoulders composed of the La Plata, while the country remote from railroads. "Red Beds" sections have not been studied in bench above them marks the base of the McElmo. SECTION OF SOUTHWESTERN COLORADO. Distribution. The Dakota sandstone is, as a rule, detail. Some of the western peaks of the Rico Mountains No strata of Lower Cretaceous age correspond­ much more highly indurated than the La Plata or The fact that vertebrate fossils have been are capped by the La Plata, but it has been eroded ing to those of the section so well developed in McElmo sandstones. It therefore resists erosion obtained at various times from the Triassic from the eastern summits. Texas have been positively recognized in this and becomes prominent in scarps facing the can­ strata of New Mexico, Arizona, Utah, and Age and correlation. The age of the La Plata part "of Colorado. The Upper Cretaceous series yons that cut below it in the plateau region. This Wyoming suggests that the fossiliferous por­ formation is indicated by its stratigraphic position is represented in southwestern Colorado by an formation is the floor of the plateau over hundreds tion of the Dolores may be represented over a rather than by any other evidence. The only fos­ important succession of formations, but is not of square miles. .> wide area. But close correlation is manifestly sils thus far obtained from it in the region adjacent lithologically divisible, for purposes of mapping, The Dakota sandstone is seldom well exposed in impossible until the local faunas have been to the San Juan Mountains are some small and uni­ into the same formation units which have been its entire thickness in the Rico quadrangle. The more thoroughly studied. This question has dentified fish remains. adopted elsewhere. In consequence of this unu­ upper part is wanting in almost all of the exposed been discussed by the writer at greater length The La Plata is undoubtedly the equivalent of sual character of the Upper Cretaceous section it sections. It is assumed that approximately 50 feet in the Telluride and La Plata folios, and still the lower sandstones of the Gunnison formation has been necessary to establish certain new for­ of the formation is.missing from the mesas west of more fully in a paper entitled "The Red Beds of the Elk Mountains, Colorado, described by mations. The whole section is characteristically Bear Creek and north of the Dolores River that of southwestern Colorado and their correlation," Eldridge in the Anthracite-Crested Butte folio. developed in the Animas Valley and its subdi­ is, erosion has not only cleared away the Mancos read before the Geological Society of America at A few minute fresh-water shells were found by vision will be fully discussed in the Durango shales from above it, but has also removed the the Philadelphia meeting, 1904. Eldridge in the limestones occurring near the base folio. upper part of the Dakota, which is less resistant of the Gunnison. The Cretaceous formations distinguished in than the lower portion on account of the presence JUKASSIC SYSTEM. southwestern Colorado are as follows: in it of many thin shale layers. LA PLATA SANDSTONE. MCELMO FORMATION. (1) Dakota. The apparent equivalent of the MANGOS SHALE. Name and definition. The La Plata formation Name and definition. The name McElmo was well-known, widespread sandstone formation. was first named in the Telluride folio from its proposed in the Telluride folio for the series of (2) Mancos. A shale complex more than 1000 Name and description. The body of shale which prominence in the La Plata Mountains. It is shales and sandstones which lie between the La feet in thickness, which from its invertebrate fos­ lies above the Dakota sandstone was named the defined as including a marked lithologic unit Plata and Dakota formations. This series is some­ sils must be supposed to represent the Benton, "Mancos shale" in the Telluride folio on account consisting principally of two massive white sand­ what variable in thickness, ranging from 400 to Niobrara, and part of the Pierre formations, as of its characteristic development in the Mancos stones with a variable calcareous member between 1000 feet in the part of southwestern Colorado commonly distinguished at the eastern base of the Valley, especially about the town of Mancos. In them, lying at the base of the fresh-water complex which has been examined. The vertical limits of Front Range in Colorado. the Telluride quadrangle the formation is present commonly assigned to the Jurassic in western the McElmo formation are accurately determinable (3) Mesaverde. A series of alternating sand­ as a succession of dark clay shales nearly .. 2000 feet Colorado. The base of the formation is the through the uniform character of the La Plata and stones and shales, with some seams of excellent in thickness, presenting no persistent lithologic or well-known plane of unconformity at which the Dakota sandstones between which it lies. The series coal. Invertebrate fossils, which are not uncom­ paleontologic horizon which can be used as a guide lower sandstone overlaps all older sedimentary itself is variable in character and no satisfactory mon at .several horizons, place this formation in to subdivision. The shales are characteristically of rocks to the Archean, as shown north of the criterion for its subdivision has been found. the lower part of the Montana group. a dark gray or lead color, and are nearly always San Juan Mountains and elsewhere. This angu­ Description. In the Rico quadrangle the (4) Lewis. A sandy clay shale, reaching an somewhat sandy. Thin calcareous layers become lar unconformity is probably responsible for the McElmo has a fairly uniform thickness of some­ observed thickness of nearly 2000 feet. The only limestones in places and are often rich in fossils. Quartz sand also locally increases in amount, but gradual decrease in the thickness of the Dolores what less than 500 feet. It is here composed identifiable fossil as «/vet found in the formation formation from the La Plata Mountains north­ more largely of shales than in the Telluride quad­ indicates that it is stratigraphically below the Fox no limestone or sandstone layer is sufficiently ward, but it is not clearly exhibited at any rangle, where its thickness on the San Miguel Hills division of the Montana group. developed to be traced for considerable distances. special locality in the Rico quadrangle. The River is nearly 1000 feet and where sandstone (5) Above the Lewis shale is a second series of Invertebrate fossils frequently occur in the Tellu­ upper limit of the La Plata is drawn at the forms the most important element of the forma­ sandstones, shales, and coals bearing some resem­ ride quadrangle at horizons about 125 and 225 feet Rico. 6 above the Dakota, The common fossils of these naturally sinks through the numerous fractures or torrential fans. It thus comprises the materials The principal deposits of calcareous tufa have horizons, Ostrea lugubris and Qryphcea newberryi, to solid bed rock and ravines caused by torrents of mixed origin covering Newman Hill and the been outlined on the maps, by reference to which are characteristic of the Ben ton shale division of the are often filled by subsequent slipping. Lateral opposite slope west of the river. their extent and distribution may be seen. Colorado group. At higher horizons other fossils trenches, lying above the larger blocks, are more At one locality the tufa has been quarried for a ALLUVIUM. are known which are characteristic of the Pierre prominent than erosional ravines. But the con­ kiln and has found a considerable use, since it is division of the Montana group. It is therefore tinual disintegration of all landslide blocks modi­ The alluvial deposits of the Rico quadrangle conveniently located and produces lime of good believed that the Mancos shale embraces equiva­ fies the topographic detail until it comes to resemble shown on the map are confined to the flood plains quality. lents of the Benton, Niobrara, and Pierre forma­ that common in areas of glacial gravels. of the principal streams. They are not contin­ Iron-bearing springs also occur at several places tions, though not extending to the top of the latter. On the illustration sheet (figs. 3, 5, and 6) will uous, being interrupted by torrential fans, land­ in the Rico Mountains, and have left local deposits Distribution. There are two areas of Mancos be found three views representing landslide areas, slides, or stretches of solid rock. : of iron oxide, cementing surface debris and form­ shale within the Rico quadrangle. One of these from which some idea may be obtained of the One notable flood-plain deposit extends for a ing what is commonly known as "iron cap." is at the head of Priest Gulch, where it has been magnitude of the movement and the characteristic mile and a half above Burns. It is clearly due to Though occurring at other places, these ferrugi­ preserved by a northeast-southwest fault as repre­ appearance of the slopes in their present condi­ the damming of the river by a landslide from C. H. nous conglomerates are especially in evidence in sented on the map. In this locality one of the tion. The explanations accompanying the views C. Hill, forcing the stream against the cliffs of Silver Creek above the Fort Wayne tunnel, in the fossiliferous bands is exposed. The other area lies call attention to their more instructive features. Sandstone Mountain. It is estimated that the depth upper part of the northern and western branches to the west of Bear Creek, on the northern slopes In the Rico report will be found nine other views of alluvium at Burns may be as much as 75 feet, of Horse Gulch, and in the lower part of Horse of the La Plata Mountains. At this place approx­ illustrating the varying characters of different slide since it seems probable that the grade of the river Gulch at the base of the northern landslide area. imately 600 feet of shales are present, but the masses. bed previous to the landslide was nearly uniform. exposures are very poor because of the dense Distribution. Important landslide areas occur On the West Dolores the valley bottom is wide IGNEOUS ROCKS. covering of spruce forest and a local slipping in the central portion of the Rico Mountains and a continuous band of alluvium follows its Introductory statement. By far the greater or creeping of the shales. The lower fossil lay­ on both sides of the Dolores River. The larger course across the quadrangle. The materials of number of the igneous rocks occurring in the ers were found in this region. areas lie to the west of the river and either side the valley deposits are, as usual, coarse gravels Rico quadrangle are directly connected with the of the great monzonite stock of Darling Ridge. QUATERNARY SYSTEM. and sands which the streams have derived from eruptive center in the Rico Mountains. A very One large area extends from near the summit of their tributaries and which have been rolled along few masses near the southern border belong to the Varieties of Quaternary deposits. The surface Telescope Mountain to the river. until deposited on the broad flats of low grade. La Plata center, and it may be assumed that the deposits of the Rico Mountains are of diverse The only slide mass of importance situated on small dikes at some distance from either mountain TORRENTIAL FANS. character and origin, and they are not in all the outer slope of peaks of the Rico dome is that group are genetically related to one or the other of cases easily separable. They add greatly to the on Landslip Mountain, south of Burnett Creek. The steeper gulches which open directly into these eruptive areas. labors of the geologist, since they cover several Other smaller slides were found in the outer zone, the Dolores Valley within the Rico Mountains A very large share of the igneous bodies occur areas in the central part of the region to such but they have usually become so greatly broken have all contributed detritus faster than the river within the bounds of the Rico special district and an extent that it has been found impossible to up and mingled with talus that they are not dis­ has been able to carry it off, so that the debris has were described in some detail in the special report. work out the geology of the solid rocks under­ tinguished on the map as landslide debris. accumulated in low conical banks at the mouths of The Telluride and La Plata folios also contain neath. The surface materials have been repre­ On the various ridges radiating from the Rico the gulches. Such accumulations are commonly descriptions of the same or similar rock types, so sented on the map in five divisions, as follows: dome the landslide phenomena soon become lim­ known as alluvial cones or torrential fans. They that it seems unnecessary to present in this folio (1) Principally landslide debris; (2) principally ited to small avalanche falls from a high cliff or are a characteristic feature of the union of streams more than a general statement as to the petro- talus and wash, with glacial gravels, soils, etc.; (3) pinnacle, such as are common in all areas of rugged of steep grade with those of low declivity, since graphic character of the various types. valley alluvium; (4) torrential fans; (5) calcareous topography. the transporting power of the steeper streams is The igneous rocks are distinguished on the map tufa. The significance of the distribution of the land­ suddenly diminished when their grade is reduced. under five heads: Hornblendie monzonite-por- Outside of the mountainous area the valley allu­ slides is considered in the section on "Geological The side streams at Rico do not at ordinary times phyry, pyroxenic monzonite-porphyry, monzonite, vium is the only one of these formations of suffi­ history." carry any appreciable load of gravel, transpor­ Calico Peak porphyry dikes, Calico Peak porphyry cient importance to be given representation on the tation being confined to times of flood. Heavy TALUS AND WASH. alunitized, and basic dikes. All are considered to map. showers and cloud-bursts sweep any available be of Tertiary age. Extensive areas in the Rico Mountains, espe­ debris into the steep gullies, and these, carried LANDSLIDE DEBRIS. HORNBLENDIC MONZONITE-PORPHYRY. cially adjacent to landslide masses, are covered by down to the main valley, are dropped and the While landslide blocks of any considerable mag­ a varying mantle of surficial rock debris, soil, etc., channel of the stream becomes inclosed by nat­ The intrusive sheets and dikes occurring in the nitude are not strictly to be classed with detrital which has effectually interfered with the mapping ural dikes, so that on becoming choked at any Rico Mountains are nearlvt/ all of one chemical and material of much finer fragments in process of of the underlying solid-rock boundaries. It has time the torrent will take a new course and, mineralogical rock type, but present many minor transportation by ordinary agencies, those of the thus been necessary to represent such areas on the changing from time to time, will finally have textural variations, which were no doubt caused by Rico Mountains are, in fact, very intimately min­ maps. swept through an arc limited by the valley walls slight local differences in the conditions attending gled with talus and wash debris and may be Talus. Accumulations from the wasting of cliffs and varying usually from 90 to 120 degrees. It their consolidation. These modifications do not described most conveniently in this place. In are related in origin to landslides, but are com­ is thus by changing its channel that the stream is obscure the similarity in composition when the the Rico report a special chapter was devoted posed of many small blocks loosened by frost able to build up the fan-shaped heap at its mouth. rocks are studied under the microscope, but some to landslides, giving many details as to their action or by heavy' rains, whereas landslides, At Rico many of the characteristics of torrential masses are too fine grained for-, the unaided eye character and distribution and discussing their though they may eventually become very much fans are beautifully illustrated. An inspection of to recognize their constitution with certainty, and origin. The treatment of these masses in this broken, are at first essentially large masses. Huge the special map will show the extent of the prin­ decomposition often renders the character obscure. folio must be concise and the reader desiring talus heaps are of frequent occurrence in the Rico cipal ones and the different relative positions of The rock of the larger sheets and of many dikes further information is referred to the report. Mountains, and while in many cases their even the stream channels upon the cones; in several is a very distinct porphyry of general light-gray Description of landslide masses. The areas rep­ slopes are covered with vegetation, in other cases cases the contouring indicates the lines of former tone, with an even balance between phenocrysts resented on the map as occupied by landslide they are entirely bare and then plainly show the channels. The typical appearance of -the tor­ and groundmass. The most abundant phenocryst debris exhibit a confused aggregate of rock masses manner in which they are formed. Such masses rential fans is shown in fig. 4, from a photo­ is a plagioclase, determined in some cases to be lal> or blocks which have slipped down the slope from entirely conceal the contacts of the alunite rock of graph of the Aztec fen. radorite, developed in the common stout prismatic some higher position. The blocks are often sev­ Calico Peak. A portion of the surface materials upon the hill­ crystals. Dark-green hornblende in small prisms eral yards, or even a few hundred feet, in diameter. Related to talus are the materials dislodged by side west of Rico may have been formed in the is the only other constant and essential pheno­ When composed of sedimentary rocks it is found avalanches and deposited where their force is spent. same manner as the fans of the lower valley, cryst. Small quartz crystals, almost invariably that dip and strike are discordant and irregular Much of the loose material upon Newman and which they very closely resemble as topographic well rounded by resorption, are sparingly present and it seldom happens that adjacent blocks con­ C. H. C. hills has been brought down in this way. features. These have not, however, been dis­ in a few cases, but can usually be detected in the sist of the same strata, for the amount of slipping The deposits of Papoose Gulch and in the head tinguished from the adjacent surface wash. hand specimen only on close scrutiny with a lens. varies greatly. of Marguerite Draw east of Mount Elliott have The gray and homogeneous-appearing groundmass CALCAREOUS TUFA. In several areas the landslide debris occupies the been considered as connected with former great consists of orthoclase and quartz. whole slope from the crest of a ridge to the stream snow banks. Probably this is, in part at least, Spring deposits. The Rico Mountains are well The most striking variation in texture noticeable bed of the adjoining gulch. In other places the their true origin, but avalanches may have been watered, and even in the driest seasons most of the in these rocks arises from the development of the upper limit is a cliff, a scar formed in part by the also concerned in their formation. gulches contain very considerable streams which plagioclase phenocrysts. These are much more detachment of the landslide blocks. Both sedi­ /Surface wash. In regions where the agents of are fed by springs. The water of the springs is abundant than hornblende and, as a rule, are mentary and igneous masses are involved, even erosion have been as active as at Rico rocks do usually impregnated either with lime or with iron, larger. But they may be nearly uniform in size the massive monzonite stock of Darling Ridge not decay in situ by surface weathering, and probably of rather superficial origin, and locally or present a gradation from the largest to those having been extensively affected. consequently residual soils, such as cover the these ingredients are frequently present in suffi­ scarcely distinguishable by the naked eye. In the The greater part of the landslide action occurred rocks in many low-lying regions, do not accum­ cient amounts to separate from solution and form sheet crossing the Dolores Valley at Montelores, long ago, but the individual blocks of the original ulate. Surface wash is composed almost entirely deposits upon the surface or in the interstices of for example, the plagioclase crystals are uniformly slips have^ been in most cases shattered by their fall of fragments derived from the higher slopes of gravel or other loose surface materials. In some 2 to 3 millimeters in diameter wherever that into minor masses which have slipped farther, and the mountains or from the disintegration of land­ cases the waters, besides their mineral contents, mass was examined. More commonly there are the movement is still in progress in many places. slide masses which, gradually moving toward the are impregnated or accompanied by gases, such numerous crystals of 5 millimeters or more, though Naturally the shattered blocks suffer rapid disin­ valleys under such effective aids to gravity as as sulphureted hydrogen and carbonic acid gas. seldom reaching a size of 1 centimeter, in diameter. tegration through the action of water and frost, snow, rain, and frost, have been spread in varying The generally calcareous nature of the spring The common color of the plagioclase is white, and so that in some areas there are now very few out­ thickness over extensive slopes, hiding the under­ water at Rico is a direct result of the richness of the centers of the larger crystals may be clear and crops to show the character of the material beneath lying formations as completely as the more mass­ the prevailing sedimentary formations of the cen­ glassy. In many places the crystals have become grassy or timbered slopes. The ridge between Bur- ive landslide materials have done. tral region in carbonate of lime, but in most cases clouded by ferritic particles and muscovite. nett and Sulphur gulches exhibits this advanced As in the case of all the surface deposits, the the amount of this substance held in solution is The predominant porphyry of the Rico Moun­ state of disintegration. PL XIX of the Rico report representation of wash on the map is generalized not sufficient to give rise to important deposits of tains, above described, is practically identical with shows the character of the crest of the ridge. and the indicated boundaries are to be taken as tufa. There are, however, several such deposits the principal rock type occurring in precisely the The detailed topography of a landslide slope approximate. The symbol under which talus and which are situated upon the lower slopes in local­ same manner in the Henry, Carriso, El Late, is eminently characteristic. There is no surface wash are included is intended to apply to all areas ities where loose materials cover the solid rock for and La Plata mountains, and in many places drainage system in most cases, because water not referable to either landslides, valley alluvium, some distance above the springs. in Colorado. 7

PYROXENIC MONZONITE-PORPHYRY. In point of time these dikes cut the earlier and but few are fresh enough to allow an accurate dissected by the tributaries of the master stream. common monzonite-porphyry, but have not been determination of their original composition. The From almost any commanding position within the Two long dikes of the Rico quadrangle belong observed in contact with the granular stock rock ferromagnesian silicates greatly predominate, com­ central part of the area the strata may be observed to a variety of monzonite-porphyry different from nor with the basic dikes described below. mon greenish augite being the most constant and to dip in all directions away from the center of the the common form of the Rico Mountains. One of Dikes of this rock in Johnny Bull Gulch have abundant, with variable amounts of olivine, red­ group. these dikes crosses the Dolores River a short dis­ undergone alteration like that of the Calico Peak dish-brown biotite, and brown hornblende. Mag­ Figs. 1, 2, and 5 of the illustration sheet show tance above the mouth of Bear Creek and has been rock, and resemble the latter very closely. netite is present rather sparingly. The feldspathic the attitude of the strata in accord with the dome traced for several miles up the valley of the latter constituents are partly orthoclase, partly plagioclase, structure on the slopes of Dolores, Blackhawk, stream. It appears to belong to the La Plata center CALICO PEAK PORPHYRY, ALUNITIZED. and analcite may have been present in some cases. Sandstone, and Telescope mountains. The gen­ of eruption. The other dike of similar rock crosses The cone of Calico Peak is made up of a light- The habit of the freshest rocks is decidedly eral structure thus exhibited in a large way is the head of Priest Gulch, and from its course colored rock which is either nearly white or stained basaltic, through the abundance and form of found to hold also in detail except in the faulted would seem to be connected with the Rico center. various shades of red and yellow, often in brilliant development of augite and olivine. area, where the dips are in various directions. These two porphyries contain a larger amount hues. The rock has either a marked porphyritic From the comparatively subordinate role ordi­ Were the whole of the deformation expressed by of feldspar than the prevalent porphyry of the structure or is highly brecciated. No contacts narily played by plagioclase, these rocks fall in such quaquaversal dips the structure of the region Rico Mountains, and the dark silicate was prob­ were seen, owing to the extensive talus slopes the group of olivine-bearing augite-vogesites. would be comparatively simple, but this is not the ably wholly augite, for cross sections of the prisms which conceal it on all sides. The alteration is Rocks similar to these occur in the La Plata case; the strata have also been faulted, and in such are octagonal, but no traces of its unaltered sub­ so extreme that it is not certain that all of the Mountains, in the Durango, Engineer Mountain, a manner that the dome effect is increased by the stance have been found. The feldspar phenocrysts rock belongs to a single mass, though it is appar­ and Telluride quadrangles, and, in fact, all over displacement of the faults. To the rule that the are all plagioclase and the felsitic groundmass ently of that character. the country adjacent to the San Juan Mountains, upthrow of faults is toward the inside of the dome mainly alkali feldspar, the two kinds being nearly The rock of the greater part of the peak was as far as it has been carefully explored. There is there are only a few exceptions. equal in amount in the rock as a whole. It is plainly porphyritic and contained many large much more variety among these dikes than is Still another factor in the deformation has been supposed that the groundmass feldspar is ortho- feldspar crystals, and from this fact it is supposed represented at Rico, where one type prevails to an the intrusion of porphyry sheets at various hori­ clase rich in soda, from the scale-like shape of that the rock was originally of the type of mon­ unusual degree. In spite of the variations, these zons. Concerning these it has been noted that the particles and the general resemblance of these zonite-porphyry with large phenocrysts of glassy rocks are apparently connected in origin with the they are more abundant in the upper horizons of rocks to certain porphyries of the La Plata Moun­ orthoclase which has been described above as intrusive sheets and stocks of monzonitic magmas the Dolores formation than in any other part of tains which were called syenite-porphyry in the Calico Peak porphyry and which occurs in fresh described above. They cut all other rocks and are the section, so that their effect has been greater La Plata folio. Quartz occurs but rarely in the form only in the vicinity of Calico Peak in long distinctly the latest igneous masses of the region. on the higher formations now removed from the dikes as represented on the maps. In its present central portion of the mountains than on the groundmass. GEOLOGIC STRUCTURE. condition the rock of the peak contains no dark lower strata which still remain. MONZONITE. silicates; the former feldspar phenocrysts are rep­ As has been stated in the general sketch of the The structure of the area, where it is not obscured The mass of the large stock west of Rico is a resented either by a mass of white kaolin or by a relations of the Rico quadrangle to the surround­ by surface deposits, may be learned from the dis­ gray granular rock containing orthoclase and pla­ granular mass of a nearly colorless mineral, ordi­ ing country, that area lies in the zone where the tribution of the formations, as exhibited on the gioclase in about equal amounts, carrying a little narily too fine grained for recognition. The altered nearly horizontal strata of the Dolores Plateau economic sheet accompanying this folio. On this quartz in most places, and having a variable devel­ groundmass is grayish in tone and may be fine come under the influence of the San Juan conti­ map strike and dip have been indicated by an opment of augite, hornblende, and biotite. The or coarse grained. In some places the rock has nental uplift. The general distribution and atti­ appropriate sign the strike by a line drawn in feldspathic constituents strongly predominate over become largely a porous quartzitic mass. The tude of ,the sedimentary formations which have its direction and the dip by a shorter line at right the ferromagnesian silicates. The rock thus belongs room of the larger feldspar crystals is seldom been described are controlled by that fact until angles to it, indicating the direction toward which in the group intermediate between the syenites and completely filled by the alteration product, which they come within the local domal uplifts of the the strata fall, the amount of deviation from the diorites to which Brogger has given the name mon- usually appears as an aggregate of rude plates, a def­ Rico or La Plata mountains. The geological map horizontal being indicated in degrees. Aside from zonite, from the type locality of Monzoni, near Pre- inite crystal outline being, however, rare. These and the profile sections of the structure-section this the manner in which the formation lines and dazzo, in Tyrol. plates are rough crystals of alunite, the basal plane sheet illustrate these conditions. It will be neces­ the sheets which lie in the stratification cross the The rock is, as a rule, of medium grain, the predominating and being bordered by the low hem- sary to give some further attention to the San contours on the mountain slopes indicates clearly variation in this respect ranging from rather ihedral pyramid commonly developed in this min­ Juan and Rico structures, but the influence the general dip of the rocks. On the east side coarse to fine grain, but not to a texture that is eral. No good crystals of polished faces were found. exerted by the La Plata dome within the Rico of the center the structure is well brought out by strictly aphanitic. With a hand lens nearly all At several places the freshly fractured rock was quadrangle is very small and is clearly expressed the lines bounding the Rico and the upper part of the mineral particles can sometimes be recognized found to exhibit a very distinct yellow color in by section C-C and the geological map. the Hermosa formation, together with the accom­ in the coarser specimens, including apatite, titanite, the porous areas representing feldspar phenocrysts, panying igneous sheets. On the west side these THE SAN JUAN STRUCTURE. and magnetite. The texture is ordinarily typically the color being due to native sulphur in minute horizons are largely hidden, but the distribution of granular, with local tendency to a development in round crystalline particles. The San Juan Mountains are flanked on the the porphyries shows the structure, as do also the which large grains of orthoclase include all other The more massive rock found in many places south, west, and north by sedimentary formations tongues of the La Plata and McElmo formations constituents. This texture is rarely very promi­ consists of a coarse-grained aggregate of irregular dipping away from the central mountainous mass, on the main ridges. nent megascopically, but almost invariably appears rude tablets with kaolin filling the interstices. which, beneath the surface volcanics, consists of A profile section of the dome is exhibited by in some degree under the microscope. Small veins of uniform fine grain also traverse granite, schist, and Algonkian sediments. On section A-A on the structure-section sheet. The The darker modifications are also the finer grained the rock locally, the character of the material the east the relations are not well known, but so profile line passes a little south of the center of the and often owe their shade to the finer particles of the being unrecognizable megascopically. far as the evidence goes it indicates that such dome, in order to avoid the larger faults and thus dark silicates. The two feldspars are distinguish­ Microscopical and chemical study of the Calico strata as escaped erosion prior to the volcanic to'bring out the amount of domal folding the more able in some places through the pinkish color of Peak rocks resulted in the identification of the eruptions dip toward the east, or that in their clearly. It shows the extent to which the forma­ the orthoclase, but this is entirely lacking in many principal substance of the mass as alunite, a absence the surface of the granite upon which tions of Eagle Peak take part in the structure, and areas and the rock has then the appearance of a hydrous sulphate of alumina and the alkalies, they once rested slopes in this direction. The the dome is specially brought out by the band rep­ diorite, the term which would have been applied and shows that kaolin and quartz are the only structure is thus seen to be that of a broad qua- resenting the Rico formation. The ,'flat position of to these masses a few years ago. While this gran­ other minerals of importance present in the spec­ quaversal fold. Its diameter in an east-west direc­ the formations in the Dolores Valley is due to the ular rock is similar to the prevalent porphyry of the imens examined. tion, along a line drawn through Rico and the strike being so nearly parallel to the course of the mountains in chemical constitution, augite is much The alteration of the porphyry of Calico Peak central portion of the Needle Mountains to the section. more abundant in it than hornblende, which is the into a rock consisting largely of alunite can be Piedra River, is upward of 60 miles. The amount The porphyry sheets of Anchor and Expectation characteristic dark silicate of the porphyries. Mon- explained only as the result of the attack of sul­ of arching along this line, estimated from a study mountains are represented as branches of one body zonites very similar to that at Rico occur in large phurous agents, and from the circumstances of of the present dip and strike of the Dakota sand­ to express the fact shown on the north face of the stocks in the Telluride and La Plata quadrangles occurrence there can be no doubt that the action stone and a hypothetical restoration to the domal former summit. It may be that they are distinct and have been fully described in the published is to be attributed to solfataric emanations of the position it occupied before it had been removed on the line of the profile. folios. Rico eruptive center in the period of waning by erosion, is approximately 10,000 feet. ' This An arm of the monzonite stock is shown as igneous activity. is equal to the total thickness of the Paleozoic cutting across the strata on the eastern slope of CALICO PEAK PORPHYRY. The general character of the Calico Peak rock is and Mesozoic formations involved. On the Rico Expectation Mountain, for the reason that a tun­ A rock of unusual character occurs in dike form similar to that of the quartz-alunite rock occurring side this amount of depression of the Dakota nel nearly on the line of the section encounters on the north slope and elsewhere in the vicinity of in the Rosita Hills, Custer County, Colo., described below the apex of the former dome is reached that rock on penetrating the landslide debris of Calico Peak, and in a sheet-like body in Dakota in the Seventeenth Annual Re-port of the United between the two branches of the Dolores, but the shoulder projecting into Sulphur Gulch. sandstone at the head of Priest Gulch. This rock States Geological Survey (pt. 2, 1896, pp. 52-56). slight northwestward dips continue for many In the Rico report another section is given further is a porphyry of most marked appearance, charac­ In the latter case the material was formed by sol­ miles beyond this to the gently warped plateau illustrating the domal structure. terized by large orthoclase phenocrysts in consid­ fataric action on rhyolite, in a small volcanic cen­ region, which, as a structural province, must be Deformation by folding. The profile section erable abundance, some of them exceeding an inch ter, and the alunite made up a much smaller part separated from that of the San Juan Mountains gives a basis on which the amount of the Rico in length. Associated with these prominent crys­ of the rock than at Calico Peak. in a somewhat arbitrary manner on some line of uplift can be approximately realized at a glance, tals are many smaller ones of plagioclase and augite, steeper dips. but a more definite conception may be gained by biotite, or hornblende. Quartz crystals are rare. BASIC DIKE ROCKS. This simple San Juan structure has been oblit­ restoring, in imagination, some particular stratum The groundmass has much plagioclase and little The geological maps represent a number of basic erated locally by the Rico and La Plata domes, to the position it may be assumed to have occupied or no quartz. On the whole it is estimated that dike rocks occurring in various parts of the Rico and the relations of the various structures can best before erosion of the uplifted rocks. the rock is much nearer the stock monzonite in Mountains, and a few others were observed in be considered after the local centers have been The most comprehensive view of the Rico struc­ composition than would be inferred at first sight. other parts of the quadrangle. These dikes are described. ture is to be obtained from a consideration of the In the development of green augite and brown bio­ seldom more than a few feet in width and their La Plata sandstone. Before the dome was dis­ THE RICO DOME. tite there is a further link connecting this peculiar length is apparently not great, though none of sected the base of this formation over the summit type with the monzonite. None was sufficiently them has been accurately traced to its end. They Elements of the domal structure. The structure was at least 4400 feet above the lowest rocks now fresh for analysis, but it is probable that the rock are often irregular in course and are effectually of the Rico dome has been well exposed by ero­ exposed at Rico on the line of section A-A, and is somewhat richer in alkali feldspar than the concealed by slight coverings of debris. sion. Directly through its center the Dolores probably somewhat higher, since the above figure monzonite, and hence approaches a quartz-bearing The basic dike rocks of the Rico Mountains are River has cut its course, dividing the mountains is estimated from the thickness of the Hermosa, syenite-porphyry. closely related, as far as can now be ascertained, into eastern and western groups, which are further Rico, Cutler, and Dolores formations and the Rico. 8

Newman Hill porphyry sheet, and takes no account can not be determined, but it would seem more It is evident that all faults obeying the above a northwesterly direction for several miles from of other probable intrusive sheets. The base of likely that they did not exist from the fact that rule have served to increase the uplift near the Blackhawk Peak. It crosses several other faults the La Plata on this estimate must have been at no central crosscutting bodies of porphyry which center of the dome. But it must be assumed that and as the dislocation south of Silver Creek is about the altitude of 13,200 feet 500 feet or more could have fed them are known; however this most of them die out gradually as they pass upward, much greater than to the north it seems plain above the highest peak of the Rico district. It is may have been, the porphyries which now remain and it may be questioned whether any of the dislo­ that much of the faulting on the Blackhawk has plain that some of this elevation may be due to reach locally an aggregate thickness of perhaps cations of less than 300 feet in amount cut through been diverted to the east-west fissures, or vice faulting, but it will be seen from the map that the 600 or 700 feet, and by this amount must have the Mancos shale, assumed to have overlain the versa. The relative ages of the various faults uplift by dislocation is mainly north of the line augmented the arching of the La Plata and higher Dakota sandstone at the time of uplift. The concerned could not be established. of section A-A. formations. Blackhawk and Nellie Bly faults are the impor­ The upthrow on the Blackhawk fault is on the The amount of local doming which is thus indi­ The formations above the La Plata doubtless tant ones following this rule, and their effect at southwest or inner side of the dome. It amounts cated may be seen by comparing the position of suffered still more deformation from injected por­ the horizon of the La Plata sandstone must have to nearly 800 feet in the vicinity of the Blackhawk the La Plata in the vicinity. About 5 miles north phyry bodies, but large masses of igneous rock are been still measurable by hundreds of feet. But if mine and diminishes to the southeast. On the of Rico the base of the formation crosses the Dolores at present to be seen only in the cap of Elliott the lateral extent of these faults be compared with northwest side of Silver Creek, beyond the Nellie River at an elevation of 9300 feet, showing a fall Mountain and in the Flattop laccolith to the the diameter of the dome, as represented in the Bly fault, the throw is only 85 or 90 feet. Its from the restored position over Rico of 3900 feet, northeast, on the line between the Telluride and section, it will be realized that the modification course is obscured by landslide material on C. H. or nearly 800 feet per mile, independent of the Engineer Mountain quadrangles. The former is of the dome by dislocations following the rule C. Hill, but there are several veins in the strike, influence of the porphyry sheet. Beyond this immediately above the La Plata sandstone, while stated above was not great and was confined to and the small faults crossing the Dolores River at outcrop the northward dips are continued for the latter lies on top of the Dakota and is capped the central portion. Burns are believed to belong to it. only a short distance, so that here we have the by the shale of the lower Mancos formation. The faults bounding the Algonkian schists and The Nellie Bly fault is an important east-west full measure of the local deformation in this direc­ The mass of monzonite which cuts through the quartzites differ from most of the other faults in fracture crossing the southwest spur of Telescope tion. In other directions the fall per mile is less, sedimentary rocks on the west side of the dome has that they limit small blocks pushed up in the heart Mountain known as Nigger Baby Hill. On the but the dips are continued for a greater distance. caused a considerable amount of metamorphism in of the dome. So far as the fractures are known south of this fault the formations have been Thus from the northwest around to the south the the adjacent rocks, but there is no evidence that they are nearly vertical. The amount of upthrust upthrown about 75 feet on the crest of the average deformation or dip slope varies from 400 to the strata have been turned up around its periph­ is indeterminable, for no remnants of the Paleozoic hill. Tilting of the block south of the fault 600 feet per mile for a distance of from 5 to 8 miles, ery. Its influence in modifying the dome must sediments lie on the Algonkian blocks. These old causes rapid decrease in the displacement to the beyond which it gradually lessens as the distance be compared to that of the block faults near Silver rocks, then, represent plugs punched up through the east. To the west the fault is obscured, but seems from the center increases. The diminished dips Creek, but there is no basis for measuring it. strata and porphyry sheets without much, if any, to be represented by the Aztec vein, west of the continue for many miles toward the west into the Deformation by faulting. In the process of visible disturbance of the adjoining beds at the Dolores, where a large but unmeasurable dislocation plateau region. Toward the south they fall at the uplift or deformation by which the Rico dome horizons seen. These blocks are comparable to has occurred. rate of about 500 feet per mile, and are met by was produced the strata were no doubt fissured the monzonite stock in this respect, but from Nearly parallel to the Nellie Bly occurs the the structure of the La Plata Mountains at a dis­ and fractured to a considerable extent. At the their small size the disturbance in the dome Last Chance fault, crossing the lower slopes of tance of about 7 miles. Also to the southeast, present time a great many< old fissures may be structure above them must have been much less Niggero<_/ Baby*j Hill. Formingo the northern border though the La Plata is missing, the lower strata detected. Most of them are filled by vein matter, than that above the stock, assuming that the stock of the Algonki'an quartzite and schist area, this fall at a rate of 300 feet per mile for about 8 miles, partly ore bearing, partly barren; and in some magma did not reach to the surface. These fault fault is clearly one of the most important, struc­ where the local structure is completely neutralized cases the evidence is clear that the veins are blocks may perhaps be regarded as quite analogous turally, in the region, but it is obscured by surface by the contrary dips away from the Needle Moun­ lines of faulting. The displacement of the faults to the stock eruption, in that both seem to be com­ wash in many places and its western portion, in tains. Four miles to the east the base of the La varies from more than a thousand feet to that paratively recent manifestations of an upward force particular, is concealed. Its displacement must he Plata is 1700 feet lower than over the top of the which is scarcely measurable. suddenly exerted, producing vertical fissures rather more than 1000 feet on the north side of the quartz­ dome and dips to the northeast, in which direction The supposition that the faults of the Rico than folding. If the fault blocks represented by ites in Silver Creek and it may be even more to it continues to fall for several miles. Mountains are fractures contemporaneous with the Algonkian rocks of existing exposures extended the west, although apparently divided among three The attitude which the formations would now the domal folding, merely expressing relief upward for some distance with nearly vertical walls, or four fissures in a zone crossing the Dolores. exhibit along the line of the Dolores Valley were of great tension by rupture of the rocks instead their disturbance of the dome structure may have The extension of the Last Chance fault to the it not for the Rico dome may be plainly seen from of further bending, seems in itself natural, but is reached to the upper shale series of the Cretaceous, Blackhawk is not seen on the surface, but seems the areal maps of the Telluride and Rico quad­ opposed by the considerations connected with the but if they wedged out upward the faulting must a natural assumption, since it provides an expla­ rangles. In the southwest corner of the former, intrusion of the igneous rocks. From the gener­ have been resolved into local tilting of adjacent nation for the sudden increase in dislocation on only 6 miles northeast of Rico, the Dolores flows ally accepted theories in regard to the relations of beds. the latter fissure south of Silver Creek. in a canyon whose rim rock is the Dakota (Creta­ igneous intrusion of laccolithic character to domal The great fault of Telescope Mountain is so The Last Chance fault is regarded as one of sev­ ceous) sandstone in almost horizontal position the uplift it is necessary to assume that the porphyry hypothetical as to its course and in its relations eral fractures bounding the small masses of Algon­ floor of the Dolores Plateau over large areas. As sheets of Rico are contemporaneous with or later to various other possible fissures that little need kian and early Paleozoic rocks which have been the valley of the Dolores leaves the Telluride quad­ than the principal uplift. But these igneous bodies be said here as to its effect on the Rico dome. upthrust near the center of the Rico Mountains. rangle the formations rise rapidly under the influ­ are cut by all faults observed to come in contact If a single fault, it must have materially modified Other named faults of this complicated area are ence of the Rico uplift, but at the mouth of Bear with th'em. No single instance was found of a the symmetry of the dome at the horizon of the La the Smelter, South Park, and Silver Creek faults, Creek, 12 miles below Rico, the stream is again porphyry dike ascending on a fault fissure. Fur­ Plata sandstone. in addition to which various transverse fractures coursing in a typical canyon of the plateau coun­ ther, the monzonite stock is traversed by many Description of faults. While the reader must divide the wedge-shaped blocks into smaller ones. try, the Dakota sandstone reappearing as the floor quartz veins, some of them bearing sulphide ores. be referred to the detailed report on the Rico On the southern side of the central area are two of the mesas or plateau remnants on either side. It is therefore necessary to disconnect the fault Mountains for full descriptions of the many faults notable faults named after Deadwood and Spruce But for the Rico uplift the Dolores would be flow­ phenomena of this region from the primary domal observed, a brief statement will be given here con­ gulches, which they cross. These fractures have a ing in a canyon like those referred to, along the uplift, although it is of course possible, or even cerning each of the more important ones, with general northwest-southeast trend and the upthrow stretch where the Rico Mountains now appear. probable, that some unidentifiable portion of the special reference to their structural significance. is in each case on the northeast, amounting to a East of the valley there would be remnants of folding accompanied the faulting. The fault producing the greatest dislocation has maximum of 250 feet on the Deadwood and 400 the Dakota forming sloping mesas like those com­ In character the faults vary from clean-cut fis­ been named the Telescope Mountain fault from its feet on the Spruce Gulch fault. They die out to mon in the Durango quadrangle. sures to zones of sheeting or brecciation many feet situation on the southeastern slope of that peak. the southeast and pass into the landslide area on Deformation by intrusion. The additional defor­ in width. Extreme breeciation is well shown in Its actual location is not positively known and yet the west side of the Dolores. mation due to sheets and laccoliths can not be esti­ various portions of the Blackhawk fault; in the the faulting movement which has evidently taken Besides the major faults which have been men­ mated except in a very crude way from the bodies veins of the Calumet, Zulu Chief, and several place on the general line indicated amounts to tioned there are many minor ones, some of which which have escaped erosion. In the central part tunnels near Rico; in the "great vein" of the nearly 2000 feet. The special map shows the are shown by the map, while there are a multitude of the area the Newman Hill porphyry, the influ­ northern part of C. H. C. Hill; and in the relations of formations demanding the assump­ of still smaller fractures of no structural impor­ ence of which has already been mentioned in the boundary zones of the Algonkian quartzites in tion of this great fault. The most striking facts tance. It is a fact, discussed by Mr. Ransome preceding section, has a thickness of 500 feet, Silver Creek. Sheeting is also seen in most of to be explained are the apparent great thickness under "Economic geology," that the numerous proved by the exposures and the drill hole sunk these localities. of the Cutler formation on the southeast face lodes of ore-bearing fissures of the region are in the Skeptical shaft. Another important sheet The distribution of the main faults is shown by of Telescope Mountain, and the evident disloca­ rarely coincident with structural faults of impor­ near the top of the Hermosa formation has a max­ the map. The greater dislocations are near the tion of beds, made plain by the distribution of tance, but seem to represent later fractures pro­ imum thickness of 250 feet where it crosses the center of the dome, but a large number of lesser the Rico formation, on the western slope. The duced in adjustment of the rocks after the larger river above Montelores, south of the area of the fractures occur in the circle of peaks about it. A Cutler appears to have almost doubled its nor­ dislocations had occurred. Some of this adjust­ special map, but thins out as it rises with the glance at the map will show that there is no pro­ mal thickness of about 1800 feet and that fact ment has taken place parallel to the bedding dome, until it finally disappears entirely in Dead- nounced systematic arrangement of the faults. In shows about where the fault must be if the dis­ planes of the sedimentary rocks and produced wood Gulch. Its distribution is such that it can certain localities the principal fissures may have a location is not distributed on several fault planes. what were called "bedding faults" in the Rico hardly be supposed to have been present in the common trend, with minor zones intersecting at A visible fault occurs at the right place to repre­ report. As Mr. Ransome points out, some of the central part of the dome. Other porphyries in oblique angles. This is illustrated in the south­ sent the one required and has been assumed to be so-called "contact" ore deposits characteristic of this formation, so far as they are exhibited in eastern part of the region, in Silver Gulch, in the in fact the fissure on which the great displacement important mines are intimately related to these surface exposures, are of minor importance and Dolores Valley near Burns, and in Newman Hill. has occurred. bedding faults. This movement may have begun could certainly in no single section aggregate more As a rule the faults are nearly vertical, but dip at The landslide area of C. H. C. Hill and Tele­ with the initial domal uplift, but proof of such than 200 feet. variable steep angles in some cases. scope Mountain conceals the western extension of origin is not now obtainable. The Dolores and Cutler formations seem to have The displacement of the faults in relation to the the great fault, although certain isolated exposures presented especially favorable conditions for sheet dome structure is subject to a simple rule for nearly shown by the map indicate nearly the line it must RELATION OF DOMES TO SAN JUAN STRUCTURE. and laccolithic intrusions; and these are localized, all of those found at some distance from the center. occupy. The Rico and La Plata secondary domes are with respect to the dome, on the eastern and When even approximately parallel to the strike of The throw of the Telescope Mountain fault is up probably genetically related to the broader San western sides. The portion of the section in the strata the upthrow is on the inside, or toward on the north or outer side of the Rico dome, mak­ Juan structure, though how close the relation­ which they occur does not contain similar intru­ the center of the dome. The only important ing it the most important exception to the general ship may have been can not be determined, since sions to the north or to the south, where" cut by exception to this rule is the great fault of Tele­ rule of upthrow on the inner side. the interconnection of igneous intrusion and con­ the Dolores Valley. Whether or not there were scope Mountain, which must be classed with the The Blackhawk fault, named from a mine where tinental and orogenic uplift of the Rocky Moun­ like intrusions over the central part of the dome block faults of Silver Creek. the displacement seems at its maximum, extends in tain type is not at present understood. In both 9

cases the preservation of the mountains as regions slopes of Dolores Mountain seen in the central the summit of Telescope Mountain. The entire springs which give off strong odors of sulphureted of high topographic relief is due to the presence part of the view, and by many lines in the higher area represented on the map as landslide terri­ hydrogen, near the head of Stoner Creek and on of igneous rocks which have been more resistant summits, due to stratification or to intercalated tory exhibits the characteristic topographic detail. Johnny Bull Creek not far from Calico Peak. to erosion than the sediments would have been sheets of porphyry. At the upper limit and on the southern border of The appearance of Calico Peak, with its talus alone. The intrusions are in the form of stocks, The higher portions of all these peaks consist of the landslide tract seen in the view there is evi­ heaps, less the vivid colors, is shown in PL VII of dikes, and sheets. To the latter, which may in the red Cutler or Dolores strata with sharply con­ dence of recent movement. In the Rico report the Rico report. some cases have sufficient thickness to be of the trasting grayish porphyries. Excellent sections of may be found a picture of a tree split in two by Anchor and Expectation mountains. Between type known as laccoliths, a certain amount of the parts of the Cutler are to be found in several movement now in progress, and even more con­ the heads of Horse and Burnett gulches are two observed deformation of the stratified rocks is cer­ places, one on the slope of Whitecap Mountain vincing evidence is exhibited in the crushed or high peaks, Anchor and Expectation mountains, tainly due. In the La Plata Mountains the mass being shown in the figure. The presence of a twisted timbers of mine workings throughout the in which the crosscutting and branching of intru­ of intruded matter of this nature shown in the thin limestone conglomerate of the fossiliferous tract. sive porphyry sheets is exemplified in many places. horizons exposed is comparable to the deforma­ section of the Dolores very near the summit Many small landslides have occurred on the Indeed, so numerous are the visible forkings of the tion which they have suffered over and above of Blackhawk Peak shows the projected horizon northern slope of Telescope Mountain, but the porphyry masses here that the conclusion seems by that affecting the lower formations, which are of the La Plata sandstone to be but a few hun­ blocks have broken up thoroughly in their fall no means far fetched that all the more or less irreg­ covered and therefore beyond observation; so that dred feet above that mountain. and can scarcely be distinguished from ordinary ular masses shown by the map in the northwest- if the porphyry included in the hidden strata The influence of faulting is not self-evident in avalanche debris. southeast zone from Johnny Bull Creek to beyond should bear the same proportion to the sedimen­ this illustration, yet the magnitude of the dis­ Landslip Mountain belong to one intrusion. The WESTERN SUMMITS. tary rocks as in the observed section, the doming placement on the Blackhawk fault is really shown, rocks are visibly different only in minor details of should be accounted for without additional uplift. for the prominent limestone band of the Dolores Mountains north of Horse Gulch. The domal texture. At Rico the structure and make-up of the dome Mountain slope is dropped on that fault to a level structure of the Rico Mountains, causing sedimen­ Peaks southwest of Burnett Gulch. The south­ is much better exhibited, and though the theory too low to permit its appearing within the field of tary beds to dip away from the center, is well western summits of the Rico group exhibit the that the observed structure might be due to a huge this view on the farther side of Allyn Gulch. shown in the high ridge leading from Sandstone Cutler and Dolores red beds in their normal posi­ laccolith lying between the Algonkian and Paleo­ The faults of this area are clearly shown in Mountain through Elliott Mountain and north­ tion dipping away from the center of the dome. zoic rocks was at one time entertained as a work­ many places by their dislocation of porphyry ward across the quadrangle line. The general On the ridge leading south from Storm Peak the ing hypothesis, it is now known that such a mass sheets, but the grassy or timbered slopes seen in attitude of the strata on this line is represented La Plata and McElmo formations are seen in of igneous rock does not exist, and that the amount fig. 1 often hinder a connected tracing out of in the view of Sandstone Mountain and the next typical development. The porphyry bodies in of deformation which the uppermost strata of the some of them. The splitting of the Blackhawk higher point on this ridge, forming fig. 2 of the the red beds have been referred to as probably region underwent was several times in excess of the fault and the gradual decrease of dislocation are illustration sheet. The Jurassic and Dakota (Cre­ connected with those of Anchor Mountain. amount of igneous material which was intruded plainly visible on the slopes of Blackhawk Peak. taceous) formations on the divides leading outward The most interesting local feature of this section into the strata below them; that is, the formation It may be seen from fig. 1 how well the occur­ from Elliott, Sockrider, and Johnny Bull moun­ is the landslide mass on the south slope of Land­ of the Rico dome is mainly due to a central uplift­ rence of intrusive porphyry masses is exhibited tains exhibit the same structure. slip Mountain. This occurrence illustrates very ing force, apart from any actual intrusions of on Whitecap Mountain and the narrow divide Elliott Mountain is conspicuous in contrast to well the various phases in the history of a land­ liquid rock material. That such a force was also at the head of Deadwood Gulch. There are other peaks of the group by reason of the light- slide area, from the newly fallen blocks seen here active in the La Plata uplift may well be believed, numerous other points at which these relations colored La Plata sandstone, which forms cliffs below adjacent to the summit of the mountain, through for there, as at Rico, the thickest laccoliths or sills can be seen to advantage. One of these is on the capping mass of porphyry. the older, partially disintegrated masses of the occupy a zone, so far as the rocks now remaining the high northern spur of Blackhawk Peak, The few faults of this area illustrate the lack middle slopes, to the forest-covered debris near are able to show, at a distance from the center of where a large sheet makes cliffs several hundred of system in these fractures, and none of them the stream below, where sinks and trenches still the dome, and it is on these peripheral intru­ feet high, shown in fig. 1. This mass extends produces results very marked in the present demonstrate the existence of slide masses. sions that the estimate of the sufficiency of the around the head of Silver Creek, covering a large topography. Darling Ridge. Between Horse and Sulphur porphyries to produce the observed structure was surface, as shown in part by the special map. The porphyries of this district illustrate several gulches is a high tract cut almost in two by the based. The crosscutting relations of these porphyries, intrusive relations of interest. The laccolithic form head of Iron Draw. Here occurs the large stock as they pass more or less obliquely < from one is fairly well shown in the mass of Elliott Moun­ of granular rock, quartz-monzonite, which appears THE RICO MOUNTAINS. horizon to another, are very plainly indicated. tain, the remnant of which is over 600 feet thick to have been one of the later intrusions, if not the It has already been pointed out that there are In the Rico report may be found several views beneath the summit, while the porphyry is not last, of the Rico center. The contacts of this stock three natural topographical and geological divi­ which will assist the reader in comprehending the present across the saddle north of the mountain. are not well shown at any point, mainly on account sions of the Rico quadrangle, viz, the Rico Moun­ character of this portion of the mountains. One There are many sheets and small dikes and the of the shattered condition of the monzonite mass, tains, the Dolores Plateau, and the main Dolores of these views presents the country lying east of forking or crosscutting of some of these bodies is which has resulted in talus or loose broken-rock Valley. The formations of the quadrangle and Blackhawk Peak. clearly exhibited. piles, where larger landslides have not taken place. the general geologic structure determining their Telescope Mountain and vicinity. The north­ In one of the branches of Horse Gulch is very The metamorphosed condition of the sedimentary attitude and distribution having been discussed, eastern quadrant of the Rico Mountains is com­ imperfectly exposed the rock of what may be one rocks on either side of the monzonite on Darling it does not seem necessary to give further descrip­ paratively simple in its geologic structure and of the principal centers of eruption. The por­ Ridge is everywhere evidence of the proximity of tive details concerning the plateau and valley possesses but one mountain summit of promi­ phyry is here seen to cut across the sediments, the contact.

areas, the geology of which is very simple. But nence Telescope Mountain. The Cutler red beds sending off numerous dikes and thin sheets. It Althougho the monzonite bodvt/ is large< j and such the Rico Mountains are so complex in structure, here assume almost exclusive surface importance, is full of apparent inclusions and is penetrated >by massive rocks usually cause rugged topography, igneous phenomena, and other respects that a through their duplication by the Telescope Moun­ many angular arms of the wall rock. Unfortu­ such is not here the case. This fact is probably resume of their prominent features is desirable. tain fault. They are overlain by the Dolores nately there has been great decomposition here due to the thoroughly shattered condition of the The Rico Mountains have been carved out of formation at a short distance east of the area and in addition the extremely complex relations stock, leading to rapid destruction of prominences the domal uplift of several elements, already covered by the special map. The high divide are obscured to a large extent by soil, forest growth, by frost. The large number of small knobs and described. Naturally the peaks exhibit most running irregularly east from Telescope Moun­ and wash, so that the representation of the map is knolls, often with pinnacled spurs or summits, clearly the formations taking part in the dome tain, which forms the watershed between the in some degree diagrammatic. In spite of these situated on the north side of Darling Ridge, are and their structure, while the deep dissection by head of the Dolores River and Hermosa Creek, conditions, this locality is an excellent one in plainly separated by zones of fracture and brec- the Dolores and its branches displays the fea­ a branch of the Animas River, has many high which to study complex intrusive relations. ciation and are themselves crumbling to pieces tures of the core of the uplift. The exhibition points above timber line in which the several Eagle Peak. The westernmost of the Rico under frost action. The assignment of these of the, latter geologic detail is, however, greatly formations may be studied. Mountains exceeding 12,000 feet in elevation is knolls to the landslide area will be discussed in obscured by the superficial landslide materials, The Rico and upper Hermosa beds form a Eagle Peak. It lies beyond the line limiting the the next section of the text. which assume a position of much local importance. scarp facing the landslide area of C. H. C. Hill distribution of visible porphyry masses and there­ THE INNER SLOPES OF THE MOUNTAINS. on the northwest ridge from Telescope Mountain fore presents in least distorted form the simple THE CIECLE OF PEAKS. as shown in fig. 5. The general structure of the structural relations of the sedimentary rocks tak­ From the preceding description of the domal The main summits of the Rico group arrange mountain may also be seen in this view from expo­ ing part in the domal structure. Passing from the structure of the Rico Mountains and of the circle themselves in harmony with the domal structure sures near the summit. peak along the ridge to the west one has excellent of prominent peaks it will be plain to the reader in a circular zone. They are remarkably uniform The minor faults of this region are conspicuous opportunity to examine sections of the La Plata, that the outer slopes of the mountain group exhibit in height, a dozen peaks exceeding 12,000 feet in through dislocation of porphyry sheets, while the McElmo, and Dakota formations and to observe simple structural relations of sedimentary forma­ elevation, while the highest, Blackhawk, is but largest fault of the mountains is scarcely identifi­ the change from the domal structure to that of the tions and that igneous masses are few. It does not 12,677 feet, or 4000 feet above the river at Rico. able on the ground. Dolores Plateau. seem necessary, therefore, to give further descrip­ The Dolores River divides the group into two The porphyry intrusions of this section of the Calico Peak. The variegated coloring exhibited tions of the peripheral portion of the Rico dome. nearly equal crescents. Rico Mountains are less in number and magnitude by the decomposed rock of this summit at the head In the heart of the mountains, where the structural than in any other part, being limited to a few thin of Horse Gulch has led to the current name Calico complexities are great, where several formations EASTERN SUMMITS. sheets and dikes in the upper half of the Dolores Peak. The original porphyry of this peak has not occurring elsewhere in the quadrangle have Mountains south of /Silver Creek. While nearly formation. It is worthy of note, however, that a been almost completely altered to a mass of alunite, been revealed by the deep erosion of the Dolores all the peaks of the Rico group exhibit many large laccolith occurs just above the Dakota sand­ kaolin, and quartz, impregnated with pyrite, the and its tributaries, and where many intrusive characteristic features of the local geology, those stone about one-half mile beyond the northeast cor­ oxidation of which has produced the vivid red bodies appear, the case is quite different. Here, lying to the south of Silver Creek are most note­ ner of the area covered by the special map, on the and yellow colors now so striking. Apparently however, the phenomena of local interest are so worthy, because they show not only the domal farther side of Barlow Creek. This mass is the the rock occurs as a small stock, although its numerous that the reader must be referred to the structure, but the effects of faulting and igneous Flattop laiccolith, a portion of which is situated in contacts are concealed by talus or slide. It is Rico report for the greater part of the detail; the intrusion, and the sedimentary section is more the Telluride quadrangle. It is not clear that this supposed that the rock was similar to the por­ present descriptions will be confined to certain of completely displayed than elsewhere, on account large intrusion has actual genetic connection with phyry of large orthoclase phenocrysts, of which the larger features of importance. In fact, it is of the comparatively insignificant development ot the Rico center, as will be explained in the discus­ a long dike crosses the slope of Johnny Bull not the fundamental relations of the formations, landslide masses. sion of the intrusions under "Geological history." Mountain, and which occurs only in this vicinity. but rather the way in which these relations have Fig. 1 illustrates many features of these peaks The landslide phenomena of Telescope Moun­ The formation of alunite is referable to sul­ been obscured, which will receive most attention. as seen from the west side of the Dolores, looking tain proper are so clearly exhibited in fig. 5 as to phurous emanations, either directly by gases or LANDSLIDE AREAS OF HORSE GULCH. nearly due east. The prevalent dip to the south­ require little further comment. The actual head indirectly through waters which have absorbed east is particularly brought out by certain massive of the slide area is on the ridge leading southwest gases. That such activity has been specially The map and figs. 3, 5, and 6 of the illustration limestones of the upper Hermosa, which cross the to Nigger Baby Hill and less than 500 feet below marked in this vicinity is shown by existing sheet show how completely the normal structure Rico. of the Horse Gulch slopes is concealed by land­ "blowout," situated on the southern slope between 2400 feet below. In its present condition this area shown by the special topographic map, the river slide debris. This is all the more striking when 10,000 and 10,500 feet, and north of the monzo­ affords few localities where landslide phenomena for some distance above Burns flows over a very compared with the cliff exposures of Sandstone nite pinnacles of Darling Ridge. The map shows are clearly exhibited, but on comparison with flat, broad bottom. Immediately below Burns the Mountain at the mouth of the gulch. closely spaced contours in a drainage channel at other areas the landslide evidence is still most stream passes into a little gorge, bounded on the Slope of Darling Ridge. The formations of the this point and a curving ravine above it, heading convincing, and the ridge is of much interest as western side by cliffs of limestone and sandstone crest of the ridge are obviously not derived by between the two monzonite areas of the ridge crest. illustrating an advanced stage in the history of and on the east by a steep bank made up of lime­ sliding from any other source, but they are in In fact, this ravine heads on the flat top of the landslide areas, when the ordinary agencies of stone, sandstone, and conglomerate in a wholly many places so shattered by important fractures ridge, and is but one of several very marked land­ degradation have nearly completed their work of confused mass of coarse slide. No outcrops of running in all directions and the blocks bounded slide trenches in that neighborhood. effacing the scars caused by the successive slips, rock in place occur on the eastern side along by these fractures are so plainly dislocated super­ Western limit of the landslides. The western leaving in the smooth slopes little evidence of the the face of this bench, and the railroad cutting ficially that the whole mass is considered as broken limit of the continuous landslide area on the confusion existing beneath. reveals most clearly the chaotic mingling of and not strictly in place. The best illustration of southern slope of Horse Gulch is rather indefi­ The upper limit of this area is a rather sharp various rocks. In PI. XVI of the Rico report the shattering is in the massive stock of monzonite nite, but lies on the east side of the south fork. line crossing the crest of the ridge almost from is an illustration of the abrupt ending of the flat opposite the head of Iron Gulch. By a glance at It is here obscured by the presence of surficial north to south at about 11,000 feet. This line at Burns against the bench of landslide material. the map it will be seen that there are here a num­ materials of later origin, part of which are derived is a well-marked trench of varying depth. On It seems necessary to assume that the present ber of sharp pinnacles and knolls, to which one or from the head of the south fork, while other por­ other sides the landslide has no close definable alluvial flat at Burns is due to the damming back two contours have been given. But a map of this tions are the result of disintegration of the land­ boundaries. The southwest slope of the ridge is of the river by the slide, and it is probable that scale fails to show the number of these knobs and slide blocks. smooth and rounded in features, entirely covered before the slide the grade of the river was very the hollows, curving ravines, and irregular depres­ The force by which the rocks of Darling Ridge by grass or timber growth, and contrasts very even from McJunkin Creek to Silver Creek. sions between them, belonging to no drainage sys­ were so shattered, producing the fissures which markedly with the opposite side of Burnett Gulch, Recent slipping in C. H. C. Hill. Evidence tem. The rock of the knobs is often fresh, but limit the main landslide blocks, was also exerted with its prominent cliffs of stratified rocks and that motion is still in progress in the surface much shattered, and the hollows between are in less degree to the west, and landslides of small porphyries. There are no outcrops of rock in materials of C. H. C. Hill is abundant in the rounded by the gravel of disintegration washed size have taken place in the angle between the place except at the head of the ridge and very prospect tunnels and shafts of various localities, into them. This topographic detail, though on south and west forks of Horse Creek, on the end near the bed of Burnett Creek, nor are there the through the crushing or the twisting of timbers. the top of the ridge, is similar to that on the of the ridge from Anchor Peak. The chief evi­ usual broken ledges characteristic of landslide Further proof and illustration of the character of landslide slopes. Below these pinnacles on the dence of this action is in more or less distinct blocks. Instead of this, the few exposures where this movement is seen in a crevice now gradually slope to Horse Creek are some other knobs of trenches of general east-west direction, below which the character of the underlying materials can be opening at the upper end of C. H. C. Hill. This monzonite, and the surface is covered by talus the strata are broken up and disturbed in strike seen and the scattered prospect tunnels reveal crevice extends from an altitude of about 10,400 and landslide heaps nearly all the way to the and dip. detrital matter of the texture of ordinary wash feet to 11,000 feet. It is most clearly shown at creek bed. North side of Horse Gulch. The landslide area or slide rock. No prospect tunnel seen has pen­ about 10,550 feet, below the trail leading from Eastward from the principal area of monzonite, on the north slope of Horse Gulch is one of the etrated to solid rock. C. H. C. Hill to the Uncle Ned saddle in the ridge along the crest of the ridge, the primary bound­ most clearly defined of the region and has very The physiographic detail of this slope is, how­ from Telescope Mountain, and on the north slope aries of the rock formations are more or less clearly characteristic details of topography. ever, most suggestive of landslides, especially when of the little spur indicated by the 10,650-foot and exposed, but from the crest, or near it, down to The greater part of this slide area is now covered seen from Landslip Mountain. There are many 10,700-foot contours on the map. The direction Horse Creek the surface is a jumble of landslide with grass or an aspen growth, and the direct evi­ projecting knolls and local benches, irregular trans­ of the crevice is here nearly east-west, curving to blocks, large or small, intact or in process of disso­ dence as to the character or attitude of the forma­ verse depressions belonging to no drainage system, the southeast in the upper part of its course. lution, and geologic boundaries showing the origi­ tions beneath is found in local outcrops, small slides and general lack of persistent drainage channels. "Where most distinct this fissure occurs on a nal relations can not be traced. of recent date within the main area, the prospect The most distinct evidences of landslides occur northerly slope which is rather thickly wooded, The general topography of this ridge is seen in holes, or the topographic details found by obser­ on or near the crest of the ridge leading from and several trees on its line have been split from fig. 3. The main feature is the great number of vation to be characteristic of landslide surfaces. Expectation Mountain. For several hundred feet the roots up to 2 or 3 feet above the ground, in trenches, most frequently parallel to the contours, Along the landslide bank of the ravine coming below the upper limit of the area the broad top of the manner shown in PL XVIII of the Rico or nearly so, yet often running diagonally across down to Horse Creek just above the Puzzle mine, the ridge is characterized by rounded knolls with report. A stump of one tree cut off at about 2 the slope. They are as a rule not persistent for on the eastern border of the area and below the flat or shallow depressions between them. More feet above the ground has been split open since long distances, being cut off by some other trench. level of 10,050 feet, the loose materials have at or less distinct ledge outcrops of sandstone, shale, the tree was felled and the parts are now seen A few of these lines are ravines of importance, several places been washed away, revealing ledge or porphyry are common on these knolls, but the about 5 feet apart. The tree was probably cut shown by the maps, and at several places they run outcrops of greenish Hermosa. sandstone. These greatest irregularity of dip and strike is found, about 1894. up to or cross the crest of the ridge. exposures belong to different blocks, some of them and the most prominent beds are clearly not con­ It was stated in describing the faults of this Outside of the trenches are mounds, knobs, fur­ 50 feet in visible length, the strike and dip chang­ tinuous. The dips observed in the mounds and district that the course of the Blackhawk fault row-like ridges, or benches, and in these are not ing abruptly from block to block, and never cor­ knolls shown in this view are quite abnormal in seemed to be indicated by the Pigeon vein, or the infrequent ledge outcrops which by various dips responding to the normal structure found on the most cases, being steep angles either down the "big fissure," and fig. 5 shows the prominent and strikes and the shattered conditions of the east side of the ravine. Some of the blocks show ridge or to the east. bench or depression marking the course of this rocks add to the evidence of the landslide action. a nearly vertical dip, while in others the beds dip No geologic boundaries can be traced across this fault in the midst of the landslide area, From No regular drainage channel exists on the south 40° or more, usually down the slope. obscure area. From the known structure of adja­ various other evidences of the mines it is plain that side of Horse Gulch between the river and the This area serves to illustrate the manner in cent areas it is plain that the vmassive limestones of the sliding movement has not greatly dislocated ravine opposite Sinbad Hill. which the complex of a slide area gradually dis­ the Carboniferous, the Montelores porphyry sheet, some of the adjacent blocks. Mr. Ransome's It is probably true that in some cases, especially integrates still further, and must eventually lose and the grits of the lower Hermosa must underlie observations in the mines of C. H. C. Hill, on the higher slopes, the dislocation of slide blocks all its distinctive surface features. Through the this mantle of loose material. recorded in his report and in this folio, lead him, is not very great, but it is sufficient to make cor­ shattered condition of such landslides they become however, to the general conclusion that even c. H. c. HILL. relation of different outcrops very uncertain except saturated with water, and at times different por­ where ore bodies have been followed for some on a basis of exhaustive study of the whole ridge. tions will slump away and break up into a mass The triangular space between the Dolores River, distances, it is doubtful if any ore has been taken Landslide block at the Puzzle mine. That there of ordinary avalanche or landslide material. Each the southwest ridge of Telescope Mountain, and from rock in place. His many observations of has been landslide action in Horse Gulch has been fresh break furnishes a point of attack for the ele­ the cliff line leading northwest is known as C. H. recent movement make it probable that nearly evident to all familiar with the ground about the ments of frost, rain, springs, snowslides, etc., and C. Hill. It is really a broad hollow on the slope the whole area of C. H. C. Hill is affected by Puzzle mine and with the experience of those who the destruction of the shattered mass goes on more of Telescope Mountain. Its character and relations landslide action at the present time. have tried to find the continuation of the ore body rapidly. are partially shown in fig. 5. originally discovered in the Puzzle; but the extent All over the central part of this landslide area While landslide trenches, ribs, and knolls are NEWMAN HILL. of the slide has not been appreciated. are very marked knolls and ridges, with shattered very plain over nearly the entire area of C. H. C. The topographic relation of Newman Hill to The ore body of the Puzzle mine, a replacement and irregular rock outcrops, back of which are Hill, the examinations have shown that the land­ Dolores Mountain is very similar to that of C. H. of a limestone stratum by galena, etc., was found the V-shaped trenches marking the fracture lines slide phenomena are, on the whole, much more C. Hill to Telescope Mountain, but, while there is in a ledge outcrop facing the stream. The strata of individual slide blocks. Some of these features superficial than might reasonably be inferred from a deep mantle of wash, it does not appear that of the ledge included crinoidal limestones typical are illustrated in fig. 6. the physiography. The peculiar topographic fea­ important landslides have occurred. The small of the middle division of the Hermosa (Carbon­ The landslide area on this side of Horse Gulch tures of the hill are principally due to landslides, landslides which have taken place on the western iferous). The beds have a general dip down­ ends upward in a point under the cliffs of red but the ground, which has slipped in blocks, has and southwestern slopes of Dolores Mountain stream, but they are irregularly dislocated on Cutler sandstones at about 11,400 feet. Chaotic been in part covered by avalanche debris and com­ have sent their debris in avalanche form down fissures now open, and in places the dip is south­ slide blocks, which are rounded and more or mon talus. The disintegration of crushed rocks has upon the upper part of Newman Hill. Local erly at a low angle. The structure is at variance less grassed over, cease at about 11,000 feet, yielded soil, and over much of the hill a growth slips have also taken place in the wash of the with that normal to the gulch, as seen on the and between this and the solid-cliff line there of spruce and aspen conceals everything. hill, but no great movement of rocks has been north side. is a small area of more angular blocks of red The part of the hill in which normal landslide detected. The wash covering of Newman Hill The ore was traced under the bench, but it was sandstone and porphyry in which there is often a phenomena are now most distinct is a broad band has been found to be several hundred feet deep soon cut off by breaks on the east, south, and west. dip toward the mountain. Each of these blocks is parallel to the cliffs on the northeast. The most in some of the shafts sunk near the steeper slope A shaft sunk in the little trough at the base of the clearly marked, and fissures of dislocation between pronounced trenches run in general parallel to of Dolores Mountain. snow-covered slope, seen in fig. 3, passed into them are like open faults with a measurable throw these cliffs, and it seems probable that an observed The geologic structure revealed in the many stream gravels at a depth probably less than 50 of 50 feet or less. Some of these blocks have sheeting of the strata in a northwest-southeast miles of mine workings in Newman Hill is that feet, proving that this ore-bearing block has fallen from the cliff in comparatively recent direction has caused not only the cliffs of to-day, to be expected so near the center of the dome. slipped down from the slope above. time, and fissures which may serve as boundary but numerous fissures bounding thick plates or Low dips to the southeast prevail, with many The workings on the original Puzzle ore body cracks of future slips are to be found here and blocks of rock which have fallen en masse at small faults, most of which are of more recent and the efforts to trace it beyond the breaks on all there in the cliff. various times. At one place observed the present date than the ore deposits, as is shown by Mr. sides have not as yet indicated the depth to which cliff exhibits a very distinctly polished and striated Ransome in his report. RIDGE BETWEEN BURNETT AND SULPHUR CREEKS. this superficial dislocation extends, nor the posi­ surface, which may be due to landslip action. The lower Hermosa beds of Newman Hill are tion of the formation in place from which the One of the largest landslide areas of the district Damming of Dolores River. The bench between intruded by some porphyry sheets, the most ore-bearing block of the Puzzle mine was detached. is the broad ridge between Burnett and Sulphur the wagon road and the river, extending from Burns important one, near the base of the hill, attain­ "The blowout." Another locality in Horse creeks, extending from about 11,000 feet on the to a point below the mouth of Horse Creek, is prob­ ing a thickness of more than 500 feet, as shown Gulch worthy of special mention is the so-called crest of the ridge down to the Dolores River, some ably composed entirely of landslide material. As by the boring at the Skeptical shaft. This mass 11 apparently splits into two or more small sheets ones, a land surface near sea level, because the Through the San Juan uplift a large area was ters in a general way with similar occurrences of before crossing the river, and many other irregu­ erosion of the interval was nowhere sufficient to elevated far above sea level and has never again the San Juan area. Porphyritic diorite, monzon­ larities may well be assumed to exist. wholly remove the Ouray limestone at any point sunk below it. Erosion became active and degra­ ite, or granite intrusions are known in the Tellu­ That this large porphyry body is in the main observed on the southern slopes of the San Juan dation of the land area continued until it was ride and Silverton quadrangles, and in some cases of laccolithic character is further indicated by the region. As stated in a preceding section the reduced to a peneplain, possibly with a small proof exists that they are later than some of the limited exposure of its base in the workings of the absence of the Ouray limestone in the valley of mountainous island rising above it to which the surface volcanics. The epoch of intrusion is, how­ South Park mine in Silver Gulch. Several very Silver Creek, near Rico, is supposed to be due to Needle Mountains area belonged. The peneplain ever, not at all clearly determinable with reference small dikes or sheets of porphyry have been erosion of this interval. That is, however, the in question is that upon which the Telluride con­ to the general time scale. The stock eruptions of encountered in the mines of Newman Hill. only point adjacent to the San Juan Mountains glomerate (Eocene?) rests in the western San Juan the Telluride and Silverton areas are later than as yet found where the Ouray is lacking at its and San Miguel mountains. any known lavas of those districts, and as the GEOLOGICAL HISTORY. appropriate place in the section. It may be that That this post-Laramie peneplain extended over Rico and La Plata stocks cut the laccolithic the area of greatest elevation and consequent ero­ the Rico quadrangle is evident. The nearest point intrusions of similar magmas, it may well be PKE-TEJEmAKY EVENTS. sion, of the time in question, was west of the San at which it can now be seen is beneath the Tellu­ that all eruptions of this type can be referred to Introductory. The visible record of pre-Ter- Juan area, in what is now the plateau district. ride conglomerate in Mount Wilson, about 5 miles the same epoch in the latter part of the Tertiary. tiary events in the geologic development of this The Pennsylvanian sedimentation was of very north of the northeast corner of the Rico quad­ It has been pointed out that the Rico Mountains area lies wholly in the sedimentary formations different character from any that preceded it in the rangle, at an elevation of 12,000 feet. In Sheep belong to the laccolithic group of the Henry Moun­ and their stratigraphic relations. From the dis­ general area of southwestern Colorado. A long- Mountain, 6.^ miles to the northeast, the sAme hori­ tains type, in spite of local structural features not cussion of the formations already given it appears continued oscillatory movement of the earth's crust zon is shown. The general position of this plane commonly supposed to exist in some of the sim­ that the section is nearly like that much better caused frequent recurrence of conditions favorable in the Telluride quadrangle is represented and its ilar centers of intrusion. These general considera­ exposed, in its lower portions at least, in the Ani- to the deposition of limestones, shales, and sand­ significance is discussed in the Telluride folio. tions as to the time of the Rico intrusions have mas Valley. No marked local characteristic has stones, forming the complex called the Hermosa undoubtedly a bearing on the question as to the been observed in the Rico formations of pre-Ter- formation. Without visible break the Hermosa TEKTIAEY PEKIOD. age of all the laccolithic groups of the plateau tiary age, so that the course of events here can beds grade into those of the Rico (Permo-Penn- No surface rocks of the Tertiary period now province. The conclusion reached here is in only be assumed to have been that of the sur­ sylvanian) and those into the overlying Cutler exist in the Rico quadrangle, but it is necessary accord with that derived from the examination rounding province, an outline of which has been red beds, here assigned to the Permian. to refer to rocks of that age which formerly cov­ of the Elk Mountains, Colorado (see Anthracite- presented in the Telluride and Silverton folios. The character of the Cutler formation is in gen­ ered the area, in order to discuss intelligently the Crested Butte folio). For the present folio it is considered sufficient to eral much like that of the lower portion of the history of the Rico Mountains. refer very briefly to the history preceding the con­ "Red Beds" in many other places where no strat­ UPLIFT OF THE RICO DOME. ACCUMULATION" OF THE TELLURIDE CONGLOMERATE. tinental uplift of the whole sedimentary section, in igraphic break separates them from strata contain­ It was brought out in describing the structure of post-Laramie time. ing a Pennsylvanian fauna. The fact that a break When the peneplain produced by erosion fol­ the Rico dome that three elements enter into its I're-Paleozoic era. From the study of the is now known to exist above the Cutler beds ren­ lowing the post-Laramie uplift had reached a constitution, namely, domal uplift by folding, Needle Mountains and the Animas Canyon sec­ ders it impossible to assume that the Paleozoic sec­ certain stage of development the local conditions igneous intrusion, and faulting. Whether or not tions it appears that the oldest rocks of this region tion of the San Juan region is complete. There changed, so that a great amount of debris from the these are all resultant phases of the action of one are certain gneisses and schists, supposed to be of may have been deposited in this district a consid­ ensuing further erosion of the adjacent mountain great force is a question of far-reaching importance. Archean age. The next younger series of rocks erable thickness of Permian strata now entirely masses was deposited upon it as a conglomerate. The evidence to be found in the Rico Mountains consists largely of igneous material, greatly met­ absent, owing to the pre-Dolores erosion. This formation, originally called the San Miguel is manifestly inadequate for the solution of this amorphosed and associated with some distinct Pre-Dolores uplift and erosion. The angular conglomerate and afterwards renamed the Tellu­ problem. It is clear, however, that the various sediments. Following the accumulation of this unconformity at Ouray between the Dolores and ride, acquired a rapidly increasing thickness west­ manifestations of deep-seated forces at this point complex came a long period of sedimentation older formations testifies to important uplift affect­ ward from its border in the Silverton quadrangle belong to different epochs and seem in some par­ during which the Uncompahgre group of con­ ing the entire known Paleozoic section. The to the San Miguel Mountains. On its border it is ticulars independent of each other. glomerates, sandstones, and shales was deposited, geographic extent of this uplift remains to be 50 feet or less thick and is a coarse conglomerate. Quaquaversal folding. It is believed that the in marked unconformity with the structures of determined. The Cutler beds were sharply folded In Mount Wilson, a few miles north of Rico, it is quaquaversal folding which seems to have been older formations. in the Ouray district, but apparently the region about 1000 feet thick and consists of fine conglom­ the principal factor in the elevation of the Rico While the sequence of events is not wholly of maximum disturbance lay to the north and east erate, sandstone, or shale, the transition in texture dome took place after the accumulation of a con­ clear, it seems probable that great folding, fault­ of the San Juan, since on the south and west no and thickness being clearly exhibited in the inter­ siderable thickness of volcanic rocks from San Juan ing, and metamorphism of all the rocks as yet relations of marked unconformity exist between mediate area. eruptions that is, in the Tertiary period and pos­ referred to was the next great step in the history the Dolores and Cutler formations. While much of the Telluride formation is well sibly in the Eocene epoch soon after the formation of the region. The gneisses and schists are pene­ The epoch of uplift and consequent erosion stratified and apparently of subaqueous origin, it of the San Juan tuff. The erosion which produced trated by a large number of granite masses, one under discussion was followed by the deposition seems possible that the whole may have been of the Telluride peneplain would surely have trun­ known important body of gabbro, and many of the fossiliferous Dolores strata, but until the fluviatile origin. In any case it is probable that cated the dome had this structure been of Mesozoic small dikes of diabasic rocks. Some of these horizon within the Triassic system represented by the conglomerate was deposited over the Rico area age. That plain is, however, nowhere seen in the rocks cut the Uncompahgre strata, and the com­ those beds has been determined it is premature to with a texture and thickness corresponding to that Rico Mountains, although Blackhawk Peak still paratively unaltered textural condition of these assign the orogenic movement to late Paleozoic exhibited in the San Miguel Mountains. rises more than 600 feet above the level at which it intrusives appears to indicate that they are all rather than to early Mesozoic time. No fossils have been found in the Telluride for­ appears in Mount Wilson, a few miles to the north. later than the time of the above-mentioned fold­ Mesozoic history. Evidence that the Dolores mation, hence its exact age is unknown. Its rela­ The greater part of the uplift which has taken ing to which the Algonkian sediments were sub­ formation is of Triassic age has been given. In tion to the San Juan volcanic deposits shows it to place has affected the whole Paleozoic section and jected. the upper Dolores Valley, as in the San Miguel to have immediately preceded them and at present it the underlying Algonkian rocks and thus the small The presence of occasional fragments of granite the north and the Animas to the east, the Triassic is assumed to be of early Eocene age, although Rico dome comes to show close relationship with or schist in the igneous intrusives of the Rico or strata are overlain with apparent conformity by the there are some reasons for thinking that the Tel­ the much broader San Juan uplift. As has been La Plata mountains shows that these same old La Plata formation, yet on the northern side of the luride conglomerate may be correlated with the stated already, the most prominent structure in the formations exist beneath later rocks in the country San Juan the La Plata transgresses the edges of Arapahoe formation of the Denver region, assigned San Juan region is pre-Tertiary in origin, but there west of the Animas. older sediments and in places rests on the Archean, to the Post-Laramie part of the Cretaceous on was also uplift in Tertiary time, and it is possible Paleozoic history. Before the earliest Paleozoic demonstrating that a period of continental uplift paleontologic evidence. A full discussion of this that the Rico dome is synchronous with the later sediments of the region were deposited there was a and great erosion intervened between the Dolores question is given in the Telluride and Silverton elevation and a result of the same force. The same period of enormous erosion which appears to have and La Plata epochs. Similar relations are known folios. is true of the La Plata Mountains. But until the affected the southern Rocky Mountain province elsewhere in Colorado. SAN JUAN VOLCANIC ERUPTIONS. structural history of the San Juan region has been and probably large areas of contiguous country. Whatever decision may ultimately be reached The volcanic complex of the San Juan region is studied in much greater detail the relation between A peneplain of marked character was produced, as to the relations of the Gunnison group as a known to be the result of outbursts of various the local uplift of the Rico and La Plata mountains which, on sinking beneath the later Cambrian whole, it is true that the upper of the assumed kinds and with various products, extending through and the more nearly continental movements of the sea, became the floor for the deposition of the Jurassic formations, the McElmo, bears such Tertiary time. w. The earliest eruptions must have San Juan region can not be thoroughly discussed. Ignacio quartzite. If that formation is of Sara- strong lithologic resemblance, in some of its followed the deposition of the Telluride conglom- Age of the laccolithic intrusions. The dikes, togan (Upper Cambrian) age, as now believed, it upper sandstone members, to the Dakota sand­ ate very closely, and it is probable, from the con­ sheets, and small laccoliths of porphyry in the is reasonable to refer this great erosion to earlier stone of the Cretaceous that it would be natural siderable thickness of tuffs and lava flows above Rico Mountains belong to the group of diorite-, Cambrian time. to assume that both formations belong to one that formation still remaining in the adjacent San monzonite-, and granite-porphyries which are so As will be clear from the description- of the Pale­ epoch of sedimentation, rather than that there Miguel and San Juan mountains, that the lower widespread in the laccolithic mountain groups of ozoic formations, the epochs of sedimentation dur­ was a great stratigraphic break between them, volcanics extended over the Rico area, with a thick­ the plateau country and also in the mountains ing the Ordovician, Silurian, and Devonian periods involving the whole of Lower Cretaceous time. ness perhaps of several thousand feet. This ques­ of Colorado. That these rocks are in all these must have been almost insignificant compared with The Upper Cretaceous section formerly present tion is particularly referred to in the Telluride folio, instances of approximately the same age is a nat­ the intervals of nondeposition. The latter, how­ in the Rico region was doubtless like that which while the explanation of the absence of the vol­ ural conclusion in harmony with all known facts, ever, were certainly not times of continental uplift has been mentioned as present south of the La canics in the Rico summits is presented below, in although the definite evidence of Tertiary age is to any great elevation above sea level, in this prov­ Plata Mountains. The alternation of shales and the discussion of the origin of the Rico domal found in but few localities. ince at least, since the thin formations of the Igna­ sandstones, with numerous coal beds, testifies to uplift. Evidence at Rico bearing on this question is cio, Elbert, and Ouray epochs, though separated general conditions similar to those prevailing in IGNEOUS INTRUSIONS OF THE RICO AND LA PLATA limited to the general considerations above stated by intervals representing long periods of land con­ the Rocky Mountain province, but differing some­ MOUNTAINS. as to the age of the domal uplift. In the adjacent ditions, are preserved in almost conformable rela­ what in detail. While no surface volcanics are now preserved in Telluride and Engineer Mountain quadrangles tions in the Animas Valley, a few miles east of the Post-Laramie uplift and erosion. That the the Rico quadrangle, the numerous intrusive rocks there are large laccolithic bodies of porphyries Rico quadrangle. A fuller discussion of this fea­ domal folding of the entire Paleozoic and Mesozoic which have been described belong undoubtedly to very similar to rocks of the Rico Mountains, ture: of Paleozoic history is given in the Needle section about the San Juan center occurred in the the Tertiary period. It is, indeed, possible that the and some of these are intruded into volcanic Mountains folio. interval succeeding the Laramie epoch has been monzonite or syenite stocks of the Rico and La rocks, proving their Tertiary age. But no evi­ Apparently the deposition of the Ouray limestone clearly established and is discussed at some length Plata centers may represent channels through dence has been found to indicate the particular was continuous from late Devonian into early Car­ in the Telluride folio. The local uplifts of the which extensive outpourings of lava took place. epoch of this period in which the intrusions took boniferous (Mississippian) time, and the succeeding Rico and La Plata mountains are imposed upon Be that as it may, there is every reason to cor­ place. elevation must have produced, as did the earlier that older structure and to some extent obscure it. relate the igneous phenomena of these local cen­ Stock eruptions and faulting. In considering Rico. 12 the nature of the forces which have produced sion. The eastern end of the monzonite is just question. Naturally the work of distinct epochs The monzonite stock on the west, side of the the Kico uplift, it is apparent that there is a close above the street in Piedmont, and there must have of Tertiary erosion can not be recognized in the river has been sufficiently resistant to form a ridge analogy between the two phases of intrusive action been fissures traversing the strata in the prolonga­ Rico district, and the present discussion is, there­ both south of Aztec Gulch and in the main divide and the two phases of structural uplift. The pri­ tion of the principal axis of the stock. These may fore, directed to the local problem as to the sculp­ south of Horse Gulch, though in neither place does mary upward pressure at this center was one to have given heated solutions the necessary access to turing of the Rico dome. This erosion began with it reach to as high an elevation as the porphyries which the whole section of Paleozoic and Meso- the limestone at the places now seen. So far as the rise of the dome, at some unknown time in the of the adjacent peaks. zoic strata accommodated itself by folding, stretch­ observed, such contact metamorphism is con­ Tertiary period, and has continued to the present The distribution of the laccolithic porphyry ing, and no doubt by minor fissuring. It would fined to the zone about the stock, with the excep­ day, although discussion is here confined to pre- masses in the upper part of the Dolores formation appear to have been a gradually exerted pressure, tion of one place in the shattered zone, between Glacial erosion. The glaciation here referred to is has determined the zonal grouping of the principal of the kind assumed to have forced the magmas the forks of the Blackhawk fault, where garnet that of which evidence is observable, and is not mountain peaks about the center of the dome struc­ of laccoliths and analogous sheets between the masses and specular iron occur near a small por­ necessarily the earliest of the region, as will be ture. In fact, it is to these porphyries that the strata of a sedimentary complex. Correspond­ phyry dike. explained in a later paragraph. Rico Mountains owe their existence. Had they ing to this idea, it is found that the distinct Solfataric action. While no evidence can ever Sculpturing of the dome. At the inception of not been encountered by the streams, the latter porphyry sheets of the Rico Mountains are the be discovered proving that the surface phenomena the Rico dome the volcanic rocks which covered in dissecting would have given to the dome a earliest intrusions. ordinarily known as volcanic attended the deep- the San Juan country were being attacked on molding scarcely different from that which they The fault blocks of the heart of the mountains, seated intrusions in the Rico dome, certain proc­ all sides by streams whose positions were prob­ have impressed upon the adjacent areas of sedi­ made up, at the exposures now seen, of Algonkian esses which are generally supposed to characterize ably determined by the distribution of the vol­ mentary rocks; the concentric outcrops of the schists and quartzites, have been thrust up through zones near the surface have been active in the canic materials. So long as eruption continued harder beds would be expressed in knolls or the folded strata with little or no evidence of con­ horizons now revealed by erosion. One of these the stream courses were constantly liable to alter­ curving ridges, but the general elevation would temporaneous folding of the adjoining beds. This processes is the decomposition of rocks by sul­ ation by lava flows, but with the temporary ces­ have been much less than at present. is also the relation of the Darling Ridge monzonite phurous vapors or by solutions that have absorbed sation of volcanic activity each stream would stock, as far as can be seen, and also of the sim­ those vapors, and the production of alunite. This maintain the course it then held, deepening its GLACIATION OF THE RICO MOUNTAINS. ilar stocks of the La Plata, Telluride, and other substance is formed at the surface in the crater of channel and sapping at its head to extend its It is known that the higher portions of the San neighboring quadrangles. Such fault blocks and Solfatara, near Naples, and is a common product canyon into the central mountainous region.. It Juan region were practically covered by an ice such masses of igneous rocks seem alike due to of the sulphurous emanations of volcanoes known seems probable that the Dolores River had assumed sheet during a late stage of the Glacial epoch. forces suddenly exerted, producing vertical frac­ from this locality as solfataric exhalations. But its present course previous to the formation of the It is, therefore, not strange to find evidence ture instead of doming. With such an analogy in the process is not necessarily connected with solfa- Rico dome, since, supposing that the surface at the of recent local glaciers in the Rico Mountains. mind, the suggestion naturally arises that a mass taras of typical volcanoes, and the term has been time the dome was formed was sufficiently smooth Reasons exist for believing that the San Juan of magma, forming a stock in greater depth, may gradually extended to cover the metamorphosing for the development of consequent drainage on Mountains were also glaciated in an earlier por­ have followed the upthrust blocks now revealed. action often consequent on eruptions which have its slopes, it is difficult to understand how one of tion of the Glacial epoch, but evidence bearing Such a hypothesis requires the assumption of very been accompanied by mineralizing agents of sul­ the radial streams thus resulting could have gained on this question is found in the Rico district direct connection between the propelling forces of phurous character, even when taking place in so distinct an advantage over the others that it only in certain high-level gravels of the Dolores magmas and those of structural uplift. depth. would finally cause their complete diversion. The Valley. Connection between folding and intrusion. If The orthoclase-bearing porphyry mass of Calico relations of hard and soft rocks in the region to Evidence of recent glaciation. The record of folding and intrusion at the Rico center be referred Peak has been almost wholly decomposed by such the north of the dome are such that it seems as if glaciers in the Rico Mountains is seen in certain to the action of the same great force, it is difficult agents, alunite and kaolin being the principal diversion of the radial streams on the northern topographic forms, in rock scoring, and in accu­ to explain why larger amounts of magma were not products. slope must have been accomplished by the west­ mulations of debris, but none of these is strikingly intruded into the strata of the Rico dome, in view Existing sulphur springs. It is especially note­ ern branch of the Dolores River long before any prominent or characteristic, from which it appears of the large porphyry masses of probably contem­ worthy, ih connection with the evidence above stream originating on the southern slope of the that because of their somewhat lower altitude and poraneous origin occurring near at hand in com­ given of former intense solfataric action, that there Rico dome could have cut its valley backward their isolation the Rico Mountains were not so paratively undisturbed beds. The Flattop mass of are numerous springs of water heavily charged through the hard core of the group to the north completely dominated by the ice as were the porphyry, exceeding in bulk all the sheets of the with sulphureted hydrogen issuing to-day from side of the dome structure. higher mountains adjacent. They formed a local Rico Mountains put together, occurs just at the the slopes of Stoner and Johnny Bull creeks and The actual amount of erosion since the Rico center of accumulation, and though the basins at northeast base of the dome, but similar large of other tributaries of the West Dolores north of uplift can not be estimated, since its effects are not Rico were probably deeply buried in snow there bodies occur on the San Miguel River in the Tel­ Johnny Bull Creek. The waters of these springs separable from those of the epoch preceding. It were but few places in which the accumulation luride quadrangle, 20 miles from the Rico uplift, are surface waters, as they are influenced directly is believed, however, that the volcanic rocks had became sufficiently deep for the consolidation of and another occurs in Hermosa Peak, a few miles by the rainfall of the season and dry up at times, not been removed entirely and that, as in Mount the snow into true glacial ice. to the east. The stocks of the Telluride quad­ but the sulphurous gases escape continuously. Wilson, to the north, sediments above those now That glaciers were not prominent for any great rangle appear likewise to be distributed without The exclusive presence of these springs on the exposed were present, up into the Mancos shale, at length of time seems clear from the absence of visible relation to any structure of the sedimentary west side of the dome, extending from the imme­ the time of uplift. marked glacial cirques or amphitheaters in the formations. In other words, it appears to. be the diate vicinity of the solfataric center at Calico Whether the Dolores was flowing in a shallow higher mountains. The basin at the head of the case that, while laccolithic intrusions and stock Peak toward 'the West Dolores, suggests that these valley or deep canyon previous to the domal uplift small gulch next east of Allyn Gulch, in the east­ eruptions have occurred at the Rico and La Plata exhalations really belong to a later solfataric period at Rico can not be surmised, but before the com­ ern part of the group, is the only one of the region centers, both forms of intrusion have also taken of this eruptive center. pletion of the structure the stream had doubtless which strongly resembles a typical cirque. It is place not far away in much greater volume, at cut a deep trench well down toward the base of also noteworthy that the side gulches of the moun­ points seemingly independent of such centers. It ORE DEPOSITION. the volcanic rocks which are supposed to have cov­ tains seldom possess the profile,'1 outline charac­ is to be hoped that more extended studies of the After the uplift of the Rico dome, the intrusion ered the region, and possibly into the Mesozoic sedi­ teristic of valleys filled by glaciers, the only two San Juan and adjacent regions may throw light of the igneous rocks, and at least a portion of the mentary rocks, upon which they probably rested. exhibiting the U-shaped form being Silver and on the relations of these various phases of intru­ fault fissuring there was a period of extensive ore This erosion belonged to the epoch of deformation. Horse gulches, the largest and deepest of the group. sion of magmas to structural movements of the deposition in the rocks now forming the Rico It was succeeded by continued erosion of the pres­ Striated or grooved rock faces have been noted earth. Mountains. While the age of the ore deposits can ent epoch as arbitrarily limited by the completion in several places, notably in Deadwood and Silver not be closely determined, it is in every way prob­ of the distinct uplift. gulches and near the head of Johnny Bull Creek, PHENOMENA CONNECTED WITH IGNEOUS INTRUSION. able that they correspond in time to the deposits With the downward cutting there has doubtless west of Calico Peak. Aside from the mechanical features of intrusion, of the La Plata Mountains and that they belong to been concomitant elevation, but of this there is no Glacial debris retains distinct morainal form which have been referred to, the principal phenom­ the great epoch of ore deposition which succeeded evidence in the immediate vicinity of Rico, though only on the southeastern slopes'of the mountains, ena connected with the igneous intrusions of the Rico the early Tertiary igneous intrusions or more typi­ some 10 miles or so to the south there are gravel at the head of two branches of Scotch Creek, in Mountains are those of contemporaneous metamor- cal volcanic eruptions in many parts of the Rocky beds about 400 feet above the present valley floor, the Engineer Mountain quadrangle. These were phism and of solfataric exhalation which appears Mountains. Apparently the more typical lacco­ showing the former position of the stream bed deposited by short glaciers of small dimensions. to have continued down to the present time. lithic mountain groups of the plateau country to and indicating an uplift since their deposition. In other places the gravels supposed to be of gla­ Contact metamorphism. Contact metamorphism the west do not contain ore deposits in an abun­ The effect of erosion within the mountains has cial origin are mingled with avalanche, landslide, of the calcareous strata adjacent to the monzonite dance at all corresponding to their development in been as if the river had cut its way at once to or wash debris, and could not be shown on the stock is very pronounced at nearly all places where the La Plata and Rico mountains, but whether the present position and then side streams and map. They occur on various ridges or mountain these rocks are exposed in the vicinity of the that fact is connected with their situation remote gullies had completed the grading of the slopes. slopes and in certain gulches, and details of their intrusive. The character of the alteration is such from the great centers of eruptive activity or with It is believed, however, that several distinct uplifts observed distribution were given in the Rico as might be expected from the action, of mineral­ local causes can not now be determined. It would, have occurred, but the pauses between them left report. izing agents, as chlorine, fluorine, and heated however, appear natural that more extensive depo­ no records because of the fact that the river was The rounded ridge at the entrance to the valley water carrying those gases and perhaps others sition of ore minerals should occur in a center like cutting its channel and not at any time widening of Silver Creek has an external appearance similar in solution. The metamorphism referred to con­ the Rico Mountains, where there has been so its valley, so that the valley was successively deep­ to that of kames or eskers, but it is really com­ sists in the formation of garnet, pyroxene, vesu- unusual an amount of fissuring, affording chan­ ened, and under conditions of heavy precipita­ posed of sedimentary rocks and intrusive porphyry vianite (?), and possibly other silicates of alumina, nels for the circulation of metal-bearing solutions. tion the slopes of the valley walls were gradually and is merely capped by gravels. It is conse­ with maknesia, iron, and lime, and in the deposi­ reduced without the production of terraces. quently a form of erosion rather than of con­ EROSION OF THE RICO DOME. tion of specular iron in scales, either impregnating The softer rocks have been carved away, leaving struction. the rocks or, more commonly, in thin crusts in General statement. The San Juan and adjacent the more indurate as cliffs or steep slopes between Collectively the phenomena observed are believed fissures. Such alteration of the calcareous strata country appears to have been a continental tract more gentle acclivities and determining the posi­ to warrant the conclusion that true glacial streams may be seen on the north side of Darling Ridge, during the whole of Tertiary time. Erosion must, tions of the main mountain masses. The rocks at one time existed on the southeastern slopes of near the blowout in Horse Gulch, and down near therefore, have been continually in progress dur­ which have been sufficiently massive to form the mountains, in the valley of Silver Creek and Piedmont. If the metamorphosed stratum is a ing that period. The work of degradation was mountain caps are mostly intruded porphyries, its tributaries, and in Deadwood Gulch, and that limestone the matrix for the silicates named is repeatedly interrupted and in great measure though the La Plata sandstone always rises as a in the upper part of Horse Gulch there were usually white crystalline marble. undone by vast volcanic accumulations in sev­ knob above the general level of the adjacent ridges. important accumulations of ice which may or may The great metamorphism of the Devonian lime­ eral different epochs. Further, the erosive power Of the few high peaks capped by other sediments not have reached into the lower part of the val­ stone in the Dolores Valley at Rico is so clearly of of streams varied greatly, according to the alter­ than La Plata, Telescope Mountain is the only one ley. If others existed, their marks have been the kind described that it is considered probable nating elevation or subsidence of the region, not protected by a very massive sheet of porphyry obscured by surface materials of another origin that this change is also due to the monzonite intru­ which probably continued during the period in lying within 100 to 200 feet of the top. or by recent erosion. 13

Valley gravels related to glacial deposits. A character of the bowlders and the meager evi­ permeable is the mass beneath to the rain that falls at an elevation of 700 feet above the Dolores River group of gravel deposits which may be tentatively dence concerning their origin scarcely warrants upon it and to the snow water. on the northern edge of the monzonite arm from referred to the close of the recent (Wisconsin) the assumption of any particular relation to One effect of this saturation by circulating waters Darling Ridge are of Glacial origin, they indicate stage of glaciation occurs in the Dolores Valley at more ancient glaciation. Gravels of high level has been to keep the fracture lines of attrition mat­ a much greater accumulation of such debris in the many points from the Rico Mountains downward. are abundant on all "sides of the San Juan, and ter and many layers of crushed sandy shale in a valley than would be suggested by any other occur­ These gravels are seen in the terrace upon which in the forthcoming Ouray folio strong evidence soft condition, favorable to the slipping of more or rences. But even if they are Glacial, the recent the town of Rico is partly built, and on the similar indicating a pre-Wisconsin glaciation will be less extensive masses whenever the support weak­ work of the river seems to have been largely the and probably corresponding bench which occurs given. ened sufficiently. Secondary slides of this sort removal of the gravels, with little cutting into the about 40 or 50 feet above the river bed north of must have been frequent ever since the original underlying rock. In Deadwood and Allyn gulches LANDSLIDES. the mouth of Sulphur Creek. The gravels are shattering of the formations, and they are still the streams have cut down through the unconsol- best exposed in the cutting for the roadway to The landslide areas of the Rico Mountains, taking place. idated gravels of Glacial origin, but this is a task the railroad station at Rico, but are known to which assume unusual importance, have been The more exposed and isolated landslide blocks, which they could have easily accomplished in a form the edge of the terrace for nearly half a described as to their character and local distri­ if prevented from further slipping en masse, break short time. Similar indications of the small effect mile to the south. Occasional remnants of cor­ bution, and it remains to refer briefly to their up gradually, while a talus slope or an avalanche of post-Glacial bed-rock erosion are seen in Silver responding gravel benches occur down the Dolores age and the evidence of their origin. A much track often denotes the course of the more rapid Creek, where the stream has locally excavated nar­ River as far as the mouth of Bear Creek. South fuller treatment of the subject is given in the disintegration. row canyons in the wider valley of Glacial origin, from Montelores the bench is from 10 to 30 feet Rico report, Origin of the landslides. The immediate cause but these canyons have in no instance exposed the above the present stream, and it seems to slope Age of the landslides. The epoch of the Rico of the Rico landslides is manifestly the very unusu­ bed rock to a depth of more than possibly 20 feet, down the valley at a slightly greater grade than landslides may be said to extend backward from the ally shattered condition of the rock formations on and in many places the stream is working on that of the Dolores River. This bench is not present day to their beginning, at a remote period steep slopes, and the discussion of origin must be debris of very recent origin, which has been thrown entirely depositional, since occasional exposures not accurately determinable. From the great num­ directed to the seat and nature of the force to which into its channel from the side gulches and ravines. show rock in place. Just north of the mouth ber of the slides in this limited region and the con­ the intense shattering is due. The evidence con­ All the evidence serves to point to the recency of of Ryman Creek the inclined and truncated ditions of their distribution it must be assumed cerning this force contained in the observations the Glacial occupation and to the small amount edges of the Cutler beds are shown to be cov­ that they are primarily due to some very unusual which have been recorded may be summarized as of erosion which has since ensued. The present ered by a thin capping of gravel, and east of force, shattering the rocks to a remarkable degree follows: topography is in no essential feature different from Montelores the eroded surface of the porphyry is and principally exerted at the beginning of the 1. The principal landslides are confined to a what it was previous to the accumulation of the but partially concealed. landslide epoch. It is therefore of prime interest small circular area in the heart of the Rico uplift, ice. Before that the streams had found their pres­ Less conspicuous remnants of a gravel terrace to ascertain when these slides began. but do not cover all of that area. ent courses and had practically assumed their pres­ occur along the Bear Creek flat. This terrace is Of all the phenomena of Quaternary age in this 2. The slides are more recent than the topo­ ent grades. at about the same elevation above the present region there is none affording definite proof as to graphic details of the mountains and valleys, In the higher parts of the mountains, however, stream as the Dolores terrace and seems to be the remoteness of the time at which the fractur­ except only some recent and minor features. the ordinary atmospheric agencies have been active closely related to it genetically. At the angle of ing of the formations took place. The principal 3. The shattering of the rock varies locally in and large amounts of talus and slide rock are seen the union of the streams a terrace remnant appears changes in the topography since the landslides degree. on many of the steeper slopes. to be common to both. The terrace gravels of began have been caused by the slides themselves. 4. The shattering is independent of lithologic Modification of topography by deposition. The Bear Creek came, of course, from the La Plata There, has been practically no erosion in the character and structural attitude of the formation, greatest change in the topography of the region Mountains. West of the mouth of Bear Creek Dolores Valley or in the more evenly graded and there is nothing in either of these conditions since the great erosion has been effected through this bench is inconspicuous or wanting. It seems reaches of its local tributaries in the landslide especially favorable to landslides. the agency of landslides. Throughout the larger probable that these Dolores Valley gravels repre­ epoch. All the distinct alluvial formations, as 5. The principal landslide slopes are in the tracts which are shown on the map the land­ sent the scanty morainal materials of the Rico flood plains and the fans or aprons at the mouths courses of many known faults, but several intensely slides have modified ( the form of the ridges and glaciers transported and deposited. The amount of streams tributary to the Dolores, are referable faulted areas of rugged topography do not exhibit mountain slopes and have'1 to some degree filled up of stream cutting below the gravel-covered ter­ to activities during the landslide epoch. Even landslides. the valley bottoms, especially of the Dolores oppo­ races is consistent with this idea. the glacial deposits seem to afford little evidence as 6. Many fault veins seem to have been opened site C. H. C. Hill and of Horse Creek. Appar­ Ancient glacial (?) gravels. Coarse gravel or to the age of the first landslides. The main traces again by the shock producing the shattering of the ently the streams in their lower courses have not bowlder beds, which from their position suggest a of glacial deposits are in the eastern portion of formations. as yet been able entirely to remove this landslide considerable former extent of such materials, occur the Rico Mountains, where landslides have not 7. The shattering extends below the surface zone debris. at numerous points in the Dolores Valley at sev­ occurred; and the gravel deposits, which seem to of actual sliding and to unknown depths. In the valley of the Dolores there are various eral hundred feet above the present stream. The be of glacial origin, have in most cases been more The consideration of all observed facts leads to deposits of stream gravels, and the map shows the most northerly of these observed occurrences is or less rearranged, so that little weight can be the comprehensive statement that in geologically distribution of the more recent deposits. Rem­ on the ridge south of Aztec Gulch, near Rico, placed on conclusions drawn from their present very recent time a part of the central portion of nants of terraces in several places indicate former where, at an elevation of 9500 feet, or about 700 position. The landslide period was apparently the Rico Mountains suffered a severe shock, shat­ deposits, but these are not always clearly distin­ feet above the river, an excavation in the wooded contemporaneous with the glaciation, or nearly so. tering the rocks at the surface and to unknown guishable from debris of other origin. surface reveals a mass of very round bowlders Relations to topography. From the details depths. As a result of this shattering many land­ While the lateral tributaries of the Dolores have lying in fine gravel. Among the rocks repre­ regarding the various slide areas which have slides have occurred where other conditions were no bottom deposits of importance, several of them sented are blue limestone, greenish sandstone, and already been given and from the illustrations, favorable. This shock must have had its source have built up very decided alluvial cones at their vein quartz. The bowlders are very unlike the it is evident that the topography of the Rico in greater or less depth, and may be referred to as mouths. The more important of these are repre­ angular fragments which are sparingly scattered Mountains had acquired almost the detail it now earthquake shock. sented on the map. about the surface. These angular blocks, often exhibits when the landslides began. The only Two important sources of earthquake shock are Small deposits of calcareous sinter or tufa have 3 feet or more in diameter, seem to have come considerable modification of that topography in specially recognized, viz, that originating in the been noted at various points on the banks of the from up the river, for red Dolores sandstone is the intervening time to the present has come relief of tension arising from structural movements Dolores, and several of them are shown on the common among them. Bowlder gravels have also directly from the landslides or indirectly through of the earth's crust, and that connected with vol­ map. At a number of these points the spring been exposed at a lower level on this same ridge in the rapid breaking down of the principal slide canic phenomena. The Rico Mountains represent waters are still highly charged with carbonate of prospects near the line of the Calumet vein, about areas. The valley of the Dolores, at the foot of a center of upheaval and intense faulting, and of lime and deposition is still going on. 300 feet above the river. C. H. C. Hill, must have been of the exact type igneous intrusions of a nature not strictly volcanic. It will be noted that the effect of nearly all of \ On the slope below the tufa bench south of Sul­ now seen above Marguerite Draw. The stream It seems natural to suppose that seismic disturb­ these recent agencies is to modify the form of the phur Creek, southwest of Rico, at about 300 feet bed of Horse Creek has plainly been interrupted ances must have taken place at the surface of the mountains existing before the Glacial epoch and above the river, there are several patches of coarse by the Puzzle slide. Rico dome during the periods of faulting and dur­ the beginning of the landslides, by producing gravel beds. Among the fragments noticed here The primary conditions for a landslide may be ing the intrusion of at least the monzonite magma gentler forms of the ridges and by filling up in was one block, nearly 3 feet in diameter, of the generally stated as a thoroughly fractured state of in the channels represented by the stocks of to-day. some degree the various valleys. peculiar hornblendic porphyry known only in the rocks on steep slopes, permitting the force of But those disturbances took place at so distant an Gas springs. Emanations of carbonic acid gas dikes in the Algonkian schists above Rico. gravity to cause the fall; and were all the rocks epoch that the connection of the shocks now under and of sulphureted hydrogen accompany many Farther down the Dolores Valley other similar of a mountain district to be uniformly shattered discussion with either of them is not plausible. springs of water in the Rico region. The former gravel patches occur at this general level of 300 the mountains of most precipitous and irregular is continually escaping in large quantities in the RECENT GEOLOGICAL HISTORY. feet above the river. They are especially well form would naturally experience the most exten­ central part of the area, while the latter is noted shown on the west side of the river between Sny- sive landslide action. But in the Rico district Many of the features of post-Glacial geology at in many places on the west side of the mountain der's ranch and Rio Lado and have been noted some of the most rugged mountains have under­ Rico are inseparable in origin from similar features group in the drainage of Stoner and Johnny Bull also near the mouth of Bear Creek. Possibly gone no visible degradation by landslips, even in of Glacial and earlier time, since in those parts creeks. Both gases doubtless have their origin in they occur in small remnants much farther down the heart of the area most affected. Sandstone of the area that were not covered by ice similar chemical changes which are going on at a greater the river. A specially good exposure was noted Mountain is the most striking instance of this processes of general erosion and of local deposition or less depth beneath the surface, and the waters near the mouth of Tenderfoot Creek, where the immunity. were active throughout the Glacial stage. For this with which they are associated may or may not be pebbles average about 4 or 5 inches in diameter, Relations to other Quaternary phenomena. The reason, in referring to certain phenomena as Recent, of deep-seated origin. In some cases they cer­ though some reach 8 or 9 inches. Among the ordinary processes of degradation operative in the there is no intention of limiting their age to the tainly are not, for at the sulphur springs the rocks represented here are porphyries, sandstone, high mountain regions of Colorado have of course post-Glacial, but rather to indicate that certain water increases and diminishes with the humidity limestone, quartzite, vein quartz, and shale. been active in the Rico Mountains during the long conditions have continued down to the present or dry ness of the season, and at certain times the These high-level bowlder beds are considered as epoch of landslide action, and it scarcely need time. The Recent phenomena of the Rico region flow of water ceases entirely, while the gas con­ mere remnants of important deposits belonging to be pointed out that all the destructive agencies are mainly erosion and deposition. The latter tinues to escape. It appears that in such instances the epoch when the floor of the valley was 300 feet must have been especially effective within the land­ includes landslides, talus, and avalanche mate­ the gases have found the same channels along or more above its present stream bed. From the slide areas. The shattered landslip blocks them­ rials, river gravels, and spring deposits, which which the waters are circulating and that the evidence, common in southwestern Colorado, of selves have been in high degree vulnerable to have been described as formations. The proc­ two mix and escape together. In the section on slight stream erosion since the last Glacial epoch, it the attacks of solvent waters, frost, etc., and have esses of their formation are so commonly known "Economic geology" Mr. Ransome tells of the would seem necessary to conclude that the gravels in many cases rapidly disintegrated. The whole that but little further reference to them seems frequent appearance of carbonic acid gas in mine under discussion are older than the recent (Wis­ slope of Darling Ridge, as of other landslide areas, necessary. workings. consin) stage of glaciation. But the waterworn is practically without surface drainage channels, so Post-Glacial erosion. If the gravels occurring June, 1904. Rico 14 ECONOMIC GEOLOGY OF THE QUADRANGLE,

By F. lu Ransome.

with various gangue minerals. In many depos­ mineralogical or commercial distinction, and are INTRODUCTION. Cable mine and to the experimental treatment of the sphaleritic ore there found. its more or less complete oxidation of the primary not necessarily of different age. The principal ore deposits of the Rico quadrangle ores has taken place, resulting in pulverulent earthy The principal minerals occurring as a direct PRODUCTION. are confined to its northeast corner, and are included ores, often very rich in silver. result of the general processes of mineralization within the area of about 35 square miles covered by The total production of the Pioneer mining are as follows: the Rico special map. The mining district is nearly district can be only roughly estimated. Accord­ DISTRIBUTION OF THE ORES. Pyrite. This is by far the most abundant sul­ coextensive with the isolated group of peaks which ing to the reports of the Director of the Mint, the In all probability more than half of the ore pro­ phide in the district. Associated Avith quartz and have been described in the foregoing pages as the output from 1879 to the end of 1903 has been duced in the Rico district has come from Newman small amounts of chalcopyrite, sphalerite, and Rico Mountains. Rico, a town of a few hundred about 73,000 ounces of gold and 9,000,000 ounces Hill. This name is applied to the slopes imme­ galena, it constitutes the practically worthless inhabitants and the seat of Dolores County, lies of silver. The value of the entire product, includ­ diately south and east of Rico, constituting the filling of most of the lodes. It is found in large nearly in the center of the district, on the Dolores ing the base metals, probably lies somewhere western flank of Dolores Mountain. Newman blanket-like masses, free from gangue, in C. H. River, which traverses the area from north to between $8,000,000 and $10,000,000. By far the Hill may be considered as bounded on the north C. Hill. In similar masses, but usually in more south. The Rio Grande Southern Railway con­ greater part of this has been silver. Present by Silver Creek, on the west by the Dolores River, solid condition, it is found as a replacement of nects the town with the Denver and Rio Grande developments indicate that the district may soon on the south by Deadwood Gulch, and on the east limestone. This is the mode of its occurrence system at Durango on the south and at Ridgeway produce considerable zinc and lead. by the cliffs formed by the massive bed of lime­ in the Blackhawk mine, where it is frequently on the north. stone characteristic of the medial division of the associated with fluorite and grades by increase" PRELIMINARY OUTLINE OF THE ORE DEPOSITS. The following general account of the ore depos­ Hermosa. On this slope, which is deeply cov­ of chalcopyrite and galena into workable ore. its is for the most part condensed from a report The ores of the Rico district show unusual ered with surface wash, are the Enterprise, Rico- Although commonly containing small quantities entitled "The Ore Deposits of the Rico Moun­ variety in their occurrence, as regards both form Aspen, Newman, Union-Carbonate, and other of silver and gold, the pyrite has hitherto proved tains, Colorado," published in the Twenty-second and genesis. It is proposed in this report to treat mines, in which the ore occurred partly in lodes too low in grade for successful treatment. Rick- Annual Report of this Survey in 1902. To that the deposits under four general heads, namely: and partly in blankets. Of the latter the princi­ ard records that the pyrite from the northwest­ report the reader is referred for detailed descrip­ (1) Lodes, (2) blankets, (3) replacements in lime­ pal one is locally known as the Newman Hill or erly lodes in the Enterprise mine usually afforded tions of individual mines. stone, and (4) stocks. This is confessedly and Enterprise "contact." on assay from 4 to 8 ounces of silver and traces obviously a rough grouping for convenience and Also on the east side of the Dolores River, but of gold. In the Gold Anchor prospect in Bull HISTORY OF MINING DEVELOPMENT. clearness of treatment, and is not intended as a north of Silver Creek, is Nigger Baby Hill, a spur Basin a large body of pyrite was found which Records of the discovery and early development scientific classification. of Telescope Mountain. This hill has produced is said to have indicated, in single assays, as of the Rico ore deposits are fragmentary and often Under the first head will be described simple or ore since 1879. The ore occurs in oxidized form much as 90 ounces of gold per ton, but which conflicting. The first recorded attempt to prospect complex veins, usually nearly vertical, which when in lodes, which in their upper portions possess so as a whole did not pay the cost of extraction. the region was in 1861, when Lieutenant Howard they occur in the sedimentary formations cut across flat a dip as to constitute essentially blanket . Galena. This very important ore mineral occurs and other members of John Baker's expedition the planes of bedding. They are fractures or fis­ deposits. abundantly in the Enterprise blanket and in most into the San Juan region made their way over the sures in the rocks, which have been afterwards C. H. C. Hill lies immediately north of Nigger of the bodies of unoxidized ore that have been mountains from the east. Eight years later Shafer filled with ore or valueless vein matter. Baby Hill. It is a landslide area, honeycombed worked in the district. It is always argentiferous, and Fearheiler built a cabin on Silver Creek, near Under the second head will be treated various with workings from which much ore has been but apparently does not constitute rich ore unless its junction with the Dolores River, and located deposits usually more nearly horizontal than ver­ taken. The ore, largely oxidized, occurs in blank­ accompanied by argentite, tetrahedrite (freiberg- several claims. One of these, the Pioneer, subse­ tical, and lying parallel to the planes of bedding ets, the continuity of which has been greatly ite?), proustite, or polybasite, as is the case in quently gave its name to the mining district. or to the surfaces of intruded sheets of igneous broken by landslide movements. the Newman Hill mines. On the other hand, it In 1872 R. C. Darling and others erected an rock. These are the deposits locally known as From the three hills mentioned has come the nowhere occurs in sufficiently large masses, unless adobe furnace and attempted to smelt ores from "contacts." This term, used in a sense that has greater part of the Rico ore. There are, however, possibly in the Atlantic Cable mine, to be work­ what are now known as the Atlantic Cable, no necessary connection with its true geological several important outlying deposits. The most able for its lead alone. It presents no unusual Aztec, Phoenix, and Yellow Jacket claims. meaning, has unfortunately found its way into prominent of these is that of the Blackhawk peculiarities in this region and is, as elsewhere, They were unsuccessful, and it was not until literature and has been so universally adopted mine, between Silver Creek and Allyn Gulch, nearly always accompanied by sphalerite. 1877 that active prospecting was resumed in the by the miners that it is difficult to altogether where the ore occurs oxidized in a lode and as Sphalerite. Zinc blende is an abundant constit­ Pioneer district. avoid its use, Wherever employed, however, the sulphide replacement deposits in massive lime­ uent of the rich ores of Newman Hill, which some­ In 1879 rich oxidized silver ore was discovered word will be placed in quotation marks, indicating stone. Another example is the Puzzle mine, on times contain over 15 per cent of zinc. Its common on Nigger Baby Hill, and the future productivity its true standing as miners' vernacular. Horse Creek, about three-fourths of a mile from associates in these ores are galena, chalcopyrite, of Newman Hill was foreshadowed by the ship­ Under the third head will be considered those its mouth, where the ore also occurred replacing rhodochrosite, and quartz, and it occurs both in ment to Swansea of some ore from the Chestnut deposits, often irregular in form, which have limestone. The Johnny Bull mine on Johnny the northeasterly lodes and in the blanket. It is vein. The town of Rico at once sprung into resulted from the metasomatic replacement of Bull Mountain, near the head of Horse Creek, has also found in massive granular form, associated existence. limestone by ore. also produced some ore. with a little chalcopyrite, galena, and fluorite, in In October, 1887, the largest and richest of the Lastly, under the fourth head, will be noticed The entire basin of Horse Creek and the eastern the Blackhawk mine, where it makes up a consid­ blanket deposits on Newman Hill was discovered a few small ore bodies, often referred to as "chim­ slope of Expectation Mountain are dotted with erable part of the large replacement bodies in lime­ by David Swickhimer in the Enterprise shaft, at a neys," of which the Johnny Bull is the principal prospects, many of which have produced small stone. In the Atlantic Cable claim it occurs in depth of 262 feet, and shortly after ore bodies were example in this region. quantities of ore, but nearly all are now. aban­ coarsely crystalline nodular masses, associated with found in the Blackhawk, Logan, and Rico-Aspen No sharp distinction exists between these various doned. chlorite, specularite, chalcopyrite, and galena, in mines. deposits. Lodes of flat dip may pass into bedding Another deposit Qf considerable interest and limestone. This sphalerite is dark brown, while The Enterprise mine was sold in 1890 to a faults along weak strata, producing breccias which, prospective value is that of the Atlantic Cable that in the Newman Hill veins is usually rosin Pittsburg company, and the same year saw the when mineralized, are classed as blankets. The mine, 011 the north side of the town, in which colored. It is also abundant in the Sambo mine advent of the Rio Grande Southern Railway. mineralization of such a breccia, particularly if galena, sphalerite, and other minerals occur as and in the Bancroft and Lily D. prospects, asso­ Vigorous exploitation was continued in various the material be calcareous shale, is likely to be replacements of the Devonian limestone. ciated with galena. The occurrence of sphalerite parts of the district until 1895, when mining largely by metasomatic replacement, producing a By reference to the geological map the pre­ has until recently been purely an objectionable activity showed signs of abating. deposit closely akin to those resulting from the ponderance of the important ore bodies occurring feature in the ores, owing to the penalty attached Since 1895 the output of the Pioneer district simple replacement of limestone. Moreover, the in the Hermosa, particularly in the lower and mid­ by the smelters to ores containing over 10 per cent has decreased. The large bodies of rich "con­ ore bodies grouped under the second and third dle divisions, will be evident. Near the periphery of zinc. But in 1900 experiments were begun to tact" ore have been mined out, and many of the heads are always intimately connected with fis­ of the dome, where the Permian, Triassic, and determine the feasibility of working some of the veins have been worked down to a depth at which sures or lodes which may or may not be themselves Jurassic sediments now constitute the surface, no sphalerite ores for zinc. At the present time zinc the ore no longer pays for shipment. Masses of productive. large ore bodies have been found. The Johnny ore is extracted in commercial quantities from the ore often proved to be curiously limited, owing to The greater part of the product of the district Bull, it is true, occurs in Dolores rocks, but the Atlantic Cable by the United Rico Mines Com­ various conditions that are characteristic of the has come from the blankets. Some of the lodes ore body, although at one time giving rise to pany and treated in a small stamp mill. The region and that will presently be described. have proved rich, but their value has invariably considerable excitement, proved to be little more galena is saved on vanners and the sphalerite The decline in the price of silver has had a fallen below the limit of profitable working at a than a pocket. concentrated by a magnetic separator. Some depressing effect on this as on other districts remarkably shallow depth, which generally bears a shipments have been made, but the plant is MINERALOGY OF THE ORES. where this metal forms a large part of the output. constant relation to some overlying blanket with essentially experimental. But nearly all the important ore bodies formerly which the lode or lodes connect. Some important The ores of the Rico district present few note­ Chalcopyrite. This mineral is not very abun­ exploited yere sufficiently rich to be workable bodies of ore have also been formed by direct worthy or peculiar mineralogical features, and dant in the Rico district, although nearly always to-day had they not been exhausted. In 1900 replacement of limestone. need receive but brief treatment under this head. present with galena and sphalerite in the workable the only ore being shipped from the district was The bulk of the ore has been found in the Car­ They may be roughly divided into (1) pyritic ores. Associated with pyrite, fluorite, and some an occasional carload taken out by leasers working boniferous sedimentary series, particularly that por­ ores, usually of very low grade, and (2) argen­ finely granular galena and sphalerite, it formed small areas of unexplored ground in the larger tion of it known as the Hermosa formation. This tiferous galena ores, sometimes with rich silver some of the best ore in the Blackhawk replace­ mines. is nearly equivalent to saying that most of the ore minerals and often containing much sphalerite. ment bodies, where it often occurred in fine con­ In 1902 practically all the important mines in. has come from the central portion of the district, The pyritic ores constitute the characteristic vein centric or irregularly curved, narrow bands. It is the district were consolidated under the name of in the heart of the dome-like uplift of the Rico filling of most of the lodes and occur in many present in small quantity in the blanket and lode the United Rico Mines Company and although no Mountains. of the blankets and other deposits. The galena ores of Newman Hill, in the Silver Swan, Aztec, material increase of production has yet resulted, The ores consist primarily of galena often ores form the workable ore bodies, deposited under and Atlantic Cable prospects, and in many other the new company has devoted itself with consider­ highly argentiferous and associated with rich various favorable circumstances of concentration. lodes and blankets throughout the district. able success to the development of the Atlantic silver-bearing minerals sphalerite, and pyrite, The two kinds of ore are not capable of sharp Tetrahedrite. Gray copper ore occurs in the rich blanket ores of the Enterprise and Rico-Aspen other gangue minerals. The lode quartz shows no morphism, occur in the altered Devonian limestone portion of a vein or lode which "consists of work* mines and in some of the northeasterly lodes. It special features peculiar to this region, and requires of the Atlantic Cable and adjacent claims. able ore is termed a pay shoot. is a valuable constituent on account of its argen­ no detailed description. In the form of jasperoid Kaolinite and serieite. These minerals are not The word blanket is used to designate a zone or tiferous character. It is here associated with sphal­ (a cryptocrystalline aggregate commonly associated abundant in this region and can scarcely be con­ lens, composed of mechanically or chemically dis­ erite, polybasite, galena, rhodochrosite, and quartz. with replacement deposits) quartz occurs in the sidered as gangue minerals. Some kaolinite, how­ integrated material, lying parallel to the bedding of It also occurs in these mines in ore that has replaced Blackhawk mine and in the blanket of the Sambo ever, occurs associated with the ore in the Johnny the sedimentary series within which it is inclosed. gypsum, the gangue in such cases being partly the mine. In the former mine, also, are found spongy, Bull mine, and serieite is found in connection Such a mass is always referred to as a "contact" transparent crystalline form of gypsum known as cavernous masses of rusty quartz, apparently due with the C. H. C. Hill blanket. by the miners of Rico. In contradistinction to a selenite. Small amounts of tetrahedrite are found to the removal of limestone, by solution, from a Paragenesis. By paragenesis is here meant the lode, a blanket is usually more nearly horizontal as a replacement of sandstone in the Gold Anchor network of quartz stringers. Quartz is abundant association of the various ore and gangue minerals, than vertical. It is normally composed of soft prospect in Bull Basin. It is probably present also in some of the blanket breccias as a replacement of with special reference to the mode and order of material as compared with the rocks which imme­ in the Aztec lode, with chalcopyrite, and may have the brecciated material. In the case of the nearly their formation. diately overlie or underlie it, and is in most cases formed part of the Johnny Bull ore. A small black Hermosa shales, many of the fragments are Although these minerals, as indicated in the pre­ a breccia, pocket of'tetrahedrite was extracted from the Iron still recognizable as dark patches in the white ceding section, are commonly found in more or less Blankets are frequently mineralized and contain lode, but the mineral apparently does not occur in quartz, although the microscope shows that they characteristic association, no constant and regular bodies of ore which may or may not be coextensive the replacement deposits of this mine. A little have been altered to cryptocrystalline quartzose order of deposition has been discovered. Careful with the blanket itself. Such ore bodies will be occurs also, with quartz, in the Eureka, a prospect aggregates. In the main blanket of the Union- study of the banded veins of Newsman Hill has termed pay shoots, as in the case of lodes. near the head of Iron Draw. It is nowhere abun­ Carbonate mine fragments of monzonite-porphyry failed to show that this banding can be explained Replacement ore bodies in limestone require no dant, and its presence is generally indicative of have been more or less completely transformed into by any simple depositional sequence of the com­ special definition. They embrace those ore masses, high-grade ore, although not necessarily in large very spongy masses of white quartz, sometimes ponent minerals. Each mineral has evidently often of irregular form, which have molecularly amount, containing pyrite. A somewhat similar silicifica- been developed at many different times during replaced the limestone through the process known Enargite. This mineral, a sulpharsenate of tion of porphyry has taken place alongside the lode the whole period of mineralization. The only as metasomatism. copper, occurs at the head of Horse Creek in fissures of the Mohawk and Marriage Stake pros­ generalizations which it appears safe to make are Stocks are those ore bodies commonly referred to the Johnny Bull mine. pects in Horse Gulch. In these cases, however, that in Newman Hill the rich silver ores proustite, as "chimneys." They are more or less solid Specularite. The crystalline variety of hematite the resulting siliceous skeleton still preserves, in a argentite, and polybasite were the last ore min­ masses of ore, roughly circular in plan, with their occurs abundantly in several mines and prospects measure, the original porphyritic appearance of the erals to form in the northeasterly lodes and in the longest dimension nearly vertical. Their forma­ in the metamorphosed Devonian beds near Rico. rock. Enterprise blanket, and that there was deposition tion, while usually initiated by two or more inter­ Among these may be named the Iron Dollar, Rhodochrosite. The carbonate of manganese is of practically barren quartz and a little pyrite, secting fissures, is accompanied by considerable Eighty-Eight, Atlantic Cable, Shamrock, and present in the rich upper portions of the northeast­ which was also subsequent to the formation of the metasomatic replacement of the country rock. Smuggler. It is closely associated with chlorite, erly lodes of Newman Hill and in the Enterprise galena, sphalerite, and rhodochrosite. Whether LODES. epidote, garnet, and wollastonite (and perhaps blanket. Its delicate pink color makes it easily this barren veining preceded, followed, or coin­ vesuvianite), as well as with galena, sphalerite, recognizable, and it is important in these mines as cided with the deposition of the rich silver min­ Fissure'systems. Partly on account of the oxi­ and chalcopyrite, and its formation was evidently a rough indication of good ore. It occurs massive, erals is not known. It is supposed to have fol­ dation of their upper portions, but more largely connected with the general metamorphism of the often irregularly but beautifully banded with the lowed it. through the concealment of their outcrops by land­ Devonian limestone. It is of no value as an ore quartz and ore of the lodes. It does not, as far as Products of oxidation or weathering. The slides and wash, the lodes of the Rico district are in this region, but has been sometimes mistaken known, occur in this region in the large, well- access of surface waters to the upper portions very poorly exposed on the surface. Their distri­ for sphalerite. formed rhombohedral crystals which characterize of many of the lodes and to some of the blank­ bution is closely connected with the general geo­ Magnetite. This mineral has been mined in this mineral in some other localities. A little resid­ ets and replacement ore bodies has resulted in a logical structure of the district. They are most small amounts for fluxing -purposes from the Mag­ ual rhodochrosite was noted in the oxidized Little variety of products, many of them earthy and of abundant in the central portion of the area and net prospect on the north side t>f Darling Ridge Maggie vein, at the Blackhawk mine, but it is not obscure mineralogical character. The rather thor­ decrease in number and importance toward the and from the Eagle prospect near the head of Sul­ generally abundant outside of Newman Hill. Some ough oxidation of the shattered pyritic blankets periphery of the dome-like uplift of strata from phur Creek. It occurs massive, with a little chal­ of the "spar" veins in other portions of the region of C. H. C. Hill has given rise to great masses of which the Rico Mountains have been carved. copyrite, replacing limestone. It may contain half have a slight pinkish tint, however, and decompose limonitic iron ore, sometimes containing gold and The most important group of fissures is undoubt­ an ounce of silver and $2 in gold per ton. in part to black oxide of manganese, showing either silver, but usually of no value. Associated with edly that of Newman Hill. Second in number Argentite and other silver-bearing minerals. that some of this mineral is present or that the this are earthy lead sulphate, pulverulent hydrous and productiveness are those of Nigger Baby Hill. Argentite, proustite, polybasite, and perhaps "spar" is manganiferous. silica, jarosite (hydrous sulphate of iron and pot­ Other notable fissures are connected with the great stephanite occur in the rich blanket and lode Calcite. This mineral, the common "spar" of ash), serieite, halloysite, gypsum (derived from dislocation known as the Blackhawk fault. Some ores of Newman Hill. They were evidently the miners, is abundant only in the veins of Nigger limestone by sulphate solutions from oxidizing of these seem to have played an important part in among the last ore minerals to form, and to Baby Hill, where it takes the usual place of quartz pyrite), silver in unknown combination, and the mineralization of C. H. C. Hill, but they are them are mainly due the richness of these as the principal gangue mineral. It is generally probably many other minerals. In Nigger Baby so disturbed and buried by landslide material that deposits. They are almost invariably found in finely crystalline, more or less impure, and often Hill the decay of the calcitic veins to a depth of thorough study of them is impossible. Many other vugs in the more solid ore, and when present resembles ordinary limestone. It is always man­ some 200 feet has resulted in soft, black earthy lode fissures occur in Horse Gulch and on Expec­ in the lodes occur along the medial plane of the ganiferous, and readily undergoes decomposition, ores consisting largely of hydrous oxides of man­ tation Mountain, but they are poorly exposed and vein in the spaces left by the comb structure of the whereby the calcium carbonate is largely removed ganese and iron, obscure hydrous compounds of none of them have proved of much economic quartz and other minerals. Argentite is found in and a soft black mass of oxidized manganiferous alumina, carbonates of copper in small amounts, importance. rounded masses, suggestive of particles of shoe­ ore left behind. Calcite is naturally often present and silver, lead, and zinc in unknown conditions, The principal fissures of Newman Hill fall into , maker's wax which have softened and fitted them­ as gangue in the replacements of limestone by ore. probably in part as carbonates. Similar products two classes, distinguished by their strikes or trends selves to the interstices between the earlier crystals. Fluorite. Fluorspar is not of widespread occur­ have resulted from the alteration of a bed of and by the characters of the veins which fill them. Polybasite and proustite also occur in vugs, but in rence in the Rico ore deposits, but is abundant in impure limestone in the Forest-Payroll mine. In The fissures of the more important class are char­ implanted crystals of the forms characterizing these the displacement ore bodies of the Blackhawk mine the Puzzle mine the argentite is stated by Puring- acterized by strikes varying from about N. 25° minerals. Stephanite was not identified at the time and in the Fortune and Duncan prospect north of ton (unpublished MS.) to have been partly altered E. to N. 65° E. They are locally known as of visit, but its occurrence has been reported by Silver Creek. In the Blackhawk it forms the to native silver and to embolite, the chlorbromide "verticals" or "pay veins," but will be referred Farish, who also mentions pyrargyrite. These gangue of pyrite, chalcopyrite, sphalerite, and of silver; and on the M. A. C. claim, adjoining to in this folio as northeasterly fissures. These rich silver-bearing minerals were not seen else­ galena in the large pay shoot outcropping at the the Puzzle, in a shaft now inaccessible, was found are occupied by the ore-bearing lodes, such as where in the district in 1900; but argentite is surface near the bunk house. It is pale lilac or a soft mass of pale-blue allophane (hydrous sili­ Swansea,4 Kitchen, Enterprise, Songbird, Hiawa­ said to have occurred in the Puzzle mine, in a colorless and is easily recognized by its hardness cate of alumina) and kaolinite in a cavity, dis­ tha, Eureka, Jumbo No. 2, Jumbo No. 3, Chest­ quartzose gangue, replacing limestone. and cleavage. In the Fortune and Duncan it solved in the limestone. nut, Klingender, Montezuma, Selenide, Star, and Silver. Native silver is reported from the Enter­ constitutes the gangue of a low-grade pyritic ore Although not strictly a product of oxidation other veins. prise and Puzzle mines, but none was seen in 1900. occupying a breccia zone between quartzite and and weathering, it may be well to mention in this The lode fissures of the second class are charac­ It was probably a product of oxidation. limestone. It is nearly colorless, with slight pink­ place the pulverulent gray mixture of dolomite and terized by strikes ranging from nearly north and Gold. Free gold is rarely detected in the Rico ish and greenish tints, and forms with pyrite and celestite which has remained as a residue after the south to about N. 45° W. They correspond to the ore deposits. Some is said to have been found chalcopyrite a friable crystalline aggregate. A solution of the gypsum of Newman Hill. "barren veins" or "cross veins" of the Newman associated with sphalerite and chalcopyrite in the small quantity of fluorite also occurs in the Hiber- Hill mines, but will be referred to in this folio Enterprise mine, and some embedded in rhodo­ nia tunnel. OCCURRENCE OF THE ORES. simply as northwesterly fissures. They are not chrosite in the Eureka vein of the same mine. A Gypsum. The hydrous sulphate of calcium DEFINITIONS. themselves ore bearing, although they exercised little free gold has also been found in prospects occurs as a gangue mineral, as far as observed, an important influence on the deposition of ore near Calico Peak, but none was seen in place. only in those ores which have replaced massive It is essential to clearness that the usage of such in an overlying blanket. The ore of the Johnny Bull mine contained con­ gypsum in Newman Hill. A portion of the gyp­ terms as are employed in the description of the The northeasterly fissures are usually simple siderable gold, with tellurium and traces of bis­ sum has in such cases recrystallized as transparent Rico ore deposits should be plainly understood. fractures, nearly vertical or dipping southeasterly muth, but it is not known whether any of the gold plates of selenite. The following definitions are intended to make at high angles. They range in width from a mere occurred native. Gold, associated with a little Barite. Heavy spar, or the sulphate of barium, clear the terminology adopted in this folio, and crack up to 2 or 3 feet; but a width of 18 inches is molybdenum, occurred in the Uncle Remus mine, is not known in this region as the gangue of any need have no currency outside of its pages. rare, and the average is probably not much over but whether free or not can not now be ascertained. workable ore body. It occurs, however, on a claim A fissure vein is the filling of a fissure. Lode is 6 inches. They traverse sandstones, shales, and Copper. Native copper \ was noted only in the adjoining the Aztec mine, in a vein supposed to be applied as a more comprehensive term, and may limestones belonging to the lower division of the California prospect, near the head of Iron Draw, the same as that worked in the latter mine. mean either a simple fissure vein or a complex Hermosa. These beds dip somewhat east of south as small crystalline sheets or\skins in the country Chlorite. This mineral occurs abundantly as a assemblage of closely spaced veins or stringers, at a general angle of from 10° to 15°. The fis­ rock. gangue for sphalerite, chalcopyrite, and specularite, often including a certain proportion of mineralized sures were originally opened by normal faulting, Quartz. This is by far the most abundant in the Atlantic Cable and other prospects in the country rock occurring alongside of and within of which the known throw has in no case exceeded gangue mineral in the region. It is nearly always Devonian limestone. It is cryptocrystalline and the fissure zone. A lode is roughly tabular in 10 feet and is usually much less. associated with pyrite, and constitutes the common massive. form, and when occurring in sedimentary rocks The fissures are limited above by the main filling of the lode fissures. Although usually pres­ Garnet, epidote, pyroxene, and wollastonite. cuts across the beds at an appreciable angle. It is Newman Hill blanket, commonly known as the ent in the workable ores, it is there associated with These minerals, all characteristic of contact meta­ usually more nearly vertical than horizontal. That "contact." Some are known above the blanket, Kico. 16 but they contain no ore, and can not, be individ­ work of Cross and Spencer has shown that this filled, has been slightly reopened along one or vein from those on Newman Hill. They are filled ually correlated with those below it. fissure is part of a pronounced zone of faulting both walls and the resulting space freshly filled, by simple veins of moderate size. In their upper The conditions which limit the northeasterly which they have called the Blackhawk fault. The usually with practically barren quartz. portions these veins have undergone decomposition fissures in length are not well known, as appar­ same fissure passes under the landslide of C. H. Though the northeasterly lodes of Newman of a kind to be more fully described hereafter, but ently none of them have been worked to the point C. Hill and probably corresponds to the Pigeon Hill are characteristically simple fissure veins, in their deeper, unoxidized portions they contain of disappearance. Toward the southwest some of lode, or so-called "big fissure" of that hill, and yet they occasionally split up into more com­ a relatively low-grade sphaleritic ore in a calcite the more continuous ones probably reach and are to the C. V. G. lode at Burns. It is quite pos­ plex forms, such as sheeted zones or stringer matrix or gangue. Quartz is either wholly lacking cut off by the Deadwood fault. Toward the north­ sible, also, that the A. B. G. lode, on the west lodes. The , transformation of the veins to or is very subordinate in amount. The bulk of east it is probable that the prominent northeasterly side of the river, is on a branch of the same fault, masses of small, irregular stringers, near their the ore from Nigger Baby Hill has been obtained fissures known in the Enterprise, Newman, and for, as the geological map shows, it occupies a fault junction with the Newman Hill blanket, has from the decomposed upper portions of these rela­ Rico-Aspen mines do not persist across the thick fissure. Details of the Blackhawk fissure are dif­ already been referred to. tively low-grade calcitic veins. intrusive sheet of monzonite-porphyry which rises ficult to obtain. It is the largest and most per­ Toward the north the material of the Newman Little can at present be learned concerning the up over the northern slope of Newman Hill. They sistent in the region, attaining in places a width Hill northeasterly veins changes. Such veins as structure of the numerous northwesterly lodes of either die out before reaching it or are deflected into of over 40 feet. It is not likely, however, that an have been followed to the vicinity of the Laura Horse Gulch, owing to the abandonment of most other courses. open space of this width existed at any one time. shaft show a notable diminution in the value of of the workings on them. The lodes of the None of the northeasterly fissures have been The great width of the lode at certain places is their contents. Immediately north of the Laura Mohawk, Zenith, and Marriage Stake claims explored to great depth, but the section afforded due to the passage of a simple fracture into a shaft the veins have not been much explored; but contain practically no vein filling, but are zones by the Lexington tunnel, about 450 feet below the sheeted zone, and also to repeated reopenings and a little farther north, in the Pro Patria, Fickle of silicifi cation and of impregnation by pyrite Newman Hill blanket, shows that they are smaller fillings along the same zone of fracture. Goddess, and other tunnels, the only northeast­ along fissures in inonzonite-porphyry. The dump at this depth than in the workings above. Intimately connected with the Blackhawk fis­ erly veins found carry little but quartz and pyrite of the Lackawanna shows that this prospect fol­ The northwesterly fissures of Newman Hill are sure are several fissures striking about N. 70° W. and are usually of too low grade to pay for working. lowed a lode containing abundant manganiferous more abundant than the northeasterly; but as they and falling into the main fissure on its northeast The northeasterly veins exploited in the Cow- carbonate. Most of the Horse Gulch lodes evi­ contain no workable ore, they are seldom drifted side. Such are the Little Maggie and Allegheny drey, Tomale, Argonaut, and Bancroft mines, on dently contained much pyrite, in association with on and are consequently not so well exposed as veins, the former dipping northeast at about 60°, the west side of the Dolores River, differ mate­ which occurred bunches of salable ore. the latter. Their dips range from vertical to about while the latter dips southwest at about 70°. rially in their filling from those in Newman Hill. The structure and filling of the nearly east-west 40° and may be northeast or south, the former On the northwestern slope of Telescope Moun­ They contain low-grade argentiferous galena, with lodes of the Nellie Bly vein, on Nigger Baby Hill, being more common. The average dip is lower tain and north of C. H. C. Hill are several north­ much sphalerite and pyrite. The gangue is quartz, are in every way similar to those of the northwest­ than that of the northeasterly fissures. They vary westerly fissures, usually with steep southwest dip. with no rhodochrosite. These veins are generally erly veins on the same hill, such as the Hope or greatly in width, from a mere crevice up to 3 or 4 The two on which are located the Golden Rod small and adherent, or "frozen," to their walls. Grand View. The Last Chance lode, lower down feet. Although often simple fractures, they very and Leap Year claims are strong and persistent They frequently split into small, irregular string­ on the south slope of the hill, is an irregular vein frequently show more complex form. They appear fractures. ers which die out in the country rock. The strik­ between walls of altered porphyry impregnated to have been opened by normal faulting, and in Northwesterly fissures also prevail in Horse ing banded structure of the Enterprise lodes is with pyrite. The vein itself is composed of some cases are reported to have faulted the overly­ Gulch, but they are poorly exposed and not at lacking in these veins of like trend on the west quartz, pyrite, and chalcopyrite. The Aztec lode, ing blanket to the extent of 25 feet throw. Verti­ present producing ore. Such are the fissures of side of the river. supposed to be on the line of the Nellie Bly fault, cal displacement to this amount is, however, rather the Mohawk, Christina, Belzora, Caledonia, Utah, The northwesterly lodes of Newman Hill form consists of banded quartz about 3 feet in width, exceptional. Moreover, as will be seen later, the and other unworked prospects. They are promi­ a notable contrast to those of northeasterly trend. accompanied by much crushed and mineralized structure of the northwesterly lodes shows that nent also in the Little Leonard mine, in monzo- They are filled with white quartz, sometimes con­ country rock on each wall. In the Zulu Chief a considerable part of the observed faulting may nite, on the eastern slope of Expectation Mountain. taining pyrite, but never any ore in commercial mine, near the head of Iron Draw, is a lode have taken place since the fissures were first In addition to the Nellie Bly lode fissure (not amounts. As opposed to the usual solidity of the exhibiting a similar structure to that of the formed. the Nellie Bly fault) on Nigger Baby Hill, which northeasterly veins, the northwesterly lodes are Aztec. It is likely that the two occupy the In the Union-Carbonate mine, on the northern has a low northerly dip, nearly east and west, crushed and accompanied by seams of gouge. same fissure. The Calumet lode, supposed to spur of Dolores Mountain, the fissures show few approximately vertical fissures are known in the The result of this crushing, in extreme cases, correspond to the Last Chance fault, is a large, resemblances to those of the more southerly por­ Lily D. mine, on C. H. C. Hill; in the Calumet is a loose mass of quartz and clay that can be irregular vein up to 5 feet in width, filled with tion of Newman Hill. The productive northeast­ mine, north of Piedmont; in the Aztec mine, in excavated with pick and shovel. quartz containing bunches of pyrite. There are erly fissures are not developed. Numerous other Aztec Gulch; in the California and Zulu Chief In the Newman mines, where the northwesterly no regular walls to the fissure and the country fissures are found striking from N. 60° W. to N. mines, near the head of Iron Draw; and appar­ lodes are better exposed and often less shattered rock is much decomposed. The whole lode is 75° W. that is, more westerly than the north­ ently also in the Eighty-Eight mine, on Silver than elsewhere, they frequently show rather com­ crushed and disturbed by recent movements. westerly veins of the Enterprise mine. Fissures Creek. plex structure, and many of them are, properly The foregoing description shows that, taking the of this general trend are dominant on the northern It appears from the foregoing that the northeast­ speaking, stringer lodes. region as a whole, fissures of like trend are not slope of Dolores Mountain, as shown in the Union- erly fissures which have proved so < productive in In the northern part of Newman Hill and on necessarily characterize.d by similar ores or by Carbonate and Forest-Payroll workings. Newman Hill are characteristic of but a small part the northern slope of Dolores Mountain the more corresponding structures. The miners who first Outside of Newman Hill, northeasterly fissures are of the district, comprising roughly the southern westerly lodes, which there predominate in the worked in Newman Hill were naturally struck of small importance. Several have been worked at half of Newman Hill and that part of the base Union-Carbonate and Forest-Payroll mines, show by the contrast between the local richness of the the eastern base of Expectation Mountain, between of Expectation Mountain lying between Sulphur less crushing or shattering. They have also fur­ northeasterly veins and the poverty of the north­ Sulphur Creek and Iron Draw, in the N. A. Cow- Creek and Iron Draw. Fissures of general north­ nished a little ore. The Forest lode is composed westerly fissures. They distinguished the two as drey, Tomale, Argonaut, and Bancroft mines. The westerly course, on the contrary, are abundant, of about a foot of quartz and low-grade sphaleritic "pay veins" and "barren veins," and this termin­ strikes of these fissures range from N. 40° E. to not only in Newman Hill, but over the entire ore. The latter occurs only in bunches. ology has obtained a certain currency throughout N. 60° E. They are fairly abundant, but small, central portion of the area. Between northeasterly On the northern spurs of Blackhawk Peak the the district. The adoption of richness and poverty rather irregular, and apparently not very persistent. fissures and northwesterly fissures the separation Little Maggie and Allegheny lodes, which also as criteria for classifying the lodes of the region is On Nigger Baby Hill the economically important is fairly definite; but between northwesterly fis­ have a northwesterly course, are simple veins, misleading, particularly as different portions of a fissures are northwesterly in trend. Such are the sures, on the one hand, and east-west fissures but so deeply oxidized that their original filling single lode would thereby frequently be placed in Hope, Cross, Grand View, Phoenix, and Butler and in a few cases north-south fissures, on the and structure can be only surmised. The Black- supposedly distinct classes. veins. Near the top of the hill and on the west­ other, the distinction is less sharp. In the main hawk lode itself is difficult to describe or define, Changes in* the ore with increase of depth. ern slope the average strike is N. 30° W. and the the fissures are simple and of small or moderate as it apparently consists of several nearly parallel There are few more striking features connected average dip northeasterly at about 25°, although size. With a few notable exceptions they either veins of quartz, forming part of the Blackhawk with the Rico ore deposits than the very limited the Cobbler vein is steeper. As these fissures are show no evidence of having been opened by fault zone. These veins are composed of practi­ vertical range of their ores. With the exception followed downward, their dips are found to increase. appreciable faulting or else the throw of the cally barren massive quartz, showing no peculiar­ of the Little Maggie and Allegheny veins and The Phoenix vein, with dips sometimes as low as fault is small. Thrust (i. e., reversed) faults of ity of structure. There are no other accessible those of Newman and Nigger Baby hills, none 15° in the upper workings, steepens to 45° in the importance are not known, and the prevalent workings on this lode until C. H. C. Hill is of the lodes in the district has produced much lower tunnels, on the southern slope of the hill. dislocation has plainly been normal in character. reached, where its probable extension, the Pigeon ore or has been explored to any considerable The strike of the fissures is also found to be more Structure and material of the lodes. The north­ lode, is encountered in the Logan and Pigeon depth. Such ore as has been found occurred in westerly as they are followed southeast, toward easterly fissures of the southern half of Newman mines. Here it greatly resembles the northwest­ isolated pockets, or is so low in grade that the Silver Creek. Hill are usually occupied by simple veins of erly lodes of Newman Hill, but is on a much various spasmodic attempts to work it have been The Iron lode, on the southeastern slope of banded ore, confined between sharply distinct larger scale. Where seen in the Pigeon mine it successively abandoned. Low-grade pyritic ores Nigger Baby Hill, strikes N. 16° W. and dips walls of sandstone or shale. The vein filling shows a width of 40 feet and consists of several extend to greater depths than are anywhere easterly at about 75°. Its trend is thus more consists of quartz, rhodochrosite, sphalerite, galena, stringers of quartz and pyrite separated by sheets reached in the mines, but no success has yet northerly than the majority of the lode fissures chalcopyrite, and pyrite, together with argentite, of country rock. The whole lode is greatly shat­ attended attempts to exploit them. In the cases on the hill. proustite, polybasite, and other highly argentif­ tered and is accompanied by much soft gouge. In of the Little Maggie, Allegheny, and Nigger The Hope, or Grand View, Cross, Phoenix, and erous minerals. These various minerals have the Logan workings the lode shows a width of 12 Baby Hill lodes, the lower limit of the workable Nellie Bly fissures are notable from the fact that been so deposited within the fissure as to give feet and contains masses of soft, crumbling pyrite. ore has thus far been found to correspond to the their strikes are very nearly parallel to those of the a pronounced but rather irregular banding to Neither the Blackhawk nor the Pigeon lode car­ bottom of the zone of oxidation. The rich ore beds which inclose them, while their dips, though the veins. Along the middle of the vein there ries any workable ore as far as known, although is here due to purely secondary processes. generally slightly steeper in angle, correspond in is usually a zone of comb structure, or vugs, some appears to have been extracted from that In the northeasterly lodes of Newman Hill, direction to those of the strataA Consequently consisting largely of quartz and of the high-grade portion of the fissure which traverses the Uncle however, there is a decided change which ante­ these fissures cross the bedding planes at a very argentiferous minerals. The last are particularly Ned ground, on Telescope Mountain. dates all superficial oxidation. Unlike secondary acute angle, causing the lodes, particularly in the abundant in the quartz vugs, which nearly always The C. V. G. lode, at Burns, not now accessible, enrichments due to ordinary oxidizing processes, upper part of the hill, to closely resemble the form occur along the medial plane of the vein, and is probably the same as the Pigeon. The A. B. G., the depth at which the Newman Hill veins change of deposit that has been termed a blanket. sometimes along planes near the wall. on the west side of the river, is a strong vein up to their character bears no relation to the topographic The main fissure, on which are located the Black- In most cases the filling of the northeasterly 6 feet in width, carrying streaks of galena, sphal­ surface, but is more or less constant with reference hawk, Argentine, and Uncle Ned mines, has a gen­ fissures from the walls to the medial zone of vugs erite, and pyrite in a gangue of quartz and calcite. to an overlying blanket. For a maximum distance eral course of N. 40° W. and dips northeast at appears to have been a continuous process; but in On Nigger Baby Hill the northwesterly fissures of about 150 feet below this blanket, the northeast­ angles varying from 50° to 80°. The geological other instances the fissure, after having once been are distinguished by an entirely different type of erly lodes contain pay shoots of rich ore. Below 17 this depth the valuable silver minerals disappear; It is highly probable that the Deadwood fault, time has elapsed after the production of the first the blanket is cut off by the Deadwood fault. As galena, sphalerite, and rhodochrosite vanish; and south of Newman Hill, cuts off the Newman Hill fracture. It is probable that in the network of it approaches the latter it turns down with a rap­ the veins, which above produced ore carrying blanket and lodes, throwing their southern contin­ intersecting and partly incipient fractures in New­ idly increasing southerly dip and passes out of many hundreds of ounces of silver per ton, uation downward and westward. A zone of faults man Hill the northeasterly fractures were as a reach of the present workings. become practically barren quartz and pyrite. So passing down the west slope of Dolores Mountain, whole the first to open sufficiently to allow of The blanket has been extensively exploited by far as their material is concerned these lower por­ past the Laura shaft, has probably offset the lodes vein deposition. If the northwesterly fissures are the Enterprise, Newman, Rico-Aspen, and other tions are indistinguishable from the northwesterly on the north. Neither of these points, however, entirely of later date than the northeasterly veins, mines and is estimated to have produced at least veins. In the Lexington tunnel, which crosscuts could be determined underground at the time of it is impossible to account for the fact that the lat­ one-third of the entire output of the district. At the Newman Hill veins at depths varying from visit. ter occasionally follow the former for short dis­ present very few known bodies of ore remain, and 350 to 550 feet below the blanket, they are small The Nellie Bly fault, which passes over the end tances, and that they sometimes die out or divide the labyrinth of contact workings has been allowed and practically barren, consisting of quartz and a of Nigger Baby Hill and brings the massive lime­ into stringers just as a northwesterly fracture is to cave in. little pyrite. stones of the medial division of the Hermosa for­ approached. That the ore changes abruptly in The blanket rests everywhere upon a bed of Oxidation of the lodes. In the southern part of mation up on the south until they abut against the value at the crossing of northwesterly fissures, limestone, familiarly known as "short lime," but Newman Hill, as no lodes are worked above the Rico beds on the north, passes through the work­ even where no faulting is perceptible, is strongly which may here be conveniently referred to as the Newman Hill blanket and as the latter is protected ings of the Grand View and Iron mines, without, insisted on by the miners. It is best explained blanket limestone. This bed varies in thickness from the free access of surface waters by an imper­ as far as can be seen, interrupting the Phoenix, by supposing that the northwesterly fractures from a few inches to about 2 feet. vious layer or "roof" of shale, oxidation has played Grand View, or Iron veins. It seems necessary to existed when the ore was being deposited in the Below the limestone are usually found 5 or 6 but a minor part in the development of the ore conclude that these veins are of later origin than more open fissures. If the northwesterly fissures feet of dark shales alternating with very thin bodies. In the Union-Carbonate mine some oxi­ the fault. were of altogether later date and simply faulted lenses of sandstone. Under these is generally a dized ore was mined from a northwesterly fissure. Relative ages of the lodes. The question of the the northeasterly veins, it should be possible to second bed of limestone similar to that above, This ore was near the surface, and extended to a relative ages of the northeasterly and northwest­ consistently match the faulted lodes on opposite and below this limestone come sandy shales and depth of 30 or 40 feet below an overlying blanket, erly lodes of Newman Hill is of theoretical and sides of the dislocation. In many cases this is sandstones, the latter predominating with increas­ also oxidized. On Nigger Baby Hill, however, practical importance. The observed phenomena utterly impossible; for whereas some lodes are ing depth. The blanket resting upon the blanket and in the Little Maggie and Allegheny lodes, that bear on the question may be briefly sum­ found to be slightly displaced and can be recog­ limestone varies in thickness from 2 to 20 feet; descending surface water has been the effective marized as follows: nized and followed, others have no continuation the average is perhaps 6 feet. It is overlain by agent in producing workable deposits of ore. (1) The lodes of the two systems are distinctly beyond the fissure. Attempts to find and identify fissile black shale, which, as far as known, is never The depth to which oxidation has penetrated in different in trend. such continuations have led to absurdities, such absent and performs an important function in Nigger Baby Hill is apparently variable. Owing (2) The northwesterly lodes are practically as supposing that a fracture which throws the over­ keeping out by its imperviousness the abundant to the steep sides and faulted structure of the hill barren, whereas the northeasterly veins usually lying blanket less than 24 feet and does not per­ surface waters. Above the shale are beds of the permanent ground-water surface is low, and contain rich ore to a depth of about 150 feet ceptibly offset the Eureka lode can offset one lode sandstone and sandy shale, all more or less oxidation has probably nowhere extended down to below the Newman Hill blanket. 175 feet and another, but a few feet away, only 20 fractured and soft, which extend up to the loose it. The process has been irregularly limited by (3) The northeasterly veins sometimes show a feet. surficial material covering the hill. very local physical and chemical factors and by later generation of quartz and pyrite. On the other hand, unless it be allowed that the As a rule the breccia constitutes but the upper time. From the extent of the old stopes it appears (4) The northwesterly veins, when not too much principal opening and filling of the northwesterly part of the blanket. The lower part, resting probable that complete oxidation extends for about crushed, usually show banding due to repeated lodes was later than the ore deposition in the directly upon the blanket limestone, is commonly 200 feet from the present surface of the hill near openings and fillings. northeasterly lodes, it is impossible to account for a gray, soft, silty material, frequently showing a its summit, growing less on its flanks. (5) The northeasterly veins are solid and often the fact that the former cross the latter, and no laminated structure suggestive of a water-laid ori­ The process of decomposition was undoubtedly adherent to their walls; the northwesterly lodes good reason appears why both should not have gin. This deposit varies in thickness and in its greatly facilitated by the fact that the veins contain are almost invariably crushed and accompanied by been equally filled with ore. The opening and upper part is mingled with fragments of shale. a calcitic and not a quartzose gangue. The impure gouge. filling of the northwesterly fissures may in part Wherever ore occurs in the blanket it is usually as calcite is dissolved and carried away, leaving behind (6) The northwesterly fissures generally cross correspond to the slight reopening and filling a replacement of this material. a sooty residue, consisting chiefly of hydrous oxides the others without being themselves deflected. with barren quartz and pyrite observed in con­ The foregoing description applies to the typical of manganese and iron, with some silica and alu­ (7) In some cases the northwesterly fissures fault nection with the northeasterly lodes. and usual appearance of the blanket, but in some mina. This soft black or brown material is some­ the northeasterly lodes, but more commonly they Finally, the shattered nature of most of the places considerable bodies of gypsum occur above times rich in silver. pass through the latter without offsetting them. northwesterly lodes and their constant accom­ the blanket limestone, occupying space usually The alteration of the upper portions of the Marked changes, in the value of the ore are said paniment by seams of gouge show that they filled by the blanket. Such a niass occurs in the Little Maggie and Allegheny veins is somewhat to occur at such crossings. have continued to be planes of more or less Enterprise mine south of the Group tunnel and similar to that described. The Little Maggie vein, (8) A northeasterly vein is sometimes lost at movement up to recent times. It is conse­ extends toward the southeast in the* direction of however, seems to have contained more copper the crossing of a northwesterly vein, as was the quently impossible to determine how much of the Vestal and Aspen shafts. Another large body than is present in the Nigger Baby Hill veins, case with the Hiawatha on the 100-foot level of the observed moderate faulting along these fis­ occurs in the Rico-Aspen mine,, near the Silver and some residual kernels of pink rhodochrosite the Enterprise mine and the Chestnut and New- sures took place prior to their complete filling Glance shaft. The maximum thickness of the gyp­ were noted. man veins in the Newman mines. It has been with quartz, and how much has taken place sum is not now revealed, but it probably exceeds Relation of the lodes to faults. The relation of commonly assumed in such cases that the north­ during the subsequent crushing of their veins. 10 feet, It sometimes rests directly upon the the lode fissures to the important structural faults easterly vein has been faulted and offset for dis­ It has been assumed in the foregoing discus­ blanket limestone, but in other places the two established by the geological mapping is neither so tances up to 175 feet. It is to be noted that the sion that lodes of like trend and similar character rocks are separated by a varying thickness of the close nor so obvious as on a priori grounds might northeasterly vein is not always sharply cut off are of practically the same age. It is possible, gray, silty material already described as a char­ be expected. at the supposed fault, but small stringers are however, that some of the northwesterly lodes acteristic constituent of the lower portion of the Productive lodes do not occur in the fissures of sometimes present, continuing beyond the latter. may be earlier and some later than the north­ blanket. Wherever this occurs, however, the considerable faults. Out of the 50 or more fault Furthermore, the supposed offset of any one of easterly veins. While it would be difficult to under side of the gypsum shows evident signs of fissures drawn on the geological map, not one has the northeasterly veins cut by a northwesterly disprove this latter view, it is not considered solution. This is well shown in the Rico-Aspen been shown to contain any large bodies of ore. fissure is not always comparable in amount with probable. mine, near the Silver Glance shaft. The Blackhawk lode, which does not itself con­ the offset of neighboring veins cut by the same Beyond the bounds of Newman Hill, it is found The gypsum is pitted with rounded, pothole tain workable ore as far as known, is close to' a fissure. that the same age relationship exists between the cavities, up to 8 feet in diameter. Lying upon large fault, but it is by no means certain that it (9) Northeasterly veins sometimes contract or northeasterly and northwesterly lodes on the west the limestone under these cavities is always more fills the actual fault fissures. The same may be divide into small stringers on approaching a north­ side of the Dolores River, between Sulphur and or less of. the silty material, which chemical analy­ said of its probable continuation, the Pigeon lode, westerly lode. Iron creeks. In other portions of the district, sis shows to be a mixture of dolomite and celes- in C. H. C. Hill. The A. B. G. lode, at Burns, is (10) None of the lode fissures displace the over­ however, there is scant opportunity for determin­ tite about 56 per cent of the former to about 37 possibly on a branch of the Blackhawk fault. It lying blanket more than 25 feet, most of them ing the sequence of lode fissures of different trend. per cent of the latter. contains some low-grade ore, which, however, has much less than this. As already noted, the northeasterly lodes are The Newman Hill gypsum was originally a bed never been worked on a commercial scale. The (11) In one case (that of the Jumbo No. 3 vein, absent or insignificant. laid down with the Hermosa shales, sandstones, Nellie Bly fault certainly does not coincide with Enterprise mine) a northeasterly vein is known to and limestones. There are good reasons for sup­ BLANKETS. the Nellie Bly vein, but appears as a close, incon­ turn into a northwesterly fissure for a short dis­ posing that it was lenticular in form and may not spicuous fissure a few feet south of the vein. It is tance and then resume nearly its normal course. Of the various blankets occurring in the Rico have extended far beyond the present bounds of possible that the western extension of this fault The deflected portion of the vein has been brec- district, the so-called Newman Hill or Enterprise Newman Hill. passes through the Aztec and Zulu Chief lodes, ciated, and subsequently healed by white quartz, "contact" is most important. This is, for the The gypsum is in process of solution and this but these properties can scarcely be classed as pro­ which was later shattered in its turn. In a few most part, an unconsolidated breccia occurring solution is accompanied step by step by the accu­ ductive. The\Last Chance fault, although taking rare instances small northwesterly fissures have nearly midway between the top and bottom of mulation of the silty residue of dolomite and its designation from a prospect of that name, is been observed containing the characteristic filling the series of sandstones, shales, and limestones celestite. The residual gypsum masses show char­ certainly not identical with the Last Chance lode of the northeasterly lodes. which make up the lower division of the Her­ acteristic cuspate solution forms on their periph­ fissure. It is shown on the map as passing through (12) The Eureka vein pursues a nearly straight mosa. It underlies the southern half of Newman eries. The blanket breccia, closely following the workings of the Calumet, an unproductive pros­ course through the principal Newman Hill work­ Hill, but its extent is only approximately known. up the process of ablation, crowds snugly against pect on the west side of the river. ings, without being offset by the northwesterly On the west it should outcrop along the hillside the wasting gypsum. The gypsum not only once The question of the coincidence of faults and fissures which cross it and which are commonly above the adits of the Rico-Aspen, Newman Hill, possessed a greater horizontal extent than at pres­ lodes may be justly summed up in the statement supposed to have offset neighboring lodes, in some and Enterprise mines, were it not for the thick ent, but was probably at one time coextensive that the most productive lode fissures of the dis­ cases as much as 175 feet. cloak of wash which conceals the rock in place. with what is now known as the Enterprise "con­ trict show very little faulting along them, while It is believed that the hypothesis which best On the east it conforms to the general southeast­ tact," or blanket. The recognition of the fact fault fissures of sufficient extent to be structurally reconciles the various observed facts is one which erly dip of the beds (about 15°) and passes under that the present gypsum masses are mere waste important contain little or no ore. It is but fair supposes the initial fractures, both northwesterly Dolores Mountain. On the north it has not been remnants of the continuous bed that once occu­ to remark, however, that the fault fissures have and northeasterly, to have been formed at practi­ followed far beyond the Laura shaft. pied the entire space is of more importance for been scarcely prospected. cally the same time. Studies of vein structure in Other blankets occur to the north in the Union- an understanding of the genesis of the ore deposit Considerable structural faults may also be this and other regions show that lode fissures may Carbonate mine, but none of them has been iden­ than is the distinct question of the origin of the important in displacing lodes which they cross. not open to their full width until some considerable tified as the Enterprise "contact." On the south gypsum itself. Rico. 18

The blanket as thus far described, consisting of fact that they Occur in lower Hermosa rocks, can a soft, banded stratum, in which are layers of an stringers. These stringers are particularly numer­ shale breccia and of pulverulent dolomite and be correlated with the Enterprise blanket. The ocherous, yellow powder containing a considerable ous and noticeable in the blanket limestone and celestite, has been locally modified through proc­ principal ore-bearing "contact" of this mine is proportion of silver, alternating with bands of the shales which underlie it. They commonly esses connected with ore deposition. The results of composed of a zone of breccia resting upon a sheet impure, sandy sulphate of lead and streaks of a consist of barren white quartz, although the these processes may be classed as (1) silicificatiou of intrusive porphyry. It is sometimes overlain white, material which proved on chemical analysis stringers above some of the northeasterly veins and (2) deposition of ore. Both of these modifi­ by porphyry and sometimes passes upward into to consist of about 83 per cent of silica, 5 per contain rich ore. The aggregate result of these cations are directly connected with the lode fis­ relatively undisturbed shales. It has a thickness cent of water, and 9 per cent of lead sulphate. small fractures is to slightly fault the overlying sures and occur where the latter meet the blanket. of 4 to 5 feet and in its less altered condition con­ The silica is probably in the opaline form. blanket, and in the case of the northwesterly lodes Workable ore is sometimes associated with such a sists of some of the mineralized fragments of shale The main blanket of C. H. C. Hill is associated there is usually also a fault plane of more recent silicified blanket, but more often as silicification and porphyry; but where certain fissures intersect with considerable alteration of the rocks between date, defined by a seam of gouge, which may pass becomes prominent the ore vanishes, or vice versa. the blanket it is entirely replaced by masses of which it lies. The limestone which usually under­ upward through the blanket. The blanket ore occurs chiefly as a replacement quartz and pyrite containing bodies of low-grade lies the blanket has been extensively attacked and In the blanket itself all the lodes, as far as of the pulverulent lower part of the blanket above ore. removed by chemical processes, with the formation could be seen or learned, entirely disappear, but both the northeasterly and northwesterly lodes, Below the blanket just described occur several of small quantities of gypsum, halloysite (an amor­ their existence below is indicated by the occur­ but it sometimes extends up into the breccia, smaller and less important ones of different char­ phous hydrous silicate of alumina), and other rence of the blanket ore, which caps them. These where it may, perhaps, have formed partly as acter. These are found in beds of dark shale, par­ obscure products. The sandstone, unlike the flat ribbons of ore, which follow the courses of the the filling of interstitial spaces as well as by ticularly in thin beds lying between relatively limestone, is not directly soluble or convertible lodes beneath them, attain a maximum width of replacement. It often partly replaces the blanket massive beds of sandstone or sheets of porphyry, into soluble substances. The change in this case 40 feet and a maximum thickness of 3 feet, being limestone, particularly where the latter is brec- and consist of plastic yellow clay. The clay is an involves the removal of some of the quartz, with usually larger over northwesterly than over north­ ciated. alteration product of the shale in place, and often the accumulation of much sericite and small quan­ easterly lodes. Above the intersections of two or In the vicinity of the Vestal shaft and in por­ preserves traces of the original shaly lamination. tities of limonite, anglesite, and some hydrous more lodes the blanket-ore body is often much tions of the Enterprise mine ore occurred in the These clayey zones sometimes contain bunches of magnesium mineral. more extensive. gypsum itself. It was found as irregular bunches oxidized ore near intersecting vertical fissures. In addition to the main blanket of C. H. C. With one or two possible exceptions, which are in the lower part of the bed, having metaso- In the Forest-Payroll mine, about 1000 feet Hill, several blanket-like masses of pyrite occur at no longer open to examination, the lodes of New­ matically replaced the gypsum. Such ore has a northeast of the Union-Carbonate, there are two other horizons in the stratigraphic series. Three man Hill do not extend above the blanket. The gangue of quartz, rhodochrosite, and selenite. blankets from 30 to 50 feet apart, neither of which such bodies of crumbling pyrite, inclosed in shales overlying rocks are much fractured and contain The usual blanket ore of the Enterprise and has as yet been correlated with any of those in the and aggregating over 50 feet in thickness, were some unimportant veins, which may have been Rico-Aspen mines is similar to that of the north­ Union-Carbonate mine, although, like the latter, passed through in the Crebec shaft before reaching formed at the same time as the lodes beneath the easterly lodes, but presents certain differences they lie in strata belonging to the lower division of the main ore blanket. Although nearer the sur­ blanket, but there is no good reason to suppose which are always sufficient to identify it. It is the Hermosa. The lower blanket is about 5 feet face than the latter, the pyritic bodies show almost that they were ever continuous with the lat­ usually less solid and shows less regular banding in thickness. It is a breccia of shale mingled with no oxidation and are too low in grade to be ter. The upward limitation of the lodes by the or none at all. It consists of galena, sphalerite, yellow clay and resembles some of the blankets of worked under present conditions. Similar but "contact" is a natural consequence of the slight and one or more silver-bearing minerals in a the Union-Carbonate mine. It is underlain by smaller bodies were noted in the Logan mine, faulting which accompanied the opening of their quartz and rhodochrosite gangue which is often sandstones and overlain by shale. This blanket extending into beds of shale on the southwest fissures and the yielding, fissile character of the subordinate in amount. Rhodochrosite is not so contains some rather low-grade ore where inter­ side of the Pigeon lode. beds in which the "contact" lies. If the lodes abundant as in the lodes. With the foregoing sected and slightly faulted by nearly vertical north­ As a whole, the blankets of C. H. C. Hill pre­ continued in full strength up to the blanket and minerals are often associated small amounts of westerly fissures. A little galena is found, but the sent a striking contrast to that of the Enterprise were there abruptly cut off, it would be natural to chalcopyrite and sometimes argentiferous tetra- ore is usually oxidized. The upper blanket is 5 mine. The persistent shale breccia, passing below suppose that they once extended to the surface and hedrite. Common pyrite is apparently very or 6 feet in thickness and rests sometimes on shaly, into the silty mixture of dolomite and celestite, is that their upper portions had been displaced by subordinate in the rich blanket ore. The rich calcareous sandstone, sometimes on a sheet of por­ absent in C. H. C. Hill, where the blankets appear faulting. But not only do they practically die silver-bearing minerals which have been identi­ phyry. It is overlain by disturbed and broken to have been originally sheet-like or lenticular out, as lodes, before the blanket is reached, but fied include polybasite, argentite, proustite, and shales. In its appearance this blanket is similar bodies consisting chiefly of pyrite. The Enter­ the relations of the blanket ore to the lodes and probably stephanite. to the lower one, but it contains irregular masses prise "contact" is practically unoxidized, while of the blanket to the gypsum and the nature of Many other blankets occur in the Rico Moun­ of limestone which bear much,the same relation to the main blanket of C. H. C. Hill is almost wholly the blanket itself all support the view that the tains, some ore bearing and some not. Most of the blanket that the gypsum bears to the Enter­ transformed to secondary products and its earlier latter has not been a plane of extensive or gen­ them are in rocks belonging to the lower division prise "contact." The limestone has been irregu­ history thereby obscured. Without such oxida­ eral faulting. of the Hermosa, and some of them occur at the larly dissolved, leaving behind a black, sooty tion and secondary enrichment, however, the ore In the Union-Carbonate and Forest-Payroll same stratigraphic horizon as the Enterprise residue consisting largely of hydrous oxides of would probably have been too poor to mine. mines the blanket ore is connected ivith north­ blanket; but certain conditions entered into the manganese and iron, with considerable magnesia On the west side of the Dolores River a small westerly fissures which apparently pass through formation of the latter which increased its ore- and alumina in unknown combinations. The ore blanket occurs in the A. B. G., a prospect at the blankets without interruption. Whether or bearing capacity and which appear to have been of the upper blanket consists mainly of galena in Burns. This is composed of partly oxidized, not the fissures are slightly faulted as they trav­ absent in the formation of all other blankets various stages of alteration to cerussite and angle- crumbling pyrite, about a foot in thickness, erse the blankets could not be satisfactorily examined. site. It occurs in small bodies at the intersection lying between the beds of the lower Hermosa on determined with the available exposures. The A small blanket, of limited horizontal extent, is of the blanket by northwesterly fissures. It is not the southwest side of the A. B. G. lode. prevalent oxidation in these mines and the recent known in the Enterprise workings from 100 to known to occur as a direct replacement of the Two blankets are known in the Great Western slipping along the lodes, as attested by the pres­ 150 feet below the main "contact.'" It is a brec­ limestone. prospect, on the north side of Horse Creek, in ence of gouge, tend to obscure the original rela­ cia of dark shale which carried a little ore along­ The South Park mine, at the northwest base of rocks that probably belong to the middle division tionship between lode and blanket. side of the Enterprise vein. Newman Hill, was unfortunately not accessible in of the Hermosa and dip 15°, to the north. The In C. 'H. C. Hill the mineralization of the main Still lower in the stratigraphic series and fully 1900. The ore is reported to have occurred partly upper "contact" is a soft, dark breccia of crushed blanket and of most of the smaller ones has plainly 400 feet below the main Enterprise blanket is the in a blanket of dark shales. If this is true this shale, sandstone, and limestone, lying iBetween two emanated from the great Pigeon-Blackhawk lode. blanket of the New Year mine. This is a strong blanket is the lowest known in Newman Hill, massive beds of limestone. The lower blanket is Here, too, subsequent movement and oxidation zone of brecciated shales resting upon an intrusive since it occurs below the gray sheet of monzonite- about 6 inches thick and consists of brecciated have obscured the details of original connection." sheet of porphyry and overlain by soft shales. It porphyry, while all the others lie above it. shale lying between shaly limestone below and In this case the blanket is without much doubt is partly silicified and contains some low-grade ore. In C. H. G. Hill there is apparently one exten­ shaly sandstone above. It is apparently of no considerably faulted by this lode, but it is impos­ On the west side of the Dolores River "contacts" sive blanket which has been exploited in the great horizontal extent. Neither of the blankets sible to say how much of the faulting took place occur in the N. A. Cowdrey, Bancroft, Little Mag­ Princeton, C. H. C., Wellington, Logan, and has produced much ore. before and how much after mineralization. All gie, and Silver Swan mines. These are all in lower Pigeon mines. All of the blanket thus far The Sambo blanket, on the northeastern spur the ore so far extracted from the blankets of C. H. Hermosa rocks, but occur at various horizons. The explored lies in a great landslide and is conse­ of Expectation Mountain, occupies a bedding C. Hill has occurred on the southwest side of the fault in lower Hermosa rocks, between black Cowdrevt/ blanket consists of two members of dis- quently much broken and disturbed. Owing to Pigeon-Blackhawk lode. On the northeast side turbed black shale, separated by a bed of limestone this fact, coupled with the caving of most of the shales below and gray calcareous shales above. of the lode the blanket horizon was probably about 3 inches thick. The entire blanket is under­ old workings, it is impossible to be sure that A zone of brecciation is replaced by quartz and dropped by the original fault; but how far its lain and overlain by massive sandstones. Some the principal blanket worked in the above-named ore for a width of about 30 feet on the southwest position has since been changed by later move­ low-grade ore occurs near the bottom of the lower mines is continuous and identical throughout. It side of a northwesterly lode. The Sambo has pro­ ment, including landsliding, is not known. shale, at its intersection by a lode. The Bancroft certainly presents a more varied character and is duced considerable rather low-grade ore. The blanket ore of the C. V. G. mine, at Burns, blanket consists of soft shale breccia, mixed with less open to a simple explanation than those thus Lastly, an unimportant blanket is known in is evidently connected with the northwesterly lode clay, which rests upon massive sandstone and is far described. The blanket is thought to lie the Montezuma mine, near Piedmont, consisting lying northeast of it, but the connection is not overlain by shale. It is from 2 to 3 inches in between beds of the upper division of the Her­ of crushed rock, gouge, and ore, resting partly exposed. . \ thickness. In the Silver Swan mine a blanket mosa, but this is by no means certain. It is usu­ on quartzite and partly on intrusive monzonite- In the Sambo mine the blanket is continuous consisting of about 6 inches of soft black clay or ally 5 feet or less in thickness and rests in some porphyry. It is overlain by shales and sand­ with a lode which faults the lower Hermosa beds gouge passes upward in an unknown thickness of places upon sandstone and in others upon lime­ stone. This is apparently a bedding fault along to the extent of at least 4 feet. The ore occurs on brecciated black shale. In the Little Maggie the stone. It is overlain by sandstone or shale. It is a bed of limestone which has been fractured and the southwest side of the lode and extends into the principal blanket consists of dark shale breccia most commonly composed of limonite, sometimes partly replaced by low-grade pyritic ore. blanket for a maximum distance of about 30 feet. resting upon a bed of limestone and overlain by as a fairly firm, cavernous mass, but often as a KELATION OP LODES AND BLANKETS. REPLACEMENTS IN LIMESTONE. shale. It more closely resembles the Enterprise loose, yellow, earthy material, which falls to blanket than do the breccia zones of the other powder between the fingers. In certain por­ As far as mining developments have shown, ore The principal examples of ore deposition by mines, but none of the powdery mixture of dol­ tions of the blanket the limonite passes into occurring in a blanket is always directly connected replacement are found in the Blackhawk, Iron, omite and celestite was observed. This breccia masses of crumbling iron pyrite, from which with some lode. The nature of the connection, and probably, also, the Puzzle mines. The Atlan­ contains small bunches of low-grade ore that has mineral it was undoubtedly formed by oxidation. however, is not always the same. tic Cable and other prospects in the Devonian been but little explored. Ore, when present, occurs in the lower part of the In the Enterprise, Newman, and Rico-Aspen limestone north of Rico must also be placed in In the Union-Carbonate mine, on the north spur blanket and is almost without exception completely mines, in Newman Hill, the northeasterly and the same category. of Dolores Mountain, several blankets are known, oxidized. northwesterly lodes, as they approach the blanket In the Blackhawk the replacement ore bodies none of which, in spite of their proximity and the In the Princeton mine workable ore occurs as from below, split up into innumerable small occur on the northeast side of a lode belonging to 19 the Blackhawk fault zone. They have irregularly The vertical extent of the original Rico uplift is Western, Bancroft, Silver Swan, Little Maggie, and relatively slight readjustment, chiefly subsidence, replaced a bed of massive limestone, belonging estimated by Cross and Spencer at about 4500 feet. New Year mines, and probably the main "con­ necessary to restore the rocks to gravitative equi­ near the top of the middle division of the Her­ A minor part of this elevation, at least 800 feet, is tact" of the Union-Carbonate mine, which seem librium after the greater faults had determined the mosa, and dipping away from the lode to the connected with the intrusion of sheets of porphyry to owe their existence chiefly to bedding faults. main structure of the region. They are a record northeast at an angle of about 25°. They attain between the beds of the sedimentary series. But It can not be positively affirmed, however, that all of the final settling down of the region under grav­ a thickness of more than 15 feet and extend to a the major part of the uplift was subsequent to of the blankets named are purely fault breccias. ity after an episode of vigorous faulting and uplift. maximum distance of 50 or 60 feet from the lode. these intrusions and was associated with profound It is quite possible that some of them may have Pay shoots. The superficial character thus far These bodies are composed in great part of massive faiilting, showing "the action at this center of a been initiated by the solution of gypsum, as in shown by the pay shoots is one of the most inter­ pyrite, of no present value, in which lie irregular powerful vertical upthrust which is not demonstra- Newman Hill, and that the traces of such genesis esting phenomena of the Rico district and vitally bodies of workable ore. The best of this ore con­ bly connected with igneous intrusion." have been obliterated by subsequent movement. concerns the permanence of its mining industry. sists of fine-grained galena, chalcopyrite, and pyrite, That the ore deposition is chiefly connected with The stratigraphic conditions under which, in As pointed out in the preceding sections, only a in a gangue of fluorite. Such ore passes toward its the later phases of uplift is shown by the fact that this region, such brecciation has taken place are relatively small number of the lodes have been periphery into lower-grade ore, large quantities of the intrusive porphyries are themselves traversed fairly constant. The fissile black shales of the themselves productive, and those to a compara­ which are still standing in the mine. This is com­ by lodes and are invariably mineralized when lower Hermosa are the rocks usually involved, tive slight depth. In the case of the Nigger Baby posed of massive compact sphalerite and galena, occurring in contact with ore bodies. not only by reason of their intrinsic weakness, Hill, Little Maggie, and Allegheny veins, the most with a little chalcopyrite, and practically no gangue. Origin of the blankets. Study of the Enterprise but on account of their present distribution with important falling off in value takes place at the This ore in turn passes into enormous masses of blanket has shown that it is essentially due to the reference to the center of orographic movement. passage of oxidized into unoxidized vein matter. nearly pure, worthless pyrite, or is directly inclosed removal, by solution, of a massive bed of gypsum Such shales are particularly susceptible to breccia­ These veins thus owe their workable portions to in limestone. As a rule there is no sharp bound­ which may have been from 15 to 30 feet in thick­ tion near their contact with some more rigid mem­ a process of enrichment which is of common occur­ ary between limestone and ore. The latter some­ ness. As the gypsum has not dissolved at an equal ber of the lithological series, such as a sheet of rence elsewhere, is wholly secondary, and is well times penetrates the white granular limestone in rate throughout,o ' and has been largely<_J t/ attacked from porphyry or massive beds of sandstone or lime­ understood in its general features. Below this small bunches, but more often the limestone next below, with the consequent formation of caverns, stone. When the bed of shale is relatively thin zone of oxidation the veins have not been suc­ the ore is changed to jasperoid. the overlying beds must have subsided unevenly and is inclosed between two massive strata, it is cessfully worked, and it is not certainly known Similar in character is the occurrence of the ore as the gypsum was removed, and were probably frequently reduced entirely to breccia. That the whether the value of the primary ore suffers a still in the Iron mine. Here middle Hermosa lime­ often let down abruptly by the enlargement and actual relative movement of the stronger beds need further diminution with increasing depth. This, stone is partly replaced by ore on both sides of final collapse of caverns of solution in the under not be very great to produce brecciation in the however, seems probable. a lode which does not noticeably fault the beds. side of the bed. -Nearly all stages of the process shales between them is well illustrated in the The rather, abrupt falling off in value of the In this mine the workable ore extends less than may yet be seen in the Newman Hill mines, from Rico-Aspen mine, near the Silver Glance shaft. northeasterly veins of Newman Hill, at a depth 12 feet from the lode, except in some cases where the usual "contact," with no remaining gypsum, Some of the shales below the blanket limestone, of less than 200. feet below the blanket, is not so replacement has worked out along minor fissures to a thick bed of gypsum showing (at least on its lying between thin beds of sandstone, are here readily explained. It will be recalled that both in the limestone. The ore, which is of low grade, under surface, which alone is visible) places where locally folded and crumpled to the verge of brec­ the northeasterly and northwesterly lodes are is usually massive, consisting chiefly of pyrite and solution has not yet been active. This irregular ciation, while the thin beds above and below them capped by pay shoots in the Enterprise blanket, chalcopyrite, with more or less calcite and quartz subsidence, proceeding at different rates at differ­ are undisturbed .and the crumpling itself has no those over the northwesterly lodes being usually as gangue. It has replaced the limestone directly, ent times throughout the area now occupied by the great horizontal extent. Similar incipient "con­ the larger and richer bodies. So far as can be with little or no formation of jasperoid. blanket, is amply sufficient to account for the brec- tacts" in various stages of development were noted determined, the blanket ore over the northeasterly The mode of occurrence of the ore in the Puzzle ciation of the overlying shale and for the generally in the Little Maggie and other mines, often dying lodes did not differ mineralogically from that over mine is at present not directly determinable. It shattered character of the rock up to the base of out within a short horizontal distance and plainly the northwesterly lodes. Furthermore, at a depth appears, however, to have been an irregular the wash covering Newman Hill. formed by only slight movements along planes of below the blanket greater than 200 feet, the con­ replacement of limestone by a siliceous ore con­ That more or less movement has taken place in bedding. tents of the two sets of lodes are mineralogically taining argentite or other rich silver minerals. the mass of shale breccia precipitated upon the soft, A part of the necessary movement probably took identical. It thus appears that the mineralogical The ore occurred in a landslide block. pulverulent residue of the gypsum is well shown place at the time of the initial doming of the beds difference between the northeasterly and north­ In the Atlantic Cable and neighboring prospects by the occurrence within it of irregular seams of by laccolithic porphyry intrusions. But the greater westerly lodes, which is so striking a feature in the Devonian limestone has been most irregularly gouge. Such movement is probably still in prog­ part, and obviously that which produced brecciation certain of the mine workings, is not, after all, so replaced by more or less isolated bunches of ore. ress, but it is chiefly local in character, owing along the contacts between porphyry and shale, must great as might at first be supposed if their compar­ It is reported that one of these bodies, discovered to varying adjustments, under gravitative stress, have been effected when the final elevation was given ison were confined to one level. The contrast is since the time of visit, has been followed for some within the plastic mass. It can not be ascer­ to the dome by upthrust and faulting. one brought about mainly by the difference in distance southward under the town of Rico. The tained that any general faulting has taken place Origin of the lode fissures. It is to this general depth to which the ore extended' in each case. deposition of this ore was closely connected with a along this soft and .structurally weak zone. period of later orogenic movement that the present Various hypotheses have suggested themselves metamorphism of the limestone, giving rise to As the gypsum was deposited in Carboniferous fissure systems of the region belong. Such earlier in explanation of this difference. Evidently dif­ chlorite, epidote, garnet, and wollastonite. The time, its solution may have begun at an early date. fractures as may have resulted from the first rela­ ference in the country rock can not be appealed to, ore consists chiefly of sphalerite, chalcopyrite, and But it was probably much accelerated, if not initi­ tively gentle doming were probably superficial in since both sets of lodes traverse the same beds. galena, associated with much specularite. It is not ated, by the original doming of the rocks coinci­ character over the central region of the dome and For the same reason, differences in pressure and visibly connected Avith any parent lode or fissure. dent with the intrusion of sheets of porphyry have been largely removed by erosion. The more temperature can scarcely have been important fac­ between the beds. The later fracturing, which deep-seated fissures which presumably opened tors. If, however, the northwesterly lodes are in STOCKS. was associated .with the final stage of uplift, must beneath the flanks of the uplift were in all like­ the main later than the northeasterly, it is inevi­ The only examples of stocks known in the Rico have still further hastened the process of removal lihood filled with,,dikes at the period of laccolithic table, from what is known of its origin, that some district are in the Johnny Bull and Gold Anchor by allowing to underground waters, heated by the intrusion. It is to the later fractures, which trav­ change in the overlying blanket must have taken mines, at the head of Horse Creek. These, how­ intruded masses of igneous rock, a more active ersed the solidified masses of monzonite-porphyry place in the interval, and it is possible that this ever, are individually of small importance and but circulation. and which served as channels through which ore- change may have been of a character to influence poorly represent a type that finds much better The upper blanket of the Forest-Payroll mine bearing solutions could penetrate to the blankets, ore deposition. exemplification in the stocks of the Red'Mountain is somewhat analogous in origin to that of the that the ore deposits are really due. It is suggested as a first hypothesis that at the district, in the San Juan Mountains. Enterprise. It is due in great measure to the That the fissuring did not all take place at one time the northeasterly lodes were being formed the The Johnny Bull stock had a diameter of 10 or local solution of a bed of limestone and the conse­ time is shown by the facts that many of the lodes, gypsum may have been only in small part dis­ 15 feet and a depth of about 120 feet. It was quent letting down of the overlying shales. In particularly northwesterly lodes, show evidence of solved. In such case the blanket ore may have inclosed in fine-grained sandstone of the Dolores this case the residue of the limestone is a sooty repeated opening; that the northwesterly fissures been originally deposited chiefly as a replacement formation, which is here cut by several dikes and material containing much oxide of manganese. of Newman Hill, although probably initiated at of gypsum (a process which is known to have irregular intrusions of porphyry. The ore, con­ Owing to the prevalent oxidation in this mine, the same time as the northeasterly fissures, subse­ operated in some portions of the blanket zone). sisting of enargite, pyrite, free gold, and probably it is impossible to determine whether the ore was quently faulted the former in several instances; The ore-bearing solutions, checked by the gyp­ other minerals, was deposited largely by replace­ originally deposited in the limestone before its and that the fissures of the Phoenix and Iron sum overlying the lodes and not finding in the ment of the sandstone, which is silicified and solution. Apparently it was not. veins are apparently younger than the Nellie Bly slow process of metasornatic replacement sufficient impregnated with pyrite in the vicinity of the Of still different origin are certain of the lesser fault. opportunity for the deposition of their metallif­ former ore body. "contacts" studied in the Union-Carbonate mine. One of the striking generalizations afforded by erous contents, may have encountered in the upper A similar, smaller stock, consisting chiefly of In these the process has been purely chemical. the study of the district has been the lack of coin­ portions of the fissures conditions favorable to the pyrite, occurs at a lower level in the Gold Anchor Certain beds of shale have been wholly or par­ cidence between those fault fissures of such extent deposit of ore. One such general condition of mine, nearly under the Johnny Bull. tially altered to a soft, ferruginous, clay-like mass as to appear as "structural faults" and the lode ore deposition is believed to be sluggishness of containing some oxidized ore. The fact that some fissures. That some faulting has taken place along circulatory motion in the ore-bearing solutions. GENESIS OF THE ORE DEPOSITS. of these soft zones are cut by dikes of porphyry lodes, both before and after their filling, is of course If mineralization through the northwesterly fis­ General relation to geologic structure. If the (intruded prior to the alteration) which have not undeniable. But the great fault fissures of the dis­ sures was of later date, it is conceivable that the preceding account of the ore deposits of the Rico been fractured or displaced shows that the forma­ trict, numerous as they are, apparently nowhere more complete removal of the gypsum may have district has succeeded in clearly and truthfully tion of the zones is not connected with faulting carry workable bodies of ore and are certainly only given all of the ore an opportunity for deposition sketching their essential features, the statement along planes of bedding. to a minor extent coincident with veins. The in the unconsolidated blanket material. that they are genetically connected with the pres­ The genesis of the main blanket of C. H. C. Hill northeasterly veins of Newman Hill and, less cer­ If the foregoing hypothesis is correct, some struc­ ent geologic structure of the region requires no is not perfectly clear, owing to later oxidation and tainly, the northwesterly veins of Nigger Baby tural evidence for it might be expected in the ore further demonstration. But it remains to investi­ disturbance. It was evidently once a large body Hill show that the favorable channels for ore of the northeasterly pay shoots of the blanket. gate this general and fundamental connection more of low-grade pyrite, which later underwent deposition were not the great fault fissures that Unfortunately, however, all of the known ore has closely in order properly to discriminate and dis­ oxidation and a concentration of its valuable gave the district its final structure, but were clean- been mined out. tribute the various effects traceable to one common constituents. This pyrite apparently occurred in cut, open fractures of but slight tangential dis­ It has been previously pointed out that some source the geologic revolution through which large part as a replacement of shale, limestone, placement. Such fissures show no clear evidence grounds exist for supposing that the northwest­ beds once nearly horizontal have been elevated and sandstone; but whether this replacement was of horizontal, compressive stress. They are, so erly fissures, while initiated at substantially the into a fissured dome and subsequently carved by preceded by brecciation is not known. far as faulting has taken place, in the main nor­ same time as the northeasterly fissures, did not erosion into the topographic forms known as the There are, however, a number of blankets in the mal faults. It is believed, although this belief is gape open until a later date, and that such open­ Rico Mountains. district, such as those found in the Sambo, Great not entirely demonstrable, that they represent the ing was gradual and intermittent. This suggests, Rico. 20 as a second hypothesis, that the blanket ore may narily capable of depositing only low-grade pyritic tains, only a few miles apart, belong to the same evolution constantly agitates the water in the bot­ have all been formed at the same time, before the ore to form relatively rich sulphide masses has been general period of igneous activity. In the San tom of the so-called "gas shaft," a shallow pros­ full opening of the northwesterly fractures, and effected not so clearly by mingling of solutions as Juan the monzonite stocks cut the Telluride con­ pect on the southwest slope of Nigger Baby Hill. that the nonoccurrence of ore in the northwest­ by the process, probably in large part selective, of glomerate (Eocene?) and the overlying volcanic As shown by bore holes on the Atlantic Cable erly fissures is due to their contracted openings at metasomatic replacement. series. Their intrusion probably took place in claim and in the Rico-Aspen mine, underground the time of ore deposition. Sources of the ores. The ores of the Rico district late Tertiary time. This relation indicates that reservoirs of gas exist under considerable pressure. Of the two hypotheses advanced, the second (not were carried in aqueous solutions and were concen­ the ore deposits of Rico are roughly of the same In fact, there is scarcely an opening in the groun.d the "former," as was inadvertently stated in the trated under the conditions already described. age as those of the San Juan Mountains prob­ near Rico that does not fill up with gas, and not a preceding report on this district) is regarded as In this, as in many other regions, stratigraphic ably late Tertiary and possibly extending into the stretch of the river between the mouths of Sulphur the more probable; but whichever one is accepted disturbance, igneous intrusion, and ore deposition Pleistocene. and Horse creeks where bubbles of carbon dioxide requires to be supplemented by further consider­ have been genetically connected. The ore-bearing may not be seen rising through the water. YALUE OF THE ORES. ations relating to the chemical causes of precipita­ solutions undoubtedly owe much of their efficiency tion before it can be regarded as complete. in gathering, transporting, and depositing the ore Most of the ore produced in the Rico district LANDSLIDES. The view that those ore bodies showing a marked constituents to heat derived from igneous activity. has been shipped crude or smelted in Rico without These are of later date than the period of ore decrease in available sulphides with depth and That some of this heat still remains is indicated by previous mechanical concentration. Consequently deposition, but have an important economic bear­ passing finally into practically barren pyrite are the thermal character of the water issuing from the the ore handled has been of rather high grade. ing on account of the hindrance which they due to the action of both ascending and descend­ west face of Nigger Baby Hill. Ore worth $20 a ton, such as was produced from impose upon successful exploitation. This is well ing solutions has come to be widely accepted. It is possible that pneumatolytic emanations the Union-Carbonate mine, is considered "low illustrated in the case of the Puzzle mine, where a The Newman Hill pay shoots appear to consti­ (that is, gaseous products given off at high tem­ grade." The ore of the Enterprise and Rico- body of rich ore occurred in a landslide mass which tute striking examples of ore bodies due to the perature) from deep-seated cooling masses of intru­ Aspen mines varied widely in value, but was has slipped down from Darling Ridge and buried mingling of solutions. That the purely ascend­ sive monzonite-porphyry may have mingled with usually rich. Thus during one year the average the former channel of Horse Creek. All attempts ing solutions rising through the fissures of this underground meteoric waters and contributed of the Enterprise was 200 ounces of silver and 2 to find the source of this block and the continua­ region have normally deposited only low-grade largely to their mineral contents. But the known ounces of gold per ton. One carload from this tion of the ore body have failed. The difficulty of pyritic ores is abundantly exemplified through­ porphyry masses had certainly solidified and prob­ mine (about 10 tons) was valued 'at $8000. The the problem is apparent on referring to the geo­ out the district. But such solutions ascending ably lost much of their initial heat before the ores general range, however, appears to have been as logical map, where it is seen that the whole north­ in the fissures under the Enterprise blanket not were deposited. The deposits that most strongly follows: Gold, from 0.2 to 1 ounce; silver, from ern slope of Darling Ridge is covered with landslide only found their upward progress barred by the suggest pneumatolytic cooperation are the excep­ 100 to 200 ounces; lead, up to 10 per cent; and material. The depth of this material is often sev­ impervious shales above, but entered a markedly tional ore bodies in the Atlantic Cable and neigh­ zinc, up to 15 per cent. eral hundred feet, and rock in place can be reached porous, unconsolidated zone traversed by laterally boring claims. These have the characteristics of Some of the oxidized ore from Nigger Baby Hill only by tunneling. There is no means of know­ moving solutions which must then, as now, have contact deposits. was rich in silver. In 1900 ore containing over ing how far the Puzzle mass has slid. Even if the carried considerable calcium sulphate in solution. The actual chemical character of the ore-deposit­ 300 ounces of silver per ton was being shipped in original source of the block should be found after Too little can be learned of the chemical nature of ing solutions is not readily determinable. Their occasional carloads. tedious and expensive prospecting, it is by no the fissure solutions to determine whether the cal­ action on the sedimentary rocks which usually The ore of the Puzzle mine is reported to have means certain that there would be any ore there. cium sulphate acted as the precipitant, but that the form the walls of the fissures is, with a few excep­ been rich, but in general the replacement bodies in Similar difficulty is encountered in the landslide precipitation was due, at least in part, to mingling tions, inconspicuous and often obscured by second­ limestone, such as those of the Blackhawk -and of C. H. C. Hill, which, as shown by the various solutions and not entirely to metasomatic replace­ ary alteration. The presence of abundant calcite Iron mines, are of relatively low grade. That mine workings, has a maximum thickness of sev­ ment within the blanket is indicated by the fact in the lodes of Nigger Baby Hill and of rhodo- from the Blackhawk contained from 10 to 30 eral hundred feet. In this case also all of the ore that the ores extend below the blanket for over chrosite in the upper part of the northeasterly ounces and that from the Iron mine from 20 to 40 thus far found has been in landslide material, and 100 feet in the northeasterly lodes. It is thus lodes of Newman Hill indicates that the solution ounces of silver. the main ore horizon has never been found in seen that the depth to which deposition of pay contained carbonates possibly alkaline carbonates. An attempt was being made in 1900 to rework the rocks in place. Even if discovered, it is by no ore extended in the lodes may have been deter­ Equally impossible of definite answer is the dump of the Enterprise mine, by concentration. means certain that it would contain workable ore. mined by the equilibrium between the ascend­ question as to the particular rocks and the pre­ Recently the United Rico Mines Company has Like conditions of obscurity obtain over a ing solutions in the lode fissures and the lateral cise depth from which the ores were derived. succeeded in concentrating and separating the ore considerable area north of Horse Creek and on descending solutions in the blanket. Evidently But it is probable that all the rocks, particu­ from the Atlantic Cable mine into three products, the southeast spur of Expectation Mountain. if the lode fissures were small and filled with larly from the top of the Rico formation down, namely, sphalerite concentrates containing from 46 Prospecting undertaken in these areas without solutions moving upward under considerable head have contributed some metalliferous constituents to 53 per cent of zinc, galena concentrates with some realization of the nature of the disturbance all the ore would be deposited in the blanket. to the ore bodies, not by the narrowly confined, about 45 per cent of lead, and pyrite concentrates which they have undergone will almost inevitably None of the other known blankets in the district academic process known as " lateral secretion," containing 59 per cent of iron, with some gold and result in disappointment. have contained such large and rich pay shoots as but by the concentration in favorable localities of silver. The sphalerite concentrates bring $25 a MISCELLANEOUS MINERAL RESOURCES. the Enterprise blanket. With the exception of an materials widely drawn from the rocks of the dis­ ton, while the galena and pyrite concentrates are Oxidized zone in the Union-Carbonate mine, the turbed region and often reaching the point of their disposed of on favorable terms to the smelters. Coal. The shaly layers of the Dakota forma­ fissures through which the mineralization of these final deposition after a roundabout journey to The Pro Patria mine in 1902 was producing con­ tion, as Mr. Cross has pointed out, usually carry blankets has taken place are barren or contain only depths far below any ever likely to be reached by centrates containing 54 per cent of zinc, from the some thin seams of lignitic coal of poor quality. a little low-grade ore. It is evident that the con­ mining operations. veins of Newman Hill, particularly the Jumbo No. Some of this coal was formerly mined on the west ditions for ore deposition in the Enterprise blanket The formation of the ore of the Atlantic Cable 3 vein. side of the Dolores River a few miles north of Rico, were usually favorable and were determined in vary­ and neighboring claims is evidently connected MINOR DIFFICULTIES IN MINING. to supply the local demand. But it has been sup­ ing degree by the following peculiarities, which are with intense contact metamorphism, as shown by planted by the better coal now brought in by the not found associated together in any other blanket the close association of the ore minerals with gar­ Carbonic acid gas. Prospecting within the Rico railroad, and is not likely to become economically known in the district: (1) The underlying blanket net, wollastonite, vesuvianite, pyroxene, chlorite, district is often much hindered by the abundance important. limestone; (2) the gypsum and its pulverulent resi­ and epidote. The character of this metamorphism of carbonic acid gas which issues from nearly every Building stone. The La Plata sandstone and due; (3) the overlying, nearly impervious bed of is such as to suggest the possibility of substantial fissure traversing'the rocks in the central portion of the thicker beds of the Dolores and Cutler forma­ black shale; and (4) the upward termination of the pneumatolytic contribution to the solutions that the dome. The gas is particularly troublesome in tions offer an abundant supply of fairly good stone, lodes at the blanket horizon. But although no deposited the Rico ores. The cause of the meta­ shafts, which become entirely filled by it. It occurs for which, however, there is no demand. Some of one of the other blankets possesses all these advan­ morphism is not evident. It may be due to the in the Lexington, Mediterranean, and Syndicate the Dolores sandstone was formerly quarried along­ tageous attributes, yet they illustrate the general fact intrusive mass of monzonite between Iron Draw and tunnels to such an extent as to render them inac­ side the railroad near the western edge of the quad­ that in this region, replacement deposits in lime­ Aztec Gulch, or more probably to some igneous cessible unless artificially ventilated, and a stream rangle; but the quarry has been abandoned, as the stone being for the present ignored, large bodies of mass which has not been exposed by erosion. of this heavy gas was noted issuing from a fissure quality of the stone was not sufficiently good to workable sulphide ores occur only where the solu­ in the Blackhawk mine. But it is in the immedi­ compensate for the long haul to the nearest market. GEOLOGICAL AGE OF THE ORE DEPOSITS. tions in the lode fissures have had opportunity ate vicinity of Rico that the evolution of the gas is Limestone. Material suitable for flux or for lime to mingle with laterally moving solutions in a The age of the ore deposits can not be deter­ most abundant. On the west bank of the Dolores is abundant in the Ouray and Hermosa formations blanket. The extent and richness of the deposit mined from a study of the Rico district alone. River, on the Riverside, 'Smuggler, and Shamrock near the town of Rico. depend largely upon the number of the above- They are plainly subsequent to the doming and claims, gas issues in many places with a bubbling WATER RESOURCES. enumerated favorable conditions which are present faulting of the region, but no definite date is noise loud enough to attract the attention of the in any one case. assigned to these structures by Cross and Spencer. passer by, and in such volume as to suffocate birds The quadrangle is well watered by the Dolores A similar statement may be made with regard to A tentative conclusion, however, may be drawn and small animals that venture too near, attracted River and its tributaries. The river supplies power replacement deposits in limestone. Lode fissures, from the similarity in character between the mon­ by the water through which the gas escapes. for lighting Rico, and undoubtedly it and the smaller which/ordinarily carry pay ore, are frequently con­ zonite of the Rico district and that of the Tellu- About 300 feet upstream from the Piedmont streams in the quadrangle might be much more nected with bodies of workable ore in limestone, as ride and Silverton regions. It may be assumed bridge the water of the river is kept in a state extensively utilized for the generation of electricity in the Blackhawk and Iron mines. In such cases as probable that the monzonitic intrusions of the of violent ebullition by the escape of gas, appar­ for mining purposes. the concentration which has enabled solutions ordi­ San Juan Mountains and those of the Rico Moun­ ently from an east-west fissure. A similar copious June, 1904. o O

Leadville limestone and other correlated horizons in map, and groups of strata change so greatly in- which have been considerably baked and metamor­ Productus semireticulatus Seniirmla subtilita. Colorado and the West, and in the lower beds of the character from place to place that horizons can phosed, but which still contain abundant identifi­ var. Hermosanus. Liinipecten occidentalis. Mississippian series in the Mississippi Valley. Some of Productus Gallatinensis. Acanthopecten carboniferus. not be definitely recognized in localities separated able fossils, which correspond with similar strata Productus cora. Myalina subquadrata. the characteristic species are: from one another by more than short distances. occurring along the western side of the Animas Productus punctatus. Schuchertella inaequalis. Spirifer centronatus. The base of the Hermosa is conformable with the Valley. Above this the rocks are green or gray The detailed studies which have recently been Chonetes Illinoisensis. Eumetria Marcyi? Molas, the line between these formations being that grits, or sandstones, alternating with gray shales Productella concentrica. Caniarotoechia iiietallica. made of the sedimentary rocks of the San Juan Productus parviformis. Straparollus Utahensis. of a change in lithologic character, as has been and containing several beds of black shale and region have led to a grouping of the Carboniferous Productus hsvicosta. stated. Its upper boundary is in general the base occasional thin, impure limestones. The ore- and Trias quite different from that employed by

CARBONIFEROUS SYSTEM. of the "Red Beds" of the region and is more accu­ bearing horizon of Newman Hill, known as the the Hayden Survey. Reference to the Hayden rately defined at Rico by the lowest fossiliferous "contact," is associated with one of the black Atlas of Colorado will show that the strata between SECTION OF SOUTHWESTERN COLORADO. layer of the Permo-Pennsylvanian. The geologic shales of this lower division. A bed of rock the Devonian and the Jurassic sandstone (corres­ According to the usage of the Geological Survey unity of the Hermosa is shown by the invertebrate gypsum, reaching in some places a thickness of ponding to La Plata) were mapped as Middle and the Carboniferous system is divided into three fauna which characterizes many of the limestones 30 feet, occurs locally above the black shales of Upper Carboniferous. The mapping of the former series, namely, the Mississippian (Lower Carbon­ and calcareous shales from the bottom to the top. the "contact" series, and was probably more widely division corresponds in general with the occurrence iferous), Pennsylvania!! (Upper Carboniferous), The name Hermosa is derived from a large creek distributed originally, since wherever it has been of the Hermosa formation, leaving the latter as and Permian. Of these the Pennsylvania!! series entering the Animas River in the Durango quad­ seen there is evidence that it has been attacked by the equivalent of the Rico, Cutler, and Dolores has long been known as represented in a large rangle, and was given to the formation in the Rico circulating waters and in part removed by solution. formations. assemblage of limestones, sandstones, and shales report, in a chapter by A. C. Spencer. Above this bed are 250 feet of rocks which are grouped in the Hermosa formation. At the time General description. Lithologically the Her­ nowhere exposed to view, and above them lie 200 RICO FORMATION. the report on the Kico Mountains was issued it mosa is composed of limestones, shale, and sand­ feet of massive and flaggy sandstones that consti­ Definition. In, the report on the geology of was supposed that the Hermosa beds rested stone, but all of these strata are more or less tute the uppermost strata of the lower division. the Rico Mountains, the name of that group of directly on the Ouray limestone, throughout calcareous throughout. The limestones are of a Containing as it does the Newman Hill "contact," peaks was given by Mr. Spencer to a formation, southwestern Colorado, making a stratigraphic blue-gray color, rather dense in texture, and usu­ and at least one other ore-bearing horizon, the assumed to be about 300 feet in thickness, occur­ break where the Mississippian beds should appear. ally very fossiliferous. They are frequently more lower division of the Hermosa formation at Rico ring between the Hermosa (Carboniferous) and This gap has been partly filled by the discovery, or less bituminous, sometimes so much so as to becomes important in the study of the geologic strata then assigned arbitrarily to the Dolores already mentioned, that the upper part of the afford a distinct odor when struck with a hammer. features of the ore deposits and numerous details (Triassic) formation, but now distinguished as Ouray limestone contains Mississippian fossils. It The shales vary from black bituminous clay shales, as to its character and distribution may be found the Cutler formation. The Rico formation is is, however, impracticable in any region thus far rather fissile, to sandy shales and sandstones of an in the Rico report. made up of sandstones and conglomerates with examined to distinguish the Carboniferous portion olive-green color. The sandstones are also of a Medial division. The second division of the intercalated shales and thin fossiliferous limestones of the Ouray as a separate formation. greenish color, and under the microscope are seen Hermosa is made up very largely of blue bitu­ which are usually sandy. In this region the for­ It has also been found that in most localities to have an amorphous green cement. The sand­ minous limestone, carrying many fossils and occur­ mation is conformable upon the Hermosa and is where the narrow zone between the Ouray lime­ stones are composed mainly of quartz, but feldspar ring in massive beds from 5 to 100 feet in thickness, followed by the Cutler with seemingly perfect par­ stone and the lowest limestone of the Hermosa and mica are common. The cementing material separated by shales and sandy shales. In the lower allelism in stratification. complex is well exposed there is a thin, reddish, is largely calcite. Sandy beds vary in grain from part there are some intercalated strata of green sand­ The prevalent reddish color of the Rico beds, as calcareous or sandy formation containing many fine to coarse, and some are conglomeratic. stone and green or black shales, and locally these well as their general lithologic character, allies chert pebbles, in some of which are Mississippian The strata of the Hermosa formation form the continue through the series, separating beds of them rather with the Cutler than with the Her­ fossils. These include the forms known in the western wall of the wider valley of the Animas limestone which elsewhere lie close together. The mosa complex, and they have undoubtedly been uppermost portion of the Ouray, as well as others, outside the canyon, from Hermosa Creek to Engi­ medial member has a thickness somewhat in excess classified hitherto as the basal part of the "Red and it seems clear that the cherts were derived neer Mountain, and thence, still northward, their of 600 feet and is a prominent feature in the stra­ Beds" in this vicinity. The fossils of the Rico from eroded parts of the Ouray. Thin limestones outcrops may be traced nearly to Silverton. In tigraphy of the region. Its massive limestones limestones have been studied by G. H. Girty in of this reddish formation contain fossils known in this distance of 25 miles the variable and incon­ appear as white ledges on the western and north­ connection with a thorough revision of the Car­ the Hermosa limestones and a few other forms of stant character of the formation is well exhibited. ern faces of Dolores Mountain and in Dead- boniferous invertebrate fauna of Colorado, the the Pennsylvania!! fauna. This lithologically pecu­ The fossiliferous limestones which occur at the base wood Gulch. The best section of this part of results of which have been published as a Pro­ liar formation, representing the^earliest sediments are found throughout this extent, but the strata the Hermosa is that seen in the precipitous fessional Paper (No. 16) of the Survey. From of the Pennsylvanian in this region and con­ above them change greatly from place to place. face of Sandstone Mountain, which is represented Mr. Girty's examination of the Rico fauna it taining evidence of a preceding erosion interval, Near Hermosa Creek the lower part of the for­ in fig. 2. appears that while many of the species are com­ was called the Molas formation in the Silverton mation above the limestones is made up of green Upper division. The upper division of the mon to the Hermosa formation, others are types folio. It is distributed through the Animas Val­ sandstones and shales with some bands of gypsif- Hermosa, which is represented separately on the usually considered Permian, and on his suggestion ley, but neither its characteristic red strata nor the erous shale; the middle and upper parts show fos­ special map, contains some bands of limestone the Rico, is referred to the Permo-Pennsylvanian. thin chert conglomerates were observed at Rico, siliferous Ograv1 */ limestones in beds from 1 to 20 feet similar to those of the medial division, but they The fauna as a whole has an aspect differing from possibly because of metamorphism or poor expo­ in thickness, interbedded with shales and sand­ are thin and unimportant in comparison. Its that of the Hermosa, in that it is largely composed sures in the very limited area where it must occur, stones. Gypsum occurs locally, but has not been strata are mainly black and gray shales alternat­ of lamellibranchs as contrasted with the brachiopod if at all. Despite its omission from the Rico map found north of the Durango quadrangle. In the ing with green grits and sandstones. Occasional assemblage predominating in the lower formation. it is possibly present. region farther north the limestones become more reddish sandstones are observed, and two black Later collections from the Rico, however, show its Above the Molas formation comes the succession massive and the intercalated shales and sandstones shales are present in the lower third of the divi­ fauna to be more closely assimilated to that of the of strata grouped as the Hermosa formation, the less important, so that for some distance south of sion. The top of the upper division, and of the Hermosa formation ; than at first appeared to be the Pennsylvanian age of which is amply demonstrated Cascade Creek the limestones of the middle section Hermosa formation as a whole, is well defined from case. by an extensive invertebrate fauna. are very prominent, 'forming the upper scarp of the base of the next higher formation. The top­ The base of the Rico formation can usually be Succeeding the Hermosa formation comes the the valley wall. Between Lime Creek and Molas most member consists of about 30 feet of fine­ very accurately located in the field by its lowest great series of sandstones, grits, etc., known as the Lake the Hermosa formation exhibits a distinct grained, mica-bearing green shales, immediately fossil-bearing stratum. The boundary between the "Red Beds," variably referred, on scanty evidence, phase, since the blue fossiliferous limestones are above which comes the red, sandy, fossiliferous Rico and Cutler formations is, however, quite to the Carboniferous or to the Trias. The Rico less massive than to the south and are distributed limestone of the Rico beds. At the base of these arbitrary, being based on the highest known district yielded the first definite evidence, in the throughout the entire thickness of the formation. throughout the Rico Mountains, a band of blue occurrence of the Rico fossils. The former is form of invertebrate fossils, that any portion of Development at Rico. At Rico the Hermosa for­ limestone, usually from 6 inches to 1 foot in thick­ made to include only that part of the section the "Red Beds" of the San Juan region belongs to mation has about its normal development, since it ness, is always present, and this stratum is charac­ characterized by the Permo-Pennsylvanian fauna, the Carboniferous. This lower fossiliferous part reaches a thickness of 1800 feet or more. It terized by the minute spindle-shaped shells formerly while the Cutler comprises the apparently unfos- of the "Red Beds," locally some 300 feet in thick­ shows an unmistakable general correspondence to known as Fmulina cylindrica, now called Triticites siliferous "Red Beds" of this region, extending to ness, has been called the Rico formation. the Animas section, and in particular has a devel­ secalicus. Detailed sections of the Hermosa forma­ the horizon at which Triassic fossils are known to Following the Rico beds comes a series of unfos- opment similar to that in the adjacent portion of tion are given in the Rico report. occur. siliferous "Red Beds," which were assigned to the the Animas region, where the limestones in the Fossils and correlation. Invertebrate fossils are Description. The general characteristics of the Triassic in the Rico report, but are here grouped as medial portion of the formation are massive and numerous and well preserved in the Hermosa for­ Rico formation in the vicinity of Rico .are, first, the Cutler formation and referred provisionally to conspicuous. At Rico, however, the gray or blue mation. By far the larger number are brachiopods, its calcareous nature, in which it resembles the the Permian series. The ground for separating limestones are even more closely segregated in the though gasteropods occur, and also the character­ strata above and below; second, the feldspathic the Cutler formation from the overlying Triassic middle third of the formation, and are of rare istic foraminifer Fusulina cylindrica. Most of the constitution and the coarseness of its sandstones, strata is the existence of an extensive angular occurrence in the upper and lower portions. This species are identical with forms occurring in the in which respect it differs from the Hermosa and unconformity between these divisions of the "Red is doubtless entirely a local facies of the formation, Missourian stage of the Carboniferous of the Mis­ resembles the Cutler; and third, its chocolate or Beds," shown in the Uncompaligre Valley at Ouray. as may be inferred from the facts observed in the sissippi Valley, which corresponds in point of age dark-maroon color, which contrasts sharply with This unconformity was discovered in 1904, in the Animas section, but it makes it possible to divide with what is commonly known as the " Upper Coal the gray or green of the Hermosa and which is work of mapping the Ouray quadrangle, and soon the Hermosa, as shown at Rico, into three approx­ Measures." The same collection of organic forms more or less distinct from the bright vermilion of afterwards the boundary line between the Cutler imately equal parts, and this division will be fol­ is found at various places in Colorado. In the the Cutler and Dolores. Locally, through meta­ and Dolores formations was traced through the lowed in the description. On the Rico special geo­ Gunnison region similar fossils are found in the morphism, the deep-red color has been changed to Rico quadrangle by Mr. W. H. Emmons. logical map the upper member has been repre­ Weber and Maroon formations, as described in green, as seen in the cliff exposures north of Silver sented by a distinct pattern, in order better to the Anthracite-Crested Butte folio of the Geolog­ Creek, in the vicinity of Uncle Ned Draw, and \ PENNSYLVANIAN SERIES. exhibit the structure, but the two lower members ical Survey, showing that the Hermosa comprises in the cliffs exposed on the northern slopes of HERMOSA FORMATION. have not been divided. This division is made the former and part of the latter formations. The Dolores Peak. Definition and name. The Hermosa formation simply as a matter of convenience, and there is following is a partial list, supplied by G. H. Girty, The bulk of the formation is made of sandstones is lithologically complex, consisting of interbedded no intention of raising the divisions to the rank of the most characteristic Hermosa fossils: and sandy shales composed of such materials as are limestones, shales, and sandstones, reaching a max­ of formations. derived from the disintegration of granite. The Triticites secalicus. Productus Nebraskensis. imum thickness of about 2000 feet in the Animas Lower division. The lower division is about Chaetetes milleporaceus. Margin if era muricata. sandstones are mostly coarse or conglomeratic, Valley. Indi vidual beds of different lithologic con­ 800 feet in thickness, excluding the porphyry Rhombopora lepidoden- Marginifera Wabashensis. always showing grains of fresh feldspar mixed stitution are too thin and too variable in develop­ sills which have been intruded between its strata. droides. Spirifer Boonensis? with mica flakes and quartz. When conglomer­ Derbya crassa. Spirifer cameratus. ment to deserve special representation on the At the base there are shales and impure limestones Chonetes inesolobus. Squamularia perplexa., atic the pebbles are chiefly of schists and quartzite. Rico.