DESCRIPTION OF THE TELLURIDE QUADRANGLE. INTRODUCTION. along the southern base, and agricultural lands water Jura of other parts of Colorado, and follow­ vents from which the lavas came are unknown, A general statement of the geography, topography, have been found in valley bottoms or on lower ing them comes the Cretaceous section, from the and the lavas themselves have been examined slopes adjacent to the snow-fed streams Economic Dakota to the uppermost coal-bearing member, the only in sufficient degree to show the predominant and geology of the San Juan region of from the mountains. With the devel- imp°rtance- Colorado. Laramie. Below Durango the post-Laramie forma­ presence of andesites, with other types ranging opment of these resources several towns of tion, made up of eruptive rock debris and known in composition from rhyolite to basalt. Pene­ The term San Juan region, or simply " the San importance have been established in sheltered as the "Animas beds," rests upon the Laramie, trating the bedded series are several massive Juan," used with variable meaning by early valleys on all sides. Railroads encircle the group and is in turn overlain by the Puerco and higher bodies of often coarsely granular rocks, such as explorers, and naturally with indefinite and penetrate to some of the mining centers of Eocene deposits. gabbro and diorite, and it now seems probable limitation during the period of settle- sa^juan the the interior. Creede, Silverton, Telluride, Ouray, Structurally, the most striking feature in the that the intrusive bodies of diorite-porphyry and ment, is. now quite. generally >-. appliedT -i to region. and Lake City, all situated in mountain valleys, present attitude of the formations described, from the allied varieties found in the sedimentary beds a large tract of mountainous country in south­ are thus connected with the main lines of traffic. the base of the Devonian upward, is Outline of adjacent to the San Juan Mountains on the west western Colorado, together with an undefined The geological history of the San Juan region is the general southerly or westerly dip structure. are also of later date than many of the surface zone of lower country bordering it on the north, too complex and as yet too imperfectly known to away from a point in the west-central part of the lavas. west, and south. The Continental Divide traverses admit of even an outline statement of satisfactory San Juan Mountains not far east of the Telluride The volcanic eruptions' in the San Juan area this area in a great bow. The principal part of accuracy. The pre-Tertiary surface of the entire quadrangle. As seen in the section of the Animas probably continued at intervals until late in the district is a deeply scored volcanic plateau, region was completely buried by the Valley, all of these formations appear to be con­ Tertiary time, although only the prod- Tertiar more than 3000 square miles in extent, drained volcanic formations which now cover knowledge formable. None of the various unconformities ucts of the earlier outbursts are well hlstory- on the north by tributaries of the Gunnison River the, main. area, and,, while, ., erosion ,has of geology. by overlap represented upon the Hayden map as known. Thus the volcanic period of building-up on the west by those of the Dolores and San again exposed some of the older rocks on all sides occurring in the area between the Animas and was in part synchronous with the third great Miguel rivers, on the south by numerous branches of the volcanic complex and even in some of the San Miguel rivers exists in fact within that terri­ period already referred to - that of sculpturing of the San Juan, and on the east by the Rio interior valleys, the reconnaissance observations tory. But at least one great orographic disturb­ by erosion by which the mountains now exist­ Grande. All but the latter drainage finds its of the Hayden and other early surveys were far ance not indicated in the Animas section is ing have been produced. Within the volcanic way to the Gulf of California through the too meager, and the present resurvey has thus far clearly shown on the northern slopes of the San area little evidence has been discovered by which Colorado River. covered too small an area, to afford solutions to Juan, and probably also on the southern side, not the sequence of events can be correlated with the The San Juan Mountains are now understood many of the problems in the earlier geologic far east of the Animas River. The red Triassic established divisions of Tertiary time. Deposits to embrace the area bounded on the north by the development of this most interesting field. formations and all older sediments are wanting in of Eocene age are known in the zone bordering generally abrupt descent to the sloping mesas In view of this condition, no attempt will be the plateau traversed by the Gunnison and its the volcanic area, but they have not been found extending for 25 miles to the canyon of the Gun­ made at this time to present a thorough review southern tributaries east of the Uncompahgre in direct contact with the lavas. While it may nison, on the west by the great plateau of Colorado of San Juan geology, but in order that the sig­ River, and the granites and gneisses are overlain be safely assumed that the closer study of the San and Utah, on the south by the more gradual nificance of the observations made in the Tellu­ by the probable equivalent of the La Plata sand­ Juan will result in the recognition of different descent to the rolling plateaus of New Mexico, ride quadrangle may be more fully appreciated, stone, of assumed Jurassic age. A similar condi­ epochs of eruptive activity and of orographic dis­ and on the east by the broad and level San Luis an outline sketch of the geologic development of tion exists east of the Animas, in the drainage of turbance, the Tertiary history of this region may Park. From this main area a broad spur leads the region will be given. This outline is particu­ the Los Pinos and Piedra rivers, according to the be summarized as a conflict between volcanic off to the southeast, losing its mountainous char­ larly applicable to the western part of the San Hayden map, but no observations appear to have forces, building up by stupendous emissions of acter near the Colorado-New Mexico line. The Juan, for it is in the valleys of this portion, near been made on either side of the San Juan Moun­ lava, and the agencies of erosion, removing the San Juan Mountains thus have an extent of nearly the mountain front, that the best exposures of the tains to show the actual position or character of igneous material and carving deep canyons to the 80 miles east and west, and from 25 to 40 miles older rocks may be found. the great overlap which must occur at the base very base of the vast lava plateau. The former north and south, and their summits form a great The Animas Valley, between Silverton and the of the La Plata sandstone, or at least below the was most effective in the earliest stages of its group rather than a range. vicinity of Durango, shows apparently a complete Dakota Cretaceous. activity, nearly the entire thickness of 5000 feet In the western part of the San Juan Mountains exposure of all formations of the San Juan, from Other periods of uplift, erosion, or subsidence of volcanic rocks found in the western San Juan the topography is very rugged. There are hun­ the Archean to the Puerco Eocene, inclusive. in Paleozoic or Mesozoic time are indicated by the being of that epoch, while the agents of degrada­ dreds of summits exceeding 13,000 feet Much of this section has now been studied in apparent absence of Cambrian and Silurian sedi­ tion are still actively at work upon the higher Topography. in elevation, and several which reach detail, but definite correlations can not as yet be ments, the insignificant development of the lower mountain masses. more than 14,000 feet above sea level. Here, too, made between the older formations here seen and and middle Carboniferous beds, the local develop­ Quantitatively, the work performed by the the bounding scarps of the group are often very the isolated exposures reported from some other ment of the fossiliferous Trias, and the absence of geologic agencies acting in this region in Cenozoic precipitous, while some of the valleys in the parts of the San Juan Mountains. the marine Jura and of recognized equivalents of time was very great, but the estimation heart of the mountains have been cut down to Ancient granites, gneisses, and schists are the great "Lower" Cretaceous section of Texas. of the post-Cretaceous disturbance, as ofMeasure erosion 9000 feet or less above the sea. To the east the known in the Animas Valley on the south, and The geologic structure and constitution of the wellTT as thei general-i decipheringi T oti» all11 accomplished. configuration is less rugged, and high table-lands, in the Uncompahgre Plateau on the Theoldest San Juan Mountains of to-day are mainly the earlier geological history, has been rendered very north. These rocks have usually been *ormations- of varying extent, represent in a measure old result of the dynamicJ forces which Post-Creta- 4 ... difficult by the mantle of volcanic rocks; and the plateau surfaces. considered as belonging to the Archean, but some were intensely active during three ceous erosion- original extent of this covering is left to specula­ Within the bordering zone of lower country, of them are probably younger than the great great periods of Tertiary time.