206 GEOLOGICAL SURVEY OF THE TERRITORIES. 



to the west. While the higher portions of the peak are composed 

 of trachyte No. 4, the last 400 leet proved to be rhyolitic material. 

 Differing from the typical rhyolite, its structure and composition, never- 

 theless, assign this place to it. Ou the fresh break the 'rock has a 

 muddy olive-greeu color, but turns brown upon exposure. Its struct- 

 ure is somewhat crystalline, owing to the large quantity of feld- 

 spathic crystals contained iu the paste. To the southwest of Mount 

 Sneffels a very extensive " drop " of the volcanic strata occurred, about 

 four miles iu length, and a little more than a mile in width. Unlike the 

 one described from Bristol Head, no erosive agent can have occasioned 

 this one, as there is no connection between the sunken area and any 

 point from whence such an agent could have operated. The strata have 

 dropped down perpendicularly for more than 2,000 feet, retaining to 

 some extent amoug the mass of debris their original connection. 

 Approaching from the south or southwest, the ridge suddenly falls off 

 in vertical bluffs, and only the accumulated mass of broken fragments 

 enables the descent. We have named this place the " Great Amphi- 

 theater," as it resembles one in shape. Those strata, that show a partial 

 connection, are cracked in every direction, traversed by large and 

 small fissures. Masses of broken rocks are constantly rolling down 

 the steep sides into the depression, so that iu course of time much of 

 the present grand aspect of the peculiar formation will be lost. The 

 ridge extending westward from Mount Sneffels drops off into the sedi- 

 mentary bluff' country, remaining volcanic throughout its entire length. 

 As at numerous other points near the edge of the volcanic area, so the 

 mountains here, too, show a regular stratification, almost horizontal. 

 It seems highly probable that subsequent erosion, to a great extent, 

 determined their present forms, as it appears incredible that flows of 

 such considerable vertical dimensions should terminate so abruptly. 

 Evidence was obtained at several places near this ridge and those 

 adjoining that the sedimentary beds held, at the time of the eruptive 

 flows, the same position they now occupy. They belong to the Cre- 

 taceous system. 



The Stin Miguel River rises in volcanic country, but soon leaves it, and 

 flows through sedimentary formations. There, as well as near the head- 

 waters of the Eio Dolores, the trachytes show the same character as 

 along the ridges upon which station 24 was located. Besides these two 

 streams the Rio Animas heads in a portion of the high volcanic district, 

 as well as its northerly tributaries. In the chapter on the mines of the 

 San Juau region the character of the rocks through which the Upper 

 Animas flows will be discussed, and for this place, therefore, only some 

 of its tributaries remain to be described. Bear Creek, heading near 

 station 30, flows in an easterly direction and joins the Animas in 

 Baker's Park, near Silverton. Ascending that creek from the park 

 mentioned, the red stratum may be observed following along Mineral 

 Creek, which branches off to the northward from Bear Creek. The 

 former rises near station 28, where the red stratutr is very well devel- 

 oped. Soon, however, after traveling up Bear Creek beyond the junc- 

 tion with Mineral, sedimentary beds set in, overlaid by the- trachyte. 

 Station 30 is located on the highest point of the ridge separating the 

 waters of the Animas from those of the San Miguel. Its elevation is 

 13,897 feet. On either side of the ridge sedimentary beds afspear, and 

 the volcanic rocks only form the capping of the ridges or isolated peaks. 

 A considerable amount of metamorphosis of the sedimentary beds has 

 been produced by the action of the overflowing volcanic material. The 

 summit of station 30 is formed by a very peculiar rock. A microcrys- 



