134 GEOLOGICAL SURVEY OF THE TEREITOEIES. . 



There is a very narrow dividing ridge in one place, between tlie drain- 

 age, which may be within one mile of the lake. 



As we have stated in the previous pages of this report, the rocks of this 

 basin are mostly volcanic, but on the south side of the divide, between the 

 Yellowstone and the sources of the Snake, the series of ridges extending 

 southward to the Tetons are largely sedimentary. Carboniferous lime- 

 stones occupy restricted areas, while some of the highest ridges are made 

 up of Cretaceous and Tertiary strata. One ridge, the summit of which 

 was over 10,000 feet above the sea, and overlooks the country for fifty 

 miles in every direction, is the exact dividing ridge which separates the 

 drainage of the two basins. On the summit and north side of the ridge 

 the rocks were smooth, as if vast masses of snow and ice had slidden 

 down for ages. The rocks are composed of somber-brown and rusty 

 grayish-brown sandstones, in which I found great quantities of leaves 

 of deciduous trees. There was one fern and a palm of huge dimensions. 

 Erom these exposures of the sedimentary beds, I draw the same conclu- 

 sion that I have done so many times previously, that the unchanged rocks 

 either now exist or have existed all over the Northwest ; that they may 

 have been removed by erosion, concealed by overflows of igneous mate- 

 rial, or thrown uj) into ridges ; but the one final conclusion is, that they 

 extended all over the region about the sources of these great rivers, in 

 a horizontal position, at a comparatively recent geological period. 



On our return to the east side of the lake from the sources of Snake 

 River, we followed down the valley of a little stream that has its origin 

 at the foot of the ridge. As it flowed toward the lake, it cut a deep chan- 

 nel into the lake deposits, sometimes 50 to 100 feet, well illustrating the 

 character of the materials. It was composed at the bottom of grayish- 

 white clays, passing up into a sort of bowlder deposit, all derived from 

 the degradation of volcanic rocks. 



We may here discuss for a moment, in general terms, the geolo- 

 gical character of the mountains on the east side of the lake. The 

 Upper Yellowstone Eiver rises in the high volcanic range which shuts 

 oft" the Yellowstone Basin from the Wind Eiver drainage, forming 

 what' is usually called the great water-shed of the continent. The 

 range of mountains on the east and south sides of the Yellowstone 

 Basin gives origin to the waters of the Snake Eiver, which flow west 

 into the Pacific, to those of Green Eiver, which flow southward into 

 the Great ColQrado, and to the numerous branches of the Yellowstone. 

 Upon the east and southeast sides, the mountains seem to be entirely 

 of volcanic origin; they are also among the ruggedest and most inac- 

 accessible ranges on the continent. From the valley of Wind Eiver 

 they present a nearly vertical wall from 1,500 to 2,000 feet high, which 

 has never been scaled by white man or Indian; but are covered with 

 perpetual snows to a greater or less extent. From any high point a 

 chaotic mass of peaks of every variety of form may be seen extending 

 from the Snake Eiver Yalley to the lower canon of the Yellowstone. 

 The general level of the summits is about 10,000 feet, but some of the 

 higher x^eaks reach 10,500 to 11,000 feet. Many of them are the nuclei 

 of old volcanic cones, composed of very compact .trachyte; others are 

 portions of the rim of a vast crater. Mounts Doane and Stevenson are 

 fragments of the rim of an immense crater, the layers of trachyte inclin- 

 ing from the basin on every side; some of the centers of effusion were 

 long fissures, forming ridges. All around these nuclei, and sometimes 

 reaching nearly to the summits, are the volcanic conglomerates or brec- 

 cias in horizontal strata. Even the highest x)ortions of the mountains, 

 the broad ridges that form the very water-shed, are composed oi these 



