22 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Eiver Mountains, both litliolog'ically and paleontologically, but the 

 Jurassic and red de])osits are, so tar as could be observed, precisely 

 alike in their character and contents. 1 believe, however, that all 

 these formations at one time extended continuously over the entire 

 divide of the Rocky Mountains. 



•'As we ilesceiid into Jackson's Hole, we find the Carboniferous lime- 

 stones with their usual lithological characters, a very hard brittle yellow 

 rock, with much cherty material, inclining 12° to 15°. l^iere is one 

 thick cherty layer, 15 feet thick, dark bluish color, inclining 12°. We 

 liud these limestones along the mountains on both sides of Jackson's 

 Hole, but the central portions of the mountain-ridges are composed of 

 eruj^tive rock. 



" Near Snake River, on the right bank, is a rather low range of hills, 

 Avhich presented the appearance at a distance of being composed of 

 stratified rocks. On examination the rocks appear to be a bluish, very 

 hard cherty limestone, apparently Carboniferous, 100 to 200 feet thick, 

 passing up into a compact siliceous gray rock with a reddish tinge. In the 

 limestone are numerous fossils, mollusca, and corals, but too much broken 

 and obscure to determine. On the left side of Snake River 1 saw lime- 

 stones charged with fossils, especially corals. These limestones are 

 scattered promiscuously along the flanks of the lower hills and ridges, 

 and while in many lilaces they are in part or entirely removed by the 

 erosive action of water, the evidence is clear that they were deposited 

 here with a thickness fully equal, and were possessed of a similar char- 

 acter, to those on the eastern slope of the mountains. The valley of 

 Snake River is broad, fertile, and beautiful, and. very few traces of the 

 Tertiary beds are seen, and 1 am now inclined to think that we can see, 

 to a very great extent, the configuration of the main portion of the 

 Snake River Basin as it was prior to the Tertiary period ; for the Tertiary 

 beds, being of a loose friable material, were easily eroded away, leaving 

 along the banks large areas covered with it. 



" June 18. — Crossing over Snake River, we ascend the pass 1,000 feet 

 above the bed of Snake Fork. The mountain-ridge over which we 

 passed could not be less than 1,000 or 1,100 feet higher, so that these 

 mountains are between 9,000 and 10,000 feet above the sea. The high- 

 est Teton, was measured with the sextant and made to be about 10,000 

 feet. All along the margins of the ridges we see a plenty of the blue, 

 cherty Carbonifei'ous limestone; also, the siliceous .rocks which lie 

 above, and a great many granitic masses, and also gray micaceous 

 slates. We have seen much of the Carboniferous rock along our road 

 to-day ; also red arenaceous beds, with now and then an erupted ridge. 

 The central portions of the mountains are composed entirely of the 

 eruptive material. 



'■'■June 19. — We traveled nearly due north twenty miles, down Pierre's 

 fork into Pierre's Hole, a beautiful valley, surrounded by mountains, 

 aijout fifteen miles wide and thirty long. On our right is the Teton 

 Range, composed entirely of eruptive rocks, with a general inclination 

 west or a little noith of west. It would seem as though this whole val- 

 ley had been formed by the drainage accumulating in a fissure of the 

 upheaval, for the mountains all seem to incline in the same direction. 

 Tlie hills are composed in part of a sort of vesicular trachyte, exceed- 

 ingly porous, some of the cavities being an iiu*-h in diameter. The 

 broad, level prairie is composed, to a large extent, of well-worn rocks, 

 basalts, obsidians, granites, &c. 



" June 20. — W>. continued our course directly north, and soon began to 

 ascend low ridges, breaking the level of the prairie. These ridges extend 



