SNAKE EIVER GORGE. 173 



stone. In outline the.se exposures are strikingly different from the sand- 

 stone, and in their mode of weathering stand out in strong contrast with 

 the surrounding rhyolite. An examination shows that these rocks are to be 

 classed as dacite. In composition they are more basic than the rhyolites, 

 and carry a larger proportion of plagioclase, and, in distinction from the 

 rhyolites of the region, hold a considerable amount of biotite. Apparently 

 they are older than the rhyolite and are exposed by the erosion of the latter 

 rock. Petrographically they are closely related to a dacite occurring in a 

 number of exposures on the west side of Snake River Valley, and then 

 interest lies in the fact that they are among the few instances known in the 

 Park of rocks with a dacitic facies related to rhyolites. 



REGION OF SNAKE RIVER GORGE. 



From Lewis River to the mouth of Coulter Creek the course of Snake 

 River makes an irregular curve of nearly 180°, closely encircling- the 

 northern end of the Wildcat Peak and Huckleberry Mountain mass, the 

 mouth of Coulter Creek lying 7J miles due east of Lewis River. Snake 

 River runs through a deeply eroded gorge, which for several miles above 

 Lewis River sharply defines the rhyolite flows of the Sheridan volcano on 

 the north side from the uplifted sedimentary . rocks on the south. Against 

 the abrupt wall of Paleozoic rocks the rhyolites were piled up to a g*reat 

 height, and it is along this contact that the river has cut its picturesque 

 gorge. On the north side the rhyolites stand out in a precipitous wall, 

 nearly 400 feet above the river, and thence rise gradually toward the central 

 portion of Mount Sheridan. On the south side a massive wall of limestone 

 rises 1,000 feet to the summit of the mountain. The beds are highly 

 crystalline, light in color, and belong to the Madison limestone. 



It is possible that older beds, represented by Three Forks and Jefferson 

 limestones, are exposed in the gorge along the base of the mountain, but 

 the mantle of drift is so heavy and the timber so dense that they have 

 nowhere been recognized. 



Above the Madison limestone come the brilliant red sandstones and 

 shales of the Teton formation, standing out conspicuously in contrast with 

 white and blue limestone. Along the summit of the mountain the Quad- 

 rant quartzite has nowhere been recognized. Arenaceous limestone lies 



