382 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



in the form of surficial flows, the orifices or fissure through which they 

 reached the surface not being' exposed to view. This is most noticeably 

 the case in the outlying remnants of thin sheets whose plane of contact 

 with the underlying rocks is frequently observed. 



The eastern face of Mount Sheridan is composed of massive white 

 and gray liparite, like the northeastern spur. The upper portion of the 

 long northern spur consists of a heavy flow of rhyolite, about 700 feet 

 thick, which appears to have the same character as the plateau rock. 



In the neighborhood of Lewis and Shoshone lakes the rhyolite exhibits 

 variations from lithoidal to glassy forms. The bluff west of Lewis River 

 at the crossing of the old trail, 2J miles below Lewis Lake, exposes 

 very fine lithoidal rhyolite, mottled dark and light gray, which is porphy- 

 ritic and banded (1971). The top of the plateau west and north of Lewis 

 Lake is mostly glassy or hyaline rhyolite — black and red obsidian and 

 perlite, which is spherulitic and porphyritic (1973). This may be observed 

 along the trail between the two lakes. The character of the rhyolite on 

 the east side of Shoshone Lake near its outlet is particularly varied. It is 

 porphyritic and is streaked with black and red glass and blue spherulitic 

 layers, besides blue lithoidal portions, weathering, pink on the exposed 

 surface. The lake beach is made up of very irregularly shaped pebbles 

 of this rhyolite, which are but partially rounded and form a beautiful 

 variegated sand. 



VICINITY OF YELLOWSTONE LAKE. 



The most striking variability in the rhyolitic lava, however, is found 

 along the shores of the Yellowstone Lake. The many miles of coast offer 

 numerous bluffs which have been cut into the surface of the great rhyolite 

 sheet, and the slightly glaciated hills of the plateau country immediately 

 west of the lake shore present all possible modifications of this variable 

 lava. 



A good example of this is found in the cliff on the second point south 

 of the mouth of the West Arm of the lake and the first one north of Flat 

 Mountain Arm. Here red and black glassy forms occur, separately and 

 also intimately mixed. The rock is porphyritic and more or less spheru- 

 litic. There are masses of black obsidian completely shattered by irregular 

 cracks, which cause it to crumble readily into small angular fragments, and 



