388 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



is dense and dark colored. But on the summit of the peak, 9,000 feet high, 

 at the head of the creek, it is light pink and gray, earthy in texture, and 

 brecciated like a tuff, with fragments of andesites, and with phenoerysts 

 like the vesicular brecciated rock lower down the creek. There is no 

 evidence, however, that it is a tuff, but it appears to be a finely brecciated 

 massive rock, of the same kind as the rhyolite opposite the mouth of Thistle 

 Creek, except that the latter is denser and not brecciated (2059 to 2064). 



Along the east shore of Yellowstone Lake the rhyolite extends from 

 the vicinity of Pelican Creek on the north as far south as Brimstone Basin, 

 a small area of hot springs 2 miles south of Columbine Creek. It forms a 

 sheet of lava which constitutes the tabledand and flat-topped spurs between 

 the lake and the Absaroka M ountains, reaching an elevation of about 8,500 

 feet. Tongues of it extend up the long valleys and are, found at still higher 

 altitudes. In the valley of Sylvan Lake it forms a massive bluff on the 

 north side, which reaches 8,700 to 8,800 feet elevation. This is several 

 hundred feet higher than the divide in Sylvan Pass; still it has not been 

 found east of the watershed in the valleys draining into the Stinkingwater 

 River. In the next valley north of that of Clear Creek the rhyolite sheet 

 is found at 9,000 feet. The rock throughout the greater part of this area is 

 massive, lithoidal, purplish, and porphyritic. The glassy and pumiceous 

 parts which probably formed its surface have been eroded away. 



VICINITY OF YELLOWSTONE RIVER. 



The surface of Central Plateau, which extends from the Elephant Back 

 west to the head of Nez Perce" Creek, and around the west end of Hayden 

 Valley northward, is covered, like the country south, with glassy rhyolite. 

 It is mostly porphyritic black obsidian, more or less spherulitic, and is in 

 places vesicular. On the south branch of Alum Creek, a short distance from 

 the road, the obsidian is spherulitic and carries lithophysse and hollow 

 spherulites from 1 to 6 inches in diameter (2072, 2073). The obsidian in 

 places is traversed by jointing planes, along which the surface of the rock 

 is smooth and polished, but slightly uneven and warped. The crystals of 

 cpiartz and feldspar have been cut across smoothly in most instances, though 

 in many cases the plane of jointing has curved around the end of a crystal, 

 or followed the cleavage of the feldspar when this was nearly coincident 

 with the plane of jointing. A small elevation and depression extends for 



