OBSIDIAN CLIFF. 359 



Mount Everts, the material from both localities being identical. The 

 rhyolite sheet continues north until it reaches the andesitic breccia forming 

 the southeast spur of Sepulchre Mountain, upon which it rests, its lower 

 portion inclosing fragments of andesite. It extends along the base of 

 the south slope of Sepulchre Mountain west to the divide between Glen 

 and Reese creeks, and forms the bench of dark-purple rock with small 

 phenocrysts south of Cache Pond, and also an isolated remnant of light- 

 reddish earthy rhyolite overlying the andesite on the west base of Sepulchre 

 Mountain. 



From this we see that in the neighborhood of the Mammoth Hot 

 Springs the rhyolite has a very uniform character, being mostly lithoidal, 

 except in one place on the old Norris road northwest of Terrace Mountain, 

 where a small mass of dark-colored rhyolitic perlite is exposed, which, as 

 Holmes has remarked, "is similar to that forming the under surface of 

 most of the rhyolitic flows in this region." In this vicinity, however, the 

 under surface of most of the rhyolite sheet has only a thin film of perlitic 

 glass along its contact with the underlying rhyolitic tuff. The tuff deposit 

 has not been observed on Glen Creek, nor at the head of Reese Creek, 

 nor does it underlie the most northern remnant of rhyolite which occurs on 

 the west side of Bear Gulch, north of the Park boundary. 



OBSIDIAN CLIFF. 



The rhyolite which forms the plateau country and the fiat-topped 

 bluffs, 300 or 400 feet high, on both sides of Willow Park along Obsidian 

 Creek is a lithoidal to earthy rock, reddish purple in darker and lighter 

 shades, and filled with brilliant phenocrysts of quartz and sanidine, the 

 latter exhibiting a blue iridescence in many localities. Rhyolite of the same 

 character continues to form the plateau as far south as the Norris Geyser 

 Basin, being well exposed all along the road. It also extends east to Lava 

 Creek and forms the west base of the mountains west of Tower Creek, 

 overlying andesitic breccia and reaching an altitude of 8,800 feet. 



At the northern end of Beaver Lake the lithoidal rhyolite is overlain 

 by a great flow of rhyolitic obsidian, which covers the high country to the 

 east in a sheet 75 to 100 feet thick and has accumulated in an ancient valley 

 to the depth of 200 feet. The stream erosion of this thicker mass has 



