FALLS EIVEE BASIN. 379 



and around the cavity the rock is lighter colored. Another modification 

 of the rock, which has numerous small phenocrysts of quartz, occurs 2 miles 

 northeast of Survey Peak. It is laminated and fissile in thin plates, resem- 

 bling a schist (1960). A similar form of rhyolite is found on the long spur 

 east of Snake River, opposite the mouth of Owl Creek. The rhyolite cap- 

 ping the limestone on the divide between Berry and Conant creeks belongs 

 to the nevadite type, being filled with phenocrysts. Near its contact with 

 the limestone it is dark slate colored, glassy, and spherulitic (1959). It 

 grades upward into lighter-colored, purplish, and yellow lithoidal nevadite, 

 full of irregularly shaped cavities. It weathers in great rounded and 

 roughened masses like granite. The same is true of the rhyolite in the 

 neighborhood of Birch Hills, in the valley to the northeast, and at Terrace 

 Falls. Here the coarsely porphyritic, reddish-purple rhyolite, or nevadite, 

 has been weathered and eroded into great rounded towers 100 feet high, 

 which resemble exposures of coarsely crystalline granite. 



It is to be remarked that while the rhyolite in the vicinity of Birch 

 Hills is lithoidal along the valleys cut by both forks of Falls River at 

 altitudes of from 6,700 to 7,000 feet, yet within the amphitheater at the 

 head of Mountain Ash Creek the stream at 7,000 feet cuts a narrow canyon 

 through porous and glassy rhyolite, which also forms the spur on the north 

 side of the amphitheater at 7,350 feet, where it is black porphyritic obsidian 

 and perlite. This is 1,200 to 1,500 feet below the edge of the Pitchstone 

 Plateau, where the rhyolite is black and red spherulitic obsidian with many 

 phenocrysts (1972). Farther down Mountain Ash Creek, at an altitude of 

 6,800 feet, Avhere two branches unite at the crest of Union Falls, 80 feet 

 high, the rhyolite is lithoidal. The occurrence of glassy forms of rhyolite 

 in the bottom of this amphitheater overlying lithoidal ones indicates that 

 this was the surface of a flow, and not the interior portion of one which 

 has been exposed by the erosion of a vast amphitheater. 



PITCHSTONE PLATEAU. 



One of the most interesting exposures of rhyolite is that furnished by 

 the high spur between the branches of Glade Creek, a tributary of the 

 Snake River, which enters the latter about 5 miles north of the forty-fourth 

 parallel of latitude. The spur is from 600 to 1,000 feet high, and at its 

 southern end presents a high bluff of lithoidal rock, exhibiting greatly 



