FALLS EIVER BASIN. 377 



sheet of rhyolite, which is finely exposed on the east wall. In the bottom 

 portion of the flow are large masses of inclosed rock. The central layer is 

 massive, with well-marked planes of flow. A short distance upstream are 

 fine double cataracts, and above these are others in continuous succession, 

 the river descending in all about 150 feet. The rocks on both sides of the 

 canyon appear to be soaked with water, which runs into the river from a 

 multitude of small streams and springs along the base of the walls. 



FALLS RIVER BASIN. 



Falls River Basin, which in reality is a terrace of the great plateau, at 

 an altitude of from 6,300 to 6,500 feet, is about 15 miles long and 8 miles 

 wide. It is a portion of the vast rhyolitic lava flow, which is partially 

 covered by a thin sheet of basalt. The river has cut its way down to the 

 rhyolite, which forms the bed of the stream from a point 1 mile below the 

 mouth of Boundary Creek down as far as explored, below Boone Creek. 

 The rl^olite is lithoidal and fissile, and at the falls below the mouth of 

 Falls River there is evidence of more than one flow of rhyolite. The upper 

 of these falls is a cataract. The middle one is a beautiful fall, 15 or 20 feet 

 high and about 200 feet wide, with a cascade in low steps above it. At the 

 west end of this fall there is a low arched cave, formed by a sheet of dense, 

 gray, glassy rhyolite, with small phenocrysts (1952), overlying a mass of 

 brecciated glassy rhyolite, which is more easily eroded. Above the gray 

 layer the rhyolite is glassy and spherulitic (1956), passing up into banded 

 lithoidal rock. The exposure appears to be that of the bottom of a lava 

 flow. The third waterfall is broad and low, not more than 5 feet high. 

 About 2 miles above the mouth of Boone Creek there is another fall, where 

 the river cuts 50 feet into the rhyolite, leaving isolated blocks of the rock 

 standing like monuments in the stream. The rhyolite forms a bench on 

 both sides of the river, with bluffs of basalt 100 feet high standing back a 

 short distance. This branch canyon of the Snake River begins to assume 

 the same geological character which the deeper canyon of the main stream 

 possesses in the neighborhood of Shoshone Falls, Idaho, where numerous 

 sheets of columnar basalt overlie a glassy and lithoidal dacite of peculiar 

 characters, which closely relate it to the rhyolite of the Yellowstone National 

 Park. 



The valley of Conant Creek, just north of the forty-fourth parallel of 



