MOUNT SHERIDAN. 381 



brown obsidian, with many phenocrysts and small spherulites. The spheru- 

 lites have crystallized around phenocrysts as nuclei in many cases. There 

 are also small crenulated cavities, which lie indiscriminately in, the glass 

 and spherulites and are coated with minute crystals (1968, 1969). These 

 curious cavities occur in the rock of Obsidian Cliff and elsewhere in 

 the Park. 



RED MOUNTAINS. 



On the edge of the plateau of rhyolite which lies between the Yellow- 

 stone Lake and Snake River rises a small group of mountains, whose highest 

 peak at the eastern end is Mount Sheridan (10,200 feet). It is an east-west 

 ridge with four prominent spurs trending north and south, separated by 

 deep amphitheaters. Its summits are from 1,500 to 2,000 feet above the 

 plateau, but the eastern peak is 2,700 feet above Heart Lake, which lies at 

 its base. The slopes and spurs on the east and north are short and steep, 

 and only the southern ones fall away gradually to the level of the plateau. 

 These mountains are of rhyolite, whose character in the body of Mount 

 Sheridan differs somewhat from that of the plateau. On the steep north- 

 eastern spur south of the Heart Lake Geyser Basin much of the rhyolite 

 is white and gray lithoidal rock, with a moderate number of small 

 phenocrysts, in some places very few, and belonging to the variety liparite. 

 The main mass of Mount Sheridan and the ridg-e immediately west is 

 composed of this dense liparite, which is light purplish gray in color and 

 fissile in thin plates (1980). Similar rhyolite occurs near the top of 

 the ridg-e and on the summit of Mount Sheridan, where it is coated with 

 hyalite in places and contains many transparent crystals of tridymite in 

 thin fissures (1984). The rhyolite on the top of the ridge and on the upper 

 northern slope and summit of Mount Sheridan is brecciated, but it is com- 

 pact, and is evidently brecciated flow rock and not an aggregation of loose 

 fragments and dust, which is the case with almost all of the andesitic breccia 

 of this region. In places on the summit the rhyolite is brown and glassy, 

 and the brecciated portion is intersected by dikes of massive banded lithoidal 

 rhyolite of no considerable extent. This appears to be the only instance 

 in which anything resembling a dike of rhyolite has been observed in the 

 Yellowstone Park. All of the rhyolite exposures observed by the writer 

 appeared to be surface flows of lava resting upon older rocks. In the cases 

 where later bodies of rhyolite have been recognized they have always been 



