614 UESCEIPTIVE GEOLOGY. 



eastward, and form north and south ridges, cropping out through the 

 horizontal beds, which form the mesa-like benches through the middle of 

 Squaw Valley: these are made up largely of re-arranged volcanic material, 

 but along the eastern end of the valley contain also a considerable admix- 

 ture of sedimentary debris from Mount Neva. 



In the western end of Squaw Valley is a broad expanse of bottom- 

 land, which, at the entrance to the canon which Eock Creek cuts through the 

 ridge south of Squaw Valley, is occupied by a shallow fresh-water lake. On 

 the summit of the ridge south of Squaw Valley is found a grayish-red, banded 

 rhyolite, in which the bands consist of alternately predominating portions of 

 a reddish felsitic groundmass, and of aggregations of crystals of sanidin and 

 quartz. The rock contains neither hornblende nor mica. Under the micro- 

 scope, the groundmass is seen to have a very sphajrulitic structure and radial 

 fibration, and to abound in ferritic microlites, to the predominance of which 

 in the sphserulites the reddish bauds owe their color. The peculiar radial 

 arrangement of the ferritic microlites is shown in the thin section of this 

 rock, represented in Vol. VI, Plate VIII, fig. 3. 



The main body of this ridge is made up of the ordinary reddish por- 

 phyritic rhyolite already described, which contains only a few crystals of 

 quartz and sanidin, porphyritically imbedded in a red felsitic paste of 

 peculiarly uneven fracture. The microscope discloses neither quartz, horn- 

 blende, or mica. Through this ridge. Rock Creek has cut a narrow wind- 

 ing canon for a distance of about 4 miles, but in Rock Creek Valley 

 runs in a broad Quaternary bottom, eroded out of the horizontal Pliocene 

 beds. To the south of this valley, the rhyolites are covered by thin flows of 

 of basalt, which form the surface of the broad table-shaped ridges of the 

 Shoshone Mesa. 



The hills to the west of Rock Creek Valley near the Warm Springs 

 are composed of a peculiar decomposed rhyolite of a light pearl-gray color, 

 which is almost identical in appearance with that which forms the little coni- 

 cal hills rising out of the Shoshone Mesa near its southeastern edge. This 

 rhyolite contains a large amoimt of small, gray, glassy sanidin crystals, and 

 large cracked quartz-grains, but is almost devoid of hornblende and mica. 

 It is remarkable for containing a large amount of tridymite. Near the 



