146 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
Southwest of Tippipah Spring 275 feet of the later rhyolite over- 
lies the Siebert lake beds, while near the center of Shoshone Moun- 
tain about 1,500 feet of rhyolite is exposed. At the first locality the 
lower 250 feet is a dense brownish-pink rock with sparse and small 
phenocrysts of glassy feldspar, biotite, and quartz. Some flow bands 
are purple or gray in color, others are vesicular, and still others 
carry numerous small inclusions of rhyolite, and are in consequence 
flow breccias. Vertical columnar joints, originating during the cooling 
of the magma, are not unusual. The broadest columns are 4 feet in 
diameter. Above this is 15 feet of a black glass, in which the pheno- 
crysts exceed the groundmass in bulk. This in turn is overlain by 10 
feet of rhyolite like that first described, except that the phenocrysts 
are larger and more numerous. In it are thin bands of the black 
glassy variety. At the south end of Shoshone Mountain similar facies 
occur, and in some of these the flow banding is strongly developed, 
the flow lines, as a rule, being straight, although in places wavy. 
The later rhyolite south of Cane Spring is a yellowish-brown rock 
with lithoidal or chalky groundmass, which is equal in bulk to the 
phenocrysts. These include medium-sized glassy unstriated feldspar 
and slightly smoky quartz, together with abundant but tiny biotite 
phenocrysts. The rhyolites west of Yucca Pass are waxy-lustered 
glasses and well-handed, phenocryst-rich flow breccias of purplish 
color. In the latter gray spherulites occur. 
The later rhyolite ordinarily forms buttes and mesas, since the 
various flow bands vary considerably in resistance to erosion. Like 
the later rhyolite of the Kawich Range, the later rhyolite of these hills 
overlies the Siebert lake beds. The two, which are rather similar 
lithologicaly, are probably about contemporaneous and are of late 
Miocene or early Pliocene age. 
Basalt. — A basalt flow caps Skull Mountain, a number of small 
areas occyr to the east of the Calico Hi IK and to the west of the Horn 
Silver mine, while three small hills lie 4 miles east of the iron tank 
in Fortymile Canyon. Basalt also occurs between Forty/mile Can- 
yon and Shoshone Mountain. It is a dense black or dark-gray rock 
with sparse phenocrysts of glassy striated feldspar and slightly 
altered glassy olivine. Both the vesicular and nonvesicular facies 
previously described in other ranges occur. The nonvesicular basalt 
weathers into spheroidal masses, while the vesicular facies usually 
break down into slabs which are elongated parallel to the flow band- 
ing. A single thin section of a dense, slightly vesicular facies proved 
on microscopic examination to be composed of dark glass containing 
many laths of a calcic plagioclase and grains and partial crystals of 
olivine more or less altered to reddish-broAvn serpentine. 
On Skull Mountain the basalt lies upon the eroded surface of the 
Siebert lake beds, while in the low isolated hills 4 miles east of the 
