186 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
of Miocene age lie upon its eroded surface. Its age is in consequence 
post-Cambrian and pre-Tertiary. The granite is also cut by dikes of 
diorite porphyry presumably of pre-Tertiary age. It is probably of 
post-Jurassic age. 
Diorite porphyry. — Dikes of diorite porphyry reaching a maximum 
observed width of 40 feet cut the Cambrian rocks and the granite of 
the Gold Mountain ridge. The rock is greenish in color, contains 
abundant phenocrj^sts of white altered feldspar, and tends to weather 
into spheroidal bowlders. It is probably of pre-Tertiary age. 
Rhyolite. — The hills to the southwest of Gold Mountain, the ridge 
east of the Rattlesnake mine, a number of isolated hills and ridges to 
the east, south, and southwest of the latter ridge, and a small area 
northwest of the granite at the west end of the Gold Mountain ridge 
are composed of rhyolite. The presence of a small outlier of rhyolite 
on granite, three-fourths of a mile southeast of Tokop, indicates that 
erosion has removed considerable portions of a once more extensive 
rhyolite flow. 
The most widely distributed type is a lilac-gray rock of lithoidal, 
in places rather incoherent, groundmass. The medium-sized pheno- 
crysts, which in bulk usually equal the groundmass, are tabular crys- 
tals of glassy, unstriated feldspars, often showing blue and purple 
color plays, and rounded grains of colorless quartz. Smaller plates 
of biotite are locally present. Pink, purple, and red facies also occur. 
Black glassy types in which feldspar greatly predominates over 
quartz, to judge from the hand specimen, may be more properly called 
latite. 
Many of the rhyolite flows are flow breccias, containing abundant 
inclusions of rhyolite of different colors and textures. Vesicular 
facies showing flow lines are unusual. Interbedded with the rhyolites 
are sandy beds which contain normal feldspar and quartz crystals 
and rounded rhyolite pebbles. These are contemporaneous tuffs of 
either aqueous or subaerial origin. It is probable that minor flows 
of basalt were contemporaneous with those of rhyolite, since vesicular 
basalt fragments are in places included in rhyolite. 
The rhyolite occurs in mesa-like ridges whose top is often coin- 
cident with the surface of a resistant rhyolite flow. This rock lies 
unconformably upon granite and Cambrian limestone, and upon its 
slightly eroded surface basalt flows rest in turn. The rhyolite is to 
be correlated with that of the Amargosa and Kawich ranges and the 
Bullfrog Hills and is probably of early Miocene age. 
Basalt.— Basalt covers the butte between Old Camp and Tokop and 
occurs in two small areas to the west of the butte and in a number of 
small areas 11 miles south of Gold Mountain, resting upon the slightly 
eroded rhyolite surface. The basalt of the butte just mentioned is 
a dense dark-gray flow rock, which is vesicular along some bands. 
