PAHUTE MESA, IGNEOUS ROCKS. 
pegmatitic quartz. The ellipsoidal form and the absence of channels 
from one mass to another in the place of observation suggest that 
the pegmatite formed in place from the local residual fluids of the 
granitic magma. 
The granite of Trappmans Camp contains inclusions of the schisl 
already described and of a black Paleozoic quartzite. It i> cut by 
dikes of the earlier rhyolite, and the later tuffs overlap it. It resem- 
bles the granite of Lone Mountain. The granite near Whiterock 
Spring is much older than the Siebert lake beds and closely resem- 
bles that .near Oak Spring in the Belted Range. Each granite is 
probably of post-Jurassic and pre-Tertiary age. 
Earliest rhyolite. — The earliest rhyolite protrudes from the basalt 
in a number of hills in the northwestern portion of the mesa. It evi- 
dently occurs for the most part as a flow, although dikes of rhyolite 
cut granite at Trappmans Camp. These dikes reach an observed maxi- 
mum width of 40 feet. They are formed of a lithoidal, brownish- 
gray rhyolite with conchoidal fracture. The feldspar, quartz, and 
biotite phenocrysts rarely exceed a length of one-tenth of an inch and 
are exceeded in bulk by the groundmass. The flow at Wilsons Camp 
is composed of a white phenocryst-rich rhyolite, in which biotite is 
present in some places and absent in others. The rock of the inlier, 
0-J miles northwest of Tolicha Peak, is a semipumiceous, brownish- 
gray rhyolitic glass, with a few small phenocrysts. Pebbles of black 
obsidian occur on the mesa near by. Other hills in the vicinity were 
not visited, but on the basis of their color and the report of pros- 
pectors some are mapped as rhyolite. Beyond the fact that the rhyo- 
lite of this vicinity is younger than the granite and older than the 
basalt, its age is unknown. It is believed, however, that the differ- 
ent masses are portions of the rhyolite of the Cactus Range buried by 
later lava flows, and are in consequence of early Miocene age. 
Biotite andesite. — Biotite andesite forms the in-lier of Gold Crater 
and several smaller inliers near by, and also occurs as dikes cutting 
the rhyolite at Wilsons Camp and the granite at Trappmans Camp. 
The biotite andesite at Gold Crater forms a group of low hills 
which protrude above the basalt, later rhyolite, and later tuffs. It is 
evident that the andesite flow was eroded into hills prior to the forma- 
tion of the younger rocks, and it is probable that the younger rocks 
never completely covered the summits of these hills, although liquid 
lava dammed up against their sides. The biotite andesite here is for 
the most part intensely altered, but appears to have been originally a 
dense gray rock with phenocrysts equaling the groundmass in bulk. 
The phenocrysts are medium sized and consist of predominant feld- 
spar in laths, biotite, and locally hornblende. Flow banding and 
floAv breccias are present. Under the microscope the microfelsitic 
groundmass is rather ill defined, but seems to be composed predomi- 
