PAHUTE MESA, IGNEOUS ROCKS. 133 
the later rhyolite and later tuffs of the Goldfield hills. ( Sec pp. 72, 71.) 
Similar beds outcrop at many points where erosion has removed the 
basalt, and it is probable that these beds underlie the greater portion 
of the basalt. No attempt has been made to separate them from 
basalt on the map. A section east of Stonewall Mountain is as 
[follows : 
Section east of Stonewall Mountain. 
Basalt. p ee t. 
Rhyolite 5 
bray tuff with small pebbles 15 
Rhyolite 7 
Yellow tuff, very rich in pebbles of pumice, which reach a maximum diame- 
ter of 1 inch 20 
Gray, rather incoherent, conglomeratic tuffaceous sediments. The pebbles, 
which reach a maximum diameter of G inches and become smaller toward 
the top, are formed of rhyolitic pumice, a quartz-poor rhyolite like the 
later rhyolite of Goldfield, and basalt. Tiny crystals of feldspar, similar 
to the phenocrysts of the rhyolite of Goldfield, are present 55 
Yellow tuffs, like above 10 
From this section and that at Goldfield it may be inferred that a 
ake covered considerable territory in the northern part of the area 
under discussion, probably in late Pliocene time. The deposits of this 
ake are separated from those of the Pahute Lake (Siebert lake beds) 
3y an erosional unconformity, and are probably older than the playa 
deposits of the older alluvium in Stonewall Flat. Between the Plio- 
cene lake beds and the older playa deposits there is little relation, 
although the deposits laid down in playas probably represent the last 
tages of the drying up of the lake in which the lake beds were 
formed. Spurr a found deposits of a late Pliocene lake widely dis- 
tributed in Nevada, and it is possible that this lake and the one here 
described are broadly contemporaneous. 
i Recent desert gravels. — Recent desert gravels mask the bed rock 
ver a considerable area east of Fortymile Canyon and extend some dis- 
ance up the main canyons. 
IGNEOUS ROCKS. 
The igneous rocks of Pahute Mesa include pre-Tertiary granular 
rocks and Tertiary dikes and lava flows. 
Pre-granite monzonite. — In the granite 5 miles north of west of 
Whiterock Spring are inclusions of a gray fine-grained granular rock. 
Both striated and unstriated feldspars are present with quartz and 
biotite, and the rock is similar to the monzonite included in the po<t- 
■Jurassic granitoid rocks of Gold Mountain and the Belted and Pana- 
mint ranges. 
t Post- Jurassic granite. — Granite forms the hills at Trappmans ( lamp 
nd also outcrops 5 miles north of west of Whiterock Spring. The 
a Spun-, J. E., Bull U. S. Geol. Survey No. l'<>s. 1903, pp. 124-125, 209-210, etc 
