72 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
is located near Goldfield. AYells at Goldfield, in the Siebert lake beds, 
have struck water, but the main water supply is piped from distant 
points. 
GENERAL GEOLOGY. 
In the detailed survey of the special area Mr. Ransome and his 
assistants recognize the following formations, named from the oldest 
to the youngest : Cambrian ( ?) jasperoid; post-Jurassic granite; first 
andesite; earlier rhyolite; andesite and dacite, the latter formation 
probably being slightly younger; quartz latite and rhyolite; Siebert 
lake beds, with associated quartz-basalt and rhyolite flows; later 
rhyolite: later tuffs, and basalt. 
Of these formations the Cambrian jasperoid and the first andesite 
and rhyolite appear to be confined to the special area and are described 
in another bulletin of the Survey." The rhyolite flow breccia occurs 
only in thin Hows and. because of the small scale employed, is mapped 
with the basalt. 
SEDIMENTARY ROCKS. 
Siebert lake beds. — The Siebert lake beds cover considerable areas 
in the central and southern portions of the Goldfield hills and form 
a band between the basalt and the Recent desert gravels. The actual 
distribution of the formation is greater than that shown on the map, 
since small areas lie on the eroded surfaces of the andesite and the 
latite and rhyolite in many places. These lake beds are gray, yellow, 
or reddish sandstones, shales, grits, and conglomerates, composed pre- 
dominantly of material derived from the Tertiary lava flows, but 
subordinately of Paleozoic sedimentary rocks and post-Jurassic gran- 
ular igneous rocks. The larger bowlders are 2 feet in diameter. 
Section- on the mesa scarp, in which the base is not exposed, show a 
vertical exposure of 240 feet. The Siebert lake beds were folded and 
the edges of the beds truncated by erosion prior to the outflow of the 
later rhyolite. The formation closely resembles the Siebert lake beds 
of Tonopah h and is presumably of Miocene age. 
LaU ■■/■ tuffs. — On the mesa scarp to the west and northwest of Gold* 
field 10 to 20 feet of tuffaceous sandstone lie above the later rhyolite 
and beneath the basalt. These sediments, probably deposited under 
water, are characterized by many bowlders of a silvery-gray pumice, 
presumably derived from the erosion of the surface of the underlying 
rhyolite. The position of these tuft's beneath the basalt with no 
marked erosional interval indicates that they are probably to be cor- 
related roughly with the lower portion of the Pliocene-Pleistocene 
older alluvium occurring elsewhere in the area studied, although they 
were probably deposited in a lake, the predecessor of the older playas] 
" Ransome, F. L., Preliminary account of Goldfield. Bullfrog, and other mining district! 
in southern Nevada, etc.: Bull. U. S. Geol. Survey No 303, 1907, 98 pp., 5 pis. 
'Spun-, J. E., Prof. Paper U. S. Geol. Survey No. 12, 1905, p. 51. 
