62 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
The siliceous igneous rocks are probably all portions of a single 
series that is older, for the most part, than the Siebert lake beds 
They are probably contemporaneous with the Tonopah rhyolite-dacite 
described by Spurr" as presumably of Miocene age. 
Later rhyolite. — Lying upon the eroded surface of the Cambrian 
rocks and protruding from the Recent alluvial deposits in the vicinity 
of Alkali Spring arc small exposures of a lavender lithoidal igneous 
rock, with fairly numerous glassy feldspar phenocrysts of medium 
<v/A\ These rocks closely resemble the later rhyolite underlying the 
basalt flows at Goldfield and are tentatively correlated with it. 
Basalt. — A basalt mesa lie- between the spurs of the Silver Peak 
Range 1 and the Goldfield hills. For convenience its description i.- 
given with that of the hill group ( p. 75). A basalt dike 50 feet wide 
cuts the Cambrian limestone near its contact with the Siebert lake 
beds on the Montezuma-Goldfield road. The basalt is a dense black- 
rock containing medium-sized glassy olivine grains and -mall striated 
feldspars and black augites. Under the microscope the rock is seen 
to be a noncrystalline olivine basalt. The plagioclase is partially 
altered to calcite and the olivine to serpentine. Basalt also covers a 
considerable area 1 \ mile- southeast of Lida. This i> evidently a flow, 
the red or black rock being full of vesicles and much of it having a 
ropy surface. This rock is contemporaneous with the basalt of Mount 
Jack-on and Goldfield, and the dike near Montezuma i- probably of 
the same age. 
Five miles northeast of Lida a flow of andesite or basalt lies upon 
the eroded surface of the Cambrian lime-tone. It has a gray lith- 
oidal groundmass in which large striated feldspar phenocrysts are 
prominent. Lithologically the rock resembles certain members 
of the basalt flow south of Goldfield. but the mature topographic 
form of it- surface may more nearly ally it to the older andesitq 
already described. 
STRUCTURE. 
The Cambrian rocks are for the most part in rather open folds 
with north or northeast axes, which are crossed by minor fold-. The 
amount of dip as :> rule is not great, although at many points in the 
isolated hill southwest of Alkali Spring the rocks are on edge and 
the folding appears close. Locally small isoclinal folds are present, 
particularly near intrusions of granite, and the northwest strike of 
the Cambrian 5 miles north of Montezuma Peak is probably due 
to the intrusion of the quartz-monzotnite porphyry. The gentle 
character of the folding of much of the Silver Peak Range is in 
marked contrast with that pi the Amaigo-a Range and Bare Moun- 
tain to the south and southwest. The folding, in part at least, pre] 
ceded the intrusion of the granite since this rock is intruded along 
Spun-, J. E., Prof. Paper 1'. s. Geol. Survey No. 41\ 1905, pp. 41-43. 
