MONITOR HILLS ; GENERAL GEOLOGY. 
leveloped in much of the rock. Some tuffaceous sediments with per- 
: ect feldspar crystals lie below the basalt on the west side of the area, 
ind it is not improbable that the younger tuffs, as well as the later 
hyolite of Goldfield, are here associated with the basalt. This rock 
s probably of late Pliocene or early Pleistocene age. 
MONITOR HILLS. 
The Monitor Hills are situated in the north-central part of the area 
urveyed. They are of gentle grade and rise 1,000 feet above the 
urrounding alluvial slopes. 
GENERAL GEOLOGY. 
The rocks of the Monitor Hills, the oldest being named first, are 
granite, Siebert lake beds, rhyolite, and basalt. 
SEDIMENTAKY ROCKS. 
Siebert lake beds. — These hills are in greater portion formed of 
ancly slopes, from the disintegration of well-bedded tuffaceous sand- 
tones. These are light gray or white in color and usually rather 
ne-grainecl, although some facies are conglomeratic. The rounded 
ebbles are of light-grayish glassy rhyolite. The tuffaceous sand- 
tones show an apparent exposure of 800 feet. These beds are litho- 
)gically similar to and are here correlated with the Siebert lake beds 
f Tonopah, situated 16 miles north of west of the Monitor Hills, 
nese Spurr a considers of Miocene age. 
IGNEOUS ROCKS. 
Granite. — Angular fragments of a medium-grained siliceous gran- 
e occur in the basalt described below, indicating that the basalt in 
js ascent to the surface passed through a mass of granite. 
Rhyolite. — Two isolated hillocks to the south of the main hills 
)pear from a distance to be an igneous rock that is lighter in color 
an basalt, and that is probably rhyolite. 
'Basalt. — A number of basalt areas are scattered over the Monitor 
ills, and this rock caps the summit of the group. The compact 
cies has a dense blue-black groundmass which is locally spotted 
Ith gray blotches one-fourth inch in diameter. The phenocry><s 
slude light-gray glassy feldspar from one-sixteenth to one-half 
h in length. Columns of a dark pyroxene and rounded grains of a 
ssy black substance, presumably olivine, occur. Under the micro- 
pe this rock proves to be a noncrystalline olivine basalt. The 
ck color of the olivine in hand specimens is due to its partial alter- 
Spurr, J. E., Prof. Paper U. S. Geol. Survey No. 42, 1905, p. GO. 
Bull. 308—07 m 7 
