BELTED RANGE, IGNEOUS ROCKS. 
tion than was possible in the present reconnaissance. The relations 
between the small rhyolite mass 2 miles south of west of Belted Peak 
and the Paleozoic rocks are poorly exposed, but the rhyolite probably 
represents the lower portion of the rhyolite flow rather than a dike 
since no dikes were observed in sedimentary rocks to (he west. 
The earlier rhyolite is similar lithologically to that of the Kawich 
and Reveille ranges, and like them lies beneath the Siebert lake beds. 
All three are practically of the same age, probably early Miocene. 
Later rhyolite. — Later rhyolite caps Oak Spring Butte (see fig. 13) 
and forms a lower bench on the same butte and a similar bench on the 
ridge north of Whiterock Spring. It also covers large areas to the 
west, on Pahute Mesa. The predominant facies is a resinous or glassy 
rhyolite composed of light-gray and black flow bands and lenses. 
Perlitic parting is locally present, Medium-sized phenocrysts equal 
the groundmass in bulk and in the hand specimen consist of unstri- 
ated feldspar, often showing beautiful color plays, and colorless 
quartz in rounded grains or dihexagonal pyramid and prism crystals. 
The microscope shows the presence also of smaller phenocrysts of 
Fig. 1A — North-south section through Oak Spring 'Butte, Belted Range. 
deep-green augite and biotite. Another facies is a reddish-gray 
japhanitic rock with a few phenocrysts of feldspar and black mica. 
iSome quartz and plagioclase phenocrysts are also visible under the 
microscope in the devitrified-glass groundmass. 
The later rhyolite is a flow, in part contemporaneous with the Sie- 
bert lake beds, but mostly younger. It is therefore of middle and 
possibly late Miocene age. It is contemporaneous with the rhyo- 
lite of Shoshone Mountain and Pahute Mesa, and probably with the 
later rhyolite of the Kawich Range and of Stonewall Mountain and 
with the later rhyolite and biotite latite of the Amargosa Range. 
Basalt.— Basalt covers considerable areas near the middle of the 
Belted Range. The low hills at the north end of the range also 
appear from a distance to be basalt. The basalt, where examined, i 
lithologically similar to and probably contemporaneous with that of 
the Reveille Range, and, like it, overlies the eroded earlier rhyolite. 
It is probably of late Pliocene age. The surface of the Belted Range 
was, however, probably less rugged than that of the Reveille Range 
at the time of the basalt effusion. 
