116 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
IGNEOUS ROCKS. 
Rhyolite. — The major portion of the Reveille Range is composed 
of rhyolite, which has a flinty, glassy groundmass, flow banding 
being common. Gray and pink are the predominant colors, although 
red, black, and brown are not unusual. Phenocrysts are abundanl 
and equal the base in mass. Colorless or smoky quartz and glassy 
feldspar phenocrysts of medium size predominate over the smaller 
individual- of biotite. Weathered surfaces show feldspar whitened 
or represented by casts, biotite flush with the surface, and quartz in 
relief. Thin sections under the microscope present a brown, glassy 
groundmass, exhibiting well-defined How flexures, and over small 
areas a spherulitic structure. A small portion of the feldspar w an 
acidic plagioclase, while ninny of the biotite phenocrysts were bent 
prior to the solidification of the lava. Magnetite, zircon, and apatite 
are also present. 
The rhyolite i- a series of (lows, and in consequence over large areas 
it weathers like a well-bedded sedimentary rock, a resemblance magni- 
fied by the presence of .1 north-south sheeting. Vertical joints arl 
locally pi L-e flov banding i- not prominent the rhyolite 
weathers into spi 1 30 to L50 feet high and from 5 to 25 feet in 
diameter. In places these are single and spring from a common basel 
Where no parting planes are prominent the rhyolite displays the well] 
rounded dome- and bosses characteristic of a granite country. 
The rhyolite of the Reveille Range is similar lithologically to that 
of the Kawich Range and has suffered equal deformation and erosioij 
Idie two are doubtless contemporaneous in age and are probably early 
Miocene. 
Basalt. — Lying upon the rhyolite are basalt flows which usuallj 
occur flanking the range, but in one case extend from the eastern base 
across the crest Line. The Hows were once much more extensive and 
perhaps completely covered the range 1 miles north of Reveille 
Peak. Basalt also forms isolated hills in the alluvial deposits. As 
shown in fig. 9, these are probably connected with (low- on the 
flanks of the range. The basalt varies from a dense black rock' to a 
vehicular slag of red color. The vesicles reach a maximum diameter 
of 1 inch and are elongated in the plane of (low. The phenocrysq 
are subordinate in bulk t'> the groundmass, and include conspicuous 
gray or white striated feldspar laths up to three-fourths of an inch 
long, rather obscure yellow altered olivine-, and black pyroxene col- 
umn-. Weathered basalt break- down into spheroidal masses sur- 
rounded by thin concentric shells. 
On microscopic examination the rock prove- to he an augite-olivine 
basalt with glassy base. Abundant plagioclase laths, fewer augite 
crystals, rounded olivine-, and magnetites are the phenocrysts. 
