180 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
south of Indian ranch are of biotite andesite. This is a dense, flinty 
or lithoidal rock of medium, pinkish, or purplish gray color. 
Black glassy facie- arc less common. The small phenocrysts, feld- 
spar laths, with slightly fewer black mica plates are subordinate to 
the groundmass in bulk. The feldspars are usually altered, but in 
some instances they are glassy and twinning striations are visible. 
From the bedded disposition of the various textural facies and the 
flow orientation, visible in many of the phenocrysts, the biotite ande- 
site is probably a flow considerably eroded and masked by coverings 
of later formations. The groundmass under the microscope is seen 
to consist of a felt of plagioclase laths in a somewhat devitrified 
glass (hyalopilitic). Besides the simple laths of plagioclase, usu- 
ally showing Carlsbad twinning, and the slightly bleached biotites, 
a rare orthoclase phenocryst is present, while certain ill-defined 
pseudomorphs, largely of serpentine, have the crystal outlines of 
hornblende. Ilmenite and apatite are present as accessory minerals. 
On the vest side of the hill northwest of Howell ranch much- 
altered pebbles and fragments of andesite are included in a tuffa- 
ceous member of the rhyolite series, and between the outflows of the 
two lava- there was apparently an erosion interval. The andesite is 
probably of late Eocene age. and is similar to and probably contem- 
poraneous with the biotite andesite of the Amargosa Range. 
Rhyolite. — The predominant formation of the Bullfrog Hills is a 
series of rhyolite flows. The most widely distributed facies is a 
lithoidal rhyolite of gray, pink, or purple color. The medium- 
sized phenocrysts. which usually exceed the groundmass in bulk, 
consist of glassy unstriated feldspar, slightly smoky quartz, and in 
many places biotite. Hornblende is also locally present. Less 
widely distributed types include aphanitic rocks with varicolored 
flow banding, resembling slaty shales; brown or greenish glasses. 
with or without phenocrysts, and gray glasses with perlitie parting 
well developed. Interbedded with the other members of the rhyo- 
lite series are rhyolitic sandstones and conglomerates, while Messrs 
Tvansome. Emmons, and Garrev found a thin limestone lens in the 
series. Through metambrphism, kaolinized and silicified facies arise. 
( Sec p. [~. ) 
The rhyolite is a series of igneous flows, with minor intercalated 
beds of sedimentary rocks, in part deposited under water and in part 
subaerially. Where the flows are thick the rhyolite in places 
weathers into bosses and low domes, but for the most part the vary- 
ing resistances of the different flows to erosion give a bench and cliff 
character to the topography. 
This rhyolite, which contains fragments of Paleozoic rocks and 
biotite andesite. on lithologic and structural grounds is correlated 
with the earlier rhyolite of the Amargosa and Kawich ranges. It is 
probably of early Miocene age. 
