106 SOUTH WESTERN NEVADA AND EASTERN CALIFORNIA. 
olivine- and augite-bearing basalt, somewhat altered. Orthoclase 
phenocrysts are also present. Small inclusions of vesicular basalt 
occur in rhyolite H miles north of Silverbow. Basaltic Hows con- 
temporaneous with rhyolite also occur in the Amargosa Range and 
the Bullfrog Hills. 
Biotite andesite. — One mile northwest of Silverbow a small hill 
surrounded by alluvium is composed of a fine-grained dense andesite 
of gray color. The small phenocrysts. which are in rude flow orien- 
tation, consist of white striated feldspars and more numerous smaller 
biotites. Southeast of Blakes Camp is a poorly exposed mass of a 
greenish-gray andesitic rock, considerably altered. Biotite and stri- 
ated feldspars one-eighth inch long are equally abundant. Under the 
microscope the glassy groundmass is seen to be subordinate to the 
phenocrysts, which consist of medium-sized simple crystals of plagio-j 
clase, biotite. and a few deeply embayed orthoclase individuals. The 
plagioclase is considerably altered to calcite, and the sericite and the 
biotite to chlorite. It is probable that these two exposures are to 
be correlated with one another and with the biotite-andesite intrusive 
in the rhyolite of the Cactus Range. 
Dacite. — In the vicinity of Rose Spring are five small areas of 
dacite. Another small ma^s lies 1 mile east of Stinking Spring and 
a rather similar rock i- exposed 3 miles south of Georges Water. 
The dacite has a compact aphanitic groundmass of medium to dark! 
gray or brown color. The phenocrysts equal the groundmass in bulla 
and consist of medium-sized glassy striated feldspars, hornblende, 
mica, and quartz. The quartz, while not abundant, is constantly 
present. A few well-formed crystals of striated feldspar, evidently 
of a later origin than those just mentioned, are from three-fourths of 
an inch to H inches Long. These are white or gray in color and 
include tiny biotite plates and hornblende columns. Weatherinl 
brings out clearly the zonal structure, while Carlsbad twins are 
sometimes ^v^w. The phenocrysts near contacts with older rocks 
possess a parallel flow orientation. Microscopic examination shows 
the- rock to have a felty groundmass of plagioclase laths with a few 
orthoclase and quartz grains. Many of the smaller plagioclasl 
phenocrysts, which are usually compound and slightly rounded by 
magmatic corrosion, show beautiful zonal growth. Plagioclase is 
ordinarily fresh, although calcite is locally developed from it. One 
or more quartz phenocrysts deeply embayed are present in mod of 
the thin sections. Biotite. usually surrounded by a magnetite reac- 
tion rim, is abundant. Forms characteristic of hornblende and 
heavily rimmed by magnetite grains are completely altered, calcite, 
in places accompanied by serpentine, and magnetite being the altera- 
tion products. Magnetite and apatite are accessory minerals. The 
dacite occurs in dikes and irregular intrusive masses which (ait the 
earlier rhyolite 2 miles southwest of Sumner Spring. The mass l\ 
