136 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
nantly of plagioclase and orthoclase. The phenocrysts include seri- 
citized feldspars, which were at least mainly plagioclase, biotites 
altered to muscovite and magnetite, and some hornblende-like forms of 
serpentine and limonite. 
The biotite andesite at Wilsons Camp is a highly altered, finely 
granular rock of pale-greenish color, containing many phenocrysts 
of white altered feldspar and some of bronze biotite. Probably to be 
correlated with this rock is a much altered greenish-gray, finely granu- 
lar rock which occurs in dikes at Trappmans Camp. Laths of altered 
feldspar, columns of hornblende, and tiny tablets of biotite are de- 
terminable in the hand specimen. The microscope shows that the 
much altered groundmass was probably originally a glass with a few 
plagioclase laths (hyalopilitic) . The only phenocrysts recognizable 
are large calcitized plagioclase and chloritized biotite and a few 
small, fresh orthoclase tablets. Aggregates of secondary minerals 
suggest the presence of hornblende or pyroxene. Apatite and mag- 
netite, probably titaniferous, are accessory minerals. 
The andesite at Wilsons Camp bears the same relation to the rhyolite 
as the biotite andesite of the Caetns Range, and the two are doubt- 
less contemporaneous and of middle Miocene age. The andesite of 
Gold Crater is much older than the latest rhyolite. Lithologically it 
is not very different from the biotite andesite and is provisionally cor- 
related with it. It i>. however, possible that it is the flow equivalent 
of the monzonite porphyry of the Kawich Range. 
Middle rhyolite. — The middle rhyolite, evidently a flow, is the pre- 
dominant formation of Pahute Mesa east of Thirsty Canyon. The 
most widely distributed t} T pe is a gray or pink lithoidal rhyolite, in 
which the groundmass is exceeded in bulk by the medium-sized pheno- 
crysts. The latter include glassy orthoclase, colorless or slightly 
smoky quartz, and, usually, biotite. This variety in many places 
contains small fragments of a white pumiceous rhyolite and in con- 
sequence is a flow breccia. Other facies include dense rocks of red, 
brownish, or gray color, with few 7 phenocrysts. Flow banding, spher- 
ulites, and perlitic parting accompany the denser groundmass in 
much of the rock. Black glasses with feldspar phenocrysts occur 
near Ammonia Tanks, and gray semipumiceous glasses with abundant 
feldspar and quartz phenocrysts are found on the east side of Silent 
Canyon, 4 miles south of its mouth. This rhyolite for the most part 
lies upon the Siebert lake beds and it is to be correlated with the later 
rhyolite of the Belted Range. It is presumably of late Miocene or 
early Pliocene age. 
Latest rhyolite.— The latest rhyolite occurs below the basalt in the 
vicinity of Stonewall Mountain and probably outcrops in many other 
places on the border of the mesa that are shown on the map as basalt. 
It is interbedded with and overlies the later tuffs. A section alreadv 
