CACTUS RANGE, [GNEOUS ROCKS. 
:hese rocks underlie the augite andesite on the east side of the range, 
rhe tuffaceous sandstone, which is well bedded and white or greenish 
n color, is but slightly consolidated and breaks down readily into a 
leep sandy sod. Interbedded with the sandstones are conglomerates 
vhich, in the area southwest of Cactus Spring, contain bowlders of 
Paleozoic limestone 3 or 4 feet in diameter. These beds arc tentatively 
considered rhyolitic tuffs deposited in local basins during the rhyolitic 
extrusion. It is recognized, however, that these sediments may in 
reality be the Siebert lake beds, in which case the earlier rhyolite of 
Dactus Range is of late Miocene age and is to be correlated with the 
ater rhyolite of the Belted Range. 
Biotite andesite, — Dikes and flows of biotite andesite are widely 
iistributed in the southern part of the Cactus Range, but do not 
ippear to extend far north of the Goldfield-Cactus Spring road. Tn 
iikes and irregular intrusive masses and possibly in flows this rock 
covers considerable areas southeast of Cactus Spring, apparent dikes 
cut the rhyolite at Wellington, and an andesite flow caps the highest 
nountain in the southern half of the range. Other areas of biotite 
indesite occur northeast of Antelope Springs, and a traverse from 
Wellington to Antelope Springs crossed numerous areas of this rock 
:oo small to show on the present map. Dikes of biotite andesite occur 
m both sides of the Goldfield-Cactus Spring road, but these also are 
:oo small to indicate on the map. 
The biotite andesite is everywhere more or less altered. The fresh- 
est rocks have a dark-gra}^ groundmass in which are embedded abun- 
dant medium-sized phenocrysts. Striated feldspars of white color and 
n many cases of zonal growth are more conspicuous than the altered 
grayish-green biotite and hornblendelike mineral. More altered 
facies are greenish gray or purplish red in color. The andesite 
3reaks into sharp joint blocks which on further alteration develop 
spheroidal weathering. Under the microscope this rock appears to 
jiave had originally a pilotaxitic groundmass. Plagioclase pheno- 
crysts are common and grade in size into the laths of the groundmass. 
3ne or more orthoclase phenocrysts are present in the majority of 
slides. Biotite is now a pseudomorph of chlorite, calcite, epidote, 
quartz, and sagenitic rutile. Aggregates of chlorite, epidote, calcite, 
Imd quartz surrounded by reaction rims of magnetite appear in some 
ftses to have the form of a pyroxene and in others that of an amphi- 
>ole. Probably both minerals were originally present. These pseudo- 
inorphs, like the plagioclase phenocrysts, grade into groundmass 
nicrolites of similar form. Magnetite is a common accessory. 
The biotite andesite cuts the rhyolite in dikes and caps it in flows; 
nclusions of rhyolite occur in the andesite, and near some of the 
thyolite masses the phenocrysts of the andesite become smaller. The 
ndesite, then, is younger than the rhyolite, and from its altered com 
