92 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
monzonite porphyry of the Kawich Range; it is therefore tentatively 
considered the effusive equivalent of that formation and is thus 
probably of Eocene age. 
Earlier rhyoliU . — The most widespread formation of the Cactus 
Range is a rhyolite which occurs in flows. Throughout the range it 
appears to bear similar relations to the other Tertiary rocks and in 
the main probably represents a single period of rhyolitic volcanism. 
More detailed work may. however, prove that some of the rhyolites 
near Cactus Peak are younger than those of the central and southern 
portions of the range. 
The rhyolite- vary among themselves in color, in character of 
groundmass, and in the relative abundance of the various phenocrysts. 
The predominant type i> a white or gray rock of lithoidal or glassy 
groundmass, in which are embedded abundant medium-sized, slightly 
smoky quartz and glassy orthoclase phenocrysts; biotite phenocrysts 
are small and inconspicuous or absent. Other phases of the rhyolite 
are black, purple, or red in color. The phenocrysts of quartz are 
typically corroded grains, although some exhibit the dihexagonal 
pyramid and prism. Wavy (low bands of slightly different color 
traverse the groundmass. In many beds irregular fragments of rhyo- 
lite are inclosed in a matrix of similar rhyolite, showing that portions 
of the magma were solidified prior to cessation of movement in the 
flow. The presence of well-rounded pebble- of Paleozoic rocks in the 
basal portion- of the rhyolite indicates that the surface upon which 
the lava flowed was covered by such pebbles. 
Microscopic examination of several thin sections shows these rock's 
to be normal rhyolites. The groundmass is a brown glass, and many 
of the phenocrysts are fractured by flow. One or two acidic plagio- 
clase phenocrysts are present in some sections. Biotite in some in- 
-i a nee- is altered to chlorite and epidote or muscovite. Accessories 
are rare, although apatite and ilmenite occur. 
On weathering the feldspar phenocrysts are removed and the quartz 
protrudes slightly. In some portions of the range the rock has the 
smooth contours of weathered granite, but in the vicinity of Cactus 
Peak the greater resistance to erosion of certain bands gives a bedded 
aspect to the series. 
Vertical columnar parting is well developed through cooling on 
Cactus Peak, while on the Goldneld-Cactus Spring road horizontal 
hexagonal joints occur. The rhyolite is so similar to that of the 
Kawich, Reveille, and Belted ranges that it is considered to be also 
of early Miocene age. 
Four areas of slightly consolidated rhyolitic sandstones and con- 
glomerates occur in the Cactus Range. Two are on the northwest 
border of the range near the Cactus Spring-Tonopah road, a third is 
situated 2 miles southwest of Cactus Spring, and in a fourth area 
