168 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
usually somewhat subordinate in bulk to the groundmass. Biotite 
and lath-shaped feldspars, many of them considerably altered, are 
as a rule equally abundant. The feldspar phenocrysts are slightlj 
larger than those of biotite, but they are nowhere over one-fourth 
inch long and are in most places much smaller. Flow lines are pres- 
ent in the groundmass and many of the phenocrysts show flow orien- 
tation. Much of the rock is vesicular. The vesicles reach a maxi- 
mum length of H inches and are more or less completely filled by 
chalcedony and chlorite. Hyalite, chalcedony, and quartz occur 
along joint planes. 
One thin section -hows under the microscope as a biotite andesite* 
with a groundmass composed of plagioclase laths in a rather ill- 
defined gently birefringent substance. The phenocrysts include 
plagioclase, biotite rimmed by beads of magnitite, orthoclase, and 
magnetite. The small plagioclase phenocrysts are simple and the 
large one- complex aggregates. Zircon i- a rare accessory mineral. 
Another thin section, although clearly from the same magma, is some- 
wliat more acidic, having allinities with latite. Quartz and ortho- 
clase are present in the microfelsitic groundmass and orthoclase 
phenocrysts are as abundant as those of plagioclase. 
The biotite andesite occurs in (lows which wore eroded prior to the 
outflow of the earlier rhyolite. On the range crest I miles south of 
Wahguyhe Peak the rhyolite contains small fragments of andesite. 
The andesite is in consequence younger than rhyolite and is probably 
of late Eocene age. The more acidic facies of the rock are mineral- 
ogically rather like the monzonite porphyry of the Kawich Range] 
which is older than the earlier rhyolite of that range, but which is 
probably contemporaneous with the earlier rhyolite of the Amargosa 
Range. The andesite and monzonite porphyry are probably, in a 
broad way. contemporaneous. 
Earlier rhyolite. — Rhyolite forms the crest and eastern slope of 
the Amargosa Range from Thorp's mill to a point 2 miles northwest 
of Willow Spring, and thence to a point 2\ miles southeast of Day- 
light Spring it form- the eastern slope of the range. East-southeast 
of this point several small rhyolite areas occur. 
The rhyolite series comprises a considerable number of siliceous 
volcanic rocks, which lie in well-defined flow beds. The predomi- 
nant facies is a lilac, gray, Avhite, or pink rhyolite, with lithoidal 
groundmass and medium-sized phenocrysts of equal volume. Of the 
phenocrysts, colorless or slightly smoky quartz and glassy, unstriated 
feldspar are, as a rule, equally abundant, although the feldspar may 
predominate. Biotite phenocrysts are usually sparse and of smaller 
size. Other facies are gray glasses without phenocrysts, but with 
highly developed perlitic parting, and black glass, with feldspar and 
fewer quartz phenocrysts. Similar rocks from other ranges prove, on 
