94 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
dition it is believed to be older than the augite andesite and basalt. It 
closely resembles the andesite of the Goldfield hills and may be con- 
temporaneous with it. If so. it is probably of early or middle Mio- 
cene age. 
Augite andesite. — A flow of dark-gray andesitic rock overlies the 
tulfaeeons facies of the earlier rhyolite 2 miles north of Antelope 
Springs. In the dense groundmass are blackish-green pyroxene and 
amphibole columns which reach a maximum length of one-eighth 
inch. The rock breaks into sharply jointed blocks, in the interstices 
of which some epidote has developed. Microscopic examination 
-hows that this is an augite andesite with glassy groundmass in which 
are numerous plagioclase laths, pyroxene crystals, and magnetite 
grains. Augite phenocrysts with slight zonal structure and twins 
parallel to the orthopinacoid are abundant. The augite is remarkably 
fresh, although a little secondary epidote and chlorite is locally 
present. .V few brown hornblende phenocrysts, some of them out- 
lined by a reaction rim of magnetite, also occur. Apatite and mag- 
netite are present as accessory minerals. 
The augite andesite forms a How which is apparently younger than 
the rhyolite and, to judge from its fresh condition, is probably also 
younger than the biotite andesite. In the Great Basin the pyroxene 
andesites are of late Pliocene-Pleistocene age," and their formation 
usually immediately preceded that of the later basalts. 
Later rhyolite (?)• — One mile northeast of Cactus Peak are some 
low hills of purplish-gray rock with rather large feldspar crystals. 
Tuffaceous beds underlie the igneous rock, and the two rocks are prob- 
ably to be correlated with the younger tun and the youngest rhyolite 
of the Goldfield hills. It is by no means impossible that the same 
series underlies the basalt in the west slope of the range. 
Basalt. — The hill 3 miles southeast of Antelope Springs is com- 
posed of rhyolite apparently overlain by basalt, and several hills along 
the edge of the range on the Cactus Spring-Silverbow road are com- 
posed of a similar rock. The dissected mesa slopes on the west side 
Gf the range north of Wellington appear from a distance to be a north- 
ward extension of the basaltic rocks of Pahute Mesa. Probably to be 
correlated with the basalt is a reddish-brown vesicular rock which 
caps a low dome H miles north of Cactus Peak. These basalts ap- 
pear to overlie the rhyolite and they are probably of late Pliocene or 
earlv Pleistocene age. 
STRUCTURE. 
The Cactus Range is predominantly formed of Tertiary rocks un- 
conformably overlying Paleozoic sedimentary rocks and granites and 
diorite porphyries probably of post -Jurassic age. The small areas of 
a Spun-, .1. I-;., Trans. Am. Inst. Min. Eng., vol. 33, 1903, \).' IVAH. 
