AMARGOSA RANGE, IGNEOUS RO 171 
ipper portion of the series a few basalt flows are apparently inter- 
bedded with the rhyolite. At another place the basalt clearly cuts 
[the Siebert lake beds and the rhyolite. (See p. 171.) This show- 
that while toward the end of the period of rhyolit* extrusion sonic 
basalt probably outflowed, for the most part the basn younger. 
The rhyolite and latite are probably of late Miocene and 
cene age. 
Later basalt. — Basalt is the youngest of the igneous rocks. I ts 
cover a considerable area at the north end of the range, and a number 
of smaller areas occur on either side of Grapevine Canyon. A dike 
of basalt 4 feet thick cuts the younger rhyolite and Siebert lake beds 
2 miles north of the Staininger ranch. This was probably one of the 
vents through which the lava outflowed, while a small, red, conical hill 
to the south of the Staininger ranch-Thorp road, 5, miles from the 
ranch, appears from a distance to be a volcanic cone. The two small 
buttes in Sarcobatus Flat, north and northeast of Currie Well, seen only 
from a distance, are probably basalt. Remnants of flows cap rhyolite 
hills southeast of Daylight Spring, and at one locality a dike of basalt 
cuts the Pogonip limestone. This dike may be the ancient vent from 
which the basalt in this vicinity flowed. The basalt of the dike is 
reddish brown in color and much altered, but on microscopic examina- 
tion appears to have been originally an olivine basalt. The basalt 
from the flows in the vicinity is black, vesicular, and without promi- 
nent phenocrysts. The basalt of Grapevine Canyon is for the most 
part black and dense, although basal portions of the flows ( in many 
places flow breccias) are vesicular and red. The more vesicular rocks 
are locally covered by a layer of basalt banded horizontally by 
flow lines, and this in turn by more massive beds which show sphe- 
roidal weathering. Phenocrysts, while usually small, are abundant 
m many outcrops and include glassy laths of plagioclase and rounded 
grains of olivine, either fresh and glassy or iron stained. Granular 
calcite, probably derived from the alteration of the basalt, fills vesi- 
cles in the basalt and veins cutting it. A thin section cut from basalt 
from the low mesas 4 miles east-northeast of the Staininger ranch 
proves to be a quartz-bearing olivine basalt with noncrystalline 
groundmass. The quartz is deeply embayed and surrounded by a 
corona of augite columns of the groundmass. 
The basalt from the south end of the Amargosa Range overlies 
the Siebert lake-bed conglomerates, and from its lithologic char- 
acter is considered to be one of the Pliocene-Pleistocene basalts. The 
basalt in the vicinity of Grapevine Canyon is probably in part con- 
temporaneous with the upper part of the later rhyolite and Siebert 
lake beds, but is for the most part younger and of late Pliocene or 
early Pleistocene age. 
