SHOSHONE AND SKULL MOUNTAINS AND HILLS NEAK BY. 1 -j-o 
and the fourth, which lies 4 miles south of Cane Spring, is small and 
is dry in summer. 
GENERAL GEOLOGY. 
The formations of these hills, from the oldest to the youngest, arc 
the following: Carboniferous (Pennsylvanian) sedimentary rocks. 
quartz-monzonite porphyry, earlier rhyolite, Siebert lake beds, later 
rhyolite, and basalt. 
SEDIMENTARY ROCKS. 
Pennsylvanian limestone. — Pennsylvanian limestone forms the ridge, 
to the northeast of Tippipah Spring and the northern face of the 
ridge northwest of Yucca Pass. A third area of limestone, 6 miles 
south of Tippipah Spring, outcrops between the desert gravels below 
and the later rhyolite above. Limestone pebbles are abundant in the 
Siebert lake beds, and small inclusions occur in the quartz-monzonite 
porphyry and the earlier rhyolite. This limestone is similar to that 
of the Belted Range. (See p. 121.) 
Siebert lake beds. — Siebert lake beds form the northern and north- 
eastern faces of Shoshone Mountain, rim the southern edge of Skull 
Mountain, and cover areas 4 miles southeast of Tippipah Spring 
and I miles south of Cane Spring. Somewhat over 1,000 feet of lake 
beds are exposed on the north side of Shoshone Mountain, a section 
being as follows : 
Section on north side of Shoshone Mountain. 
Feet. 
White (locally yellow through iron stains), fine-grained, incoherent, im- 
pure sandstone. Bedding planes near the base J00 feet apart. Crystals 
of biotite, glassy feldspar, and quartz occur in upper 200 feet. Some 
bands contain pebbles of glassy rhyolite less than ."» inches in diameter 650 
Same, but without the small crystals. Some pink layers. Bedding 
planes well developed from 40 feet to 1 foot or less apart. Pebbles of 
glassy rhyolite up to 3 inches in diameter occur in certain beds__ — 100 
Same, with pebbles of Carboniferous limestones and quartzites one- 
fourth inch in diameter 200 
Same, with abundant crystals of biotite, glassy feldspar, and quartz. 
Pebbles up to 4 inches in diameter, largely of glassy or semipumi- 
eeous rhyolite 100 
In other areas these sediments ai;e more brilliantly colored, pinks 
and reds not being unusual. The bowlders of the conglomeratic 
beds in some instances reach a diameter of 5 feet. In addition to 
the pebbles already mentioned, others are present of quartz-monzonite 1 
porphyry, the earlier rhyolite, and a vesicular basalt. The basalt 
was not seen in place, but is perhaps contemporaneous with the 
earlier rhyolite and corresponds to the earlier basalt of the Amar- 
gosa Range. The Siebert lake beds, which characteristically form 
depressed domes, break down readily into a sand. Huge globular 
cavities, due in part to exfoliation and in part to wind erosion, a 
