444 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
pillars capped and protected by huge bowlders add fantastic ele- 
ments to many hills. Joints affect only the minor topographic fea- 
tures. In certain instances silicification has followed joints and 
chalcedony has been deposited in their fractures. Such joints, being 
resistant, weather in relief. 
The Siebert lake beds appear to have been deposited in a lake of 
some depth, to which much fragmental material was carried. The 
size of the material varied considerably from time to time, the varia- 
tion in coarseness being an expression in part of the depth of the 
lake and in part of the topography of the lake shores. Much of the 
material was derived from disintegrating rhyolite, but at times there 
were apparently also explosive rhvolitic eruptions, which threw into 
the lake many small crystals of biotite, feldspar, and quartz. The 
sediments on Shoshone Mountain overlie the quartz-monzonite por- 
phyry ami earlier rhyolite unconformably and are overlain appar- 
ently conformably by later rhyolite. while on Skull Mountain the 
basalt overlies them unconformably. They are without much doubt 
to be correlated with the Sieberft lake beds at Tonopah ° of Miocene 
age. 
IGNEOUS ROCKS. 
Quartz-monzonite porphyry. — Quartz-monzonite porphyry is the 
predominant formation in the hills to the east, north, and northeast 
of Cane Spring. The texture of the rock indicates that it was prob- 
ably intruded in older rocks, which are, however, not at present 
exposed. 
The quartz-monzonite porphyry has a gray or greenish-gray, finely 
crystallized or lithoidal groundmass, which is somewhat subordinate 
in bulk to the phenocrysts. The latter include feldspar, usually 
altered but in rare instances fresh and then clearly striated ; biotite, 
fresh to altered, and serpentinized hornblende. In many cases horn- 
blende is very subordinate to biotite. The feldspars reach a maxi- 
mum length of one-fourth inch ; the other phenocrysts are slightly 
smaller. The microscope shows that the rock is a quartz-monzonite 
porphyry almost approaching granodiorite porphyry. The ground- 
mass consists usually of a microgranitic, but over small areas micro- 
pegmatitic mosaic of orthoclase,' quartz, and a little plagioclase. The 
plagioclase phenocrysts, complex crystals, are usually twinned accord- 
ing to the Carlsbad law and many of them are zonally built. One 
determination proved this mineral to be basic andesine. A few phe- 
nocrysts of quartz, deeply embayed by magma tic corrosion, and of 
orthoclase are also present, in addition to the phenocrysts micro- 
scopically determined. Magnetite, zircon, and apatite are present as 
accessory minerals. The secondary minerals include kaolin and epi- 
a Spurr, J. E., Prof. Paper U. S. Geol. Survey No. 42, 1905, p. 54. 
