58 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
The lake beds at Montezuma Peak are about 1,100 feet thick. On 
the western border of the area mapped they are probably 200 or 300 
feet thicker. The formation was for the most part deposited in an 
inland lake, although some portions were possibly laid down sub- 
aerially by explosive volcanic action. 
Mr. R. H. Chapman found a piece of silicified wood at a point 4 
miles west of Montezuma Peak. Mr. F. H. Knowlton, who kindly 
examined it. states that it is wood of a deciduous tree not older than 
the Tertiary. These sandstones are correlated with the Siebert lake 
beds of Tonopah. described by Spurr." 
Travertine. — A small area of travertine at Alkali Spring, presum- 
ably of Pleistocene age, has already been described (p. 35). 
IGNEOUS ROCKS. 
Post- Jurassic granite, granite /><>rj>]ii/r;/. <in<l quartz-monzonite por- 
phyry. — The dikes and small irregular intrusive masses of Avhite fine- 
grained alaskite and granite porphyry '2 miles southwest of Alkali 
Spring are usually injected parallel to the bedding of the upturned 
Cambrian rocks, but here and there cut across it. The porphyritic 
habit is better developed at the contact and next to the border pheno- 
crysts are in flow orientation. The rock breaks down into angular 
joint blocks, many of which are stained by limonite. A single thin 
section under the microscope proves to be a granite porphyry with 
microgranitic groundmass of quartz and orthoclase. Coronae of mi- 
cropegmatitic quartz and orthoclase surround the abundant pheno- 
crysts of dominant quartz and orthoclase and subordinate biotite and 
plagioclase (oligoclase). Some phenocrysts are composed of quartz 
and feldspar in a graphic-granite intergrowth. The phenocrysts all 
show undulatory extinction, and in consequence the rock has suffered 
some deformation. Biotite has been completely altered to muscovite 
shreds, rutile and an iron ore forming simultaneously. 
The dikes on the east slope of this hill are composed of medium- 
grained gray biotite granite. On weathering the granite breaks 
down into spheroidal masses. It includes numerous small Cambrian 
fragments, while basic segregations of biotite and probably horn- 
blende are present. This rock proves on microscopic examination to 
have a granular, hypidiomorphic texture, plagioclase individuals be- 
ing of columnar habit. The orthoclase. much of which is microper- 
thitic, is usually twinned according to the Carlsbad law. The feld- 
spars are partially replaced by kaolin and sericite and the biotite by 
chlorite. Zircon and magnetite are accessories. Fine-grained pink 
aplitic dikes, from 1 inch to 4 inches wide, cut the granite and locally 
fault the Cambrian inclusions. These more acidic dikes are probably 
genetically related to the main granitic intrusion. 
« Spurr, J. E., Prof. Paper U. S. Geol. Survey No. 42, 1905, pp. ."1 55. 
