38 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
amounts. If quartz is present, the rock is called a quartz monzonite. 
It is usually of a gray color. 
Monzonite porphyry bears the same relation to monzonite as granite 
porphyry to granite. 
Pegmatite is here applied to a coarsely crystalline rock which oc- 
curs as dikes in some other rock — granite, for example — with which 
it practically coincides in composition. Such rocks are probably 
formed by segregations from the original molten mass of the inclos- 
ing igneous rock. 
Rhyolite is a porphyritic rock with glassy base. It has the same 
chemical composition as granite. The most abundant phenocrysts are 
orthoclase and quartz, although biotite, augite, and hornblende also 
occur. 
Syenite is a granular rock usually of gray color. The component 
minerals are orthoclase and hornblende or mica. If quartz is pres- 
ent, the rock i- called a quartz syenite, and with increase in this con- 
stituent quartz syenite passes into granite. Where the component 
minerals are rich in soda, the rock is a soda syenite. 
GKOLOGIC HISTORY. 
As has been stated, no pre-Cambrian formations have been found 
in the area studied, but the Prospect Mountain quartzite in the 
Specter Range contains pebbles which are in part derived from sedi- 
mentary rock- and in part from granite. It may be inferred, there- 
fore, that pre-Cambrian rocks, both sedimentary and igneous, at 
one time covered areas in southwestern Nevada and eastern Cali- 
fornia. 
In Cambrian and Ordovician time this portion of the Great Basils 
was part of a sea bottom upon which sandstones and then limestone 
and finally sandstones were laid down. The sinking of the sea bot- 
tom progressed step by step with the deposition of the sediments. 
King" has shown that the eastern border of the land surface from 
which the sediments were derived was. in the latitude of the fortieth 
parallel, near 117° 30' west longitude, while the shore line farther 
south appears from Spurr's map 7 ' to have been at about 118° 15'. 
The early Paleozoic rocks of the area studied contain a greater propor- 
tion of sandy sediments, and the limestones are more impure than 
those described by Hague at Eureka, Nev., facts in harmony with the 
closer proximity of tin- area to the old continental shore line. The 
presence in the lower Paleozoic rocks of ripple marks, cross-beddingj 
intraformational conglomerate-, and oolitic limestones indicates that 
the sea was for the greater portion of the time shallow, and locally, 
as evidenced by sun cracks, the deposits were even raised above sei 
a Kins, Clarence. T\ S. Geol. Explor. -HUh Par., vol. 1. 1878, p. 247. 
6 Spurr, J. E., Bull. U. S. Geol. Survey No. 208, 1903, PI. I. 
