SILVER PEAK RANGE, NORTHEASTERN EXTENSION. 
Dikes in the Cambrian rocks. 6 miles west of south of Alkali Spring, 
are composed of a dense white granite porphyry with rather numerous 
small phenocrysts. These dikes are too small to show on a map of 
the scale employed in this report. Under the microscope the around- 
mass appears as a mosaic of quartz and orthoclase which arc in micro- 
pegmatitic intergrowth over small areas. A little plagioclase is also 
present. The phenocrysts include orthoclase, usually microperthitic, 
quartz deeply embayed, and plagioclase. In some specimens rutile 
rods are so grouped as to simulate somewhat rudely biotite crystal-, 
and it may be that biotite phenocrysts present in the magma were ab- 
sorbed prior to its solidification. Magnetite is an accessory. 
The mass of quartz-monzonite porphyry 4 miles west of south of 
Alkali Spring and the dikes extending from it into the Cambrian 
ocks to the northeast are composed of a gray rock of porphyritic 
habit in which numerous medium-sized phenocrysts lie in a finely 
granular groundmass. The constituents in the order of their abun- 
dance are feldspar, both striated and unstriated, biotite, and horn- 
blende. The rock weathers into rounded blocks. Inclusions of Cam- 
brian rocks are common, as are basic segregations. Microscopic 
examination shows a microgranitic groundmass, composed of plagi- 
oclase laths, orthoclase tablets, and quartz grains. The phenocrysts 
consist of plagioclase; orthoclase, much of which shows excellent 
zonal growths; ragged biotite tablets, and hornblende columns. Mag- 
netite and apatite are accessory minerals. The quartz-monzonite por- 
phyry is cut by dikes of diorite porphyry and white quartz-monzonite 
porphyry. It is believed that this rock is a variant of the gran- 
itic magma intruded contemporaneously with the granites of the 
vicinity. 
The Cambrian limestone in contact with the quartz-monzonite por- 
phyry just described has been metamorphosed to a white, medium- 
drained marble in which epidote and other lime silicates are devel- 
oped. The shales near the granitic rocks 2 miles southwest of Alkali 
Spring have been metamorphosed to slates. A dense, greenish-gray 
•late shows under the microscope as a mosaic of quartz and orthoclase 
n which are shreds of sericite and chlorite. Locally the sericite 
nd chlorite form large skeleton growths containing numerous quart/ 
nd feldspar individuals. Another facies is a dense, black slate of 
onchoidal fracture. In thin section this is formed of large, rounded 
ireas of orthoclase lying in a ramifying network of orthoclase 
grains intimately cut by biotite and muscovite shreds. Imperfect an- 
alusite crystals are scattered throughout the rock, and some of these 
ave the black centers characteristic of the variety called chiastolite. 
These igneous rocks intrude Cambrian rocks and occur as pebbles 
n the rhyolite and Siebert lake beds. They are believed to be mem- 
ers of the post-Jurassic granite series. 
