SLATE RIDGE, IGNEOUS RO< 193 
to be molybdite (molybdenum trioxide). Purple fluorite occurs in 
crevices in the quartz, and fluorite cubes one-fourth inch in diameter 
also line vugs in the quartz. Microscopic examination shows that the 
quartz individuals in contact with fluorite po- sharp out line-. 
showing that they were but little corroded by the introduction of the 
fluorite, which is probably of pneumatolitic origin. 
The granite is similar lithologieally to that of Gold Mountain and 
has like relations to the Cambrian rocks and the Tertiary rhyolite. 
It is probably of post-Jurassic age. 
Pre-Tertiary quartz-monzonite porphyry. — The Cambrian sedi- 
mentary rocks on the road from Tokop to (loldfield are cut by dikes 
which reach a maximum width of 10 feet. The rock is white and 
dense and contains a few feldspar phenocrysts. It is similar to the 
quartz-monzonite porphyry already described from the vicinity of 
Lida and, like it, is probably genetically related to the granite. 
Pre-Tertiary diorite porphyry. — The Cambrian rocks and the gran- 
ite are cut by narrow dikes of a greenish-gray much-altered rock 
in which some of the prominent kaolinized feldspar phenocrysts are 
one-half inch long. This is probably the pre-Tertiary diorite por- 
phyry. 
Earlier rhyolite. — The earlier rhyolite forms a considerable portion 
of the east end of Slate Ridge, and small areas of it occur, widely dis- 
tributed over the central and western portions. In some places east of 
the Bullfrog-Goldfield road it appears from a distance to protrude 
from the later rhyolite. The earlier rhyolite is a flow rock of purple, 
gray, or white color. The groundmass, which is usually lithoidal, is 
equaled in bulk by the medium-sized phenocrysts of glassy unstriated 
feldspar, quartz, and biotite. Semituffaceous layers are interbeclded ' 
with the flows northeast of Tokop. The earlier rhyolite lies upon 
the eroded surface of the Cambrian rocks and the granites, and in 
turn is overlain by basalt and the younger rhyolite. If is similar to 
that of the Amargosa Range and is probably of earlier Miocene age. 
Later rhyolite. — The dissected mesa at the east end of Slate Ridge 
and a number of outliers in the recent gravels east of it are formed 
of flows of rhyolitic rocks. This later rhyolite has a dense ground- 
mass of deep-brown color, in which are abundant glassy feldspar 
phenocrysts; in some facies very small and in others medium sized. 
Microscopic examination proves the groundmass to be a brown glass 
with well-developed flow lines. The phenocrysts are orthoclase, some- 
what corroded, rare and small greenish augite crystals, and very 
rare quartz grains. While this rock is tentatively here called a rhyo- 
lite, chemical analysis might prove it to be of less acidic composition. 
Beneath this rhyolite are tuffaceous sandstones in which are rhyo- 
lite pebbles. This is probably the later tuff (Pliocene) of the Gold- 
Bull, 308—07 m 13 
