SHOSHONE AND SKULL MOUNTAINS, IGNEOUS ROCKS. 
dote after the feldspars, chlorite and epidote after biotite, and pseudo- 
morphs of serpentine after hornblende or possibly augite. 
The rock resembles lithologically and in its - ratigraphic position 
the monzonite porphyry of the Kawich Range, and the two rock- 
are doubtless approximately contemporaneous. Each is strikingly 
similar to the quartz-monzonite porphyry of Stonewall Mountain. 
The porphyry of Skull Mountain and that of the Kawich Range are 
older than a rhyolite, which is supposed to be of early Miocene age. 
The quartz-monzonite porphyry also closely resembles rocks which 
on the western border of the area surveyed appear to be a variant of 
the post-Jurassic granite magma. While it is possible that they are 
pre-Tertiary in age they are here tentatively assigned to the late 
P^ocene. 
Earlier rhyolite.— -The earlier rhyolite and its kaolinized and iron- 
stained alteration products form the Calico Hills, remarkable for 
their brilliant pink, red, and white coloring. A number of smaller 
areas lie in the group of hills in which the Horn Silver mine is situ- 
ated, and some of the rhyolite to the west of Yucca Pass may pos- 
sibly belong to the earlier series. 
The earlier rhyolite includes a number of facies, most of whie.h 
are characterized by abundant phenocrysts that equal or exceed the 
groundmass in bulk. The groundmass is white or gray in color 
and is usually dense. The phenocrysts are of medium size. Tabu- 
lar crystals of glassy unstriated feldspar and rounded grains of 
slightly smoky quartz are much more abundant than the somewhat 
smaller hexagonal plates of black mica. Other facies are black 
glasses with perlitic parting, but without phenocrysts, and gray 
glasses with a few quartz phenocrysts. The rock is a flow, since cer- 
tain beds show distinct flow banding and others contain many frag- 
ments of rhyolites and are evidently flow breccias. 
The earlier rhyolite appears to lie upon the uneven surface of 
the quartz-monzonite porphyry. The later rhyolite apparently over- 
lies the earlier, while pebbles of the earlier rhyolite are contained in 
the Siebert lake beds. The same relations exist in the Kawich Range, 
and the earlier rhyolites of the two ranges are doubtless approxi- 
mately contemporaneous and of early Miocene age. 
Later rhyolite: — The later rhyolite flow overlies the Siebert lake 
beds on Shoshone Mountain, while several small masses to the south 
of Cane Spring cap the same formation. In the latter locality the 
former extension of the rhyolite was much greater and the south- 
ward-clipping flow of the Shoshone Mountain may at one time have 
been connected with it. The rhyolites west of Yucca Pass are. from 
their lithologic character and relation to the Siebert lake beds, prob 
ably to be correlated with the later rhyolite. 
Bull. 308—07 m 10 
