BULLFROG HILLS, IGNEOUS ROCKS. 
predominant. The most widely distributed facies of pegmatii 
formed of white feldspar, less muscovite, still less smoky quartz, 
and a little pyrite. The largest feldspar individuals reach a diam- 
eter of 4 inches, while muscovite is in aggregates of plates from one- 
fourth to 1 inch across. Quartz veins of pegmatitic origin are also 
present and similar veins, probably of the same origin, cut the more 
feldspathic pegmatite. From this it is probable that a rock of 
granitic composition separated from the magma and at a later period 
a more quartzose rock was deposited from the residual liquid in 
fractures in the older pegmatite. Lens-like veins of practically pure 
white feldspar grade on the one hand into normal pegmatite, and 
on the other into the eyelike areas of the schist already mentioned. 
The associated alaskite is a white, medium-grained rock composed 
of quartz and feldspar, with muscovite and a little altered biotite. 
The quartz and orthoclase grains are micropegn atitically inter- 
grown along their borders and the biotite shreds are largely chlori- 
tized. Accessory minerals are not abundant, but include apatite, 
zircon, and magnetite, and apparently tourmaline. In the hand 
specimen the rock is considerably mashed and the microscope shows 
phenomena of granulation and recrystallization, including the depo- 
sition of muscovite, derived from feldspar, in fractures. 
The pegmatite and alaskite inject the schist bed by bed, although 
in rare instances the planes of schistosity are crossed. The wider peg- 
matitic bands are 100 or more feet thick, while the thinner pegma- 
titic layers are but one-fourth inch wide, and many of them are sepa- 
rated only by an equal thickness of schist. The pegmatite cuts 
schists probably of p re-Silurian age and is older than the unmashed 
rhyolite of Tertiary age. On lithologic grounds its age is provi- 
sionally believed to be post- Jurassic and pre-Tertiary. 
Pre-Tertiary diorite porphyry. — A few narrow dikes of diorite 
porphyry cut the pegmatite and schist 3 miles west of Bullfrog. The 
diorite porphyry is a dark grayish-green rock speckled with tiny 
white areas, and is evidently composed predominantly of a ferro- 
magnesian mineral and feldspar. The rock has a medium-grained 
microgranitic groundmass of plagioclase, orthoclase, and some 
quartz. Columns of brown hornblende are very numerous, while 
larger phenocrysts of sericitized plagioclase are fewer. The acces- 
sory minerals, titanite, apatite, and magnetite, are very abundant. 
Epidote and chlorite are developed from hornblende or from horn- 
blende and plagioclase. This rock is, without much doubt, to be 
correlated with the pre-Tertiary diorite porphyry widely distributed 
in the mountains on the western border of the area surveyed, 
p. 31.) 
Biotite andesite.—A. hill 2 miles northwest of Howell ranch and 
a small knob rising from the older alluvium three-fourt 
