STONEWALL MOUNTAIN. S5 
'ow dike of granite porphyry cuts the easternmost limestone. This 
:ock has a finely granular yellowish groundmass in which are inclosed 
nedium-sized quartz and feldspar phenocrysts. In the field this re- 
sembles the granite porphyry near Alkali Spring in the Silver Peak 
Range and it may be genetically related to the post- Jurassic granites. 
Vngular fragments of a gray monzonite porphyry occur in the earlier 
'hyolite near the summit of the group and these may bo correlated 
vith similar rocks of the Silver Peak Range that are also related in 
>rigin to the post-Jurassic granites. 
Earlier rhyolite. — The most widely distributed formation of Stone- 
wall Mountain is a rhyolite characterized by many small colorless 
tnd glassy or white and opaque unstriated feldsp; r phenocrysts. 
Sear Stonewall Spring the rhyolite is medium gray and compact; 
hat of the highest peak is also compact and of a faded red color; 
;he rhyolite of the small hill in the detrital apron on the southwest 
tide of the mountain is a gray lithoidal rock. The rhyolite from 
he last two localities contains biotite phenocrysts. 
An indistinct original banding is locally present and some facies 
ire a flow breccia. The rhyolite is probably a flow, although the 
vertical position of the banding in places and the texture of some 
'acies indicate that the rock may possibly be an intrusive mass. The 
'hyolite as exposed at present is several thousand feet thick, and the 
granitoid habit of the quartz syenite which injects it suggests that 
t was probably once much thicker. The rhyolite has well-developed 
oints over large areas which express themselves in the topography 
is straight elements. Where the joints are less closely spaced the 
diyolite weathers in rounded forms like a granite. 
Microscopic examination shows the rhyolite to have either a 
levitrified glassy or a microgranitic groundmass composed of pre- 
iominant orthoclase with fewer quartz grains and many small mag- 
netite cubes. The groundmass varies widely in grain from one part 
If the thin section to another, but is everywhere rather fine grained, 
prthoclase phenocrysts. many of them microperthitic, with ragged 
orders which inclose some of the groundmass anhedra, are abundant. 
arely plagioclase laths, well-formed biotites, very ragged horn- 
lende, and in some thin sections one or more quartz phenocrysts 
re associated. Zircon and apatite are accessories. 
The mature topography of Stonewall Mountain shows that the 
yolite is old, and pebbles of it occur in the Siebert lake beds. It 
s believed to be older than the rhyolite of the Kawich Range, with 
diich it has no lithologic affinities, and is probably of early Eocene 
r even late Cretaceous age. 
Quartz syenite and quartz-monzonite porphyry. — Dikes and elon- 
gated masses of quartz syenite and quartz-monzonite porphyry, 
fhich grade into one another and are hence contemporaneous, intrude 
