86 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
the earlier rhyolite at widely separated localities in the mountain. 
Dikes of quartz syenite less than 200 feet wide intrude the earlier 
rhyolite on the fault scar]) just east of Stonewall Spring. The 
quartz syenite is a grayish-white, medium-grained granitoid rock 
composed of unstriatecl feldspar with a little biotite. Microscopic 
examination shows the presence of a few small wedges of quartz 
between the feldspar grains and the biotite tablets. Tlmenite, apa- 
tite, and zircon are accessories. The granitoid habit of these small 
masses indicate- that its magma solidified beneath a considerable 
depth of rhyolite now removed by erosion. 
The quartz-monzonite porphyry is typically porphyrinic in habit, 
with a gray and black speckled noncrystalline groundmass. The 
phenocrysts, which predominate over the groundmass, include white 
or gray glassy feldspar, striated or unstriated, and black or bronze- 
brown biotite. The largest phenocrysts are one-fourth inch in 
length. The contact facies of the dike three-fourths of a mile 
southeast of the summit of Stonewall Mountain is of slightly finer 
grain and includes many fragments of the earlier rhyolite! The 
rock weathers into rounded bosses and disintegrates into spheroidal 
bowlders stained brown or reddish by iron oxides. The microscope 
shows that the microgranitic groundmass is composed of ortho- 
clase grains, many of them twinned according to the Carlsbad law, 
with a few associated quartz wedges and biotite shreds. The phenol 
crysts include both plagioclase. in large complex individuals, and 
orthoclase. Phenocrysts of biotite and smaller ones of light-greed 
augite are also present. Magnetite, apatite, and zircon are the acces- 
sory minerals. Calcite, sericite, and kaolin are present as alteration 
products of the feldspars. 
One and three-fourths miles upstream from the easternmost area of 
Cambrian limestone a poorly exposed black glass with rather numer- 
ous feldspar phenocrysts apparently cuts the earlier rhyolite. The 
exposure is too small to map. Under the microscope this proves to 
be a glass with orthoclase and a few bluish-green augite phenocrysts. 
It may well be a glassy form of the quartz syenite A small frag- 
ment of hornblende andesite, unlike any rock exposed in the moun- 
tain, is included in the thin section. 
The quartz syenite and quartz-monzonite porphyry of Stonewall 
Mountain intrude the earlier rhyolite, and pebbles are embedded in 
the Siebert lake beds. They most closely resemble the post -Jurassic 
monzonite porphyry of the Silver Peak Range (see p. 59), although 
they are not very different from the monzonite porphyry of the 
Kawich Range and their correlation with the latter is more nearll 
correct. The rocks are believed to be of late Eocene age. 
Later rhyolite. — The later rhyolite form- the low. massive brown 
and gray domes of the eastern part of Stonewall Mountain. Two 
