STONEWALL MOUNTAIN. 87 
aitlying hills on Pahute Mesa to the south are of the same formation. 
rhe later rhyolite has a dense gray stony to glassy groundmass. In 
t are numerous medium-sized glassy unstriated feldspar and fewer 
ounded quartz phenocrysts. The feldspar crystals often exhibit 
feautiful plays of color, purples and blues being refracted from nu- 
merous cracks. The glassy facies locally show well-developed flow 
ines and spherulites, while slightly vesicular facies also occur. The 
ater rhyolite breaks into platy fragments and in places a granite- 
ike weathering is developed. The rlwolite flow, as now exposed, 
,200 feet thick, is in broad, gentle flexures which may be due in part 
o the slightly uneven surface upon which it flowed. Microscopic 
xamination shows a turbid glass in which orthoclase and quartz and 
ewer acidic plagioclase phenocrysts are associated. A little magnet- 
ic is also present. 
For a distance of 2 miles west of the rhyolite, pebbles of black 
glassy obsidian are common upon the surface of Stonewall Mountain. 
Lnese pebbles may indicate that the rhyolite once extended some dis- 
ance farther west, although similar rocks were not noted in the later 
hyolite section. 
The later rhyolite overlies and is in consequence younger than the 
)iebert lake beds. The contact between the rhyolite and the basalt 
piich forms the top of Pahute Mesa at the northeast corner of Stone- 
wall Mountain was examined at one point. The contact is slightly 
mdulating, and the base of the overlying rhyolite for 5 to 6 feet is a 
low breccia of massive blocks of vesicular rhyolite. This apparently 
ndicates that the rhyolite is younger than the basalt beneath it. The 
opography of the rhyolite, however, is so much more mature than 
hat of the Pahute Mesa basalt that it is believed that the rock here 
nderlying the rhyolite is a portion of an older basalt which ex- 
Bnded from Stonewall Mountain as a shelf and which happened 
o be on a level with Pahute Mesa. The later rhyolite is probably to 
e approximately correlated with the later rhyolite of the Southern 
Klondike hills. 
Basalt. — A small mass of a grayish-black basalt outcrops 1 mile east 
f south of Stonewall Spring. Basalt breccias indicate that it is a 
6w. The age of this rock is unknown, but from the apparent 
bsence of its bowlders in the Siebert lake beds it is probably to be 
Drrelated with the later basalt of the Goldfield hills. 
STRUCTURE. 
The Cambrian limestone is practically horizontal, the folding being 
road and open like that of the hills of the Cuprite mining district, 
i restricted areas, however, the rock is closely folded. Prior t<> the 
rtrusion of the earlier rhyolite the Paleozoic rocks appear to have 
.d a gently accentuated topography. The most striking structural 
