SOUTHERN KLONDIKE HILLS, IGNEOUS ROCKS. 
than the present Southern Klondike hills. Since that extrusion 
erosion has removed considerable masses of the once more extensive 
rhyolite and partially exhumed the sedimentary rocks. The rhyo- 
lite at the northern boundary of the area mapped, on the west side of 
the hills, is also probably the earlier rhyolite. It occurs in flows and 
dikes, the flows overlying the Cambrian rocks, the dikes clearly cut- 
ting them. The white or pinkish or lilac-white lithoidal ground- 
mass predominates in bulk over the small phenocrysts, among which 
feldspar is more abundant than quartz. Narrow wavy flow bands 
wrap around the phenocrysts. One-half mile north of Southern 
Klondike a resin-colored glassy fades was noted. Rhyolite tuffs and 
flow breccias are associated with the normal igneous rock near the 
boundary of the area. 
The rhyolite is cut by two sets of joints from 2 to 5 inches apart, 
and the flow parting constitutes a third plane of weakness. In con- 
sequence the residual fragments are rectangular or platv. Weath- 
ered fragments are characterized by small pits, the casts of feldspar 
phenocrysts. The rhyolite forms yellowish or slightly pinkish-white 
conical hills or low areas. The single slide examined has a brown 
devitrified groundmass showing flow lines and spherulites. Quartz 
and orthoclase predominate in the groundmass. while the rather 
sparse phenocrysts are orthoclase, quartz, and plagioclase. The rock 
is a rhyolite tending toward latite. The earlier rhyolite is tentatively 
correlated with the Miocene Tonopah rhyolite-daeite a of Tonopah, a 
orrelation already suggested by Spurr h for the northern mass. 
Earlier quartz latite. — In the northwest portion of the hills, near 
:he areal boundary, is an outcrop of quartz latite" of dull-lilac or 
nedium-gray color, which, however, as mapped, may include some 
basalt. The dense groundmass contains numerous crystals of white 
striated and unstriated feldspar reaching a maximum length of 
me-eighth inch, and black mica. Under the microscope the pre- 
ominant groundmass shows as a devitrified glass containing con- 
siderable quartz. Plagioclase and orthoclase are equally abundant 
phenocrysts, while biotite is in some specimens surrounded by a re- 
iction rim of magnetite. Magnetite, apatite, and zircon are acc- 
essories. The latite flow overlies the Siebert lake beds. The rock 
Is rather similar to the later rhyolite of the Goldfield hills, although 
ess siliceous, and the two are perhaps contemporaneous. The Gold- 
teld rhyolite underlies and is older than the basalt. 
I Basalt— The large mass of volcanic rock capping the Siebert 
ake beds northeast of Southern Klondike appears to be an eroded 
tasalt flow, as are the hills between this mass and the large playa 
jrossed by the road from Tonopah to Cactus Spring. It is probable 
" Spurr, J. E., Prof. Paper U. S. Geol. Survey No. 42, 1905, pp. 51-55. 
°Op. cit., p. 99. 
