SILVER PEAK RANGE, NORTHEASTERN EXTENSION. 
latter rock is apparently younger than the former. Pebbles ol the 
diorite porphyry occur in the Siebert lake beds — a fact which leads 
to the belief that these dikes were intruded in post-Jurassic and pre- 
Miocene time. 
Andesite. — The andesite masses 4 miles north of Lida were seen 
by the writer only at a distance, but Turner, who mapped their 
western extension, believes the andesite to be of " early Neocene " age, 
and it is probably older than the formation described in the follow- 
ing paragraphs. 
Rhyolites and dacites. — Thin flows of rhyolite are interbedded with 
the lower portions of the Siebert lake beds of Montezuma Peak and 
of the area 4 miles north of it. The lake beds seem to lie on a sili- 
ceous effusive igneous rock which forms a narrow north-south band to 
the east of Montezuma Peak. Small areas of rhyolite are exposed 
by the erosion of the overlying basalt mesa 3^ miles east and 4 and 5 
miles southeast of Montezuma Peak. A small poorly exposed mas^ 
of rhyolite — possibly a dike, to judge from the disturbed condition 
of the surrounding limestone — lies 5 miles north of west of Mon- 
tezuma Peak. Rhyolite is also exposed 3^ miles north of Lida. 
The rhyolite flows in the Siebert lake beds are dense gray or pink 
rocks, in which are sparse and small phenocrysts of quartz, orthoclase, 
and biotite. Black glasses are also present. Flow banding is highly 
developed, and spherulites form freely in cavities and impinge against 
one another in the more dense facies. The spherulites, some of which 
ire simple and others compound, are of radial structure. The larger 
3nes are 2 inches in diameter. Lithophysse occur in the area 4 miles 
north of Montezuma Peak. Under the microscope the glassy ground- 
less appears somewhat devitrifiecl. The flows here contain numerous 
oebbles of Cambrian jasperoid and quartz-monzonite porphyry. 
The siliceous eruptive rock east of the Siebert lake beds of Monte- 
zuma Peak is a red semivitreous rock in which some small pheno- 
crysts of striated feldspar and fewer of unstriated feldspar and 
ijuartz are sparsely distributed. The microscope shows numerous 
dagioclase microlites in the glassy groundmass, and orthoclase is so 
subordinate that the rock is a siliceous dacite. 
The rhyolite of the other areas mentioned has abundant pheno- 
rysts of quartz, orthoclase, and biotite in a light-colored groundmass 
khieh under the microscope appears as a glass somewhat devitrifiecl, 
mowing flow banding and spherulites. In one slide a few plagio- 
lase phenocrysts are present with those already mentioned. Zircon 
|nd magnetite are accessories. This rhyolite grades into a white 
ipxv glass without phenocrvsts in the area 4 miles east of Montezuma 
?eak.^ 
a Turner, II. W., Silver Teak folio ; unpublished. 
