GOLDFIELD HILLS, GENERAL GEOLOGY. 
IGNEOUS ROCKS. 
Post-Jurassic granite. — In addition to the granite mapped by Kan- 
some in the special area, pebbles of alaskite and a somewhat less 
siliceous rock, probably to be correlated with the darker monzonite 
porphyry of the northeast spur of the Silver Peak Range, occur in 
the Siebert lake beds. 
Andesite and dacite. — Ransome found that andesite flows and 
dacite intrusions, the latter probably being slightly younger, followed 
the extrusion of the earlier rhyolite. These formations have been 
separated in the detailed study of the Goldfield special area, but since 
the andesite is by far the more abundant rock, the dacite is here 
grouped with it. It is the most widely distributed formation in the 
Goldfield hills and appears from a distance to cover a large area in 
their northeast portion. The later andesite is a rather dark-gray to 
light-pinkish rock with dense lithoidal groundmass. Phenocrysts, 
which equal the groundmass in bulk, consist of feldspar and horn- 
blende, augite, and biotite, one of these latter being present in many 
cases to the almost total exclusion of the others. The dacite, which 
:overs considerable areas within the district shown on the special 
nap, extends eastward beyond its borders, is usually slightly lighter 
n color than the andesite and contains a few quartz phenocrysts. 
The andesite and dacite are younger than the earlier rhyolite and 
)lder than the biotite latite and rhyolite. The later andesite is simi- 
ar in many ways to the later andesite of Tonopah a and the two for- 
nations may be roughly contemporaneous. The dacite is correlated 
with that of the Kawich Range. These rocks are probably of early 
>r middle Miocene age. 
Quartz latite and rhyolite. — Several outcrops of quartz latite and 
hyolite occur in the hills surrounding the Goldfield special area. 
Vhile some of these may belong to the earlier rhyolite, the majority 
hould certainly be assigned to the quartz latite and rhyolite. Quartz 
atite outcrops 2 miles south of Preble Mountain. The rock here has 
white stony groundmass, slightly exceeded in bulk by the medium- 
ized phenocrysts of which black mica is most conspicuous, while 
;lassy feldspar striated or unstriated and slightly smoky quartz also 
jccur. Many small ellipsoidal areas of pumiceous latite are pres- 
nt, indicating that the rock was in part a flow breccia. Under the 
licroscope the quartz latite is seen to have a turbid glassy ground- 
Lass, with many eddying flow lines. The larger feldspar pheno- 
rysts are andesine and in total bulk the plagioclase phenocrysts 
lightly exceed those of orthoclase. Orthoclase in particular i 
pnally built. Quartz has been greatly and biotite slightly corroded 
the magma. The magma must have been very viscous prior to the 
"Spurr, J. E., Prof. Paper U. S. Geol. Survey No. 42, 1905, p. 33. 
