GOLDFIELD HILLS, IGNEOUS ROCKS. 75 
is too thin to be shown on the map, but it forms a narrow band from 
50 to 100 feet thick beneath the basalt capping of the mesa and occurs 
ilso to the south of Golclfield in the area here mapped as Siebert lake 
ceds. 
The later rhyolite includes a number of facies of acidic flow rocks. 
On the Goldlield-Bullfrog road, 3 miles east of south of Golclfield, 
[he formation is 50 feet thick and consists of 4 feet of black glass. 
ibove which is 20 feet of faded brick-red dense rhyolite and above 
this 25 feet of lavender lithoidal rhyolite. The black glass has an ex- 
cellent flow parting. The middle rhyolite carries rather numerous 
glassy feldspar phenocrysts, which are, however, subordinate in bulk 
to the groundmass. It contains many vesicles elongated parallel to 
the flow, and in these spherulites are developed. The spherulites 
in some instances form shelf-like partitions in the cavities. Verti- 
cal columnar parting is fairly well developed in this portion of the 
flow. Under the microscope the groundmass, which has many eddj 7- - 
ing flow lines, shows as a slightly devitrified turbid glass. The pheno- 
rysts are orthoclase, with which are associated a few of plagio- 
lase and bluish-green augite. The presence of considerable quartz 
n the devitrified glass probably justifies the use of the name rhyo- 
ite. The upper member is a flow breccia with lithophysa? here and 
;here throughout its mass. The phenocrysts in the glassy base are 
een under the microscope to be orthoclase. bluish-green augite, and 
juartz. A rhyolite similar to the middle member of this section is 
xposed 1J miles southwest of the type locality and north of the Gold- 
field-Cactus Spring road, 1 mile west of the edge of the alluvial slope. 
Similar rocks occur beneath the basalt of the mesa west of Gold- 
ield. One facies from this mesa is similar to the upper portion of the 
low at the type locality, although phenocrysts, particularly quartz, 
ire more abundant. Apatite is also microscopically visible. Such 
acies locally include fragments of quartz basalt and the Cambrian 
sasperoid. 
i The extrusion of this rhyolite occurred after the folding and ero- 
Jon of the Siebert lake beds. It is presumably of Pliocene age. Sim- 
ilar rocks occur in the Silver Peak Range, in Slate Ridge, and beneath 
lie basalt of Pahute Mesa. 
j Basalt. — The youngest formation of the Goldfield hills is the basalt 
fhich forms the mesa west of Goldfield, with which the basalt 
p the northwest of Diamondfield is probably contemporaneous. At 
■lie time basalt covered the greater portion of the Goldfield hills. 
I The basalt is for the most part a dense, compact, dark-gray or black 
ock, in some instances with phenocrysts equal to the groundmass in 
julk and in others without phenocrysts. The phenocrysts, which 
Hch a maximum length of one-fourth inch, include glassy striated 
kths of feldspar, stocky black columns of augite, and rounded grams 
