BULLFROG HILLS, SEDIMENTARY ROCKS. 
Tertiary diorite porphyry, andesite, rhyolite, earlier basalt contem- 
poraneous with rhyolite, Siebert lake beds( ?), and later basalt. 
SEDIMENTARY ROCKS. 
P re-Silurian schist. — Three miles west of Bullfrog is a low ridge 
of post-Jurassic pegmatite containing a vast number of schist inclu- 
sions. The rocks are present in practically equal amounts, and the 
area could as well be described as an area of schist intensely injected 
by pegmatite. The pegmatite, however, is more resistant to erosion 
and hence gives its white color to the low rounded ridge. 
The schist varies from a finely foliated biotite schist to an ** eye " 
schist, containing many lenses of pegmatitic feldspar from one- 
sixteenth inch to 2 inches long. These eyes are elongated parallel to 
the schistosity, the shortest diameter usually being one-fourth the 
length of the longest. Some of the schist is composed of minute 
scales of biotite of common orientation, while other portions also con- 
tain quartz, a little feldspar, and the large feldspar lenses already 
mentioned. The planes of schistosity dip at considerable angles, but 
are rarely minutely crenulated. A single thin section of the " eye " 
schist under the microscope proves to be a schist in which shreds of 
biotite and a little muscovite are arranged in folia. Between these 
is a fine-grained mosaic of orthoclase and quartz, with here and there 
a partial crystal of apatite or zircon. The pegmatitic " eyes," which 
are usually acidic plagioclase and in only a few cases quartz or ortho- 
clase, are closely encircled by the biotite films. 
The schists are presumably metamorphosed sedimentary rocks, but 
they may possibly be mashed igneous rocks. Mr. G. H. Garrey." 
who examined the relations between the schist and the Pogonip lime- 
stone on the western border of the Bullfrog special area, states that 
the contact between the two rocks is not exposed, but that outcrops 
of limestone adjacent to the schist are comparatively unmetamor- 
phosed and that the schist in consequence is presumably pre-Silurian. 
Lithologically this rock resembles Cambrian schists near Tokop, and 
although possibly of pre-Cambrian age it is tentatively correlated 
with the Cambrian. 
Eureka quartzite. — A hillock rising from the rhyolite to the 
southeast of the limestone last described and a hill 2 miles south of 
Indian ranch are formed of quartzite interbedded with shale. Frag- 
ments of similar shale are included in Tertiary rhyolite ?>h miles 
northeast of Bullfrog. The Eureka quartzite of Bare Mountain 
extends across Amargosa River northwest of Gold Center. The 
quartzite of the hills first mentioned is gray, pink, or white in color, 
and of fine to medium grain. It appears to be a practically pure 
a Oral communication. 
Bull. 308—07 m 12 
