90 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
cut by numerous small white calcite veins. On lithologic grounds it is 
probably the Pogonip limestone of Ordovician age. Bowlders of sim- 
ilar limestone are embedded in rhyolitic tuffs near by and fragments 
of limestone and jasperoid are inclosed in the rhyolite at several 
places and in a granite porphyry in the northern part of the range. 
These fragments were probably derived from Ordovician or Cam- 
brian limestones. 
Eureka quartzite. — Two miles southwest of Antelope Spring an 
area of quartzite lies unconformably below the surrounding rhyolite. 
This is a fine- to medium-grained quartzite of white, yellow, or red 
color and is cut by small stringers of white quartz. From its litho- 
logic character it is considered to be the Eureka quartzite (Ordo- 
vician ) . 
Weber conglomerate, — Three miles west of south of Cactus Spring 
a mass of rusty-looking sedimentary rocks, one-half mile in diameter, 
protrudes through rhyolite. About 150 feet of conglomeratic beds, 
with the matrix of sand and small pebbles thoroughly cemented, are 
exposed. Embedded in this matrix are many beautifully rounded 
pebbles of green and black flint and jasperoid, white quartzite, and 
black limestone. The roundness of the pebbles, the largest of which 
are 1 inches in diameter, indicates a long period of attrition and the 
conglomerate is probably of marine origin. Prior to their inclusion 
in the conglomerates the pebbles, probably derived from Cambrian, 
Ordovician, and Silurian rocks, were cut by quartz and calcito 
veinlet- and the lime-tone silicified to jasperoid. The conglomerate 
is thus much younger than the Pogonip limestone, but since it con- 
lain- no granite or diorite pebble- it is believed that it is of Car- 
boniferous age and that it i- to be correlated with the Weber con- 
glomerate of the Belted Range. 
[GNEOUS ROCKS. 
Post-Jurassic granite j><>rj>Ji;iry and granite. — Midway between 
('act n- Peak and ("actus Spring is a small area of granite porphyry 
that i- poorly exposed, but lithologically distinct from the surround- 
ing rhyolite. The pinkish-gray rock is of well-developed porphy- 
ritic habit, with a finely crystalline groundmass. The phenocrystsj 
which exceed the groundmass in bulk, are pink glassy feldspar 
tablets up to three-fourths of an inch in length, a few smaller quartz 
grains, and fairly abundant biotite flakes. In it are fragments of 
Paleozoic sedimentary rocks. Under the micro-cope the groundmass 
is seen to be a microgranitic mosaic of orthoclase and quartz with 
here and there a little plagioclase and biotite. The orthoclase pheno- 
crysts, many of which have zonal structure, are in places twinned 
according to the Carlsbad law. Quartz, biotite. and a few plagio- 
clase phenocrysts are associated. Magnetite and zircon occur as 
