68 
SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
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features. (See fig. 5.) The more crystalline rhyolite was sheared 
and fractured along the limbs of the fold, while the glassy facies 
thickened at the troughs and crests. The thinner flows weather some- 
what like a horizontal sedimentary rock, while the thicker show the 
rounded topographic forms of granite. 
Minor rhyolitic effusions started while the Siebert lake beds were 
being deposited, although the ma- 
jor outflow was later. The rhyo- 
lite is probably to be correlated 
with the younger rhyolite of the 
A.margosa Range and is of late 
Miocene or early Pliocene age. 
Basalt. — Slightly eroded benches 
of basalt flank Mount Jackson on 
the south and a small basalt area 
is located 5 miles to the east. The 
basalt is a dense black vesicular 
(low rock which microscopic exam- 
ination proves to be an ordinary 
olivine basalt. As Spurr states, 65 
erosion carved Mount Jackson approximately to its present form 
prior to the outflow of basalt. At the north end of the ridge 
east of Cuprite and in several small areas west of the Goldfieldj 
Bullfrog road is basalt which is an extension of the younger basalt 
of the Goldfield hills. (See p. 75.) It is probable that small areas 
of younger rhyolite and late tuffs of these hills are here mapped 
with the basalt. The basalt northeast of Cuprite clearly overlies 
and is younger than the rhyolite, being probably of late Pliocene or 
early Pleistocene age. 
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i -Flow folds in rhyolite (hill H) miles east 
of north ol Mount Jackson), showing thicken- 
ing of more glassy rhyolite (white) on crests 
and troughs of folds and cross fractures in 
more lithoidal rhyolite on limbs of folds. 
STRUCTURE. 
The Cambrian rocks are in rather open folds of northeast-south- 
west axes (fig. 6). The angle of dip rarely exceeds 30°, although 
Desert B 
gravel 
k 
Siebert lake 
Lambnan limestone beds 
3 miles 
Fig. 6. — East-west section across Mount Jackson hills 1 mile south of Halfway Station. 
4 miles west of Cuprite isoclinal folds occur. The highest hills of 
the two main ridges are situated on the axes of synclines. The rocks 
on the south face of the hills toward the valley north of Slate Ividge 
dip gently to the west. Faults, usually normal and across the strike 
•Spurr, J. E., Bull. U. S. Geol. Survey No. 208 1903, p. 1 33. 
