88 
SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
o fcZvJ 
>^f 
u- 
and topographic feature of Stonewall Mountain is the fault on the 
front of the mountain group, near Stonewall Spring. This fault 
strikes X. C>:)° E. and dips 70° N. Minor faults 
and sheeting parallel to the main fault occur for 
a distance of one-half mile south of the mountain 
front. The Siebert lake beds strike east and 
west and dip 5° to 50° X. This fact probably 
indicates that Stonewall Mountain is on the up- 
thrown side of the fault. The fault fissure was 
healed by a quartz vein and has been reopened 
several times and again healed. (See p. 88.) 
Since the fissure filling erosion has uncovered the 
fault and a secondary fault scarp, whose position 
is determined by the resistant quartz vein, has 
been formed* The topographic form of the valley 
of the Stonewall Spring drainage line (see p. 84) 
indicates that in comparatively recent time the 
mountains behind the fault line have been up- 
lifted. Fig. 7 is a section through the north side 
of Stonewall Mountain. 
ECONOMIC GEOLOGY. 
Quartz veins and stringers filling faults, joints, 
and the cavities of brecciation are very abundant 
in the earlier rhyolite, quartz-monzonite por- 
phyry, ami quartz syenite near Stonewall Spring. 
The most prominent vein follows the fault scarp 
immediately south of the spring. The quartz 
vein is in some place- simple and 40 feet wide; 
in others it is complex and composed of many 
parallel veins. This vein or other veins of ap- 
proximately similar strike extend -2 miles west 
and 1 mile east of the spring. The quartz is 
white or colorless, rarely greenish } r ellow, and 
is beautifully crust ified. Vugs with quartz crys- 
tals or mammillary quartz are common. Move] 
nient reoccurred along this fault and the quartz 
has locally been fractured and slightly displaced, 
later quartz filling the cavities. Heavy stains 
of limonite and slight stains of azurite were 
noted in the quartz, and pyrite is locally de- 
veloped. Prospectors report from a trace to $6 
in gold per ton at a number of places on this 
vein. Similar but smaller quartz veins occur 
throughout the areas of earlier rhyolite and quartz-monzonite 
porphyry. 
V 
- 
m 
u 
VI 
~ 
- 
a 
> 
c 

: 
■j. 

q-l 
I. 

L -_ 
I 
