SHOSHONE AND SKULL MOUNTAINS, ECONOMIC GEOLOGY. 147 
iron tank it appears to overlie the later rhyolite. It is without doubt 
the youngest formation of these hills, and is probably of late Pliocene 
or early Pleistocene age. 
STRUCTURE. 
The Pennsylvanian limestone is folded into rather open folds, the 
dips of which rarely exceed 45°. The predominant axes trend from 
northeast to southwest, although minor folds with east-west axes also 
occur. The ridge to the northeast of Tippipah Spring is synclinal 
in structure. The Pennsylvanian rocks appear to have had a rather 
uneven surface prior to the deposition of the Siebert lake beds. Not 
only do many of the lake beds contain large bowlders of limestone, but 
west of Tippipah Spring a number of hillocks of the older limestoiu 
protrude through the Tertiary sediments. 
In a broad way the Siebert lake beds and the later rhyolite are 
slightly flexed and the beds on Shoshone Mountain dip gently to the 
south. Normal faults of 50 feet displacement are not unusual in the 
tuffs. The latest deformation is that of the basalt. On Skull Moun- 
tain it dips gently to the south, while on the ridge to the east it dips 
with a similar angle to the north, the two blocks being separated by 
a north-south fault. 
ECONOMIC GEOLOGY, 
The quartz-monzonite porphyry and the earlier rhyolite have been 
silicified and kaolinized in restricted areas, the silicification being 
comparable to the alterations in the vicinity of Kawich and the kaolin- 
ization to those of Silverbow. These areas, which are worthy of pros- 
pecting are delineated in fig. 4 (p. 43). The quartz-monzonite por- 
phyry near the Horn Silver mine has not only been altered to a spongy 
mass of brownish-gray silicified porphyry like that of Kawich, but it 
is cut by quartz stringers with associated gypsum. Other veins and 
irregular cavities are partially filled with chalcedony, which in turn 
is incrusted by quartz crystals. The rhyolite of the Calico Hills has 
been in places intensely silicified. The silicified rhyolite '2 miles north 
of the Horn Silver mine is cut by thin quartz seams. A similar yel- 
low stain (a basic ferric-alkali sulphate?) to that noted near the 
prospects at Silverbow is present on joint surfaces. 
SPECTER RANGE AND SKELETON HILLS. 
TOPOGRAPHY AND GEOGRAPHY. 
The hills in the southeast corner of the area surveyed may be con- 
sidered as the westward extension of the irregular Spring Mountain 
Range. They consist of a number of mountains and hills now iso- 
lated from one another by narrow canyons and broad valleys filled 
with desert gravels. The ridges and hills are rugged and sharp and 
are crowned by many peaks. The highest point (5,800 feet above 
