42 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
end of this period the basalt extrusion reached its climax. Uplift 
accompanied by normal faulting and folding followed the deposi- 
tion of the older alluvium, and at this time erosion appears to have 
been very active. 
In Recent time the erosion characteristic of arid lands has partially 
filled the inclosed valleys with bowlders, gravels, and sands — debris 
from the wasting mountains — and the process is still going on. 
The ranges of this portion of the Great Basin were delineated by 
post-Jurassic folding and Cretaceous erosion. Later uplift, coupled 
with volcanic outbursts along these same structural lines, has added 
much t<> their height, and the effect of these processes has been em- 
phasized by the deepening of the intermontane valleys by erosion. 
In Death Valley alone doe- faulting appear to have been the dominant 
process in blocking out the existing topographic forms. The unim- 
portance of faulting in the topographic evolution elsewhere is well 
exhibited in the northeast quarter of the area surveyed. The Paleo- 
zoic rocks -)i' the Kawieh and the Belted and Reveille ranges are folded 
in a synclinewith it- trough in the Kawieh and Reveille valleys. The 
presence of early Miocene rhyolites in the ('actus. Kawieh, Reveille, 
and Belted range- at approximately equal elevations further proves 
that, unless compensating faults are hidden in the intervening in- 
closed valleys, faults have been of little moment. The projection of 
minor transverse ridge- from the main ranges into the desert valleys 
and the frequently observed gradation from small erosional buttes to 
groups of rounded hills, such as the Monitor Hills, and from these to 
such elevations as the Cactus Range, are also opposed to the view that 
the principal mountain masses have been bodily uplifted by great 
faults. The ascendency of erosion over faulting is further indicated 
by the broad and rounded detrital embayments which notch many of 
the ranges. 
ECONOMIC GEOLOGY. 
HISTORY OF MINING DEVELOPMENT. 
Two waves of activity in prospecting and mining have passed over 
the portions of Nevada and California under consideration. The 
first started in the late sixties, reached its maximum height in the 
seventies, and died out before 1890. The hardy prospector and miner 
in these years confined his attention to the Paleozoic lime-tones and 
the post-Jurassic granites. Lida, Old Camp, and Montezuma were 
flourishing mining centers. Mining in the area studied was dormant 
in the nineties, but with the discovery of Tonopah's phenomenal 
veins in 1900 the desert region was attacked with new ardor. The 
Tertiary eruptives have been more especially the chosen held of the 
prospector, although the limestones and granite- have been by no 
means neglected. 
