LATER DEFORMATIONS IN GREAT BASIN RANGES 275 
erosion surface, the formation of which antedated the latest 
deformation. The processes of desert erosion and deposition in 
an arid climate may mantle the rock surfaces of the ranges almost 
to their summits, and yet give only a very thin veneer above the 
bed rock, as mining operations at Tonopah and elsewhere have 
shown. In the immediate proximity of great fault scarps, the 
- piedmont alluvial fans are, however, usually of great thickness, 
as on both sides of Owens and Death valleys. 
The second uplift followed the axes of the first later Tertiary 
deformation but it seems to have been less intense, for the non- 
competent later Tertiary sediments, which were intensely folded in 
places and even overthrust during the first deformation, have 
been only gently warped during the second. This fact can be 
determined by the shape of the deformations in the peneplain 
produced during the first cycle of erosion. How much of the 
original folding of the non-competent later Tertiary beds has 
been. due to a movement laterally over the basement of com- 
petent rocks which may have deformed mainly by fracture, is 
not known. 
Zones of faulting along the bases of ranges have been examined 
by the writer in the Calico Mountains of the Mohave Desert in 
the Silver Peak Range in western Nevada and on the south base 
of the Sierra Nevada east of Tehachapi Pass. The fault planes 
in these localities approach the vertical and some even overhang. 
There is in the Silver Peak Range a zone of faulting rather than 
a single plane of faulting. The upthrown side forming the scarp 
is made up of the more competent and more erosion-resistant 
rocks, while the less competent strata on the downthrown side 
are closely compressed and overthrust contiguous to the faulting. 
In the Death Valley region, where faulting has probably taken 
place on as great a scale as anywhere in the Great Basin, closely 
folded Tertiary strata, referred to the Miocene by Spurr and Ball, 
bound the valley side of the Funeral Range fault. It is difficult 
to conceive how this folding and thrusting can have been caused 
by tensional stresses manifested in normal faults. 
Gilbert’s original conception of block-faulted mountains is 
incontestable. Gilbert never held, to the knowledge of the writer, 
