EROSIONAL ORIGIN OF GREAT BASIN RANGES 33 
vigorous erosion, the effects of the latter being fully as important as the 
faulting in producing the present relief expression. Being fresh 
from the humid regions all denuding agents spelled water-action. 
All of the above allusions to the erosional agencies in the desert 
ranges of the West have referred solely to the work of water. Of 
the possibility of the existence of any other effective denuding power 
there has been small hint. Only very recently have other erosive 
influences been suggested to account for some of the most conspicuous 
of the desert features. It now seems probable that wind-scour must 
be regarded as the chief erosive power in the dry regions. 
In the Great Basin area the salient aspects of the country are 
quite different from the larger relief features of other parts of the west- 
ern desert region. There is some of the faulting that is more recent 
and more profound than elsewhere. Late orogenic movements are 
perhaps more extensive. Evidences of much greater precipitation 
than now at no distant geologic date are manifest. No noteworthy 
streams traverse the district to disguise the effects of typical desert- 
leveling. Little is yet known in regard to the probable nature of the 
surface relief prior to the commencement of the present arid period. 
The general conditions are such as to present little critical evidence 
in support of any hypothesis yet proposed concerning the genesis of 
the desert ranges. 
In other portions of the desert region far to the south and south- 
east of the Great Basin, in Arizona, New Mexico, and Old Mexico, 
there are many features suggestive of structures and conditions which 
formerly prevailed but of which there is small hint to be derived in 
the more northern area. The most noteworthy of these characteristics 
are the mesas, or plateau-plains, many of which now stand high above 
the present level of the intermont plains, or general plains-surface of 
the region. These mesas manifestly represent, as recently shown,! 
former positions of the general plains-surface. Their greater resist- 
ance to erosional influences and the general lowering of the region 
is due mainly to the protection afforded by extensive lava-flows which 
are now the capping-rock of the remnantal levels. The surfaces on 
which the lavas rest are true beveled rock-floors, just as in the cases 
of the present plains. 
t Bull. Geol. Soc: America, XIX, p. 63, 1908. 
