EROSIONAL ORIGIN OF GREAT BASIN RANGES Bi 
plicable by any method of water-sculpturing. ‘They are all readily 
accounted for, however, through eolian activities under conditions 
of an arid climate. 
The close tectonic pattern of the American desert region has 
done much to obscure the real significance of its most striking features. 
In South Africa, where the great elevated plateau of the dry region 
has been long relatively free from orogenic movement, there are the 
same isolated mountains, the same vast and even intermont plains, 
and the same streamless country. In that region the vigor of wind- 
scour action and the impotency of water corrasion is amply attested 
by the recent observations of Passarge,’ and others. In the Great 
Basin the wind as a factor in general erosion appears not less effective. 
It seems probable that we must now regard wind-scour not only the 
most important erosive agent under conditions of aridity but more 
potent than all other agencies combined. Moreover, in the production 
of the present desert mountains its influence doubtless very far sur- 
passes that of any recent deformation or dislocation. 
From a view-point of true desert conditions all late observations in 
the arid country of southwestern United States go to show: (1) 
That there existed at the beginning of the present geographic cycle a 
broad peneplain at a level of about 4,000 feet above the present plains- 
surface; (2) that the major faulting and gentle folding of the region 
took place chiefly before the beginning of the present cycle; (3) that 
water-action is unimportant; (4) that wind-scour is very potent; 
(5) that the belts of hard and soft strata produced by the deformation 
in former cyles allowed the winds to erode the latter much more rapidly 
than the former, producing the present plains, and leaving many of 
the former as monadnock ranges; and (6) that while differential move- 
ments of the rock-masses have in all likelihood taken place recently 
and locally their direct effects compared with those of eolian erosion 
have been relatively unimportant in the formation of the present oro- 
graphic expression of the desert country; and (7) that, all things con- 
sidered, the comparative values of deflation and corrasion in the arid 
regions may be expressed by the estimate that the volume of rock- 
waste brought down by the waters from the mountains in a year may 
be removed by the winds in a single day. 
1 Zeitsch. d. deutschen geol. Gesellschaft, LVI Bd., Protokol, pp. 193-209, 1904. 
