OROGENY AND EARTH’S ROTATION 
65 
the light of thrust-fault activities before the isostatic explanation 
can be properly considered or fully accepted. 
Many of the physiographic features of Great Basin deserts, 
where the isostatic hypothesis had its birth, are now known not to 
be the effects of profound faulting at all, but to be typical develop¬ 
ments of wind-scour. As in moist lands, under the influence of 
the rains and ordinary stream corrosion, weak rock-belts are re¬ 
moved faster than the resistant rock-masses, and the latter are thus 
left standing in bold relief. Under dry-climate conditions the 
resistant rocks are the mountain-rocks to a more notable extent 
than in moist climes. 
In order to measure quantitatively the grander work of the 
winds, as a geologic process, to compare its relative efficiency 
with that of other erosional agencies, and to view it where it is 
most energetically active, we have to turn from our moist climate 
homes to regions of excessive dryness. Typical of such lands is 
the Great Basin. 
Since all arid lands do not display the differential effects of 
eolic erosion equally well the local substructure becomes a prime 
factor in the determination of the relief expression. In a tract 
like the South African plateau, where the rocks are homogeneous 
and unfolded, relief contrasts produced by wind-action are not 
usually very well marked. On the other hand, in a region of 
close-patterned orogeny, such as the Great Basin, the elevated 
country of Southwestern United States, and the Mexican table¬ 
land, the larger effects of the wind in re-forming the facial ex¬ 
pression of the earth are most striking, and there wind-work is at 
its best. It is these very features which were once thought to be 
peculiarly the effects of isostatic movement. 
The attractive feature of the Gilbertian explanation of Basin 
Range structure is that it furnishes the basis of that brilliant con¬ 
ception which Dutton dominated Isostatic Compensation. As, 
however, the hypothesis is tested critically the loading and un¬ 
loading of contiguous areas, or regional depression and uprising, 
appear not to be strictly compensatory. 
It was with Gilbert’s fault-block idea in mind that I approached,^ 
a decade and a half ago, the desert mountains of New Mexico 
and the Mexican tableland.^® Keen curiosity was aroused to ex- 
26 American Geologist, Vol. XXXIII, p. 19, 1904, 
