OROGENY AND EARTH’S ROTATION 
63 
The recent conclusions concerning the facial expression of the 
Great Basin region are that the folded structures belong chiefly 
to an ancient system of tectonics, probably mainly early Mesozoic 
in date; the major faulting is also chiefly old, its relief effects 
being long since completely mastered by desert erosion. It seems 
to have small direct connection with the rearing of the present 
ranges. The entire flexed and faulted region appears to have 
been smoothed off to the condition of a peneplain before it was 
upraised and presented to the sculpturing agencies through which 
the existing mountains took form. The tectonics of the region, 
therefore, have merely an unimportant, or accidental, role, rather 
than a direct genetic influence in the shaping and production of 
the Desert ranges. 
Since recently the commonly ascribed fault-origin of the Great 
Basin ranges has been so sharply challenged special attention is 
directed to the review of many of the published details and to a 
search for data of a critical character. There now seems to be 
discernible not only in the Great Basin, but throughout the arid 
region, two distinct and general sets of geologic structures.^^ . In 
point of time these periods of diastrophism are widely separated 
from each other. The older of the two appears to have been 
pressed mainly in Jurassic times; the younger during Late Tertic 
times. Neither, therefore, seems to contribute materially to the 
shaping of the existing mountains. The latter appear to owe their 
existence not so much to the general flexing, or to the late pro¬ 
found faulting as they do to the differential effects of true desert 
leveling in which the resistant rock-belts retain longest the higher 
elevations. 
The fact that the majority of the major faults bounding the 
orographic blocks are not recent manifestly militates strongly 
against the idea of their being expressions of isostatic conditions. 
Another important consideration tending to invalidate the Dut- 
tonian isostasy hypothesis is the discovery that the basal truncation 
of the transverse ridges of desert mountains is not a fault-scarp, 
but simply the direct result of desert sand-blast action. In New 
Mexico evidence of the existence of faults bounding orographic 
blocks are strangely wanting. When located at all they are sit¬ 
uated miles out on the plains, far beyond the base of the moun- 
23 Bull. Geol. Soc. America, Vol. XXI, p. 547, 1910. 
