64 
OROGENY AND EARTH’S ROTATION 
tains.^^ This corresponds closely with the observation of the late 
W J McGee, in the Sierra Madre, of the Sonoran region, of Old 
Mexico.Thus does Gilbert’s interpretation of the Basin ranges 
fail at every step to support the theory of isostasy. 
The original impressions of King, and other members of the 
Fortieth Parallel Survey, concerning the genesis and structure of 
the Great Basin ranges may not be after all so remote from fact 
as later writers would have us believe. Leaving out of consider¬ 
ation present configuration of these mountains the strictly tectonic 
features display some curious anomalies. Because of the fact 
that these mountains are widely believed to be buried in their own 
debris much is left to surmise concerning the alleged under¬ 
ground structure. Direct corroboration of theoretical conclusions 
is not supported so strongly as it might be by observation. 
It is a singular circumstance that when so much had to be left 
to inference that the possibility of extensive thrust-movement in¬ 
stead of normal displacement never suggested itself. If ever there 
were a situation where thrust-faulting should leave impress upon 
the local topography that place is certainly the arid region. At 
this day almost any side-view of desert ranges gives first impres¬ 
sion of tilting and enormous overriding of mountain-blocks. Far¬ 
ther north in Montana and Alberta thrust movements are clearly 
in evidence. Such phenomena as those displayed at Lombard and 
Crow’s ‘Nest seem to furnish worth-while clues to the possible 
wider application of this type of crustal movement and disturb¬ 
ance. 
It is probable that the main reason why normal faulting came 
be stressed is that it happened to suggest itself first, and then the 
idea of isostatic adjustment which followed precluded all other 
working hypotheses. However, such facts as the occurrence of 
fault-lines which often bound the mountain-blocks miles away 
from the piedmont, the absence of ore-bodies along the supposed 
lines of displacement at the foot of the mountains, the usual mis¬ 
interpretation of the origin of the truncated cross-ridges of the 
desert ranges, the well-known girdling of desert mountains by 
natural sand-blast action, all tend to show that the older tectonics 
of the Great Basin ranges have to be critically examined again in 
24 Bull. Geol. Soc. America, Vol. XXI, p. 562, 1910. 
25 Ibid., Vol. VIII, p. 90, 1897. 
