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IOWA ACADEMY OF SCIENCE 
we may kave to ascribe to tbe mountains of the region generally, .even when 
normal faulting is in evidence, an initiatory genesis through reversed faulting. 
Second only in importance to this important observation is that of the 
possible radical difference in expression of compressive strains in great rock- 
sections composed entirely of brittle masses and these of masses made up of 
alternating beds of weak and resistant rocks, especially when the former 
greatly predominates. Through great compression the tectonics of the western 
country would thus be quite distinctive from that of the eastern, or Appa- 
lachian, field. In the last mentioned region the rock-masses include numerous 
thick zones of yielding shales. Under compression from the sides such masses 
would easily flow, as it were, enabling the more rigid and brittle layers to 
become severely flexed into a series of close folds without loss of their con- 
tinuity. In the Great Basin region there are practically no shale beds except 
the late clays. In the entire Paleozoic section, which is upwards of 15,000 feet 
in thickness, there are very few layers that would yield to flexing. Under 
these conditions folding would in any case be very slight. With brittle rock- 
masses only breakage and shearing would be possible. As shown by Irving* 
for the Wisconsin region, great movement in such rocks, is likely to take place 
only through a narrow belt. Thousands of feet of movement may be expressed 
within a space of a few hundred feet with no other break within a distance of 
many miles on either side. 
In the desert region of the West the conditions are not so‘ favorable as they 
might be for obtaining critical evidences bearing upon this point. The foots 
of the mountains are almost invariably covered by ever-shifting loose materials. 
So far as known the desired data have never been especially sought. The only 
published suggestion that is now recalled of this mode of mountain origin is 
in the cases of the Sierra de los Caballos, the Sierra Oscura, and the Sierra 
San Andreas, in south-central New Mexicot. 
If the thrust be the true explanation of some of the Great Basin ranges then 
the mountains of this region are not to be put in a class so very different from 
that to which the Appalachians and the Juras belong. The specific difference 
is only in the character of the rocks composing the mountains. This, however, 
may give rise to very diverse geologic structures. 
It is also now known that there have been great tortional movements in many 
parts of the American arid region, by which long ranges have undergone con- 
siderable lateral displacement. Abundant evidences of this phenomenon are 
to be seen in the Sandia and Manzano ranges, and in the Caballos, Cochillo 
and Fra Cristobal mountains!. This phase of the subject is of very great in- 
terest and is worthy of careful and extended inquiry. 
*U. S. Geol. Surv., 7th Ann. Kept, p. 390, 1888. 
tJoumal of Geology, Vol, XIII, p. 64, 1905. 
tProc. Iowa Acad. Sci., Vol. XII, p. 163, 1905. 
