LATER DEFORMATIONS IN GREAT BASIN RANGES . 277 
back’s main contributions; it was noted in several ranges by the 
geologists of the Fortieth Parallel Survey, and the writer has 
recently noted the same divergence of earlier and later axes of 
folding in two other Nevada ranges, the Pilot Range and the 
Cedar Mountain Range. 
The recency of the movement to which the existing Basin 
Ranges owe their forms is such as to leave intact, in large measure, 
the superficial rocks of the lithosphere, even when these lie high 
in the zone that was affected by fracture during this deformation. 
We know this because there is still preserved much of the erosion 
surface developed previous to this deformation. It also happens 
that a large portion of this superficial rock is of a competent nature 
and, without any load, seems quite as likely, or more likely, to 
break than to fold. The Basin Ranges may very possibly have 
originated by tangential compression. Their present elevations 
and structures may be a joint product of the initial intensity of the 
deformative forces and of the relative resistance to deformation 
of the strata involved. This is a very elementary conception but 
is as far as the writer is willing to go on his present data. 
The mid-Mesozoic deformation was apparently the most 
intense, the folds in the Death Valley region and in the Pilot 
Range being as close as those of the central Appalachians. The 
first later Tertiary deformation was probably on the whole less 
intense, although locally non-competent beds are closely crumpled 
and overthrust. The writer’s studies have not been of such a 
detailed nature as clearly to separate the effects of these two 
movements on the older rocks. The most recent deformation is 
the least intense, at any rate as exhibited on the surface, but is 
the one which is responsible for the present orographic features of the 
Basin Ranges. What has happened in the zone of flow, or in the 
zone of combined fracture and flow, during this latest deformation, 
we have no means of knowing, since erosion has not yet laid bare 
these zones. But it is probable that in the southwestern portion 
of the Great Basin the competent brittle rocks at the surface or 
close to the surface have deformed somewhat differently from the 
dominantly sedimentary rocks of the Rocky Mountains, the Jura, 
