274 CHARLES LAURENCE BAKER 
term was first used by Powell and C. A. White—as by the old 
conception of normal or gravity faults. Such of the faults as 
are really of the tensional type are probably those formed by the 
stretching of competent brittle rock along the flanks or summits 
of anticlines. There is so much of very competent rock in that 
region, and this is so commonly deformed by fracture rather than 
by flexure, that faults are very greatly developed and are often of 
great displacement. 
The key to the problem is an old erosion surface, locally 
approaching the condition of a peneplain. This bevels the folded 
strata, and these contain a fairly abundant mammalian fauna 
of later Tertiary age. These folded and beveled later Tertiary 
beds have now been found in three widely separated localities of 
the southwestern Great Basin, namely in central western Nevada, 
in the central Mohave Desert, and in a region on the border line 
between the Mohave Desert proper and the rest of the Great 
Basin. The erosion surface cut out of these beds is in places 
warped into synclines, which form the valleys between many 
Basin Ranges, and into anticlines, which form the isolated 
ranges. In other places it is faulted, forming grabens in the cases 
of Death and Owens valleys and possibly elsewhere, and in other 
and more numerous cases tilted blocks bounded on one side by a 
fault scarp. The longitudinal profile of a block-faulted range, as 
determined by Louderback and the writer, is essentially that of 
the longer axis of an anticline. From a point of maximum move- 
ment the amount of displacement gradually dies out in either direc- 
tion into monoclinal flexures and finally into undisturbed strata. 
Some of the Basin Ranges, as first pointed out by Spurr and Ball, 
do not owe their present forms in any sense to faulting but are 
merely structural upwarps or a series of anticlines more or less 
modified by erosion. Many of the intermontane basins are 
plainly seen to be true synclinal spoon-shaped basins. Gilbert’s 
original suggestion that many of the Basin Ranges have their 
greater portions buried under their own débris is true only in a 
limited degree, for the original bed rock is found exposed at various 
places from the centers to the peripheries of some basins. In 
many of the basins the alluvium has not buried the post-Miocene 
