560 C. K. KEYES PROFILES AND STRUCTURES IN DESERT RANGES 



The soft rocks would be acted upon much faster than the hard ones, just 

 as in a humid country. The tendency would be naturally to greatly 

 accentuate the larger geologic structures. The latter would be expressed 

 in the topographic forms as clearly as they are in such districts as the 

 Appalachians or the Juras. Desert erosion has gone on so long and so 

 extensively in the region under consideration that all the landscape 

 shapes now may be regarded as the direct products of deflative erosion, 

 except in a comparatively few instances where there is some modification 

 on account of recent minor faulting. 



First reviewing deductively the field of Basin-range genesis, as dis- 

 played in the northern part of the Mexican tableland, it seems wholly 

 inadequate, with existing conditions of an arid climate imposed, to postu- 

 late a development of the present mountains by direct rearing through 

 profound fault movements. 



-. r , ^ '^'^^ An,Srea.s A/I h ^CLCi-aiTxento A/)U 



Figure 15. — Apparent Basin-range Structure in the Mexican Tableland 



Assuming the initial factor of an upraised peneplain in a region which 

 had been previously gently flexed and profoundly faulted, such as the 

 New Mexican area appears to have been, with great thicknesses of weak 

 strata 10,000 to 15,000 feet in vertical measurement, overlying an equally 

 thick section of beds of very resistant character, the most noteworthy 

 feature in the areal distribution of the geological formations would be 

 the marked alternation of belts of hard and soft rocks. The ideal gen- 

 eral tectonics of the region is, therefore, that of the typical Basin-range 

 structure. This is represented below (figure 15) of an actual section 

 across the Jornada del Muerto, in southern New Mexico,^^ in which the 

 dark band is the great Carbonic limestone formation. 



When, however, we come to look for evidences of actual fault-lines we 

 do not find them where we would expect them to occur, on the theory 

 that the mountains were reared by fault-movements of 4,000 to 10,000 

 feet — that is, at the bases of the so-called range-blocks, where mountain 

 sharply meets plain — ^but when they occur it is invariably far out on the 



isjournal of Geology, vol. xiii, 1905, p. 67. 



