DEFLATION OF AKID LANDS 583 



DEGRADATIONAL CHARACTER OF DEFLATION 



The denuding tendencies of deflation under conditions of aridity are of 

 such extent that humid regions give small suggestion of their effective- 

 ness. When a large arid area has become a plain, deflative action as- 

 sumes mainly degradational proclivities. The plains have very gener- 

 ally rock floors, often but thinly veneered by debris and soils. In lately 

 calling special attention to the extensive rock floor of the Jornada del 

 Muerto, in New Mexico,^^ I noted also similar conditions existing in other 

 bolsons of the Mexican tableland. McGee^" expresses his surprise at find- 

 ing on the great interment plains of Sonora, Mexico, at distances of sev- 

 eral miles from the mountains and without intervening foothills, that the 

 horses' shoes beat on planed granite and schist or other hard rocks, in- 

 stead of yielding sands. Throughout all of the American arid regions^® 

 the phenomenon is shown to be widespread. Passarge^^ especially de- 

 scribes similar clean swept plains in South Africa. 



Playas and salinas also now appear to be chiefly areas of degradation 

 rather than of aggradation, as was formerly generally believed. The man- 

 ner in which playa deposits are transported I have fully explained in an- 

 other place.*^ 



It is the differential effects of eolative action that are most remarkable. 

 To some of these effects attention has been already called.*^ Wind scour 

 on the desert surface naturally wears away the areas of soft rocks much 

 more rapidly than it does the hard rocks, in the same way as in the case 

 of running water. After the more resistant rock-masses have been 

 brought out in strong bas-relief eolian erosion continues to act most vigor- 

 ous at the plains level, and eats back into the highlands at this horizon in 

 the same way as does the sea on an exposed coast. The rugged seashore 

 and its broad marine shelf has its exact counterpart in the desert in the 

 mountain piles rising sharply, isle-like, out of a boundless sea of earth. 

 It is the sharp meeting of mountain and plain without the usual transi- 

 tionary foothills that has frequently led to the mistake of ascribing nor- 

 mal eolation features to faulting on a gigantic scale. 



EXTRALIMITAL EFFECTS OF DEFLATION 



General desert-leveling, chiefly through eolation, is doubtless much 

 more extensive than has been commonly supposed. In the presence of 



38 American Journal of Science (4), vol. xv, 1903, p. 207. 



3^ Bulletin of the Geological Society of America, vol. 8, 1897, p. 90. 



38 Ibid., vol. 19, 1908, p. 63. 



3» Zeitsch. d. deut. geol. Gesellschaft, LVI Bd., Protokol, 1904, p. 193. 



*» Bulletin of the Geological Society of America, vol. 19, 1908, p. 83. 



*^ Ante, p. 581 et seq. 



