696 C. R. KEYES MID-COfNTINENTAL EOLATIOK 



vegetation. There is general agreement among recent writers on the 

 subject that the conditions thus imposed are productive of terranal and 

 topographic contrasts much stronger than those displayed in humid lands. 

 Certain it is that in arid areas such features as rock-wasting mainly me- 

 chanical in nature, dry and pulverulent soils, plant growth which does 

 not bind the soils, indefinite waterways and few, and winds constant and 

 strong, have no counterpart in moist countries. Erosional conditions 

 with which we are most familiar are absent to an extent which is hard 

 for us to fully realize. They are in pressing need of critical review from 

 vantage points other than those commonly selected. 



DEFLATION IN DRY REGIONS 



Since the appearance of the recent publications of Walther,^® Par- 

 sarge,^"^ Bornhardt,^^ Cross,^^ and others on the various phases of erosion 

 displayed in arid region, wind action as a degradational process is made 

 in every way comparable to general stream planation under the most 

 favorable circumstances. Under the stimulus of aridity, wind-scour be- 

 comes, as lately set forth in some detail,*^ an erosive agent more constant 

 than any workings of the rain, more potent than corrasion of streams, 

 and more persistent than the encroachment of the sea. 



As is now well known, areas in which the annual rainfall is less than 

 10 inches, in which typical desert conditions prevail, are little eroded by 

 water, while the degrading effects of wind-scour reach their maximum 

 efficiency. Under these conditions the wind is often the sole eroding 

 agency. It has been lately urged*^ that in the case of arid tracts of 

 southwestern United States general desert-leveling and lowering of the 

 country has been accomplished largely by the wind. The desert ranges 

 are regarded as developed through means of differential eolic effects upon 

 alternating belts of resistant and weak rocks. Between the initial plains 

 level, represented apparently by the tops of existing ranges, and the 

 present general plains surface, represented by the even intermont plains, 

 no less than 5,000 feet of rock material seems to have been removed. 



In the transportation and exportation of this rock waste by means of 

 the wind, quantitative data are now available. The air stream carries 

 fine dust debris many times the volume that rivers do. This being the 



3« Das Gesetz der Wiistenbildung im Gegenwart und Vorzeit, Berlin, 1900. 

 s^Zeitsch. d. dent. geol. Gesellschaft, Ivi Bd., Protokol, 1904, p. 193. 

 38 Zur Oberflachengestallung u. Geol. Deutscli-Ostafrikas, Berlin, 1900. 

 29 Bull. Geol. Soc. America, vol. 19, 1908, p. 53. 

 *o Bull. Geol. Soc. America, vol. 21, 1910, p. 566. 



*i Bull. Geol. Soc. America, vol. 21, 1910, p. 587 ; also Journal of Geology, vol. xvii, 

 1909, p. 31. 



