390 W. M. DAVIS 



sufficient volume to represent the great volume of material removed 

 in the degradation of the plateaus. 



In the case of truncated uplands elsewhere — that is, uplands whose 

 surface truncates their structure, as in the central plateau of France — 

 it is generally a tacit postulate, if not a proved conclusion, that the 

 climate during their truncation was not arid, and hence it is inferred 

 that they were worn down as peneplains with respect to normal 

 baselevel, and that they have been uplifted since; this aspect of the 

 problem will be considered farther on. In the meantime, there is 

 another aspect of erosion in arid regions which, to my knowledge, 

 has not, until recently, received attention. 



The beginning of old age. — During the advance of drainage inte- 

 gration the exportation of wind-borne waste is continued. At the 

 same time, the tendency of wind-action to form hollows wherever 

 the rocks weather most rapidly to a dusty texture would be favored 

 by the general decrease of surface slopes, and by the decrease of 

 rainfall and of stream-action resulting from the general wearing- 

 down of the highlands. Thus it may well happen that wind-blown 

 hollows should be produced here and there, through the mature and 

 later stages of the cycle, and that they should even during early 

 maturity interfere, to a greater or less degree, with the development 

 of the integrated drainage, described above. In any case, it may be 

 expected that wind-blown hollows would in late maturity seriously 

 interfere with the maintenance of an integrated drainage system. 

 Thus it appears that, along with the processes which tend toward 

 the mature integration of drainage, there are other processes which 

 tend toward a later disintegration, and that the latter gain efficiency 

 as the former begin to weaken. A strong initial relief of large pat- 

 tern, a quality of rock not readily reducible to dusty waste, and an 

 irregular movement of light winds might give the control of sculpture 

 to the intermittent streams through youth and into maturity; in such 

 a case maturity might be characterized by a fully integrated system 

 of drainage slopes, with insignificant imperfections in the way of 

 wind-blown hollows. In a second region an initial form of weaker 

 relief, a quality of rock readily reducible to dust, and a steady flow 

 of strong winds might favor the development of wind-blown hollows 

 or basins, and here the process of drainage disintegration would set 



