PLAINS OF ARID REGIONS 853 



tinguislicd ; llius . . . the winds in lime would appear to gain the 

 upper hand as agents of erosion and transportation. If such were 

 the case, it would seem that great inequalities of level might be 

 produced by the excavation of wide and deep hollows in areas of 

 weak rocks. As long as the exportation of wind-swept sand and 

 of wind-borne dust continued, no easily defined limit would be 

 found for the depth of the hollows that might thus be developed in 

 the surface, for the sweeping and lifting action of the wind is not 

 controlled by any general baselevel. In an absolutely rainless re- 

 gion there appears to be no reason for doubting that these abnormal 

 inequalities of surface might eventually produce a strong relief in 

 a still-standing land of unchanging climate ; but in the actual deserts 

 of the world there appears to be no absolutely rainless region; and 

 even small and occasional rainfalls will suffice, especially when 

 they occur suddenly and cause floods, as is habitual in deserts, to 

 introduce an altogether dififerent regime in the development of sur- 

 face forms from the rock hills and hollows which would prevail 

 under the control of the wands alone. The prevailing absence of 

 such hill-and-hollow forms, and the general presence of graded 

 wadies and of drainage slopes in desert regions, confirm this state- 

 ment." 



'*As soon as a shallow wand-blown hollow is formed, that part 

 of the integrated drainage system which leads to the hollow will 

 supply waste to it whenever rain falls there ; the finer waste will be 

 blown away, the coarser waste will accumulate, and thus the ten- 

 dency of the winds to overdeepen local hollows will be sponta- 

 neously and effectively counteracted. As incipient hollows are 

 formed in advancing old age, and the maturely integrated drainage 

 system disintegrates into many small and variable systems, each 

 system will check the deepening of a hollow by wind action ; hence 

 no deep hollow can be formed anywhere, so long as occasional rain 

 falls." {12-391-392.) 



With the continuance of the processes and the further disin- 

 tegration of the drainage, the surface is slowly lowered, leaving 

 only those rock masses projecting as monadnocks or "Inselberge" 

 which most eiTectually resist dry weathering. The production of 

 the Inselberg landscape chiefly by eolian agencies has already been 

 considered in an early chapter. 



"At last, as the waste is more completely exported, the desert 

 plain may be reduced to a lower level than that of the deepest 

 initial basin" which originally was a temporary recipient of the 

 waste, "and then a rock-floor, thinly veneered with waste, unre- 

 lated to normal baselevel, will prevail throughout — except where 



