THE GEOGRAPHICAL CYCLE IN AN ARID CLIMATE 385 



in Arizona and Oregon — while fine dust carried aloft in whirlwinds 

 and dust-storms is spread about by the upper currents with little 

 regard to the slopes of the land surface far below. Sand may be 

 drifted, and dust may be in this way carried outside of the arid region 

 from which it was derived. Wind-erosion may, furthermore, tend to 

 produce shallow depressions or hollows; for the whole region is the 

 bed of the wind, and is therefore to a certain extent analogous to the 

 bed of a river, where hollows are common enough; but in the early 

 stages of the cycle in a region where the initial relief was strong, the 

 action of the wind is not able to make hollows on the original slopes 

 that are actively worked upon, and for a time even steepened, by 

 streams and floods. Hence in the youthful stage wind-blown hollows 

 are not likely to be formed. 



It is important to notice that a significant, though small, share of 

 wind-swept or wind-borne waste may be carried entirely outside of 

 or "exported" from an arid region. It may be deposited on neigh- 

 boring lands, where it will be held among the grass of a less arid 

 climate, as long ago suggested by Richthofen; it may even be held 

 down on coastal lands by the dew, as has been suggested for certain 

 districts in Morocco by Fischer; it may fall into the sea, as is proved 

 by the sand that gives a ruddy tinge to the sails of vessels in the 

 Atlantic to leeward of the Sahara, and by the sand grains that are 

 dredged up with true pelagic deposits from the bottom of that part 

 of the Atlantic. It may therefore be expected that the progress of 

 erosion and waste exportation in a desert region should be associated 

 with the deposit of fine waste, as in loess sheets, on the neighboring 

 less arid regions, especially down the course of the prevailing winds. 

 In regions of weak and variable winds the process of sand and dust 

 exportation must be extremely slow; in regions of steady winds it 

 must still be vastly slower than the ordinary rate of waste removal 

 in young or mature regions of plentiful rainfall and normal rivers. 

 Yet it is by this slow process of exportation that the mean altitude of 

 an arid region, such as is here considered, will be continually 

 decreased; hence the earlier stages of the arid cycle are expectably 

 longer than the corresponding stages of the normal cycle. 



In the normal cycle the youthful stage is - characterized by the 

 headward growth of many subsequent streams, chiefly along belts 



