EXPLANATIONS OF PLAINS FORMATION 695 



tracts, where the exported dusts of the deserts come to rest under condi- 

 tions of moist climate and abundance of vegetation. Such deposits are 

 now believed to be very much more extensive than there is at the present 

 time any general notion of. Among the examples specifically enumerated 

 are the so-called fresh-water Tertiaries of the Great Plains, certain loamy 

 deposits around the Caspian Sea, extensive siltlike formations in north- 

 ern China, and some of the coarser deposits bordering the Nile. 



Origin of folic Continental Deposits 

 relations of areas of denudation and deposition 



Although the activity of the wind as a geologic agent is widely ad- 

 mitted, its potency as an erosive power comparable to that of the river or 

 the sea is not so generally recognized. From the recent literature on 

 eolic formations it is to be inferred that wind-formed deposits are laid 

 down within the limits of the arid regions themselves. Such inference 

 is quite incorrect. The areas of eolic denudation and deposition are as 

 distinct and as widely separated from each other as are the corresponding 

 fields of streams. 



Since deserts are mainly areas of rapid degradation, extensive terranal 

 accumulation is not to be expected ; they are the tracts of maximum defla- 

 tion. The dust-ladened air currents flow outward in all directions from 

 the desert. Just as the rivers radially leave a chief mountain uplift and 

 pursue their courses toward the sea. — their areas of deposition. The loads 

 of the air streams are finally dropped in the semi-arid and moist climate 

 belts far outside of the areas where desert conditions prevail. In every 

 desert area great sand-dunes there are on every hand, low mounds and 

 low ridges of finer materials frequently occur, and the entire surface is 

 mantled by a fine pulverulent loam ; but all these are quite ephemeral in 

 character. They are soon swept away or exported beyond the desert 

 boundaries. They are constantly replaced by other accumulations of 

 like nature and magnitude. 



It can not be too often emphasized that areas of deflation and of aero- 

 position are not identical; that they overlap at but few points, and that 

 the one represents destructive erosion, the other constructive gradation. 



CLIMATIC PECULIARITIES 



Of the climatic conditions peculiar to desert regions only one or two 

 points need be referred to here. The factors highly influential in the 

 production of materials for eolic deposits are deficient rainfall, clear 

 skies, high evaporation, great range of diurnal temperatures, and sparse 



