RELATION OF GLACIAL AND EROSION CONDITIONS 543 



tion effects might be soon rendered quite conspicuous and even the eolic 

 erosion of the protruding rock hills might soon appear appreciable. 



The association of the origin of the continental glaciers with eolic 

 acti\dties is no doubt more intimate and far-reaching in its physio- 

 graphic bearing than might be at first glance supposed. During a 

 greater part of the year arctic conditions are essentially desert condi- 

 tions. The identity and nature of the rock weathering in the two cli- 

 mates has been already noted. The peculiar dry, powdery character of 

 arctic snows are comparable in all respects to the fine dusts and sands 

 of arid regions. In both instances the main effects of the wind on the 

 loose materials are the same. Until the dry snows, through partial melt- 

 ing, consolidate into ice, they remain in the same condition as desert 

 dusts before they become exposed to moisture. That the one should 

 accumulate into vast continental ice-fields and the other into vast even- 

 surfaced mantles of continental sedimentaries is a fact which strictly 

 accords with the theoretic expectations of eolic action. 



It appears that considerably over one-half of the land area of the 

 globe is profoundly affected by eolic agencies. Murray-''' estimates that 

 not less than one-fifth of the land surface is occupied by desert. At 

 least another one-fifth is subject to greater or less accumulations of con- 

 tinental deposits of one sort or another in which wind-borne dusts form 

 no inconsiderable part.^' Perhaps another one-fifth is or was within 

 recent geological times covered by snow-fields and physiographically is 

 to be considered as truly desert as the Sahara. 



Essential Features of Aridity 



The peculiarities of an arid climate are generally described in terms 

 of normally humid conditions. Contrasted with those of moist climate 

 they have been lately especially characterized by Davis. -^ With particu- 

 lar reference to southwestern United States they have also been briefly 

 noted by me.^® To these papers further reference is subsequently made. 



Contrasted Characteristics of arid and humid initial Stages 



SURFACE OFFERED TO SCULPTURING AGENCIES 



Although the earth's crust with any structure, any form, and any alti- 

 tude is postulated in the beginning for the normal or moist geographic 

 cycle, the ideal and most complete cycle demands a recently upraised 



28 Science, n. s., vol. xvi, 1890, p. 106. 

 27 Bull. Geol. Soc. America, vol. 22, 1911. p. 688. 

 2« Journal of Geology, vol. xlil, 1905, p. 382. 

 »BuIl. Geol. Soc. America, vol. 21, 1910, p. 568. 



