568 C. R. KEYES — RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



6. Soft substructure of the plains which constitute four-fifths of the 

 entire desert area;® 



7. Eemarkable beveled rock structure of the plains surface;^® 



8. Plains character of the interment rock-floors themselves ;^^ 



9. Eepresentation of former plains-levels by the plateau-plains;^^ 



10. Eemarkable thinness of the surface mantle of many plains; 



11. Transported nature of the surface materials ; 



12. Marked tendency of the surface mantle to make the plains 

 smoother ; 



13. Gravelly character of the surface mantle only apparent; 

 • 14. Notable absence of normal rock-weathering; 



15. Absence of distinct waterways on the plains ;^^ 



16. Extent of sheetflood transportation.^* 



There are other major features which need not be enumerated in the 

 present connection, and many minor characteristics of which even a list 

 would be of great interest. Some of these desert features have not been 

 especially noted. The origin of many of them has been ascribed to the 

 same erosive processes which are most active in the more familiar humid 

 regions. Only recently have the older interpretations of desert structures 

 been questioned. The later writings of Walther, Spurr, McGee, Passarge, 

 Davis, Penck, Keyes, and Cross contain many statements suggesting 

 that the new explanations must now be sought if we are to satisfactorily 

 account for many of the physiographic aspects of the desert regions. 

 That the features mentioned are distinctive of arid countries and find no 

 counterpart in the humid lands is only beginning to be generally appre- 

 ciated. Eolative phenomena are as varied and as peculiar to the desert 

 as are the ordinary erosional effects by running waters under normally 

 wet climatic conditions. 



The critical criteria for recognizing eolative effects are discussed at 

 length in another place. Typical views of some of the most character- 

 istic relief features I have presented in a recent bulletin of the Geological 

 Society of America,^^ to which reference may be made. 



Peculiarities of an Arid Climate 



In the present connection only the more striking features of an arid 

 climate need be mentioned as bearing directly upon the phenomenon of 



9 Bulletin of the Geological Society of America, vol. 19, 1908, p, 76. 



10 American Journal of Science (4), vol. xv, 1903, p. 207. 



" Engineering and Mining Journal, vol. Ixviii, 1904, p. 670. 



^ Proceedings of the Iowa Academy of Sciences, vol. xiii, 1908, p. 221. 



" American Geologist, vol. xxxiv, 1904, p. 160. 



"American Journal of Science (4), xxiv, 1907, p. 467. 



15 Bulletin of the Geological Society of America, vol. 19, 1908, p. 570. 



