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



the mutation of land forms in the humid countries. One of the great 

 principles thus formulated has been so suggestive of tangible results in 

 the land of its birth that it has done more, perhaps, than any one factor 

 to cast into shadow the other. The geologic processes whereby the second 

 basic principle manifests itself have been until very lately almost wholly 

 overlooked. 



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Figure 1. — Geographic Provinces of Arid United States and Northern Mexico 



It is the arid region which has introduced to us an erosive agent more 

 potent than corrasion, more constant that the working of the rains, more 

 extensive and persistent than the encroachment of the sea. Eolian ero- 

 sion, wind scour, or eolation, must ever grow in effectiveness as the his- 

 tory of our globe goes on and erosion lasts. It may be that in the linea- 

 ments of our moon we see the final effects of eolic powers. 



