WIND-GRAVED MESAS 237 



By wind-action alone there appears now to be incontestable testi- 

 mony that from the entire area of the vast arid region there has been 

 lifted and exported by the winds in very recent geologic times a prodi- 

 gious layer of rock not less than 5,000 feet in thickness. With a con- 

 tinuance of the present climatic conditions another volume of rock- 

 materials of equal dimensions seems allotted to be similarly transported 

 before the effects of the process shall become appreciably diminished. 



Thus it is that the arid regions have introduced to us an erosive 

 agent more potent than stream-corrasion, more constant than the wash- 

 ings of the rains, more extensive and persistent than the encroachments 

 of the sea. The present conception of general eolic erosion, a sculptur- 

 ing power in every way comparable to erosion by river and by ocean, 

 appears destined to take its place among the first half-dozen great and 

 new thoughts which shall especially distinguish geologic science of the 

 twentieth century. 



