320 THE JOURNAL OF GEOLOGY. 



Wind erosion becomes geologically importaiit o?ily in certain locali- 

 ties, and the conditions favori?ig it are a dry climate and a topography 

 of abrupt a?id broken reliefs. 



On plains where the ground is dry and vegetation scanty or 

 absent, ordinary strong winds are apt to slowly wear into the 

 soil, where the roots of plants do not protect it. If such soil 

 contains sand which is too coarse to be lifted up and carried 

 away, dunes are formed, and the uneven topography thus devel- 

 oped still more favors wind erosion; for it is evident that the slopes 

 of the dunes will be struck with greater force than the even sur- 

 face of a level plain. In such places the sand grains are tritura- 

 ted and worn, and the abraded material is promptly removed. 

 It is also evident that where a country is traversed by vertical 

 escarpments and cliffs, and steep slopes, strong eddies are set up 

 as the wind strikes these reliefs. Where the rocks are of fine 

 materials and but little indurated, like most of the Mesozoic and 

 Cenozoic beds of the west, it would be singular if such eddies 

 did not erode the bare surfaces of their outcrops. It does not 

 appear practicable to estimate separately the erosion produced 

 by impact of the air alone, and the abrasion produced by the 

 materials carried. The ratio between the two will, of course, 

 vary with the quantity of the load. Where this is considerable, 

 abrasion is no doubt proportionally greater than in water, for the 

 speed of the impinging particles is here much higher, and their 

 striking force consequently greater. Occasionally this circum- 

 stance greatly intensifies aerial erosion and produces a natural 

 sand-blast, which is very effective in its action on solid rock. 

 That such abrasion becomes appreciable and important along the 

 escarpments of " mesas " in dry regions appears not to admit of 

 a doubt. In such places the driven sand may sometimes be felt 

 smiting the exposed skin of the traveler. 



The speed of the wi?id being lowest ?iear the surface of the ground, 

 materials m.ust by some means be lifted through this zone of low veloc- 

 ity in order to be tra?isported any considerable distance by the atmos- 

 phere. 



According to some observations made by Stevenson, the aver- 



