160 



DYNA3IICAL GEOLOGY. 



of the moving air ; and so it is when the hurricane tears up trees, prostrates 

 forests, unroofs houses, or moves them from their foundations. These de- 

 structive effects are dependent, as already explained, not merely on velocity, 

 but also on the extent, form, and position of the object against which it 

 strikes. The adhesion of the hardened mud along the ruts of a country 

 road may not be overcome by a gale that prostrates forests. 



Besides lifting and transporting loose sands, the heavier winds tear off 

 grains from exposed ledges or bluffs of rock, which the action of the sun, 

 or oxidation, or saline efflorescences, or other means have loosened, and thus 

 carry on the work of denudation. 



2. By means of the material transported. — But the sand, gravel, or stones 

 borne by the winds give them their chief denuding power. Attention was 

 first called to this wind work by W. P. Blake, who described the granite 

 of the Pass of San Bernardino, Cal., as scratched like rocks of glacier 

 regions, even quartz and tourmaline being finely polished, and the garnets 

 left projecting on pedicels of feldspar, inclined in the direction of the wind; 

 limestone as eroded and channeled as if by dissolving waters. Mr. Blake 

 observed, further, that the scratching and polishing effects were not confined 

 to the Pass, but were visible over all parts of the Colorado desert to the 

 eastward, where hard rocks were exposed ; and he dwells on the great impor- 

 tance of this action of the winds as a means of denudation (1855). Later 

 observers have shown that many of the bluffs, needles, and towers of soft 

 sandstone characterizing the scenery in different parts of the Eocky Moun- 

 tain region have been more or less shaped by this means. Moreover, scratches 

 made by drifted sand, long since noticed on the glass of windows on Cape 

 Cod, have been observed in Maine where it is not arid (G-. H. Stone, 1886). 

 In arid parts of India, according to Mr. E,. D. Oldham, they differ from 

 those of glaciers in being deepest at the end facing the wind. 



Eolian denudation has its best examples in the Egyptian and other true 

 deserts where the annual fall of water is very small. The following fig- 

 ures of Egyptian denuda- 

 1^^- tion are from the work 



of J. Walther (1891), 

 which treats the subject 

 with great fullness and 

 gives many illustrations, 

 after personal observa- 

 tions. The differences in 

 hardness of the layers de- 

 termines the rate of wear 

 and leads to nearly the 

 same forms that are produced by running water. In Fig. 155 the beds are 

 Eocene limestone and other kinds. In Pig. 156 Cretaceous beds are upturned, 

 and the harder limestone caps each elevation. The deflation leaves silicified 

 fossils (Exogyra and Corals) projecting over the surface, as in Pig. 156. 



Southwest end of Mokkatam. "Walther. 



