EROSION PERFORMED BY THE ATMOSPHERE. 329 



progress is transverse to the bounding highlands. It may also 

 be presumed that the wind retards its velocity, when going down 

 an inclined plane. The greater depth of the atmospheric ocean 

 in these instances ought to have the same influence on the gen- 

 eral current as the widening or deepening of a river channel. If 

 this be the case with extensive continental depressions, valleys 

 of rivers and smaller depressions of the earth's surface ought to 

 produce somewhat similar effects in retarding the passing wind 

 and inducing it to give up a part of the dust it may happen to 

 carry along. On the other hand, when the wind passes over 

 land covered by a growth of timber or only tall grass, its lowest 

 part will be held comparatively still and will drop its load. Did 

 the same air remain among the vegetation all the time this 

 unloading process would stop with the first deposit, but as the 

 eddies no doubt keep up a slow but constant exchange with the 

 air above, the accumulation continues as long as there is any 

 dust left. 



Several important deductions can be drawn from the forego- 

 ing considerations. 



The velocities in the atmosphere being so much greater than 

 those obtaining in rivers, lakes, and seas, the distances over which 

 materials may be transported in it will be correspondingly 

 greater. In the sea sediments are carried < out 200 miles and 

 even farther. In the atmosphere, where the velocities often are 

 100 times greater than those in the sea, dust may, no doubt, be 

 transferred a distance of several hundreds, if not a few thousands 

 of miles. The very finest particles may be borne round the 

 earth, as shown by the dust of Krakatoa, or may, indeed, circle 

 about it for some time. 



The greater depth of the aerial ocean renders it but little 

 dependent in its movements on smaller elevations of the land. 

 In a sea five miles deep an elevation of the bottom 8,000 feet 

 high would interpose no serious obstacle to a general forward 

 movement of the whole body of the fluid. Few of our mountain 

 ranges exceed this height, and it would not seem impossible, 

 therefore, that dust in some notable quantities should be carried 



