92 GEOLOGY 



Since dust is carried to a considerable extent in the upper air, 

 its movements and its deposition are little affected by obstacles 

 on the surface of the land, and when it falls it is spread more or less 

 uniformly over the surface where it settles. 



Much of the dust transported by the wind is carried out over 

 seas or lakes and falls into them, causing sedimentation over the 

 bottom of the whole ocean, and at the bottoms of all lakes. The 



Fig. 54. Shows the effect of sage-brush or other similar vegetation in hold- 

 ing sand or earth, or in causing its lodgment in dry regions. 



amount of dust blown into the sea cannot well be determined, 

 but it is safe to say that, if such determinations were possible, the 

 result, if stated in terms of weight, would be surprisingly great. 



Sand 



The wind does not commonly lift sand far above the surfnco 

 of the land, and its movement is therefore interfered with by sur- 

 face obstacles, more than are the movements of dust. A shrub, 

 a tree, a fence, a building, or even a stone may occasion the lodg- 

 ment of sand in considerable quantity, though it has little effect 

 on the lodgment of dust. If the obstacle which occasions the 

 lodgment of sand presents a surface which the wind cannot pen- 

 etrate, such as a wall, sand is dropped abundantly on its wind- 



