60 THE PRINCIPLES OF SOIL MANAGEMENT 



Further west, where the country is less rugged and 

 the rocks less refractory, the soil covering is of greater 

 depth and generally less stony. In the states of the 

 Mississippi valley, the broad, level areas of excellent 

 agricultural soil are very largely the result of these 

 glacial influences. 



21. Wind or seolian soils. Attention has been di- 

 rected to the transporting power of wind. It is continu- 

 ally picking up particles, which are deposited in accord 

 with the same general laws which govern water deposits. 

 The material thus carried, often to great heights, is 

 again brought to the surface by gravity. These particles 

 are frequently accelerated in their fall by rain and 

 snow. Every particle of fog, of rain and of snow has 

 for its nucleus a particle of dust around which con- 

 densation began, and for this reason the atmosphere 

 is always most clear after precipitation. Large amounts 

 of material are, in the course of time, brought to earth 

 in this way. 



This continual deposition from the atmosphere is 

 illustrated by the layer of dust that quickly accumu- 

 lates in any unoccupied building, however tightly it 

 may be closed. 



Besides this general filtering of dust particles from 

 the atmosphere, there is the definite drifting of soil 

 by wind, of which sand-dunes are the most common 

 illustration. These occur in many parts of the world. 

 They are likely to be developed wherever dry sand is 

 exposed to the wind. 



Related to these modern wind deposits are immense 

 areas of soil of great agricultural value, the origin of 



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