18 THE PRINCIPLES OF SOIL MANAGEMENT 



forms, as a result of the varying hardness of the rock 

 and the uneven distribution of the particles. The 

 abraded particles are born along by the wind and be- 

 come new tools of destruction. 



In humid regions this form of disintegration is less 

 prominent, but in sandy regions it performs some ef- 

 fective work. As an example of this effectiveness, 

 Merrill describes a large sheet of plate-glass, once a 

 window, in a lighthouse on Cape Cod, well known for 

 its sand-dunes. During a severe storm, of not above 

 forty-eight hours' duration, this became on its exposed 

 surface so ground by the impact of grains of sand 

 blown against it as to be no longer transparent, and to 

 necessitate its removal. He reports that window-panes 

 in dwelling-houses in the vicinity are frequently drilled 

 quite through by the same means. 



Material blown about by wind is very much rounded 

 and smoothed by the impacts to which it has heen sub- 

 ject, a characteristic very much less in evidence in 

 water-moved material of the same fineness. 



Winds also act in conjunction with plants where the 

 roots have penetrated into a crevice or joint, using the 

 tops as a lever to push off or further fracture masses of 

 rock. This process is most effective in rough mountainous 

 regions where the larger vegetation is just getting a foot- 

 hold. In passing, attention may be called to this process 

 of overturning plants as one of nature's cultural methods, 

 whereby the soil is subjected to very thorough, if long- 

 drawn-out, tillage. 



7. Heat and cold. In general, heat accelerates all 

 chemical processes. It greatly increases the solvent 



