2 >\ ll:^\-y^R:Tl^L FARMING 



The breaking down of the original rocks has been accom- 

 plished by very simple means, the action of which has been very 

 powerful. 



Work of the Atmosphere. Everyone knows how a piece of 

 iron is attacked and falls into a powdery iron dust, which is 

 nothing but iron combined with oxygen taken from the air to 

 form a different substance called iron oxide. The oxygen, 

 carbon dioxide and other gases of the air attack the iron, lime 

 and other elements in the rocks, forming new substances and 

 causing the particles to fall apart, as is the case of the iron 



U. S. G. S. 



EFFECT OF WIND ACTION ON ROCKS 



rusting. Rocks are also carved, eroded and worn away by the 

 cutting and sawing action of the wind, especially when it carries 

 with it any considerable amount of dust or sharp sand particles. 

 In this way large rocks are sometimes entirely worn away. In 

 some localities in this country the sand is swept across the level 



