14 AGRONOMY 



readily distinguish the newly quarried blocks from older ones, 

 by their fresh look. The carbon dioxide in the air unites with 

 water to form carbonic acid, and this also attacks various 

 elements in the rocks and tears them from their compounds. 



Water breaks up the rocks by dissolving out the more 

 soluble compounds. Pure water wears the rock very slowly, 

 but when it contains, as it often does, carbon dioxide or 

 other acids derived from the humus or the roots of plants. 



FIG. 2. Underground channel in limestone made by water, Joliet, Illinois 

 A portion of the rock has heen dissolved and carried away 



it is a most powerful agent in weathering. Limestones, espe- 

 cially, are quickly dissolved by such means, and the occur- 

 rence of caves and underground channels in these rocks is 

 thus explained. Here and there water charged with dissolved 

 minerals may come to the surface and we then have min- 

 eral springs. Sandstones, though usually more enduring than 

 limestones, are often rapidly disintegrated by having the ma- 

 terials which bind the sand grains together dissolved. Rain 

 water usually contains small amounts of ammonia and nitric 

 acid, and these, like the carbonic acid, are active in dissolving 



