12 AGRICULTURE. 



II. Water. 



Among the factors of soil formation none is 

 o-reater than that of water in its various phases 

 — as, rain, rill, river, lake, and sea ; frost, ice, 

 avalanche, and glacier. 



I. Chemical Action. — In many of its forms 

 water exerts a violent and stupendous force, 

 but there is a silent and subtle force whose re- 

 sults are often overlooked. It is the great sol- 

 vent power of water. It absorbs both oxygen 

 and carbon dioxide from the air, and these give 

 it great chemical power in dissolving, or de- 

 composing, rock substances.* The simplest 

 effects are the uniting of oxygen and of water 

 with the minerals composing the rocks. But 

 as the rain sinks into the ground it is provided 

 with new weapons through the absorption of 

 the humic acids and, possibly, of alkaline sub- 

 stances. For this reason, many rocks disinte- 

 grate more rapidly under ground than they do 

 when exposed to the atmosphere. 



Calcium carbonate, or limestone, is the sub- 

 stance dissolved or decomposed in the greatest 

 quantity ; but magnesium carbonate (MgCOj), 

 organic matter, silica (SiO_,), and many other 

 substances are held in solution by clear river 

 water. (See " Field Exercise No. i," Part 2.) 



*" Perfectly pure water has very little effect, but perfectly 

 pure water does not exist in nature." — Scott's Geology. 



