186 OUTLINES OF FORESTRY. 



Soils maybe divided into gravelly, sandy, clayey, 

 calcareous, and peaty. 



The agencies by which the hard crystalline 

 rocks may be broken up or disintegrated to form 

 soil are, 



1. The expansion produced during the sprout- 

 ing or growing of vegetation. 



2. The alternate contractions and expansions 

 that attend the freezing or thawing of the water 

 that sinks into the rocks. 



3. The cutting or eroding power of running- 

 water charged with suspended mineral matters. 



4. The eroding or cutting power of glaciers. 



5. The solvent power of water containing such 

 gases as oxygen or carbonic acid. 



A plant, during its vigorous growth, by the ex- 

 pansion of its roots, may break or rend a rock, 

 and thus aid in its disintegration. 



The alternate expansions and contractions that 

 attend the thawing or freezing of the water which 

 sinks into a rock gradually break the rock into 

 fragments, and thus aids in the formation of soil. 



During the gradual movements of glaciers down 

 the mountain valleys, the fragments of hard rocks 

 lodged in the ice cut or grind the rocks which 

 form the sides of the valleys through which the 



