52 OUTLINES OF FORESTRY. 



"Where the soil remains directly on the surface of 

 the rocks from which it was derived, it is interest- 

 ing to trace the gradual changes that occur in 

 passing downward from the loose soil to the hard, 

 unchanged rock below. On top is the loose soil, 

 with the admixture of vegetable humus so neces- 

 sary to the growth of the higher forms of vege- 

 table life. Under this is thoroughly broken-up 

 rock, which contains less vegetable matter. Under 

 this is coarser and less broken-up rock. Under 

 this the rock is intact and merely softened by 

 the agencies effecting the disintegration. Finally, 

 lying under all, is the still untouched virgin rock. 



The principal agencies causing the disintegration 

 of rocks are : 



1. The expansive force which sprouting or grow- 

 ing vegetation exerts on the rock. 



2. The alternate expansions and contractions 

 that attend the freezing and thawing of the water 

 which flows into the crevices between the rocks, 

 or sinks into their porous structure. 



3. The erosion or cutting power of water charged 

 with suspended matter or sediment. 



4. The erosion or cutting of glaciers or masses 

 of moving ice. 



5. The solvent power of water, especially when 



