THE FORMATION OF SOIL. 53 



aided by the chemical action of such gases as 

 oxygen and carbonic acid gas dissolved in the 

 water. 



During the vigorous growth of any form of 

 plant life the increase in the length and diameter 

 of the roots will break up or disintegrate even the 

 hardest of rocks. This action is especially effec- 

 tive from the fact that in many cases the roots ex- 

 tend for the greater part through crevices or cracks 

 where the quantity of moisture is greater than 

 elsewhere. 



The effects of alternate expansion and contrac- 

 tion are limited to climates where the temperature 

 occasionally falls below the freezing-point of water. 

 The water sinking into the porous rocks, and fill- 

 ing the crevices and cracks between them, expands 

 on freezing and breaks the rock into fragments. 

 These fragments are afterwards broken into smaller 

 fragments, until the pieces are sufficiently small to 

 be carried by the winds and waters to distant 

 localities. 



The ability of running water charged with sus- 

 pended mineral matter to cut or wear away hard 

 rocks is very great. The moving water carries 

 mechanically suspended in it minute fragments of 

 such hard minerals as angular fragments of quartz 



6* 



