334 



GENERAL SCIENCE 



nature of these soils, aside from the organic matter in them 

 and the material in the rock originally, also depends upon 

 the chemical changes that may have accompanied or fol- 

 lowed the disintegration of the rocks. 



Water is perhaps the chief agent by means of which the 

 rocks have been broken down and reduced to a finely divided 



* 



FIG. 112. Stratified rock, the origin of which dates back to a time 

 when the material in it was deposited as a sediment in layers originally 

 horizontal. 



condition. By its freezing and thawing, by its solvent action 

 and its powers of erosion when in motion, even the hardest 

 of rocks are slowly worn away. 



Rocks exposed to the direct rays of the sun on mountain 

 sides are subject to rapid expansion and contraction of their 

 surface layers due to daily temperature changes. As they 

 crumble the fragments are blown away, or are washed down 

 the slopes. Carbon dioxide gas in solution, and waters 



