St4 



THE GUIDE TO NATURE 



FIG. 1. GI.ALIAL JJi iL l.DER. BURLlXCrfOX COXXECTICL'T. 



into fragments. But the process is 

 partly chemical. Oxygen, carbon di- 

 oxide, and water decompose many of 

 the minerals of which the rocks are 

 made. As rain water penetrates down- 

 ward in cracks, it carries in solution 

 the oxygen and carbon dioxide of the 

 atmosphere, and so this chemical de- 

 composition or rotting of the rocks may 



go on, not only at the surface, but in 

 cracks many feet below the surface. 

 Some parts of a rock mass may disin- 

 tegrate more rapidly than others. So 

 blocks of comparatively sound rock 

 may be left, while other portions of the 

 mass have crumbled into a coarse pow- 

 der. The rain, especially on slopes 

 of some degree of steepness, washes 



FIG. 2. GLACIAL BOULDER, BURLINGTON, CONNECTICUT. 



