SURFACE OF THE EAETH 



219. Effect of plants and animals. When the roots of plants 

 grow into crevices of rock, they often exert sufficient force to 

 split it (fig. 98). Many lowly plants, as lichens and mosses, 



FIG. 97. The continental glacier 



The map shows the extent of the continental glacier in North America. After 

 United States Geological Survey 



grow upon the surfaces of rock, and the organic products 

 (acids) formed by them assist in disintegrating the rock. Ani- 

 mal and plant bodies contribute much to soil formation. 



