278 THE EARTH'S SURFACE AND PLANT LIFE 



the same forces are doing both. It is impossible to tell 

 which set of forces is of the greatest service to man; be- 

 cause without either, life could not continue. 



It is believed that the whole surface of the earth originally 

 hardened from a molten condition, just as lava from a volcano 

 hardens when it cools. We have seen that the waters of the 

 sea and the waters that run over the land are wearing away 



ROCK SPLIT BY ROOTS OF TREE 



the rocks, grinding them together, pulverizing them, and 

 carrying the wreckage to other places. This eroding must 

 have begun as soon as the earth's crust became cool enough 

 for the waters of the atmosphere to condense. 



It is necessary, however, to take into account not only the 

 power of water " to wear away the stones," but also its 

 ability to hold many substances in solution and to carry them 

 away to places where the water is evaporated and the dis- 



