THE SURFACE OF THE EARTH 



169 



constituents of the air may also attack rocks chemi- 

 cally, but oxygen is the most powerful. 



Work of rain in causing 1 relief. All have observed 

 that streams are muddy after a heavy rainfall. This 



PIG. 65. 



Permission of United States Forest Service. 

 Gullies in a hillside formed by running water. 



muddiness is caused by the rain carrying soil into 

 the streams. With every rain more or less earth is 

 discharged into streams; the total effect of rain ero- 

 sion is enormous. Perhaps rain has had more to do 

 with producing relief features than any other one 

 agent. Its effect as an erosive agent is more notice- 



