192 ELKMKNTS OF GENERAL SCIENCE 



\\hirh makes our commercial gardens possible; in others, un- 

 drained swamp lands ; and in still others, sand and bare rock. 

 How have these regions come to be as they are ? 



215. The surface and water. If the earth's surface were 

 perfectly level, water which falls upon it would not flow off 

 in any definite direction. With the unevenness that exists, 

 water runs from higher to lower levels, except where absorbed 

 and held by the surface materials. Thus, in the development 

 of the earth's surface, water has accumulated, making pools 



FIG. 93. The solid rock 



Solid rock may always be found at a relatively short distance from the surface. 

 Here the layer of soil is particularly thin. (Presque Isle, Michigan) 



in some places, lakes in others, and oceans in others. Where 

 water runs over the surface of the earth it follows the depres- 

 sions until it finally comes to a place of so low a level that it 

 no longer flows. When the difference in level is great enough 

 between two places which are near one another, water runs 

 rapidly from one to the other. If the difference is abrupt, 

 a waterfall is formed. Falling water wears away the hardest 

 rock if the falling continues long enough. Not only does the 

 water itself wear the rock upon which it falls, but many large 

 and small pieces of rock are carried by it, and these add 

 greatly to its cutting power. The wearing power of water 

 will be discussed more fully later. 



