280 ELEMENTS OF GENERAL SCIENCE 



short slopes, long ridges or round knolls, etc. gives dis- 

 tinctive character to a landscape and does much to determine 

 the type of agriculture in a region. 



In places the surface is readily seen to consist of solid 

 rock (fig. 136), and solid rock may be found anywhere if the 

 surface material is penetrated. It is thought that ages ago, 

 when the earth's surface was newly exposed, the surfaces 

 were all solid rock. A study of the development of condi- 

 tions which we now see presents most interesting questions. 



Fns. 18<i. 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 parts we find good soil and abundant natural vege- 

 tation ; in others, rich garden soil which makes our com- 

 mercial gardens possible ; in others, undrained swamp lands ; 

 and in still others, sand and bare rock. How have these 

 regions come to be as they are ? 



281. 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 

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



