PART IY. THE EARTH'S CRUST 



CHAPTER XVIII 

 NATURAL FORCES UPON THE SURFACE OF THE EARTH 



214. The surface of the earth. In previous studies both 

 land and water have been discussed in connection with other 

 topics. It is a matter of general knowledge that approximately 

 two thirds of the earth's surface is water and the rest is land ; 

 or, to state it differently, about two thirds of the solid surface 

 of the earth is covered by water. The water has different 

 depths in different places. The floors of oceans and lakes, 

 like the uncovered land surfaces, are undulating in nature. 

 Sometimes whole mountain ranges and the intervening valleys 

 are covered by the same body of water. 



Conspicuous irregularities in the land surfaces are easily 

 noted in mountain regions, and even in the most level plains 

 some irregularities in the surface appear. Such unevenness 

 may be abrupt or gentle in slope, and its nature often gives 

 the distinctive character to a region. 



In places the surface is readily seen to consist of solid rock 

 (fig. 93), and solid rock may be found anywhere if the surface 

 material is penetrated. For example, the area upon which 

 the city of Chicago is located varies from places where solid 

 rock lies bare upon the surface to places where the sand and 

 clay must be penetrated more than 100 feet before solid rock 

 is found. It is thought that ages ago, when the earth's sur- 

 face was newly exposed, the surfaces were all solid rock. A 

 study of how the present conditions have developed presents 

 most interesting questions. In some parts we find good soil 

 and abundant natural vegetation; in others, rich garden soil 



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