CHAPTER II 

 PHYSICAL CHANGES OF THE OUTER SHELL 



The earth's crust. In studying the solid part of the earth, 

 we are necessarily limited to a thin shell near the surface. 

 In the deepest mines and canons we may go down to a depth 

 of a little more than a mile. By the slow denudation of the 

 uplifted lands, rocks which were once buried to a depth of 

 several miles may be uncovered at the surface. This outer 

 shell, which alone is open to investigation, is the subject of 

 the present Chapter. It has often been called the " crust of 

 the earth," in allusion to an older theory that the interior is 

 so hot as to be liquid, but is covered by a thin, solid crust. 

 Although this theory has been largely abandoned, the 

 term is convenient, and we shall use it to mean simply 

 the outer part of the earth, which is partially open to obser- 

 vation. 



Surface features of the crust. The outside of the earth 

 has an irregular surface. A glance at a model of the globe 

 shows that there are several broad, smooth tracts, which are 

 sunk on an average of about two and one half miles below the 

 surface of the sea. These are the great ocean basins. Be- 

 tween them large plateaus stand out in relief (Fig. 43). The 

 so-called continents are merely the portions of these plateaus 

 that are now out of water, and hence are land. The great 

 surface features of the earth are, then, the oceanic depressions 

 and the continental plateaus. 



Upon examining these major features in more detail, we 

 find that the surface of the land is notably rougher than that 

 of the sea bottoms. On the former we see mountains, ridges, 

 and minor plateaus, with their complementary valleys, basins, 



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