GENERAL SURFACE CONFIGURATION OF THE EARTH. 169 



in a globe of two feet in diameter would be less than one one-hundredth 

 of an inch— an amount that would be scarcely perceptible. If a 

 globe of clay or of stone of this size were heated to incandescence and 

 in this condition ground to a true sphere and then allowed to cool, it 

 is probable that the inequality would be as great as or greater than the 

 above.* 



It is only the greatest inequalities, viz., land-surfaces and sea-bot- 

 toms, which we account for in this way. Mountain-chains are certainly 

 formed by a different process, which we will discuss under that head 

 (p. 250) ; and it is even possible that the causes which operate to 

 produce mountain-chains may also produce these greatest inequalities. 



The continuance of these causes would tend constantly to increase 

 the extent and height of the land, and to increase the depth, but dimin- 

 ish the extent of the sea. This, on the whole, seems to have been the 

 fact, during the history of the earth, as will be shown in Part III. 

 Nevertheless, local causes, both aqueous and igneous, as already shown 

 in Part I, have greatly modified the general contour, both map and 

 profile, given by secular contraction. 



Laws of Continental Form. — That the general contour of continents 

 and sea-bottoms has been determined by some general cause, such as 

 secular contraction, affecting the whole earth, is further shown by the 

 laws of continental form. The most important of these are as follows : 



1. Continents consist of a great interior basin, bordered by elevated 

 coast-chain rims. This typical form is most conspicuously seen in 

 North and South America, Africa, and Australia. Europe-Asia is 

 more irregular, and therefore the typical form is less distinct. We 

 give in Fig. 134, A and B, an east-and-west section of North America 

 and of Australia, as typical examples of continental structure. 



Fig. 134.-^4, Section across North America; B, Section across Australia (after Guyot). 



The great rivers of the world, e. g., the Nile, Mississippi, Amazon, 

 La Plata, etc., drain these interior continental basins. 



* To this, according to Faye (Comptes Rendus, vol. xc, p. 1185, 1880), must be added 

 still another cause. As soon as the water collects in the depressions formed by un- 

 equal radial contraction, its very presence would tend to increase the cooling and con- 

 tracting of these parts, and thus to deepen still further the depressions. This effect 

 results (1) from the greater conductivity of water as compared with rock, and (2) from 

 the circulation of ice-cold water from the poles along the sea-bottom (p. 40). 



