176 STRUCTURAL GEOLOGY. 



insignificant portion of the earth upon which to found a 

 science of the earth. Bnt it must be remembered that on 

 this superficial portion has been enacted, and in its struc- 

 ture has been recorded, the whole history of the earth. 



General Surface Configuration of the Crust. The 

 crust of the earth is diversified by greater and smaller 

 features. The greater features are due to interior or 

 elevating, the lesser to exterior or leveling agencies. Under 

 the former head come those greatest features, constitut- 

 ing continental surfaces and oceanic bottoms, and those 

 next greatest, viz., mountain-chains and great valleys. 

 Under the latter come all those peaks and ridges, valleys 

 and ravines, which have been produced by subsequent 

 erosion. 



The mean height above the sea-level of the continents 

 is about 1,200 to 1,300 feet, or less than one fourth mile, 

 and the mean depth of the ocean-bottoms below the same 

 level is about 15,000 or 16,000 feet, or nearly three miles. 

 The ocean-surface being nearly three times as great as the 

 land-surface, it is evident that, if the inequalities of the 

 crust-surface were removed, there is water enough to 

 cover the whole earth more than two miles deep. 



General Laws of Continental Form. There are 

 certain general laws of continental form which have a 

 bearing on the question of the origin of continents, and 

 which, therefore, must be briefly mentioned. 



1. Continents consist essentially of Interior 

 Basins, with Coast-Chain Kims. The interior basins 

 are drained by the great rivers of the world. This typical 

 structure is well shown in America, North and South, in 

 Australia, and in Africa. For example, in North America 

 we have the great interior basin drained by the Mississippi 

 River, bordered on the Atlantic side by the Appalachian, 

 and on the Pacific side' by the great Rocky Mountain sys- 

 tem or American Cordilleras, consisting of many ranges, 

 of which Colorado, Wahsatch, and the Sierra and Coast 



