GENERAL SURFACE CONFIGURATION OF THE EARTH. 167 



operated atmospheric, aqueous, and organic agencies — and therefore 

 on this insignificant crust have been recorded all the most important 

 events in the history of the earth. 



4. General Surface Configuration of the Earth. 



The surface inequalities of the earth are of two general kinds, 

 which may be called greater and lesser. The one is due to interior, 

 the other to exterior causes ; the one to igneous, the other to aqueous 

 or erosive agencies. The lesser inequalities we will treat under the 

 head of forms of sculpture (p. 266). Our discussion now is limited 

 to the greater. Again, these are of two orders of greatness, viz., those 

 which constitute land-masses and ocean basins, and those which con- 

 stitute mountain-ranges and intervening valleys. These latter we 

 shall treat fully hereafter (p. 250) ; we are therefore specially con- 

 cerned now with the former. 



Nearly three quarters of the whole surface of the earth is covered by 

 the ocean. The mean height of the continents, according to the most 

 recent results, is as follows : Europe, 984 feet ; Asia and Africa, 1,640 feet ; 

 America, North and South, 1,083 feet ; Australia, 820 feet. The mean 

 height of all land is given as about 1,378 feet.* These figures are con- 

 siderably greater than those given by Humboldt and heretofore adopted. 



The mean depth of the ocean is probably 12,000 to 15,000 feet 

 (Thompson). There is probably water enough in the ocean, if the 

 inequalities of the earth's surface were removed, to cover the earth to 

 a depth of about two miles. 



The extreme height of the land above the sea-level is five miles, 

 and the extreme depth of the ocean is at least as much. The extreme 

 relief of the solid earth is therefore not less than ten miles. 



Cause of Land-Surfaces and Sea-Bottoms.— The most usual idea 

 among geologists as to the general constitution of the earth is that the 

 earth is still essentially a liquid mass, covered by a solid shell of twen- 

 ty-five to thirty miles in thickness ; and that the great inequalities, 

 constituting land-surfaces and ocean-bottoms, are produced by the 

 upbending and down-bending of this crust into convex and concave 

 arches, as shown in Fig. 132. The clear statement of this view is 

 sufficient to refute 



the arches with Fl6 132 "^" 



which we are here 



dealing have a span of nearly a semi-circumference of the earth, it 



becomes evident that no such arch, either above or below the mean 



* Kriimmel, American Naturalist, vol. xiii, p. 464, 1879. 



