V 8 GEOLOGY 



of about 26.8 miles more. Its equatorial circumference is 24,902 

 miles, its meridional circumference 24,860 miles, and its surface 

 area about 196,940,700 square miles. Its average specific gravity 

 is about 5.57. The oblateness of the spheroid is the result of the 

 rotation of the earth. Computations seem to indicate that the 

 equatorial bulge is very nearly what it would be if the earth were 

 in a liquid condition. From this the inference has been drawn 

 that the earth was in that condition when it assumed its present 

 form. It is thought by others, however, that the plasticity of the 

 earth is such that it would assume this form under the influence 

 of rotation at the present rate, even if the interior is solid. 



Irregularities. It is only in a general view, however, that there 

 is a close approximation to a perfect spheroidal surface. In detail 

 there are very notable variations from it. The equatorial diameters 

 are not exactly equal, and the continental protuberances are, on 

 the average, some three miles above the bottoms of the oceans. 

 The continental platforms and ocean basins do not correspond accu- 

 rately with the present land and water surfaces, for about the con- 

 tinental land there are submerged borders, the continental shelves, 

 beyond which the surface of the lithosphere descends rapidly to 

 the depths of the ocean. The continental shelf belongs properly 

 to the continent, but its outer edge is covered by 100 fathoms or 

 less of water. If the upper 600 feet of the ocean were removed, 

 the outlines of the land would correspond quite closely with the 

 border of the true continental platforms. The agencies which 

 produced the continental platforms and abysmal basins, and the 

 great undulations, foldings, and volcanic extrusions of both, are 

 yet subjects of debate. 



It is customary to look upon the protrusions of the continents 

 as the great features of the earth's surface, but in reality the 

 oceanic depressions are the master feature. Both in breadth and 

 depth they much exceed the continental protrusions, and if the 

 earth be regarded as a shrunken body, the settling of the ocean 

 bottoms has doubtless been the greatest diastropic movement. 



The following tables show the relative areas of the lithosphere 

 above, below, and between certain levels. 1 



1 Article on The Earth, in Johnson's Encyclopaedia (Gilbert) ; also Smiths. 

 An. Kept., 1899, p. 312 (Murray); and Scot. Geog. Mag., Vol. XV, 1899, p. 511. 



