6 PRELIMINARY OUTLINE 



oblate spheroid with a polar diameter of 7,899.7 miles, and an 

 equatorial diameter of about 26.8 miles more. Its equatorial cir- 

 cumference 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. 



The earth is not a perfect spheroid. Its equatorial diameters 

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

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

 forces or 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 continents as the great features 

 of the earth's surface, but in reality the oceanic depressions are the 

 master feature. They exceed the continental protrusions in breadth, 

 and they are much farther below sea-level than the continents are 

 above it. If the earth be regarded as a shrunken body, the settling 

 of the ocean bottoms has doubtless been the greatest diastrophic 

 movement. 



The following table shows the relative areas of the lithosphere 

 above, below, and between certain levels. 



Per cent 



More than 6,000 feet above sea-level 2.3 



Between sea-level and 6,000 feet above 25.5 



Between sea-level and 6,000 feet below 14 . 8 



Between 6,000 and 12,000 feet below sea-level 14 . 8 



Between 12,000 and 18,000 feet below sea-level 39-4 



Between 18,000 feet and 24,000 feet 3.1 



From these estimates it appears that if the surface of the litho- 

 sphere were graded to a common level by cutting away the conti- 

 nental platforms and dumping the material in the ocean basins, 

 bringing all to a common level, this level would be about 9,000 feet 

 below sea-level. The continental platforms may be conceived as 

 rising from this common plane rather than from the sea-level. 



The bottoms of the ocean basins have broad undulations ranging 

 through many thousands of feet; but they have not those irregu- 

 larities of form that give variety to land surfaces. The ocean bot- 

 toms are also diversified by volcanic peaks, many of which consti- 

 tute islands. From many of them, the solid surface slopes down 

 rapidly to abysmal depths. Many of the volcanic islands are 



