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A TEXTBOOK OF OCEANOGRAPHY 



per cent, of the sea bottom. The land surface over 4,000 

 mètres high is only of 3 million square kilomètres extent ; that 

 over 5,000 mètres only ^ million. 



The sea bottom below 4,000 mètres depth is 185 million 

 square kilomètres — that is, 36 million more than the whole 



Mean Height oF Land 



2ÛÏÏ tO'O " 



FiG. 2. — Hypsographical Curve of the Earth's Surface. 



surface of dry land. Depths below 5,000 mètres comprise 

 72 million square kilomètres, nearly half the dry land area. 

 Nearly 5J million square kilomètres lie below the 6,000-metre 

 line. 



The océans represent, therefore, tremendous cavities in the 

 earth's surface. 



The General Features of the Océan Bottom. 



The great océan bottoms are not concave, but convex, as 

 portions of a spherical surface should be. Concavities are 

 only met with exceptionally ; they are especially characteristic 

 of continental seas. The bed of the océan consists for the most 

 part of flat plains of enormous extent. The least déviation 

 from the horizontal appréciable to the naked eye is one in 200 

 or 0° 17', and this slope is seldom met with in the beds of the 

 great océans, 



