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A TEXTBOOK OF OCEANOGRAPHY 



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

 metres high is only of 3 million square kilometres extent ; that 

 over 5,000 metres only J million. 



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

 square kilometres that is, 36 million more than the whole 



$00 400 300 200 100 



FIG. 2. HYPSOGRAPHICAL CURVE OF THE EARTH'S SURFACE. 



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

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

 Nearly 5J million square kilometres lie below the 6,ooo-metre 

 line. 



The oceans represent, therefore, tremendous cavities in the 

 earth's surface. 



THE GENERAL FEATURES OF THE OCEAN BOTTOM. 



The great ocean 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 ocean consists for the most 

 part of flat plains of enormous extent. The least deviation 

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

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

 great oceans. 



