CHAPTER XXXIX 

 DISTRIBUTION OF LIFE IN THE OCEAN 



Physical Characters. — Two-thirds of the surface 

 of the earth is covered by oceans. Throughout four-fifths 

 of their extent the water is over a mile in depth, more 

 than two miles over two-thirds of their area, while one- 

 fifteenth is over three miles deep. The greatest depth 

 ever sounded, 32,089 feet, is located east of the Philippine 

 Islands. 



Sea water, as we know, is salty, containing about 27 

 grams of common salt to the liter. It also contains, in 

 much smaller quantities, salts of magnesium, calcium, 

 potassium and traces of a considerable number of other 

 substances, some of which are of prime importance in 

 the growth of many animals and plants. Oxygen, nitro- 

 gen and carbon dioxide gases are also dissolved in it. 



In the neighborhood of large land masses the sea bot- 

 tom usually slopes rather gradually to a point where the 

 water becomes approximately 600 feet in depth. Here 

 this continental plateau, as it is termed, unites with the 

 continental slope that falls away with comparative 

 abruptness into the abyss. In these deeper oceanic re- 

 gions the configuration of the bottom varies widely. It 

 may present the appearance of a gently undulating plain, 

 hundreds or thousands of square miles in extent, with 

 here and there a tremendous saucer-like depression known 

 as a deep. Or there may be rolling hills with wide inter- 

 vening valleys, or narrow gorges between mountains 

 whose lofty summits, rising to the surface, form islands 

 such as those of the Hawaiian Group. 



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