30 THE OCEAN 



fathoms, and on some coasts there appear to 

 be submerged terraces, but as a rule even 

 these submarine slopes do not exceed the 

 gradients found on our roads and railways, 

 while the ocean-floor throughout the abyssal 

 region mr«y be regarded as consisting of vast 

 gently undulating plains, interrupted here 

 and there by elevations, some of which rise 

 above the sea-surface to form oceanic islands, 

 as, for instance, Bermuda in the Atlantic, 

 Christmas Island in the Indian Ocean and 

 many Pacific islands. 



Sub-Oceanic Regions. — The mean depth of 

 the ocean is estimated at 2080 fathoms (12,480 

 feet) and the mean height of the land at 

 375 fathoms (2250 feet). If we now suppose 

 the higher portions of the continental eleva- 

 tions to be cut away and filled into the oceanic 

 depressions, the depth of the ocean around 

 the whole world would then be about 1700 

 fathoms (say 10,000 feet). This depth of 

 1700 fathoms has been called the mean sphere 

 level. The region lying deeper than 1700 

 fathoms may be regarded as the great abyssal 

 region or abyssal plain, covering an area 

 exceeding 100 millions of square miles, or 

 more than one-half of the earth's surface. 

 Turning now to the continental shores, we 

 find the continental shelf extending out from 

 the shore-line to an average depth of 100 



