IV DEPTHS AND DEPOSITS OF THE OCEAN 133 



This table shows at a glance that the greater portion of 

 the ocean-floor is covered by deep water, i.e. by water exceed- 

 ing 1000 fathoms in depth, equal to more than four- fifths of 

 the entire superficies of the ocean, two-thirds being occupied by 

 water exceeding 2000 fathoms in depth, while only one-fifteenth 

 of the entire sea-floor is covered by water exceeding 3000 

 fathoms in depth. 



Those parts of the ocean in which depths greater than 3000 

 fathoms have been recorded are called "deeps," and have had "Deeps." 

 distinctive names conferred upon them, just as mountain ranges 

 and peaks on the dry land (Mount Everest, for example) are 

 distinguished by names. These deeps are shown on Map H., 

 and will presently be dealt with in some detail. 



The table also shows that a comparatively large area, about Areas of the 

 one-sixth of the ocean-floor, is covered by water less than 1000 SanT^ 

 fathoms in depth, of which by far the greater proportion is continental 

 covered by still shallower water. Thus if we divide this area ^°p^* 

 into two portions by the 500-fathoms line, we find that the 

 area within that line is about 17 million square miles (or 

 over 12 per cent of the entire ocean) compared with only 

 4|- million square miles (or 3 per cent of the entire ocean) 

 beyond that line, i.e. having depths between 500 and 1000 

 fathoms. Again, of the area covered by less than 500 fathoms 

 of water, more than one-half is occupied by the continental 

 shelf or continental plateau lying between the shore-line and 

 the loo-fathoms line, which has elsewhere^ been estimated at 7 

 per cent of the whole ocean. The relatively large area covered 

 by the gentle slopes of the continental shelf in depths less than 

 100 fathoms, as compared with the relatively small area covered 

 by the steeper gradients of the continental slope in depths 

 greater than 100 fathoms, is strikingly shown by these figures, 

 for while about 7 per cent of the ocean-floor lies within the 

 lOO-fathoms line, only about 5 per cent occurs within the next 

 succeeding 400 fathoms (between the 100- and 500-fathoms 

 lines), and only about 3 per cent within the next succeeding 

 500 fathoms (between the 500- and looo-fathoms lines). 



The position occupied by the junction of the continental Continental 

 shelf with the continental slope, as indicated by the change nSSne 

 of gradient, has been called the continental edge (see Fig. 144, 

 p. 198), and varies in depth according to circumstances, but on 

 the average all over the world is not far from the lOO-fathoms 



1 Sir John Murray, Presidential Address to the Geographical Section of the British Associa- 

 tion, Dover, 1899. 



