208 THE SEA FLOOR 



depths.* The area between ioo fathoms and the next 

 900 fathoms, it will be observed, covers only g per 

 cent, of the whole floor of the ocean. This area is 

 called the continental slope. It represents the sides 

 of the ocean basins, and shows a relatively rapid 

 descent into deep water ; it is the only area of the 

 ocean's floor which can be regarded as concave 

 upwards. 



Beyond 1,000 fathoms the floor of the ocean becomes 

 more and more flat and uniform : 



84 per cent, lies below 1,000 fathoms, 

 65 ,, ,, 2,000 



arid 7 ,, ,, 3,000 



These great submerged plains sink, as we have 

 seen, in some instances to depths of nearly six English 

 miles, and at times they are interrupted by cone-like 

 elevations, some of which do not rise to the surface 

 of the ocean, while others do, and then form groups 

 of volcanic or coral islands. Although a great many 

 soundings have been recorded exceeding 3,000 

 fathoms, and a great many submarine elevations have 

 been also recorded in recent years, still the general 

 result of recent explorations has been to increase the 

 area of the ocean lying between 2,000 and 3,000 

 fathoms. 



Deposits on the Floor of the Ocean. — We have very 

 few indications of the nature of the solid rocks forming 



* On the Wyville Thomson Ridge, between the North of 

 Scotland and the Faeroe Bank, mud is not deposited at depths 

 of 250 and 300 fathoms. The tidal wave, being here confined, 

 apparently sweeps the top of the ridge at these great depths, 

 and we have evidence of the same phenomenon on the saddle- 

 backs between contiguous oceanic islands, and on very deeply 

 submerged ridges. Further observations in this direction are 

 much desired. 



