CONTINENTAL BORDER 227 



less favourable to organisms than the conditions 

 existing in the open ocean. The deposits in those 

 seas which interpenetrate the continental masses are 

 terrigenous in origin, although there may be an 

 approach to pelagic conditions towards the more 

 central parts. 



It has already been observed that the continental 

 shelf facing the great oceans is continually swept by 

 waves, tides, and oceanic currents, down to an average 

 depth of 100 fathoms. Just beyond this depth all 

 the minute mineral and organic fragments from the 

 continental shelf come to rest on the bottom, and 

 form what I have called the " mud line." This con- 

 stitutes the great feeding ground of the ocean. Targe 

 numbers of Holothurians and other marine creatures 

 here eat the mud to obtain the organic matter associ- 

 ated with it. Indeed, it is more than probable that all 

 marine deposits are in this way passed through the 

 intestines of organisms. Very many Crustaceans 

 frequent this area to pick up the little particles of 

 organic matter which are just settling on the bottom, 

 and some of them — like Nyctiphanes — are provided 

 with phosphorescent organs to enable them to do this 

 more effectively. All these mud-eating creatures are 

 in turn the prey of carnivorous animals, both vertebrate 

 and invertebrate. 



The continental slope extends from the mud line 

 (100 fathoms) down to the mean sphere level 

 (1,450 fathoms).* The continental slope, and similar 

 areas around oceanic islands, present a greater variety 

 of conditions than is found elsewhere on the ocean bed. 



* If^all the elevated portions of the earth's crust were cut 

 away, and rilled into the hollows till the whole surface was 

 uniform, then the whole earth would be covered by an ocean 

 1,450 fathoms in depth — the mean sphere level. 



