THE DEPTH AND MARINE DEPOSITS OF THE PACIFIC OCEAN. 161 



ground of the ocean. Large numbers of Holothurians and other marine 

 creatures here eat the mud to obtain the organic matter associated with it j 

 indeed, it is more than probable that all marine deposits are in this way 

 passed through the intestines of organisms. Very many Crustaceans fre- 

 quent 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 plw)sphorescent organs to enable them to do this more effec- 

 tively. All these mud-eating creatures are in turn the prey of carnivorous 

 animals, both vertebrate and invertebrate. 



The continental sbpe extends from the mud-line (100 fathoms) down to 

 the mean sphere level, 1450 fathoms.^ The continental slope, and similar 

 areas around oceanic islands, present a greater variety of conditions than is 

 found elsewhere on the ocean-bed beyond the continental shelf. At the 

 upper limit the sun's rays may produce twilight, but elsewhere there must 

 be total darkness, except where this is relieved by the phosphorescent light 

 of organisms. The temperature conditions are likewise widely different at 

 the upper and lower limits of the region. At some points the descent from 

 the 100-fathoms line is known to be almost perpendicular; at other points 

 outcrops of stratified and volcanic rocks are indicated ; generally, however, 

 the slope is by no means pronounced, though much steeper than between the 

 deeper contour-lines. 



The deposits now being laid down over the continental slope vary 

 greatly according to position : off large rivers they are chiefly made up 

 of detritus from the land ; at other places, especially where cold and warm 

 currents alternately occupy the surface, pelagic conditions are more or less 

 approached, and Green Sand and phosphatic deposits are being laid down ; 

 quartz and other continental minerals predominate. Generally it may be 

 said that in enclosed seas, and along the continental shelf and slope, de- 

 posits are now forming which are chiefly made up of continental detritus, 

 and are quite similar to those which have made up the stratified rocks of 

 past ages. Indeed, it seems as if inland seas and the borders of conti- 

 nental masses had again and again been pushed up into dry land, and again 

 and again been torn down and transported to the ocean by the same denud- 

 ing and disintegrating agents, the final result being that quartz particles 



1 If all the elevated portion of the earth's crust were cut away and tilled into the hollows till the 

 whole surface were uniform, then the whole earth would be covered by an ocean 1450 fathoms in 

 depth — the mean sphere level. 



11 



