228 THE SEA FLOOR 



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 ioo- 

 fathom line is known to be almost perpendicular ; at 

 other points outcrops of stratified and volcanic rocks 

 are indicated. 



The deposits now being laid down over the conti- 

 nental 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 pronounced, and greensand 

 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, deposits are being formed 

 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 continental 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 denuding and disintegrating agents, the final 

 result being that quartz particles accumulate on the 

 continental areas, rendering these rocks specifically 

 lighter than the deposits on the oceanic areas. 



In the abysmal regions, beyond the depth of the 

 mean sphere level, covering about one-half of the 

 earth's surface, the physical conditions are uniform 

 and widespread ; the temperature everywhere ap- 

 proaches zero Centigrade ; the darkness is relieved only 

 by phosphorescent light ; motion of all kinds must be 

 extremely slow, and there is little evidence of transport 



