THE GEOSPHERES 231 



ocean, forming oceanic islands. Sometimes 

 they are capped at the sea-surface by coral 

 reefs in the form of atolls, or they may not 

 rise to the surface, and then form submerged 

 banks covered with a white mantle of living 

 and dead calcium-carbonate organisms. 



Along the sides of these great cones and 

 between them, as well as on the tops of sub- 

 merged cones, there is evidence of marine 

 currents, probably due to the tidal wave, 

 but with this exception there is no evidence 

 of transport or erosion over the surface of the 

 great abyssal plain ; it is essentially an area of 

 deposition. The sun's rays never reach this 

 deeply submerged part of the lithosphere, 

 and the temperature over the whole of the 

 deep ocean-floor never rises higher than two 

 or three degrees above the freezing point of 

 fresh water. 



If we put our finger on a map of the South 

 Pacific half way between South America and 

 Australia, we indicate an area farther re- 

 moved from continental land than any other 

 area on the globe. If a successful trawling 

 be made in this area at a depth of over 

 2400 fathoms, the net will contain several 

 hundreds of sharks' teeth (Car char odon, Oxy- 

 rhina, Lamna) and dozens of earbones of 

 whales, a few beaks of ziphioid whales and a 

 few fragments of the more areolar bones of 



