THE AFTER-HISTORY OF THE ATOLL 277 



sand which is produced around the reef. A vast quantity never 

 reaches the barrier flats, but falls down the submarine slope, 

 and adds to the sedimentation bank upon which the reef rests. 

 Another great contribution is removed again from the lagoon 

 by the currents which run out of the leeward channels, and this 

 too is added to the submarine bank. 



In the Cocos-Keeling atoll the current that runs from the 

 lagoon mouth impinges upon another island — Pulu Luar — 

 and on its windward shores sand-building is taking place with 

 some rapidity. This is not, however, the common fate of 

 sediment removed from a lagoon, for in ordinary cases it would 

 be swept to sea, and dropped so as to form an elongation of 

 the bank to leeward. By the addition of sand and fragments 

 that drop from the edge of the reef, the submarine bank is 

 therefore always tending to grow ; but there is every reason to 

 imagine that its growth will be very slow. The additions to 

 the bank will enable the reef margin to advance seawards and 

 so the whole atoll may become enlarged, but this must be an 

 extremely slow process in an atoll that has its origin in very 

 deep water. But I do not doubt that all atolls, and 

 especially those in shallower seas, grow outwards upon banks 

 of their own talus, and that this outward growth is an im- 

 portant factor in atoll construction. 



The atoll has therefore reached its full development, and, 

 short of any actual change of land-level, it will remain at this 

 stage for an incalculable time, its only alteration being a very 

 tardy spreading of its boundaries. In these final stages of 

 atoll development the presence of sediment is again the 

 dominant factor. Sediment played an all-important part in 

 the early stages of the formation of the structure, and in its 

 closing scenes it again becomes the guiding and directing 

 influence. 



