CHAPTER XXIII 



THE AFTER-HISTOEY OF THE ATOLL 



The atoll, as far as we have at present considered its develop- 

 ment, consists of a wave-washed barrier crescent surmounted 

 by a series of piles of debris which are also crescent-shaped. 

 The islands, so far, are mere masses of broken coral branches 

 and loose boulders, heaped up in confusion by the waves, and a 

 Very long period of time may be passed without any advance 

 from this condition being made. 



The " Button Islands " (between Pulu Pasir and Pulu 

 Bras) of the eastern side of the Cocos atoll are to-day mere 

 piles of debris and their development has not proceeded in 

 any way since they were first charted by Van der Jagt in 

 1829; but this is not the usual history of such structures. 

 As a rule, the heap of loose coral fragments becomes a con- 

 solidated and vegetated island ; and in the process the winds 

 are the most influential agents. 



It is obvious that the height to which the seas may pile 

 the boulders is limited, and the limit depends upon the 

 amount of tidal variation, and the violence of the storms that 

 pass over the tract of ocean in which the atoll lies. The 

 influence of the range of tidal variation is easy to account for, 

 and the greater the rise and fall of the tides, the higher in all 

 probability will the island be ; for since the tides have a 

 greater excursion upon either side of their level of maximum 

 wave force, it follows that the range of possible building 

 will be greater from this level when the tidal variation is a 

 large one. When the tides have an excursion of 3 feet it may 

 be roughly said that 1 foot 6 inches of broken water will be 

 available for land-building above the level of the barrier : 

 Avhen the excursion is 5 feet another foot is added to its range 



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