CHAPTER XXII 



DEVELOPMENT OF THE ATOLL 



The Formation of the Barrier. — The rim of the reef is now- 

 lying a httle way below the surface of the sea, and it is obvious 

 that, since it is raised by the agency of living coral colonies 

 only, there it must stay, so long as no other building factor 

 comes into play. 



The corals themselves can never grow beyond the tide 

 limit, and form dry land of their own efforts ; and a submerged 

 reef would stay submerged for ever if its rising depended 

 upon coral growth alone. With the advent of the reef near to 

 the surface of the sea, however, a very different set of building 

 agencies comes into play, and a very great change is introduced 

 in the history of the coral-clad plateau. 



We have seen that the whole structure of the atoll consists 

 in reality of a level, and slightly raised, rim around the 

 margins of an extensive reef, and that on this level rim islands 

 of coral debris are placed at intervals. It is therefore necessary, 

 first of all, to follow the method of formation of this level rim, 

 — the barrier flats. This process of formation, again, is one 

 that can always be seen in the making, and the composition 

 of the barrier rock itself easily gives the explanation of its 

 origin. 



When the reef- edge has risen so near to the surface of the 



sea that the waves may act upon it powerfully, some colonies, 



or some portions of colonies, will be uprooted from the extreme 



edge, and shifted on to the tops of their fellows that lie nearer 



to the centre. These accumulations of broken corals will tend 



to increase, and the reef edge will become the site of many 



irregular elevations, so that in time waves will break upon 



some part of the edge of the bank. This is the beginning of the 



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