THE STRUCTURE OF THE BARRIER 163 



storms. It is this belt of wreckage, which happens to be 

 composed of coral, that forms the barrier of coral islands. 

 Now this boulder-strewn belt of shore varies in different parts 

 of its breadth ; to the ocean edge are the large masses and 

 deep pools with their sheltered and living corals, and inside 

 this is the broad smooth belt formed by the perpetual sea 

 wear. 



Every block on this belt becomes firmly embedded in the 

 platform, and cemented fast to its neighbour, and the whole 

 surface is levelled by the waves and the fragments which they 

 carry. Coral sand, formed in various ways, is driven into 

 every chink, and a fine breccia of dead and altered coral is 

 formed. The breccia becomes for the most part covered with 

 living Nulli'porne, but it is not, as a rule, the site of any living 

 coral growth. The landward edge remains loose, for there the 

 fragments are trundled beyond the reach of ordinary waves, 

 and on the breccia which has been formed in the past the 

 piles of rounded sea-worn coral fragments lie ; and of such 

 fragments the coral island is formed. The structure of the 

 barrier reef varies in different parts of the atoll ring, and it is 

 different on the leeward and windward sides, as might be 

 imagined. On the windward side, in such places as the 

 neighbourhood of Pulu Atas, the barrier reefs are of great 

 width, and stretch lagoonwards as long sand flats, which 

 extend in between the islets as spits and shallows, far out 

 into the lagoon. The smaller particles are swept on the 

 farthest, and the fine sand formed from disintegrated corals 

 and shells makes a gradually extending silt which has filled 

 the ancient lagoonlets of the windward islands, and is gradually 

 shallowing the windward side of the lagoon itself. Here too 

 the barrier rock is evidently the most changed by extreme age, 

 and the coral forming the breccia is often so altered that, in the 

 finely tessellated fracture of the windward barrier, its original 

 structure is often difficult to recognise. Amongst the other 

 factors which tend to modify the structure of the barrier reef 

 are the entrance and exit places of the currents of the lagoon, 

 and remarkable changes take place in the more variable 



