CORAL-REEFS. 65 



channel ; they generally occur in front of the main valleys, 

 a circumstance which can be accounted for, as will be 

 seen in the fourth chapter, without much difficulty. The 

 breaches being situated in front of the valleys, which 

 descend indifferently on all sides, explains their more 

 frequent occurrence through the windward side of barrier- 

 reefs than through the windward side of atolls, — for in 

 atolls there is no included land to influence the position of 

 the breaches. 



It is remarkable, that the lagoon -channels round 

 mountainous islands have not in every instance been long 

 ago filled up with coral and sediment ; but it is more easily 

 accounted for than appears at first sight. In cases like that 

 of Hogoleu and the Gambier Islands, where a few small 

 peaks rise out of a great lagoon, the conditions scarcely 

 differ from those of an atoll, and I have already shown, at 

 some length, that the filling up of a true lagoon must be an 

 extremely slow process. Where the channel is narrow, the 

 agency, which on unprotected coasts is most productive of 

 sediment, namely the force of the breakers, is here entirely 

 excluded, and the reef being breached in the front of the 

 main valleys, much of the finer mud from the rivers must 

 be transported into the open sea. As a current is formed 

 by the water thrown over the edge of atoll-formed reefs, 

 which carries sediment with it through the deep-water 

 breaches, the same thing probably takes place in barrier- 

 reefs, and this would greatly aid in preventing the lagoon- 

 channel from being filled up. The low alluvial border, 

 however, at the foot of the encircled mountains, shows that 

 the work of filling up is in progress ; and at Maurua (Plate 

 III., Fig. 1), in the Society group, it has been almost effected, 

 so that there remains only one harbour for small craft. 



If we look at a set of charts of barrier-reefs, and leave out 



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