Ch. II. BARKIER EEEFS. 61 



accounted for without much difficulty. In cases like 

 that of Hogoleu and the Grambier 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 lagoon-channel is narrow, that agency, which on 

 unprotected coasts is the most productive of sediment, 

 namely the force of the breakers, is here entirely ex- 

 cluded ; and owing to 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. The 

 water which is thrown over the edges of atoll-formed 

 reefs causes a current which carries sediment from the 

 lagoon through the breaches into the sea ; and the 

 same thing probably takes place in barrier-reefs. This 

 would greatly aid in preventing the lagoon-channels 

 from being filled up. The low alluvial border, how- 

 ever, at the foot of the encircled mountains, shows 

 that the work of filling up is in progress; and at 

 Maurua (Plate I., fig. 6), 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 in imagination the encircled land, we shall 

 see that besides the many points already noticed of 

 resemblance or rather of identity in structure with atolls, 

 there is a close general agreement in form, average 

 dimensions, and grouping. Encircling reefs, like 

 atolls, are generally elongated, and have an irregularly 



