Oh. II. 
BARRIER-REEFS. 
61 
that of Hogoleu and the Gamhier 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, aver- 
age dimensions, and grouping. Encircling reefs, like 
atolls, are generally elongated, and have an irregularly 
rounded, though sometimes angular outline. There are 
6 
