58 
BARRIER-REEFS. 
Ch. II. 
writings of the Eevs. W. Ellis and J. Williams, I con- 
clude that this peculiar structure is common to most 
of the encircled islands of the Society Archipelago. 
The reef within this mound or breakwater, has an ex- 
tremely irregular surface, even more so than between 
the islets on the reef of Keeling atoll, with which 
alone (as there are no islets on the reef of Tahiti) it 
can properly be compared. At Tahiti the reef is very 
irregular in width ; but round many other encircled 
islands, for instance Vanikoro or Gambier Islands 
(figs. 1 and 8, Plate I.), it is quite as regular, and of 
the same average width, as in true atolls. Most 
barrier-reefs on the inner side slope irregularly into 
the lagoon-channel, (as the space of deep water sepa- 
rating the reef from the included land may be called,) 
but at Vanikoro the reef slopes only for a short dis- 
tance, and then terminates abruptly in a submarine 
wall forty feet high,— a structure absolutely similar to 
that described by Chamisso in the Marshall atolls. 
In the Society Archipelago, Ellis' states that the 
reefs generally lie at the distance of from one to one 
and a-half miles, and, occasionally, even at more than 
three miles from the shore. The central mountains 
are generally bordered by a fringe of flat, and often 
marshy alluvial land, from one to four miles in width. 
This fringe consists of coral-sand and detritus thrown 
up from the lagoon-channel, and of soil washed down 
from the hills ; it is an encroachment on the channel, 
1 Consult, on this and other points, the Polynesian Researches 
by the Rev. W. Ellis, an admirable work, full of curious information. 
