58 BARRIER REEFS. Ch. II. 



writings of the Kevs. 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 Grambier 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 



1 Consult, on this and other points, the Polynesian Eesearches by 

 the Eer. W. Ellis, an admirable work, full of curious information. 



