CORAL-REEFS. 63 



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 

 (Plate I., Fig. 1, and Plate II., Fig. 5), 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 separating the reef from 

 the included land may be called), but at Vanikoro the reef 

 slopes only for a short distance, 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 1 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, analogous to that low and 

 inner part of the islets in many atolls which is formed 

 by the accumulation of matter from the lagoon. At 

 Hogoleu (Fig. 3, Plate II.), in the Caroline Archipelago, 2 

 the reef on the south side is no less than twenty miles; 

 on the east side, five; and on the north side, fourteen 

 miles from the encircled high islands. 



The lagoon channels may be compared in every respect 



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

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



2 See Hydrographical Mem. and the Atlas of the Voyage of the 

 Astrolabe by Captain Dumont D'Urville, p. 428. 



