ORIGIN OF THE BARRIER REEF. 223 



— it is a narrow platform of rock at the surface, dropping off 

 at its edge to shallow depths, and then some distance out, 

 declining more abruptly. Let the same island become sub- 

 merged till II is the water line: — the reef extends itself up- 

 ward, as submergence goes on, and may have the character at 

 the surface represented by b' f b' /'. There is here a fringing 

 reef and also a barrier reef, with a narrow channel between, 

 such as we have described as existmg on the shores of Tahiti 

 (see p. 206) ; b' is a section of the barrier, c of the channel, 

 and /' of the fringing reef. Suppose a farther submergence, 

 till III is the water line : then the channel {c" c") within the 

 barrier becomes quite broad, as in the island of Nairai or 

 Angau ; on one side (/'") the fringing reef remains, but on tlie 

 other it has disappeared, owing perhaps, to some change of 



SECTION ILLUSTRATING THE ORIGIN OF THE BARRIER REEFS. 



circumstance as regards currents, which retarded its growth, 

 and prevented its keeping pace with the subsidence. With the 

 water at IV, there are two islets of rock in a wide lagoon, along 

 with other islets (/"' i"') of reef over two peaks which have dis- 

 appeared. The coral reef-rock by gradual growth has attained 

 a great thickness, and envelops nearly the whole of the former 

 land. Nanuku, the Argo Reef, and Exploring Isles are here 

 exemplified, for the view is a good transverse section of either 

 of them. //" b"' are sections of the distant inclosing barrier, 

 and c" c"\ and other intermediate spots, of the water within. 



The supposed similarity between these ideal sections and 

 existing islands is fully sustained by actual comparison. The 

 figure beyond is a map of the island of Aiva, in the Feejee 

 Group. There are two peaks in the lagoon, precisely as above; 



