260 CORALS AND CORAL ISLANDS. 



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 V f b' f. There is here a fringing 

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



SECTION ILLUSTRATING THE ORIGIN OF BARRIER REEFS. 



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

 (see p. 242) ; V is a section of the barrier, d of the channel, and 

 f of the fringing reef. Suppose a farther submergence, till III 

 is the water line : then the channel {c" c") within the barrier be- 

 comes quite broad, as in the island of Nairai or Angau ; on one 

 side (f' ; ) the fringing reef remains, but on the other it has dis- 

 appeared, owing, perhaps, to some change of 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!" i f ") of reef over two peaks which have disappeared. 

 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 exem- 

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

 them. V" V" are sections of the distant enclosing barrier, and 

 c /v 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. 



