90 ON CORAL REEFS AND ISLANDS. 



in many cases it is probably much greater. Now as reef corals 

 do not grow below twenty fathoms, there is no way in which 

 this thousand feet of reef could have been formed except by a 

 gradual subsiding of the land upon which it stands. The large 

 number of instances of distant barriers in the Pacific remove any 

 doubt with regard to these conclusions. The map of the Feejees 

 abounds in them through its eastern part, and we may infer with 

 reason that this has been a large area of subsidence, like that 

 which is now going on in Greenland. 



Evidence of subsidence still more conclusive, if possible, is 

 obtained by actual observation at Metia and some of the elevated 

 coral islands. This island is 250 feet in height, full twice the 

 coral-growing depth. At another island in the Hervey Group, 

 Mangaia, the coral rock is raised 300 feet out of water. 



The fact of subsidence having actually taken place during the 

 formation of many reefs, is therefore put beyond doubt. It must 

 form a part of any true theory of reefs, whether it be the crater 

 hypothesis or the view here advocated. The latter has this ad- 

 vantage, that it explains all the facts, and requires no other ele- 

 ment but this single one of subsidence. It rests on a simple fact 

 and demands no hypothesis whatever. 



The manner in which subsidence would operate is shown in 

 the following sketches, representing ideal transverse sections of 

 an island and its reefs. In figure 1, if I be the water line, the 

 island, like Goro, has a simple fringing reef,/,/: — it is a narrow 

 platform of rock at the surface, dropping off at its edge to shal- 

 low depths, and then some distance out, declining more abruptly. 

 Let the same island become submerged till II is the water line : — 

 the reef extends itself upward, as submergence goes on, and 

 may have the character at the surface represented by b' f b' f. 

 There is here a fringing reef and also a barrier reef, with a nar- 

 row channel between, such as we have described as existing 

 on the shores of Tahiti;* b' is a section of the barrier, c' of 

 the channel, and f of the fringing reef. Suppose a farther sub- 

 mergence, till III is the water line: then the channel (c" c") 

 within the barrier is quite broad, as in the island of Nairai or 

 Angau ; on one side (f") the fringing reef remains, but on the 

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

 cumstance 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 disap- 

 peared. 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 ex- 



* See Paere 14. 



