ORIGIN OF THE BARRIER REEF. 259 



some instances than a thousand feet ; and in many cases it is 

 probably much greater. Now as reef corals do not grow be- 

 low eighteen or 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 re- 

 move 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 over this extended area there 

 has occurred, since the reefs began to form, a slowly progress- 

 ing subsidence, like that which is now going on in Green- 

 land. 



Again, the island of Metia is 250 feet in height, full twice 

 the coral-growing depth. At the island of Mangaia, in the 

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

 Such thick beds could not have been made by corals growing 

 in depths not exceeding 120 feet without a sinking of many 

 scores of feet during their progress. 



The fact that subsidence has 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 advantage, that it explains all the facts, and requires no 

 other element 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 the annexed figure, if I be the wa- 

 ter 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 shallow depths, and then some distance out, de- 

 clining more abruptly. Let the same island become sub- 



