222 CORALS AND CORAL ISLANDS. 



proved, and accounts of some of them which are now in pro- 

 gress, as ihat of Sweden and that of Greenland, are to be 

 found in any geological treatise. 



But it admits of direct demonstration that such a subsidence 

 has actually taken place. It has been stated that the depth ot 

 the reef at different distances from the shore it encircles may 

 generally be estimated from the slope of the shore. On this 

 principle it has been shown on a former page (p. 125) that the 

 thickness of the distant barrier reef cannot be less in some 

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

 ably much greater. Now as reef corals do not grow below 

 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 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 over this extended area there has 

 occurred, since the reefs began to form, a slowly progressing 

 subsidence, like that which is now going on in Greenland. 



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 

 water line, the island, like Goro, has a simple fringing reef,//.- 



