1836.] THEORY OF CORAL-REEFS. 345 



formed on the reef; and a ship is anchored in the lagoon-channel. 

 This channel will be more or less deep, according to the rate of subsi- 

 dence, to the amount of sediment accumulated in it, and to the growth 

 of the delicately branched corals which can live there. The section in 

 this state resembles in every respect one drawn through an encircled 

 island ; in fact, it is a real section (on the scale of -517 of an inch to a 

 mile) through Bolabola in the Pacific. We can now at once see why 

 encircling barrier-reefs stand so far from the shores which they front. 

 We can also perceive, that a line drawn perpendicularly down from the 

 outer edge of the new reef, to the foundation of solid rock beneath the 

 old fringing-reef, will exceed by as many feet as there have been feet of 

 subsidence, that small limit of depth at which the effective corals can 

 live : the little architects having built up their great wall-like mass, aa 

 the whole sank down, upon a basis formed of other corals and their 

 consolidated fragments. Thus the difficulty on this head, which 

 appeared so great, disappears. 



If, instead of an island, we had taken the shore of a continent fringed 



AA. Outer edges of the fringing-reef, at the level of the sea. BB. Tne shores of the 



fringed island. 



A' A'. Outer edges of the reef, after its upward growth during a period of subsidence, 

 now converted into a barrier, with islets on it. B'B'. The shores of the now encircled 

 island. CC. Lagoon-channel. 



N.B. In this and the following woodcut, the subsidence of the land could b* repre- 

 sented only by an apparent rise in the level of the sea. 



with reefs, and had imagined it to have subsided, a great straight 

 barrier, like that of Australia or New Caledonia, separated from the 

 land by a wide and deep channel, would evidently have been the result. 

 Let us take our new encircling barrier-reef, of which the section is 

 now represented by unbroken lines, and which, as I have said, is a real 

 section through Bolabola, and let it go on subsiding. As the barrier- 

 reef slowly sinks down, the corals will go on vigorously growing 

 upwards ; but as the island sinks, the water will gain inch by inch on 

 the shore the separate mountains first forming separate islands within 

 one great reef and finally, the last and highest pinnacle disappearing. 

 The instant this takes place, a perfect atoll is formed: I have said, 

 remove the high land from within an encircling barrier-reef, and an 

 atoll is left, and the land has been removed. We can now perceive 

 how it comes that atolls, having sprung from encircling barrier-reefs, 

 resemble them in general size, form, in the manner in which they are 

 grouped together, and in their arrangement in single or double lines ; 



