1836.] THEORY OP CORAL-REEFS, 455 



feet as there have been feet of subsidence, that small limit of deptli 

 at which the effective corals can live : the little architects having 

 built tip their great wall-liko mags, as 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, disap- 

 pears. 



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

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

 straight barrier, like that of Australia or New Caledonia, separated 

 from the laud 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 



A" 







A' A'. Outer edges of the barrier-reef at the level of the sea, with Islets on it. B'B'. The 

 shores of the included island. CC. The lagoon-channel. 



A"A." Outer edges of the reef, now converted into an atoll. C. The lagoon of the 

 new atoll. 



N.B. According to the true scale, the depths of the lagoon-channel and lagoon are much 

 exaggerated. 



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 ; for 

 they may be called rude outline charts of the sunken islands over 



