1836.] THEORY OF CORAL REEFS. 465 



the island becoming lower and smaller, and the space 

 between the inner edge of the reef and the beach pro- 

 portionally broader. A section of the reef and island in 

 this state, after a subsidence of several hundred feet, is 

 given by the dotted lines. Coral islets are supposed to 

 have been 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 subsidence, to the amount of 

 sediment accumulated in it, and to the giowth 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, thai 

 small limit of depth at which the effective corals can live : — 

 the little archiiects having built up their great wall-like 

 mass, 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, disappears. 

 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 land by a wide 

 and deep channel, would evidently have been the result. 

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

 ciion is now represented by unbroken lines, and which, 

 -, 1 have said, is a real section through Bolabola, and let 

 it go on subsiding. As the barrier-reef slowly sinks down, 

 tlie corals will go on vigorously growing upwards ; but 

 as the island sinks, the water will gain inch by inch on 

 tlie shore — the separate mountains tirst forming separate 

 lands within one great reef — and finally, the last and 

 i^Hiest pinnacle disappearing. The instant this takes 

 place, a perfect atoll is formed : I have said, remove the 

 liigh land from within an encirciing barrier-reef, and an 

 'oil is left, and the land has been removed. We can now 

 :ceive how it comes that atolls, having sprung from 

 ' iicircling barrier-reefs, resemble them in general size, 

 lorm, in the manner in which they are giox»ped together, 



