. Journal of researches into the natural history and geology of the countries visited during the voyage round the world of 'Beagle,' under the command of Captain Fitz Roy. y broader. A section of the reef andisland in this state, after a subsidence of several hundred feet,is given by the dotted lines. Coral islets are supposed to havebeen formed on the reef; and a ship is anchored in the lagoon- THEORY OF CORAL-REEFS 5o3 channel. This channel will be more or less deep, according tothe rate of subsidence, to the amount of sediment accumulatedin it, and to the growth of the delicately bra
. Journal of researches into the natural history and geology of the countries visited during the voyage round the world of 'Beagle,' under the command of Captain Fitz Roy. y broader. A section of the reef andisland in this state, after a subsidence of several hundred feet,is given by the dotted lines. Coral islets are supposed to havebeen formed on the reef; and a ship is anchored in the lagoon- THEORY OF CORAL-REEFS 5o3 channel. This channel will be more or less deep, according tothe rate of subsidence, to the amount of sediment accumulatedin it, and to the growth of the delicately branched corals whichcan live there. The section in this state resembles in everyrespect one drawn through an encircled island : in fact, it is areal section (on the scale of .5 17 of an inch to a mile) throughBolabola in the Pacific. We can now at once see why encirclingbarrier-reefs stand so far from the shores which they can also perceive that a line drawn perpendicularly downfrom the outer edge of the new reef, to the foundation of solidrock beneath the old fringing-reef, will exceed by as many feetas there have been feet of subsidence, that small limit of depth. AA, Outer edges of the barrier-reef at the level of the sea, with islets on it. BB, The shoresof the included island. CC, The lagoon-channel. AA, Outer edges of the reef, now converted into an atoll. C, The lagoon of the new atoll. —According to the true scale, the depths of the lagoon-channel and lagoon are muchexaggerated. SECTION OF CORAL-REEF. at which the effective corals can live:—the little architectshaving built up their great wall-like mass, as the whole sankdown, upon a basis formed of other corals and their consolidatedfragments. Thus the difficulty on this head, which appearedso great, disappears. If, instead of an island, we had taken the shore of acontinent fringed with reefs, and had imagined it to havesubsided, a great straight barrier, like that of Australia orNew Caledonia, separated
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