THEORY OF CORAL-REEFS 



503 



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 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 .5 17 of an inch to a mile) through 

 Bolabola in the Pacific. We can now at once see wh}- encircling 

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

 \\'e 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 



A" 





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. 



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

 exaggerated. 



SECTION OF CORAL-REEF. 



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

 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 

 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 



