474 



THEORY OF CORAL-REEFS. 



[chap. XX. 



lidated fragments. Thus the difficulty on this head, which ap- 

 peared 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, sepa- 

 rated 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 sec- 

 tion is now represented by unbroken lines, and which, as I have 

 said, is a real section through Bolabola, and let it go on sub- 

 siding. As the barrier-reef slowly sinks down, the corals will 



A" 



jg^JtL 



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 

 exagger .ted. 



go on vigorously growing upwards ; but as the island sinks, the 

 water will gain inch by inch on the shore the separate moun- 

 tains 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 per- 

 ceive 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 which they stand. We can 

 further see how it arises that the atolls in the Pacific and Indian 

 oceans extend in lines parallel to the generally prevailing strike of 

 the high islands and great coast-lines of those oceans. I venture, 



