460 BARRIER-REEFS. [chap. xx. 



profouiidest parts of the Pacific and Indian Oceans, at an 

 immense distance from any continent, and where the 

 water is perfectly limpid. It is equally improbable that the 

 elevatory forces should have uplifted, throughout the above 

 vast areas, innumerable great rocky banks within twenty 

 to thirty fathoms, or 120 to 180 feet, of the surface of the 

 sea, and not one single point above that level; for where 

 on the whole face of the globe can we find a single chain 

 of mountains, even a few hundred miles in length, with 

 their many summits rising within a few feet of a given 

 level, and not one pinnacle above it? If then the founda- 

 tions, whence the atoll-building corals sprang, were not 

 formed of sediment, and if they were not lifted up to the 

 required level, they must of necessity have subsided into it ; 

 and this at once solves the difficulty. For as mountain 

 after mountain, and island after island, slowly sank beneath 

 the water, fresh bases would be successively afforded for 

 the growth of the corals. It is impossible here to enter 

 into all the necessary details, but I venture to defy* any 

 one to explain in any other manner, how it is possible that 

 numerous islands should be distributed throughout vast 

 areas — all the islands being low — all being built of corals,, 

 absolutely requiring a foundation within a limited depth 

 from the surface. 



Before explaining how atoll-formed reefs acquire their 

 peculiar structure, we must turn to the second great 

 class, namely, barrier-reefs. These either extend in straight 

 lines in front of the shores of a continent or of a large 

 island, or they encircle smaller islands ; in both cases, 

 being separated from the land by a broad and rather deep 

 channel of water, analogous to the lagoon within an 

 atoll. It is remarkable how little attention has been paid 

 to encircling barrier-reefs ; yet they are truly wonderful 

 structures. For instance, in the barrier encircling the 

 island of Bolabola in the Pacific, seen from one of the 

 central peaks, the whole line of reef has been converted 

 into land ; but usually a snow-white line of great breakers, 

 with only here and there a single low islet crowned with 

 cocoa-nut trees, divides the dark heaving waters of the 

 ocean from the light-green xpanse of the lagoon-channel. 



* It is remarkable that Mr, Lyell, even in the first Edition of his " Principles 

 of Geology," inferred that the amount of subsidence in the Pacific must have 

 exceeded that of elevation, from the area of land being very small relatively to 

 the agents there tending to form it, namely, the growth of coral and volcanic 

 action. 



