1836.] BARRIER-REEFS. 341 



and profoundest 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 hun- 

 dred 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 struc- 

 ture, we must turn to the second great class, namely, Barrier-reefs. 

 These either extend in straight lines in front of the shores of a c^itinent 

 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 it remarkable how 

 little attention has been paid to encircling barrier-reefs ; yet they are 

 truly wonderful structures. The following sketch represents part of 

 the' barrier encircling the island of Bolabola in the Pacific, as seen from 



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

 ples 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. 



