1836.] OR ATOLLS. 449 



others, it may be safely inferred that the utmost depth at which 

 corals can construct reefs is between 20 and 30 fathoms. Now 

 there are enormous areas in the Pacific and Indian Oceans, in 

 which every single island is of coral formation, and is raised only 

 to that height to which the waves can throw up fragments, and 

 the winds pile up sand. Thus the Eadack group of atolls is an 

 irregular square, 520 miles long and 240 broad ; the Low Archi- 

 pelago is elliptic-formed, 840 miles Jn its longer, and 420 in its 

 shorter axis : there are other small groups and single low islands 

 between these two archipelagoes, making a linear space of ocean 

 actually more than 4000 miles in length, in which not one single 

 island rises above the specified height. Again, in the Indian Ocean 

 there is a space of ocean 1500 miles in length, including three 

 archipelagoes, in which every island is low and of coral formation. 

 From the fact of the reef-building corals not living at great depths, 

 it is absolutely certain that throughout these vast areas, wherever 

 there is now an atoll, a foundation must have originally existed 

 within a depth of from 20 to 30 fathoms from, the surface. It is 

 improbable in the highest degree that broad, lofty, isolated, steep- 

 sided banks of sediment, arranged in groups and lines hundreds of 

 leagues in length, could have been deposited in the central 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 20 to 30 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 foundations, 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 



* It is remarkable that Mr. Lyell, even in the first edition of liis 

 " Principles of Geology," inferred that the amount of subsidence in the 



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