vi.] ON CORAL AND CORAL REEFS. 125 



How again, on this hypothesis, are atolls to be 

 accounted for, unless, as some have done, we take refuge 

 in the wild supposition that every atoll corresponds with 

 the crater of a submarine volcano 1 And what explana- 

 tion does it afford of the fact that, in some parts of the 

 ocean, only atolls and encircling reefs occur, while others 

 present none but fringing reefs ? 



These and other puzzling facts remained insoluble 

 until the publication, in the year 1840, of Mr. Darwin's 

 famous work on coral reefs ; in which a key was given 

 to all the difficult problems connected with the subject, 

 and every difficulty was shown to be capable of solution 

 by deductive reasoning from a happy combination of 

 certain well-established geological and biological truths. 

 Mr. Darwin, in fact, showed, that so long as the level of 

 the sea remains unaltered in any area in which coral 

 reefs are being formed, or if the level of the sea relatively 

 to that of the land is falling, the only reefs which can 

 be formed are fringing reefs. While if, on the contrary, 

 the level of the sea is rising relatively to that of the 

 land, at a rate not faster than that at which the upward 

 growth of the coral can keep pace with it, the reef will 

 gradually pass from the condition of a fringing, into that 

 of an encircling or barrier reef. And, finally, that if the 

 relative level of the sea rise so much that the encircled 

 land is completely submerged, the reef must necessarily 

 pass into the condition of an atoll. 



For, suppose the relative level of the sea to remain 

 stationary, after a fringing reef has reached that distance 

 from the land at which the depth of water amounts to 

 150 feet. Then the reef cannot extend seaward by the 

 migration of coral germs, because these coral germs 

 would find the bottom of the sea to be too deep for 

 them to live in. And the only manner in which the 

 reef could extend outwards, would be by the gradual 



