130 ON CORAL AND CORAL REEFS 



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 rela- 

 tively 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 dis- 

 tance from the land at which the depth of water 

 amounts to one hundred and fifty 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 accumulation, at the foot of 

 its seaward face, of a talus of coral fragments torn off 

 by the violence of the waves, which talus might, in 

 course of time, become high enough to bring its upper 

 surface within the limits of coral growth, and in that 

 manner provide a sort of factitious sea-bottom upon 



