558 



CORAL FORMATIONS. 



April, 1836. 



of ocean, of more than a thousand miles in one direction and 

 several hundreds in another, scattered over with islands, 

 none of which rise to a greater height than that to which 

 waves can throw fragments, or the wind heap up sand. 

 Now if we leave out of the question subsidence, the 

 foundation on which these reefs are built, must in every 

 case come to the surface within that small limit (we may 

 say twenty fathoms) at which corals can live. This con- 

 clusion is so extremely improbable that it may at once be 

 rejected : for in what country can there be found a broad and 

 grand range of mountains of the same height within a hun- 

 dred and twenty feet ? But on the idea of subsidence, the 

 case is at once clear; as each point, one after the other 

 according to its altitude, was submerged, the coral grew 

 upwards, and formed the many islets now standing at one 

 level. 



Having endeavoured on general grounds not only to re- 

 move any extreme degree of improbability in the belief of 

 a general subsidence, but likewise to show that it is almost 

 necessary to account for the existence of a vast number of 

 reefs on one level, we v/ill now see how far the same idea 

 will apply to the peculiar configuration in the several classes. 

 Let us imagine an island merely fringed by reefs extending 

 to a short distance from the shore ; in which case, as we have 

 before remarked, there is no difficulty in understanding their 

 structure. Now let this island subside by a series of move- 

 ments of extreme slowness, the coral at each interval grow- 

 ing up to the surface. Without the aid of sections it is not 

 very easy to follow out the result, but a little reflection will 

 show that a reef encircling the shore at a greater or less dis- 

 tance, according to the amount of subsidence, would be 

 produced. If we suppose the sinking to continue, the 

 encircled island must, by the submergence of the central 

 land but upward growth of the ring of coral, be converted 

 into a lagoon island. If we take a section of some encircled 

 island on a true scale, as for instance Gambier, which has 

 been so well described by Captain Beech ey, we shall not find 



