vi.] ON CORAL AND CORAL REEFS. 121 



But the corals which are to be seen growing in the 

 shallow waters of the lagoon are of a different kind from 

 those which abound on the outer edge of the reef, and of 

 which the reef is built up. Close to the seaward edge of 

 the reef, over which, even in calm weather, a surf almost 

 always breaks, the coral rock is encrusted with a thick 

 coat of a singular vegetable organism, which contains a 

 great deal of lime the so-called Nullipora. Beyond 

 this, in the part of the edge of the reef which is always 

 covered by the breaking waves, the living, true, reef- 

 polypes make their appearance ; and, in different forms, 

 coat the steep seaward face of the reef to a depth of 100 

 or even 150 feet. Beyond this depth the sounding-lead 

 rests, not upon the wall-like face of the reef, but on the 

 ordinary - shelving sea-bottom. And the distance to 

 which a fringing reef extends from the land corresponds 

 with that at which the sea has a depth of twenty or five- 

 and-twenty fathoms. 



If, as we have supposed, the sea could be suddenly 

 withdrawn from arouncl an island provided with a 

 fringing reef, such as the Mauritius, the reef would 

 present the aspect of a terrace, its seaward face, 100 feet 

 or more high, blooming with the animal flowers of the 

 coral, while its surface would be hollowed out into a 

 shallow and irregular moat-like excavation. 



The coral mud, which occupies the bottom of the 

 lagoon, and with which all the interstices of the coral 

 skeletons which accumulate to form the reef are filled up, 

 does not proceed from the washing action of the waves 

 alone ; innumerable fishes, and other creatures which 

 prey upon the coral, add a very important contribution 

 of finely-triturated calcareous matter ; and the corals and 

 mud becoming incorporated together, gradually harden 

 and give rise to a sort of limestone rock, which may vary 

 a good deal in texture. Sometimes it remains friable 



