300 CORALS AND CORAL ISLANDS. 



There is no reason to believe that such aid from shells or 

 Rhizopods is consistent with the grouping of living corals 

 thickly enough to form reefs. 



The other kind of Umestone beds referred to, where unmixed 

 with the former, grow up in compact layers to the surface, as a 

 necessary consequence of wave-action ; and limestones are 

 made in such regions, instead of sandstones and shales, 

 because the material exposed to degradation is corals and 

 shells, instead of common rocks. 



The facts show that there are formed about coral-reefs, in 

 indefinite amount, all the ordinary products of degradation by 

 wave-action — fragments large and small, down to sand, and 

 even mud. With such an agent as the ocean's waves, driven 

 often by the storm, so powerful and so persistent at lifting, 

 rending, grinding, and transporting, it is of little account, at 

 least about outer reefs, that some coral stems or masses are 

 first weakened below by the boring sponge or moUusk ; and 

 neither fish, nor holothurian, nor alcyonoid is needed, in order 

 to keep up the supply of particles for sand or mud-beds. In 

 accordance with these facts, the reef-formations illustrate that 

 not only coral conglomerates, or coral rag, may be made of 

 corals, but also the very finest and most compact unfossiliferous 

 limestones ; that fine compact limestone, as flint-like in fracture 

 as any of Silurian time, is one of the most common of coral- 

 reef rocks, and is nothing but consolidated mud, or fine sand, 

 of coral origin. 



The elevated portion of the island of Matea, which consists 

 largely of this kind of white, compact, coral-made limestone, 

 appears to correspond to the interior of the original lagoon ol 

 the island ; it exemplifies the kind of rock-making which is 

 going forward in most coral-island lagoons. In archipelagos 

 like that of the Feejees, where the reef channels are very 

 broad, there is an opportunity for the formation of very large 

 areas of this compact white limestone, and also for others of 

 impure or argillaceous limestones. 



Besides the kinds of coral-rocks above mentioned, there are 

 also the Beach and Drift Sand-rocks, which are accumulated 



