102 CORAL-REEFS. 



would generally be slow, except under peculiarly favourable 

 circumstances. Almost the only natural condition, allowing 

 a quick upward growth of the whole surface of a reef, 

 would be a slow subsidence of the area in which it stood ; — 

 if, for instance, Keeling atoll were to subside two or three 

 feet, can we doubt that the projecting margin of live coral, 

 about half an inch in thickness, which surrounds the dead 

 upper surfaces of the mounds of Porites, would in this case 

 form a concentric layer over them, and the reef thus 

 increase upwards, instead of, as at present, outwards? 

 The Nulliporse are now encroaching on the Porites and 

 Millepora, but in this case might we not confidently expect 

 that the latter would, in their turn, encroach on the Nulli- 

 poras ? After a subsidence of this kind, the sea would gain 

 on the islets, and the great fields of dead but upright corals 

 in the lagoon, would be covered by a sheet of clear water ; 

 and might we not then expect that these reefs would rise to 

 the surface, as they anciently did when the lagoon was less 

 confined by islets, and as they ditf. within a period of ten 

 years in the schooner-channel, cut by the inhabitants ? In 

 one of the Maldiva atolls, a reef, which within a very few 

 years existed as an islet bearing cocoa-nut trees, was found 

 by Lieut. Prentice "entirely covered with live coral and 

 Madrepore." The natives believe that the islet was washed 

 away by a change in the currents, but if, instead of this, it 

 had quietly subsided, surely every part of the island which 

 offered a solid foundation, would in a like manner have 

 become coated with living coral. 



Through steps such as these, any thickness of rock, 

 composed of a singular intermixture of various kinds of 

 corals, shells, and calcareous sediment, might be formed; 

 but without subsidence, the thickness would necessarily 

 be determined by the depth at which the reef-building 



