Sect. II. RATE OF GROWTH. 103 



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

 Nulliporge 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 Nulliporae ? 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 an- 

 ciently did when the lag*oon was less confined by islets, 

 and as they did 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, in- 

 stead 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 polypifers can exist. If it be asked, at 

 what rate in years I suppose a reef of coral favourably 

 circumstanced could grow up from a given depth ; I 

 must answer that we have no precise evidence on 

 this head. It will, however, be hereafter shown that 



