i34 CORAL-REEFS. 



and the new annular reef thus formed on the old one, 

 would have its summit inclined inwards, and be terminated 

 by a subaqueous wall, formed by the upward growth of the 

 coral (before being much checked), from the inner edge of 

 the solid parts of the old reef. The inner portion of the 

 new reef, from not having grown to the surface, would be 

 covered by the waters of the lagoon. Should a subsidence 

 of the same kind be repeated, the corals would again grow 

 up in a wall, from all the solid parts of the resunken reef, 

 and, therefore, not from within the sandy shores of the 

 lagoon ; and the inner part of the new annular reef would, 

 from being as before checked in its upward growth, be of 

 less height than the exterior parts, and therefore would not 

 reach the surface of the lagoon. In this case the shores of 

 the lagoon would be surrounded by two inclined ledges, 

 one beneath the other, and both abruptly terminated by 

 subaqueous cliffs. 1 



The ring or bason-formed reefs of the northe7'n Maldiva 

 atolls. — I may first observe, that the reefs within the 

 lagoons of atolls and within lagoon-channels, would, if 

 favourably circumstanced, grow upwards during subsidence 

 in the same manner as the annular rim ; and, therefore, we 

 might expect that such lagoon-reefs, when not surrounded 



1 According to Mr. Couthouy (p. 26), the external reef round many 

 atolls descends by a succession of ledges or terraces. He attempts, I 

 doubt whether successfully, to explain this structure somewhat in the 

 same manner as I have attempted, with respect to the internal ledges 

 round the lagoons of some atolls. More facts are wanted regarding 

 the nature both of the interior and exterior step-like ledges: are all the 

 ledges, or only the upper ones, covered with living coral ? If they are 

 all covered, are the kinds different on the ledges according to the 

 depth? Do the interior and exterior ledges occur together in the same 

 atolls; if so, what is their total width, and is the intervening surface- 

 reef narrow, etc. ? 



