CORAL-REEFS. 135 



and buried by an accumulation of sediment more rapid 

 than the rate of subsidence, would rise abruptly from a 

 greater depth than that at which the efficient polypifers can 

 flourish : we see this well exemplified in the small abruptly- 

 sided reefs, with which the deep lagoons of the Chagos and 

 Southern Maldiva atolls are studded. With respect to the 

 ring or bason-formed reefs of the Northern Maldiva atolls, 

 it is evident, from the perfectly continuous series which 

 exists, that the marginal rings, although wider than the 

 exterior or bounding reef of ordinary atolls, are only modi- 

 fied portions of such a reef ; it is also evident that the cen- 

 tral rings, although wider than the knolls or reefs which 

 commonly occur in lagoons, occupy their place. The ring- 

 like structure has been shown to be contingent on the 

 breaches into the lagoon being broad and numerous, so 

 that all the reefs which are bathed by the waters of the 

 lagoon are placed under nearly the same conditions with 

 the outer coast of an atoll standing in the open sea. 

 Hence the exterior and living margins of these reefs must 

 have been favourably circumstanced for growing outwards, 

 and increasing beyond the usual breadth ; and they must 

 likewise have been favourably circumstanced for growing 

 vigorously upwards, during the subsiding movements, to 

 which by our theory the whole archipelago has been sub- 

 jected; and subsidence with this upward growth of the 

 margins would convert the central space of each little reef 

 into a small lagoon. This, however, could only take place 

 with those reefs, which had increased to a breadth sufficient 

 to prevent their central spaces from being almost imme- 

 diately filled up with the sand and detritus driven inwards 

 from all sides : hence it is that few reefs, which are less 

 than half a mile in diameter, even in the atolls where the 

 bason-like structure is most strikingly exhibited, include 



