Ch. V. OF CORAL-REEFS. 139 



reefs would sometimes be found rising abruptly from 

 a greater depth than that at which the efficient poly- 

 pifers 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 (see Plate II. fig. 4), 

 it is evident from the perfectly continuous series which 

 exists, that the rings on the margin, although broader 

 than the exterior or bounding reef of an ordinary atoll, 

 are only modified portions of such a reef; it is also 

 evident that the central rings, although broader than 

 the knolls or reefs which commonly occur within 

 lagoons, occupy the same relative position. The ring- 

 like structure has been shown to be contingent on 

 the breaches into the lagoon being wide and numerous, 

 thus causing the inner side of the marginal reef and 

 the central reefs to be placed under nearly the same 

 conditions with the outside of an ordinary atoll which 

 is exposed to the open sea. Hence the margins of these 

 reefs have been favourably circumstanced for growing 

 outwards and increasing beyond their usual breadth ; 

 and the conditions have likewise been favourable for 

 their growing vigorously upwards, during that subsiding 

 movement to which by our theory the whole archi- 

 pelago has been subjected; and subsidence together 

 with the upward growth of the margin would convert 

 the central space of each little reef into a small 

 lagoon. This, however, could only take place with 

 reefs which had increased in breadth sufficiently to 



