CORAL-REEFS. 183 



expanse; and a barrier-reef differs from a fringing-reef, in 

 being placed at a much greater distance from the land 

 with reference to the probable inclination of its submarine 

 foundation, and in the presence of a deep-water lagoon- 

 like space or moat within the reef. In the fourth chapter 

 the growing powers of the reef-constructing polypifers were 

 discussed; and it was shown, that they cannot flourish 

 beneath a very limited depth. In accordance with this 

 limit, there is no difficulty respecting the foundations on 

 which fringing-reefs are based ; whereas, with barrier- 

 reefs and atolls, there is a great apparent difficulty on 

 this head ; — in barrier-reefs from the improbability of 

 the rock of the coast or of banks of sediment extend- 

 ing, in erery instance, so far seaward within the required 

 depth ; — and in atolls, from the immensity of the spaces 

 over which they are interspersed, and the apparent 

 necessity for believing that they are all supported on 

 mountain-summits, which, although rising very near to the 

 surface-level of the sea, in no one instance emerge above 

 it. To escape this latter most improbable admission, which 

 implies the existence of submarine chains of mountains 

 of almost the same height, extending over areas of many 

 thousand square miles, there is but one alternative ; namely, 

 the prolonged subsidence of the foundations, on which 

 the atolls were primarily based, together with the upward 

 growth of the reef-constructing corals. On this view every 

 difficulty vanishes : fringing-reefs are thus converted into 

 barrier-reefs ; and barrier-reefs, when encircling islands, are 

 thus converted into atolls, the instant the last pinnacle of 

 land sinks beneath the surface of the ocean. 



Thus the ordinary forms and certain peculiarities in the 

 structure of atolls and barrier-reefs can be explained; — 

 namely, the wall-like structure on their inner sides, the 



