CORAL-REEFS. 131 



washed inwards by the breakers ; and as the space becomes 

 shallower, their growth will, also, be checked by the 

 impurities of the water, and probably by the small amount 

 of food brought by the enfeebled currents, in proportion 

 to the surface of living reefs studded with innumerable 

 craving mouths : the subsidence of a reef based on a bank 

 of this kind, would give depth to its central expanse or 

 lagoon, steepness to its flanks, and through the free growth 

 of the coral, symmetry to its outline : — I may here repeat 

 that the larger groups of atolls in the Pacific and Indian 

 Oceans cannot be supposed to be founded on banks of this 

 nature. 



If, instead of the island in the diagram, the shore of a 

 continent fringed by a reef had subsided, a great barrier- 

 reef, like that on the N.E. coast of Australia, would have 

 necessarily resulted; and it would have been separated 

 from the main land by a deep-water channel, broad in pro- 

 portion to the amount of subsidence, and to the less or 

 greater inclination of the neighbouring coast-line. The 

 effect of the continued subsidence of a great barrier-reef 

 of this kind, and its probable conversion into a chain of 

 separate atolls, will be noticed, when we discuss the 

 apparent progressive disseverment of the larger Maldiva 

 atolls. 



We now are able to perceive that the close similarity in 

 form, dimensions, structure, and relative position (which 

 latter point will hereafter be more fully noticed) between 

 fringing and encircling barrier-reefs, and between these 

 latter and atolls, is the necessary result of the transforma- 

 tion, during subsidence of the one class into the other. 

 On this view, the three classes of reefs ought to graduate 

 into each other. Reefs having an intermediate character 

 between those of the fringing and barrier classes do exist ; 



