Ch. V. OF COKAL-KEEFS, 135 



of the less massive kinds on the central expanse, would 

 be checked by the sediment formed there, and by that 

 washed inwards by the breakers ; and as the space be- 

 came shallower, their growth would also be checked by 

 the impurities of the water, and probably by the small 

 amount of food brought to them by the enfeebled cur- 

 rents. The subsidence of a reef based on a bank of this 

 kind, would give depth to the central expanse or lagoon, 

 steepness to the flanks, and through the free growth of 

 the coral, symmetry to the whole outline ; but, as we 

 have seen, the larger groups of atolls in the Pacific and 

 Indian Oceans cannot have been formed on banks of 

 this nature. 



If, instead of an island, as in the diagram, the shore 

 of a continent fringed by a reef were to subside, a great 

 barrier-reef like that on the N.E. coast of Australia, 

 would be the necessary result; and it would be separated 

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

 proportion to the amount of subsidence, and to the less 

 or greater inclination of the bed of the sea. The effect 

 of the continued subsidence of a barrier -reef, and its 

 probable conversion into a chain of separate atolls, will 

 be considered when we discuss the progressive dissever- 

 ment of the larger Maldiva atolls. 



We now are able to perceive that the close similarity 

 in form, dimensions, structure, and relative position 

 between fringing and encircling barrier-reefs, and be- 

 tween these latter reefs and atolls, is the necessary result 

 of the transformation, during subsidence, of the one 

 class into the other. On this view, the three classes of 



