MR. DARWIN'S REEF-FORMATION THEORIES. 8i 



of subsidence go on, and the doubly pointed liill will form two small islands included within 

 one annular reef. Let the island continue to subside, and the coral-reef will continue growing 

 lip on its own foundation, whilst the water gains inch by inch on the land until the last 

 and highest pinnacle is covered, and there remains a perfect atoll. A vertical section ot 

 this atoll is shown in the woodcut by the dotted lines: a ship is anchored in its lagoon, 

 but islets are not supposed yet to have been formed on the reef. The depths ol the lagoon 

 and the width and slope of the reef will depend on the different circumstances to which it has 

 been exposed, as just stated with respect to barrier reefs. Any further subsidence will 

 produce no change in the atoll except a diminution in its size from the reef not growing 

 vertically upwards. I may here observe that a bank, either of rock or of hardened sedi- 

 ment, level with the surface of the sea and fringed with living coral, would be immediately 

 converted by subsidence with an atoll, without passing, as is the case of a reef fringing 

 the shore of an island, through the intermediate form of a barrier reef. As before remarked, 

 if such a bank lay a few fathoms submerged the simple growth of the coral, without the aid 

 of subsidence, would produce a structure scarcely to be distinguished from a true atoll ; tor 

 the corals on the outer margin, from being freely exposed to the open sea, would grow 

 vigorously, and tend to form a continuous ring, whilst the growth of the less massive kinds 

 in the central expanse would be checked by the sediment formed there, and by that 

 washed inwards by the breakers ; and, as the space became shallower, their growth would 

 also be checked by the impurities of the water, and probably by the small amount oi 

 food brought to them by the enfeebled currents. 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 diagrams, the shore of a continent fringed by a reel 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 mainland by a deep-water channel, broad in propor- 

 tion to the amount of subsidence, and to the less or greater inclination of the bed ol the 

 sea." 



What the Great Barrier Reef of Australia has to say in support of the deduction implied 

 in the foregoing paragraph we shall see a little later. The reader should now be in a posi- 

 tion to comprehend and appreciate the highly logical and sagacious line of reasoning 

 brought to bear by the illustrious naturalist, on the interpretation of this exceedingly difficult 

 subject, and to apprehend the cogency of the evidence, for and against it, that has been 

 brought forward by later observers. As the piienomenon of geological subsidence is neces- 

 sarily bound up with this Darwinian theory of the primary origin ot atolls and barrier-reefs, 



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