76 THE GREAT BARRIER REEF. 



level surface, a reef not to be distinguished from an atoll might be formed ; and I believe 

 some such exist in the West Indies. But in this view it must be assumed that in every case 

 the basis consists of a flat bank ; for if it were conically formed, like a mountainous mass, 

 we can see no reason why the corals should spring up from the flanks instead ot from the 

 central and highest parts. As the lagoons of atolls are sometimes forty fathoms deep, it 

 must, also, be assumed, in this view, that at a depth at which the waves do not break, the coral 

 grows more vigorously on the edges of a bank than on its central part; and this is an 

 assumption without any evidence." The objection raised by Mr. Darwin to the foregoing 

 theory of atoll formation, in so far as it relates to positive evidence being at that time wanting 

 to prove " that coral grows most vigorously on the edges of a bank," is somewhat weakened 

 by the results of investigations prosecuted and recorded since his decease. Many of the 

 photographic reef-views in this volume, moreover, unmistakably indicate a more retarded 

 central, and most vigorous peripheral, growth. 



No special theory worthy of notice at the date of publication of Mr. Darwin's book had 

 been brought forward to account for the presence of those barrier reefs which encircle islands 

 of moderate dimensions. It is mentioned, however, by this authority that " the great reef 

 which fronts the coast of Australia has been supposed, but witliout any evidence, to rest on 

 the edge of a submarine precipice parallel to the shore." 



Mr. Darwin's original theories concerning the origin of atolls and barrier reefs may now 

 be submitted /;/ cxtciiso. " What cause," he writes, " then, has given to atolls and barrier 

 reefs their characteristic form ? Let us see whether an important deduction does not follow 

 from the following facts: first, that reef-building corals only flourish at a very limited depth ; 

 and, secondly, that throughout areas of vast dimensions, none of the coral reefs and coral 

 islets rise to a greater height above the level of the sea than that attained by matter thrown 

 up by the wind and waves." In vindication of this last assertion the respective superficial 

 areas of the Low, Marshall, Caroline, and Maldivas Archipelagoes, situated in the Indian and 

 the Pacific Oceans, arc described at length — no one of them contains an islet which rises 

 above the height to which waves and wind or open sea can heap up matter. One of these 

 areas, situated between the Low and the Marshall Archipelagoes, is shown to include a narrow 

 band of ocean more than 4,000 miles in length and to embrace a vast number of islands, all 

 of which present the same low character. 



The argument is continued thus: "On what foundations, then, have these reefs and islets 

 of coral been constructed ? A foundation must originally have been present beneath each 

 atoll, at that limited depth which is indispensable for the first group of the reef-building polyps. 

 A conjecUirc will perhaps be hazarded, that the requisite bases may have been afforded by the 

 accumulation of great banks of sediment which did not quite reach the surface owing to the 

 action of superficial currents aided possibly by the undul.-itory mo\cmcnt of the sea. This 



