86 A". H '. 8kt W.v— The ( \>ral-reef Problem 



"moat" filling he ignores the activity of organisms and devel- 

 ops his destructive criticism on the assumption that the moat 

 is filled entirely by means of transported and deposited sedi- 

 ment. While it may he granted that Daly's criticism would 

 have some force if sediment were the only factor in moat fill- 

 ing, the argument is very seriously weakened if not completely 

 vitiated by ignoring the part played by organisms. One of the 

 best established facts in modern work on coral reefs is that any 

 effects due to solution from lagoon waters, which was an im- 

 portant element in Murray's picture of the development ,of 

 lagoons, is negligible in amount and quite overbalanced by 

 organic growth and the deposition of sediment. In fact 

 measurements show that the lagoons of many atolls are becom- 

 ing shallower owing to these causes. If it can be shown that 

 the growth and deposition of organisms within the waters of 

 the lagoon are quantitatively much more important than the 

 deposition of sediment the weight of Daly's destructive criti- 

 cism would be removed and the relatively fiat floors of many 

 lagoons could be attributed to the deposition of organisms. 

 To decide between the opposing hypotheses the appeal is to the 

 facts so far as they are known. The Funafuti report* again 

 provides the most definite evidence of the nature of lagoon 

 deposits. Two bores were put down from the waters of the 

 lagoon and penetrated to depths of 113 and 144 feet respec- 

 tively below the floor of the lagoon. The record of the deeper 

 boring, |_ , shows that of the material from the first 70 feet 

 between 80 and 95 per cent consists of loose uncemented fronds 

 of the calcareous alga, Halimeda ; below 70 feet the organisms 

 are cemented by calcite into a white limestone and for about 

 20 feet consist of about one third of Halimeda and two thirds 

 of foraminifera. In the lower 50 feet the rock is mainly com- 

 posed of corals and foraminifera while Halimeda is scarce. It 

 will be noted that for the first 60 feet the filling of the lagoon 

 "moat" consists of loose fronds of Halimeda practically devoid 

 of sediment; below this level the calcite cement, small in 

 amount, may or may not represent recrystallized calcareous 

 sediment. At Funafuti, therefore, it is clear that of the two 

 factors, organisms and sediment, which have filled the lagoon 

 "moat", the organisms are of overwhelming importance. The 

 deposits of Halimeda in the upper part of the lagoon bore must 

 have been accumulated fairly rapidly since from the floor of 

 the lagoon down to a depth of 8&J feet the fronds of Halimeda 

 were still sufficiently preserved to show the peripheral cells on 

 decalcification. We may therefore conclude, at any rate, so far 

 as Funafuti is concerned, that Daly's objections to the subsi- 

 dence theory of atolls so far as they are based on difficulties 

 connected with the filling of the lagoon "moat", have no weight 

 * The Atoll of Funafuti, London, 1904, pp. 310-315. 



