July 8, 1922] 



N-A TURE 



57 



more particularly to account for the atolls and barrier 

 reefs of the supposedly quiescent central Pacific than 

 for the barrier reefs of much disturbed regions like 

 the Louisiade archipelago : nevertheless the Tagula 

 reef in particular affords critical evidence against that 

 theory, as will be made clear by the following con- 

 siderations. 



The Glacial-control theory appears to be based on 

 the conviction that it is the smooth lagoon floors 

 rather than their enclosing reefs which are most in 

 need of explanation, and that the bathymetric relation 

 of the lagoon floors to the level of the ocean reached 

 by the enclosing reefs is normally so nearly constant 

 in all the coral seas that their explanation by Darwin's 

 theory in terms of reef upgrowth and lagoon aggrada- 

 tion on subsiding foundations of irregular form is 

 impossible. A long period of nearly perfect stability 

 of the mid-ocean floor is therefore assumed, although 

 instability is admitted for islands in the south- 

 western Pacific ; and instead of postulating that 

 lagoon floors represent " moats " that have been 

 heavily aggraded behind the upgrowing reefs during 

 the subsidence of their foundations, a series of in- 

 genious suppositions is invented, of which the chief 

 are : that during Preglacial time many still-standing 

 islands, more or less reef-surrounded, were either 

 worn down to low relief by subaerial erosion or cut 

 down to shallow platforms by marine abrasion ; that 

 during the Glacial epochs of the Glacial period the 

 ocean surface was lowered by about 35 fathoms by 

 the withdrawal of water to form continental ice sheets : 

 that the surface waters of the ocean were then so 

 chilled as to kill or greatly to weaken reef-building 

 organisms ; that islands were then attacked by the 

 waves, which cut low-level benches around them if 

 they were high, or if they were low completely trun- 

 cated them in platforms at a depth of 35 to 40 fathoms 

 below normal sea level ; that as the waters warmed 

 and rose, reefs grew up on the margins of the benches 

 and platforms, whereupon the lagoons behind them 

 were moderately aggraded ; and that the thickness 

 of the aggrading deposits is greater, and consequently 

 the lagoon depth is less in small than in large lagoons, 

 because the detritus supplied from a linear front-foot 

 of a reef has a smaller interior sector to aggrade in a 

 small lagoon than in a large one. In brief, the long- 

 continued stability of reef foundations and the 

 abrasion of sub-lagoon platforms upon them are 

 leading factors of the Glacial-control theory. 



It should be noted here that neither the stability 

 of reef foundations nor the abrasion of sub-lagoon 

 platforms is proved by any direct evidence ; both of 

 these leading factors are, like the subsidence of atoll 

 foundations in Darwin's theory, assumed because 

 they are thought to be necessary for the explanation 

 of observed facts ; and both assumptions are believed 

 to be true because of the apparent success of the 

 explanation that they provide. Hence if it be shown, 

 even in a single instance, that a lagoon floor of typical 

 form and depth has been produced around an island 

 which provides independent evidence contradictory 

 to stability and abrasion and, indeed, requires strong 

 subsidence, the fundamental assumptions of the 

 Glacial-control theory will be seriously invalidated. 



The bearing of Tagula reef and lagoon on the 

 Glacial-control theory may now be apprehended. 

 Tagula is, as has already been shown, not in a region 

 of long-continued and nearly perfect stability, but 

 in one of marked instability ; and as will next be 

 shown, it has not suffered abrasion by the lowered 

 ocean ; yet its lagoon floor is smooth and of a depth 

 accordant with that of other large lagoons in various 

 parts of the Pacific. Hence long-continued stability 

 and extensive low-level abrasion are not essential 



NO. 2749, VOL. I 10] 



factors in the production of this fine example of a 

 barrier-reef lagoon floor. But if these factors are 

 not essential in Tagula, they should not be regarded 

 as essential anywhere else ; and their adoption as 

 the leading postulates of the Glacial-control theory 

 is therefore unnecessary ; flatness of lagoon floors 

 and their accordant depths may be explained else- 

 where as well as in Tagula as the result of long- 

 continued aggradation on subsiding foundations of 

 uneven surface. 



The evidence that Tagula has not suffered abrasion 

 by the low Glacial ocean, and hence that the reef- 

 building organisms around Tagula were not seriously 

 weakened by the lowered temperatures of the lowered 

 ocean in the Glacial epochs, is found partly in the 

 absence of chartered cliffs on the shores of the main 

 island where the barrier reef becomes a fringe, partly 

 in the absence of similar cliffs on the exposed sides 

 of the satellite islands at either end of the Calvados 

 chain where it approaches the barrier reef, and partly 

 in the presence of the outpost islands in the barrier- 

 reef loops around the northern lagoon compartment. 



As to the first line of evidence based on the absence 

 of cliffs on Tagula : If abrasion by the lowered ocean 

 had operated long enough to cut a platform 10 or 

 20 miles wide beneath the present floor of the southern 

 compartment, it ought at the same time to have cut 

 spur-end cliffs on the north shore of the main island, 

 where the defending reef is a fringe only half a mile 

 wide ; and these cliffs ought still to show the upper 

 part of their faces as plunging cliffs, now that the 

 ocean has resumed its normal level ; but the charts 

 show no such cliffs. 



The second line of evidence based on the absence 

 of cliffs on the Calvados islands is similarly argued. 

 It may be added that the absence of cliffs at these 

 significant points on the charts of the Louisiade 

 islands does not appear to be due to poor charting ; 

 for on the coast of Misima, where Maitland observed 

 the white limestone scarps of elevated reefs, the 

 charts clearly show a shore cliff, and a legend is 

 printed along it : " Cliffs 100 feet high." 



As to the third line of evidence : The little outpost 

 islands are so numerous in the Tagula barrier-reef 

 loops around the northern lagoon compartment and 

 around the western part of the southern compart- 

 ment that it seems unreasonable to believe the waves 

 of the lowered Glacial ocean could have cut their 

 way behind the outposts efficiently enough to abrade 

 a platform 10 miles in width. Not only so, the 

 outpost islands show no sign of having cliffs on their 

 outer sides. One of them, L'tian, a mile across and 

 480 feet high, is reported by Maitland to consist of 

 volcanic rocks ; but it is not a young volcanic cone 

 built up in Postglacial time, for the chart shows it 

 to have well-dissected form, with three slender points 

 enclosing two small bays turned toward the outer 

 ocean ; yet the points are not cut back in plunging 

 cliffs. Another outpost not far away is said by the 

 same observer to consist of limestone ; this island 

 cannot have been made and elevated since an 

 assumed platform was abraded, for the height of the 

 island, 530 feet, is so great that in such case the 

 platform thereabouts ought to be more or less 

 emerged ; and it cannot have been made and raised 

 before the platform was cut, for in such case the 

 limestone ought to have been consumed by the waves 

 that cut the platform. 



The small outpost islands of the Tagula barrier 

 reef therefore give strong confirmation of the evidence 

 against abrasion derived from the absence of plunging 

 cliffs on the north side of the main islands and on 

 the terminal members of the Calvados chain. But if 

 the northern compartment of the Tagula lagoon. 



