Davis. — Significniif Fealures uf Beef-hordered Coasts. 15 



already smothered where the great delta of the Fly Kiver has advanced 

 far into the sea. 



Most barrier reefs are, however, in no immediate danger of such a 

 fate ; their lagoons are far from being filled with alluvium ; deltas, indeed, 

 as a rule do not fill the embayments in which they head. Both the Rewa 

 and the Fly are exceptional in being much larger than the rivers of most 

 islands within barrier reefs. The prevalence of open lagoons shows that 

 no such constant relation of land and sea level has been maintained as 

 was provisionally postulated above, but that submergence has prevented 

 lagoon-filling not only by providing new depths to be filled up, but also 

 in the case of islands by diminishing the area and the altitude of the land 

 from which ])art of the filling-material should come. 



On the other hand, in view of the many possible changes of land and 

 sea level, and of their many possible combinations with periods of rest, 

 it is surprising that, among the many examples of reef-encircled islands, 

 none are found with mature reef plains approaching or realizing the stage 

 of smothering the corals on the reef-face. For if the development of 

 barrier reefs depended only on the subsidence of their foundations, and if 

 their foundations were of dift'erent ages and had subsided by different 

 measures and at different dates, we should expect to see all stages of reef- 

 development to-day — some close-set, discontinuous barriers ; others broader 

 and farther from their central island, tlie embayments of which should 

 contam good-sized deltas ; and so on through all the stages to a mature 

 reef plain, the alluvium of which is just overlapping the reef ; and then 

 an old reef plain, much reduced from its original breadth by abrasion. 

 But, with such exceptions as the Rewa and Fly deltas, no mature reef 

 plains are known. 



Combination of I stand -subsidence ivith Changes of Ocean-level. — The 

 absence of completed reef plains cannot be due to lack of detritus for their 

 formation, for the amounts of detritus that have been discharged from 

 many deeply denuded volcanic islands are vastly greater than the volumes 

 of the lagoons enclosed by their barrier reefs. Hence the prevalence to-day 

 of young barrier reefs with open lagoons must be taken as suggesting that 

 some recent and widespread cause has produced a more general submergence 

 than should be expected from island-subsidence alone ; and this cause is 

 perhaps to be found in the post-Glacial rise of ocean-level, for a rise of 

 ocean-level combined with a prevalent but intermittent subsidence of 

 reef-foundations would tend to maintain the barrier reefs of to-day in an 

 early stage of their development and prevent the attainment of the more 

 mature stage which they would reach during a long period of fixed levels 

 of islands and ocean. 



On the other hand, a fall of ocean-level, such as must have accom- 

 panied the oncoming of the last glacial epoch, would have tended to 

 lessen or even to neutralize the submergence due to prevalent subsidence ; 

 hence during a glacial epoch lagoons may have been more generally filled 

 than during an interglacial epoch or during the present post-Glacial epoch 

 (Davis, 1915, p. 267 ; 1916a, p. 565). These somewhat transcendental 

 aspects of the coral-reef problem are mentioned here in hopes that they 

 may incite special observations, by means of which the possibilities here 

 sketched may be assigned their proper values. 



That the post-Glacial rise of ocean-level is not the entire or even the 

 chief cause of the submergence under which barrier reefs as well as uncon- 

 formable fringing reefs have been developed is proved by the great diversity 



