552 W. M. DAVIS SUBSIDENCE OF REEF-ENCIRCLED ISLANDS 



be less than 30 or 40 fathoms^ unless postglacial subsidence lias taken 

 place, or unless the glacial lowering of ocean level was more than 40 

 fathoms, which seems improbable. 



THE REQUIREMENTS OF THE SUBSIDENCE THEORY 



It is well, when we attempt to determine the features of lagoons formed 

 according to Darwin's theory, to bear in mind his original statements 

 regarding the fundamental postulate of subsidence. Many statements 

 indicate intermittent movements; thus, ^^Subsidence supervening after 

 long intervals of rest ... is probably the ordinary course of events" 

 (1842, 130). Subsidence of different islands at different times is clearly 

 conceived. Eecent subsidence is illustrated by Yanikoro, in the Santa 

 Cruz group of the western Pacific, where ^^the unusual depth of the chan- 

 nel [lagoon] between the shore and the [barrier] reef, the entire absence 

 of islets on the reef, its wall-like structure on the inner side, and the 

 small amount of low alluvial land at the foot of the mountains, all seem 

 to show that this island has not long remained at its present level, with 

 the lagoon channel subjected to the accumulation of sediment, and the 

 reef to the wear and tear of the breakers.'^ More remote subsidence is 

 inferred for the Society Islands, "where . . . the shoalness of the 

 lagoon channels round some of the islands, the number of islets formed 

 on the reefs of others, and the broad belt of low land at the foot of the 

 mountain indicate that, although there must have been great subsidence 

 to have produced the barrier reefs, there has since elapsed a long station- 

 ary period" (128). The relation of subsidence to lagoon depth is ex- 

 plicitly stated: "The lagoon channel will be deeper or shallower, in pro- 

 portion to . . . the accumulation of sediment; . . . also to the 

 rate of subsidence and the length of the intervening stationary periods" 

 (99), and the effect of a long stationary period in nearly filling a lagoon 

 with sediment is mentioned (102). 



The effect of unusually rapid subsidence in producing fringing reefs 

 of a new generation, as was so clearly, though briefly, explained by Dar- 

 win, has already been sufficiently considered. The effect of rapid subsi- 

 dence on atolls was more fully stated and has been more generally recog- 

 nized : "There is nothing improbable in the death . . . from the sub- 

 sidence being great or sudden, of the corals on the whole, or on portions 

 of some of the atolls. . . . Further subsidence [of a submerged atoll] , 

 together with the accumulation of sediment, would often obliterate its 

 atoll-like structure [form?] and leave only a bank with a level surface" 

 (108, 107). Elevation also has its due in Darwin's discussion, as well as 

 oscillations of level (145, 146). In thus accepting the possibility of both 



