W. M. Davis — Shaler Memorial Study of Coral Beefs. 237 



period, and in not considering successive depressions of sea 

 level separately, although it must of course be understood 

 that several oscillations of sea level must have taken place. 



Under this theory the smooth floor of a large lagoon is 

 regarded as preserving with little change the flat surface of the 

 platform abraded during the lower stand of the sea ; and the 

 embayments of the central islands within a barrier reef are 

 explained as drowned valleys that were eroded while the sea 

 level was lowered. Like all the other theories of coral reefs, 

 this one will account for the visible features of the reefs them- 

 selves, if its postulates and processes are accepted. The pos- 

 tulate of changes in the level of the sea during the glacial 

 period seems undeniable; but as recently set forth the theory 

 includes, on what seem to be unproved, hypothetical grounds, 

 the unessential assumption of a practically never-subsiding 

 ocean bottom ; for Daly writes : — " There is no reason to doubt 

 that the volcanoes [of the Pacific] here considered are of many 

 different ages, possibly from the pre-Cambrian to the Tertiary. 

 For the older ones, subaerial denudation must have . . . 

 approached or reached peneplanation. . . . Such denudation, 

 combined with marine erosion during pre- Pleistocene time, had 

 reduced most of the volcanic masses to the plateau form " 

 (304); and it is on such platforms that atolls are supposed to 

 stand, although not one of several uplifted atolls shows any 

 trace of such a platform. Thus the theory really belongs with 

 the still-stand theories already considered, as far as the stabil- 

 ity of the ocean bottom is concerned, and therefore stands 

 in unnecessary opposition to Darwin's theory of subsidence, 

 instead of, as appears to me more reasonable, working in coop- 

 eration with it. 



Our knowledge of ocean basins is by no means sufficient to 

 warrant the assumption that the ocean floors have been so 

 steady through Paleozoic, Mesozoic and Cenozoic time as to 

 allow the subaerial erosion and abrasion to truncate the cones 

 of pre-Cambrian or Paleozoic volcanoes and thus produce shal- 

 low platforms, on which the depth of water has changed only 

 as the climate of the glacial period changed the level of the 

 ocean surface. If some truncated cones had been revealed by 

 uplift, it would be easier to imagine that other cones, similarly 

 truncated, served as the foundations for hundreds of atolls ; 

 but, as above stated, not a single example of a truncated cone 

 has been observed in the coral seas. 



Again, it is not reasonable to assign the special value, zero, 

 to a process like ocean-floor subsidence, of which we know so 

 little, particularly in an ocean which contains many uplifted 

 reefs, still more in an ocean which in its western part, where 

 coral reefs are most abundant, contains several islands of con- 



