236 W. M. Davis — Shaler Memorial Study of Coral Reefs. 



none the less expectable in the total mass of a coral reef ram- 

 part, formed along a similar coast in warmer latitudes. Therein, 

 indeed, must lie one of the chief contrasts between the total reef 

 mass of a continental border and the total reef mass around a 

 small oceanic island. The contrast might be so great as to war- 

 rant different names for the two structures ; but whatever the 

 nomenclature of the problem, it is reasonable to suppose that a 

 barrier reef was the distinguishing feature of the Queensland 

 border in the long still-stand period before the recent sub- 

 mergence, just as it is in the present period after the sub- 

 mergence. Hence, it seems to me unreasonable to conclude, 

 without decisive evidence, that the Queensland border had no 

 barrier reef while the continental shelf was forming along the 

 border of New South Wales. 



Coral Reefs and the Glacial Period. — It remains to inquire 

 which one of the theories that postulate a change in the rela- 

 tive level of land and sea best accounts for the facts of barrier 

 reefs and their associated central islands. One of these is Dar- 

 win's simple theory of a slowly subsiding ocean bottom, as a 

 result of which the islands that stand on the ocean bottom 

 gradually sink, diminish in size, and eventually disappear, 

 while their fringing reefs grow upwards and are converted 

 into barrier reefs and atolls. The other is the more complicated 

 "glacial-control theory," independently suggested by Belt and 

 by Upham some years ago, briefly discussed by Penck in 1894, 

 and lately elaborated by Daly* with especial reference to atolls. 

 It begins, as above stated, by assuming essentially still-standing 

 foundations, above and around which outgrowing atolls of less 

 or greater size, standing near sea level, A, fig. 4, with shallow 

 lagoons or none, were developed in preglacial times ; a lower- 

 ing of sea level is then inferred during the glacial period, 

 when a significant amount of sea water was withdrawn to form 

 the continental ice sheets, and when in consequence of lowered 

 ocean temperature the corals of most reefs were killed ; next 

 follows an abrasion of the undefended preglacial reefs by wave 

 attack on their flanks so as to reduce them to flat platforms, D, 

 a little below the lowered sea level ; and finally, when a rising 

 temperature melts the continental ice-sheets and the sea surface 

 is raised and warmed, and the corals are permitted to grow 

 again, reefs are built up to the present sea level, E, around the 

 margin of the abraded platforms, producing barrier reefs or 

 atolls. A critical discussion of this theory would require a 

 consideration of the several glacial and interglacial epochs 

 into which the glacial period as a whole is divided, but I 

 shall here follow Daly in using only the general term, glacial 



* Pleistocene Glaciation and the Coral Eeef Problem, this Journal, xxx, 

 297-308, 1910. 



