ARTICLES 423 



submergence of stationary reef foundations and the upgrowth 

 of their reefs, this " Glacial-control " theory, like various other 

 theories, gives a possible means of accounting for the few 

 observable features of atolls, provided the various assumptions 

 that go with the theory are accepted ; but when the conse- 

 quences of the theory are confronted with the more exacting 

 requirements of barrier reefs and their encircled islands, the 

 theory proves unsatisfactory, because the emergence preceding 

 the submergence is of too short a duration, and because the 

 submergence that it provides is small and uniform in amount, 

 and everywhere of the same date ; also because the postulate 

 of still-standing islands and reef foundations is extremely im- 

 probable, and for other reasons as well. This aspect of the 

 problem also I have endeavoured to set forth in an article 

 already published. 1 



Unconformable Contact of Reef Limestones and their Foun- 

 dations. — Attention will be directed in the present article chiefly 

 to one highly significant structural feature of coral reefs ; a 

 feature that, in spite of its strong testimony for subsidence, 

 has been even more generally neglected than the embayed shore- 

 lines of reef-encircled islands : namely, the unconformable 

 contact of reef and lagoon limestones with their foundations, 

 as illustrated in section M of sector L, Fig. 1 . This is a matter 

 which is, one may say, tacitly involved in all theories that 

 recognise the association of submergence with reef upgrowth, 

 and the nature and the extent of the unconformity are of 

 critical importance in estimating the amount of submergence 

 and the duration of the preceding emergence of the reef foun- 

 dations ; but, in spite of the century-long establishment of the 

 elementary principle here involved, its importance has been 

 almost universally overlooked, because the origin of coral reefs 

 has been too generally treated as a zoological instead of as a 

 geological problem. 



It is true that a few small and discontinuous fringing reefs 

 which border certain young volcanic islands, and a few other 

 small and discontinuous reefs which occur in association with 

 certain delta deposits may be regarded as lying conformably 

 on a non-eroded under-structure ; but with these unimportant 

 exceptions the limestones of all fringing and barrier reefs, 



1 " Problems Associated with the Study of Coral Reefs," Set. Monthly, ii. 1916, 

 , 313-333, 479-5oi. 557-572 ; see p. 563. 



