W. M. Davis — Shaler Memorial Study of Coral Reefs. 257 



complicated than that offered by the embayed shorelines of 

 the central islands within sea-level barrier reefs, it is no less 

 consistently in favor of upgrowth during submergence. But 

 by very reason of the variety and complication of the evidence 

 furnished by elevated reefs, the need of studying each example 

 for itself is the greater. 



Submergence and Subsidence. — In view of the foregoing dis- 

 cussion it appears reasonable to conclude that a submergence of 

 the foundations of barrier reefs during the formation of the reefs 

 is demonstrated, and that a similar submergence of foundations 

 took place during the formation of various other reefs now 

 elevated ; thus the long-lasting issue between the several still- 

 stand theories of coral reefs and certain other theories which 

 postulate submergence may be regarded as settled in favor of 

 the latter, as far as reefs that have provided independent testi- 

 mony about their origin are concerned. As to atolls, which 

 afford no such testimony, and as to barrier reefs and elevated 

 reefs which have not been examined, it is still logically permis- 

 sible to maintain a belief in their formation by some still-stand 

 process, if any one wishes to do so. 



Among the theories which postulate submergence as the 

 cause of upgrowth for existing sea-level reefs, the glacial-con- 

 trol theory, acting alone, appears to be insufficient ; and it does 

 not seem to have been in operation at the time of the formation 

 of certain elevated reefs. Hence some other cause of submerg- 

 ence, strong in value and long in intermittent operation, must 

 be sought for. 



It might seem, at first thought, that no other cause than sub- 

 sidence, as postulated in Darwin's theory, could be suggested ; 

 yet it must soon become evident that the embayed shoreline 

 of a barrier-reef island, together with the upgrowth of the sur- 

 rounding barrier reefs, may be quite as well explained by a rise 

 of the sea surface around still-standing islands as by a sinking 

 of the islands beneath a still-standing sea surface ; and likewise 

 that all the indications of submergence given by the structure 

 of uplifted reefs can be explained as well by a contemporaneous 

 rise of sea level as by a subsidence of their foundations. It is 

 in order to avoid begging the question thus opened that the 

 word, submergence, has been used on the foregoing pages in- 

 stead of subsidence. Truly, if signs of recent submergence 

 were found around all the coasts of the world, and if the measure 

 and date of the submergence were everywhere the same, so 

 uniform and universal a change would have to be explained by a 

 rise of sea surface ; but signs of submergence are by no means 

 universal. If indications of recent and uniform submergence 

 were found on a majority of coasts, insular and continental, so 

 extended a change of sea level might also be best explained by 



