W M. Davis — Shaler Memorial Study of Coral Reefs. 261 



where would be required to raise the ocean surface by such 

 amounts. 



A happy escape from the embarrassment of these super- 

 extravagant measures may be found by assuming that the area 

 of compensating subsidence includes the district of the reefs 

 themselves, as in the third section of fig. 8 ; for then the sub- 

 sidence need be no greater than the measure of submergence, 

 except for any small rise of sea level that may result from an 



Fig. 9. 



Fig. 9. Deduced stages of an upgrowing reef on a subsiding island. 



excess in a neighboring, compensating uplift, or from unrelated 

 uplifts, and except for a very slight fall of sea level due to the 

 withdrawal of a small volume of water from all the rest of the 

 ocean in order to cover the snbmei'ged part of the subsiding 

 islands. But behold, in this escape from the embarrassments 

 inherent in the proposed alternative for Darwin's theory of 

 subsidence, we are led back to that very theory ; hence it may 

 now be more directly considered. 



Darwin's Theory of Subsidence. — In Darwin's first book on 

 Coral Reefs (1842) he explicitly recognized the idea of com- 

 pensating uplifts and depressions, for in describing areas of 

 unlike movement in the Pacific he said they were related " as 

 if the sinking of one area balanced the rising of another " (p. 

 145). Thus the condition postulated in the preceding para- 

 graph should be regarded as a characteristic element in his 

 treatment of the coral reef problem, although it is not one that 

 he greatly emphasized. It is here further considered, its 

 deduced consequences being illustrated in fig. 9 ; especial atten- 

 tion should be given to the section, M, of the atoll sector, L. 



If a barrier-reef group like the Society islands sinks with a 

 subsiding ocean bottom, and a neighboring part of the ocean 



