W. M. Davis — Shaler Memorial Study of Coral Reefs. 267 



greater uniformity of subsidence over certain large areas than 

 now seems necessary, in view of later discovered facts ; but, as 

 noted above, the ease with which the first outline of the theory 

 may be modified so as to account for the new facts is to its 

 advantage, not to its discredit ; and after all the new facts are 

 considered, subsidence of the ocean bottom in areas where bar- 

 rier reefs and atolls prevail is to-day, as it was in Darwin's 

 time, the best explanation that has been offered for the 

 observed phenomena 



As to the first deficiency above-named, it seems that after 

 Darwin had invented his theory of subsidence, he came to 

 know that the central islands of barrier reefs were often 

 embayed, but he did not see that the embayments must neces- 

 sarily result from the partial subsidence of a previously dis- 

 sected volcanic island, probably because he ascribed many large 

 valleys to marine erosion. It was Dana who, as has already 

 been pointed out, first showed that subsidence must produce 

 embayments ; and Darwin's theory thus extended is now 

 regarded by an increasing number of students as more com- 

 petent to explain coral reefs in general than any other theory 

 yet proposed ; but like Darwin, Dana did not clearly distin- 

 guish between subsiding islands and a rising ocean surface. 

 However, in view of the considerations presented above under 

 the heading, " Submergence and Subsidence," repeated eleva- 

 tions of ocean level by the amount demanded by many coral 

 reefs appears improbable to the point of inadmissibility ; thus 

 the competence of Darwin's theory becomes all the greater. 

 Dana's name should, therefore, be linked with Darwin's in the 

 discussion of coral reefs, not only because he observed various 

 reefs that Darwin did not visit, not chiefly because he was 

 thus led to support Darwin's views, but far more because he 

 brought to light a necessary but unexpected consequence, dedu- 

 cible from Darwin's theory, that Darwin himself always over- 

 looked : for this deduced consequence — the embayment of 

 barrier-reef islands — is confirmed by observation, and, taken 

 with the structural features of elevated reefs, supplies the most 

 compelling evidence of the whole problem. 



Subsidence Modified by Glacial Changes of Sea level. — A 

 suggestion made in an earlier section regarding Reefs and Reef 

 Platforms deserves expansion. It has been recognized that 

 changes of sea level and of sea temperature must have taken 

 place in some undetermined measure during the glacial period ; 

 but it has been shown that a change of level caused by regional 

 subsidence seems on the whole to have been the dominating 

 factor in the formation of the heavy limestone masses of bar- 

 rier reefs and probably also of atolls. Let the attempt now be 

 made to discover the results that will follow from the com- 

 bined action of these two changes ; the temporary and excep- 



