190 J. D. Dana — Origin of Coral Reefs and Islands. 



V. Conclusion. 



With the theory of abrasion and solution incompetent, all 

 the hypotheses of objectors to Darwin's theory are alike weak ; 

 for all have made these processes their chief reliance, whether 

 appealing to a calcareous, or volcanic, or mountain-peak base- 

 ment for the structure. The subsidence which the Darwinian 

 theory requires has not been opposed by the mention of any 

 fact at variance with it, nor by setting aside Darwin's argu- 

 ments in its favor; and it has found new support in the facts 

 from the Challenger's soundings off Tahiti that had been put 

 in array against it, and strong corroboration in the facts from 

 the West Indies. 



Darwin's theory therefore remains as the theory that accounts 

 for the origin of coral reefs and islands. 



VI. Central-Pacific Subsidence. 



Darwin, as has been said, took a step beyond direct observa- 

 tion in his inference that the subsidence attested to by each 

 atoll extended over the intermediate seas and characterized a 

 large central area of the ocean. He may be wrong here (and 

 the writer with him) while not wrong in his theory. But, con- 

 sidering the distribution of the Pacific atolls in the ocean, their 

 relation in this respect to the distribution of other Pacific lands, 

 and the facts connected with the history of coral reefs and 

 islands, the generalization appears to be well sustained. The 

 question is here left without further argument, to be considered 

 over the best geographical map of the ocean to be had, and the 

 best bathy metrical map that can be made, only asking that the 

 doubts which physical theory has set afloat may not be al- 

 lowed by the geologist to warp the judgment or cripple inves- 

 tigation.* 



My own agreement with Darwin as to the area of Coral-reef 

 subsidence was promoted by an early personal study of the 

 oceanic lands. For more than five years previous to passing 

 my third decade I was ranging over the oceans — receiving im- 



* One point often encountering an a-priori doubt is the slowness of the required 

 subsidence. The subsidence over the Appalachian region which preceded the 

 making of the Appalachian mountains amounted, according to well ascertained facts 

 (as stated by Hall and Lesley), to at least 30,000 feet. The great trough, nearly a 

 thousand miles long, was in progress through all of Paleozoic time. If the Paleo- 

 zoic ages covered only 20,000,000 years (a low estimate) the mean annual rate was 

 0*018 inch, which is less than half a millimeter per year. Such a fact is no evi- 

 dence as to the rate of the atoll-making subsidence ; but, whatever the cause to- 

 which the Appalachian subsidence is to be attributed, it is suggestive as to possi- 

 bilities and probabilities connected with the earth's movements. 



