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



Part I. — The Darwinian Theory and its Evidences. 



1. According to the Darwinian theory, islands with fringing 

 reefs have been often changed through a slow subsidence of 

 the region into islands with barrier reefs ; and, as the last sum- 

 mit of the sinking land disappeared, the latter have become 

 atolls, that is, barrier reefs enclosing simply a piece of the 

 ocean (or a lagoon). Darwin added to this conclusion, a second, 

 in view of the wide distribution of atolls and their relations to 

 other islands : that the subsidence indicated involved the whole 

 central Pacific, besides other large areas. He also expressed 

 the opinion that a Pacific continent had disappeared through 

 the subsidence. The proofs of the first and the second con- 

 clusions are partly different and should not be confounded. 

 The third is no necessary part of the general theory, was not 

 adopted in my Eeport, and need not be further considered. 



2. Darwin did not hold that atolls were necessarily evidence 

 of a subsidence now in progress, but allowed that in some 

 regions they may have reached a state of rest, and may per- 

 haps have undergone an elevation since the cessation of the 

 subsidence ; and also that subsidence and elevation may have 

 alternated. 



3. Darwin found what he believed to be almost certain proof 

 of subsidence in the features of the large barrier-islands and 

 atolls. He perceived in the rocky islets that dot the great in- 

 terior lagoon-like waters of the Gambier group, Hogoleu, and 

 other similar barrier-islands of the Pacific, and the general 

 resemblance of such islands to atolls, strong evidence, " leav- 

 ing scarcely any doubt on the mind," that the islets were the 

 emerged points of sunken lands ; that such barrier-islands were 

 no less lagoon islands than Keeling atoll (the atoll which he 

 investigated) ; and if evidence of subsidence, the atoll was 

 proof of further subsidence, that is, one that had continued to 

 tbe disappearance of the sinking peaks. 



The evidence which had satisfied him was satisfactory to me 

 when I first learned of his views in Australia (in 1839), after a 

 cruise among the Paumotu atolls and the Tahitian and Samoan 

 reef-regions ; and more decidedly so later, when I had been 

 among the Friendly, Feejee and other Pacific Islands. 



That the argument may be appreciated I here introduce a 

 map of the eastern half of the Feejee Archipelago.* Several 

 of the great barrier reefs in this map, 10 to 20 miles in length, 

 have but one or two peaks of the sunken land remaining ; 

 Nanuku has but one little point near its southeastern angle, a 

 mile of peak within a barrier island 200 square miles in area ; 



* Reduced from the general map of the archipelago in the Atlas of the Wilkes 

 Expedition. 



