THE GEOLOGY OF BERMUDA. IT 



tlie soft drift-rock around the shores suffered extensive marine erosion^ 

 and the shore platform and clifis already' described were formed. 



On this hypothesis, the peculiarities of Bermuda mentioned by Dar- 

 win as rendering its atoll character at least doubtful,* admit of ready 

 explanation. The absence of the usual horizontal reef-platform, and 

 the gradual shoaling of the water for a mile or more around the islands^ 

 may be accounted for by the supi)osition that the last subsidence was 

 too rapid and too recent to allow the growth of the reef into its usual 

 and typical form.t The original atoll character has, indeed, been greatly 

 modified by the subsequent changes; and the gradually sloping bottom 

 for some distance from the shore presents, instead of the typical hori- 

 zontal reef-platform, a plane of marine denudation formed by the rapid 

 erosion of the soft calcareous sand-rock during the progressive subsi- 

 dence. Dana has shown that a subsidence too rapid for the growth of 

 the reef to keep pace with it may lead to the formation of narrow fring- 

 ing reefs, producing thus an effect which may counterfeit the effects of 

 elevation.^ Darwin is inclined to regard the fringing reefs on the south, 

 shore of Bermuda as evidence of recent elevation ;§ but I believe alE 

 the facts taken together are far more satisfactorily explained on the 

 hypothesis that the latest movement has been one of subsidence. The 

 extraordinary size and elevation of Bermuda, as compared with other 

 atolls, is accounted for by the vast accumulation of drift-sand during a 

 period of elevation. Darwiu, indeed, admits that the probable iEolian 

 formation of most of the Bermudian rock renders the unusual height of 

 the islands immaterial as an objection to their atoll character. || 



The difference in the amount of dry land between the northern and 

 southern sides of the ellipse is doubtless due, as suggested by Dana,^ 

 in part to the prevailing southerly winds, the windward side of the atoll 

 being the more favorable both for the growth of the reef proper and for 

 the accumulation of beach and drift sand-rock; and partly to difler- 

 eucies in the configuration of the lands around which the reefs were 

 formed. 



It is a profound and comprehensive suggestion of Professor Dana 



* Coral Eeefs, p. 264. 



tFor estimates illustrative of the extreme slowness of the growth of coral reefs, see 

 Dana, Corals and Coral Islands, pp. 249-254. 



t Notes on the now edition of Mr. Darwin's work on the Structure and Distribution 

 of Coral Reefs: in Nature, Vol. X., pp. 408,409. 



$ Coral Reefs, p. 265. 



II Coral Reefs, p. 265. 



IT Corals and Coral Islands, p. 221. 



Bull. Nat. Mus. No. 25 2 



