THE GEOLOGY OF BERMUDA. 17 
the soft drift-rock around the shores suffered extensive marine erosion,, 
and the shore platform and cliffs 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 supposition 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.t Darwin is inclined to regard the fringing reefs on the south 
shore of Bermuda as evidence of recent elevation;§ but I believe all 
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. Darwin, indeed, admits that the probable Holian 
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 differ- 
encies in the configuration of the lands around which the reefs were 
formed. 
It is a profound and comprehensive suggestion of Professor Dana 
* Coral Reefs, p. 264. 
’ +¥For estimates illustrative of the extreme slowness of the growth of coral reefs, see 
Dana, Corals and Coral Islands, pp. 249-254. 
{Notes on the new 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. 
|| Coral Reefs, p. 265. 
4] Corals and Coral Islands, p. 221. 
Bull. Nat. Mus. No. 25——2 
