CORAL REEFS. APPENDIX. 217 



first brought up by upheaval to or above the sea-level. Lat- 

 eral extensions or wings grow out on either side, so as to 

 ultimately form a horse-shoe reef. Such a reef presents its 

 convexity against the prevailing surface currents, to which in 

 truth it owes its shape " (loc. cit., p. 900 ; this view of the forma- 

 tion of atollons or horse-shoe reefs is further elaborated in Mr. 

 Guppy's paper "Preliminary Note on Keeling Atoll," Nature, 

 Jan. 3, 1889). Such are seemingly the conditions that we find 

 on Santa Anna Island, but the examination of the 100-fathom 

 contour line, which closely conforms to the actual bounda- 

 ries of the island, even to the indentation of the 17-fathom 

 Port Mary concerning which Mr. Guppy expresses himself 

 as having " been unable to obtain any satisfactory explana- 

 tion " ("Solomon Islands," p. 117) proves conclusively, I be- 

 lieve, that the surface exposed above water is merely the cor- 

 respondent of that which is below it, in other words, the island 

 has grown up on a base of its own form, which base is seem- 

 ingly a breached crateral cone of a volcano. It repeats on a 

 larger scale what is still presented by its own highest elevation, 

 the eastern volcanic cone, which carries " a small circular hol- 

 low, between 100 and 150 yards across and 35 or 40 feet in 

 depth. There was a time in its history, when the present sum- 

 mit alone appeared at the surface of the Sea as a tiny ring of 

 coral reef, capping a submerged volcanic peak, the remains of 

 which still exist in the shallow basin on the highest part of 

 the island " (op. cit., p. 113). I think we are well justified from 

 this evidence in assuming that the large breached-ring is sim- 

 ilarly only an upgrowth from a larger crateral border, upon 

 which the small cone is perched. Mr. Darwin early recognized 

 the possibility of such a structure, and he guardedly affirmed 

 his belief that under suitable conditions a " reef like a perfectly 

 characterized atoll " might be formed over the rim of a crater 

 (" Structure and Distribution of Coral-Reefs," 1842, p. 89). 



It is therefore in no way surprising that the thickness of the 

 coral-made rock on this island should be comparatively slight, 

 and nowhere exceeding 150 feet. 



