488 Mr Gardiner, The Coral Reefs of [Mar. 7, 



PART VI. 

 The Formation of Coral Reefs. 



The generally accepted theory of the formation of atolls, 

 before Darwin brought forward his well-known theory of subsi- 

 dence, was that they were situated on the rims of submarine 

 volcanoes. It is possible to explain the formation of atolls quite 

 satisfactorily on this theory, if such craters really can and do exist 

 at the requisite depths. Prof. Sollas, without expressing au 

 opinion, has drawn attention to the striking similarity of the 

 slopes of Funafuti to those of a volcano 1 . "The conical mountain 

 below the 140 fathoms line, with its parabolic slope is suggestively 

 similar to a volcano; but if so its crater must have been immense, 

 10 miles across at least." Many arguments can be advanced 

 against the possibility of there being such large craters and such 

 a large number, as would be requisite to account for the founda- 

 tions of the atolls of the Indian and Pacific Oceans. No volcanoes 

 with craters even as large as the medium sized atolls and atoll- 

 reefs exist at the present day ; and there are no indications, 

 except in the Andes, of such large chains of volcanoes as would be 

 requisite to form the Ellice, Gilbert and Marshall Groups, which 

 form a line as if of one mountain chain. Another great objection 

 to this theory lies in the fact that it leaves the formation of 

 barrier reefs completely out of account, and anyone, who has 

 examined both atoll and barrier reefs, cannot fail to have been 

 struck by their striking similarity to one another. Again, there is 

 found every intermediate variety from the narrow fringing reef to 

 the atoll with perhaps the greater part of its rim converted into 

 land. 



It is, however, very doubtful whether a submarine crater could 

 be formed as such ; lava meeting the water would usually break 

 up into loose vesicular cinders and ash. Falcon Island, Tonga, is 

 said 2 to be "just a bare brown heap of ashes." Sir A. Geikie 

 mentions 3 Graham's Island, Sicily, erupted 1831, Sabrina Island, 

 Azores, 1811, a new island in the basin of Santorin, 1866, and an 

 island formed in 1783, 30 miles west of Cape Reykjanaes, Iceland, 

 all of ash, and either completely w r orn away or being rapidly 

 destroyed. The only permanent cone, known to have been thrown 

 up by a submarine eruption, is the island of Johanna Bogoslawa, 



1 Loc. cit., p. 375. 



2 "The Newly Emerged Falcon Island." By J. J. Lister. Proc. R. G. Soc, 

 March, 1890. 



3 Text-Book of Geology, 1893. 





