68 BARRIER REEFS. Ch. II. 



end had been worn away until it was a little beneath 

 the level of the sea, why should the coral-reefs have 

 become attached not on the central crest, but in the 

 same line with the reefs which still front the existing 

 shores? We shall hereafter see, that there is one, and 

 I believe only one solution of this difficulty. 



One other supposition to account for the position 

 of encircling reefs remains, but it is almost too pre- 

 posterous to be mentioned ; — namely, that they rest 

 on enormous submarine craters surrounding the in- 

 cluded islands. When the size, height, and form of 

 the islands in the Society group are considered, 

 together with the fact that all are thus encircled, 

 such a notion will be rejected by every one. New 

 Caledonia, moreover, besides its size, is composed of 

 primitive formatious, as are some of the Comoro 

 Islands ; l and Aitutaki consists of calcareous rock. 

 We must, therefore, reject these several explanations, 

 and conclude that the vertical thickness of barrier- 

 reefs, from their outer edges to the foundation on 

 which they rest (from AA in the sections No. 4 to 

 the dotted lines), is really great : but this presents 

 no real difficulty, as I hope to show hereafter when 

 the upward growth of coral-reefs, during the slow 

 subsidence of their foundation, is discussed. 



1 I have been informed that this is the case by Dr. Allen of Forres, 

 who has visited this group. 



