CORAL-REEFS. 71 



shall soon see, there is a whole class of reefs, which take 

 their name from growing closely attached (especially where 

 the sea is deep) to the beach. At New Caledonia (see 

 Plate III., Fig. 3) the reefs which run in front of the west 

 coast are prolonged in the same line 150 miles beyond the 

 northern extremity of the island, and this shows that some 

 explanation, quite different from any of those just suggested, 

 is required. The continuation of the reefs on each side of 

 the submarine prolongation of New Caledonia is an exceed- 

 ingly interesting fact, if this part formerly existed as the 

 northern extremity of the island, and before the attachment 

 of the coral had been worn down by the action of the sea, 

 or if it originally existed at its present height, with or 

 without beds of sediment on each flank, how can we 

 possibly account for the reefs, not growing on the crest of 

 this submarine portion, but fronting its sides, in the same 

 line with the reefs which front the shores of the lofty island ? 

 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 barrier-reefs remains, but it is almost too pre- 

 posterous to be mentioned; — namely, that they rest on 

 enormous submarine craters, surrounding the included 

 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 

 almost every one. New Caledonia, moreover, besides its 

 size, is composed of primitive formations, as are some of 

 the Comoro Islands \ x and Aitutaki consists of calcareous 

 rock. We must, therefore, reject these several explanations, 

 and conclude that the vertical thickness of barrier-reefs, 



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

 who has visited this group. 



