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 everyone. New 
Caledonia, moreover, besides its size, is composed of 
primitive formations, as are some of the Comoro 
Islands ; 1 and Aitutaki consists of calcareous rock. 
We must, therefore, reject the several explanations, 
and conclude that the vertical thickness of barrier- 
reefs, from their outer edges to the foundation on 
which they rest (from A A in the sections No. 4 to 
the dotted lines), is really great : hut 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. Allan oi 
Forres, who has visited this group. 
