66 CORAL-REEFS. 



in imagination the encircled land, we shall find that, besides 

 the many points already noticed of resemblance, or rather 

 of identity in structure with atolls, there is a close general 

 agreement in form, average dimensions, and grouping. 

 Encircling barrier-reefs, like atolls, are generally elongated, 

 with an irregularly rounded, though sometimes angular out- 

 line. There are atolls of all sizes, from less than two miles 

 in diameter to sixty miles (excluding Tilla-dou-Matte, as it 

 consists of a number of almost independent atoll-formed 

 reefs) ; and there are encircling barrier-reefs from three 

 miles and a half to forty-six miles in diameter, — Turtle 

 Island being an instance of the former, and Hogoleu of the 

 latter. At Tahiti the encircled island is thirty-six miles in 

 its longest axis, whilst at Maurua it is only a little more than 

 two miles. It will be shown, in the last chapter in this 

 volume, that there is the strictest resemblance in the group- 

 ing of atolls and of common islands, and consequently there 

 must be the same resemblance in the grouping of atolls and 

 of encircling barrier-reefs. 



The islands lying within reefs of this class are of very 

 various heights. Tahiti 1 is 7,000 feet ; Maurua about 800; 

 Aitutaki 360, and Manouai only 50. The geological nature 

 of the included land varies : in most cases it is of ancient 

 volcanic origin, owing apparently to the fact that islands of 

 this nature are most frequent within all great seas ; some, 

 however, are of madreporitic limestone, and others of 

 primary formation, of which latter kind New Caledonia 



1 The height of Tahiti is given from Captain Beechey ; Maurua from 

 Mr. F. D. Bennett {GeograJ>h. Jour., vol. viii. p. 220); Aitutaki from 

 measurements made on board the Beagle ; and Manouai or Harvey 

 Island, from an estimate by the Rev. J. Williams. The two latter 

 islands, however, are not in some respects well characterised examples 

 of the encircled class. 



