62 BARRIER REEFS. Ch. II. 



rounded, though sometimes angular outline. There are 

 atolls of all sizes, from less than two miles in diameter 

 to sixty miles (excluding Tilla-dou-Matte, which 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 also be shown in 

 the last chapter, that there is the strictest resemblance 

 between the grouping of atolls and of common islands, 

 and there is the same resemblance between atolls and 

 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 geo- 

 logical nature of the included land also varies ; in most 

 cases it is of ancient volcanic origin, owing apparently to 

 the fact that islands of this nature are the most frequent 

 within all great seas ; some, however, are of madreporitic 

 limestone, and others of primary formation, of which 

 latter kind New Caledonia offers the best example. 

 The central land consists either of one island, or of 

 several ; thus in the Society group, Eimeo stands by 



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

 Mr. F. D. Bennett (Greograph. Journ. vol. viii. p. 220) ; Aitutaki from 

 measurements made on board the Beagle; and Manouai, or Harvey 

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

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

 of the encircled class. 



