G2 
BAERIER-IiEEFS. 
Cn. IL 
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 
geological nature of the included land also varies ; in 
most cases it is of ancient volcanic origin, owing appa- 
rently 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 forma- 
tion, 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 
itself; while -Taha and Eaiatea (fig. 3, Plate I.), both 
1 The height of Tahiti is given from Captain Beechey ; Maurua 
from Mr. F. D. Bennett (Geograph. Journ. vol. viii. p. 220); Aitutaki 
from measurements made on board the Beagle ; and Manouai, or 
Harvey Island, from an estimate by the Bev. J. Williams. The two 
latter islands, however, are not in some respects well characterised 
examples of the encircled class. 
