Ch. n. 
BARTMETM1EEES. 
63 
moderated large islands, of nearly equal size, are in- 
cluded in ne reef. Within the reef of the Gambier 
group thee are four large and some smaller iskuids 
(fig. 8, Fate I.) ; within that of Hogoleu (fig. 2, 
Plate I.) jearly a dozen small islands are scattered over 
the expaise of one vast lagoon. 
Afte 7 the details now given, it may he asserted that 
there is not one point of essential difference between 
encirclug barrier -reefs and atolls; — the latter enclose 
a simpe sheet of water, the former encircle an expanse 
with cue or more islands rising from it. I was much 
struc’ with this fact, when viewing, from the heights 
of T hiti, the distant island of Eimeo standing within 
sme All wa ter, and encircled by a ring of snow-white 
Lr takers. Remove the central land, and an annular 
r f like that of an atoll in an early stage of its forma- 
tion is left ; remove Bolabola, and there remains a 
circle of linear coral-islets crowned with tall cocoa- 
rut trees, like one of the many atolls scattered over the 
Pacific and Indian oceans. 
The barrier-reefs of Australia and of New Caledonia 
deserve a separate notice from their great dimensions. 
The reef on the west coast of New Caledonia (fig. 5, 
Plate II.) is 400 miles in length ; and for a length of 
many leagues seldom approaches within eight miles of 
the shore. Near the southern end of the island, the 
space between the reef and the land is sixteen miles in 
width. The Australian barrier extends, with a few 
interruptions, for about eleven hundred miles ; its 
average distance from the land is between twenty and 
