1836.] BARRIER-REEFS. 469 
a broad and rather deep channel of water, analogous to the 
lagoon within an atoll. It is remarkable how little attention 
has been paid to encircling barrier-reefs; yet they are truly 
wonderful structures. The following sketch represents part of 
the barrier encircling the island of Bolabola in the Pacific, as 
seen from one of the central peaks. In this instance the whole line 
of reef has been converted into land ; but usually a snow-white line 
of great breakers, with only here and there a single low islet 
crowned with cocoa-nut trees, divides the dark heaving waters 
of the ocean from the light-green expanse of the lagoon-channel. 
And the quiet waters of this channel generally bathe a fringe of 
low alluvial soil, loaded with the most beautiful productions of 
the tropics, and lying at the foot of the wild, abrupt, central 
mountains. 
Encircling barrier-reefs are of all sizes, from three miles to 
no less than forty-four miles in diameter ; and that which fronts 
one side, and encircles both ends, of New Caledonia, is 400 miles 
long. Each reef includes one, two, or several rocky islands of 
various heights ; and in one instance, even as many as twelve 
separate islands. The reef runs at a greater or less distance 
from the included land; in the Society archipelago generally 
from one to three or four miles; but at Hogoleu the reef is 
20 miles on the southern side, and 14 miles on the opposite 
or northern side, from the included islands. The depth within 
the lagoon-channel also varies much; from 10 to 30 fathoms 
may be taken as an average; but at Vanikoro there are spaces no 
less than 56 fathoms or 336 feet deep. Internally the reef either 
slopes gently into the lagoon-channel, or ends in a perpendicular 
