498 
BARRIER-REEFS 
CHAP. 
of the corals. It is impossible here to enter into all the 
necessary details, but I venture to defy 1 any one to explain in 
any other manner, how it is possible that numerous islands 
should be distributed throughout vast areas—all the islands 
being low—all being built of corals, absolutely requiring a 
foundation within a limited depth from the surface. 
Before explaining how atoll-formed reefs acquire their 
peculiar structure, we must turn to the second great class, 
namely Barrier-reefs. These either extend in straight lines in 
front of the shores of a continent or of a large island, or they 
encircle smaller islands ; in both cases, being separated from 
the land by a broad and rather deep channel of water, 
analogous to the lagoon within an atoll. It is remarkable how 
BARRIER-REEF, BOLABOLA. 
little attention has been paid to encircling barrier-reefs ; yet 
they are truly wonderful structures. The accompanying 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. 
1 It is remarkable that Mr. Lyell, even in the first Edition of his Principles of 
Geology , inferred that the amount of subsidence in the Pacific must have exceeded 
that of elevation, from the area of land being very small relatively to the agents 
there tending to form it, namely, the growth of coral and volcanic action. 
