Externally the reef rises, like an atoll, with
extreme abruptness out of the profound depths of the ocean.
What can be more singular than these structures? We see
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an island, which may be compared to a castle situated on the
summit of a lofty submarine mountain, protected by a great
wall of coral-rock, always steep externally and sometimes
internally, with a broad level summit, here and there breached
by a narrow gateway, through which the largest ships can
enter the wide and deep encircling moat.
As far as the actual reef of coral is concerned, there is not
the smallest difference, in general size, outline, grouping,
and even in quite trifling details of structure, between a
barrier and an atoll. The geographer Balbi has well remarked,
that an encircled island is an atoll with high land rising out
of its lagoon; remove the land from within, and a perfect
atoll is left.
But what has caused these reefs to spring up at such
great distances from the shores of the included islands? It
cannot be that the corals will not grow close to the land;
for the shores within the lagoon-channel, when not surrounded
by alluvial soil, are often fringed by living reefs;
and we shall presently see that there is a whole class, which
I have called Fringing Reefs from their close attachment
to the shores both of continents and of islands. Again, on
what have the reef-building corals, which cannot live at
great depths, based their encircling structures? This is a
great apparent difficulty, analogous to that in the case of
atolls, which has generally been overlooked. It will be
perceived more clearly by inspecting the following sections
which are real ones, taken in north and south lines, through
the islands with their barrier-reefs, of Vanikoro, Gambier,
and Maurua; and they are laid down, both vertically and
horizontally, on the same scale of a quarter of an inch to
a mile.
It should be observed that the sections might have been
taken in any direction through these islands, or through
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many other encircled islands, and the general features would
have been the same. Now, bearing in mind that reef-building
coral cannot live at a greater depth than from 20 to 30
fathoms, and that the scale is so small that the plummets on
the right hand show a depth of 200 fathoms, on what are
these barrier-reefs based? Are we to suppose that each
island is surrounded by a collar-like submarine ledge of rock,
or by a great bank of sediment, ending abruptly where the
reef ends?
