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
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