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.
(PLATE 95. SECTIONS OF BARRIER-REEFS. 1. Vanikoro. 2. Gambier
Islands. 3. Maurua.
The horizontal shading shows the barrier-reefs and lagoon-channels.
The inclined shading above the level of the sea (AA) shows the
actual form of the land; the inclined shading below this line shows
its probable prolongation under water.
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