A section of the
reef and island in this state, after a subsidence of several
hundred feet, is given by the dotted lines. Coral islets are
supposed to have been formed on the reef; and a ship is anchored in
the lagoon-channel. This channel will be more or less deep,
according to the rate of subsidence, to the amount of sediment
accumulated in it, and to the growth of the delicately branched
corals which can live there. The section in this state resembles in
every respect one drawn through an encircled island: in fact, it is
a real section (on the scale of .517 of an inch to a mile) through
Bolabola in the Pacific. We can now at once see why encircling
barrier-reefs stand so far from the shores which they front. We can
also perceive that a line drawn perpendicularly down from the outer
edge of the new reef, to the foundation of solid rock beneath the
old fringing-reef, will exceed by as many feet as there have been
feet of subsidence, that small limit of depth at which the
effective corals can live:--the little architects having built up
their great wall-like mass, as the whole sank down, upon a basis
formed of other corals and their consolidated fragments.
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