se skeletons accumulate on
their summits, and they further receive a constant rain of the
calcareous and silicious skeletons of minute organisms which teem in the
waters above. By these agencies the banks are gradually raised to the
lowest depth at which reef-building corals can flourish, and once these
establish themselves they will grow more rapidly on the periphery of the
bank, because they are more favourably situated as regards food-supply.
Thus the reef will rise to the surface as an atoll, and the nearer it
approaches the surface the more will the corals on the exterior faces be
favoured, and the more will those in the centre of the reef decrease,
for experiment has shown that the minute pelagic organisms on which
corals feed are far less abundant in a lagoon than in the sea outside.
Eventually, as the margin of the reef rises to the surface and material
is accumulated upon it to form islets or continuous land, the coral
growth in the lagoon will be feeble, and the solvent action of sea-water
and the scour of the tide will tend to deepen the lagoon. Thus the
considerable depth of some lagoons, amounting to 40 or 50 fathoms, may
be accounted for. The observations of Guppy in the Solomon islands have
gone far to confirm Murray's conclusions, since he found in the islands
of Ugi, Santa Anna and Treasury and Stirling islands unmistakable
evidences of a nucleus of volcanic rock, covered with soft earthy bedded
deposits several hundred feet thick. These deposits are highly
fossiliferous in parts, and contain the remains of pteropods,
lamellibranchs and echinoderms, embedded in a foraminiferous deposit
mixed with volcanic debris, like the deep-sea muds brought up by the
"Challenger." The flanks of these elevated beds are covered with
coralline limestone rocks varying from 100 to 16 ft. in thickness. One
of the islands, Santa Anna, has the form of an upraised atoll, with a
mass of coral limestone 80 ft. in vertical thickness, resting on a
friable and sparingly argillaceous rock resembling a deep-sea deposit.
A. Agassiz, in a number of important researches on the Florida reefs,
the Bahamas, the Bermudas, the Fiji islands and the Great Barrier Reef
of Australia, has further shown that many of the peculiar features of
these coral formations cannot be explained on the theory of subsidence,
but are rather attributable to the natural growth of corals on banks
formed by prevailing currents, or on extensive shore platforms or
sub