[Illustration: Coral Polyp]

It was at one time supposed that these coral reefs were erected on
the edges of the craters of submarine volcanoes, an opinion to
which their annular form, and the lagoon in the centre, lent some
countenance; but the vast size of some of them, united to several
other particulars connected with them, threw great doubts over this
supposition.

More recently it has been shown by Mr. Darwin that, while volcanic
agency does perform a part in their formation, it is different from
what had been formerly imagined. His supposition is, that these
coral reefs were built round the coasts of islands which had once
stood very much higher above water than they do now. He conceives
that the bottom of the sea under them being very volcanic, and
containing large collections of molten lava beneath a thin solid
crust, the islands have gradually sunk down into the lava, until
their central parts have become covered with a considerable depth
of water. The central parts thus submerged, he imagines, form the
lagoons in the middle of the islands, while the ring of coral reefs
has gradually grown upwards, as the ground on which it rested sank
downwards.

[Illustration: Coral Reef.]

The corals thus rise to near the surface, but immediately on their
being uncovered by the water they die, and the reef ceases to grow.
Then the waves by their action break the upper part of it into
pieces, which thus become heaped up by degrees on the remainder,
until the mass attain so great a height that the sea can no longer