Here they form a ring, surrounding a little inland lake,
that communicates with the sea by means of gaps. There they make
barriers of reefs like those on the coasts of New Caledonia and the
various Pomoton islands. In other places, like those at Reunion and
at Maurice, they raise fringed reefs, high, straight walls, near
which the depth of the ocean is considerable.
Some cable-lengths off the shores of the Island of Clermont I
admired the gigantic work accomplished by these microscopical
workers. These walls are specially the work of those madrepores
known as milleporas, porites, madrepores, and astraeas. These polypi
are found particularly in the rough beds of the sea, near the
surface; and consequently it is from the upper part that they begin
their operations, in which they bury themselves by degrees with the
debris of the secretions that support them. Such is, at least,
Darwin's theory, who thus explains the formation of the _atolls_, a
superior theory (to my mind) to that given of the foundation of the
madreporical works, summits of mountains or volcanoes, that are
submerged some feet below the level of the sea.
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