assemblages of polypes which spring by budding, or by dividing, from a
single polype, occasionally attain very considerable dimensions. Such
skeletons are sometimes great plates, many feet long and several feet
in thickness; or they may form huge half globes, like the brainstone
corals, or may reach the magnitude of stout shrubs, or even small
trees. There is reason to believe that such masses as these take a
long time to form, and hence that the age a polype tree, or polype
turf, may attain, may be considerable. But, sooner or later, the coral
polypes, like all other things, die; the soft flesh decays, while the
skeleton is left as a stony mass at the bottom of the sea, where it
retains its integrity for a longer or a shorter time, according as its
position affords it more or less protection from the wear and tear of
the waves.

The polypes which give rise to the white coral are found, as has been
said, in the seas of all parts of the world; but in the temperate and
cold oceans they are scattered and comparatively small in size,
so that the skeletons of those which die do not accumulate in any
considerable quantity. But it is otherwise in the greater part of the
ocean which lies in the warmer parts of the world, comprised within a
distance of about 1,800 miles on each side of the equator. Within the
zone thus bounded, by far the greater part of the ocean is inhabited
by coral polypes, which not only form very strong and large skeletons,
but associate together into great masses, like the thickets and the
meadow turf, or, better still, the accumulations of peat, to which
plants give rise on the dry land. These masses of stony matter, heaped
up beneath the waters of the ocean, become as dangerous to mariners
as so much ordinary rock, and to these, as to common rock ridges, the
seaman gives the name of "reefs."