have a very peculiar appearance. They are called Atolls or Coral
Islands. Although not exactly of volcanic origin, yet the manner in
which they are formed has some connexion with submarine volcanic
action.

An atoll consists essentially of a ring of coral rocks but little
elevated above the level of the sea, and having in its centre a
lagoon or salt-water lake, which generally communicates by a deep
narrow channel with the sea. The ring of rocks is flat on the
surface, which is composed of friable soil, and sustains a
luxuriant vegetation, chiefly of cocoa-nut palms. It is seldom more
than half a mile in breadth between the sea and lagoon, sometimes
only three or four hundred yards. The outer margin of the ring is
the highest, and it slopes gradually down towards the lagoon; but
on the outside of the ledge of rocks is a beach of dazzling
whiteness, composed of powdered and broken coral and shells. The
appearance they present is thus not less beautiful than singular.
Some of these islands are of large size, from thirty to fifty miles
long, and from twenty to thirty broad, but they are in general
considerably smaller. Their most frequent form is either round or
oval. The rocks composing them are all formed by different species
of coral. The animal which constructs them is of the polyp tribe,
and so small that it can be seen only under the higher powers of
the microscope. It multiplies by means of buds like those of a
tree, the individuals all combining to form a composite stony mass,
which is called a polypidom. A number of such polypidoms growing
close together form a coral reef. See woodcuts.

[Illustration: Coral]