e Oceania is applied to all of the islands in
the Pacific, but in a more limited sense only to those lying between the
American continent and Australasia.

The chief divisions of Oceania are Australasia, Melanesia, Micronesia,
and Polynesia. Australia, the largest body of land, is usually regarded
as a continent. Nearly all the smaller islands are of coral or of
volcanic origin; in many instances both agencies have contributed to
their formation. The coral and volcanic islands seem to be the tops of
mountain ranges that, little by little, have sunk, until only their
higher summits are now above sea level.

The central part of the Pacific Ocean is pre-eminently the home of the
reef-building coral. Countless islands and reefs, wholly or partly built
up by these tiny creatures, are found widely scattered over an immense
area limited to one thousand eight hundred miles on each side of the
equator. All these formations are composed of the compact limestone
remains of coral polyps.

These polyps have the power of extracting carbonate of lime from the
sea-water and building it into massive formations which, for the most
part, are nearly or completely submerged.

The reef-building coral differs very materially in form and appearance
from the precious or red coral; the former is confined to comparatively
shallow water, while the latter is found most commonly at a depth of
six hundred feet or more, and it occurs chiefly in the Mediterranean
Sea. The common or reef-building coral has but little use except as a
source of lime, and no intrinsic value except as an object of curiosity.

Coral reefs may be arranged under three classes; namely, fringing reefs,
barrier reefs, and atolls. The first class embraces the shallow-water
reefs found close to land, either surrounding islands or skirting the
shores of continents. The reefs of the second class likewise skirt
islands or continents, but at such distances as to leave a deep channel
between them and the shore. The third class are called atolls; each is
irregularly ring-shaped and almost entirely encloses a sheet of water,
called a lagoon.

The ring-shaped reef, or atoll, is broken in one or more places,
generally on the leeward side, and built up higher on the windward side.
The reason for such omissions and buildings is obvious when we remember
that the coral animal cannot move from its fixed position to seek food,
but must depend upon the waves to bring it within reach. The wat