Continental islands are situated in the immediate vicinity of the
continents, and form properly a part of the continental structure. They
have the same kinds of rocks and mountain forms, and the same varieties
of plants and large animals, which are found on the neighboring coasts
of the mainland.
The size of this class of islands varies extremely. Some are mere
isolated rocks, while others occupy large areas, like the British Isles,
Japan Islands and Madagascar; or, more extensive still, Papua and
Borneo, each of which has an area exceeding two hundred thousand square
miles.
The distinctive character of Oceanic islands is that they lie at a
distance from the continents, in the midst of the ocean basins. They are
always small, and, though sometimes forming lines, or bands, they more
frequently occur in groups.
The rocks which make up the body of the continents and continental
islands--sandstone, slate, granite, and the various metamorphic
rocks--are entirely wanting in oceanic islands. The latter are composed
either of volcanic substances, or of limestone. Hence they present much
less variety in relief forms than the continental islands.