Corals and coral islands.

346

CORALS AND CORAL ISLANDS8.

The amount of subsidence determines in some cases the distance of barrier reefs from shore; but it by no means accounts
for the difference in their extent in different parts of a single
group of islands. Indeed, if this cause be considered alone,
every grade of extent, from no subsidence to the largest amount,
might in many instances be proved as having occurred on a
single island. Of far greater importance, as has appeared, is
the volcanic character of the land. At whatever time the existing reefs in the Pacific commenced their growth, they began about those of the igneous islands whose fires had become
nearly or quite extinct; and as others in succession were extinguished, these became in their turn, the sites of corals, and
of coral reefs. Those lands whose volcanoes still burn, are
yet without corals, or there are only limited iatches on sonme
favored spots. Zoophytes and volcanoes are the land-making
agents of the Pacific. The latter prepare the way by pouring forth the liquid rock, and building up the lofty summit.
Quiet succeeds, and then commences the work of the zoophyte
beneath the sea, while verdure covers the exposed heights.
We may add a few mnore illustrations fromn other parts of
the coral-reef seas.
Along the north and northwest coast of Australia, there
appears to be little or no coral in the Gulf of Carpentaria,
while some extensive patches occur on the shores west of this
Gulf, as far as the northwest cape in latitude 23~ S.
In the East Indies, there are large, scattered reef-islands
south of Borneo and Celebes, about some of the Molluccas,
and near the west end of New Guinea. The islands of Timorlaut, ind Timor, with many of those intermediate, have large
reefs. The Arru Group consists wholly of coral. This sea,
fromn Arru, to the islands south of Borneo, is more thriving in
corals than anv other in the East Indies.