ISLANDS.

 

far as the Tonga Islands, cutting the Australasian
chain at right angles.

96. Isolated Oceanic Islands are mainly of two
kinds: the Volcanic and the Coral. As a rule, the
Volcanic islands are high, while Coral islands sel-
‘dom rise more than twelve feet above the water.

Volcanic Islands are not confined to isolated
groups, but occur also in long chains. The Poly-
nesian, Sandwich, and New Zealand Chains con-
tain numerous volcanic peaks. But the high, iso-
lated oceanic islands are almost always of voleanie
origin, and, consisting of the summits of subma-
rine volcanoes, are generally small. Some of the
Canary and Sandwich Islands, which are of this
class, rise nearly 14,000 feet above the sea.

97. Coral Islands, or Atolls, though of a great
variety of shapes, agree in one particular:

They consist of a low, narrow rim of coral rock,
enclosing a body of water called a lagoon.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 36, A Coral Island,

98. Mode of Formation of Coral Islands.—The
reef forming the island is of limestone, derived
from countless skeletons of minute polyps that
once lived beneath the surface of the waters.
The skeletons, however, are not separate. The
polyp propagates its species by a kind of bud-
‘ding; that is, a new polyp grows out of the body
of the old. In this way the skeletons of count-
less millions of polyps are united in one mass and
assume a great variety of shapes.

One of the most common species of reef-forming corals,
the madrepora, is shown in Fig. 37. Many other forms
exist.

The delicate coral structures, together with
shells from various shellfish, are ground into frag-
ments by the action of the waves, and by the in-

 

Fig, 37, Coral,

filtration of water containing lime in solution,
they become compacted into hard limestone, on
which new coral formations grow.

The growth of the coral mass is directed up-
ward, and ceases when low-water mark is reached,
because exposure to a tropical sun kills the polyps.
But the action of the waves continues, and the
broken fragments are gradually thrown up above
the general level of the water. In this way a reef
is formed, whose height is limited by the force of
the waves, and seldom exceeds twelve feet.

On the bare rock, which has thus emerged, a
soil is soon formed and a scanty vegetation ap-
pears, planted by the hardy seeds scattered over
it by the winds and waves.

The coral island never affords a very comfortable resi-
dence for man. The palm tree is almost the only valuable
vegetable species; the animals are few and small, and the
arable soil is limited. Moreover, the island is subject to
occasional inundations by huge waves from the ocean.

99. Distribution of Coral Islands. —According
to Dana, the reef-forming coral polyp is found
only in regions where the winter temperature
of the waters is never lower than 68° Fahr.
Some varieties, however, will grow in colder
water. Coral islands are confined to those parts
of tropical waters where the depth does not greatly
exceed 100 feet, and which are protected from cold
ocean-currents, from the influence of fresh river-
waters, muddy bottoms, and remote from active vol-
canoes, whose occasional submarine action causes
the death of the coral polyp. Though some coral
polyps grow in quiet water, the greater part thrive
best when exposed to the breakers. The growth ts
therefore more rapid on the side toward the ocean
than on the side toward the island.

‘