The attention of seamen and navigators has long been attracted by the
number of circular islands in the warm parts of the Pacific and Indian
oceans. Generally each one of these circular islands contains a lake of
quiet water extending almost to its outer shores, so that the island
looks like a fairy ring of land floating in the ocean, and adorned with
tropical trees and plants.
Happily for the boys and girls of the present day, this subject, with
other equally fascinating branches of science, has now been studied by
naturalists, who give us the rich results of their labors. It seems
scarcely possible that the dainty beautiful corals which we examined not
long ago in YOUNG PEOPLE can have anything to do with the making of
islands, but so it is. Coral reefs are vast masses of coral which have
grown in warm oceans. Their formation must have been slow, yet they
sometimes extend hundreds of miles. Florida and many other parts of our
solid continents are known to have been formed from coral reefs.
Let us now try to picture to ourselves the beginning of one of these
reefs, and by following its growth step by step we may at least
understand how it has been formed. There are hills and valleys on the
bottom of the ocean as well as on the land. We will fancy that some
young coral polyps which have been swimming about in the sea settle on
the sides of one of these hills, and begin to grow and spread all around
it. They will increase also by the deposit of eggs until they form a
circular wall.
As the coral wall grows, the lower polyps and the inner ones die, their
skeletons forming a solid foundation for all that grow above them. There
may be only about an inch of living coral on the outside of the reef.
[Illustration: Fig. 1.--a, a, SURFACE OF THE WATER; b, NATURAL RED OF
THE OCEAN; c, c, CORAL FORMATION; d, LAGOON.]
These walls rise nearly straight, and you will see that in doing so they
inclose a circular basin of quiet water, and now you can understand why
it is that a coral island mostly has a lake in the centre, as is shown
in Fig. 1. The lakes are called lagoons.
The bottom of the wall is formed of brain-coral and other solid kinds
which live only in deep water, and they die when a certain height is
reached. The formation of the new island does not stop with their death,
however. The wall having now reached the proper height to suit branching
corals, which require shallower water, their young polyps will settle
upon it, and finish the structure. We might suppose a reef formed of
branching corals would be open and unsubstantial, but in their growth
the branches are thickly interlaced. The spaces between them become
filled with substances floating in the ocean, and with pieces of coral
which are broken from the reef by the fierce dashing of the waves. The
whole forms a solid mass, stronger, perhaps, than any stone masonry. The
fragments of coral suffer no serious injury by breaking, but if lodged
in some favorable spot they continue to grow.
The outer edge of the wall is steep and abrupt. Soundings taken just
outside show very deep water. In this portion of the wall the corals
live and thrive, always supplied with clear water. The breakers dash
against it with such fury that apparently the hardest rock must in time
yield to the tremendous force of the waves. But, strange as it may
appear, the soft jelly-like bodies of the polyps give to the reef the
power of resisting the billows.
The inner surface of the wall slopes gently to the land, and being
washed by quiet waters often containing sand and mud, it is not
favorable to the growth of polyps. Still, there are certain kinds of
coral which thrive within the lagoons; some of them are exceedingly
brilliant and beautiful.
The coral polyps die before they reach the surface of the ocean, as no
corals can live out of water. The remainder of the island is built up by
shells, pieces of broken coral, sea-weed, and other floating materials
which are washed upon it, raising the wall higher and higher. The
never-ceasing action of the waves grinds up these shells and broken
coral, until at last they form a soil of sand and mud which is now ready
to receive any seeds that may float on the water or be brought by the
winds and the birds. The seeds take root in the new soil, and young
plants begin to appear on the glistening white surface. Floating
cocoa-nuts often lodge on the shores, and cocoa-nut-trees are among the
first to grow upon them. As the plants drop their leaves and decay, the
soil is enriched little by little, and fitted for the home of various
animals and birds, which in some mysterious manner find their way to
these lonely spots far out at sea. In time our coral reef may become a
beautiful tropical island fringed with waving trees and plants, and
inhabited by man.
[Illustration: Fig. 2.--AN ATOLL.]
Circular islands seldom form complete rings. There is generally an
opening into the lake on the side most sheltered from the wind. A safe
harbor in mid-ocean is thus made, in which vessels may take shelter, but
it requires an expert navigator to pass the perils at its entrance. To
anchor on the outer shore would be impossible. In Fig. 2 is a pretty
little coral island with ships in its lagoon. If a lake is entirely
inclosed by the coral wall, it may in time be changed to fresh water by
the rains that fall into it.
Coral reefs often extend to a depth of three hundred feet below the
surface of the ocean, and formerly persons were puzzled to know how they
could have grown in such deep water, as no coral polyps can live at a
greater depth than twenty or thirty fathoms. This puzzling question was
settled by the late Charles Darwin, who first showed that coral islands
occur where there has been a gradual sinking of the bottom of the ocean.
As the reef rises in height, the sinking of the foundation partly
counteracts the upward growth of the coral; consequently the proper
depth of water is secured, and the reef appears to be stationary,
whereas it is really growing upward.
Whenever a coral reef rises above the surface of the ocean, we may know
that the coral, which grew under water, has been lifted above the level
of the sea by a rising of the ocean-bed.
These circular reefs are called "atolls." They are quite different from
the "fringing reefs," which extend along the shores of continents and
islands. There are usually openings or breaks in fringing reefs directly
opposite the mouths of rivers and fresh-water streams, as the corals can
not endure currents which carry mud or sediment. Perhaps the grandest
reef to be found in any part of the world is the one extending along the
northeast coast of Australia. It is nearly one thousand miles in length,
and proves to us that the helpless coral polyps have played no trifling
part in the formation of our earth. All they have accomplished has been
done merely by their living and growing.
[Illustration: WINTER.]