Coral belongs to the class of those surprising productions of nature,
which are named _zoöphytes_, or plant-animals, on account of their
filling up the intermediate space between the animal and vegetable
kingdoms; and in treating of them this curious substance will be
distinctly considered. In the mean time, the production of coral reefs
and islands presents one of those geological changes, by which the
earth’s surface has been modified, and has received a new accession from
the sea.
The common foundation of the clusters of islands discovered by modern
navigators in the Pacific ocean, as well as of those belonging to New
South Wales, is evidently of coral structure, immense reefs of which
shoot out in all directions. There is every reason to believe that the
islands which are occasionally raised by the tremendous agency of
subterraneous volcanoes, do not bear any proportion to those which are
perpetually forming, by the silent but persevering efforts of the sea
worms by which coral is produced. Banks of coral are found at all
depths, and at all distances from the shore, entirely unconnected with
the land, and detached from each other. By a quick progression, they
grow up toward the surface; while the winds, heaping up the coral from
deeper water, rapidly accelerate the formation of these banks into
shoals and islands. They become gradually shallower; and when once the
sea meets with resistance, the coral is quickly thrown up by the force
of the waves breaking against the bank. These coral banks have been seen
in all their stages: some in deep water; others with a few rocks
appearing above the surface, just formed into islands without the least
appearance of vegetation; and, lastly, others covered with soil and
weeds.
The loose corals, rolled inward by the billows in large pieces, strike
upon the grounds, and, the reflux being unable to carry them away,
become a bar to the coagulated sand with which they are always
intermixed. This sand, being easiest raised, is lodged at top; and when
its accumulated mass is elevated by violent storms, and no longer within
the reach of common waves, it becomes a resting-place to birds whom the
search of prey draws thither. Their excrement, feathers, &c., augment
the soil, and prepare it for the reception of accidental roots, branches
and seeds cast up by the waves, or brought thither by birds. Thus
islands are formed: the leaves and rotten branches, intermixing with the
sand, produce in time a light black mold, in which trees and shrubs
vegetate and thrive. Cocoa-nuts, which continue long in the sea without
losing their vegetative powers, having been thrown on such islands,
produce trees which are particularly adapted to all soils, whether
sandy, rich or rocky. The violence of the waves, within the tropics,
must generally be directed to two points, according to the monsoons.
Hence the islands formed from coral banks must be long and narrow, and
lie nearly in a meridional direction. Even supposing the banks to be
round, as they seldom are when large, the sea meeting most resistance in
the middle, must heave up the matter in greater quantities there, than
toward the extremities; and, by the same rule, the ends will generally
be open, or at least lowest. They will also commonly have soundings
there, as the remains of the banks, not accumulated, will be under
water. Where the coral banks are not exposed to the common monsoon, they
will alter their direction, and become either round, or extended in the
parallel, or of irregular forms, according to accidental circumstances.
Captain Flinders, in his voyage to the South Pacific, gives an account
of an unbroken reef of coral, three hundred and fifty miles long, on the
coast of New Holland; and he states, that between that country and New
Guinea, the coral formations extend through a distance of seven hundred
miles, interrupted by no intervals of more than thirty miles in length.
He also gives us a lively and interesting description of a coral reef on
the southern coast of New South Wales. On this reef he landed, and the
water being very clear round the edges, a new creation, as it were, but
imitative of the old, was presented to the view. Wheat sheaves,
mushrooms, stags’ horns, cabbage leaves, and a variety of other forms,
were glowing under water with vivid tints of every shade betwixt green,
purple, brown and white; equaling in beauty, and excelling in grandeur,
the most favorite _parterre_ of the curious florist. These were
different species of coral and fungus, growing, as it were, out of the
solid rock, and each had its peculiar form and shade of coloring; but,
whilst contemplating the richness of the scene, the destruction with
which it was pregnant could not be forgotten. Different corals in a dead
state, concreted into a solid mass of a dull white color, composed the
stone of the reef. The negro heads were lumps which stood higher than
the rest; and being generally dry, were blackened by the weather; but
even in these the forms of the different corals and some shells were
distinguishable. The edges of the reef, but particularly on the outside
where the sea broke, were the lightest parts; within these were pools
and holes containing live corals, sponges, sea-eggs and cucumbers; and
many enormous cockles were scattered upon different parts of the reef.
At low water, these cockles seem most commonly to lie half open, but
frequently close with much noise, and the water within the shells then
spouts up in a stream, three or four feet high: it is from this noise
and the spouting of the water that they are discovered, for, in other
respects, they are scarcely to be distinguished from the coral rock.
His description of a coral island which he afterward visited on the same
coast, is truly philosophical and throws great light on these surprising
productions of nature.
“This little island, or rather the surrounding reef, which is three or
four miles long, affords shelter from the south-east winds. It is
scarcely more than a mile in circumference, but appears to be increasing
both in elevation and extent. At no very distant period of time, it was
one of those banks produced by the washing up of sand and broken coral,
of which most reefs afford instances, and those of Torres’ strait a
great many. These banks are in different stages of progress: some, like
this, are become islands, but not yet habitable; some are above
high-water mark, but destitute of vegetation; whilst others are
overflowed with every returning tide. It seems to me, that, when the
animalcules which form the corals at the bottom of the ocean, cease to
live, their structures adhere to each other, by virtue either of the
glutinous remains within, or of some property in salt water; and the
interstices being gradually filled up with sand and broken pieces of
coral washed by the sea, which also adhere, a mass of rock is at length
formed. Future races of these animalcules erect their habitations upon
the rising bank, and die in their turn, to increase, but principally to
elevate, this monument of their wonderful labors. The care taken to work
perpendicularly in the early stages, marks a surprising instinct in
these diminutive creatures. Their wall of coral is for the most part in
situations where the winds are constant, and when it reaches the
surface, it affords a shelter, to the leeward of which their infant
colonies may be safely sent forth; and to this their instinctive
foresight it seems to be owing, that the windward side of a reef exposed
to the open sea, is generally, if not always, the highest part, and
rises almost perpendicular, sometimes from the depth of two hundred, and
perhaps many more fathoms. To be constantly covered with water, seems
necessary to the existence of the animalcules, for they do not work,
except in holes upon the reef, beyond low-water mark; but the coral sand
and other broken remnants thrown up by the sea adhere to the rock, and
form a solid mass with it, as high as the common tides reach. That
elevation surpassed, the future remnants, being rarely covered, lose
their adhesive property; and remaining in a loose state, form what is
usually called a _key_ upon the tops of the reef. The new bank is not
long in being visited by sea-birds; salt plants take root upon it, and a
soil begins to be formed; a cocoa-nut is thrown on shore; land birds
visit it and deposit the seeds of shrubs and trees; every high tide, and
still more every gale, adds something to the bank; the form of an island
is gradually assumed, and last of all comes man to take possession. This
island is well advanced in the above progressive state; having been many
years, probably some ages, above the reach of the highest spring-tides,
or the wash of the surf in the heaviest gales. I distinguished, however,
in the rock which forms its basis, the sand, coral and shells formerly
thrown up, in a more or less perfect state of cohesion; small pieces of
wood, pumice-stone, and other extraneous bodies, which chance had mixed
with the calcareous substances when the cohesion began, were inclosed in
the rock, and, in some cases, were still separable from it without much
force. The upper part of the island is a mixture of the same substances
in a loose state, with a little vegetable soil, and is covered with the
_casuarina_ and a variety of other trees and shrubs, which give food to
paroquets, pigeons, and some other birds; to whose ancestors it is
probable the island was originally indebted for this vegetation.”
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