interesting observations on this subject. He informs us that the
low islands of the South Sea and Indian Ocean owe their origin
principally to the operations of several species of coral. Their
situation with respect to each other, as they often form rows, their
union in several places in large groups, and their total absence in
other parts of the same seas, induce us to conclude, that the corals
have founded their building on shoals of the sea; or, to speak more
correctly, on the tops of mountains lying under water. On the one
side, as they increase, they continue to approach the surface of
the sea, on the other side they enlarge the extent of their earth.
The larger species of corals, which form blocks, measuring several
fathoms in thickness, seem to prefer the more violent surf on the
external edge of the reef; this, and the obstacles opposed to the
continuation of their life, in the middle of a broad reef, by the
amassing of the shells abandoned by the animals, and fragments of
corals, are probably the reason that the outer edge of the reef first
approaches the surface. As soon as it has reached such a height, that
it remains almost dry at low water, the corals leave off building
higher; sea-shells, fragments of coral, shells of echini, and their
broken-off prickles, are united by the burning sun, through the
medium of the cementing calcareous sand, which has arisen from
the pulverization of the above mentioned shells into one whole or
solid stone, which, strengthened by the continual throwing up of new
materials, gradually increases in thickness till it at last becomes
so high, that it is covered only during some seasons of the year by
the high tides. The heat of the sun so penetrates the mass of stone
when it is dry, that it splits in many places, and breaks off in
flakes. These flakes, so separated, are raised one upon another by
the waves at the time of high water. The always active surf throws
blocks of coral, (frequently of a fathom in length, and three or
four feet thick,) and shells of marine animals, between and upon the
foundation stones; after this the calcareous sand lies undisturbed,
and offers to the seeds of trees and plants, cast upon it by the
waves, a soil upon which they rapidly grow, to overshadow its
dazzling white surface. Entire trunks of trees, which are carried by
the rivers from other countries and islands, find here, at length, a
resting place after their long wanderings; with them come some small
animals, such as lizards and insects, as the first inhabitants. Even
before the trees form a wood, the real sea-birds nestle here; strayed
land-birds take refuge in the bushes; and at a much later period,
when the work has been long since completed, man also appears, builds
his hut on the fruitful soil formed by the corruption of the leaves
of the trees, and calls himself lord and proprietor of this new
creation.
In the preceding account, we have seen how the exterior edge of a
submarine coral edifice first approaches the surface of the water,
and how this reef gradually assumes the properties of land; the
island, therefore, necessarily has a circular form, and in the
middle of it an inclosed lake. This lake, however, is not entirely
inclosed; (and it could not be, for without supply from the sea it
would soon be dried up by the rays of the sun,) but the exterior
wall consists of a great number of smaller islands, which are
separated from each other by sometimes larger, sometimes smaller
spaces. The number of these islets amounts, in the larger coral
islands, to sixty; and between them it is not so deep but that it
becomes dry at the time of ebb. The interior sea has in the middle
generally a depth of from thirty to five-and-thirty fathoms; but on
all sides towards the land the depth gradually increases. In those
seas where the constant monsoons prevail, where, consequently, the
waves beat only on one side of the reef or island, it is natural
that this side of the reef, exposed to the unremitting fury of the
ocean, should be formed chiefly by broken-off blocks of coral, and
fragments of shells, and first rise above the elements that created
it. It is only these islands respecting the formation and nature
of which we hitherto know any thing with certainty; we are almost
entirely without any observations on those in the Indian and Chinese
Sea, which lie in the regions of the six months’ monsoons. From the
charts given of them, it is to be inferred that every side is equally
advanced in formation. The lee side of such a coral reef in the
Pacific Ocean, which is governed by the constant monsoons, frequently
does not shew itself above the water, when the opposite side, from
time immemorial, has attained perfection in the atmospheric region;
the former reef is even interrupted in many places by intervals
tolerably broad, and of the same depth as the inner sea, which have
been left by nature, like open gates, for the exploring mariner
to enter the internal calm and secure harbour. In their external
form the coral islands do not resemble each other; but this, and
the extent of each, probably depends on the size of the submarine
mountain tops, on which their basis is founded. Those islands which
have more length than breadth, and are opposed in their greatest
extent to the winds and waves, are richer in fruitful islets than
those whose situation is not so adapted to a quick formation.
In the large island-chains, there are always some single islets
which have the appearance of high land; these lie upon an angle
projecting into the sea, are exposed to the surf upon two sides,
consist therefore almost entirely of large blocks of coral, and are
destitute of smaller fragments of shells and coral sand to fill up
the intervals. They are, therefore, not adapted to support plants
requiring a depth of soil, and only afford a basis to high trees,
provided with fibrous roots, (as the Pisonia, Cordia Sebastiana, L.;
Morinda citrifolia, L.; and Pandanus odoratissimus, L.), which, at
a distance, give to these, always very small islands, the form of a
hill. The inner shores of the island, exposed to the surf, consist
of fine sand, which is washed up by the tide. Between the small
islands under their protection, and even in the middle of the inner
sea, are found smaller pieces of coral, which seek a quiet abode,
form in time, though very slowly, banks, till they at last reach the
surface of the water; gradually increase in extent; unite with the
islands that surround them; and at length fill up the minor seas,
so that what was at first a ring of islands, becomes one connected
land. The islands which are so far formed, retain in the middle a
flat plain, which is always lower than the wall that surrounds them
on the banks; for which reason pools of water are formed in them,
after a continued rain,--the only springs and wells they possess. One
of the peculiarities of these islands is, that no dew falls in the
evening, that they cause no tempests, and do not check the course of
the wind. The very low situation of the country sometimes exposes the
inhabitants to great danger, and threatens their lives when the waves
roll over their islands, if it happens that the equinox and full
moon fall on the same day (consequently when the water has reached
its greatest height), and a storm agitates the sea at the same time.
These islands are said to be also shaken by earthquakes.
MM. Quoy and Gaimard, in a lately published memoir, propose, _1st_,
To examine how corals raise their habitations upon rocks, and what
circumstances are favourable or unfavourable to their growth. _2d_,
To shew that there are no islands of any extent, constantly inhabited
by man, which are entirely formed of corals; and that far from
raising from the depths of the ocean perpendicular walls, as has been
alleged, these animals form only layers or crusts of a few fathoms
thickness.
The following, according to the French naturalists, is the manner in
which this addition or superposition of madrepores is effected. In
the places where the heat is constantly intense, where the land is
indented by bays containing shallow and quiet water, which is not
liable to be agitated by great surges, or by the regular breezes
of the tropics, there also the saxigenous polypi multiply. They
construct their habitations on the submarine rocks, envelope these
rocks in whole or in part, but do not form them properly speaking.
Thus, all those reefs, those girdles of madrepore, which are so
frequently met with in the South Sea, to the leeward of islands,
are shoals depending upon the conformation of the original ground,
which will be perceived to belong to it when the direction of the
mountains and hills has been attentively observed. It is always where
the slopes are gentle, and the sea shallowest, that the greatest
masses of madrepores are found. They sprout up if it is calm; in the
contrary case, they form only scattered tufts, belonging to species
which seem to be least affected by the agitation of the waters.
It has been said, and it is even a matter of general belief among
mariners, say MM. Quoy and Gaimard, that there occur in the
equatorial seas shoals composed of _corals_, which rise from the
greatest depths, like walls at the bottom of which the sounding
line finds no ground. The fact certainly does exist in so far as
regards the depth spoken of; and it is this very circumstance which
is productive of so much danger to vessels, which, when taken in a
calm and carried away by currents, cannot cast anchor in such places.
But it is not correct to say that these reefs are entirety formed of
madrepores. First, because the species which always form the most
considerable banks, such as some meandrinæ, certain caryophylleæ, but
especially the astreæ, adorned with the most beautiful and velvety
colours, require the influence of light to perfect them; because
they are not seen to grow beyond a few yards of depth; and because
they cannot consequently be developed at a depth of ten or twelve
hundred feet, as they would necessarily be, did they raise the cliffs
in question. Besides, these different species of animals would then
almost exclusively enjoy the privilege of living at all depths,
under all degrees of pressure, and, so to speak, in all temperatures.
Another circumstance to which navigators have not adverted, which
corroborates the opinion here stated, is, that, in depths so great
as those to which we allude, the sea, always agitated at the
surface, breaks with force upon these reefs, without requiring for
that purpose any additional impulse from the winds. And by merely
attending to the necessary consequences of the observations of these
same navigators, who say (what is very true) that, wherever the waves
are agitated, the lithophytes are unable to go on with their work,
because they destroy their frail edifices, we shall acquire the moral
certainty that these submarine steeps are not produced by these
animalcules. But, in these same places, let there occur a hollow, a
sheltered spot of some kind, and then they will immediately raise
their habitations, and will contribute to diminish the little depth
that already exists there. And this is what may be seen in almost all
the places where an elevated temperature permits these animals to
grow in abundance.
In the localities where the tides are sensible, their currents alone
may sometimes form irregular canals between the madrepores, without
their ever being encumbered with their species, from the twofold
cause united, of the motion and the coldness of the water; while, on
the other hand, the flexible alcyonia are seen to multiply there.
When these geological dispositions are carefully observed, we see
that the zoophytes rise to the surface of the waves, never beyond
it; after which the generation which has attained thus far appears
to die. It is destroyed much sooner, if, from the effect of the
tides, these frail animalcules are exposed naked to the action of a
burning sun. When there occur small hollows in these heaps of inert
spoils, deprived of their inhabitants, which are always covered by
the water, several tufts of those lithophytes are still remarked,
which, having escaped from the almost general destruction, glow with
the most lively colours. Then, the families which are developed anew,
not being able to build on the outside of those reefs on which the
sea is constantly breaking, draw nearer and nearer the shore, where
the waves now deadened have scarcely any more action upon them, as in
the Isle of France, at Timor, the Papua, the Marian, and the Sandwich
Islands; provided always the waters had not a great depth, as is
the case at Turtle Island, of which Cook speaks, where no bottom is
found between the madrepore reefs and the island, notwithstanding the
shortness of the space which exists between these two points.
If we examine these animals in the places best adapted to their
growth, we shall see their different species, the forms of which, as
varied as they are elegant, become rounded into balls, spread out
into fans, or ramify into trees, mingling together, blending with
each other, and reflecting the varied hues of red, yellow, blue and
violet.
It is well known that all these alleged walls, exclusively formed of
corals, are intersected with openings through which the sea enters
and retires with violence; and every body knows the danger which
Captain Cook ran on one occasion, on the coast of New Holland, when
he had no other resource, in order to save himself from destruction,
than to take the sudden resolution of attempting one of these narrow
passes, where one is always sure of finding plenty of water. And this
circumstance also comes in support of what we have advanced; for, if
these perpendicular walls were entirely composed of madrepores, they
would present no deep openings in their continuity, because it is the
property of zoophytes to build in masses that have no interruption;
and because, again, could they raise themselves from very great
depths, they would end with encumbering and shutting up these
passages; a circumstance which does not take place, and probably
never will, from the causes which we have related.
If these facts prove, that madrepores cannot exist at very great
depths, the submarine rocks, which they only increase in height, are
not, therefore, exclusively formed by them.
We now come to the second part of the argument; and we assert, that
there are no islands of any magnitude and constantly inhabited by
man, that are formed by corals; and that the layers which they
construct under the water, are not more than a few fathoms in
thickness.
We shall commence with the second part of this question. The
impossibility of penetrating to the bottom of the sea to examine
at what precise depth the solid zoophytes establish themselves,
constrains us to confine ourselves to what has taken place in former
times; and the monuments which the ancient revolutions of the globe
have disclosed to our view, will serve to prove what is going on in
our own days. We shall mention what has been seen in several places,
and we shall first speak of the island which Peron took for the
theatre of the great works of these polypi, namely the island of
Timor.
The banks of coral which the sea has left exposed in the land,
as it retired, are remarkable for their uncommon magnitude. The
whole shores of Coupang are formed of them, and the low hills in
its vicinity are enveloped in them; but a few hundred yards from
the town, they disappear, when distinct strata of slate make their
appearance. The corals form a bed over the subjacent rocks from 25 to
80 feet thick.
Every thing announces that, in the Island of Timor, there exist
no mountains exclusively formed of corals. As in all extensive
countries, they are composed of various substances. Quoy and Gaimard
having coasted it for about fifty leagues, sufficiently near to
enable them to form an idea of its geography, were able to see that
it exhibited volcanic appearances in several parts. Besides it
abounds in mines of gold and copper, which, in conjunction with what
we have already mentioned, shews in a general way the nature of the
rocks of which it is composed.
Perhaps, remarks Quoy and Gaimard, the Bald-Head, a mountain of
King George’s harbour in New Holland, which Vancouver has described
in passing, and on the summit of which he saw perfectly preserved
branches of coral, might be adduced as a fact in opposition to the
opinion here advanced. Yet the phenomenon exhibited there, is still
precisely the same as at Timor, and in a thousand other places[382].
The zoophytes have built upon a basis previously existing, and
they occupy only the surface of it. For why should this Bald-Head
differ from Mount Gardner, which, although close by it, is formed of
primitive rocks? Besides, Peron says, that it has the same geological
constitution. (T. ii. p. 133.)
At Rota, one of the Marian Isles, M. Gaudichaud, detached from the
limestone rock, at about a hundred toises above the level of the
sea, branches of true madrepores, in perfect preservation. Here are,
then, three localities in which they are found at great heights. We
have observed them, say the French naturalists, at infinitely lower
elevations in several other places, as at the Isle of France, where
they form a bed more than six feet thick, between two streams of
lava; at Wahou, one of the Sandwich Islands, where they have not
a greater elevation, but extend for several hundred toises over
the surface of the island. In all these cases, it is necessary to
distinguish between the lithophytes, which have, by their living
powers, formed continuous masses, from those which, after having been
rolled about, broken down by the water, and mixed with sea shells,
contribute to form those deposits known by the name of _madrepore
limestone_. The latter sort is nothing but the debris of the former.
Deposits of this description occur in the Marian Isles, and in those
of the Papous; they occur also on the coasts of France, and in
several other places.
It would appear from observations made in Timor and other places,
that the species of the genus Astræa which are the only ones capable
of covering immense extents of surface, do not commence their
operations at a greater depth than twenty-five or thirty feet, in
order to raise their habitations to near the surface of the sea.
Fragments of these species are never obtained, either with the
sounding line, or upon the anchors; nor do we ever see them, unless
in places where the water is shallow; while the branched madrepores,
which do not form thick and continuous beds, either on the elevated
places which the ocean has left, or on the shores where they still
exist, live at considerable depths.
It is evident, then, that these corals have erected their fabrics
on the summits of submarine hills and mountains; and that all those
reefs of Taiti, the Dangerous Archipelago, Navigators’ Islands, the
Friendly Islands, &c. are composed of madrepores only at the surface.
We thus consider it demonstrated, that the rocks of the solid
zoophytes or coral, are not capable of forming the immense bases on
which the greater number of the islands that occur in the Pacific
Ocean rest.
There now remains for us to state how these animals, by their
union, are capable of raising small islets. Forster, as already
stated, has given a very good description of the manner in which
this is effected. In fact, when these animalcules have raised their
habitations to the surface of the water, under the shelter of the
land, and they remain uncovered during the reflux of the tide, the
hurricanes which sometimes supervene, by the agitation which they
produce in those shallow waters, throw up from the bottom sand and
mud. These substances are detained in the sinuosities and cavities
formed between the corals, and thus serve to fix them together, and
connect them into masses. Whenever the summit of this new island
can remain constantly uncovered by the sea, and the waves can no
longer destroy what they themselves have contributed to form, then
its extent is enlarged, and its edges are gradually raised by the
successive addition of sand. According to the direction of the
winds and currents they may long remain sterile; but if the seeds
of vegetables be transported to them from the neighbouring shores,
by the action of these two causes, then, in latitudes favourable to
their development, we presently see these islands becoming covered
with verdure, the successively accumulated remains of which form
layers of soil, which contribute to the elevation of the surface.
But, in order that this phenomenon of growth be accomplished,
the distance from land must not be too great, because then the
vegetables cannot get so easily to the islets in question, which
then remain almost always bare and sterile. And for this reason what
navigators report of those madrepore Islands of the Great Ocean,
which are covered with verdure, and are yet at a great distance
from any known land, has always appeared to us extraordinary; and
that so much the more, that, in those vast spaces, the violence
of the waves, which nothing can break there, must disturb the
operations of the zoophytes. We do not, however, deny the existence
of these islands, which it would be interesting carefully to examine
anew; for, whenever navigators meet with low islands between the
Tropics, they do not hesitate, in compliance with the generally
received opinion, to say that they consist of madrepores. Yet how
many islands, which scarcely rise above the surface of the water,
recognise no such origin? We may mention, as an example, the Island
of Boni, situated under the equator, the beautiful vegetation of
which rises upon limestone. Cocoa Island, near Guam, is in the same
condition, being also composed of limestone. In general, if they are
inhabited, consequently they have springs or lakes of fresh water,
we may almost be certain that they are not composed of lithophytes,
or are only so in part, because springs could not be formed in their
porous substances. Some of the Caroline Isles are excessively low; we
supposed them encrusted with madrepores; and as they have inhabitants
there must be somewhere in them a soil favourable to the accumulation
of fresh water[383].
In restraining the power of these animalcules, concludes Quoy and
Gaimard, and in pointing out the limits which nature has prescribed
them, we have no other object than to furnish more correct data to
the naturalists who aspire to great hypothetical considerations,
regarding the conformation of the globe. On reconsidering these
zoophytes with greater attention, they will no longer be seen
filling up the basins of the seas, raising islands, increasing
the size of the continents, threatening future generations with a
solid equatorial circle formed of their spoils. Their influence,
with regard to the road-steads or harbours, in which they multiply,
is already great enough, without adding more to it. But, compared
with the masses on which they rest, what are their layers, often
interrupted, and which must be searched for with care, before they
can be recognised, to the enormous volcanic peaks of the Sandwich
Islands, the Island of Bourbon, the Moluccas, the Marian Islands, the
mountains of Timor, New Guinea, &c. &c.? Nothing, certainly; and the
solid zoophytes are in no degree capable of being compared with the
testaceous mollusca, with reference to the materials which they have
furnished, and still continue to furnish to the crust of the Globe.
NOTE I, p. 33.
ON THE LEVEL OF THE BALTIC.
About the middle of the last century, a controversy took place
among the natural philosophers of the north of Europe, regarding
the alleged gradual lowering of the level of the sea in general,
and of the Baltic Sea in particular. Celsius was the first who
introduced this idea to notice. He generalised it by applying it
to all the planets, and was supported by the authority of the
celebrated Linnæus. He soon perceived, however, that the point could
never be settled by mere discussion, and that facts alone could
lead to any certain result. Observation was therefore had recourse
to; and thus the dispute in question had at least one good effect,
that of directing to the subject the attention of men of science,
whose situation might enable them to mark the variations of level
that take place along the coasts of the North Sea. The results of
investigations undertaken for this purpose, are now beginning to be
collected.
In the course of 1820 and 1821, Mr Bruncrona, assisted by the
officers of the Pilotage Establishment, and other qualified persons,
undertook the examination of all the authentic measures that had been
established upon the west coast of the Baltic, during the last half
century. The results of this examination are given in a short memoir,
inserted in the Swedish Transactions for 1823. The following table
indicates the degree to which the level of the sea has fallen during
the last forty years, on the coast of Sweden, at various latitudes.
It is proper to remark, that, in some of the places observed, the
measures were much older, and in some others much more recent,
than the period of forty years. In both these cases, the change of
level that must have been effected during this period, has been
estimated, by calculating the mean annual depression furnished by the
observations.