Coral Reefs

Chapter IV. On The Distribution And Growth Of Coral-Reefs

In this chapter I will give all the facts which I have collected, relating
to the distribution of coral-reefs,--to the conditions favourable to their
increase,--to the rate of their growth,--and to the depth at which they are
formed.

These subjects have an important bearing on the theory of the origin of the
different classes of coral-reefs.


SECTION 4.I.--ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE.

With regard to the limits of latitude, over which coral-reefs extend, I
have nothing new to add. The Bermuda Islands, in 32 deg 15' N., is the
point furthest removed from the equator, in which they appear to exist; and
it has been suggested that their extension so far northward in this
instance is owing to the warmth of the Gulf Stream. In the Pacific, the
Loo Choo Islands, in latitude 27 deg N., have reefs on their shores, and
there is an atoll in 28 deg 30', situated N.W. of the Sandwich Archipelago.
In the Red Sea there are coral-reefs in latitude 30 deg. In the southern
hemisphere coral-reefs do not extend so far from the equatorial sea. In
the Southern Pacific there are only a few reefs beyond the line of the
tropics, but Houtmans Abrolhos, on the western shores of Australia in
latitude 29 deg S., are of coral formation.

The proximity of volcanic land, owing to the lime generally evolved from
it, has been thought to be favourable to the increase of coral-reefs.
There is, however, not much foundation for this view; for nowhere are
coral-reefs more extensive than on the shores of New Caledonia, and of
north-eastern Australia, which consist of primary formations; and in the
largest groups of atolls, namely the Maldiva, Chagos, Marshall, Gilbert,
and Low Archipelagoes, there is no volcanic or other kind of rock,
excepting that formed of coral.

The entire absence of coral-reefs in certain large areas within the
tropical seas, is a remarkable fact. Thus no coral-reefs were observed,
during the surveying voyages of the "Beagle" and her tender on the west
coast of South America south of the equator, or round the Galapagos
Islands. It appears, also, that there are none (I have been informed that
this is the case, by Lieutenant Ryder, R.N., and others who have had ample
opportunities for observation.) north of the equator; Mr. Lloyd, who
surveyed the Isthmus of Panama, remarked to me, that although he had seen
corals living in the Bay of Panama, yet he had never observed any reefs
formed by them. I at first attributed this absence of reefs on the coasts
of Peru and of the Galapagos Islands (The mean temperature of the surface
sea from observations made by the direction of Captain Fitzroy on the
shores of the Galapagos Islands, between the 16th of September and the 20th
of October, 1835, was 68 deg Fahr. The lowest temperature observed was
58.5 deg at the south-west end of Albemarle Island; and on the west coast
of this island, it was several times 62 deg and 63 deg. The mean
temperature of the sea in the Low Archipelago of atolls, and near Tahiti,
from similar observations made on board the "Beagle", was (although further
from the equator) 77.5 deg, the lowest any day being 76.5 deg. Therefore
we have here a difference of 9.5 deg in mean temperature, and 18 deg in
extremes; a difference doubtless quite sufficient to affect the
distribution of organic beings in the two areas.), to the coldness of the
currents from the south, but the Gulf of Panama is one of the hottest
pelagic districts in the world. (Humboldt's "Personal Narrative," volume
vii., page 434.) In the central parts of the Pacific there are islands
entirely free from reefs; in some few of these cases I have thought that
this was owing to recent volcanic action; but the existence of reefs round
the greater part of Hawaii, one of the Sandwich Islands, shows that recent
volcanic action does not necessarily prevent their growth.

In the last chapter I stated that the bottom of the sea round some islands
is thickly coated with living corals, which nevertheless do not form reefs,
either from insufficient growth, or from the species not being adapted to
contend with the breaking waves.

I have been assured by several people, that there are no coral-reefs on the
west coast of Africa (It might be concluded, from a paper by Captain Owen
("Geographical Journal", volume ii., page 89), that the reefs off Cape St.
Anne and the Sherboro' Islands were of coral, although the author states
that they are not purely coralline. But I have been assured by Lieutenant
Holland, R.N., that these reefs are not of coral, or at least that they do
not at all resemble those in the West Indies.), or round the islands in the
Gulf of Guinea. This perhaps may be attributed, in part, to the sediment
brought down by the many rivers debouching on that coast, and to the
extensive mud-banks, which line great part of it. But the islands of St.
Helena, Ascension, the Cape Verdes, St. Paul's, and Fernando Noronha, are,
also, entirely without reefs, although they lie far out at sea, are
composed of the same ancient volcanic rocks, and have the same general
form, with those islands in the Pacific, the shores of which are surrounded
by gigantic walls of coral-rock. With the exception of Bermuda, there is
not a single coral-reef in the central expanse of the Atlantic Ocean. It
will, perhaps, be suggested that the quantity of carbonate of lime in
different parts of the sea, may regulate the presence of reefs. But this
cannot be the case, for at Ascension, the waves charged to excess
precipitate a thick layer of calcareous matter on the tidal rocks; and at
St. Jago, in the Cape Verdes, carbonate of lime not only is abundant on the
shores, but it forms the chief part of some upraised post-tertiary strata.
The apparently capricious distribution, therefore, of coral-reefs, cannot
be explained by any of these obvious causes; but as the study of the
terrestrial and better known half of the world must convince every one that
no station capable of supporting life is lost,--nay more, that there is a
struggle for each station, between the different orders of nature,--we may
conclude that in those parts of the intertropical sea, in which there are
no coral-reefs, there are other organic bodies supplying the place of the
reef-building polypifers. It has been shown in the chapter on Keeling
atoll that there are some species of large fish, and the whole tribe of
Holothuriae which prey on the tenderer parts of the corals. On the other
hand, the polypifers in their turn must prey on some other organic beings;
the decrease of which from any cause would cause a proportionate
destruction of the living coral. The relations, therefore, which determine
the formation of reefs on any shore, by the vigorous growth of the
efficient kinds of coral, must be very complex, and with our imperfect
knowledge quite inexplicable. From these considerations, we may infer that
changes in the condition of the sea, not obvious to our senses, might
destroy all the coral-reefs in one area, and cause them to appear in
another: thus, the Pacific or Indian Ocean might become as barren of
coral-reefs as the Atlantic now is, without our being able to assign any
adequate cause for such a change.

It has been a question with some naturalists, which part of a reef is most
favourable to the growth of coral. The great mounds of living Porites and
of Millepora round Keeling atoll occur exclusively on the extreme verge of
the reef, which is washed by a constant succession of breakers; and living
coral nowhere else forms solid masses. At the Marshall islands the larger
kinds of coral (chiefly species of Astraea, a genus closely allied to
Porites) "which form rocks measuring several fathoms in thickness," prefer,
according to Chamisso (Kotzebue's "First Voyage" (English Translation),
volume iii., pages 142, 143, 331.), the most violent surf. I have stated
that the outer margin of the Maldiva atolls consists of living corals (some
of which, if not all, are of the same species with those at Keeling atoll),
and here the surf is so tremendous, that even large ships have been thrown,
by a single heave of the sea, high and dry on the reef, all on board thus
escaping with their lives.

Ehrenberg (Ehrenberg, "Uber die Natur und Bildung der Corallen Banke im
rothen Meere," page 49.) remarks, that in the Red Sea the strongest corals
live on the outer reefs, and appear to love the surf; he adds, that the
more branched kinds abound a little way within, but that even these in
still more protected places, become smaller. Many other facts having a
similar tendency might be adduced. (In the West Indies, as I am informed
by Captain Bird Allen, R.N., it is the common belief of those, who are best
acquainted with the reefs, that the coral flourishes most, where freely
exposed to the swell of the open sea.) It has, however, been doubted by
MM. Quoy and Gaimard, whether any kind of coral can even withstand, much
less flourish in, the breakers of an open sea ("Annales des Sciences
Naturelles," tome vi., pages 276, 278.--"La ou les ondes sont agitees, les
Lytophytes ne peuvent travailler, parce qu'elles detruiraient leurs
fragiles edifices," etc.): they affirm that the saxigenous lithophytes
flourish only where the water is tranquil, and the heat intense. This
statement has passed from one geological work to another; nevertheless, the
protection of the whole reef undoubtedly is due to those kinds of coral,
which cannot exist in the situations thought by these naturalists to be
most favourable to them. For should the outer and living margin perish, of
any one of the many low coral-islands, round which a line of great breakers
is incessantly foaming, the whole, it is scarcely possible to doubt, would
be washed away and destroyed, in less than half a century. But the vital
energies of the corals conquer the mechanical power of the waves; and the
large fragments of reef torn up by every storm, are replaced by the slow
but steady growth of the innumerable polypifers, which form the living zone
on its outer edge.

From these facts, it is certain, that the strongest and most massive corals
flourish, where most exposed. The less perfect state of the reef of most
atolls on the leeward and less exposed side, compared with its state to
windward; and the analogous case of the greater number of breaches on the
near sides of those atolls in the Maldiva Archipelago, which afford some
protection to each other, are obviously explained by this circumstance. If
the question had been, under what conditions the greater number of species
of coral, not regarding their bulk and strength, were developed, I should
answer,--probably in the situations described by MM. Quoy and Gaimard,
where the water is tranquil and the heat intense. The total number of
species of coral in the circumtropical seas must be very great: in the Red
Sea alone, 120 kinds, according to Ehrenberg (Ehrenberg, "Uber die Natur,"
etc., etc., page 46.), have been observed.

The same author has observed that the recoil of the sea from a steep shore
is injurious to the growth of coral, although waves breaking over a bank
are not so. Ehrenberg also states, that where there is much sediment,
placed so as to be liable to be moved by the waves there is little or no
coral; and a collection of living specimens placed by him on a sandy shore
died in the course of a few days. (Ibid., page 49.) An experiment,
however, will presently be related in which some large masses of living
coral increased rapidly in size, after having been secured by stakes on a
sandbank. That loose sediment should be injurious to the living
polypifers, appears, at first sight, probable; and accordingly, in sounding
off Keeling atoll, and (as will hereafter be shown) off Mauritius, the
arming of the lead invariably came up clean, where the coral was growing
vigorously. This same circumstance has probably given rise to a strange
belief, which, according to Captain Owen (Captain Owen on the Geography of
the Maldiva Islands, "Geographical Journal", volume ii., page 88.), is
general amongst the inhabitants of the Maldiva atolls, namely that corals
have roots, and therefore that if merely broken down to the surface, they
grow up again; but, if rooted out, they are permanently destroyed. By this
means the inhabitants keep their harbours clear; and thus the French
Governor of St. Mary's in Madagascar, "cleared out and made a beautiful
little port at that place." For it is probable that sand would accumulate
in the hollows formed by tearing out the corals, but not on the broken and
projecting stumps, and therefore, in the former case, the fresh growth of
the coral might be thus prevented.

In the last chapter I remarked that fringing-reefs are almost universally
breached, where streams enter the sea. (Lieutenant Wellstead and others
have remarked that this is the case in the Red Sea; Dr. Ruppell ("Reise in
Abyss." Band. i., page 142) says that there are pear-shaped harbours in the
upraised coral-coast, into which periodical streams enter. From this
circumstance, I presume, we must infer that before the upheaval of the
strata now forming the coast-land, fresh water and sediment entered the sea
at these points; and the coral being thus prevented growing, the pear-shaped
harbours were produced.) Most authors have attributed this fact to
the injurious effects of the fresh water, even where it enters the sea only
in small quantity, and during a part of the year. No doubt brackish water
would prevent or retard the growth of coral; but I believe that the mud and
sand which is deposited, even by rivulets when flooded, is a much more
efficient check. The reef on each side of the channel leading into Port
Louis at Mauritius, ends abruptly in a wall, at the foot of which I sounded
and found a bed of thick mud. This steepness of the sides appears to be a
general character in such breaches. Cook (Cook's "First Voyage," volume
ii., page 271 (Hawkesworth's edition).), speaking of one at Raiatea, says,
"like all the rest, it is very steep on both sides." Now, if it were the
fresh water mingling with the salt which prevented the growth of coral, the
reef certainly would not terminate abruptly, but as the polypifers nearest
the impure stream would grow less vigorously than those farther off, so
would the reef gradually thin away. On the other hand, the sediment
brought down from the land would only prevent the growth of the coral in
the line of its deposition, but would not check it on the side, so that the
reefs might increase till they overhung the bed of the channel. The
breaches are much fewer in number, and front only the larger valleys in
reefs of the encircling barrier class. They probably are kept open in the
same manner as those into the lagoon of an atoll, namely, by the force of
the currents and the drifting outwards of fine sediment. Their position in
front of valleys, although often separated from the land by deep water
lagoon-channels, which it might be thought would entirely remove the
injurious effects both of the fresh water and the sediment, will receive a
simple explanation when we discuss the origin of barrier-reefs.

In the vegetable kingdom every different station has its peculiar group of
plants, and similar relations appear to prevail with corals. We have
already described the great difference between the corals within the lagoon
of an atoll and those on its outer margin. The corals, also, on the margin
of Keeling Island occurred in zones; thus the Porites and Millepora
complanata grow to a large size only where they are washed by a heavy sea,
and are killed by a short exposure to the air; whereas, three species of
Nullipora also live amidst the breakers, but are able to survive uncovered
for a part of each tide; at greater depths, a strong Madrepora and
Millepora alcicornis are the commonest kinds, the former appearing to be
confined to this part, beneath the zone of massive corals, minute
encrusting corallines and other organic bodies live. If we compare the
external margin of the reef at Keeling atoll with that on the leeward side
of Mauritius, which are very differently circumstanced, we shall find a
corresponding difference in the appearance of the corals. At the latter
place, the genus Madrepora is preponderant over every other kind, and
beneath the zone of massive corals there are large beds of Seriatopora.
There is also a marked difference, according to Captain Moresby (Captain
Moresby on the Northern Maldiva atolls, "Geographical Journal", volume v.,
page 401.), between the great branching corals of the Red Sea, and those on
the reefs of the Maldiva atolls.

These facts, which in themselves are deserving of notice, bear, perhaps,
not very remotely, on a remarkable circumstance which has been pointed out
to me by Captain Moresby, namely, that with very few exceptions, none of
the coral-knolls within the lagoons of Peros Banhos, Diego Garcia, and the
Great Chagos Bank (all situated in the Chagos group), rise to the surface
of the water; whereas all those, with equally few exceptions, within
Solomon and Egmont atolls in the same group, and likewise within the large
southern Maldiva atolls, reach the surface. I make these statements, after
having examined the charts of each atoll. In the lagoon of Peros Banhos,
which is nearly twenty miles across, there is only one single reef which
rises to the surface; in Diego Garcia there are seven, but several of these
lie close to the margin of the lagoon, and need scarcely have been
reckoned; in the Great Chagos Bank there is not one. On the other hand, in
the lagoons of some of the great southern Maldiva atolls, although thickly
studded with reefs, every one without exception rises to the surface; and
on an average there are less than two submerged reefs in each atoll; in the
northern atolls, however, the submerged lagoon-reefs are not quite so rare.
The submerged reefs in the Chagos atolls generally have from one to seven
fathoms water on them, but some have from seven to ten. Most of them are
small with very steep sides (Some of these statements were not communicated
to me verbally by Captain Moresby, but are taken from the MS. account
before alluded to, of the Chagos Group.); at Peros Banhos they rise from a
depth of about thirty fathoms, and some of them in the Great Chagos Bank
from above forty fathoms; they are covered, Captain Moresby informs me,
with living and healthy coral, two and three feet high, consisting of
several species. Why then have not these lagoon-reefs reached the surface,
like the innumerable ones in the atolls above named? If we attempt to
assign any difference in their external conditions, as the cause of this
diversity, we are at once baffled. The lagoon of Diego Garcia is not deep,
and is almost wholly surrounded by its reef; Peros Banhos is very deep,
much larger, with many wide passages communicating with the open sea. On
the other hand, of those atolls, in which all or nearly all the lagoon-reefs
have reached the surface, some are small, others large, some shallow,
others deep, some well-enclosed, and others open.

Captain Moresby informs me that he has seen a French chart of Diego Garcia
made eighty years before his survey, and apparently very accurate; and from
it he infers, that during this interval there has not been the smallest
change in the depth on any of the knolls within the lagoon. It is also
known that during the last fifty-one years, the eastern channel into the
lagoon has neither become narrower, nor decreased in depth; and as there
are numerous small knolls of living coral within it, some change might have
been anticipated. Moreover, as the whole reef round the lagoon of this
atoll has been converted into land--an unparalleled case, I believe, in an
atoll of such large size,--and as the strip of land is for considerable
spaces more than half a mile wide--also a very unusual circumstance,--we
have the best possible evidence, that Diego Garcia has remained at its
present level for a very long period. With this fact, and with the
knowledge that no sensible change has taken place during eighty years in
the coral-knolls, and considering that every single reef has reached the
surface in other atolls, which do not present the smallest appearance of
being older than Diego Garcia and Peros Banhos, and which are placed under
the same external conditions with them, one is led to conclude that these
submerged reefs, although covered with luxuriant coral, have no tendency to
grow upwards, and that they would remain at their present levels for an
almost indefinite period.

From the number of these knolls, from their position, size, and form, many
of them being only one or two hundred yards across, with a rounded outline,
and precipitous sides,--it is indisputable that they have been formed by
the growth of coral; and this makes the case much more remarkable. In
Peros Banhos and in the Great Chagos Bank, some of these almost columnar
masses are 200 feet high, and their summits lie only from two to eight
fathoms beneath the surface; therefore, a small proportional amount more of
growth would cause them to attain the surface, like those numerous knolls,
which rise from an equally great depth within the Maldiva atolls. We can
hardly suppose that time has been wanting for the upward growth of the
coral, whilst in Diego Garcia, the broad annular strip of land, formed by
the continued accumulation of detritus, shows how long this atoll has
remained at its present level. We must look to some other cause than the
rate of growth; and I suspect it will be found in the reefs being formed of
different species of corals, adapted to live at different depths.

The Great Chagos Bank is situated in the centre of the Chagos Group, and
the Pitt and Speaker Banks at its two extreme points. These banks resemble
atolls, except in their external rim being about eight fathoms submerged,
and in being formed of dead rock, with very little living coral on it: a
portion nine miles long of the annular reef of Peros Banhos atoll is in the
same condition. These facts, as will hereafter be shown, render it very
probable that the whole group at some former period subsided seven or eight
fathoms; and that the coral perished on the outer margin of those atolls
which are now submerged, but that it continued alive, and grew up to the
surface on those which are now perfect. If these atolls did subside, and
if from the suddenness of the movement or from any other cause, those
corals which are better adapted to live at a certain depth than at the
surface, once got possession of the knolls, supplanting the former
occupants, they would exert little or no tendency to grow upwards. To
illustrate this, I may observe, that if the corals of the upper zone on the
outer edge of Keeling atoll were to perish, it is improbable that those of
the lower zone would grow to the surface, and thus become exposed to
conditions for which they do not appear to be adapted. The conjecture,
that the corals on the submerged knolls within the Chagos atolls have
analogous habits with those of the lower zone outside Keeling atoll,
receives some support from a remark by Captain Moresby, namely, that they
have a different appearance from those on the reefs in the Maldiva atolls,
which, as we have seen, all rise to the surface: he compares the kind of
difference to that of the vegetation under different climates. I have
entered at considerable length into this case, although unable to throw
much light on it, in order to show that an equal tendency to upward growth
ought not to be attributed to all coral-reefs,--to those situated at
different depths,--to those forming the ring of an atoll or those on the
knolls within a lagoon,--to those in one area and those in another. The
inference, therefore, that one reef could not grow up to the surface within
a given time, because another, not known to be covered with the same
species of corals, and not known to be placed under conditions exactly the
same, has not within the same time reached the surface, is unsound.


SECTION 4.II.--ON THE RATE OF GROWTH OF CORAL-REEFS.

The remark made at the close of the last section, naturally leads to this
division of our subject, which has not, I think, hitherto been considered
under a right point of view. Ehrenberg (Ehrenberg, as before cited, pages
39, 46, and 50.) has stated, that in the Red Sea, the corals only coat
other rocks in a layer from one to two feet in thickness, or at most to a
fathom and a half; and he disbelieves that, in any case, they form, by
their own proper growth, great masses, stratum over stratum. A nearly
similar observation has been made by MM. Quoy and Gaimard ("Annales des
Sciences Nat." tom. vi., page 28.), with respect to the thickness of some
upraised beds of coral, which they examined at Timor and some other places.
Ehrenberg (Ehrenberg, ut sup., page 42.) saw certain large massive corals
in the Red Sea, which he imagines to be of such vast antiquity, that they
might have been beheld by Pharaoh; and according to Mr. Lyell (Lyell's
"Principles of Geology," book iii., chapter xviii.) there are certain
corals at Bermuda, which are known by tradition, to have been living for
centuries. To show how slowly coral-reefs grow upwards, Captain Beechey
(Beechey's "Voyage to the Pacific," chapter viii.) has adduced the case of
the Dolphin Reef off Tahiti, which has remained at the same depth beneath
the surface, namely about two fathoms and a half, for a period of
sixty-seven years. There are reefs in the Red Sea, which certainly do not
appear (Ehrenberg, ut sup., page 43.) to have increased in dimensions during
the last half-century, and from the comparison of old charts with recent
surveys, probably not during the last two hundred years. These, and other
similar facts, have so strongly impressed many with the belief of the
extreme slowness of the growth of corals, that they have even doubted the
possibility of islands in the great oceans having been formed by their
agency. Others, again, who have not been overwhelmed by this difficulty,
have admitted that it would require thousands, and tens of thousands of
years, to form a mass, even of inconsiderable thickness; but the subject
has not, I believe, been viewed in the proper light.

That masses of considerable thickness have been formed by the growth of
coral, may be inferred with certainty from the following facts. In the
deep lagoons of Peros Banhos and of the Great Chagos Bank, there are, as
already described, small steep-sided knolls covered with living coral.
There are similar knolls in the southern Maldiva atolls, some of which, as
Captain Moresby assures me, are less than a hundred yards in diameter, and
rise to the surface from a depth of between two hundred and fifty and three
hundred feet. Considering their number, form, and position, it would be
preposterous to suppose that they are based on pinnacles of any rock, not
of coral formation; or that sediment could have been heaped up into such
small and steep isolated cones. As no kind of living coral grows above the
height of a few feet, we are compelled to suppose that these knolls have
been formed by the successive growth and death of many individuals,--first
one being broken off or killed by some accident, and then another, and one
set of species being replaced by another set with different habits, as the
reef rose nearer the surface, or as other changes supervened. The spaces
between the corals would become filled up with fragments and sand, and such
matter would probably soon be consolidated, for we learn from Lieutenant
Nelson ("Geological Transactions," volume v., page 113.), that at Bermuda a
process of this kind takes place beneath water, without the aid of
evaporation. In reefs, also, of the barrier class, we may feel sure, as I
have shown, that masses of great thickness have been formed by the growth
of the coral; in the case of Vanikoro, judging only from the depth of the
moat between the land and the reef, the wall of coral-rock must be at least
three hundred feet in vertical thickness.

It is unfortunate that the upraised coral-islands in the Pacific have not
been examined by a geologist. The cliffs of Elizabeth Island, in the Low
Archipelago, are eighty feet high, and appear, from Captain Beechey's
description, to consist of a homogeneous coral-rock. From the isolated
position of this island, we may safely infer that it is an upraised atoll,
and therefore that it has been formed by masses of coral, grown together.
Savage Island seems, from the description of the younger Forster (Forster's
"Voyage round the World with Cook," volume ii., pages 163, 167.), to have a
similar structure, and its shores are about forty feet high: some of the
Cook Islands also appear (Williams's "Narrative of Missionary Enterprise,"
page 30.) to be similarly composed. Captain Belcher, R.N., in a letter
which Captain Beaufort showed me at the admiralty, speaking of Bow atoll,
says, "I have succeeded in boring forty-five feet through coral-sand, when
the auger became jammed by the falling in of the surrounding CREAMY
matter." On one of the Maldiva atolls, Captain Moresby bored to a depth of
twenty-six feet, when his auger also broke: he has had the kindness to
give me the matter brought up; it is perfectly white, and like finely
triturated coral-rock.

In my description of Keeling atoll, I have given some facts, which show
that the reef probably has grown outwards; and I have found, just within
the outer margin, the great mounds of Porites and of Millepora, with their
summits lately killed, and their sides subsequently thickened by the growth
of the coral: a layer, also, of Nullipora had already coated the dead
surface. As the external slope of the reef is the same round the whole of
this atoll, and round many other atolls, the angle of inclination must
result from an adaption between the growing powers of the coral, and the
force of the breakers, and their action on the loose sediment. The reef,
therefore, could not increase outwards, without a nearly equal addition to
every part of the slope, so that the original inclination might be
preserved, and this would require a large amount of sediment, all derived
from the wear of corals and shells, to be added to the lower part.
Moreover, at Keeling atoll, and probably in many other cases, the different
kinds of corals would have to encroach on each other; thus the Nulliporae
cannot increase outwards without encroaching on the Porites and Millepora
complanata, as is now taking place; nor these latter without encroaching on
the strongly branched Madreporet, the Millepora alcicornis, and some
Astraeas; nor these again without a foundation being formed for them within
the requisite depth, by the accumulation of sediment. How slow, then, must
be the ordinary lateral or outward growth of such reefs. But off Christmas
atoll, where the sea is much more shallow than is usual, we have good
reason to believe that, within a period not very remote, the reef has
increased considerably in width. The land has the extraordinary breadth of
three miles; it consists of parallel ridges of shells and broken corals,
which furnish "an incontestable proof," as observed by Cook (Cook's "Third
Voyage," book III., chapter x.), "that the island has been produced by
accessions from the sea, and is in a state of increase." The land is
fronted by a coral-reef, and from the manner in which islets are known to
be formed, we may feel confident that the reef was not three miles wide,
when the first, or most backward ridge, was thrown up; and, therefore, we
must conclude that the reef has grown outwards during the accumulation of
the successive ridges. Here then, a wall of coral-rock of very
considerable breadth has been formed by the outward growth of the living
margin, within a period during which ridges of shells and corals, lying on
the bare surface, have not decayed. There can be little doubt, from the
account given by Captain Beechey, that Matilda atoll, in the Low
Archipelago, has been converted in the space of thirty-four years, from
being, as described by the crew of a wrecked whaling vessel, a "reef of
rocks" into a lagoon-island, fourteen miles in length, with "one of its
sides covered nearly the whole way with high trees." (Beechey's "Voyage to
the Pacific," chapter vii. and viii.) The islets, also, on Keeling atoll,
it has been shown, have increased in length, and since the construction of
an old chart, several of them have become united into one long islet; but
in this case, and in that of Matilda atoll, we have no proof, and can only
infer as probable, that the reef, that is the foundation of the islets, has
increased as well as the islets themselves.

After these considerations, I attach little importance, as indicating the
ordinary and still less the possible rate of OUTWARD growth of coral-reefs,
to the fact that certain reefs in the Red Sea have not increased during a
long interval of time; or to other such cases, as that of Ouluthy atoll in
the Caroline group, where every islet, described a thousand years before by
Cantova was found in the same state by Lutke (F. Lutke's "Voyage autour du
Monde." In the group Elato, however, it appears that what is now the islet
Falipi, is called in Cantova's Chart, the Banc de Falipi. It is not stated
whether this has been caused by the growth of coral, or by the accumulation
of sand.),--without it could be shown that, in these cases, the conditions
were favourable to the vigorous and unopposed growth of the corals living
in the different zones of depth, and that a proper basis for the extent of
the reef was present. The former conditions must depend on many
contingencies, and in the deep oceans where coral formations most abound, a
basis within the requisite depth can rarely be present.

Nor do I attach any importance to the fact of certain submerged reefs, as
those off Tahiti, or those within Diego Garcia not now being nearer the
surface than they were many years ago, as an indication of the rate under
favourable circumstances of the UPWARD growth of reefs; after it has been
shown, that all the reefs have grown to the surface in some of the Chagos
atolls, but that in neighbouring atolls which appear to be of equal
antiquity and to be exposed to the same external conditions, every reef
remains submerged; for we are almost driven to attribute this to a
difference, not in the rate of growth, but in the habits of the corals in
the two cases.

In an old-standing reef, the corals, which are so different in kind on
different parts of it, are probably all adapted to the stations they
occupy, and hold their places, like other organic beings, by a struggle one
with another, and with external nature; hence we may infer that their
growth would generally be slow, except under peculiarly favourable
circumstances. Almost the only natural condition, allowing a quick upward
growth of the whole surface of a reef, would be a slow subsidence of the
area in which it stood; if, for instance, Keeling atoll were to subside two
or three feet, can we doubt that the projecting margin of live coral, about
half an inch in thickness, which surrounds the dead upper surfaces of the
mounds of Porites, would in this case form a concentric layer over them,
and the reef thus increase upwards, instead of, as at present, outwards?
The Nulliporae are now encroaching on the Porites and Millepora, but in
this case might we not confidently expect that the latter would, in their
turn, encroach on the Nulliporae? After a subsidence of this kind, the sea
would gain on the islets, and the great fields of dead but upright corals
in the lagoon, would be covered by a sheet of clear water; and might we not
then expect that these reefs would rise to the surface, as they anciently
did when the lagoon was less confined by islets, and as they did within a
period of ten years in the schooner-channel, cut by the inhabitants? In
one of the Maldiva atolls, a reef, which within a very few years existed as
an islet bearing cocoa-nut trees, was found by Lieutenant Prentice
"ENTIRELY COVERED WITH LIVE CORAL AND MADREPORE." The natives believe that
the islet was washed away by a change in the currents, but if, instead of
this, it had quietly subsided, surely every part of the island which
offered a solid foundation, would in a like manner have become coated with
living coral.

Through steps such as these, any thickness of rock, composed of a singular
intermixture of various kinds of corals, shells, and calcareous sediment,
might be formed; but without subsidence, the thickness would necessarily be
determined by the depth at which the reef-building polypifers can exist.
If it be asked, at what rate in years I suppose a reef of coral favourably
circumstanced could grow up from a given depth; I should answer, that we
have no precise evidence on this point, and comparatively little concern
with it. We see, in innumerable points over wide areas, that the rate has
been sufficient, either to bring up the reefs from various depths to the
surface, or, as is more probable, to keep them at the surface, during
progressive subsidences; and this is a much more important standard of
comparison than any cycle of years.

It may, however, be inferred from the following facts, that the rate in
years under favourable circumstances would be very far from slow. Dr.
Allan, of Forres, has, in his MS. Thesis deposited in the library of the
Edinburgh University (extracts from which I owe to the kindness of Dr.
Malcolmson), the following account of some experiments, which he tried
during his travels in the years 1830 to 1832 on the east coast of
Madagascar. "To ascertain the rise and progress of the coral-family, and
fix the number of species met with at Foul Point (latitude 17 deg 40')
twenty species of coral were taken off the reef and planted apart on a
sand-bank THREE FEET DEEP AT LOW WATER. Each portion weighed ten pounds,
and was kept in its place by stakes. Similar quantities were placed in a
clump and secured as the rest. This was done in December 1830. In July
following, each detached mass was nearly level with the sea at low water,
quite immovable, and several feet long, stretching as the parent reef, with
the coast current from north to south. The masses accumulated in a clump
were found equally increased, but some of the species in such unequal
ratios, as to be growing over each other." The loss of Dr. Allan's
magnificent collection by shipwreck, unfortunately prevents its being known
to what genera these corals belonged; but from the numbers experimented on,
it is certain that all the more conspicuous kinds must have been included.
Dr. Allan informs me, in a letter, that he believes it was a Madrepora,
which grew most vigorously. One may be permitted to suspect that the level
of the sea might possibly have been somewhat different at the two stated
periods; nevertheless, it is quite evident that the growth of the ten-pound
masses, during the six or seven months, at the end of which they were found
immovably fixed (It is stated by De la Beche ("Geological Manual," page
143), on the authority of Mr. Lloyd, who surveyed the Isthmus of Panama,
that some specimens of Polypifers, placed by him in a sheltered pool of
water, were found in the course of a few days firmly fixed by the secretion
of a stony matter, to the bottom) and several feet in length, must have
been very great. The fact of the different kinds of coral, when placed in
one clump, having increased in extremely unequal ratios, is very
interesting, as it shows the manner in which a reef, supporting many
species of coral, would probably be affected by a change in the external
conditions favouring one kind more than another. The growth of the masses
of coral in N. and S. lines parallel to the prevailing currents, whether
due to the drifting of sediment or to the simple movement of the water, is,
also, a very interesting circumstance.

A fact, communicated to me by Lieutenant Wellstead, I.N., in some degree
corroborates the result of Dr. Allan's experiments: it is, that in the
Persian Gulf a ship had her copper bottom encrusted in the course of twenty
months with a layer of coral, TWO FEET in thickness, which it required
great force to remove, when the vessel was docked: it was not ascertained
to what order this coral belonged. The case of the schooner-channel choked
up with coral in an interval of less than ten years, in the lagoon of
Keeling atoll, should be here borne in mind. We may also infer, from the
trouble which the inhabitants of the Maldiva atolls take to root out, as
they express it, the coral-knolls from their harbours, that their growth
can hardly be very slow. (Mr. Stutchbury ("West of England Journal", No.
I., page 50.) has described a specimen of Agaricia, "weighing 2 lbs. 9 oz.,
which surrounds a species of oyster, whose age could not be more than two
years, and yet is completely enveloped by this dense coral." I presume
that the oyster was living when the specimen was procured; otherwise the
fact tells nothing. Mr. Stutchbury also mentions an anchor, which had
become entirely encrusted with coral in fifty years; other cases, however,
are recorded of anchors which have long remained amidst coral-reefs without
having become coated. The anchor of the "Beagle", in 1832, after having
been down exactly one month at Rio de Janeiro, was so thickly coated by two
species of Tubularia, that large spaces of the iron were entirely
concealed; the tufts of this horny zoophyte were between two and three
inches in length. It has been attempted to compute, but I believe
erroneously, the rate of growth of a reef, from the fact mentioned by
Captain Beechey, of the Chama gigas being embedded in coral-rock. But it
should be remembered, that some species of this genus invariably live, both
whilst young and old, in cavities, which the animal has the power of
enlarging with its growth. I saw many of these shells thus embedded in the
outer "flat" of Keeling atoll, which is composed of dead rock; and
therefore the cavities in this case had no relation whatever with the
growth of coral. M. Lesson, also, speaking of this shell (Partie Zoolog.
"Voyage de la 'Coquille'"), has remarked, "que constamment ses valves
etaient engages completement dans la masse des Madrepores.")

From the facts given in this section, it may be concluded, first, that
considerable thicknesses of rock have certainly been formed within the
present geological area by the growth of coral and the accumulation of its
detritus; and, secondly, that the increase of individual corals and of
reefs, both outwards or horizontally and upwards or vertically, under the
peculiar conditions favourable to such increase, is not slow, when referred
either to the standard of the average oscillations of level in the earth's
crust, or to the more precise but less important one of a cycle of years.


SECTION 4.III.--ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.

I have already described in detail, which might have appeared trivial, the
nature of the bottom of the sea immediately surrounding Keeling atoll; and
I will now describe with almost equal care the soundings off the
fringing-reefs of Mauritius. I have preferred this arrangement, for the sake
of grouping together facts of a similar nature. I sounded with the wide
bell-shaped lead which Captain Fitzroy used at Keeling Island, but my
examination of the bottom was confined to a few miles of coast (between
Port Louis and Tomb Bay) on the leeward side of the island. The edge of
the reef is formed of great shapeless masses of branching Madrepores, which
chiefly consist of two species,--apparently M. corymbosa and pocillifera,--
mingled with a few other kinds of coral. These masses are separated from
each other by the most irregular gullies and cavities, into which the lead
sinks many feet. Outside this irregular border of Madrepores, the water
deepens gradually to twenty fathoms, which depth generally is found at the
distance of from half to three-quarters of a mile from the reef. A little
further out the depth is thirty fathoms, and thence the bank slopes rapidly
into the depths of the ocean. This inclination is very gentle compared
with that outside Keeling and other atolls, but compared with most coasts
it is steep. The water was so clear outside the reef, that I could
distinguish every object forming the rugged bottom. In this part, and to a
depth of eight fathoms, I sounded repeatedly, and at each cast pounded the
bottom with the broad lead, nevertheless the arming invariably came up
perfectly clean, but deeply indented. From eight to fifteen fathoms a
little calcareous sand was occasionally brought up, but more frequently the
arming was simply indented. In all this space the two Madrepores above
mentioned, and two species of Astraea, with rather large stars, seemed the
commonest kinds (Since the preceding pages were printed off, I have
received from Mr. Lyell a very interesting pamphlet, entitled "Remarks upon
Coral Formations," etc., by J. Couthouy, Boston, United States, 1842.
There is a statement (page 6), on the authority of the Rev. J. Williams,
corroborating the remarks made by Ehrenberg and Lyell (page 71 of this
volume), on the antiquity of certain individual corals in the Red Sea and
at Bermuda; namely, that at Upolu, one of the Navigator Islands,
"particular clumps of coral are known to the fishermen by name, derived
from either some particular configuration or tradition attached to them,
and handed down from time immemorial." With respect to the thickness of
masses of coral-rock, it clearly appears, from the descriptions given by
Mr. Couthouy (pages 34, 58) that Mangaia and Aurora Islands are upraised
atolls, composed of coral rock: the level summit of the former is about
three hundred feet, and that of Aurora Island is two hundred feet above the
sea-level.); and it must be noticed that twice at the depth of fifteen
fathoms, the arming was marked with a clean impression of an Astraea.
Besides these lithophytes, some fragments of the Millepora alcicornis,
which occurs in the same relative position at Keeling Island, were brought
up; and in the deeper parts there were large beds of a Seriatopora,
different from S. subulata, but closely allied to it. On the beach within
the reef, the rolled fragments consisted chiefly of the corals just
mentioned, and of a massive Porites, like that at Keeling atoll, of a
Meandrina, Pocillopora verrucosa, and of numerous fragments of Nullipora.
From fifteen to twenty fathoms the bottom was, with few exceptions, either
formed of sand, or thickly covered with Seriatopora: this delicate coral
seems to form at these depths extensive beds unmingled with any other kind.
At twenty fathoms, one sounding brought up a fragment of Madrepora
apparently M. pocillifera, and I believe it is the same species (for I
neglected to bring specimens from both stations) which mainly forms the
upper margin of the reef; if so, it grows in depths varying from 0 to 20
fathoms. Between 20 and 23 fathoms I obtained several soundings, and they
all showed a sandy bottom, with one exception at 30 fathoms, when the
arming came up scooped out, as if by the margin of a large Caryophyllia.
Beyond 33 fathoms I sounded only once; and from 86 fathoms, at the distance
of one mile and a third from the edge of the reef, the arming brought up
calcareous sand with a pebble of volcanic rock. The circumstance of the
arming having invariably come up quite clean, when sounding within a
certain number of fathoms off the reefs of Mauritius and Keeling atoll
(eight fathoms in the former case, and twelve in the latter) and of its
having always come up (with one exception) smoothed and covered with sand,
when the depth exceeded twenty fathoms, probably indicates a criterion, by
which the limits of the vigorous growth of coral might in all cases be
readily ascertained. I do not, however, suppose that if a vast number of
soundings were obtained round these islands, the limit above assigned would
be found never to vary, but I conceive the facts are sufficient to show,
that the exceptions would be few. The circumstance of a GRADUAL change, in
the two cases, from a field of clean coral to a smooth sandy bottom, is far
more important in indicating the depth at which the larger kinds of coral
flourish than almost any number of separate observations on the depth, at
which certain species have been dredged up. For we can understand the
gradation, only as a prolonged struggle against unfavourable conditions.
If a person were to find the soil clothed with turf on the banks of a
stream of water, but on going to some distance on one side of it, he
observed the blades of grass growing thinner and thinner, with intervening
patches of sand, until he entered a desert of sand, he would safely
conclude, especially if changes of the same kind were noticed in other
places, that the presence of the water was absolutely necessary to the
formation of a thick bed of turf: so may we conclude, with the same
feeling of certainty, that thick beds of coral are formed only at small
depths beneath the surface of the sea.

I have endeavoured to collect every fact, which might either invalidate or
corroborate this conclusion. Captain Moresby, whose opportunities for
observation during his survey of the Maldiva and Chagos Archipelagoes have
been unrivalled, informs me, that the upper part or zone of the steep-sided
reefs, on the inner and outer coasts of the atolls in both groups,
invariably consists of coral, and the lower parts of sand. At seven or
eight fathoms depth, the bottom is formed, as could be seen through the
clear water, of great living masses of coral, which at about ten fathoms
generally stand some way apart from each other, with patches of white sand
between them, and at a little greater depth these patches become united
into a smooth steep slope, without any coral. Captain Moresby, also,
informs me in support of his statement, that he found only decayed coral on
the Padua Bank (northern part of the Laccadive group) which has an average
depth between twenty-five and thirty-five fathoms, but that on some other
banks in the same group with only ten or twelve fathoms water on them (for
instance, the Tillacapeni bank), the coral was living.

With regard to the coral-reefs in the Red Sea, Ehrenberg has the following
passage:--"The living corals do not descend there into great depths. On
the edges of islets and near reefs, where the depth was small, very many
lived; but we found no more even at six fathoms. The pearl-fishers at
Yemen and Massaua asserted that there was no coral near the pearl-banks at
nine fathoms depth, but only sand. We were not able to institute any more
special researches." (Ehrenberg, "Uber die Natur," etc., page 50.) I am,
however, assured both by Captain Moresby and Lieutenant Wellstead, that in
the more northern parts of the Red Sea, there are extensive beds of living
coral at a depth of twenty-five fathoms, in which the anchors of their
vessels were frequently entangled. Captain Moresby attributes the less
depth, at which the corals are able to live in the places mentioned by
Ehrenberg, to the greater quantity of sediment there; and the situations,
where they were flourishing at the depth of twenty-five fathoms, were
protected, and the water was extraordinarily limpid. On the leeward side
of Mauritius where I found the coral growing at a somewhat greater depth
than at Keeling atoll, the sea, owing apparently to its tranquil state, was
likewise very clear. Within the lagoons of some of the Marshall atolls,
where the water can be but little agitated, there are, according to
Kotzebue, living beds of coral in twenty-five fathoms. From these facts,
and considering the manner in which the beds of clean coral off Mauritius,
Keeling Island, the Maldiva and Chagos atolls, graduated into a sandy
slope, it appears very probable that the depth, at which reef-building
polypifers can exist, is partly determined by the extent of inclined
surface, which the currents of the sea and the recoiling waves have the
power to keep free from sediment.

MM. Quoy and Gaimard ("Annales des Sci. Nat." tom. vi.) believe that the
growth of coral is confined within very limited depths; and they state that
they never found any fragment of an Astraea (the genus they consider most
efficient in forming reefs) at a depth above twenty-five or thirty feet.
But we have seen that in several places the bottom of the sea is paved with
massive corals at more than twice this depth; and at fifteen fathoms (or
twice this depth) off the reefs of Mauritius, the arming was marked with
the distinct impression of a living Astraea. Millepora alcicornis lives in
from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0 to 20
fathoms. Captain Moresby has given me a specimen of Sideropora scabra
(Porites of Lamarck) brought up alive from 17 fathoms. Mr. Couthouy
("Remarks on Coral Formations," page 12.) states that he has dredged up on
the Bahama banks considerable masses of Meandrina from 16 fathoms, and he
has seen this coral growing in 20 fathoms. A Caryophyllia, half an inch in
diameter, was dredged up alive from 80 fathoms off Juan Fernandez (latitude
33 deg S.) by Captain P.P. King (I am indebted to Mr. Stokes for having
kindly communicated this fact to me, together with much other valuable
information.): this is the most remarkable fact with which I am
acquainted, showing the depth at which a genus of corals often found on
reefs, can exist.

We ought, however, to feel less surprise at this fact, as Caryophyllia
alone of the lamelliform genera, ranges far beyond the tropics; it is found
in Zetland (Fleming's "British Animals," genus Caryophyllia.) in Latitude
60 deg N. in deep water, and I procured a small species from Tierra del
Fuego in Latitude 53 deg S. Captain Beechey informs me, that branches of
pink and yellow coral were frequently brought up from between twenty and
twenty-five fathoms off the Low atolls; and Lieutenant Stokes, writing to
me from the N.W. coast of Australia, says that a strongly branched coral
was procured there from thirty fathoms; unfortunately it is not known to
what genera these corals belong.

(I will record in the form of a note all the facts that I have been able to
collect on the depths, both within and without the tropics, at which those
corals and corallines can live, which there is no reason to suppose ever
materially aid in the construction of a reef.

(In the following list the name of the Zoophyte is followed by the depth in
fathoms, the country and degrees S. latitude, and the authority. Where no
authority is given, the observation is Darwin's own.)

SERTULARIA, 40, Cape Horn 66.

CELLARIA, 40, Cape Horn 66.

CELLARIA, A minute scarlet encrusting species, found living, 190, Keeling
Atoll, 12.

CELLARIA, An allied, small stony sub-generic form, 48, St Cruz Riv. 50.

A coral allied to VINCULARIA, with eight rows of cells, 40, Cape Horn.

TUBULIPORA, near to T. patima, 40, Cape Horn.

TUBULIPORA, near to T. patima, 94, East Chiloe 43.

CELLEPORA, several species, and allied sub-generic forms, 40, Cape Horn.

CELLEPORA, several species, and allied sub-generic forms, 40 and 57, Chonos
Archipelago 45.

CELLEPORA, several species, and allied sub-generic forms, 48, St Cruz 50.

ESCHARA, 30, Tierra del Fuego 53.

ESCHARA, 48, St Cruz R. 50.

RETEPORA, 40, Cape Horn.

RETEPORA, 100, Cape of Good Hope 34, Quoy and Gaimard, "Ann. Scien. Nat."
tome vi., page 284.

MILLEPORA, a strong coral with cylindrical branches, of a pink colour,
about two inches high, resembling in the form of its orifices M. aspera of
Lamarck, 94 and 30, E. Chiloe 43, Tierra del Fuego 53.

CORALIUM, 120, Barbary 33 N., Peyssonel in paper read to Royal Society May
1752.

ANTIPATHES, 16, Chonos 45.

GORGONIA (or an allied form), 160, Abrolhos on the coast of Brazil 18,
Captain Beechey informed me of this fact in a letter.

Ellis ("Nat. Hist. of Coralline," page 96) states that Ombellularia was
procured in latitude 79 deg N. STICKING to a LINE from the depth of 236
fathoms; hence this coral either must have been floating loose, or was
entangled in stray line at the bottom. Off Keeling atoll a compound
Ascidia (Sigillina) was brought up from 39 fathoms, and a piece of sponge,
apparently living, from 70, and a fragment of Nullipora also apparently
living from 92 fathoms. At a greater depth than 90 fathoms off this coral
island, the bottom was thickly strewed with joints of Halimeda and small
fragments of other Nulliporae, but all dead. Captain B. Allen, R.N.,
informs me that in the survey of the West Indies it was noticed that
between the depth of 10 and 200 fathoms, the sounding lead very generally
came up coated with the dead joints of a Halimeda, of which he showed me
specimens. Off Pernambuco, in Brazil, in about twelve fathoms, the bottom
was covered with fragments dead and alive of a dull red Nullipora, and I
infer from Roussin's chart, that a bottom of this kind extends over a wide
area. On the beach, within the coral-reefs of Mauritius, vast quantities
of fragments of Nulliporae were piled up. From these facts it appears,
that these simply organized bodies are amongst the most abundant
productions of the sea.)

Although the limit of depth, at which each particular kind of coral ceases
to exist, is far from being accurately known; yet when we bear in mind the
manner in which the clumps of coral gradually became infrequent at about
the same depth, and wholly disappeared at a greater depth than twenty
fathoms, on the slope round Keeling atoll, on the leeward side of the
Mauritius, and at rather less depth, both without and within the atolls of
the Maldiva and Chagos Archipelagoes; and when we know that the reefs round
these islands do not differ from other coral formations in their form and
structure, we may, I think, conclude that in ordinary cases, reef-building
polypifers do not flourish at greater depths than between twenty and thirty
fathoms.

It has been argued ("Journal of the Royal Geographical Society," 1831, page
218.) that reefs may possibly rise from very great depths through the means
of small corals, first making a platform for the growth of the stronger
kinds. This, however, is an arbitrary supposition: it is not always
remembered, that in such cases there is an antagonist power in action,
namely, the decay of organic bodies, when not protected by a covering of
sediment, or by their own rapid growth. We have, moreover, no right to
calculate on unlimited time for the accumulation of small organic bodies
into great masses. Every fact in geology proclaims that neither the land,
nor the bed of the sea retain for indefinite periods the same level. As
well might it be imagined that the British Seas would in time become choked
up with beds of oysters, or that the numerous small corallines off the
inhospitable shores of Tierra del Fuego would in time form a solid and
extensive coral-reef.