Chapter 4 - Section 4.I

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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.