Section 4.II

Image via Wikipedia

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.