Goose Green

Wednesday, August 06, 2008

Section 4.III

Portion of a Pacific atoll showing two islets ...Image via Wikipedia

I have already described in
detail, which might have appeared
trivial, the nature of the bottom
of the sea immediately surrounding
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

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

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.

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