Section 4.III

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

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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
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.

Chapter 2 - Barrier Reefs

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CHAPTER II.--BARRIER REEFS.














Closely resemble in general form and structure atoll-reefs.--Width and
depth of the lagoon-channels.--Breaches through the reef in front of
valleys, and generally on the leeward side.--Checks to the filling up of
the lagoon-channels.--Size and constitution of the encircled islands.--
Number of islands within the same reef.--Barrier-reefs of New Caledonia and
Australia.--Position of the reef relative to the slope of the adjoining
land.--Probable great thickness of barrier-reefs.

The term "barrier" has been generally applied to that vast reef which
fronts the N.E. shore of Australia, and by most voyagers likewise to that
on the western coast of New Caledonia.  At one time I thought it convenient
thus to restrict the term, but as these reefs are similar in structure, and
in position relatively to the land, to those, which, like a wall with a
deep moat within, encircle many smaller islands, I have classed them
together.  The reef, also, on the west coast of New Caledonia, circling
round the extremities of the island, is an intermediate form between a
small encircling reef and the Australian barrier, which stretches for a
thousand miles in nearly a straight line.

The geographer Balbi has in effect described those barrier-reefs, which
encircle moderately sized islands, by calling them atolls with high land
rising from within their central expanse.  The general resemblance between
the reefs of the barrier and atoll classes may be seen in the small, but
accurately reduced charts on Plate I. (The authorities from which these
charts have been reduced, together with some remarks on them and
descriptive of the Plates, are given separately.), and this resemblance can
be further shown to extend to every part of the structure.  Beginning with
the outside of the reef; many scattered soundings off Gambier, Oualan, and
some other encircled islands, show that close to the breakers there exists
a narrow shelving margin, beyond which the ocean becomes suddenly
unfathomable; but off the west coast of New Caledonia, Captain Kent
(Dalrymple, "Hydrog. Mem." volume iii.) found no bottom with 150 fathoms,
at two ships' length from the reef; so that the slope here must be nearly
as precipitous as off the Maldiva atolls.

I can give little information regarding the kinds of corals which live on
the outer margin.  When I visited the reef at Tahiti, although it was low
water, the surf was too violent for me to see the living masses; but,
according to what I heard from some intelligent native chiefs, they
resemble in their rounded and branchless forms, those on the margin of
Keeling atoll.  The extreme verge of the reef, which was visible between
the breaking waves at low water, consisted of a rounded, convex,
artificial-like breakwater, entirely coated with Nulliporae, and absolutely
similar to that which I have described at Keeling atoll.  From what I heard
when at Tahiti, and from the writings of the Revs. W. Ellis and J.
Williams, I conclude that this peculiar structure is common to most of the
encircled islands of the Society Archipelago.  The reef within this mound
or breakwater, has an extremely irregular surface, even more so than
between the islets on the reef of Keeling atoll, with which alone (as there
are no islets on the reef of Tahiti) it can properly be compared.  At
Tahiti, the reef is very irregular in width; but round many other encircled
islands, for instance, Vanikoro or Gambier Islands (Figures 1 and 8, Plate
I.), it is quite as regular, and of the same average width, as in true
atolls.  Most barrier-reefs on the inner side slope irregularly into the
lagoon-channel (as the space of deep water separating the reef from the
included land may be called), but at Vanikoro the reef slopes only for a
short distance, and then terminates abruptly in a submarine wall, forty
feet high,--a structure absolutely similar to that described by Chamisso in
the Marshall atolls.

In the Society Archipelago, Ellis (Consult, on this and other points, the
"Polynesian Researches," by the Rev. W. Ellis, an admirable work, full of
curious information.) states, that the reefs generally lie at the distance
of from one to one and a half miles, and, occasionally, even at more than
three miles, from the shore.  The central mountains are generally bordered
by a fringe of flat, and often marshy, alluvial land, from one to four
miles in width.  This fringe consists of coral-sand and detritus thrown up
from the lagoon-channel, and of soil washed down from the hills; it is an
encroachment on the channel, analogous to that low and inner part of the
islets in many atolls which is formed by the accumulation of matter from
the lagoon.  At Hogoleu (Figure 2, Plate I.), in the Caroline Archipelago
(See "Hydrographical Mem." and the "Atlas of the Voyage of the
'Astrolabe'," by Captain Dumont D'Urville, page 428.), the reef on the
south side is no less than twenty miles; on the east side, five; and on the
north side, fourteen miles from the encircled high islands.

The lagoon channels may be compared in every respect with true lagoons.  In
some cases they are open, with a level bottom of fine sand; in others they
are choked up with reefs of delicately branched corals, which have the same
general character as those within the Keeling atoll.  These internal reefs
either stand separately, or more commonly skirt the shores of the included
high islands.  The depth of the lagoon-channel round the Society Islands
varies from two or three to thirty fathoms; in Cook's (See the chart in
volume i. of Hawkesworth's 4to edition of "Cook's First Voyage.") chart of
Ulieta, however, there is one sounding laid down of forty-eight fathoms; at
Vanikoro there are several of fifty-four and one of fifty-six and a half
fathoms (English), a depth which even exceeds by a little that of the

interior of the great Maldiva atolls.  Some barrier-reefs have very few
islets on them; whilst others are surmounted by numerous ones; and those
round part of Bolabola (Plate I., Figure 5) form a single linear strip.
The islets first appear either on the angles of the reef, or on the sides
of the breaches through it, and are generally most numerous on the windward
side.  The reef to leeward retaining its usual width, sometimes lies
submerged several fathoms beneath the surface; I have already mentioned
Gambier Island as an instance of this structure.  Submerged reefs, having a
less defined outline, dead, and covered with sand, have been observed (see
Appendix) off some parts of Huaheine and Tahiti.  The reef is more
frequently breached to leeward than to windward; thus I find in
Krusenstern's "Memoir on the Pacific," that there are passages through the
encircling reef on the leeward side of each of the seven Society Islands,
which possess ship-harbours; but that there are openings to windward
through the reef of only three of them.  The breaches in the reef are
seldom as deep as the interior lagoon-like channel; they generally occur in
front of the main valleys, a circumstance which can be accounted for, as
will be seen in the fourth chapter, without much difficulty.  The breaches
being situated in front of the valleys, which descend indifferently on all
sides, explains their more frequent occurrence through the windward side of
barrier-reefs than through the windward side of atolls,--for in atolls
there is no included land to influence the position of the breaches.

It is remarkable, that the lagoon-channels round mountainous islands have
not in every instance been long ago filled up with coral and sediment; but
it is more easily accounted for than appears at first sight.  In cases like
that of Hogoleu and the Gambier Islands, where a few small peaks rise out
of a great lagoon, the conditions scarcely differ from those of an atoll,
and I have already shown, at some length, that the filling up of a true
lagoon must be an extremely slow process.  Where the channel is narrow, the
agency, which on unprotected coasts is most productive of sediment, namely
the force of the breakers, is here entirely excluded, and the reef being
breached in the front of the main valleys, much of the finer mud from the
rivers must be transported into the open sea.  As a current is formed by
the water thrown over the edge of atoll-formed reefs, which carries
sediment with it through the deep-water breaches, the same thing probably
takes place in barrier-reefs, and this would greatly aid in preventing the
lagoon-channel from being filled up.  The low alluvial border, however, at
the foot of the encircled mountains, shows that the work of filling up is
in progress; and at Maura (Plate I., Figure 6), in the Society group, it
has been almost effected, so that there remains only one harbour for small
craft.

If we look at a set of charts of barrier-reefs, and leave out in
imagination the encircled land, we shall find that, besides the many points
already noticed of resemblance, or rather of identity in structure with
atolls, there is a close general agreement in form, average dimensions, and
grouping.  Encircling barrier-reefs, like atolls, are generally elongated,
with an irregularly rounded, though sometimes angular outline.  There are
atolls of all sizes, from less than two miles in diameter to sixty miles
(excluding Tilla-dou-Matte, as it consists of a number of almost
independent atoll-formed reefs); and there are encircling barrier-reefs
from three miles and a half to forty-six miles in diameter,--Turtle Island
being an instance of the former, and Hogoleu of the latter.  At Tahiti the
encircled island is thirty-six miles in its longest axis, whilst at Maurua
it is only a little more than two miles.  It will be shown, in the last
chapter in this volume, that there is the strictest resemblance in the
grouping of atolls and of common islands, and consequently there must be
the same resemblance in the grouping of atolls and of encircling
barrier-reefs.

The islands lying within reefs of this class, are of very various heights.
Tahiti is 7,000 feet (The height of Tahiti is given from Captain Beechey;
Maurua from Mr. F.D. Bennett ("Geograph. Journ." volume viii., page 220);
Aitutaki from measurements made on board the "Beagle"; and Manouai or
Harvey Island, from an estimate by the Rev. J. Williams.  The two latter
islands, however, are not in some respects well characterised examples of
the encircled class.); Maurua about 800; Aitutaki 360, and Manouai only 50.
The geological nature of the included land varies: in most cases it is of
ancient volcanic origin, owing apparently to the fact that islands of this
nature are most frequent within all great seas; some, however, are of
madreporitic limestone, and others of primary formation, of which latter
kind New Caledonia offers the best example.  The central land consists
either of one island, or of several: thus, in the Society group, Eimeo
stands by itself; while Taha and Raiatea (Figure 3, Plate I.), both
moderately large islands of nearly equal size, are included in one reef.
Within the reef of the Gambier group there are four large and some smaller
islands (Figure 8, Plate I.); within that of Hogoleu (Figure 2, Plate I.)
nearly a dozen small islands are scattered over the expanse of one vast
lagoon.

After the details now given, it may be asserted that there is not one point
of essential difference between encircling barrier-reefs and atolls: the
latter enclose a simple sheet of water, the former encircle an expanse with
one or more islands rising from it.  I was much struck with this fact, when
viewing, from the heights of Tahiti, the distant island of Eimeo standing
within smooth water, and encircled by a ring of snow-white breakers.
Remove the central land, and an annular reef like that of an atoll in an
early stage of its formation is left; remove it from Bolabola, and there
remains a circle of linear coral-islets, crowned with tall cocoa-nut trees,
like one of the many atolls scattered over the Pacific and Indian Oceans.

The barrier-reefs of Australia and of New Caledonia deserve a separate
notice from their great dimensions.  The reef on the west coast of New
Caledonia (Figure 5, Plate II.) is 400 miles in length; and for a length of
many leagues it seldom approaches within eight miles of the shore; and near
the southern end of the island, the space between the reef and the land is
sixteen miles in width.  The Australian barrier extends, with a few
interruptions, for nearly a thousand miles; its average distance from the
land is between twenty and thirty miles; and in some parts from fifty to
seventy.  The great arm of the sea thus included, is from ten to twenty-five
fathoms deep, with a sandy bottom; but towards the southern end, where
the reef is further from the shore, the depth gradually increases to forty,
and in some parts to more than sixty fathoms.  Flinders (Flinders' "Voyage
to Terra Australis," volume ii., page 88.) has described the surface of
this reef as consisting of a hard white agglomerate of different kinds of
coral, with rough projecting points.  The outer edge is the highest part;
it is traversed by narrow gullies, and at rare intervals is breached by
ship-channels.  The sea close outside is profoundly deep; but, in front of
the main breaches, soundings can sometimes be obtained.  Some low islets
have been formed on the reef.

(PLATE: UNNAMED, THREE VERTICAL SECTIONS (WOODCUT DIAGRAMS):

1. VANIKORO, from the "Atlas of the Voyage of the 'Astrolabe'," by D.
D'Urville.

2. GAMBIER ISLAND, from Beechey.

3. MAURUA, from the "Atlas of the Voyage of the 'Coquille'," by Duperrey.

The horizontal line is the level of the sea, from which on the right hand a
plummet descends, representing a depth of 200 fathoms, or 1,200 feet.  The
vertical shading shows the section of the land, and the horizontal shading
that of the encircling barrier-reef: from the smallness of the scale, the
lagoon-channel could not be represented.

AA.--Outer edge of the coral-reefs, where the sea breaks.

BB.--The shore of the encircled islands.)

There is one important point in the structure of barrier-reefs which must
here be considered.  The accompanying diagrams represent north and south
vertical sections, taken through the highest points of Vanikoro, Gambier,
and Maurua Islands, and through their encircling reefs.  The scale both in
the horizontal and vertical direction is the same, namely, a quarter of an
inch to a nautical mile.  The height and width of these islands is known;
and I have attempted to represent the form of the land from the shading of
the hills in the large published charts.  It has long been remarked, even
from the time of Dampier, that considerable degree of relation subsists
between the inclination of that part of the land which is beneath water and
that above it; hence the dotted line in the three sections, probably, does
not widely differ in inclination from the actual submarine prolongation of
the land.  If we now look at the outer edge of the reef (AA), and bear in
mind that the plummet on the right hand represents a depth of 1,200 feet,
we must conclude that the vertical thickness of these barrier coral-reefs
is very great.

I must observe that if the sections had been taken in any other direction
across these islands, or across other encircled islands (In the fifth
chapter an east and west section across the Island of Bolabola and its
barrier-reefs is given, for the sake of illustrating another point.  The
unbroken line in it (woodcut No. 5) is the section referred to.  The scale
is .57 of an inch to a mile; it is taken from the "Atlas of the Voyage of
the 'Coquille'," by Duperrey.  The depth of the lagoon-channel is
exaggerated.), the result would have been the same.  In the succeeding
chapter it will be shown that reef-building polypifers cannot flourish at
great depths,--for instance, it is highly improbable that they could exist
at a quarter of the depth represented by the plummet on the right hand of
the woodcut.  Here there is a great APPARENT difficulty--how were the basal
parts of these barrier-reef formed?  It will, perhaps, occur to some, that
the actual reefs formed of coral are not of great thickness, but that
before their first growth, the coasts of these encircled islands were
deeply eaten into, and a broad but shallow submarine ledge thus left, on
the edge of which the coral grew; but if this had been the case, the shore
would have been invariably bounded by lofty cliffs, and not have sloped
down to the lagoon-channel, as it does in many instances.  On this view
(The Rev. D. Tyerman and Mr. Bennett ("Journal of Voyage and Travels,"
volume i., page 215) have briefly suggested this explanation of the origin
of the encircling reefs of the Society Islands.), moreover, the cause of
the reef springing up at such a great distance from the land, leaving a
deep and broad moat within, remains altogether unexplained.  A supposition
of the same nature, and appearing at first more probable is, that the reefs
sprung up from banks of sediment, which had accumulated round the shore
previously to the growth of the coral; but the extension of a bank to the
same distance round an unbroken coast, and in front of those deep arms of
the sea (as in Raiatea, see Plate II., Figure 3) which penetrate nearly to
the heart of some encircled islands, is exceedingly improbable.  And why,
again, should the reef spring up, in some cases steep on both sides like a
wall, at a distance of two, three or more miles from the shore, leaving a
channel often between two hundred and three hundred feet deep, and rising
from a depth which we have reason to believe is destructive to the growth
of coral?  An admission of this nature cannot possibly be made.  The
existence, also, of the deep channel, utterly precludes the idea of the
reef having grown outwards, on a foundation slowly formed on its outside,
by the accumulation of sediment and coral detritus.  Nor, again, can it be
asserted, that the reef-building corals will not grow, excepting at a great
distance from the land; for, as we shall soon see, there is a whole class
of reefs, which take their name from growing closely attached (especially
where the sea is deep) to the beach.  At New Caledonia (see Plate II.,
Figure 5) the reefs which run in front of the west coast are prolonged in
the same line 150 miles beyond the northern extremity of the island, and
this shows that some explanation, quite different from any of those just
suggested, is required.  The continuation of the reefs on each side of the
submarine prolongation of New Caledonia, is an exceedingly interesting
fact, if this part formerly existed as the northern extremity of the
island, and before the attachment of the coral had been worn down by the
action of the sea, or if it originally existed at its present height, with
or without beds of sediment on each flank, how can we possibly account for
the reefs, not growing on the crest of this submarine portion, but fronting
its sides, in the same line with the reefs which front the shores of the
lofty island?  We shall hereafter see, that there is one, and I believe
only one, solution of this difficulty.

One other supposition to account for the position of encircling barrier-reefs
remains, but it is almost too preposterous to be mentioned; namely,
that they rest on enormous submarine craters, surrounding the included
islands.  When the size, height, and form of the islands in the Society
group are considered, together with the fact that all are thus encircled,
such a notion will be rejected by almost every one.  New Caledonia,
moreover, besides its size, is composed of primitive formations, as are
some of the Comoro Islands (I have been informed that this is the case by
Dr. Allan of Forres, who has visited this group.); and Aitutaki consists of
calcareous rock.  We must, therefore, reject these several explanations,
and conclude that the vertical thickness of barrier-reefs, from their outer
edges to the foundation on which they rest (from AA in the section to the
dotted lines) is really great; but in this, there is no difficulty, for it
is not necessary to suppose that the coral has sprung up from an immense
depth, as will be evident when the theory of the upward growth of
coral-reefs, during the slow subsidence of their foundation, is discussed.

Chapter I, Section 1.1

CHAPTER I.--ATOLLS OR LAGOON-ISLANDS.

SECTION 1.I.--KEELING ATOLL.

Corals on the outer margin.--Zone of Nulliporae.--Exterior reef.--Islets.--
Coral-conglomerate.--Lagoon.--Calcareous sediment.--Scari and Holuthuriae
subsisting on corals.--Changes in the condition of the reefs and islets.--
Probable subsidence of the atoll.--Future state of the lagoon.

(PLATE: UNTITLED WOODCUT, VERTICAL SECTION THROUGH KEELING ATOLL.)

A.--Level of the sea at low water: where the letter A is placed, the depth
is twenty-five fathoms, and the distance rather more than one hundred and
fifty yards from the edge of the reef.

B.--Outer edge of that flat part of the reef, which dries at low water:
the edge either consists of a convex mound, as represented, or of rugged
points, like those a little farther seaward, beneath the water.

C.--A flat of coral-rock, covered at high water.

D.--A low projecting ledge of brecciated coral-rock washed by the waves at
high water.

E.--A slope of loose fragments, reached by the sea only during gales: the
upper part, which is from six to twelve feet high, is clothed with
vegetation.  The surface of the islet gently slopes to the lagoon.

F.--Level of the lagoon at low water.

KEELING or COCOS atoll is situated in the Indian Ocean, in 12 deg 5' S.,
and longitude 90 deg 55' E.: a reduced chart of it was made from the
survey of Captain Fitzroy and the Officers of H.M.S. "Beagle," is given in
Plate I., Figure 10.  The greatest width of this atoll is nine miles and a
half.  Its structure is in most respects characteristic of the class to
which it belongs, with the exception of the shallowness of the lagoon.  The
accompanying woodcut represents a vertical section, supposed to be drawn at
low water from the outer coast across one of the low islets (one being
taken of average dimensions) to within the lagoon.

The section is true to the scale in a horizontal line, but it could not be
made so in a vertical one, as the average greatest height of the land is
only between six and twelve feet above high-water mark.

I will describe the section, commencing with the outer margin.  I must
first observe that the reef-building polypifers, not being tidal animals,
require to be constantly submerged or washed by the breakers.  I was
assured by Mr. Liesk, a very intelligent resident on these islands, as well
as by some chiefs at Tahiti (Otaheite), that an exposure to the rays of the
sun for a very short time invariably causes their destruction.  Hence it is
possible only under the most favourable circumstances, afforded by an
unusually low tide and smooth water, to reach the outer margin, where the
coral is alive.  I succeeded only twice in gaining this part, and found it
almost entirely composed of a living Porites, which forms great irregularly
rounded masses (like those of an Astraea, but larger) from four to eight
feet broad, and little less in thickness.  These mounds are separated from
each other by narrow crooked channels, about six feet deep, most of which
intersect the line of reef at right angles.  On the furthest mound, which I
was able to reach by the aid of a leaping-pole, and over which the sea
broke with some violence, although the day was quite calm and the tide low,
the polypifers in the uppermost cells were all dead, but between three and
four inches lower down on its side they were living, and formed a
projecting border round the upper and dead surface.  The coral being thus
checked in its upward growth, extends laterally, and hence most of the
masses, especially those a little further inwards, had broad flat dead
summits.  On the other hand I could see, during the recoil of the breakers,
that a few yards further seaward, the whole convex surface of the Porites
was alive; so that the point where we were standing was almost on the exact
upward and shoreward limit of existence of those corals which form the
outer margin of the reef.  We shall presently see that there are other
organic productions, fitted to bear a somewhat longer exposure to the air
and sun.

Next, but much inferior in importance to the Porites, is the Millepora
complanata.  (This Millepora (Palmipora of Blainville), as well as the M.
alcicornis, possesses the singular property of stinging the skin where it
is delicate, as on the face and arm.)

It grows in thick vertical plates, intersecting each other at various
angles, and forms an exceedingly strong honeycombed mass, which generally
affects a circular form, the marginal plates alone being alive.  Between
these plates and in the protected crevices on the reef, a multitude of
branching zoophytes and other productions flourish, but the Porites and
Millepora alone seem able to resist the fury of the breakers on its upper
and outer edge: at the depth of a few fathoms other kinds of stony corals
live.  Mr. Liesk, who was intimately acquainted with every part of this
reef, and likewise with that of North Keeling atoll, assured me that these
corals invariably compose the outer margin.  The lagoon is inhabited by
quite a distinct set of corals, generally brittle and thinly branched; but
a Porites, apparently of the same species with that on the outside, is
found there, although it does not seem to thrive, and certainly does not
attain the thousandth part in bulk of the masses opposed to the breakers.

The woodcut shows the form of the bottom off the reef: the water deepens
for a space between one and two hundred yards wide, very gradually to
twenty-five fathoms (A in section), beyond which the sides plunge into the
unfathomable ocean at an angle of 45 deg.  (The soundings from which this
section is laid down were taken with great care by Captain Fitzroy himself.
He used a bell-shaped lead, having a diameter of four inches, and the
armings each time were cut off and brought on board for me to examine.  The
arming is a preparation of tallow, placed in the concavity at the bottom of
the lead.  Sand, and even small fragments of rock, will adhere to it; and
if the bottom be of rock it brings up an exact impression of its surface.)
To the depth of ten or twelve fathoms the bottom is exceedingly rugged, and
seems formed of great masses of living coral, similar to those on the
margin.  The arming of the lead here invariably came up quite clean, but
deeply indented, and chains and anchors which were lowered, in the hopes of
tearing up the coral, were broken.  Many small fragments, however, of
Millepora alcicornis were brought up; and on the arming from an eight-fathom
cast, there was a perfect impression of an Astraea, apparently
alive.  I examined the rolled fragments cast on the beach during gales, in
order further to ascertain what corals grew outside the reef.  The
fragments consisted of many kinds, of which the Porites already mentioned
and a Madrepora, apparently the M. corymbosa, were the most abundant.  As I
searched in vain in the hollows on the reef and in the lagoon, for a living
specimen of this Madrepore, I conclude that it is confined to a zone
outside, and beneath the surface, where it must be very abundant.
Fragments of the Millepora alcicornis and of an Astraea were also numerous;
the former is found, but not in proportionate numbers, in the hollows on
the reef; but the Astraea I did not see living.  Hence we may infer, that
these are the kinds of coral which form the rugged sloping surface
(represented in the woodcut by an uneven line), round and beneath the
external margin.  Between twelve and twenty fathoms the arming came up an
equal number of times smoothed with sand, and indented with coral: an
anchor and lead were lost at the respective depths of thirteen and sixteen
fathoms.  Out of twenty-five soundings taken at a greater depth than twenty
fathoms, every one showed that the bottom was covered with sand; whereas,
at a less depth than twelve fathoms, every sounding showed that it was
exceedingly rugged, and free from all extraneous particles.  Two soundings
were obtained at the depth of 360 fathoms, and several between two hundred
and three hundred fathoms.  The sand brought up from these depths consisted
of finely triturated fragments of stony zoophytes, but not, as far as I
could distinguish, of a particle of any lamelliform genus: fragments of
shells were rare.

At a distance of 2,200 yards from the breakers, Captain Fitzroy found no
bottom with a line of 7,200 feet in length; hence the submarine slope of
this coral formation is steeper than that of any volcanic cone.  Off the
mouth of the lagoon, and likewise off the northern point of the atoll,
where the currents act violently, the inclination, owing to the
accumulation of sediment, is less.  As the arming of the lead from all the
greater depths showed a smooth sandy bottom, I at first concluded that the
whole consisted of a vast conical pile of calcareous sand, but the sudden
increase of depth at some points, and the circumstance of the line having
been cut, as if rubbed, when between five hundred and six hundred fathoms
were out, indicate the probable existence of submarine cliffs.

On the margin of the reef, close within the line where the upper surface of
the Porites and of the Millepora is dead, three species of Nullipora
flourish.  One grows in thin sheets, like a lichen on old trees; the second
in stony knobs, as thick as a man's finger, radiating from a common centre;
and the third, which is less common, in a moss-like reticulation of thin,
but perfectly rigid branches.  (This last species is of a beautiful bright
peach-blossom colour.  Its branches are about as thick as crow-quills; they
are slightly flattened and knobbed at the extremities.  The extremities
only are alive and brightly coloured.  The two other species are of a dirty
purplish-white.  The second species is extremely hard; its short knob-like
branches are cylindrical, and do not grow thicker at their extremities.)
The three species occur either separately or mingled together; and they
form by their successive growth a layer two or three feet in thickness,
which in some cases is hard, but where formed of the lichen-like kind,
readily yields an impression to the hammer: the surface is of a reddish
colour.  These Nulliporae, although able to exist above the limit of true
corals, seem to require to be bathed during the greater part of each tide
by breaking water, for they are not found in any abundance in the protected
hollows on the back part of the reef, where they might be immersed either
during the whole or an equal proportional time of each tide.  It is
remarkable that organic productions of such extreme simplicity, for the
Nulliporae undoubtedly belong to one of the lowest classes of the vegetable
kingdom, should be limited to a zone so peculiarly circumstanced.  Hence
the layer composed by their growth merely fringes the reef for a space of
about twenty yards in width, either under the form of separate mammillated
projections, where the outer masses of coral are separate, or, more
commonly, where the corals are united into a solid margin, as a continuous
smooth convex mound (B in woodcut), like an artificial breakwater.  Both
the mound and mammillated projections stand about three feet higher than
any other part of the reef, by which term I do not include the islets,
formed by the accumulation of rolled fragments.  We shall hereafter see
that other coral reefs are protected by a similar thick growth of
Nulliporae on the outer margin, the part most exposed to the breakers, and
this must effectually aid in preserving it from being worn down.

The woodcut represents a section across one of the islets on the reef, but
if all that part which is above the level of C were removed, the section
would be that of a simple reef, as it occurs where no islet has been
formed.  It is this reef which essentially forms the atoll.  It is a ring,
enclosing the lagoon on all sides except at the northern end, where there
are two open spaces, through one of which ships can enter.  The reef varies
in width from two hundred and fifty to five hundred yards, its surface is
level, or very slightly inclined towards the lagoon, and at high tide the
sea breaks entirely over it: the water at low tide thrown by the breakers
on the reef, is carried by the many narrow and shoal gullies or channels on
its surface, into the lagoon: a return stream sets out of the lagoon
through the main entrance.  The most frequent coral in the hollows on the
reef is Pocillopora verrucosa, which grows in short sinuous plates, or
branches, and when alive is of a beautiful pale lake-red: a Madrepora,
closely allied or identical with M. pocillifera, is also common.  As soon
as an islet is formed, and the waves are prevented breaking entirely over
the reef, the channels and hollows in it become filled up with cemented
fragments, and its surface is converted into a hard smooth floor (C of
woodcut), like an artificial one of freestone.  This flat surface varies in
width from one hundred to two hundred, or even three hundred yards, and is
strewed with a few large fragments of coral torn up during gales: it is
uncovered only at low water.  I could with difficulty, and only by the aid
of a chisel, procure chips of rock from its surface, and therefore could
not ascertain how much of it is formed by the aggregation of detritus, and
how much by the outward growth of mounds of corals, similar to those now
living on the margin.  Nothing can be more singular than the appearance at
low tide of this "flat" of naked stone, especially where it is externally
bounded by the smooth convex mound of Nulliporae, appearing like a
breakwater built to resist the waves, which are constantly throwing over it
sheets of foaming water.  The characteristic appearance of this "flat" is
shown in the foregoing woodcut of Whitsunday atoll.

The islets on the reef are first formed between two hundred and three
hundred yards from its outer edge, through the accumulation of a pile of
fragments, thrown together by some unusually strong gale.  Their ordinary
width is under a quarter of a mile, and their length varies from a few
yards to several miles.  Those on the south-east and windward side of the
atoll, increase solely by the addition of fragments on their outer side;
hence the loose blocks of coral, of which their surface is composed, as
well as the shells mingled with them, almost exclusively consist of those
kinds which live on the outer coast.  The highest part of the islets
(excepting hillocks of blown sand, some of which are thirty feet high), is
close to the outer beach (E of the woodcut), and averages from six to ten
feet above ordinary high-water mark.  From the outer beach the surface
slopes gently to the shores of the lagoon, which no doubt has been caused
by the breakers the further they have rolled over the reef, having had less
power to throw up fragments.  The little waves of the lagoon heap up sand
and fragments of thinly-branched corals on the inner side of the islets on
the leeward side of the atoll; and these islets are broader than those to
windward, some being even eight hundred yards in width; but the land thus
added is very low.  The fragments beneath the surface are cemented into a
solid mass, which is exposed as a ledge (D of the woodcut), projecting some
yards in front of the outer shore and from two to four feet high.  This
ledge is just reached by the waves at ordinary high-water: it extends in
front of all the islets, and everywhere has a water-worn and scooped
appearance.  The fragments of coral which are occasionally cast on the
"flat" are during gales of unusual violence swept together on the beach,
where the waves each day at high-water tend to remove and gradually wear
them down; but the lower fragments having become firmly cemented together
by the percolation of calcareous matter, resist the daily tides longer, and
hence project as a ledge.  The cemented mass is generally of a white
colour, but in some few parts reddish from ferruginous matter; it is very
hard, and is sonorous under the hammer; it is obscurely divided by seams,
dipping at a small angle seaward; it consists of fragments of the corals
which grow on the outer margin, some quite and others partially rounded,
some small and others between two and three feet across; and of masses of
previously formed conglomerate, torn up, rounded, and re-cemented; or it
consists of a calcareous sandstone, entirely composed of rounded particles,
generally almost blended together, of shells, corals, the spines of echini,
and other such organic bodies; rocks, of this latter kind, occur on many
shores, where there are no coral reefs.  The structure of the coral in the
conglomerate has generally been much obscured by the infiltration of
spathose calcareous matter; and I collected a very interesting series,
beginning with fragments of unaltered coral, and ending with others, where
it was impossible to discover with the naked eye any trace of organic
structure.  In some specimens I was unable, even with the aid of a lens,
and by wetting them, to distinguish the boundaries of the altered coral and
spathose limestone.  Many even of the blocks of coral lying loose on the
beach, had their central parts altered and infiltrated.

The lagoon alone remains to be described; it is much shallower than that of
most atolls of considerable size.  The southern part is almost filled up
with banks of mud and fields of coral, both dead and alive, but there are
considerable spaces, between three and four fathoms, and smaller basins,
from eight to ten fathoms deep.  Probably about half its area consists of
sediment, and half of coral-reefs.  The corals composing these reefs have a
very different aspect from those on the outside; they are very numerous in
kind, and most of them are thinly branched.  Meandrina, however, lives in
the lagoon, and great rounded masses of this coral are numerous, lying
quite or almost loose on the bottom.  The other commonest kinds consist of
three closely allied species of true Madrepora in thin branches; of
Seriatapora subulata; two species of Porites (This Porites has somewhat the
habit of P. clavaria, but the branches are not knobbed at their ends.  When
alive it is of a yellow colour, but after having been washed in fresh water
and placed to dry, a jet-black slimy substance exuded from the entire
surface, so that the specimen now appears as if it had been dipped in ink.)
with cylindrical branches, one of which forms circular clumps, with the
exterior branches only alive; and lastly, a coral something like an
Explanaria, but with stars on both surfaces, growing in thin, brittle,
stony, foliaceous expansions, especially in the deeper basins of the
lagoon.  The reefs on which these corals grow are very irregular in form,
are full of cavities, and have not a solid flat surface of dead rock, like
that surrounding the lagoon; nor can they be nearly so hard, for the
inhabitants made with crowbars a channel of considerable length through
these reefs, in which a schooner, built on the S.E. islet, was floated out.
It is a very interesting circumstance, pointed out to us by Mr. Liesk, that
this channel, although made less than ten years before our visit, was then,
as we saw, almost choked up with living coral, so that fresh excavations
would be absolutely necessary to allow another vessel to pass through it.

The sediment from the deepest parts in the lagoon, when wet, appeared
chalky, but when dry, like very fine sand.  Large soft banks of similar,
but even finer grained mud, occur on the S.E. shore of the lagoon,
affording a thick growth of a Fucus, on which turtle feed: this mud,
although discoloured by vegetable matter, appears from its entire solution
in acids to be purely calcareous.  I have seen in the Museum of the
Geological Society, a similar but more remarkable substance, brought by
Lieutenant Nelson from the reefs of Bermuda, which, when shown to several
experienced geologists, was mistaken by them for true chalk.  On the
outside of the reef much sediment must be formed by the action of the surf
on the rolled fragments of coral; but in the calm waters of the lagoon,
this can take place only in a small degree.  There are, however, other and
unexpected agents at work here: large shoals of two species of Scarus, one
inhabiting the surf outside the reef and the other the lagoon, subsist
entirely, as I was assured by Mr. Liesk, the intelligent resident before
referred to, by browsing on the living polypifers.  I opened several of
these fish, which are very numerous and of considerable size, and I found
their intestines distended by small pieces of coral, and finely ground
calcareous matter.  This must daily pass from them as the finest sediment;
much also must be produced by the infinitely numerous vermiform and
molluscous animals, which make cavities in almost every block of coral.
Dr. J. Allan, of Forres, who has enjoyed the best means of observation,
informs me in a letter that the Holothuriae (a family of Radiata) subsist
on living coral; and the singular structure of bone within the anterior
extremity of their bodies, certainly appears well adapted for this purpose.
The number of the species of Holothuria, and of the individuals which swarm
on every part of these coral-reefs, is extraordinarily great; and many
shiploads are annually freighted, as is well-known, for China with the
trepang, which is a species of this genus.  The amount of coral yearly
consumed, and ground down into the finest mud, by these several creatures,
and probably by many other kinds, must be immense.  These facts are,
however, of more importance in another point of view, as showing us that
there are living checks to the growth of coral-reefs, and that the almost
universal law of "consumed and be consumed," holds good even with the
polypifers forming those massive bulwarks, which are able to withstand the
force of the open ocean.

Considering that Keeling atoll, like other coral formations, has been
entirely formed by the growth of organic beings, and the accumulation of
their detritus, one is naturally led to inquire how long it has continued,
and how long it is likely to continue, in its present state.  Mr. Liesk
informed me that he had seen an old chart in which the present long island
on the S.E. side was divided by several channels into as many islets; and
he assures me that the channels can still be distinguished by the smaller
size of the trees on them.  On several islets, also, I observed that only
young cocoa-nut trees were growing on the extremities; and that older and
taller trees rose in regular succession behind them; which shows that these
islets have very lately increased in length.  In the upper and south-eastern
part of the lagoon, I was much surprised by finding an irregular
field of at least a mile square of branching corals, still upright, but
entirely dead.  They consisted of the species already mentioned; they were
of a brown colour, and so rotten, that in trying to stand on them I sank
halfway up the leg, as if through decayed brushwood.  The tops of the
branches were barely covered by water at the time of lowest tide.  Several
facts having led me to disbelieve in any elevation of the whole atoll, I
was at first unable to imagine what cause could have killed so large a
field of coral.  Upon reflection, however, it appeared to me that the
closing up of the above-mentioned channels would be a sufficient cause; for
before this, a strong breeze by forcing water through them into the head of
the lagoon, would tend to raise its level.  But now this cannot happen, and
the inhabitants observe that the tide rises to a less height, during a high
S.E. wind, at the head than at the mouth of the lagoon.  The corals, which,
under the former condition of things, had attained the utmost possible
limit of upward growth, would thus occasionally be exposed for a short time
to the sun, and be killed.

Besides the increase of dry land, indicated by the foregoing facts, the
exterior solid reef appears to have grown outwards.  On the western side of
the atoll, the "flat" lying between the margin of the reef and the beach,
is very wide; and in front of the regular beach with its conglomerate
basis, there is, in most parts, a bed of sand and loose fragments with
trees growing out of it, which apparently is not reached even by the spray
at high water.  It is evident some change has taken place since the waves
formed the inner beach; that they formerly beat against it with violence
was evident, from a remarkably thick and water-worn point of conglomerate
at one spot, now protected by vegetation and a bank of sand; that they beat
against it in the same peculiar manner in which the swell from windward now
obliquely curls round the margin of the reef, was evident from the
conglomerate having been worn into a point projecting from the beach in a
similarly oblique manner.  This retreat in the line of action of the
breakers might result, either from the surface of the reef in front of the
islets having been submerged at one time, and afterward having grown
upwards, or from the mounds of coral on the margin having continued to grow
outwards.  That an outward growth of this part is in process, can hardly be
doubted from the fact already mentioned of the mounds of Porites with their
summits apparently lately killed, and their sides only three or four inches
lower down thickened by a fresh layer of living coral.  But there is a
difficulty on this supposition which I must not pass over.  If the whole,
or a large part of the "flat," had been formed by the outward growth of the
margin, each successive margin would naturally have been coated by the
Nulliporae, and so much of the surface would have been of equal height with
the existing zone of living Nulliporae: this is not the case, as may be
seen in the woodcut.  It is, however, evident from the abraded state of the
"flat," with its original inequalities filled up, that its surface has been
much modified; and it is possible that the hinder portions of the zone of
Nulliporae, perishing as the reef grows outwards, might be worn down by the
surf.  If this has not taken place, the reef can in no part have increased
outwards in breadth since its formation, or at least since the Nulliporae
formed the convex mound on its margin; for the zone thus formed, and which
stands between two and three feet above the other parts of the reef, is
nowhere much above twenty yards in width.

Thus far we have considered facts, which indicate, with more or less
probability, the increase of the atoll in its different parts: there are
others having an opposite tendency.  On the south-east side, Lieutenant
Sulivan, to whose kindness I am indebted for many interesting observations,
found the conglomerate projecting on the reef nearly fifty yards in front
of the beach: we may infer from what we see in all other parts of the
atoll, that the conglomerate was not originally so much exposed, but formed
the base of an islet, the front and upper part of which has since been
swept away.  The degree to which the conglomerate, round nearly the whole
atoll, has been scooped, broken up, and the fragments cast on the beach, is
certainly very surprising, even on the view that it is the office of
occasional gales to pile up fragments, and of the daily tides to wear them
away.  On the western side, also, of the atoll, where I have described a
bed of sand and fragments with trees growing out of it, in front of an old
beach, it struck both Lieutenant Sulivan and myself, from the manner in
which the trees were being washed down, that the surf had lately
recommenced an attack on this line of coast.  Appearances indicating a
slight encroachment of the water on the land, are plainer within the
lagoon: I noticed in several places, both on its windward and leeward
shores, old cocoa-nut trees falling with their roots undermined, and the
rotten stumps of others on the beach, where the inhabitants assured us the
cocoa-nut could not now grow.  Captain Fitzroy pointed out to me, near the
settlement, the foundation posts of a shed, now washed by every tide, but
which the inhabitants stated, had seven years before stood above high
watermark.  In the calm waters of the lagoon, directly connected with a
great, and therefore stable ocean, it seems very improbable that a change
in the currents, sufficiently great to cause the water to eat into the land
on all sides, should have taken place within a limited period.  From these
considerations I inferred, that probably the atoll had lately subsided to a
small amount; and this inference was strengthened by the circumstance, that
in 1834, two years before our visit, the island had been shaken by a severe
earthquake, and by two slighter ones during the ten previous years.  If,
during these subterranean disturbances, the atoll did subside, the downward
movement must have been very small, as we must conclude from the fields of
dead coral still lipping the surface of the lagoon, and from the breakers
on the western shore not having yet regained the line of their former
action.  The subsidence must, also, have been preceded by a long period of
rest, during which the islets extended to their present size, and the
living margin of the reef grew either upwards, or as I believe outwards, to
its present distance from the beach.

Whether this view be correct or not, the above facts are worthy of
attention, as showing how severe a struggle is in progress on these low
coral formations between the two nicely balanced powers of land and water.
With respect to the future state of Keeling atoll, if left undisturbed, we
can see that the islets may still extend in length; but as they cannot
resist the surf until broken by rolling over a wide space, their increase
in breadth must depend on the increasing breadth of the reef; and this must
be limited by the steepness of the submarine flanks, which can be added to
only by sediment derived from the wear and tear of the coral.  From the
rapid growth of the coral in the channel cut for the schooner, and from the
several agents at work in producing fine sediment, it might be thought that
the lagoon would necessarily become quickly filled up.  Some of this
sediment, however, is transported into the open sea, as appears from the
soundings off the mouth of the lagoon, instead of being deposited within
it.  The deposition, moreover, of sediment, checks the growth of coral-reefs,
so that these two agencies cannot act together with full effect in
filling it up.  We know so little of the habits of the many different
species of corals, which form the lagoon-reefs, that we have no more
reasons for supposing that their whole surface would grow up as quickly as
the coral did in the schooner-channel, than for supposing that the whole
surface of a peat-moss would increase as quickly as parts are known to do
in holes, where the peat has been cut away.  These agencies, nevertheless,
tend to fill up the lagoon; but in proportion as it becomes shallower, so
must the polypifers be subject to many injurious agencies, such as impure
water and loss of food.  For instance, Mr. Liesk informed me, that some
years before our visit unusually heavy rain killed nearly all the fish in
the lagoon, and probably the same cause would likewise injure the corals.
The reefs also, it must be remembered, cannot possibly rise above the level
of the lowest spring-tide, so that the final conversion of the lagoon into
land must be due to the accumulation of sediment; and in the midst of the
clear water of the ocean, and with no surrounding high land, this process
must be exceedingly slow.