Coral Reefs
by
Charles Darwin
Part 2 out of 5
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 III.--FRINGING OR SHORE-REEFS.
Reefs of Mauritius.--Shallow channel within the reef.--Its slow filling
up.--Currents of water formed within it.--Upraised reefs.--Narrow
fringing-reefs in deep seas.--Reefs on the coast of East Africa and of
Brazil.--Fringing-reefs in very shallow seas, round banks of sediment and
on worn-down islands.--Fringing-reefs affected by currents of the sea.--
Coral coating the bottom of the sea, but not forming reefs.
Fringing-reefs, or, as they have been called by some voyagers, shore-reefs,
whether skirting an island or part of a continent, might at first be
thought to differ little, except in generally being of less breadth, from
barrier-reefs. As far as the superficies of the actual reef is concerned
this is the case; but the absence of an interior deep-water channel, and
the close relation in their horizontal extension with the probable slope
beneath the sea of the adjoining land, present essential points of
difference.
The reefs which fringe the island of Mauritius offer a good example of this
class. They extend round its whole circumference, with the exception of
two or three parts (This fact is stated on the authority of the Officier du
Roi, in his extremely interesting "Voyage a l'Isle de France," undertaken
in 1768. According to Captain Carmichael (Hooker's "Bot. Misc." volume
ii., page 316) on one part of the coast there is a space for sixteen miles
without a reef.), where the coast is almost precipitous, and where, if as
is probable the bottom of the sea has a similar inclination, the coral
would have no foundation on which to become attached. A similar fact may
sometimes be observed even in reefs of the barrier class, which follow much
less closely the outline of the adjoining land; as, for instance, on the
south-east and precipitous side of Tahiti, where the encircling reef is
interrupted. On the western side of the Mauritius, which was the only part
I visited, the reef generally lies at the distance of about half a mile
from the shore; but in some parts it is distant from one to two, and even
three miles. But even in this last case, as the coast-land is gently
inclined from the foot of the mountains to the sea-beach, and as the
soundings outside the reef indicate an equally gentle slope beneath the
water, there is no reason for supposing that the basis of the reef, formed
by the prolongation of the strata of the island, lies at a greater depth
than that at which the polypifers could begin constructing the reef. Some
allowance, however, must be made for the outward extension of the corals on
a foundation of sand and detritus, formed from their own wear, which would
give to the reef a somewhat greater vertical thickness, than would
otherwise be possible.
The outer edge of the reef on the western or leeward side of the island is
tolerably well defined, and is a little higher than any other part. It
chiefly consists of large strongly branched corals, of the genus Madrepora,
which also form a sloping bed some way out to sea: the kinds of coral
growing in this part will be described in the ensuing chapter. Between the
outer margin and the beach, there is a flat space with a sandy bottom and a
few tufts of living coral; in some parts it is so shallow, that people, by
avoiding the deeper holes and gullies, can wade across it at low water; in
other parts it is deeper, seldom however exceeding ten or twelve feet, so
that it offers a safe coasting channel for boats. On the eastern and
windward side of the island, which is exposed to a heavy surf, the reef was
described to me as having a hard smooth surface, very slightly inclined
inwards, just covered at low-water, and traversed by gullies; it appears to
be quite similar in structure to the reefs of the barrier and atoll
classes.
The reef of Mauritius, in front of every river and streamlet, is breached
by a straight passage: at Grand Port, however, there is a channel like
that within a barrier-reef; it extends parallel to the shore for four
miles, and has an average depth of ten or twelve fathoms; its presence may
probably be accounted for by two rivers which enter at each end of the
channel, and bend towards each other. The fact of reefs of the fringing
class being always breached in front of streams, even of those which are
dry during the greater part of the year, will be explained, when the
conditions unfavourable to the growth of coral are considered. Low
coral-islets, like those on barrier-reefs and atolls, are seldom formed on
reefs of this class, owing apparently in some cases to their narrowness, and
in others to the gentle slope of the reef outside not yielding many fragments
to the breakers. On the windward side, however, of the Mauritius, two or
three small islets have been formed.
It appears, as will be shown in the ensuing chapter, that the action of the
surf is favourable to the vigorous growth of the stronger corals, and that
sand or sediment, if agitated by the waves, is injurious to them. Hence it
is probable that a reef on a shelving shore, like that of Mauritius, would
at first grow up, not attached to the actual beach, but at some little
distance from it; and the corals on the outer margin would be the most
vigorous. A shallow channel would thus be formed within the reef, and as
the breakers are prevented acting on the shores of the island, and as they
do not ordinarily tear up many fragments from the outside, and as every
streamlet has its bed prolonged in a straight line through the reef, this
channel could be filled up only very slowly with sediment. But a beach of
sand and of fragments of the smaller kinds of coral seems, in the case of
Mauritius, to be slowly encroaching on the shallow channel. On many
shelving and sandy coasts, the breakers tend to form a bar of sand a little
way from the beach, with a slight increase of depth within it; for
instance, Captain Grey (Captain Grey's "Journal of Two Expeditions," volume
i. page 369.) states that the west coast of Australia, in latitude 24 deg.,
is fronted by a sand bar about two hundred yards in width, on which there
is only two feet of water; but within it the depth increases to two
fathoms. Similar bars, more or less perfect, occur on other coasts. In
these cases I suspect that the shallow channel (which no doubt during
storms is occasionally obliterated) is scooped out by the flowing away of
the water thrown beyond the line, on which the waves break with the
greatest force. At Pernambuco a bar of hard sandstone (I have described
this singular structure in the "London and Edinburgh Phil. Mag." October
1841.), which has the same external form and height as a coral-reef,
extends nearly parallel to the coast; within this bar currents, apparently
caused by the water thrown over it during the greater part of each tide,
run strongly, and are wearing away its inner wall. From these facts it can
hardly be doubted, that within most fringing-reefs, especially within those
lying some distance from the land, a return stream must carry away the
water thrown over the outer edge; and the current thus produced, would tend
to prevent the channel being filled up with sediment, and might even deepen
it under certain circumstances. To this latter belief I am led, by finding
that channels are almost universally present within the fringing-reefs of
those islands which have undergone recent elevatory movements; and this
could hardly have been the case, if the conversion of the very shallow
channel into land had not been counteracted to a certain extent.
A fringing-reef, if elevated in a perfect condition above the level of the
sea, ought to present the singular appearance of a broad dry moat within a
low mound. The author ("Voyage a l'Isle de France, par un Officier du
Roi," part i., pages 192, 200.) of an interesting pedestrian tour round the
Mauritius, seems to have met with a structure of this kind: he says
"J'observai que la, ou la mer etale, independamment des rescifs du large,
il y a terre UNE ESPECE D'EFFONCEMENT ou chemin couvert naturel. On y
pourrait mettre du canon," etc. In another place he adds, "Avant de passer
le Cap, on remarque un gros banc de corail eleve de plus de quinze pieds:
c'est une espece de rescif, que la mer abandonne, il regne au pied une
longue flaque d'eau, dont on pourrait faire un bassin pour de petits
vaisseaux." But the margin of the reef, although the highest and most
perfect part, from being most exposed to the surf, would generally during a
slow rise of the land be either partially or entirely worn down to that
level, at which corals could renew their growth on its upper edge. On some
parts of the coast-land of Mauritius there are little hillocks of coral-rock,
which are either the last remnants of a continuous reef, or of low
islets formed on it. I observed that two such hillocks between Tamarin Bay
and the Great Black River; they were nearly twenty feet high, about two
hundred yards from the present beach, and about thirty feet above its
level. They rose abruptly from a smooth surface, strewed with worn
fragments of coral. They consisted in their lower part of hard calcareous
sandstone, and in their upper of great blocks of several species of Astraea
and Madrepora, loosely aggregated; they were divided into irregular beds,
dipping seaward, in one hillock at an angle of 8 deg., and in the other at
18 deg. I suspect that the superficial parts of the reefs, which have been
upraised together with the islands they fringe, have generally been much
more modified by the wearing action of the sea, than those of Mauritius.
Many islands are fringed by reefs quite similar to those of Mauritius (I
may give Cuba, as another instance; Mr. Taylor ("Loudon's Mag. of Nat.
Hist." volume ix., page 449) has described a reef several miles in length
between Gibara and Vjaro, which extends parallel to the shore at the
distance of between half and the third part of a mile, and encloses a space
of shallow water, with a sandy bottom and tufts of coral. Outside the edge
of the reef, which is formed of great branching corals, the depth is six
and seven fathoms. This coast has been upheaved at no very distant
geological period."); but on coasts where the sea deepens very suddenly the
reefs are much narrower, and their limited extension seems evidently to
depend on the high inclination of the submarine slope; a relation, which,
as we have seen, does not exist in reefs of the barrier class. The
fringing-reefs on steep coasts are frequently not more than from fifty to
one hundred yards in width; they have a nearly smooth, hard surface,
scarcely uncovered at low water, and without any interior shoal channel,
like that within those fringing-reefs, which lie at a greater distance from
the land. The fragments torn up during gales from the outer margin are
thrown over the reef on the shores of the island. I may give as instances,
Wateeo, where the reef is described by Cook as being a hundred yards wide;
and Mauti and Elizabeth Islands (Mauti is described by Lord Byron in the
voyage of H.M.S. "Blonde", and Elizabeth Island by Captain Beechey.), where
it is only fifty yards in width: the sea round these islands is very deep.
Fringing-reefs, like barrier-reefs, both surround islands, and front the
shores of continents. In the charts of the eastern coast of Africa, by
Captain Owen, many extensive fringing-reefs are laid down; thus, for a
space of nearly forty miles, from latitude 1 deg 15' to 1 deg 45' S., a
reef fringes the shore at an average distance of rather more than one mile,
and therefore at a greater distance than is usual in reefs of this class;
but as the coast-land is not lofty, and as the bottom shoals very gradually
(the depth being only from eight to fourteen fathoms at a mile and a half
outside the reef), its extension thus far from the land offers no
difficulty. The external margin of this reef is described, as formed of
projecting points, within which there is a space, from six to twelve feet
deep, with patches of living coral on it. At Mukdeesha (latitude 2 deg 1'
N.) "the port is formed," it is said (Owen's "Africa," volume i., page 357,
from which work the foregoing facts are likewise taken.) "by a long reef
extending eastward, four or five miles, within which there is a narrow
channel, with ten to twelve feet of water at low spring-tides;" it lies at
the distance of a quarter of a mile from the shore. Again, in the plan of
Mombas (latitude 4 deg S.), a reef extends for thirty-six miles, at the
distance of from half a mile to one mile and a quarter from the shore;
within it, there is a channel navigable "for canoes and small craft,"
between six and fifteen feet deep: outside the reef the depth is about
thirty fathoms at the distance of nearly half a mile. Part of this reef is
very symmetrical, and has a uniform breadth of two hundred yards.
The coast of Brazil is in many parts fringed by reefs. Of these, some are
not of coral formation; for instance, those near Bahia and in front of
Pernambuco; but a few miles south of this latter city, the reef follows
(See Baron Roussin's "Pilote du Bresil," and accompanying hydrographical
memoir.) so closely every turn of the shore, that I can hardly doubt it is
of coral; it runs at the distance of three-quarters of a mile from the
land, and within it the depth is from ten to fifteen feet. I was assured
by an intelligent pilot that at Ports Frances and Maceio, the outer part of
the reef consists of living coral, and the inner of a white stone, full of
large irregular cavities, communicating with the sea. The bottom of the
sea off the coast of Brazil shoals gradually to between thirty and forty
fathoms, at the distance of between nine and ten leagues from the land.
From the description now given, we must conclude that the dimensions and
structure of fringing-reefs depend entirely on the greater or less
inclination of the submarine slope, conjoined with the fact that
reef-building polypifers can exist only at limited depths. It follows from
this, that where the sea is very shallow, as in the Persian Gulf and in
parts of the East Indian Archipelago, the reefs lose their fringing
character, and appear as separate and irregularly scattered patches, often
of considerable area. From the more vigorous growth of the coral on the
outside, and from the conditions being less favourable in several respects
within, such reefs are generally higher and more perfect in their marginal
than in their central parts; hence these reefs sometimes assume (and this
circumstance ought not to be overlooked) the appearance of atolls; but they
differ from atolls in their central expanse being much less deep, in their
form being less defined, and in being based on a shallow foundation. But
when in a deep sea reefs fringe banks of sediment, which have accumulated
beneath the surface, round either islands or submerged rocks, they are
distinguished with difficulty on the one hand from encircling barrier-reefs,
and on the other from atolls. In the West Indies there are reefs,
which I should probably have arranged under both these classes, had not the
existence of large and level banks, lying a little beneath the surface,
ready to serve as the basis for the attachment of coral, been occasionally
brought into view by the entire or partial absence of reefs on them, and
had not the formation of such banks, through the accumulation of sediment
now in progress, been sufficiently evident. Fringing-reefs sometimes coat,
and thus protect the foundations of islands, which have been worn down by
the surf to the level of the sea. According to Ehrenberg, this has been
extensively the case with the islands in the Red Sea, which formerly ranged
parallel to the shores of the mainland, with deep water within them: hence
the reefs now coating their bases are situated relatively to the land like
barrier-reefs, although not belonging to that class; but there are, as I
believe, in the Red Sea some true barrier-reefs. The reefs of this sea and
of the West Indies will be described in the Appendix. In some cases,
fringing-reefs appear to be considerably modified in outline by the course
of the prevailing currents. Dr. J. Allan informs me that on the east coast
of Madagascar almost every headland and low point of sand has a coral-reef
extending from it in a S.W. and N.E. line, parallel to the currents on that
shore. I should think the influence of the currents chiefly consisted in
causing an extension, in a certain direction, of a proper foundation for
the attachment of the coral. Round many intertropical islands, for
instance the Abrolhos on the coast of Brazil surveyed by Captain Fitzroy,
and, as I am informed by Mr. Cuming, round the Philippines, the bottom of
the sea is entirely coated by irregular masses of coral, which although
often of large size, do not reach the surface and form proper reefs. This
must be owing, either to insufficient growth, or to the absence of those
kinds of corals which can withstand the breaking of the waves.
The three classes, atoll-formed, barrier, and fringing-reefs, together with
the modifications just described of the latter, include all the most
remarkable coral formations anywhere existing. At the commencement of the
last chapter in the volume, where I detail the principles on which the map
(Plate III.) is coloured, the exceptional cases will be enumerated.
CHAPTER IV.--ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.
In this chapter I will give all the facts which I have collected, relating
to the distribution of coral-reefs,--to the conditions favourable to their
increase,--to the rate of their growth,--and to the depth at which they are
formed.
These subjects have an important bearing on the theory of the origin of the
different classes of coral-reefs.
SECTION 4.I.--ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE.
With regard to the limits of latitude, over which coral-reefs extend, I
have nothing new to add. The Bermuda Islands, in 32 deg 15' N., is the
point furthest removed from the equator, in which they appear to exist; and
it has been suggested that their extension so far northward in this
instance is owing to the warmth of the Gulf Stream. In the Pacific, the
Loo Choo Islands, in latitude 27 deg N., have reefs on their shores, and
there is an atoll in 28 deg 30', situated N.W. of the Sandwich Archipelago.
In the Red Sea there are coral-reefs in latitude 30 deg. In the southern
hemisphere coral-reefs do not extend so far from the equatorial sea. In
the Southern Pacific there are only a few reefs beyond the line of the
tropics, but Houtmans Abrolhos, on the western shores of Australia in
latitude 29 deg S., are of coral formation.
The proximity of volcanic land, owing to the lime generally evolved from
it, has been thought to be favourable to the increase of coral-reefs.
There is, however, not much foundation for this view; for nowhere are
coral-reefs more extensive than on the shores of New Caledonia, and of
north-eastern Australia, which consist of primary formations; and in the
largest groups of atolls, namely the Maldiva, Chagos, Marshall, Gilbert,
and Low Archipelagoes, there is no volcanic or other kind of rock,
excepting that formed of coral.
The entire absence of coral-reefs in certain large areas within the
tropical seas, is a remarkable fact. Thus no coral-reefs were observed,
during the surveying voyages of the "Beagle" and her tender on the west
coast of South America south of the equator, or round the Galapagos
Islands. It appears, also, that there are none (I have been informed that
this is the case, by Lieutenant Ryder, R.N., and others who have had ample
opportunities for observation.) north of the equator; Mr. Lloyd, who
surveyed the Isthmus of Panama, remarked to me, that although he had seen
corals living in the Bay of Panama, yet he had never observed any reefs
formed by them. I at first attributed this absence of reefs on the coasts
of Peru and of the Galapagos Islands (The mean temperature of the surface
sea from observations made by the direction of Captain Fitzroy on the
shores of the Galapagos Islands, between the 16th of September and the 20th
of October, 1835, was 68 deg Fahr. The lowest temperature observed was
58.5 deg at the south-west end of Albemarle Island; and on the west coast
of this island, it was several times 62 deg and 63 deg. The mean
temperature of the sea in the Low Archipelago of atolls, and near Tahiti,
from similar observations made on board the "Beagle", was (although further
from the equator) 77.5 deg, the lowest any day being 76.5 deg. Therefore
we have here a difference of 9.5 deg in mean temperature, and 18 deg in
extremes; a difference doubtless quite sufficient to affect the
distribution of organic beings in the two areas.), to the coldness of the
currents from the south, but the Gulf of Panama is one of the hottest
pelagic districts in the world. (Humboldt's "Personal Narrative," volume
vii., page 434.) In the central parts of the Pacific there are islands
entirely free from reefs; in some few of these cases I have thought that
this was owing to recent volcanic action; but the existence of reefs round
the greater part of Hawaii, one of the Sandwich Islands, shows that recent
volcanic action does not necessarily prevent their growth.
In the last chapter I stated that the bottom of the sea round some islands
is thickly coated with living corals, which nevertheless do not form reefs,
either from insufficient growth, or from the species not being adapted to
contend with the breaking waves.
I have been assured by several people, that there are no coral-reefs on the
west coast of Africa (It might be concluded, from a paper by Captain Owen
("Geographical Journal", volume ii., page 89), that the reefs off Cape St.
Anne and the Sherboro' Islands were of coral, although the author states
that they are not purely coralline. But I have been assured by Lieutenant
Holland, R.N., that these reefs are not of coral, or at least that they do
not at all resemble those in the West Indies.), or round the islands in the
Gulf of Guinea. This perhaps may be attributed, in part, to the sediment
brought down by the many rivers debouching on that coast, and to the
extensive mud-banks, which line great part of it. But the islands of St.
Helena, Ascension, the Cape Verdes, St. Paul's, and Fernando Noronha, are,
also, entirely without reefs, although they lie far out at sea, are
composed of the same ancient volcanic rocks, and have the same general
form, with those islands in the Pacific, the shores of which are surrounded
by gigantic walls of coral-rock. With the exception of Bermuda, there is
not a single coral-reef in the central expanse of the Atlantic Ocean. It
will, perhaps, be suggested that the quantity of carbonate of lime in
different parts of the sea, may regulate the presence of reefs. But this
cannot be the case, for at Ascension, the waves charged to excess
precipitate a thick layer of calcareous matter on the tidal rocks; and at
St. Jago, in the Cape Verdes, carbonate of lime not only is abundant on the
shores, but it forms the chief part of some upraised post-tertiary strata.
The apparently capricious distribution, therefore, of coral-reefs, cannot
be explained by any of these obvious causes; but as the study of the
terrestrial and better known half of the world must convince every one that
no station capable of supporting life is lost,--nay more, that there is a
struggle for each station, between the different orders of nature,--we may
conclude that in those parts of the intertropical sea, in which there are
no coral-reefs, there are other organic bodies supplying the place of the
reef-building polypifers. It has been shown in the chapter on Keeling
atoll that there are some species of large fish, and the whole tribe of
Holothuriae which prey on the tenderer parts of the corals. On the other
hand, the polypifers in their turn must prey on some other organic beings;
the decrease of which from any cause would cause a proportionate
destruction of the living coral. The relations, therefore, which determine
the formation of reefs on any shore, by the vigorous growth of the
efficient kinds of coral, must be very complex, and with our imperfect
knowledge quite inexplicable. From these considerations, we may infer that
changes in the condition of the sea, not obvious to our senses, might
destroy all the coral-reefs in one area, and cause them to appear in
another: thus, the Pacific or Indian Ocean might become as barren of
coral-reefs as the Atlantic now is, without our being able to assign any
adequate cause for such a change.
It has been a question with some naturalists, which part of a reef is most
favourable to the growth of coral. The great mounds of living Porites and
of Millepora round Keeling atoll occur exclusively on the extreme verge of
the reef, which is washed by a constant succession of breakers; and living
coral nowhere else forms solid masses. At the Marshall islands the larger
kinds of coral (chiefly species of Astraea, a genus closely allied to
Porites) "which form rocks measuring several fathoms in thickness," prefer,
according to Chamisso (Kotzebue's "First Voyage" (English Translation),
volume iii., pages 142, 143, 331.), the most violent surf. I have stated
that the outer margin of the Maldiva atolls consists of living corals (some
of which, if not all, are of the same species with those at Keeling atoll),
and here the surf is so tremendous, that even large ships have been thrown,
by a single heave of the sea, high and dry on the reef, all on board thus
escaping with their lives.
Ehrenberg (Ehrenberg, "Uber die Natur und Bildung der Corallen Banke im
rothen Meere," page 49.) remarks, that in the Red Sea the strongest corals
live on the outer reefs, and appear to love the surf; he adds, that the
more branched kinds abound a little way within, but that even these in
still more protected places, become smaller. Many other facts having a
similar tendency might be adduced. (In the West Indies, as I am informed
by Captain Bird Allen, R.N., it is the common belief of those, who are best
acquainted with the reefs, that the coral flourishes most, where freely
exposed to the swell of the open sea.) It has, however, been doubted by
MM. Quoy and Gaimard, whether any kind of coral can even withstand, much
less flourish in, the breakers of an open sea ("Annales des Sciences
Naturelles," tome vi., pages 276, 278.--"La ou les ondes sont agitees, les
Lytophytes ne peuvent travailler, parce qu'elles detruiraient leurs
fragiles edifices," etc.): they affirm that the saxigenous lithophytes
flourish only where the water is tranquil, and the heat intense. This
statement has passed from one geological work to another; nevertheless, the
protection of the whole reef undoubtedly is due to those kinds of coral,
which cannot exist in the situations thought by these naturalists to be
most favourable to them. For should the outer and living margin perish, of
any one of the many low coral-islands, round which a line of great breakers
is incessantly foaming, the whole, it is scarcely possible to doubt, would
be washed away and destroyed, in less than half a century. But the vital
energies of the corals conquer the mechanical power of the waves; and the
large fragments of reef torn up by every storm, are replaced by the slow
but steady growth of the innumerable polypifers, which form the living zone
on its outer edge.
From these facts, it is certain, that the strongest and most massive corals
flourish, where most exposed. The less perfect state of the reef of most
atolls on the leeward and less exposed side, compared with its state to
windward; and the analogous case of the greater number of breaches on the
near sides of those atolls in the Maldiva Archipelago, which afford some
protection to each other, are obviously explained by this circumstance. If
the question had been, under what conditions the greater number of species
of coral, not regarding their bulk and strength, were developed, I should
answer,--probably in the situations described by MM. Quoy and Gaimard,
where the water is tranquil and the heat intense. The total number of
species of coral in the circumtropical seas must be very great: in the Red
Sea alone, 120 kinds, according to Ehrenberg (Ehrenberg, "Uber die Natur,"
etc., etc., page 46.), have been observed.
The same author has observed that the recoil of the sea from a steep shore
is injurious to the growth of coral, although waves breaking over a bank
are not so. Ehrenberg also states, that where there is much sediment,
placed so as to be liable to be moved by the waves there is little or no
coral; and a collection of living specimens placed by him on a sandy shore
died in the course of a few days. (Ibid., page 49.) An experiment,
however, will presently be related in which some large masses of living
coral increased rapidly in size, after having been secured by stakes on a
sandbank. That loose sediment should be injurious to the living
polypifers, appears, at first sight, probable; and accordingly, in sounding
off Keeling atoll, and (as will hereafter be shown) off Mauritius, the
arming of the lead invariably came up clean, where the coral was growing
vigorously. This same circumstance has probably given rise to a strange
belief, which, according to Captain Owen (Captain Owen on the Geography of
the Maldiva Islands, "Geographical Journal", volume ii., page 88.), is
general amongst the inhabitants of the Maldiva atolls, namely that corals
have roots, and therefore that if merely broken down to the surface, they
grow up again; but, if rooted out, they are permanently destroyed. By this
means the inhabitants keep their harbours clear; and thus the French
Governor of St. Mary's in Madagascar, "cleared out and made a beautiful
little port at that place." For it is probable that sand would accumulate
in the hollows formed by tearing out the corals, but not on the broken and
projecting stumps, and therefore, in the former case, the fresh growth of
the coral might be thus prevented.
In the last chapter I remarked that fringing-reefs are almost universally
breached, where streams enter the sea. (Lieutenant Wellstead and others
have remarked that this is the case in the Red Sea; Dr. Ruppell ("Reise in
Abyss." Band. i., page 142) says that there are pear-shaped harbours in the
upraised coral-coast, into which periodical streams enter. From this
circumstance, I presume, we must infer that before the upheaval of the
strata now forming the coast-land, fresh water and sediment entered the sea
at these points; and the coral being thus prevented growing, the pear-shaped
harbours were produced.) Most authors have attributed this fact to
the injurious effects of the fresh water, even where it enters the sea only
in small quantity, and during a part of the year. No doubt brackish water
would prevent or retard the growth of coral; but I believe that the mud and
sand which is deposited, even by rivulets when flooded, is a much more
efficient check. The reef on each side of the channel leading into Port
Louis at Mauritius, ends abruptly in a wall, at the foot of which I sounded
and found a bed of thick mud. This steepness of the sides appears to be a
general character in such breaches. Cook (Cook's "First Voyage," volume
ii., page 271 (Hawkesworth's edition).), speaking of one at Raiatea, says,
"like all the rest, it is very steep on both sides." Now, if it were the
fresh water mingling with the salt which prevented the growth of coral, the
reef certainly would not terminate abruptly, but as the polypifers nearest
the impure stream would grow less vigorously than those farther off, so
would the reef gradually thin away. On the other hand, the sediment
brought down from the land would only prevent the growth of the coral in
the line of its deposition, but would not check it on the side, so that the
reefs might increase till they overhung the bed of the channel. The
breaches are much fewer in number, and front only the larger valleys in
reefs of the encircling barrier class. They probably are kept open in the
same manner as those into the lagoon of an atoll, namely, by the force of
the currents and the drifting outwards of fine sediment. Their position in
front of valleys, although often separated from the land by deep water
lagoon-channels, which it might be thought would entirely remove the
injurious effects both of the fresh water and the sediment, will receive a
simple explanation when we discuss the origin of barrier-reefs.
In the vegetable kingdom every different station has its peculiar group of
plants, and similar relations appear to prevail with corals. We have
already described the great difference between the corals within the lagoon
of an atoll and those on its outer margin. The corals, also, on the margin
of Keeling Island occurred in zones; thus the Porites and Millepora
complanata grow to a large size only where they are washed by a heavy sea,
and are killed by a short exposure to the air; whereas, three species of
Nullipora also live amidst the breakers, but are able to survive uncovered
for a part of each tide; at greater depths, a strong Madrepora and
Millepora alcicornis are the commonest kinds, the former appearing to be
confined to this part, beneath the zone of massive corals, minute
encrusting corallines and other organic bodies live. If we compare the
external margin of the reef at Keeling atoll with that on the leeward side
of Mauritius, which are very differently circumstanced, we shall find a
corresponding difference in the appearance of the corals. At the latter
place, the genus Madrepora is preponderant over every other kind, and
beneath the zone of massive corals there are large beds of Seriatopora.
There is also a marked difference, according to Captain Moresby (Captain
Moresby on the Northern Maldiva atolls, "Geographical Journal", volume v.,
page 401.), between the great branching corals of the Red Sea, and those on
the reefs of the Maldiva atolls.
These facts, which in themselves are deserving of notice, bear, perhaps,
not very remotely, on a remarkable circumstance which has been pointed out
to me by Captain Moresby, namely, that with very few exceptions, none of
the coral-knolls within the lagoons of Peros Banhos, Diego Garcia, and the
Great Chagos Bank (all situated in the Chagos group), rise to the surface
of the water; whereas all those, with equally few exceptions, within
Solomon and Egmont atolls in the same group, and likewise within the large
southern Maldiva atolls, reach the surface. I make these statements, after
having examined the charts of each atoll. In the lagoon of Peros Banhos,
which is nearly twenty miles across, there is only one single reef which
rises to the surface; in Diego Garcia there are seven, but several of these
lie close to the margin of the lagoon, and need scarcely have been
reckoned; in the Great Chagos Bank there is not one. On the other hand, in
the lagoons of some of the great southern Maldiva atolls, although thickly
studded with reefs, every one without exception rises to the surface; and
on an average there are less than two submerged reefs in each atoll; in the
northern atolls, however, the submerged lagoon-reefs are not quite so rare.
The submerged reefs in the Chagos atolls generally have from one to seven
fathoms water on them, but some have from seven to ten. Most of them are
small with very steep sides (Some of these statements were not communicated
to me verbally by Captain Moresby, but are taken from the MS. account
before alluded to, of the Chagos Group.); at Peros Banhos they rise from a
depth of about thirty fathoms, and some of them in the Great Chagos Bank
from above forty fathoms; they are covered, Captain Moresby informs me,
with living and healthy coral, two and three feet high, consisting of
several species. Why then have not these lagoon-reefs reached the surface,
like the innumerable ones in the atolls above named? If we attempt to
assign any difference in their external conditions, as the cause of this
diversity, we are at once baffled. The lagoon of Diego Garcia is not deep,
and is almost wholly surrounded by its reef; Peros Banhos is very deep,
much larger, with many wide passages communicating with the open sea. On
the other hand, of those atolls, in which all or nearly all the lagoon-reefs
have reached the surface, some are small, others large, some shallow,
others deep, some well-enclosed, and others open.
Captain Moresby informs me that he has seen a French chart of Diego Garcia
made eighty years before his survey, and apparently very accurate; and from
it he infers, that during this interval there has not been the smallest
change in the depth on any of the knolls within the lagoon. It is also
known that during the last fifty-one years, the eastern channel into the
lagoon has neither become narrower, nor decreased in depth; and as there
are numerous small knolls of living coral within it, some change might have
been anticipated. Moreover, as the whole reef round the lagoon of this
atoll has been converted into land--an unparalleled case, I believe, in an
atoll of such large size,--and as the strip of land is for considerable
spaces more than half a mile wide--also a very unusual circumstance,--we
have the best possible evidence, that Diego Garcia has remained at its
present level for a very long period. With this fact, and with the
knowledge that no sensible change has taken place during eighty years in
the coral-knolls, and considering that every single reef has reached the
surface in other atolls, which do not present the smallest appearance of
being older than Diego Garcia and Peros Banhos, and which are placed under
the same external conditions with them, one is led to conclude that these
submerged reefs, although covered with luxuriant coral, have no tendency to
grow upwards, and that they would remain at their present levels for an
almost indefinite period.
From the number of these knolls, from their position, size, and form, many
of them being only one or two hundred yards across, with a rounded outline,
and precipitous sides,--it is indisputable that they have been formed by
the growth of coral; and this makes the case much more remarkable. In
Peros Banhos and in the Great Chagos Bank, some of these almost columnar
masses are 200 feet high, and their summits lie only from two to eight
fathoms beneath the surface; therefore, a small proportional amount more of
growth would cause them to attain the surface, like those numerous knolls,
which rise from an equally great depth within the Maldiva atolls. We can
hardly suppose that time has been wanting for the upward growth of the
coral, whilst in Diego Garcia, the broad annular strip of land, formed by
the continued accumulation of detritus, shows how long this atoll has
remained at its present level. We must look to some other cause than the
rate of growth; and I suspect it will be found in the reefs being formed of
different species of corals, adapted to live at different depths.
The Great Chagos Bank is situated in the centre of the Chagos Group, and
the Pitt and Speaker Banks at its two extreme points. These banks resemble
atolls, except in their external rim being about eight fathoms submerged,
and in being formed of dead rock, with very little living coral on it: a
portion nine miles long of the annular reef of Peros Banhos atoll is in the
same condition. These facts, as will hereafter be shown, render it very
probable that the whole group at some former period subsided seven or eight
fathoms; and that the coral perished on the outer margin of those atolls
which are now submerged, but that it continued alive, and grew up to the
surface on those which are now perfect. If these atolls did subside, and
if from the suddenness of the movement or from any other cause, those
corals which are better adapted to live at a certain depth than at the
surface, once got possession of the knolls, supplanting the former
occupants, they would exert little or no tendency to grow upwards. To
illustrate this, I may observe, that if the corals of the upper zone on the
outer edge of Keeling atoll were to perish, it is improbable that those of
the lower zone would grow to the surface, and thus become exposed to
conditions for which they do not appear to be adapted. The conjecture,
that the corals on the submerged knolls within the Chagos atolls have
analogous habits with those of the lower zone outside Keeling atoll,
receives some support from a remark by Captain Moresby, namely, that they
have a different appearance from those on the reefs in the Maldiva atolls,
which, as we have seen, all rise to the surface: he compares the kind of
difference to that of the vegetation under different climates. I have
entered at considerable length into this case, although unable to throw
much light on it, in order to show that an equal tendency to upward growth
ought not to be attributed to all coral-reefs,--to those situated at
different depths,--to those forming the ring of an atoll or those on the
knolls within a lagoon,--to those in one area and those in another. The
inference, therefore, that one reef could not grow up to the surface within
a given time, because another, not known to be covered with the same
species of corals, and not known to be placed under conditions exactly the
same, has not within the same time reached the surface, is unsound.
SECTION 4.II.--ON THE RATE OF GROWTH OF CORAL-REEFS.
The remark made at the close of the last section, naturally leads to this
division of our subject, which has not, I think, hitherto been considered
under a right point of view. Ehrenberg (Ehrenberg, as before cited, pages
39, 46, and 50.) has stated, that in the Red Sea, the corals only coat
other rocks in a layer from one to two feet in thickness, or at most to a
fathom and a half; and he disbelieves that, in any case, they form, by
their own proper growth, great masses, stratum over stratum. A nearly
similar observation has been made by MM. Quoy and Gaimard ("Annales des
Sciences Nat." tom. vi., page 28.), with respect to the thickness of some
upraised beds of coral, which they examined at Timor and some other places.
Ehrenberg (Ehrenberg, ut sup., page 42.) saw certain large massive corals
in the Red Sea, which he imagines to be of such vast antiquity, that they
might have been beheld by Pharaoh; and according to Mr. Lyell (Lyell's
"Principles of Geology," book iii., chapter xviii.) there are certain
corals at Bermuda, which are known by tradition, to have been living for
centuries. To show how slowly coral-reefs grow upwards, Captain Beechey
(Beechey's "Voyage to the Pacific," chapter viii.) has adduced the case of
the Dolphin Reef off Tahiti, which has remained at the same depth beneath
the surface, namely about two fathoms and a half, for a period of
sixty-seven years. There are reefs in the Red Sea, which certainly do not
appear (Ehrenberg, ut sup., page 43.) to have increased in dimensions during
the last half-century, and from the comparison of old charts with recent
surveys, probably not during the last two hundred years. These, and other
similar facts, have so strongly impressed many with the belief of the
extreme slowness of the growth of corals, that they have even doubted the
possibility of islands in the great oceans having been formed by their
agency. Others, again, who have not been overwhelmed by this difficulty,
have admitted that it would require thousands, and tens of thousands of
years, to form a mass, even of inconsiderable thickness; but the subject
has not, I believe, been viewed in the proper light.
That masses of considerable thickness have been formed by the growth of
coral, may be inferred with certainty from the following facts. In the
deep lagoons of Peros Banhos and of the Great Chagos Bank, there are, as
already described, small steep-sided knolls covered with living coral.
There are similar knolls in the southern Maldiva atolls, some of which, as
Captain Moresby assures me, are less than a hundred yards in diameter, and
rise to the surface from a depth of between two hundred and fifty and three
hundred feet. Considering their number, form, and position, it would be
preposterous to suppose that they are based on pinnacles of any rock, not
of coral formation; or that sediment could have been heaped up into such
small and steep isolated cones. As no kind of living coral grows above the
height of a few feet, we are compelled to suppose that these knolls have
been formed by the successive growth and death of many individuals,--first
one being broken off or killed by some accident, and then another, and one
set of species being replaced by another set with different habits, as the
reef rose nearer the surface, or as other changes supervened. The spaces
between the corals would become filled up with fragments and sand, and such
matter would probably soon be consolidated, for we learn from Lieutenant
Nelson ("Geological Transactions," volume v., page 113.), that at Bermuda a
process of this kind takes place beneath water, without the aid of
evaporation. In reefs, also, of the barrier class, we may feel sure, as I
have shown, that masses of great thickness have been formed by the growth
of the coral; in the case of Vanikoro, judging only from the depth of the
moat between the land and the reef, the wall of coral-rock must be at least
three hundred feet in vertical thickness.
It is unfortunate that the upraised coral-islands in the Pacific have not
been examined by a geologist. The cliffs of Elizabeth Island, in the Low
Archipelago, are eighty feet high, and appear, from Captain Beechey's
description, to consist of a homogeneous coral-rock. From the isolated
position of this island, we may safely infer that it is an upraised atoll,
and therefore that it has been formed by masses of coral, grown together.
Savage Island seems, from the description of the younger Forster (Forster's
"Voyage round the World with Cook," volume ii., pages 163, 167.), to have a
similar structure, and its shores are about forty feet high: some of the
Cook Islands also appear (Williams's "Narrative of Missionary Enterprise,"
page 30.) to be similarly composed. Captain Belcher, R.N., in a letter
which Captain Beaufort showed me at the admiralty, speaking of Bow atoll,
says, "I have succeeded in boring forty-five feet through coral-sand, when
the auger became jammed by the falling in of the surrounding CREAMY
matter." On one of the Maldiva atolls, Captain Moresby bored to a depth of
twenty-six feet, when his auger also broke: he has had the kindness to
give me the matter brought up; it is perfectly white, and like finely
triturated coral-rock.
In my description of Keeling atoll, I have given some facts, which show
that the reef probably has grown outwards; and I have found, just within
the outer margin, the great mounds of Porites and of Millepora, with their
summits lately killed, and their sides subsequently thickened by the growth
of the coral: a layer, also, of Nullipora had already coated the dead
surface. As the external slope of the reef is the same round the whole of
this atoll, and round many other atolls, the angle of inclination must
result from an adaption between the growing powers of the coral, and the
force of the breakers, and their action on the loose sediment. The reef,
therefore, could not increase outwards, without a nearly equal addition to
every part of the slope, so that the original inclination might be
preserved, and this would require a large amount of sediment, all derived
from the wear of corals and shells, to be added to the lower part.
Moreover, at Keeling atoll, and probably in many other cases, the different
kinds of corals would have to encroach on each other; thus the Nulliporae
cannot increase outwards without encroaching on the Porites and Millepora
complanata, as is now taking place; nor these latter without encroaching on
the strongly branched Madreporet, the Millepora alcicornis, and some
Astraeas; nor these again without a foundation being formed for them within
the requisite depth, by the accumulation of sediment. How slow, then, must
be the ordinary lateral or outward growth of such reefs. But off Christmas
atoll, where the sea is much more shallow than is usual, we have good
reason to believe that, within a period not very remote, the reef has
increased considerably in width. The land has the extraordinary breadth of
three miles; it consists of parallel ridges of shells and broken corals,
which furnish "an incontestable proof," as observed by Cook (Cook's "Third
Voyage," book III., chapter x.), "that the island has been produced by
accessions from the sea, and is in a state of increase." The land is
fronted by a coral-reef, and from the manner in which islets are known to
be formed, we may feel confident that the reef was not three miles wide,
when the first, or most backward ridge, was thrown up; and, therefore, we
must conclude that the reef has grown outwards during the accumulation of
the successive ridges. Here then, a wall of coral-rock of very
considerable breadth has been formed by the outward growth of the living
margin, within a period during which ridges of shells and corals, lying on
the bare surface, have not decayed. There can be little doubt, from the
account given by Captain Beechey, that Matilda atoll, in the Low
Archipelago, has been converted in the space of thirty-four years, from
being, as described by the crew of a wrecked whaling vessel, a "reef of
rocks" into a lagoon-island, fourteen miles in length, with "one of its
sides covered nearly the whole way with high trees." (Beechey's "Voyage to
the Pacific," chapter vii. and viii.) The islets, also, on Keeling atoll,
it has been shown, have increased in length, and since the construction of
an old chart, several of them have become united into one long islet; but
in this case, and in that of Matilda atoll, we have no proof, and can only
infer as probable, that the reef, that is the foundation of the islets, has
increased as well as the islets themselves.
After these considerations, I attach little importance, as indicating the
ordinary and still less the possible rate of OUTWARD growth of coral-reefs,
to the fact that certain reefs in the Red Sea have not increased during a
long interval of time; or to other such cases, as that of Ouluthy atoll in
the Caroline group, where every islet, described a thousand years before by
Cantova was found in the same state by Lutke (F. Lutke's "Voyage autour du
Monde." In the group Elato, however, it appears that what is now the islet
Falipi, is called in Cantova's Chart, the Banc de Falipi. It is not stated
whether this has been caused by the growth of coral, or by the accumulation
of sand.),--without it could be shown that, in these cases, the conditions
were favourable to the vigorous and unopposed growth of the corals living
in the different zones of depth, and that a proper basis for the extent of
the reef was present. The former conditions must depend on many
contingencies, and in the deep oceans where coral formations most abound, a
basis within the requisite depth can rarely be present.
Nor do I attach any importance to the fact of certain submerged reefs, as
those off Tahiti, or those within Diego Garcia not now being nearer the
surface than they were many years ago, as an indication of the rate under
favourable circumstances of the UPWARD growth of reefs; after it has been
shown, that all the reefs have grown to the surface in some of the Chagos
atolls, but that in neighbouring atolls which appear to be of equal
antiquity and to be exposed to the same external conditions, every reef
remains submerged; for we are almost driven to attribute this to a
difference, not in the rate of growth, but in the habits of the corals in
the two cases.
In an old-standing reef, the corals, which are so different in kind on
different parts of it, are probably all adapted to the stations they
occupy, and hold their places, like other organic beings, by a struggle one
with another, and with external nature; hence we may infer that their
growth would generally be slow, except under peculiarly favourable
circumstances. Almost the only natural condition, allowing a quick upward
growth of the whole surface of a reef, would be a slow subsidence of the
area in which it stood; if, for instance, Keeling atoll were to subside two
or three feet, can we doubt that the projecting margin of live coral, about
half an inch in thickness, which surrounds the dead upper surfaces of the
mounds of Porites, would in this case form a concentric layer over them,
and the reef thus increase upwards, instead of, as at present, outwards?
The Nulliporae are now encroaching on the Porites and Millepora, but in
this case might we not confidently expect that the latter would, in their
turn, encroach on the Nulliporae? After a subsidence of this kind, the sea
would gain on the islets, and the great fields of dead but upright corals
in the lagoon, would be covered by a sheet of clear water; and might we not
then expect that these reefs would rise to the surface, as they anciently
did when the lagoon was less confined by islets, and as they did within a
period of ten years in the schooner-channel, cut by the inhabitants? In
one of the Maldiva atolls, a reef, which within a very few years existed as
an islet bearing cocoa-nut trees, was found by Lieutenant Prentice
"ENTIRELY COVERED WITH LIVE CORAL AND MADREPORE." The natives believe that
the islet was washed away by a change in the currents, but if, instead of
this, it had quietly subsided, surely every part of the island which
offered a solid foundation, would in a like manner have become coated with
living coral.
Through steps such as these, any thickness of rock, composed of a singular
intermixture of various kinds of corals, shells, and calcareous sediment,
might be formed; but without subsidence, the thickness would necessarily be
determined by the depth at which the reef-building polypifers can exist.
If it be asked, at what rate in years I suppose a reef of coral favourably
circumstanced could grow up from a given depth; I should answer, that we
have no precise evidence on this point, and comparatively little concern
with it. We see, in innumerable points over wide areas, that the rate has
been sufficient, either to bring up the reefs from various depths to the
surface, or, as is more probable, to keep them at the surface, during
progressive subsidences; and this is a much more important standard of
comparison than any cycle of years.
It may, however, be inferred from the following facts, that the rate in
years under favourable circumstances would be very far from slow. Dr.
Allan, of Forres, has, in his MS. Thesis deposited in the library of the
Edinburgh University (extracts from which I owe to the kindness of Dr.
Malcolmson), the following account of some experiments, which he tried
during his travels in the years 1830 to 1832 on the east coast of
Madagascar. "To ascertain the rise and progress of the coral-family, and
fix the number of species met with at Foul Point (latitude 17 deg 40')
twenty species of coral were taken off the reef and planted apart on a
sand-bank THREE FEET DEEP AT LOW WATER. Each portion weighed ten pounds,
and was kept in its place by stakes. Similar quantities were placed in a
clump and secured as the rest. This was done in December 1830. In July
following, each detached mass was nearly level with the sea at low water,
quite immovable, and several feet long, stretching as the parent reef, with
the coast current from north to south. The masses accumulated in a clump
were found equally increased, but some of the species in such unequal
ratios, as to be growing over each other." The loss of Dr. Allan's
magnificent collection by shipwreck, unfortunately prevents its being known
to what genera these corals belonged; but from the numbers experimented on,
it is certain that all the more conspicuous kinds must have been included.
Dr. Allan informs me, in a letter, that he believes it was a Madrepora,
which grew most vigorously. One may be permitted to suspect that the level
of the sea might possibly have been somewhat different at the two stated
periods; nevertheless, it is quite evident that the growth of the ten-pound
masses, during the six or seven months, at the end of which they were found
immovably fixed (It is stated by De la Beche ("Geological Manual," page
143), on the authority of Mr. Lloyd, who surveyed the Isthmus of Panama,
that some specimens of Polypifers, placed by him in a sheltered pool of
water, were found in the course of a few days firmly fixed by the secretion
of a stony matter, to the bottom) and several feet in length, must have
been very great. The fact of the different kinds of coral, when placed in
one clump, having increased in extremely unequal ratios, is very
interesting, as it shows the manner in which a reef, supporting many
species of coral, would probably be affected by a change in the external
conditions favouring one kind more than another. The growth of the masses
of coral in N. and S. lines parallel to the prevailing currents, whether
due to the drifting of sediment or to the simple movement of the water, is,
also, a very interesting circumstance.
A fact, communicated to me by Lieutenant Wellstead, I.N., in some degree
corroborates the result of Dr. Allan's experiments: it is, that in the
Persian Gulf a ship had her copper bottom encrusted in the course of twenty
months with a layer of coral, TWO FEET in thickness, which it required
great force to remove, when the vessel was docked: it was not ascertained
to what order this coral belonged. The case of the schooner-channel choked
up with coral in an interval of less than ten years, in the lagoon of
Keeling atoll, should be here borne in mind. We may also infer, from the
trouble which the inhabitants of the Maldiva atolls take to root out, as
they express it, the coral-knolls from their harbours, that their growth
can hardly be very slow. (Mr. Stutchbury ("West of England Journal", No.
I., page 50.) has described a specimen of Agaricia, "weighing 2 lbs. 9 oz.,
which surrounds a species of oyster, whose age could not be more than two
years, and yet is completely enveloped by this dense coral." I presume
that the oyster was living when the specimen was procured; otherwise the
fact tells nothing. Mr. Stutchbury also mentions an anchor, which had
become entirely encrusted with coral in fifty years; other cases, however,
are recorded of anchors which have long remained amidst coral-reefs without
having become coated. The anchor of the "Beagle", in 1832, after having
been down exactly one month at Rio de Janeiro, was so thickly coated by two
species of Tubularia, that large spaces of the iron were entirely
concealed; the tufts of this horny zoophyte were between two and three
inches in length. It has been attempted to compute, but I believe
erroneously, the rate of growth of a reef, from the fact mentioned by
Captain Beechey, of the Chama gigas being embedded in coral-rock. But it
should be remembered, that some species of this genus invariably live, both
whilst young and old, in cavities, which the animal has the power of
enlarging with its growth. I saw many of these shells thus embedded in the
outer "flat" of Keeling atoll, which is composed of dead rock; and
therefore the cavities in this case had no relation whatever with the
growth of coral. M. Lesson, also, speaking of this shell (Partie Zoolog.
"Voyage de la 'Coquille'"), has remarked, "que constamment ses valves
etaient engages completement dans la masse des Madrepores.")
From the facts given in this section, it may be concluded, first, that
considerable thicknesses of rock have certainly been formed within the
present geological area by the growth of coral and the accumulation of its
detritus; and, secondly, that the increase of individual corals and of
reefs, both outwards or horizontally and upwards or vertically, under the
peculiar conditions favourable to such increase, is not slow, when referred
either to the standard of the average oscillations of level in the earth's
crust, or to the more precise but less important one of a cycle of years.
SECTION 4.III.--ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.
I have already described in detail, which might have appeared trivial, the
nature of the bottom of the sea immediately surrounding Keeling atoll; and
I will now describe with almost equal care the soundings off the
fringing-reefs of Mauritius. I have preferred this arrangement, for the sake
of grouping together facts of a similar nature. I sounded with the wide
bell-shaped lead which Captain Fitzroy used at Keeling Island, but my
examination of the bottom was confined to a few miles of coast (between
Port Louis and Tomb Bay) on the leeward side of the island. The edge of
the reef is formed of great shapeless masses of branching Madrepores, which
chiefly consist of two species,--apparently M. corymbosa and pocillifera,--
mingled with a few other kinds of coral. These masses are separated from
each other by the most irregular gullies and cavities, into which the lead
sinks many feet. Outside this irregular border of Madrepores, the water
deepens gradually to twenty fathoms, which depth generally is found at the
distance of from half to three-quarters of a mile from the reef. A little
further out the depth is thirty fathoms, and thence the bank slopes rapidly
into the depths of the ocean. This inclination is very gentle compared
with that outside Keeling and other atolls, but compared with most coasts
it is steep. The water was so clear outside the reef, that I could
distinguish every object forming the rugged bottom. In this part, and to a
depth of eight fathoms, I sounded repeatedly, and at each cast pounded the
bottom with the broad lead, nevertheless the arming invariably came up
perfectly clean, but deeply indented. From eight to fifteen fathoms a
little calcareous sand was occasionally brought up, but more frequently the
arming was simply indented. In all this space the two Madrepores above
mentioned, and two species of Astraea, with rather large stars, seemed the
commonest kinds (Since the preceding pages were printed off, I have
received from Mr. Lyell a very interesting pamphlet, entitled "Remarks upon
Coral Formations," etc., by J. Couthouy, Boston, United States, 1842.
There is a statement (page 6), on the authority of the Rev. J. Williams,
corroborating the remarks made by Ehrenberg and Lyell (page 71 of this
volume), on the antiquity of certain individual corals in the Red Sea and
at Bermuda; namely, that at Upolu, one of the Navigator Islands,
"particular clumps of coral are known to the fishermen by name, derived
from either some particular configuration or tradition attached to them,
and handed down from time immemorial." With respect to the thickness of
masses of coral-rock, it clearly appears, from the descriptions given by
Mr. Couthouy (pages 34, 58) that Mangaia and Aurora Islands are upraised
atolls, composed of coral rock: the level summit of the former is about
three hundred feet, and that of Aurora Island is two hundred feet above the
sea-level.); and it must be noticed that twice at the depth of fifteen
fathoms, the arming was marked with a clean impression of an Astraea.
Besides these lithophytes, some fragments of the Millepora alcicornis,
which occurs in the same relative position at Keeling Island, were brought
up; and in the deeper parts there were large beds of a Seriatopora,
different from S. subulata, but closely allied to it. On the beach within
the reef, the rolled fragments consisted chiefly of the corals just
mentioned, and of a massive Porites, like that at Keeling atoll, of a
Meandrina, Pocillopora verrucosa, and of numerous fragments of Nullipora.
From fifteen to twenty fathoms the bottom was, with few exceptions, either
formed of sand, or thickly covered with Seriatopora: this delicate coral
seems to form at these depths extensive beds unmingled with any other kind.
At twenty fathoms, one sounding brought up a fragment of Madrepora
apparently M. pocillifera, and I believe it is the same species (for I
neglected to bring specimens from both stations) which mainly forms the
upper margin of the reef; if so, it grows in depths varying from 0 to 20
fathoms. Between 20 and 23 fathoms I obtained several soundings, and they
all showed a sandy bottom, with one exception at 30 fathoms, when the
arming came up scooped out, as if by the margin of a large Caryophyllia.
Beyond 33 fathoms I sounded only once; and from 86 fathoms, at the distance
of one mile and a third from the edge of the reef, the arming brought up
calcareous sand with a pebble of volcanic rock. The circumstance of the
arming having invariably come up quite clean, when sounding within a
certain number of fathoms off the reefs of Mauritius and Keeling atoll
(eight fathoms in the former case, and twelve in the latter) and of its
having always come up (with one exception) smoothed and covered with sand,
when the depth exceeded twenty fathoms, probably indicates a criterion, by
which the limits of the vigorous growth of coral might in all cases be
readily ascertained. I do not, however, suppose that if a vast number of
soundings were obtained round these islands, the limit above assigned would
be found never to vary, but I conceive the facts are sufficient to show,
that the exceptions would be few. The circumstance of a GRADUAL change, in
the two cases, from a field of clean coral to a smooth sandy bottom, is far
more important in indicating the depth at which the larger kinds of coral
flourish than almost any number of separate observations on the depth, at
which certain species have been dredged up. For we can understand the
gradation, only as a prolonged struggle against unfavourable conditions.
If a person were to find the soil clothed with turf on the banks of a
stream of water, but on going to some distance on one side of it, he
observed the blades of grass growing thinner and thinner, with intervening
patches of sand, until he entered a desert of sand, he would safely
conclude, especially if changes of the same kind were noticed in other
places, that the presence of the water was absolutely necessary to the
formation of a thick bed of turf: so may we conclude, with the same
feeling of certainty, that thick beds of coral are formed only at small
depths beneath the surface of the sea.
I have endeavoured to collect every fact, which might either invalidate or
corroborate this conclusion. Captain Moresby, whose opportunities for
observation during his survey of the Maldiva and Chagos Archipelagoes have
been unrivalled, informs me, that the upper part or zone of the steep-sided
reefs, on the inner and outer coasts of the atolls in both groups,
invariably consists of coral, and the lower parts of sand. At seven or
eight fathoms depth, the bottom is formed, as could be seen through the
clear water, of great living masses of coral, which at about ten fathoms
generally stand some way apart from each other, with patches of white sand
between them, and at a little greater depth these patches become united
into a smooth steep slope, without any coral. Captain Moresby, also,
informs me in support of his statement, that he found only decayed coral on
the Padua Bank (northern part of the Laccadive group) which has an average
depth between twenty-five and thirty-five fathoms, but that on some other
banks in the same group with only ten or twelve fathoms water on them (for
instance, the Tillacapeni bank), the coral was living.
With regard to the coral-reefs in the Red Sea, Ehrenberg has the following
passage:--"The living corals do not descend there into great depths. On
the edges of islets and near reefs, where the depth was small, very many
lived; but we found no more even at six fathoms. The pearl-fishers at
Yemen and Massaua asserted that there was no coral near the pearl-banks at
nine fathoms depth, but only sand. We were not able to institute any more
special researches." (Ehrenberg, "Uber die Natur," etc., page 50.) I am,
however, assured both by Captain Moresby and Lieutenant Wellstead, that in
the more northern parts of the Red Sea, there are extensive beds of living
coral at a depth of twenty-five fathoms, in which the anchors of their
vessels were frequently entangled. Captain Moresby attributes the less
depth, at which the corals are able to live in the places mentioned by
Ehrenberg, to the greater quantity of sediment there; and the situations,
where they were flourishing at the depth of twenty-five fathoms, were
protected, and the water was extraordinarily limpid. On the leeward side
of Mauritius where I found the coral growing at a somewhat greater depth
than at Keeling atoll, the sea, owing apparently to its tranquil state, was
likewise very clear. Within the lagoons of some of the Marshall atolls,
where the water can be but little agitated, there are, according to
Kotzebue, living beds of coral in twenty-five fathoms. From these facts,
and considering the manner in which the beds of clean coral off Mauritius,
Keeling Island, the Maldiva and Chagos atolls, graduated into a sandy
slope, it appears very probable that the depth, at which reef-building
polypifers can exist, is partly determined by the extent of inclined
surface, which the currents of the sea and the recoiling waves have the
power to keep free from sediment.
MM. Quoy and Gaimard ("Annales des Sci. Nat." tom. vi.) believe that the
growth of coral is confined within very limited depths; and they state that
they never found any fragment of an Astraea (the genus they consider most
efficient in forming reefs) at a depth above twenty-five or thirty feet.
But we have seen that in several places the bottom of the sea is paved with
massive corals at more than twice this depth; and at fifteen fathoms (or
twice this depth) off the reefs of Mauritius, the arming was marked with
the distinct impression of a living Astraea. Millepora alcicornis lives in
from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0 to 20
fathoms. Captain Moresby has given me a specimen of Sideropora scabra
(Porites of Lamarck) brought up alive from 17 fathoms. Mr. Couthouy
("Remarks on Coral Formations," page 12.) states that he has dredged up on
the Bahama banks considerable masses of Meandrina from 16 fathoms, and he
has seen this coral growing in 20 fathoms. A Caryophyllia, half an inch in
diameter, was dredged up alive from 80 fathoms off Juan Fernandez (latitude
33 deg S.) by Captain P.P. King (I am indebted to Mr. Stokes for having
kindly communicated this fact to me, together with much other valuable
information.): this is the most remarkable fact with which I am
acquainted, showing the depth at which a genus of corals often found on
reefs, can exist.
We ought, however, to feel less surprise at this fact, as Caryophyllia
alone of the lamelliform genera, ranges far beyond the tropics; it is found
in Zetland (Fleming's "British Animals," genus Caryophyllia.) in Latitude
60 deg N. in deep water, and I procured a small species from Tierra del
Fuego in Latitude 53 deg S. Captain Beechey informs me, that branches of
pink and yellow coral were frequently brought up from between twenty and
twenty-five fathoms off the Low atolls; and Lieutenant Stokes, writing to
me from the N.W. coast of Australia, says that a strongly branched coral
was procured there from thirty fathoms; unfortunately it is not known to
what genera these corals belong.
(I will record in the form of a note all the facts that I have been able to
collect on the depths, both within and without the tropics, at which those
corals and corallines can live, which there is no reason to suppose ever
materially aid in the construction of a reef.
(In the following list the name of the Zoophyte is followed by the depth in
fathoms, the country and degrees S. latitude, and the authority. Where no
authority is given, the observation is Darwin's own.)
SERTULARIA, 40, Cape Horn 66.
CELLARIA, 40, Cape Horn 66.
CELLARIA, A minute scarlet encrusting species, found living, 190, Keeling
Atoll, 12.
CELLARIA, An allied, small stony sub-generic form, 48, St Cruz Riv. 50.
A coral allied to VINCULARIA, with eight rows of cells, 40, Cape Horn.
TUBULIPORA, near to T. patima, 40, Cape Horn.
TUBULIPORA, near to T. patima, 94, East Chiloe 43.
CELLEPORA, several species, and allied sub-generic forms, 40, Cape Horn.
CELLEPORA, several species, and allied sub-generic forms, 40 and 57, Chonos
Archipelago 45.
CELLEPORA, several species, and allied sub-generic forms, 48, St Cruz 50.
ESCHARA, 30, Tierra del Fuego 53.
ESCHARA, 48, St Cruz R. 50.
RETEPORA, 40, Cape Horn.
RETEPORA, 100, Cape of Good Hope 34, Quoy and Gaimard, "Ann. Scien. Nat."
tome vi., page 284.
MILLEPORA, a strong coral with cylindrical branches, of a pink colour,
about two inches high, resembling in the form of its orifices M. aspera of
Lamarck, 94 and 30, E. Chiloe 43, Tierra del Fuego 53.
CORALIUM, 120, Barbary 33 N., Peyssonel in paper read to Royal Society May
1752.
ANTIPATHES, 16, Chonos 45.
GORGONIA (or an allied form), 160, Abrolhos on the coast of Brazil 18,
Captain Beechey informed me of this fact in a letter.
Ellis ("Nat. Hist. of Coralline," page 96) states that Ombellularia was
procured in latitude 79 deg N. STICKING to a LINE from the depth of 236
fathoms; hence this coral either must have been floating loose, or was
entangled in stray line at the bottom. Off Keeling atoll a compound
Ascidia (Sigillina) was brought up from 39 fathoms, and a piece of sponge,
apparently living, from 70, and a fragment of Nullipora also apparently
living from 92 fathoms. At a greater depth than 90 fathoms off this coral
island, the bottom was thickly strewed with joints of Halimeda and small
fragments of other Nulliporae, but all dead. Captain B. Allen, R.N.,
informs me that in the survey of the West Indies it was noticed that
between the depth of 10 and 200 fathoms, the sounding lead very generally
came up coated with the dead joints of a Halimeda, of which he showed me
specimens. Off Pernambuco, in Brazil, in about twelve fathoms, the bottom
was covered with fragments dead and alive of a dull red Nullipora, and I
infer from Roussin's chart, that a bottom of this kind extends over a wide
area. On the beach, within the coral-reefs of Mauritius, vast quantities
of fragments of Nulliporae were piled up. From these facts it appears,
that these simply organized bodies are amongst the most abundant
productions of the sea.)
Although the limit of depth, at which each particular kind of coral ceases
to exist, is far from being accurately known; yet when we bear in mind the
manner in which the clumps of coral gradually became infrequent at about
the same depth, and wholly disappeared at a greater depth than twenty
fathoms, on the slope round Keeling atoll, on the leeward side of the
Mauritius, and at rather less depth, both without and within the atolls of
the Maldiva and Chagos Archipelagoes; and when we know that the reefs round
these islands do not differ from other coral formations in their form and
structure, we may, I think, conclude that in ordinary cases, reef-building
polypifers do not flourish at greater depths than between twenty and thirty
fathoms.
It has been argued ("Journal of the Royal Geographical Society," 1831, page
218.) that reefs may possibly rise from very great depths through the means
of small corals, first making a platform for the growth of the stronger
kinds. This, however, is an arbitrary supposition: it is not always
remembered, that in such cases there is an antagonist power in action,
namely, the decay of organic bodies, when not protected by a covering of
sediment, or by their own rapid growth. We have, moreover, no right to
calculate on unlimited time for the accumulation of small organic bodies
into great masses. Every fact in geology proclaims that neither the land,
nor the bed of the sea retain for indefinite periods the same level. As
well might it be imagined that the British Seas would in time become choked
up with beds of oysters, or that the numerous small corallines off the
inhospitable shores of Tierra del Fuego would in time form a solid and
extensive coral-reef.
CHAPTER V.--THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEFS.
The atolls of the larger archipelagoes are not formed on submerged craters,
or on banks of sediment.--Immense areas interspersed with atolls.--Their
subsidence.--The effects of storms and earthquakes on atolls.--Recent
changes in their state.--The origin of barrier-reefs and of atolls.--Their
relative forms.--The step-formed ledges and walls round the shores of some
lagoons.--The ring-formed reefs of the Maldiva atolls.--The submerged
condition of parts or of the whole of some annular reefs.--The disseverment
of large atolls.--The union of atolls by linear reefs.--The Great Chagos
Bank.--Objections from the area and amount of subsidence required by the
theory, considered.--The probable composition of the lower parts of atolls.
The naturalists who have visited the Pacific, seem to have had their
attention riveted by the lagoon-islands, or atolls,--those singular rings
of coral-land which rise abruptly out of the unfathomable ocean--and have
passed over, almost unnoticed, the scarcely less wonderful encircling
barrier-reefs. The theory most generally received on the formation of
atolls, is that they are based on submarine craters; but where can we find
a crater of the shape of Bow atoll, which is five times as long as it is
broad (Plate I., Figure 4); or like that of Menchikoff Island (Plate II.,
Figure 3.), with its three loops, together sixty miles in length; or like
Rimsky Korsacoff, narrow, crooked, and fifty-four miles long; or like the
northern Maldiva atolls, made up of numerous ring-formed reefs, placed on
the margin of a disc,--one of which discs is eighty-eight miles in length,
and only from ten to twenty in breadth? It is, also, not a little
improbable, that there should have existed as many craters of immense size
crowded together beneath the sea, as there are now in some parts atolls.
But this theory lies under a greater difficulty, as will be evident, when
we consider on what foundations the atolls of the larger archipelagoes
rest: nevertheless, if the rim of a crater afforded a basis at the proper
depth, I am far from denying that a reef like a perfectly characterised
atoll might not be formed; some such, perhaps, now exist; but I cannot
believe in the possibility of the greater number having thus originated.
An earlier and better theory was proposed by Chamisso (Kotzebue's "First
Voyage," volume iii., page 331.); he supposes that as the more massive
kinds of corals prefer the surf, the outer portions, in a reef rising from
a submarine basis, would first reach the surface and consequently form a
ring. But on this view it must be assumed, that in every case the basis
consists of a flat bank; for if it were conically formed, like a
mountainous mass, we can see no reason why the coral should spring up from
the flanks, instead of from the central and highest parts: considering the
number of the atolls in the Pacific and Indian Oceans, this assumption is
very improbable. As the lagoons of atolls are sometimes even more than
forty fathoms deep, it must, also, be assumed on this view, that at a depth
at which the waves do not break, the coral grows more vigorously on the
edges of a bank than on its central part; and this is an assumption without
any evidence in support of it. I remarked, in the third chapter, that a
reef, growing on a detached bank, would tend to assume an atoll-like
structure; if, therefore, corals were to grow up from a bank, with a level
surface some fathoms submerged, having steep sides and being situated in a
deep sea, a reef not to be distinguished from an atoll, might be formed: I
believe some such exist in the West Indies. But a difficulty of the same
kind with that affecting the crater theory, runners, as we shall presently
see, this view inapplicable to the greater number of atolls.
No theory worthy of notice has been advanced to account for those
barrier-reefs, which encircle islands of moderate dimensions. The great
reef which fronts the coast of Australia has been supposed, but without any
special facts, to rest on the edge of a submarine precipice, extending
parallel to the shore. The origin of the third class or of fringing-reefs
presents, I believe, scarcely any difficulty, and is simply consequent on
the polypifers not growing up from great depths, and their not flourishing
close to gently shelving beaches where the water is often turbid.
What cause, then, has given to atolls and barrier-reefs their
characteristic forms? Let us see whether an important deduction will not
follow from the consideration of these two circumstances, first, the
reef-building corals flourishing only at limited depths; and secondly, the
vastness of the areas interspersed with coral-reefs and coral-islets, none
of which rise to a greater height above the level of the sea, than that
attained by matter thrown up by the waves and winds. I do not make this
latter statement vaguely; I have carefully sought for descriptions of every
island in the intertropical seas; and my task has been in some degree
abridged by a map of the Pacific, corrected in 1834 by MM. D'Urville and
Lottin, in which the low islands are distinguished from the high ones (even
from those much less than a hundred feet in height) by being written
without a capital letter; I have detected a few errors in this map,
respecting the height of some of the islands, which will be noticed in the
Appendix, where I treat of coral formations in geographical order. To the
Appendix, also, I must refer for a more particular account of the data on
which the statements on the next page are grounded. I have ascertained,
and chiefly from the writings of Cook, Kotzebue, Bellinghausen, Duperrey,
Beechey, and Lutke, regarding the Pacific; and from Moresby (See also
Captain Owen's and Lieutenant Wood's papers in the "Geographical Journal",
on the Maldiva and Laccadive Archipelagoes. These officers particularly
refer to the lowness of the islets; but I chiefly ground my assertion
respecting these two groups, and the Chagos group, from information
communicated to me by Captain Moresby.) with respect to the Indian Ocean,
that in the following cases the term "low island" strictly means land of
the height commonly attained by matter thrown up by the winds and the waves
of an open sea. If we draw a line (the plan I have always adopted) joining
the external atolls of that part of the Low Archipelago in which the
islands are numerous, the figure will be a pointed ellipse (reaching from
Hood to Lazaref Island), of which the longer axis is 840 geographical
miles, and the shorter 420 miles; in this space (I find from Mr. Couthouy's
pamphlet (page 58) that Aurora Island is about two hundred feet in height;
it consists of coral-rock, and seems to have been formed by the elevation
of an atoll. It lies north-east of Tahiti, close without the line bounding
the space coloured dark blue in the map appended to this volume. Honden
Island, which is situated in the extreme north-west part of the Low
Archipelago, according to measurements made on board the "Beagle", whilst
sailing by, is 114 feet from the SUMMIT OF THE TREES to the water's edge.
This island appeared to resemble the other atolls of the group.) none of
the innumerable islets united into great rings rise above the stated level.
The Gilbert group is very narrow, and 300 miles in length. In a prolonged
line from this group, at the distance of 240 miles, is the Marshall
Archipelago, the figure of which is an irregular square, one end being
broader than the other; its length is 520 miles, with an average width of
240; these two groups together are 1,040 miles in length, and all their
islets are low. Between the southern end of the Gilbert and the northern
end of Low Archipelago, the ocean is thinly strewed with islands, all of
which, as far as I have been able to ascertain, are low; so that from
nearly the southern end of the Low Archipelago, to the northern end of the
Marshall Archipelago, there is a narrow band of ocean, more than 4,000
miles in length, containing a great number of islands, all of which are
low. In the western part of the Caroline Archipelago, there is a space of
480 miles in length, and about 100 broad, thinly interspersed with low
islands. Lastly, in the Indian Ocean, the archipelago of the Maldivas is
470 miles in length, and 60 in breadth; that of the Laccadives is 150 by
100 miles; as there is a low island between these two groups, they may be
considered as one group of 1,000 miles in length. To this may be added the
Chagos group of low islands, situated 280 miles distant, in a line
prolonged from the southern extremity of the Maldivas. This group,
including the submerged banks, is 170 miles in length and 80 in breadth.
So striking is the uniformity in direction of these three archipelagoes,
all the islands of which are low, that Captain Moresby, in one of his
papers, speaks of them as parts of one great chain, nearly 1,500 miles
long. I am, then, fully justified in repeating, that enormous spaces, both
in the Pacific and Indian Oceans, are interspersed with islands, of which
not one rises above that height, to which the waves and winds in an open
sea can heap up matter.
On what foundations, then, have these reefs and islets of coral been
constructed? A foundation must originally have been present beneath each
atoll at that limited depth, which is indispensable for the first growth of
the reef-building polypifers. A conjecture will perhaps be hazarded, that
the requisite bases might have been afforded by the accumulation of great
banks of sediment, which owing to the action of superficial currents (aided
possibly by the undulatory movement of the sea) did not quite reach the
surface,--as actually appears to have been the case in some parts of the
West Indian Sea. But in the form and disposition of the groups of atolls,
there is nothing to countenance this notion; and the assumption without any
proof, that a number of immense piles of sediment have been heaped on the
floor of the great Pacific and Indian Oceans, in their central parts far
remote from land, and where the dark blue colour of the limpid water
bespeaks its purity, cannot for one moment be admitted.
The many widely-scattered atolls must, therefore, rest on rocky bases. But
we cannot believe that the broad summit of a mountain lies buried at the
depth of a few fathoms beneath every atoll, and nevertheless throughout the
immense areas above-named, with not one point of rock projecting above the
level of the sea; for we may judge with some accuracy of mountains beneath
the sea, by those on the land; and where can we find a single chain several
hundred miles in length and of considerable breadth, much less several such
chains, with their many broad summits attaining the same height, within
from 120 to 180 feet? If the data be thought insufficient, on which I have
grounded my belief, respecting the depth at which the reef-building
polypifers can exist, and it be assumed that they can flourish at a depth
of even one hundred fathoms, yet the weight of the above argument is but
little diminished, for it is almost equally improbable, that as many
submarine mountains, as there are low islands in the several great and
widely separated areas above specified, should all rise within six hundred
feet of the surface of the sea and not one above it, as that they should be
of the same height within the smaller limit of one or two hundred feet. So
highly improbable is this supposition, that we are compelled to believe,
that the bases of the many atolls did never at any one period all lie
submerged within the depth of a few fathoms beneath the surface, but that
they were brought into the requisite position or level, some at one period
and some at another, through movements in the earth's crust. But this
could not have been effected by elevation, for the belief that points so
numerous and so widely separated were successively uplifted to a certain
level, but that not one point was raised above that level, is quite as
improbable as the former supposition, and indeed differs little from it.
It will probably occur to those who have read Ehrenberg's account of the
Reefs of the Red Sea, that many points in these great areas may have been
elevated, but that as soon as raised, the protuberant parts were cut off by
the destroying action of the waves: a moment's reflection, however, on the
basin-like form of the atolls, will show that this is impossible; for the
upheaval and subsequent abrasion of an island would leave a flat disc,
which might become coated with coral, but not a deeply concave surface;
moreover, we should expect to see, in some parts at least, the rock of the
foundation brought to the surface. If, then, the foundations of the many
atolls were not uplifted into the requisite position, they must of
necessity have subsided into it; and this at once solves every difficulty
(The additional difficulty on the crater hypothesis before alluded to, will
now be evident; for on this view the volcanic action must be supposed to
have formed within the areas specified a vast number of craters, all rising
within a few fathoms of the surface, and not one above it. The supposition
that the craters were at different times upraised above the surface, and
were there abraded by the surf and subsequently coated by corals, is
subject to nearly the same objections with those given above in this
paragraph; but I consider it superfluous to detail all the arguments
opposed to such a notion. Chamisso's theory, from assuming the existence
of so many banks, all lying at the proper depth beneath the water, is also
vitally defective. The same observation applies to an hypothesis of
Lieutenant Nelson's ("Geolog. Trans." volume v., page 122), who supposes
that the ring-formed structure is caused by a greater number of germs of
corals becoming attached to the declivity, than to the central plateau of a
submarine bank: it likewise applies to the notion formerly entertained
(Forster's "Observ." page 151), that lagoon-islands owe their peculiar form
to the instinctive tendencies of the polypifers. According to this latter
view, the corals on the outer margin of the reef instinctively expose
themselves to the surf in order to afford protection to corals living in
the lagoon, which belong to other genera, and to other families!), for we
may safely infer, from the facts given in the last chapter, that during a
gradual subsidence the corals would be favourably circumstanced for
building up their solid frame works and reaching the surface, as island
after island slowly disappeared. Thus areas of immense extent in the
central and most profound parts of the great oceans, might become
interspersed with coral-islets, none of which would rise to a greater
height than that attained by detritus heaped up by the sea, and
nevertheless they might all have been formed by corals, which absolutely
required for their growth a solid foundation within a few fathoms of the
surface.
It would be out of place here to do more than allude to the many facts,
showing that the supposition of a gradual subsidence over large areas is by
no means improbable. We have the clearest proof that a movement of this
kind is possible, in the upright trees buried under the strata many
thousand feet in thickness; we have also every reason for believing that
there are now large areas gradually sinking, in the same manner as others
are rising. And when we consider how many parts of the surface of the
globe have been elevated within recent geological periods, we must admit
that there have been subsidences on a corresponding scale, for otherwise
the whole globe would have swollen. It is very remarkable that Mr. Lyell
("Principles of Geology," sixth edition, volume iii., page 386.), even in
the first edition of his "Principles of Geology," inferred that the amount
of subsidence in the Pacific must have exceeded that of elevation, from the
area of land being very small relatively to the agents there tending to
form it, namely, the growth of coral and volcanic action. But it will be
asked, are there any direct proofs of a subsiding movement in those areas,
in which subsidence will explain a phenomenon otherwise inexplicable?
This, however, can hardly be expected, for it must ever be most difficult,
excepting in countries long civilised, to detect a movement, the tendency
of which is to conceal the part affected. In barbarous and semi-civilised
nations how long might not a slow movement, even of elevation such as that
now affecting Scandinavia, have escaped attention!
Mr. Williams (Williams's "Narrative of Missionary Enterprise," page 31.)
insists strongly that the traditions of the natives, which he has taken
much pains in collecting, do not indicate the appearance of any new
islands: but on the theory of a gradual subsidence, all that would be
apparent would be, the water sometimes encroaching slowly on the land, and
the land again recovering by the accumulation of detritus its former
extent, and perhaps sometimes the conversion of an atoll with coral islets
on it, into a bare or into a sunken annular reef. Such changes would
naturally take place at the periods when the sea rose above its usual
limits, during a gale of more than ordinary strength; and the effects of
the two causes would be hardly distinguishable. In Kotzebue's "Voyage"
there are accounts of islands, both in the Caroline and Marshall
Archipelagoes, which have been partly washed away during hurricanes; and
Kadu, the native who was on board one of the Russian vessels, said "he saw
the sea at Radack rise to the feet of the cocoa-nut trees; but it was
conjured in time." (Kotzebue's "First Voyage," volume iii., page 168.) A
storm lately entirely swept away two of the Caroline islands, and converted
them into shoals; it partly, also, destroyed two other islands. (M.
Desmoulins in "Comptes Rendus," 1840, page 837.) According to a tradition
which was communicated to Captain Fitzroy, it is believed in the Low
Archipelago, that the arrival of the first ship caused a great inundation,
which destroyed many lives. Mr. Stutchbury relates, that in 1825, the
western side of Chain Atoll, in the same group, was completely devastated
by a hurricane, and not less than 300 lives lost: "in this instance it was
evident, even to the natives, that the hurricane alone was not sufficient
to account for the violent agitation of the ocean." ("West of England
Journal", No. I., page 35.) That considerable changes have taken place
recently in some of the atolls in the Low Archipelago, appears certain from
the case already given of Matilda Island: with respect to Whitsunday and
Gloucester Islands in this same group, we must either attribute great
inaccuracy to their discoverer, the famous circumnavigator Wallis, or
believe that they have undergone a considerable change in the period of
fifty-nine years, between his voyage and that of Captain Beechey's.
Whitsunday Island is described by Wallis as "about four miles long, and
three wide," now it is only one mile and a half long. The appearance of
Gloucester Island, in Captain Beechey's words (Beechey's "Voyage to the
Pacific," chapter vii., and Wallis's "Voyage in the 'Dolphin'," chapter
iv.), has been accurately described by its discoverer, but its present form
and extent differ materially." Blenheim reef, in the Chagos group,
consists of a water-washed annular reef, thirteen miles in circumference,
surrounding a lagoon ten fathoms deep: on its surface there were a few
worn patches of conglomerate coral-rock, of about the size of hovels; and
these Captain Moresby considered as being, without doubt, the last remnants
of islets; so that here an atoll has been converted into an atoll-formed
reef. The inhabitants of the Maldiva Archipelago, as long ago as 1605,
declared, "that the high tides and violent currents were diminishing the
number of the islands" (See an extract from Pyrard's Voyage in Captain
Owen's paper on the Maldiva Archipelago, in the "Geographical Journal",
volume ii., page 84.): and I have already shown, on the authority of
Captain Moresby, that the work of destruction is still in progress; but
that on the other hand the first formation of some islets is known to the
present inhabitants. In such cases, it would be exceedingly difficult to
detect a gradual subsidence of the foundation, on which these mutable
structures rest.
Some of the archipelagoes of low coral-islands are subject to earthquakes:
Captain Moresby informs me that they are frequent, though not very strong,
in the Chagos group, which occupies a very central position in the Indian
Ocean, and is far from any land not of coral formation. One of the islands
in this group was formerly covered by a bed of mould, which, after an
earthquake, disappeared, and was believed by the residents to have been
washed by the rain through the broken masses of underlying rock; the island
was thus rendered unproductive. Chamisso (See Chamisso, in Kotzebue's
"First Voyage," volume iii., pages 182 and 136.) states, that earthquakes
are felt in the Marshall atolls, which are far from any high land, and
likewise in the islands of the Caroline Archipelago. On one of the latter,
namely Oulleay atoll, Admiral Lutke, as he had the kindness to inform me,
observed several straight fissures about a foot in width, running for some
hundred yards obliquely across the whole width of the reef. Fissures
indicate a stretching of the earth's crust, and, therefore, probably
changes in its level; but these coral-islands, which have been shaken and
fissured, certainly have not been elevated, and, therefore, probably they
have subsided. In the chapter on Keeling atoll, I attempted to show by
direct evidence, that the island underwent a movement of subsidence, during
the earthquakes lately felt there.
The facts stand thus;--there are many large tracts of ocean, without any
high land, interspersed with reefs and islets, formed by the growth of
those kinds of corals, which cannot live at great depths; and the existence
of these reefs and low islets, in such numbers and at such distant points,
is quite inexplicable, excepting on the theory, that the bases on which the
reefs first became attached, slowly and successively sank beneath the level
of the sea, whilst the corals continued to grow upwards. No positive facts
are opposed to this view, and some general considerations render it
probable. There is evidence of change in form, whether or not from
subsidence, on some of these coral-islands; and there is evidence of
subterranean disturbances beneath them. Will then the theory, to which we
have thus been led, solve the curious problem,--what has given to each
class of reef its peculiar form?
(PLATE: WOODCUT NO. 4.
AA--Outer edge of the reef at the level of the sea.
BB--Shores of the island.
A'A'--Outer edge of the reef, after its upward growth during a period of
subsidence.
CC--The lagoon-channel between the reef and the shores of the now encircled
land.
B'B'--The shores of the encircled island.
N.B.--In this, and the following woodcut, the subsidence of the land could
only be represented by an apparent rise in the level of the sea.
PLATE: WOODCUT NO. 5.
A'A'--Outer edges of the barrier-reef at the level of the sea. The
cocoa-nut trees represent coral-islets formed on the reef.
CC--The lagoon-channel.
B'B'--The shores of the island, generally formed of low alluvial land and
of coral detritus from the lagoon-channel.
A"A"--The outer edges of the reef now forming an atoll.
C'--The lagoon of the newly formed atoll. According to the scale, the
depth of the lagoon and of the lagoon-channel is exaggerated.)
Let us in imagination place within one of the subsiding areas, an island
surrounded by a "fringing-reef,"--that kind, which alone offers no
difficulty in the explanation of its origin. Let the unbroken lines and
the oblique shading in the woodcut (No. 4) represent a vertical section
through such an island; and the horizontal shading will represent the
section of the reef. Now, as the island sinks down, either a few feet at a
time or quite insensibly, we may safely infer from what we know of the
conditions favourable to the growth of coral, that the living masses bathed
by the surf on the margin of the reef, will soon regain the surface. The
water, however, will encroach, little by little, on the shore, the island
becoming lower and smaller, and the space between the edge of the reef and
the beach proportionately broader. A section of the reef and island in
this state, after a subsidence of several hundred feet, is given by the
dotted lines: coral-islets are supposed to have been formed on the new
reef, and a ship is anchored in the lagoon-channel. This section is in
every respect that of an encircling barrier-reef; it is, in fact, a section
taken (The section has been made from the chart given in the "Atlas of the
Voyage of the 'Coquille'." The scale is .57 of an inch to a mile. The
height of the island, according to M. Lesson, is 4,026 feet. The deepest
part of the lagoon-channel is 162 feet; its depth is exaggerated in the
woodcut for the sake of clearness.) east and west through the highest point
of the encircled island of Bolabola; of which a plan is given in Plate I.,
Figure 5. The same section is more clearly shown in the following woodcut
(No. 5) by the unbroken lines. The width of the reef, and its slope, both
on the outer and inner side, will have been determined by the growing
powers of the coral, under the conditions (for instance the force of the
breakers and of the currents) to which it has been exposed; and the
lagoon-channel will be deeper or shallower, in proportion to the growth of
the delicately branched corals within the reef, and to the accumulation of
sediment, relatively, also, to the rate of subsidence and the length of the
intervening stationary periods.
It is evident in this section, that a line drawn perpendicularly down from
the outer edge of the new reef to the foundation of solid rock, exceeds by
as many feet as there have been feet of subsidence, that small limit of
depth at which the effective polypifers can live--the corals having grown
up, as the whole sank down, from a basis formed of other corals and their
consolidated fragments. Thus the difficulty on this head, which before
seemed so great, disappears.
As the space between the reef and the subsiding shore continued to increase
in breadth and depth, and as the injurious effects of the sediment and
fresh water borne down from the land were consequently lessened, the
greater number of the channels, with which the reef in its fringing state
must have been breached, especially those which fronted the smaller
streams, will have become choked up with the growth of coral: on the
windward side of the reef, where the coral grows most vigorously, the
breaches will probably have first been closed. In barrier-reefs,
therefore, the breaches kept open by draining the tidal waters of the
lagoon-channel, will generally be placed on the leeward side, and they will
still face the mouths of the larger streams, although removed beyond the
influence of their sediment and fresh water;--and this, it has been shown,
is commonly the case.
Referring to the diagram shown above, in which the newly formed barrier-reef
is represented by unbroken lines, instead of by dots as in the former
woodcut, let the work of subsidence go on, and the doubly pointed hill will
form two small islands (or more, according to the number of the hills)
included within one annular reef. Let the island continue subsiding, and
the coral-reef will continue growing up on its own foundation, whilst the
water gains inch by inch on the land, until the last and highest pinnacle
is covered, and there remains a perfect atoll. A vertical section of this
atoll is shown in the woodcut by the dotted lines;--a ship is anchored in
its lagoon, but islets are not supposed yet to have been formed on the
reef. The depth of the lagoon and the width and slope of the reef, will
depend on the circumstances just referred to under barrier-reefs. Any
further subsidence will produce no change in the atoll, except perhaps a
diminution in its size, from the reef not growing vertically upwards; but
should the currents of the sea act violently upon it, and should the corals
perish on part or on the whole of its margin, changes would result during
subsidence which will be presently noticed. I may here observe, that a
bank either of rock or of hardened sediment, level with the surface of the
sea, and fringed with living coral, would (if not so small as to allow the
central space to be quickly filled up with detritus) by subsidence be
converted immediately into an atoll, without passing, as in the case of a
reef fringing the shore of an island, through the intermediate form of a
barrier-reef. If such a bank lay a few fathoms submerged, the simple
growth of the coral (as remarked in the third chapter) without the aid of
subsidence, would produce a structure scarcely to be distinguished from a
true atoll; for in all cases the corals on the outer margin of a reef, from
having space and being freely exposed to the open sea, will grow vigorously
and tend to form a continuous ring whilst the growth of the less massive
kinds on the central expanse, will be checked by the sediment formed there,
and by that washed inwards by the breakers; and as the space becomes
shallower, their growth will, also, be checked by the impurities of the
water, and probably by the small amount of food brought by the enfeebled
currents, in proportion to the surface of living reefs studded with
innumerable craving mouths: the subsidence of a reef based on a bank of
this kind, would give depth to its central expanse or lagoon, steepness to
its flanks, and through the free growth of the coral, symmetry to its
outline:--I may here repeat that the larger groups of atolls in the Pacific
and Indian Oceans cannot be supposed to be founded on banks of this nature.
If, instead of the island in the diagram, the shore of a continent fringed
by a reef had subsided, a great barrier-reef, like that on the north-east
coast of Australia, would have necessarily resulted; and it would have been
separated from the main land by a deep-water channel, broad in proportion
to the amount of subsidence, and to the less or greater inclination of the
neighbouring coast-line. The effect of the continued subsidence of a great
barrier-reef of this kind, and its probable conversion into a chain of
separate atolls, will be noticed, when we discuss the apparent progressive
disseverment of the larger Maldiva atolls.
We now are able to perceive that the close similarity in form, dimensions,
structure, and relative position (which latter point will hereafter be more
fully noticed) between fringing and encircling barrier-reefs, and between
these latter and atolls, is the necessary result of the transformation,
during subsidence of the one class into the other. On this view, the three
classes of reefs ought to graduate into each other. Reefs having
intermediate character between those of the fringing and barrier classes do
exist; for instance, on the south-west coast of Madagascar, a reef extends
for several miles, within which there is a broad channel from seven to
eight fathoms deep, but the sea does not deepen abruptly outside the reef.
Such cases, however, are open to some doubts, for an old fringing-reef,
which had extended itself a little on a basis of its own formation, would
hardly be distinguishable from a barrier-reef, produced by a small amount
of subsidence, and with its lagoon-channel nearly filled up with sediment
during a long stationary period. Between barrier-reefs, encircling either
one lofty island or several small low ones, and atolls including a mere
expanse of water, a striking series can be shown: in proof of this, I need
only refer to the first plate in this volume, which speaks more plainly to
the eye, than any description could to the ear. The authorities from which
the charts have been engraved, together with some remarks on them and
descriptive of the plates, are given above. At New Caledonia (Plate II.,
Figure 5.) the barrier-reefs extend for 150 miles on each side of the
submarine prolongation of the island; and at their northern extremity they
appear broken up and converted into a vast atoll-formed reef, supporting a
few low coral-islets: we may imagine that we here see the effects of
subsidence actually in progress, the water always encroaching on the
northern end of the island, towards which the mountains slope down, and the
reefs steadily building up their massive fabrics in the lines of their
ancient growth.
We have as yet only considered the origin of barrier-reefs and atolls in
their simplest form; but there remain some peculiarities in structure and
some special cases, described in the two first chapters, to be accounted
for by our theory. These consist--in the inclined ledge terminated by a
wall, and sometimes succeeded by a second ledge with a wall, round the
shores of certain lagoons and lagoon-channels; a structure which cannot, as
I endeavoured to show, be explained by the simple growing powers of the
corals,--in the ring or basin-like forms of the central reefs, as well as
of the separate marginal portions of the northern Maldiva atolls,--in the
submerged condition of the whole, or of parts of certain barrier and
atoll-formed reefs; where only a part is submerged, this being generally to
leeward,--in the apparent progressive disseverment of some of the Maldiva
atolls,--in the existence of irregularly formed atolls, some being tied
together by linear reefs, and others with spurs projecting from them,--and,
lastly, in the structure and origin of the Great Chagos Bank.
STEP-FORMED LEDGES ROUND CERTAIN LAGOONS.
If we suppose an atoll to subside at an extremely slow rate, it is
difficult to follow out the complex results. The living corals would grow
up on the outer margin; and likewise probably in the gullies and deeper
parts of the bare surface of the annular reef; the water would encroach on
the islets, but the accumulation of fresh detritus might possibly prevent
their entire submergence. After a subsidence of this very slow nature, the
surface of the annular reef sloping gently into the lagoon, would probably
become united with the irregular reefs and banks of sand, which line the
shores of most lagoons. Should, however, the atoll be carried down by a
more rapid movement, the whole surface of the annular reef, where there was
a foundation of solid matter, would be favourably circumstanced for the
fresh growth of coral; but as the corals grew upwards on its exterior
margin, and the waves broke heavily on this part, the increase of the
massive polypifers on the inner side would be checked from the want of
water. Consequently, the exterior parts would first reach the surface, and
the new annular reef thus formed on the old one, would have its summit
inclined inwards, and be terminated by a subaqueous wall, formed by the
upward growth of the coral (before being much checked), from the inner edge
of the solid parts of the old reef. The inner portion of the new reef,
from not having grown to the surface, would be covered by the waters of the
lagoon. Should a subsidence of the same kind be repeated, the corals would
again grow up in a wall, from all the solid parts of the resunken reef,
and, therefore, not from within the sandy shores of the lagoon; and the
inner part of the new annular reef would, from being as before checked in
its upward growth, be of less height than the exterior parts, and therefore
would not reach the surface of the lagoon. In this case the shores of the
lagoon would be surrounded by two inclined ledges, one beneath the other,
and both abruptly terminated by subaqueous cliffs. (According to Mr.
Couthouy (page 26) the external reef round many atolls descends by a
succession of ledges or terraces. He attempts, I doubt whether
successfully, to explain this structure somewhat in the same manner as I
have attempted, with respect to the internal ledges round the lagoons of
some atolls. More facts are wanted regarding the nature both of the
interior and exterior step-like ledges: are all the ledges, or only the
upper ones, covered with living coral? If they are all covered, are the
kinds different on the ledges according to the depth? Do the interior and
exterior ledges occur together in the same atolls; if so, what is their
total width, and is the intervening surface-reef narrow, etc.?)
THE RING OR BASIN-FORMED REEFS OF THE NORTHERN MALDIVA ATOLLS.
I may first observe, that the reefs within the lagoons of atolls and within
lagoon-channels, would, if favourably circumstanced, grow upwards during
subsidence in the same manner as the annular rim; and, therefore, we might
expect that such lagoon-reefs, when not surrounded and buried by an
accumulation of sediment more rapid than the rate of subsidence, would rise
abruptly from a greater depth than that at which the efficient polypifers
can flourish: we see this well exemplified in the small abruptly-sided
reefs, with which the deep lagoons of the Chagos and Southern Maldiva
atolls are studded. With respect to the ring or basin-formed reefs of the
Northern Maldiva atolls, it is evident, from the perfectly continuous
series which exists that the marginal rings, although wider than the
exterior or bounding reef of ordinary atolls, are only modified portions of
such a reef; it is also evident that the central rings, although wider than
the knolls or reefs which commonly occur in lagoons, occupy their place.
The ring-like structure has been shown to be contingent on the breaches
into the lagoon being broad and numerous, so that all the reefs which are
bathed by the waters of the lagoon are placed under nearly the same
conditions with the outer coast of an atoll standing in the open sea.
Hence the exterior and living margins of these reefs must have been
favourably circumstanced for growing outwards, and increasing beyond the
usual breadth; and they must likewise have been favourably circumstanced
for growing vigorously upwards, during the subsiding movements, to which by
our theory the whole archipelago has been subjected; and subsidence with
this upward growth of the margins would convert the central space of each
little reef into a small lagoon. This, however, could only take place with
those reefs, which had increased to a breadth sufficient to prevent their
central spaces from being almost immediately filled up with the sand and
detritus driven inwards from all sides: hence it is that few reefs, which
are less than half a mile in diameter, even in the atolls where the
basin-like structure is most strikingly exhibited, include lagoons. This
remark, I may add, applies to all coral-reefs wherever found. The
basin-formed reefs of the Maldiva Archipelago may, in fact, be briefly
described, as small atolls formed during subsidence over the separate
portions of large and broken atolls, in the same manner as these latter were
formed over the barrier-reefs, which encircled the islands of a large
archipelago now wholly submerged.
SUBMERGED AND DEAD REEFS.
In the second section of the first chapter, I have shown that there are in
the neighbourhood of atolls, some deeply submerged banks, with level
surfaces; that there are others, less deeply but yet wholly submerged,
having all the characters of perfect atolls, but consisting merely of dead
coral-rock; that there are barrier-reefs and atolls with merely a portion
of their reef, generally on the leeward side, submerged; and that such
portions either retain their perfect outline, or they appear to be quite
effaced, their former place being marked only by a bank, conforming in
outline with that part of the reef which remains perfect. These several
cases are, I believe, intimately related together, and can be explained by
the same means. There, perhaps, exist some submerged reefs, covered with
living coral and growing upwards, but to these I do not here refer.
As we see that in those parts of the ocean, where coral-reefs are most
abundant, one island is fringed and another neighbouring one is not
fringed; as we see in the same archipelago, that all the reefs are more
perfect in one part of it than in another, for instance, in the southern
half compared with the northern half of the Maldiva Archipelago, and
likewise on the outer coasts compared with the inner coasts of the atolls
in this same group, which are placed in a double row; as we know that the
existence of the innumerable polypifers forming a reef, depends on their
sustenance, and that they are preyed on by other organic beings; and,
lastly, as we know that some inorganic causes are highly injurious to the
growth of coral, it cannot be expected that during the round of change to
which earth, air, and water are exposed, the reef-building polypifers
should keep alive for perpetuity in any one place; and still less can this
be expected, during the progressive subsidences, perhaps at some periods
more rapid than at others, to which by our theory these reefs and islands
have been subjected and are liable. It is, then, not improbable that the
corals should sometimes perish either on the whole or on part of a reef; if
on part, the dead portion, after a small amount of subsidence, would still
retain its proper outline and position beneath the water. After a more
prolonged subsidence, it would probably form, owing to the accumulation of
sediment, only the margin of a flat bank, marking the limits of the former
lagoon. Such dead portions of reef would generally lie on the leeward side
(Mr. Lyell, in the first edition of his "Principles of Geology," offered a
somewhat different explanation of this structure. He supposes that there
has been subsidence; but he was not aware that the submerged portions of
reef were in most cases, if not in all, dead; and he attributes the
difference in height in the two sides of most atolls, chiefly to the
greater accumulation of detritus to windward than to leeward. But as
matter is accumulated only on the backward part of the reef, the front part
would remain of the same height on both sides. I may here observe that in
most cases (for instance, at Peros Banhos, the Gambier group and the Great
Chagos Bank), and I suspect in all cases, the dead and submerged portions
do not blend or slope into the living and perfect parts, but are separated
from them by an abrupt line. In some instances small patches of living
reef rise to the surface from the middle of the submerged and dead parts.),
for the impure water and fine sediment would more easily flow out from the
lagoon over this side of the reef, where the force of the breakers is less
than to windward; and therefore the corals would be less vigorous on this
side, and be less able to resist any destroying agent. It is likewise
owing to this same cause, that reefs are more frequently breached to
leeward by narrow channels, serving as by ship-channels, than to windward.
If the corals perished entirely, or on the greater part of the
circumference of an atoll, an atoll-shaped bank of dead rock, more or less
entirely submerged, would be produced; and further subsidence, together
with the accumulation of sediment, would often obliterate its atoll-like
structure, and leave only a bank with a level surface.
In the Chagos group of atolls, within an area of 160 miles by 60, there are
two atoll-formed banks of dead rock (besides another very imperfect one),
entirely submerged; a third, with merely two or three very small pieces of
living reef rising to the surface; and a fourth, namely, Peros Banhos
(Plate I., Figure 9), with a portion nine miles in length dead and
submerged. As by our theory this area has subsided, and as there is
nothing improbable in the death, either from changes in the state of the
surrounding sea or from the subsidence being great or sudden, of the corals
on the whole, or on portions of some of the atolls, the case of the Chagos
group presents no difficulty. So far indeed are any of the above-mentioned
cases of submerged reefs from being inexplicable, that their occurrence
might have been anticipated on our theory, and as fresh atolls are supposed
to be in progressive formation by the subsidence of encircling barrier-reefs,
a weighty objection, namely that the number of atolls must be
increasing infinitely, might even have been raised, if proofs of the
occasional destruction and loss of atolls could not have been adduced.
THE DISSEVERMENT OF THE LARGER MALDIVA ATOLLS.
The apparent progressive disseverment in the Maldiva Archipelago of large
atolls into smaller ones, is, in many respects, an important consideration,
and requires an explanation. The graduated series which marks, as I
believe, this process, can be observed only in the northern half of the
group, where the atolls have exceedingly imperfect margins, consisting of
detached basin-formed reefs. The currents of the sea flow across these
atolls, as I am informed by Captain Moresby, with considerable force, and
drift the sediment from side to side during the monsoons, transporting much
of it seaward; yet the currents sweep with greater force round their
flanks. It is historically known that these atolls have long existed in
their present state; and we can believe, that even during a very slow
subsidence they might thus remain, the central expanse being kept at nearly
its original depth by the accumulation of sediment. But in the action of
such nicely balanced forces during a progressive subsidence (like that, to
which by our theory this archipelago has been subjected), it would be
strange if the currents of the sea should never make a direct passage
across some one of the atolls, through the many wide breaches in their
margins. If this were once effected, a deep-water channel would soon be
formed by the removal of the finer sediment, and the check to its further
accumulation; and the sides of the channel would be worn into a slope like
that on the outer coasts, which are exposed to the same force of the
currents. In fact, a channel precisely like that bifurcating one which
divides Mahlos Mahdoo (Plate II., Figure 4.), would almost necessarily be
formed. The scattered reefs situated near the borders of the new
ocean-channel, from being favourably placed for the growth of coral, would,
by their extension, tend to produce fresh margins to the dissevered portions;
such a tendency is very evident (as may be seen in the large published
chart) in the elongated reefs on the borders of the two channels
intersecting Mahlos Mahdoo. Such channels would become deeper with
continued subsidence, and probably from the reefs not growing up
perpendicularly, somewhat broader. In this case, and more especially if
the channels had been formed originally of considerable breadth, the
dissevered portions would become perfect and distinct atolls, like Ari and
Ross atolls (Plate II., Figure 6), or like the two Nillandoo atolls, which
must be considered as distinct, although related in form and position, and
separated from each other by channels, which though deep have been sounded.
Further subsidence would render such channels unfathomable, and the
dissevered portions would then resemble Phaleedoo and Moluque atolls, or
Mahlos Mahdoo and Horsburgh atolls (Plate II., Figure 4), which are related
to each other in no respect except in proximity and position. Hence, on
the theory of subsidence, the disseverment of large atolls, which have
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