have not in every instance been long ago filled up with coral and
sediment; but it is more easily accounted for than appears at first
sight. In cases like that of Hogoleu and the Gambier Islands, where a
few small peaks rise out of a great lagoon, the conditions scarcely
differ from those of an atoll, and I have already shown, at some
length, that the filling up of a true lagoon must be an extremely slow
process. Where the channel is narrow, the agency, which on unprotected
coasts is most productive of sediment, namely the force of the
breakers, is here entirely excluded, and the reef being breached in the
front of the main valleys, much of the finer mud from the rivers must
be transported into the open sea. As a current is formed by the water
thrown over the edge of atoll-formed reefs, which carries sediment with
it through the deep-water breaches, the same thing probably takes place
in barrier-reefs, and this would greatly aid in preventing the
lagoon-channel from being filled up. The low alluvial border, however,
at the foot of the encircled mountains, shows that the work of filling
up is in progress; and at Maura (Plate I., Figure 6), in the Society
group, it has been almost effected, so that there remains only one
harbour for small craft.
If we look at a set of charts of barrier-reefs, and leave out in
imagination the encircled land, we shall find that, besides the many
points already noticed of resemblance, or rather of identity in
structure with atolls, there is a close general agreement in form,
average dimensions, and grouping. Encircling barrier-reefs, like
atolls, are generally elongated, with an irregularly rounded, though
sometimes angular outline. There are atolls of all sizes, from less
than two miles in diameter to sixty miles (excluding Tilla-dou-Matte,
as it consists of a number of almost independent atoll-formed reefs);
and there are encircling barrier-reefs from three miles and a half to
forty-six miles in diameter,—Turtle Island being an instance of the
former, and Hogoleu of the latter. At Tahiti the encircled island is
thirty-six miles in its longest axis, whilst at Maurua it is only a
little more than two miles. It will be shown, in the last chapter in
this volume, that there is the strictest resemblance in the grouping of
atolls and of common islands, and consequently there must be the same
resemblance in the grouping of atolls and of encircling barrier-reefs.
The islands lying within reefs of this class, are of very various
heights. Tahiti[6] is 7,000 feet; Maurua about 800; Aitutaki 360, and
Manouai only 50. The geological nature of the included land varies: in
most cases it is of ancient volcanic origin, owing apparently to the
fact that islands of this nature are most frequent within all great
seas; some, however, are of madreporitic limestone, and others of
primary formation, of which latter kind New Caledonia offers the best
example. The central land consists either of one island, or of several:
thus, in the Society group, Eimeo stands by itself; while Taha and
Raiatea (Figure 3, Plate I.), both moderately large islands of nearly
equal size, are included in one reef. Within the reef of the Gambier
group there are four large and some smaller islands (Figure 8, Plate
I.); within that of Hogoleu (Figure 2, Plate I.) nearly a dozen small
islands are scattered over the expanse of one vast lagoon.
[6] The height of Tahiti is given from Captain Beechey; Maurua from
Mr. F.D. Bennett (“Geograph. Journ.” volume viii., page 220); Aitutaki
from measurements made on board the _Beagle;_ and Manouai or Harvey
Island, from an estimate by the Rev. J. Williams. The two latter
islands, however, are not in some respects well characterised examples
of the encircled class.
After the details now given, it may be asserted that there is not one
point of essential difference between encircling barrier-reefs and
atolls: the latter enclose a simple sheet of water, the former encircle
an expanse with one or more islands rising from it. I was much struck
with this fact, when viewing, from the heights of Tahiti, the distant
island of Eimeo standing within smooth water, and encircled by a ring
of snow-white breakers. Remove the central land, and an annular reef
like that of an atoll in an early stage of its formation is left;
remove it from Bolabola, and there remains a circle of linear
coral-islets, crowned with tall cocoa-nut trees, like one of the many
atolls scattered over the Pacific and Indian Oceans.
The barrier-reefs of Australia and of New Caledonia deserve a separate
notice from their great dimensions. The reef on the west coast of New
Caledonia (Figure 5, Plate II.) is 400 miles in length; and for a
length of many leagues it seldom approaches within eight miles of the
shore; and near the southern end of the island, the space between the
reef and the land is sixteen miles in width. The Australian barrier
extends, with a few interruptions, for nearly a thousand miles; its
average distance from the land is between twenty and thirty miles; and
in some parts from fifty to seventy. The great arm of the sea thus
included, is from ten to twenty-five fathoms deep, with a sandy bottom;
but towards the southern end, where the reef is further from the shore,
the depth gradually increases to forty, and in some parts to more than
sixty fathoms. Flinders[7] 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.
[7] Flinders’ “Voyage to Terra Australis,” volume ii., page 88.
[Illustration]
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,[8]
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,[9] 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.
[8] 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.
[9] 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.
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;[10] 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.
[10] I have been informed that this is the case by Dr. Allan of
Forres, who has visited this group.
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,[1] 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.
[1] This fact is stated on the authority of the Officier du Roi, in
his extremely interesting “Voyage à 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.
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[2] states that the west coast of Australia, in latitude 24°, 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,[3] 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.
[2] Captain Grey’s “Journal of Two Expeditions,” volume i. page 369.
[3] I have described this singular structure in the “London and
Edinburgh Phil. Mag.” October 1841.
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[4] of an interesting pedestrian tour
round the Mauritius, seems to have met with a structure of this kind:
he says “J’observai que là, où la mer étale, indépendamment des rescifs
du large, il y à terre _une espèce 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 elévé de
plus de quinze pieds: c’est une espèce de rescif, que la mer abandonné,
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 Astræa and Madrepora,
loosely aggregated; they were divided into irregular beds, dipping
seaward, in one hillock at an angle of 8°, and in the other at 18° 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.
[4] “Voyage à l’Isle de France, par un Officier du Roi,” part i.,
pages 192, 200.
Many islands[5] are fringed by reefs quite similar to those of
Mauritius; 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,[6] where
it is only fifty yards in width: the sea round these islands is very
deep.
[5] 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.”
[6] Mauti is described by Lord Byron in the voyage of H.M.S. _Blonde_,
and Elizabeth Island by Captain Beechey.
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° 15′ to 1° 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° 1′ N.) “the port is formed,” it is
said,[7] “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° 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.
[7] Owen’s “Africa,” volume i., page 357, from which work the
foregoing facts are likewise taken.
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[8] 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.
[8] See Baron Roussin’s “Pilote du Brésil,” and accompanying
hydrographical memoir.
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 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° 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° N., have reefs on their shores, and
there is an atoll in 28° 30′, situated N.W. of the Sandwich
Archipelago. In the Red Sea there are coral-reefs in latitude 30°. 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° 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[1] 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,[2]
to the coldness of the currents from the south, but the Gulf of Panama
is one of the hottest pelagic districts in the world.[3] 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.
[1] I have been informed that this is the case, by Lieutenant Ryder,
R.N., and others who have had ample opportunities for observation.
[2] 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° Fahr. The lowest temperature observed was 58.5° at the
south-west end of Albemarle Island; and on the west coast of this
island, it was several times 62° and 63°. 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°, the lowest any day being 76.5°. Therefore we have
here a difference of 9.5° in mean temperature, and 18° in extremes; a
difference doubtless quite sufficient to affect the distribution of
organic beings in the two areas.
[3] Humboldt’s “Personal Narrative,” volume vii., page 434.
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,[4] 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 Holothuriæ 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.
[4] 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.
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 Astræa,
a genus closely allied to Porites) “which form rocks measuring several
fathoms in thickness,” prefer, according to Chamisso,[5] 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.
[5] Kotzebue’s “First Voyage” (English Translation) volume iii., pages
142, 143, 331.
Ehrenberg[6] 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.[7] 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:[8] 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.
[6] Ehrenberg, “Über die Natür und Bildung der Corallen Bänke im
rothen Meere,” page 49.
[7] 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.
[8] “Annales des Sciences Naturelles,” tome vi., pages 276, 278.—“Là
où les ondes sont agitées, les Lytophytés ne peuvent travailler, parce
qu’elles détruiraient leurs fragiles édifices,” &c.
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,[9] have been observed.
[9] Ehrenberg, “Über die Natür,” etc., etc., page 46.
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.[10] 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,[11] 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.
[10] Ibid., page 49.
[11] Captain Owen on the Geography of the Maldiva Islands,
“Geographical Journal”, volume ii., page 88.
In the last chapter I remarked that fringing-reefs are almost
universally breached, where streams enter the sea.[12] 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,[13] 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.
[12] Lieutenant Wellstead and others have remarked that this is the
case in the Red Sea; Dr. Rüppell (“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.
[13] Cook’s “First Voyage,” volume ii., page 271 (Hawkesworth’s
edition).)
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,[14] between the great
branching corals of the Red Sea, and those on the reefs of the Maldiva
atolls.
[14] Captain Moresby on the Northern Maldiva atolls, “Geographical
Journal”, volume v., page 401.
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;[15] 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.
[15] 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.
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 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[16] 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,[17] with respect to the thickness of
some upraised beds of coral, which they examined at Timor and some
other places. Ehrenberg[18] 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[19] 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[20] 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[21] 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.
[16] Ehrenberg, as before cited, pages 39, 46, and 50.
[17] “Annales des Sciences Nat.” tom. vi., page 28.
[18] Ehrenberg, ut sup., page 42.
[19] Lyell’s “Principles of Geology,” book iii., chapter xviii.
[20] Beechey’s “Voyage to the Pacific,” chapter viii.
[21] Ehrenberg, ut sup., page 43.
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,[22] 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.
[22] “Geological Transactions,” volume v., page 113.
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,[23] to have a similar structure, and its shores are
about forty feet high: some of the Cook Islands also appear[24] 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.
[23] Forster’s “Voyage round the World with Cook,” volume ii., pages
163, 167.
[24] Williams’s “Narrative of Missionary Enterprise,” page 30.
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 Nulliporæ 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
Astræas; 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,[25] “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.”[26] 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.
[25] Cook’s “Third Voyage,” book III., chapter x.
[26] Beechey’s “Voyage to the Pacific,” chapter vii. and viii.
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 Lutké,[27]—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.
[27] F. Lutké’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.
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 Nulliporæ 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 Nulliporæ? 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° 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[28] 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.
[28] 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.
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.[29]
[29] 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 étaient
engagés complètement 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 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 Astræa, with
rather large[30] stars, seemed the commonest kinds; and it must be
noticed that twice at the depth of fifteen fathoms, the arming was
marked with a clean impression of an Astræa. 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.
[30] 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.
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.”[31] 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.
[31] Ehrenberg, “Über die Natür,” &c., page 50.
MM. Quoy and Gaimard[32] believe that the growth of coral is confined
within very limited depths; and they state that they never found any
fragment of an Astræa (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 Astræa. _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[33] 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°
S.) by Captain P.P. King:[34] 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.[35] 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° N. in deep
water, and I procured a small species from Tierra del Fuego in Latitude
53° 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.
[32] “Annales des Sci. Nat.” tom. vi.
[33] “Remarks on Coral Formations,” page 12.
[34] I am indebted to Mr. Stokes for having kindly communicated this
fact to me, together with much other valuable information.
[35] 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.
NAME OF ZOOPHYTE Depth in fathoms COUNTRY and S. LATITUDE AUTHORITY
SERTULARIA 40 Cape Horn 66° Where none is given,
the observation is my own
CELLARIA ditto Ditto
CELLARIA, A minute 190 Keeling Atoll, 12°
scarlet encrusting
species found living
CELLARIA, An allied, 48 St Cruz Riv. 50°
small stony sub-generic
form
A coral allied to 40 Cape Horn
VINCULARIA, with eight
rows of cells
TUBULIPORA, near to 40 Cape Horn
T. patina
TUBULIPORA, near to 94 East Chiloe 43°
T. patina
CELLEPORA, several 40 Cape Horn
species, and allied
sub-generic forms
CELLEPORA, several 40 and 57 Chonos Archipelago 45°
species, and allied
sub-generic forms
CELLEPORA, several 48 St Cruz 50°
species, and allied
sub-generic forms
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 94 and 30 E. Chiloe 43°
coral with cylindrical
branches, of a pink
colour, about two
inches high,
resembling in the
form of its orifices
M. aspera of Lamarck
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 160 Abrolhos on the Captain Beechey informed me of this fact in a letter.
form) coast of Brazil 18°
Ellis (“Nat. Hist. of Coralline,” page 96) states that Ombellularia was
procured in latitude 79° 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 Nulliporæ, 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 Nulliporæ
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[36] 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.
[36] “Journal of the Royal Geographical Society,” 1831, page 218.
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;[1] 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.
[1] Kotzebue’s “First Voyage,” volume iii., page 331.
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
Lutké, regarding the Pacific; and from Moresby[2] 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[3] 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.
[2] 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.
[3] 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.
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,[4] 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.
[4] 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!