CHAPTER VI.—ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE

THEORY OF THEIR FORMATION

APPENDIX

INDEX

DETAILED TABLE OF CONTENTS

DESCRIPTION OF THE PLATES

INTRODUCTION

CHAPTER I.—ATOLLS OR LAGOON-ISLANDS

SECTION I.—DESCRIPTION OF KEELING ATOLL. Corals on the outer

margin.—Zone of Nulliporæ.—Exterior

reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari

and Holuthuriæ subsisting on corals.—Changes in the condition of the

reefs and islets.—Probable subsidence of the atoll.—Future state of the

lagoon.

SECTION II.—GENERAL DESCRIPTION OF ATOLLS. General form and size of

atolls, their reefs and islets.—External slope.— Zone of

Nulliporæ.—Conglomerate.—Depth of lagoons.—Sediment.—Reefs submerged

wholly or in part.—Breaches in the reef.—Ledge-formed shores round

certain lagoons.—Conversion of lagoons into land.

SECTION III.—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK.

Maldiva Archipelago.—Ring-formed reefs, marginal and central.—Great

depths in the lagoons of the southern atolls.—Reefs in the lagoons all

rising to the surface.—Position of islets and breaches in the reefs,

with respect to the prevalent winds and action of the

waves.—Destruction of islets.—Connection in the position and submarine

foundation of distinct atolls.—The apparent disseverment of large

atolls.—The Great Chagos Bank.—Its submerged condition and

extraordinary structure.

CHAPTER II.—BARRIER REEFS

Closely resemble in general form and structure atoll-reefs.—Width and

depth of the lagoon-channels.—Breaches through the reef in front of

valleys, and generally on the leeward side.—Checks to the filling up of

the lagoon-channels.—Size and constitution of the encircled islands.—

Number of islands within the same reef.—Barrier-reefs of New Caledonia

and Australia.—Position of the reef relative to the slope of the

adjoining land.—Probable great thickness of barrier-reefs.

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 E. 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.

CHAPTER IV.—ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS

SECTION I.—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS

FAVOURABLE TO THEIR INCREASE.

SECTION II.—ON THE RATE OF GROWTH OF CORAL-REEFS.

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

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.—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.

CHAPTER VI.—ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE

THEORY OF THEIR FORMATION

Description of the coloured map.—Proximity of atolls and

barrier-reefs.— Relation in form and position of atolls with ordinary

islands.—Direct evidence of subsidence difficult to be detected.—Proofs

of recent elevation where fringing-reefs occur.—Oscillations of

level.—Absence of active volcanoes in the areas of

subsidence.—Immensity of the areas which have been elevated and have

subsided.—Their relation to the present distribution of the land.—Areas

of subsidence elongated, their intersection and alternation with those

of elevation.—Amount and slow rate of the subsidence.—Recapitulation.

APPENDIX

INDEX

DESCRIPTION OF THE PLATES.

PLATE I.

Illustration: PLATE I.—MAP SHOWING THE RESEMBLANCE IN FORM BETWEEN

BARRIER CORAL-REEFS SURROUNDING MOUNTAINOUS ISLANDS, AND ATOLLS OR

LAGOON ISLANDS.

In the several original surveys, from which the small plans on this

plate have been reduced, the coral-reefs are engraved in very different

styles. For the sake of uniformity, I have adopted the style used in

the charts of the Chagos Archipelago, published by the East Indian

Company, from the survey by Captain Moresby and Lieutenant Powell. The

surface of the reef, which dries at low water, is represented by a

surface with small crosses: the coral-islets on the reef are marked by

small linear spaces, on which a few cocoa-nut trees, out of all

proportion too large, have been introduced for the sake of clearness.

The entire _annular reef_, which when surrounding an open expanse of

water, forms an “atoll,” and when surrounding one or more high islands,

forms an encircling “barrier-reef,” has a nearly uniform structure. The

reefs in some of the original surveys are represented merely by a

single line with crosses, so that their breadth is not given; I have

had such reefs engraved of the width usually attained by coral-reefs. I

have not thought it worth while to introduce all those small and very

numerous reefs, which occur within the lagoons of most atolls and

within the lagoon-channels of most barrier-reefs, and which stand

either isolated, or are attached to the shores of the reef or land. At

Peros Banhos none of the lagoon-reefs rise to the surface of the water;

a few of them have been introduced, and are marked by plain dotted

circles. A few of the deepest soundings are laid down within each reef;

they are in fathoms, of six English feet.

Figure 1.—VANIKORO, situated in the western part of the South Pacific;

taken from the survey by Captain D’Urville in the _Astrolabe;_ the

soundings on the southern side of the island, namely, from thirty to

forty fathoms, are given from the voyage of the Chev. Dillon; the other

soundings are laid down from the survey by D’Urville; height of the

summit of the island is 3,032 feet. The principal small detached reefs

within the lagoon-channel have in this instance been represented. The

southern shore of the island is narrowly fringed by a reef: if the

engraver had carried this reef entirely round both islands, this figure

would have served (by leaving out in imagination the barrier-reef) as a

good specimen of an abruptly-sided island, surrounded by a reef of the

fringing class.

Figure 2.—HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the

atlas of the voyage of the _Astrolabe_, compiled from the surveys of

Captains Duperrey and D’Urville; the depth of the immense lagoon-like

space within the reef is not known.

Figure 3.—RAIATEA, in the Society Archipelago; from the map given in

the quarto edition of “Cook’s First Voyage;” it is probably not

accurate.

Figure 4.—BOW, or HEYOU ATOLL (or lagoon-island), in the Low

Archipelago, from the survey by Captain Beechey, R.N.; the lagoon is

choked up with reefs, but the average greatest depth of about twenty

fathoms, is given from the published account of the voyage.

Figure 5.—BOLABOLA, in the Society Archipelago, from the survey of

Captain Duperrey in the _Coquille:_ the soundings in this and the

following figures have been altered from French feet to English

fathoms; height of highest point of the island 4,026 feet.

Figure 6.—MAURUA, in the Society Archipelago; from the survey by

Captain Duperrey in the _Coquille:_ height of land about eight hundred

feet.

Figure 7.—POUYNIPÈTE, or SENIAVINE, in the Caroline Archipelago; from

the survey by Admiral Lutké.

Figure 8.—GAMBIER ISLANDS, in the southern part of the Low Archipelago;

from the survey by Captain Beechey; height of highest island, 1,246

feet; the islands are surrounded by extensive and irregular reefs; the

reef on the southern side is submerged.

Figure 9.—PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in

the Indian Ocean; from the survey by Captain Moresby and Lieutenant

Powell; not nearly all the small submerged reefs in the lagoon are

represented; the annular reef on the southern side is submerged.

Figure 10.—KEELING, or COCOS ATOLL (or lagoon-island), in the Indian

Ocean; from the survey by Captain Fitzroy; the lagoon south of the

dotted line is very shallow, and is left almost bare at low water; the

part north of the line is choked up with irregular reefs. The annular

reef on the north-west side is broken, and blends into a shoal

sandbank, on which the sea breaks.

PLATE II.—GREAT CHAGOS BANK, NEW CALEDONIA, MENCHIKOFF ATOLL, ETC.

Illustration: Plate 2

Figure 1.—GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey

by Captain Moresby and Lieutenant Powell; the parts which are shaded,

with the exception of two or three islets on the western and northern

sides, do not rise to the surface, but are submerged from four to ten

fathoms; the banks bounded by the dotted lines lie from fifteen to

twenty fathoms beneath the surface, and are formed of sand; the central

space is of mud, and from thirty to fifty fathoms deep.

Figure 2.—A vertical section, on the same scale, in an eastern and

western line across the Great Chagos Bank, given for the sake of

exhibiting more clearly its structure.

Figure 3.—MENCHIKOFF ATOLL (or lagoon-island), in the Marshall

Archipelago, Northern Pacific Ocean; from Krusenstern’s “Atlas of the

Pacific;” originally surveyed by Captain Hagemeister; the depth within

the lagoons is unknown.

Figure 4.—MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the

Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant

Powell; the white spaces in the middle of the separate small reefs,

both on the margin and in the middle part, are meant to represent

little lagoons; but it was found not possible to distinguish them

clearly from the small islets, which have been formed on these same

small reefs; many of the smaller reefs could not be introduced; the

nautical mark (dot over a dash) over the figures 250 and 200, between

Mahlos Mahdoo and Horsburgh atoll and Powell’s island, signifies that

soundings were not obtained at these depths.

Figure 5.—NEW CALEDONIA, in the western part of the Pacific; from

Krusenstern’s “Atlas,” compiled from several surveys; I have slightly

altered the northern point of the reef, in accordance with the “Atlas

of the Voyage of the _Astrolabe_.” In Krusenstern’s “Atlas,” the reef

is represented by a single line with crosses; I have for the sake of

uniformity added an interior line.

Figure 6.—MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by

Captain Moresby and Lieutenant Powell.

PLATE III.—MAP SHOWING THE DISTRIBUTION OF CORAL-REEFS AND ACTIVE

VOLCANOES.

Illustration: Plate 3 Shewing the Distribution of the Different Kinds

of Coral Reefs, Together with the Position of the Active Volcanoes in

the Map.

The principles, on which this map was coloured, are explained in the

beginning of Chapter VI.; and the authorities for each particular spot

are detailed in the Appendix to “Coral Reefs.” The names not printed in

upper case in the Index refer to the Appendix.

THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.

INTRODUCTION.

The object of this volume is to describe from my own observation and

the works of others, the principal kinds of coral-reefs, more

especially those occurring in the open ocean, and to explain the origin

of their peculiar forms. I do not here treat of the polypifers, which

construct these vast works, except so far as relates to their

distribution, and to the conditions favourable to their vigorous

growth. Without any distinct intention to classify coral-reefs, most

voyagers have spoken of them under the following heads:

“lagoon-islands,” or “atolls,” “barrier” or “encircling reefs,” and

“fringing” or “shore-reefs.” The lagoon-islands have received much the

most attention; and it is not surprising, for every one must be struck

with astonishment, when he first beholds one of these vast rings of

coral-rock, often many leagues in diameter, here and there surmounted

by a low verdant island with dazzling white shores, bathed on the

outside by the foaming breakers of the ocean, and on the inside

surrounding a calm expanse of water, which from reflection, is of a

bright but pale green colour. The naturalist will feel this

astonishment more deeply after having examined the soft and almost

gelatinous bodies of these apparently insignificant creatures, and when

he knows that the solid reef increases only on the outer edge, which

day and night is lashed by the breakers of an ocean never at rest. Well

did François Pyrard de Laval, in the year 1605, exclaim, “C’est une

merueille de voir chacun de ces atollons, enuironné d’un grand banc de

pierre tout autour, n’y ayant point d’artifice humain.” The

accompanying sketch of Whitsunday island, in the South Pacific, taken

from Captain Beechey’s admirable “Voyage,” although excellent of its

kind, gives but a faint idea of the singular aspect of one of these

lagoon-islands.

[Illustration]

Whitsunday Island is of small size, and the whole circle has been

converted into land, which is a comparatively rare circumstance. As the

reef of a lagoon-island generally supports many separate small islands,

the word “island,” applied to the whole, is often the cause of

confusion; hence I have invariably used in this volume the term

“atoll,” which is the name given to these circular groups of

coral-islets by their inhabitants in the Indian Ocean, and is

synonymous with “lagoon-island.”

[Illustration]

Barrier-reefs, when encircling small islands, have been comparatively

little noticed by voyagers; but they well deserve attention. In their

structure they are little less marvellous than atolls, and they give a

singular and most picturesque character to the scenery of the islands

they surround. In the accompanying sketch, taken from the “Voyage of

the ‘Coquille’,” the reef is seen from within, from one of the high

peaks of the island of Bolabola.[1] Here, as in Whitsunday Island, the

whole of that part of the reef which is visible is converted into land.

This is a circumstance of rare occurrence; more usually a snow-white

line of great breakers, with here and there an islet crowned by

cocoa-nut trees, separates the smooth waters of the lagoon-like channel

from the waves of the open sea. The barrier-reefs of Australia and of

New Caledonia, owing to their enormous dimensions, have excited much

attention: in structure and form they resemble those encircling many of

the smaller islands in the Pacific Ocean.

[1] I have taken the liberty of simplifying the foreground, and

leaving out a mountainous island in the far distance.

With respect to fringing, or shore-reefs, there is little in their

structure which needs explanation; and their name expresses their

comparatively small extension. They differ from barrier-reefs in not

lying so far from the shore, and in not having within a broad channel

of deep water. Reefs also occur around submerged banks of sediment and

of worn-down rock; and others are scattered quite irregularly where the

sea is very shallow; these in most respects are allied to those of the

fringing class, but they are of comparatively little interest.

I have given a separate chapter to each of the above classes, and have

described some one reef or island, on which I possessed most

information, as typical; and have afterwards compared it with others of

a like kind. Although this classification is useful from being obvious,

and from including most of the coral-reefs existing in the open sea, it

admits of a more fundamental division into barrier and atoll-formed

reefs on the one hand, where there is a great apparent difficulty with

respect to the foundation on which they must first have grown; and into

fringing-reefs on the other, where, owing to the nature of the slope of

the adjoining land, there is no such difficulty. The two blue tints and

the red colour (replaced by numbers in this edition.) on the map (Plate

III.), represent this main division, as explained in the beginning of

the last chapter. In the Appendix, every existing coral-reef, except

some on the coast of Brazil not included in the map, is briefly

described in geographical order, as far as I possessed information; and

any particular spot may be found by consulting the Index.

Several theories have been advanced to explain the origin of atolls or

lagoon-islands, but scarcely one to account for barrier-reefs. From the

limited depths at which reef-building polypifers can flourish, taken

into consideration with certain other circumstances, we are compelled

to conclude, as it will be seen, that both in atolls and barrier-reefs,

the foundation on which the coral was primarily attached, has subsided;

and that during this downward movement, the reefs have grown upwards.

This conclusion, it will be further seen, explains most satisfactorily

the outline and general form of atolls and barrier-reefs, and likewise

certain peculiarities in their structure. The distribution, also, of

the different kinds of coral-reefs, and their position with relation to

the areas of recent elevation, and to the points subject to volcanic

eruptions, fully accord with this theory of their origin.[2]

[2] A brief account of my views on coral formations, now published in

my Journal of Researches, was read May 31st, 1837, before the

Geological Society, and an abstract has appeared in the Proceedings.

CHAPTER I.

ATOLLS OR LAGOON-ISLANDS.

SECTION I.—KEELING ATOLL.

Corals on the outer margin.—Zone of Nulliporæ.—Exterior

reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari

and Holuthuriæ subsisting on corals.—Changes in the condition of the

reefs and islets.—Probable subsidence of the atoll.—Future state of the

lagoon.

[Illustration]

A.—Level of the sea at low water: where the letter A is placed, the

depth is twenty-five fathoms, and the distance rather more than one

hundred and fifty yards from the edge of the reef.

B.—Outer edge of that flat part of the reef, which dries at low water:

the edge either consists of a convex mound, as represented, or of

rugged points, like those a little farther seaward, beneath the water.

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

D.—A low projecting ledge of brecciated coral-rock washed by the waves

at high water.

E.—A slope of loose fragments, reached by the sea only during gales:

the upper part, which is from six to twelve feet high, is clothed with

vegetation. The surface of the islet gently slopes to the lagoon.

F.—Level of the lagoon at low water.

Keeling or Cocos atoll is situated in the Indian Ocean, in 12° 5′ S.,

and longitude 90° 55′ E.: a reduced chart of it was made from the

survey of Captain Fitzroy and the Officers of H.M.S. “Beagle,” is given

in Plate I., Figure 10. The greatest width of this atoll is nine miles

and a half. Its structure is in most respects characteristic of the

class to which it belongs, with the exception of the shallowness of the

lagoon. The accompanying woodcut represents a vertical section,

supposed to be drawn at low water from the outer coast across one of

the low islets (one being taken of average dimensions) to within the

lagoon.

The section is true to the scale in a horizontal line, but it could not

be made so in a vertical one, as the average greatest height of the

land is only between six and twelve feet above high-water mark.

I will describe the section, commencing with the outer margin. I must

first observe that the reef-building polypifers, not being tidal

animals, require to be constantly submerged or washed by the breakers.

I was assured by Mr. Liesk, a very intelligent resident on these

islands, as well as by some chiefs at Tahiti (Otaheite), that an

exposure to the rays of the sun for a very short time invariably causes

their destruction. Hence it is possible only under the most favourable

circumstances, afforded by an unusually low tide and smooth water, to

reach the outer margin, where the coral is alive. I succeeded only

twice in gaining this part, and found it almost entirely composed of a

living Porites, which forms great irregularly rounded masses (like

those of an Astræa, but larger) from four to eight feet broad, and

little less in thickness. These mounds are separated from each other by

narrow crooked channels, about six feet deep, most of which intersect

the line of reef at right angles. On the furthest mound, which I was

able to reach by the aid of a leaping-pole, and over which the sea

broke with some violence, although the day was quite calm and the tide

low, the polypifers in the uppermost cells were all dead, but between

three and four inches lower down on its side they were living, and

formed a projecting border round the upper and dead surface. The coral

being thus checked in its upward growth, extends laterally, and hence

most of the masses, especially those a little further inwards, had

broad flat dead summits. On the other hand I could see, during the

recoil of the breakers, that a few yards further seaward, the whole

convex surface of the Porites was alive; so that the point where we

were standing was almost on the exact upward and shoreward limit of

existence of those corals which form the outer margin of the reef. We

shall presently see that there are other organic productions, fitted to

bear a somewhat longer exposure to the air and sun.

Next, but much inferior in importance to the Porites, is the _Millepora

complanata_.[1] It grows in thick vertical plates, intersecting each

other at various angles, and forms an exceedingly strong honeycombed

mass, which generally affects a circular form, the marginal plates

alone being alive. Between these plates and in the protected crevices

on the reef, a multitude of branching zoophytes and other productions

flourish, but the Porites and Millepora alone seem able to resist the

fury of the breakers on its upper and outer edge: at the depth of a few

fathoms other kinds of stony corals live. Mr. Liesk, who was intimately

acquainted with every part of this reef, and likewise with that of

North Keeling atoll, assured me that these corals invariably compose

the outer margin. The lagoon is inhabited by quite a distinct set of

corals, generally brittle and thinly branched; but a Porites,

apparently of the same species with that on the outside, is found

there, although it does not seem to thrive, and certainly does not

attain the thousandth part in bulk of the masses opposed to the

breakers.

[1] This Millepora (Palmipora of Blainville), as well as the _M.

alcicornis_, possesses the singular property of stinging the skin

where it is delicate, as on the face and arm.

The woodcut shows the form of the bottom off the reef: the water

deepens for a space between one and two hundred yards wide, very

gradually to twenty-five fathoms (_A_ in section), beyond which the

sides plunge into the unfathomable ocean at an angle of 45°.[2] To the

depth of ten or twelve fathoms the bottom is exceedingly rugged, and

seems formed of great masses of living coral, similar to those on the

margin. The arming of the lead here invariably came up quite clean, but

deeply indented, and chains and anchors which were lowered, in the

hopes of tearing up the coral, were broken. Many small fragments,

however, of _Millepora alcicornis_ were brought up; and on the arming

from an eight-fathom cast, there was a perfect impression of an Astræa,

apparently alive. I examined the rolled fragments cast on the beach

during gales, in order further to ascertain what corals grew outside

the reef. The fragments consisted of many kinds, of which the Porites

already mentioned and a Madrepora, apparently the _M. corymbosa_, were

the most abundant. As I searched in vain in the hollows on the reef and

in the lagoon, for a living specimen of this Madrepore, I conclude that

it is confined to a zone outside, and beneath the surface, where it

must be very abundant. Fragments of the _Millepora alcicornis_ and of

an Astræa were also numerous; the former is found, but not in

proportionate numbers, in the hollows on the reef; but the Astræa I did

not see living. Hence we may infer, that these are the kinds of coral

which form the rugged sloping surface (represented in the woodcut by an

uneven line), round and beneath the external margin. Between twelve and

twenty fathoms the arming came up an equal number of times smoothed

with sand, and indented with coral: an anchor and lead were lost at the

respective depths of thirteen and sixteen fathoms. Out of twenty-five

soundings taken at a greater depth than twenty fathoms, every one

showed that the bottom was covered with sand; whereas, at a less depth

than twelve fathoms, every sounding showed that it was exceedingly

rugged, and free from all extraneous particles. Two soundings were

obtained at the depth of 360 fathoms, and several between two hundred

and three hundred fathoms. The sand brought up from these depths

consisted of finely triturated fragments of stony zoophytes, but not,

as far as I could distinguish, of a particle of any lamelliform genus:

fragments of shells were rare.

[2] The soundings from which this section is laid down were taken with

great care by Captain Fitzroy himself. He used a bell-shaped lead,

having a diameter of four inches, and the armings each time were cut

off and brought on board for me to examine. The arming is a

preparation of tallow, placed in the concavity at the bottom of the

lead. Sand, and even small fragments of rock, will adhere to it; and

if the bottom be of rock it brings up an exact impression of its

surface.

At a distance of 2,200 yards from the breakers, Captain Fitzroy found

no bottom with a line of 7,200 feet in length; hence the submarine

slope of this coral formation is steeper than that of any volcanic

cone. Off the mouth of the lagoon, and likewise off the northern point

of the atoll, where the currents act violently, the inclination, owing

to the accumulation of sediment, is less. As the arming of the lead

from all the greater depths showed a smooth sandy bottom, I at first

concluded that the whole consisted of a vast conical pile of calcareous

sand, but the sudden increase of depth at some points, and the

circumstance of the line having been cut, as if rubbed, when between

five hundred and six hundred fathoms were out, indicate the probable

existence of submarine cliffs.

On the margin of the reef, close within the line where the upper

surface of the Porites and of the Millepora is dead, three species of

Nullipora flourish. One grows in thin sheets, like a lichen on old

trees; the second in stony knobs, as thick as a man’s finger, radiating

from a common centre; and the third, which is less common, in a

moss-like reticulation of thin, but perfectly rigid branches.[3] The

three species occur either separately or mingled together; and they

form by their successive growth a layer two or three feet in thickness,

which in some cases is hard, but where formed of the lichen-like kind,

readily yields an impression to the hammer: the surface is of a reddish

colour. These Nulliporæ, although able to exist above the limit of true

corals, seem to require to be bathed during the greater part of each

tide by breaking water, for they are not found in any abundance in the

protected hollows on the back part of the reef, where they might be

immersed either during the whole or an equal proportional time of each

tide. It is remarkable that organic productions of such extreme

simplicity, for the Nulliporæ undoubtedly belong to one of the lowest

classes of the vegetable kingdom, should be limited to a zone so

peculiarly circumstanced. Hence the layer composed by their growth

merely fringes the reef for a space of about twenty yards in width,

either under the form of separate mammillated projections, where the

outer masses of coral are separate, or, more commonly, where the corals

are united into a solid margin, as a continuous smooth convex mound

(_B_ in woodcut), like an artificial breakwater. Both the mound and

mammillated projections stand about three feet higher than any other

part of the reef, by which term I do not include the islets, formed by

the accumulation of rolled fragments. We shall hereafter see that other

coral reefs are protected by a similar thick growth of Nulliporæ on the

outer margin, the part most exposed to the breakers, and this must

effectually aid in preserving it from being worn down.

[3] This last species is of a beautiful bright peach-blossom colour.

Its branches are about as thick as crow-quills; they are slightly

flattened and knobbed at the extremities. The extremities only are

alive and brightly coloured. The two other species are of a dirty

purplish-white. The second species is extremely hard; its short

knob-like branches are cylindrical, and do not grow thicker at their

extremities.

The woodcut represents a section across one of the islets on the reef,

but if all that part which is above the level of _C_ were removed, the

section would be that of a simple reef, as it occurs where no islet has

been formed. It is this reef which essentially forms the atoll. It is a

ring, enclosing the lagoon on all sides except at the northern end,

where there are two open spaces, through one of which ships can enter.

The reef varies in width from two hundred and fifty to five hundred

yards, its surface is level, or very slightly inclined towards the

lagoon, and at high tide the sea breaks entirely over it: the water at

low tide thrown by the breakers on the reef, is carried by the many

narrow and shoal gullies or channels on its surface, into the lagoon: a

return stream sets out of the lagoon through the main entrance. The

most frequent coral in the hollows on the reef is _Pocillopora

verrucosa_, which grows in short sinuous plates, or branches, and when

alive is of a beautiful pale lake-red: a Madrepora, closely allied or

identical with _M. pocillifera_, is also common. As soon as an islet is

formed, and the waves are prevented breaking entirely over the reef,

the channels and hollows in it become filled up with cemented

fragments, and its surface is converted into a hard smooth floor (_C_

of woodcut), like an artificial one of freestone. This flat surface

varies in width from one hundred to two hundred, or even three hundred

yards, and is strewed with a few large fragments of coral torn up

during gales: it is uncovered only at low water. I could with

difficulty, and only by the aid of a chisel, procure chips of rock from

its surface, and therefore could not ascertain how much of it is formed

by the aggregation of detritus, and how much by the outward growth of

mounds of corals, similar to those now living on the margin. Nothing

can be more singular than the appearance at low tide of this “flat” of

naked stone, especially where it is externally bounded by the smooth

convex mound of Nulliporæ, appearing like a breakwater built to resist

the waves, which are constantly throwing over it sheets of foaming

water. The characteristic appearance of this “flat” is shown in the

foregoing woodcut of Whitsunday atoll.

The islets on the reef are first formed between two hundred and three

hundred yards from its outer edge, through the accumulation of a pile

of fragments, thrown together by some unusually strong gale. Their

ordinary width is under a quarter of a mile, and their length varies

from a few yards to several miles. Those on the south-east and windward

side of the atoll, increase solely by the addition of fragments on

their outer side; hence the loose blocks of coral, of which their

surface is composed, as well as the shells mingled with them, almost

exclusively consist of those kinds which live on the outer coast. The

highest part of the islets (excepting hillocks of blown sand, some of

which are thirty feet high), is close to the outer beach (_E_ of the

woodcut), and averages from six to ten feet above ordinary high-water

mark. From the outer beach the surface slopes gently to the shores of

the lagoon, which no doubt has been caused by the breakers the further

they have rolled over the reef, having had less power to throw up

fragments. The little waves of the lagoon heap up sand and fragments of

thinly-branched corals on the inner side of the islets on the leeward

side of the atoll; and these islets are broader than those to windward,

some being even eight hundred yards in width; but the land thus added

is very low. The fragments beneath the surface are cemented into a

solid mass, which is exposed as a ledge (_D_ of the woodcut),

projecting some yards in front of the outer shore and from two to four

feet high. This ledge is just reached by the waves at ordinary

high-water: it extends in front of all the islets, and everywhere has a

water-worn and scooped appearance. The fragments of coral which are

occasionally cast on the “flat” are during gales of unusual violence

swept together on the beach, where the waves each day at high-water

tend to remove and gradually wear them down; but the lower fragments

having become firmly cemented together by the percolation of calcareous

matter, resist the daily tides longer, and hence project as a ledge.

The cemented mass is generally of a white colour, but in some few parts

reddish from ferruginous matter; it is very hard, and is sonorous under

the hammer; it is obscurely divided by seams, dipping at a small angle

seaward; it consists of fragments of the corals which grow on the outer

margin, some quite and others partially rounded, some small and others

between two and three feet across; and of masses of previously formed

conglomerate, torn up, rounded, and re-cemented; or it consists of a

calcareous sandstone, entirely composed of rounded particles, generally

almost blended together, of shells, corals, the spines of echini, and

other such organic bodies; rocks, of this latter kind, occur on many

shores, where there are no coral reefs. The structure of the coral in

the conglomerate has generally been much obscured by the infiltration

of spathose calcareous matter; and I collected a very interesting

series, beginning with fragments of unaltered coral, and ending with

others, where it was impossible to discover with the naked eye any

trace of organic structure. In some specimens I was unable, even with

the aid of a lens, and by wetting them, to distinguish the boundaries

of the altered coral and spathose limestone. Many even of the blocks of

coral lying loose on the beach, had their central parts altered and

infiltrated.

The lagoon alone remains to be described; it is much shallower than

that of most atolls of considerable size. The southern part is almost

filled up with banks of mud and fields of coral, both dead and alive,

but there are considerable spaces, between three and four fathoms, and

smaller basins, from eight to ten fathoms deep. Probably about half its

area consists of sediment, and half of coral-reefs. The corals

composing these reefs have a very different aspect from those on the

outside; they are very numerous in kind, and most of them are thinly

branched. Meandrina, however, lives in the lagoon, and great rounded

masses of this coral are numerous, lying quite or almost loose on the

bottom. The other commonest kinds consist of three closely allied

species of true Madrepora in thin branches; of _Seriatapora subulata;_

two species of Porites[4] with cylindrical branches, one of which forms

circular clumps, with the exterior branches only alive; and lastly, a

coral something like an Explanaria, but with stars on both surfaces,

growing in thin, brittle, stony, foliaceous expansions, especially in

the deeper basins of the lagoon. The reefs on which these corals grow

are very irregular in form, are full of cavities, and have not a solid

flat surface of dead rock, like that surrounding the lagoon; nor can

they be nearly so hard, for the inhabitants made with crowbars a

channel of considerable length through these reefs, in which a

schooner, built on the S.E. islet, was floated out. It is a very

interesting circumstance, pointed out to us by Mr. Liesk, that this

channel, although made less than ten years before our visit, was then,

as we saw, almost choked up with living coral, so that fresh

excavations would be absolutely necessary to allow another vessel to

pass through it.

[4] This Porites has somewhat the habit of _P. clavaria_, but the

branches are not knobbed at their ends. When alive it is of a yellow

colour, but after having been washed in fresh water and placed to dry,

a jet-black slimy substance exuded from the entire surface, so that

the specimen now appears as if it had been dipped in ink.

The sediment from the deepest parts in the lagoon, when wet, appeared

chalky, but when dry, like very fine sand. Large soft banks of similar,

but even finer grained mud, occur on the S.E. shore of the lagoon,

affording a thick growth of a Fucus, on which turtle feed: this mud,

although discoloured by vegetable matter, appears from its entire

solution in acids to be purely calcareous. I have seen in the Museum of

the Geological Society, a similar but more remarkable substance,

brought by Lieutenant Nelson from the reefs of Bermuda, which, when

shown to several experienced geologists, was mistaken by them for true

chalk. On the outside of the reef much sediment must be formed by the

action of the surf on the rolled fragments of coral; but in the calm

waters of the lagoon, this can take place only in a small degree. There

are, however, other and unexpected agents at work here: large shoals of

two species of Scarus, one inhabiting the surf outside the reef and the

other the lagoon, subsist entirely, as I was assured by Mr. Liesk, the

intelligent resident before referred to, by browsing on the living

polypifers. I opened several of these fish, which are very numerous and

of considerable size, and I found their intestines distended by small

pieces of coral, and finely ground calcareous matter. This must daily

pass from them as the finest sediment; much also must be produced by

the infinitely numerous vermiform and molluscous animals, which make

cavities in almost every block of coral. Dr. J. Allan, of Forres, who

has enjoyed the best means of observation, informs me in a letter that

the Holothuriæ (a family of Radiata) subsist on living coral; and the

singular structure of bone within the anterior extremity of their

bodies, certainly appears well adapted for this purpose. The number of

the species of Holothuria, and of the individuals which swarm on every

part of these coral-reefs, is extraordinarily great; and many shiploads

are annually freighted, as is well-known, for China with the trepang,

which is a species of this genus. The amount of coral yearly consumed,

and ground down into the finest mud, by these several creatures, and

probably by many other kinds, must be immense. These facts are,

however, of more importance in another point of view, as showing us

that there are living checks to the growth of coral-reefs, and that the

almost universal law of “consumed and be consumed,” holds good even

with the polypifers forming those massive bulwarks, which are able to

withstand the force of the open ocean.

Considering that Keeling atoll, like other coral formations, has been

entirely formed by the growth of organic beings, and the accumulation

of their detritus, one is naturally led to inquire how long it has

continued, and how long it is likely to continue, in its present state.

Mr. Liesk informed me that he had seen an old chart in which the

present long island on the S.E. side was divided by several channels

into as many islets; and he assures me that the channels can still be

distinguished by the smaller size of the trees on them. On several

islets, also, I observed that only young cocoa-nut trees were growing

on the extremities; and that older and taller trees rose in regular

succession behind them; which shows that these islets have very lately

increased in length. In the upper and south-eastern part of the lagoon,

I was much surprised by finding an irregular field of at least a mile

square of branching corals, still upright, but entirely dead. They

consisted of the species already mentioned; they were of a brown

colour, and so rotten, that in trying to stand on them I sank halfway

up the leg, as if through decayed brushwood. The tops of the branches

were barely covered by water at the time of lowest tide. Several facts

having led me to disbelieve in any elevation of the whole atoll, I was

at first unable to imagine what cause could have killed so large a

field of coral. Upon reflection, however, it appeared to me that the

closing up of the above-mentioned channels would be a sufficient cause;

for before this, a strong breeze by forcing water through them into the

head of the lagoon, would tend to raise its level. But now this cannot

happen, and the inhabitants observe that the tide rises to a less

height, during a high S.E. wind, at the head than at the mouth of the

lagoon. The corals, which, under the former condition of things, had

attained the utmost possible limit of upward growth, would thus

occasionally be exposed for a short time to the sun, and be killed.

Besides the increase of dry land, indicated by the foregoing facts, the

exterior solid reef appears to have grown outwards. On the western side

of the atoll, the “flat” lying between the margin of the reef and the

beach, is very wide; and in front of the regular beach with its

conglomerate basis, there is, in most parts, a bed of sand and loose

fragments with trees growing out of it, which apparently is not reached

even by the spray at high water. It is evident some change has taken

place since the waves formed the inner beach; that they formerly beat

against it with violence was evident, from a remarkably thick and

water-worn point of conglomerate at one spot, now protected by

vegetation and a bank of sand; that they beat against it in the same

peculiar manner in which the swell from windward now obliquely curls

round the margin of the reef, was evident from the conglomerate having

been worn into a point projecting from the beach in a similarly oblique

manner. This retreat in the line of action of the breakers might

result, either from the surface of the reef in front of the islets

having been submerged at one time, and afterward having grown upwards,

or from the mounds of coral on the margin having continued to grow

outwards. That an outward growth of this part is in process, can hardly

be doubted from the fact already mentioned of the mounds of Porites

with their summits apparently lately killed, and their sides only three

or four inches lower down thickened by a fresh layer of living coral.

But there is a difficulty on this supposition which I must not pass

over. If the whole, or a large part of the “flat,” had been formed by

the outward growth of the margin, each successive margin would

naturally have been coated by the Nulliporæ, and so much of the surface

would have been of equal height with the existing zone of living

Nulliporæ: this is not the case, as may be seen in the woodcut. It is,

however, evident from the abraded state of the “flat,” with its

original inequalities filled up, that its surface has been much

modified; and it is possible that the hinder portions of the zone of

Nulliporæ, perishing as the reef grows outwards, might be worn down by

the surf. If this has not taken place, the reef can in no part have

increased outwards in breadth since its formation, or at least since

the Nulliporæ formed the convex mound on its margin; for the zone thus

formed, and which stands between two and three feet above the other

parts of the reef, is nowhere much above twenty yards in width.

Thus far we have considered facts, which indicate, with more or less

probability, the increase of the atoll in its different parts: there

are others having an opposite tendency. On the south-east side,

Lieutenant Sulivan, to whose kindness I am indebted for many

interesting observations, found the conglomerate projecting on the reef

nearly fifty yards in front of the beach: we may infer from what we see

in all other parts of the atoll, that the conglomerate was not

originally so much exposed, but formed the base of an islet, the front

and upper part of which has since been swept away. The degree to which

the conglomerate, round nearly the whole atoll, has been scooped,

broken up, and the fragments cast on the beach, is certainly very

surprising, even on the view that it is the office of occasional gales

to pile up fragments, and of the daily tides to wear them away. On the

western side, also, of the atoll, where I have described a bed of sand

and fragments with trees growing out of it, in front of an old beach,

it struck both Lieutenant Sulivan and myself, from the manner in which

the trees were being washed down, that the surf had lately recommenced

an attack on this line of coast. Appearances indicating a slight

encroachment of the water on the land, are plainer within the lagoon: I

noticed in several places, both on its windward and leeward shores, old

cocoa-nut trees falling with their roots undermined, and the rotten

stumps of others on the beach, where the inhabitants assured us the

cocoa-nut could not now grow. Captain Fitzroy pointed out to me, near

the settlement, the foundation posts of a shed, now washed by every

tide, but which the inhabitants stated, had seven years before stood

above high watermark. In the calm waters of the lagoon, directly

connected with a great, and therefore stable ocean, it seems very

improbable that a change in the currents, sufficiently great to cause

the water to eat into the land on all sides, should have taken place

within a limited period. From these considerations I inferred, that

probably the atoll had lately subsided to a small amount; and this

inference was strengthened by the circumstance, that in 1834, two years

before our visit, the island had been shaken by a severe earthquake,

and by two slighter ones during the ten previous years. If, during

these subterranean disturbances, the atoll did subside, the downward

movement must have been very small, as we must conclude from the fields

of dead coral still lipping the surface of the lagoon, and from the

breakers on the western shore not having yet regained the line of their

former action. The subsidence must, also, have been preceded by a long

period of rest, during which the islets extended to their present size,

and the living margin of the reef grew either upwards, or as I believe

outwards, to its present distance from the beach.

Whether this view be correct or not, the above facts are worthy of

attention, as showing how severe a struggle is in progress on these low

coral formations between the two nicely balanced powers of land and

water. With respect to the future state of Keeling atoll, if left

undisturbed, we can see that the islets may still extend in length; but

as they cannot resist the surf until broken by rolling over a wide

space, their increase in breadth must depend on the increasing breadth

of the reef; and this must be limited by the steepness of the submarine

flanks, which can be added to only by sediment derived from the wear

and tear of the coral. From the rapid growth of the coral in the

channel cut for the schooner, and from the several agents at work in

producing fine sediment, it might be thought that the lagoon would

necessarily become quickly filled up. Some of this sediment, however,

is transported into the open sea, as appears from the soundings off the

mouth of the lagoon, instead of being deposited within it. The

deposition, moreover, of sediment, checks the growth of coral-reefs, so

that these two agencies cannot act together with full effect in filling

it up. We know so little of the habits of the many different species of

corals, which form the lagoon-reefs, that we have no more reasons for

supposing that their whole surface would grow up as quickly as the

coral did in the schooner-channel, than for supposing that the whole

surface of a peat-moss would increase as quickly as parts are known to

do in holes, where the peat has been cut away. These agencies,

nevertheless, tend to fill up the lagoon; but in proportion as it

becomes shallower, so must the polypifers be subject to many injurious

agencies, such as impure water and loss of food. For instance, Mr.

Liesk informed me, that some years before our visit unusually heavy

rain killed nearly all the fish in the lagoon, and probably the same

cause would likewise injure the corals. The reefs also, it must be

remembered, cannot possibly rise above the level of the lowest

spring-tide, so that the final conversion of the lagoon into land must

be due to the accumulation of sediment; and in the midst of the clear

water of the ocean, and with no surrounding high land, this process

must be exceedingly slow.

SECTION II.—GENERAL DESCRIPTION OF ATOLLS.

General form and size of atolls, their reefs and islets.—External

slope.— Zone of Nulliporæ.—Conglomerate.—Depth of

lagoons.—Sediment.—Reefs submerged wholly or in part.—Breaches in the

reef.—Ledge-formed shores round certain lagoons.—Conversion of lagoons

into land.

I will here give a sketch of the general form and structure of the many

atolls and atoll-formed reefs which occur in the Pacific and Indian

Oceans, comparing them with Keeling atoll. The Maldiva atolls and the

Great Chagos Bank differ in so many respects, that I shall devote to

them, besides occasional references, a third section of this chapter.

Keeling atoll may be considered as of moderate dimensions and of

regular form. Of the thirty-two islands surveyed by Captain Beechey in

the Low Archipelago, the longest was found to be thirty miles, and the

shortest less than a mile; but Vliegen atoll, situated in another part

of the same group, appears to be sixty miles long and twenty broad.

Most of the atolls in this group are of an elongated form; thus Bow

Island is thirty miles in length, and on an average only six in width

(See Figure 4, Plate I.), and Clermont Tonnere has nearly the same

proportions. In the Marshall Archipelago (the Ralick and Radack group

of Kotzebue) several of the atolls are more than thirty miles in

length, and Rimsky Korsacoff is fifty-four long, and twenty wide, at

the broadest part of its irregular outline. Most of the atolls in the

Maldiva Archipelago are of great size, one of them (which, however,

bears a double name) measured in a medial and slightly curved line, is

no less than eighty-eight geographical miles long, its greatest width

being under twenty, and its least only nine and a half miles. Some

atolls have spurs projecting from them; and in the Marshall group there

are atolls united together by linear reefs, for instance Menchikoff

Island (See Figure 3, Plate II.), which is sixty miles in length, and

consists of three loops tied together. In far the greater number of

cases an atoll consists of a simple elongated ring, with its outline

moderately regular.

The average width of the annular wreath may be taken as about a quarter

of a mile. Captain Beechey[5] says that in the atolls of the Low

Archipelago it exceeded in no instance half a mile. The description

given of the structure and proportional dimensions of the reef and

islets of Keeling atoll, appears to apply perfectly to nearly all the

atolls in the Pacific and Indian Oceans. The islets are first formed

some way back either on the projecting points of the reef, especially

if its form be angular, or on the sides of the main entrances into the

lagoon—that is in both cases, on points where the breakers can act

during gales of wind in somewhat different directions, so that the

matter thrown up from one side may accumulate against that before

thrown up from another. In Lutké’s chart of the Caroline atolls, we see

many instances of the former case; and the occurrence of islets, as if

placed for beacons, on the points where there is a gateway or breach

through the reef, has been noticed by several authors. There are some

atoll-formed reefs, rising to the surface of the sea and partly dry at

low water, on which from some cause islets have never been formed; and

there are others on which they have been formed, but have subsequently

been worn away. In atolls of small dimensions the islets frequently

become united into a single horse-shoe or ring-formed strip; but Diego

Garcia, although an atoll of considerable size, being thirteen miles

and a half in length, has its lagoon entirely surrounded, except at the

northern end, by a belt of land, on an average a third of a mile in

width. To show how small the total area of the annular reef and the

land is in islands of this class, I may quote a remark from the voyage

of Lutké, namely, that if the forty-three rings, or atolls, in the

Caroline Archipelago, were put one within another, and over a steeple

in the centre of St. Petersburg, the whole world would not cover that

city and its suburbs.

[5] Beechey’s “Voyage to the Pacific and Beering’s Straits,” chapter

viii.

The form of the bottom off Keeling atoll, which gradually slopes to

about twenty fathoms at the distance of between one and two hundred

yards from the edge of the reef, and then plunges at an angle of 45°

into unfathomable depths, is exactly the same[6] with that of the

sections of the atolls in the Low Archipelago given by Captain Beechey.

The nature, however, of the bottom seems to differ, for this officer[7]

informs me that all the soundings, even the deepest, were on coral, but

he does not know whether dead or alive. The slope round Christmas atoll

(Lat. 1° 4′ N., 157° 45′ W.), described by Cook,[8] is considerably

less, at about half a mile from the edge of the reef, the average depth

was about fourteen fathoms on a fine sandy bottom, and at a mile, only

between twenty and forty fathoms. It has no doubt been owing to this

gentle slope, that the strip of land surrounding its lagoon, has

increased in one part to the extraordinary width of three miles; it is

formed of successive ridges of broken shells and corals, like those on

the beach. I know of no other instance of such width in the reef of an

atoll; but Mr. F.D. Bennett informs me that the inclination of the

bottom round Caroline atoll in the Pacific, is like that off Christmas

Island, very gentle. Off the Maldiva and Chagos atolls, the inclination

is much more abrupt; thus at Heawandoo Pholo, Lieutenant Powell[9]

found fifty and sixty fathoms close to the edge of the reef, and at 300

yards distance there was no bottom with a 300-yard line. Captain

Moresby informs me, that at 100 fathoms from the mouth of the lagoon of

Diego Garcia, he found no bottom with 150 fathoms; this is the more

remarkable, as the slope is generally less abrupt in front of channels

through a reef, owing to the accumulation of sediment. At Egmont

Island, also, at 150 fathoms from the reef, soundings were struck with

150 fathoms. Lastly, at Cardoo atoll, only sixty yards from the reef,

no bottom was obtained, as I am informed by Captain Moresby, with a

line of 200 fathoms! The currents run with great force round these

atolls, and where they are strongest, the inclination appears to be

most abrupt. I am informed by the same authority, that wherever

soundings were obtained off these islands, the bottom was invariably

sandy: nor was there any reason to suspect the existence of submarine

cliffs, as there was at Keeling Island.[10] Here then occurs a

difficulty; can sand accumulate on a slope, which, in some cases,

appears to exceed fifty-five degrees? It must be observed, that I speak

of slopes where soundings were obtained, and not of such cases, as that

of Cardoo, where the nature of the bottom is unknown, and where its

inclination must be nearly vertical. M. Élie de Beaumont[11] has

argued, and there is no higher authority on this subject, from the

inclination at which snow slides down in avalanches, that a bed of sand

or mud cannot be formed at a greater angle than thirty degrees.

Considering the number of soundings on sand, obtained round the Maldiva

and Chagos atolls, which appears to indicate a greater angle, and the

extreme abruptness of the sand-banks in the West Indies, as will be

mentioned in the Appendix, I must conclude that the adhesive property

of wet sand counteracts its gravity, in a much greater ratio than has

been allowed for by M. Élie de Beaumont. From the facility with which

calcareous sand becomes agglutinated, it is not necessary to suppose

that the bed of loose sand is thick.

[6] The form of the bottom round the Marshall atolls in the Northern

Pacific is probably similar: Kotzebue (“First Voyage,” volume ii.,

page 16.) says: “We had at a small distance from the reef, forty

fathoms depth, which increased a little further so much that we could

find no bottom.”)

[7] I must be permitted to express my obligation to Captain Beechey,

for the very kind manner in which he has given me information on

several points, and to own the great assistance I have derived from

his excellent published work.

[8] Cook’s “Third Voyage,” volume ii., chapter 10.

[9] This fact is taken from a MS. account of these groups lent me by

Captain Moresby. See also Captain Moresby’s paper on the Maldiva

atolls in the “Geographical Journal”, volume v., page 401.

[10] Off some of the islands in the Low Archipelago the bottom appears

to descend by ledges. Off Elizabeth Island, which, however, consists

of raised coral, Captain Beechey (p. 45, quarto edition) describes

three ledges: the first had an easy slope from the beach to a distance

of about fifty yards: the second extended two hundred yards with

twenty-five fathoms on it, and then ended abruptly, like the first;

and immediately beyond this there was no bottom with two hundred

fathoms.

[11] “Mémoires pour servir à une description Géolog. de France,” tome

iv., page 216.

Captain Beechey has observed, that the submarine slope is much less at

the extremities of the more elongated atolls in the Low Archipelago,

than at their sides; in speaking of Ducie’s Island he says[12] the

buttress, as it may be called, which “has the most powerful enemy (the

S.W. swell) to oppose, is carried out much further, and with less

abruptness than the other.” In some cases, the less inclination of a

certain part of the external slope, for instance of the northern

extremities of the two Keeling atolls, is caused by a prevailing

current which there accumulates a bed of sand. Where the water is

perfectly tranquil, as within a lagoon, the reefs generally grow up

perpendicularly, and sometimes even overhang their bases; on the other

hand, on the leeward side of Mauritius, where the water is generally

tranquil, although not invariably so, the reef is very gently inclined.

Hence it appears that the exterior angle varies much; nevertheless in

the close similarity in form between the sections of Keeling atoll and

of the atolls in the Low Archipelago, in the general steepness of the

reefs of the Maldiva and Chagos atolls, and in the perpendicularity of

those rising out of water always tranquil, we may discern the effects

of uniform laws; but from the complex action of the surf and currents,

on the growing powers of the coral and on the deposition of sediment,

we can by no means follow out all the results.

[12] Beechey’s “Voyage,” 4to edition, page 44.

Where islets have been formed on the reef, that part which I have

sometimes called the “flat” and which is partly dry at low water,

appears similar in every atoll. In the Marshall group in the North

Pacific, it may be inferred from Chamisso’s description, that the reef,

where islets have not been formed on it, slopes gently from the

external margin to the shores of the lagoon; Flinders states that the

Australian barrier has a similar inclination inwards, and I have no

doubt it is of general occurrence, although, according to Ehrenberg,

the reefs of the Red Sea offer an exception. Chamisso observes that

“the red colour of the reef (at the Marshall atolls) under the breakers

is caused by a Nullipora, which covers the stone _wherever the waves

beat;_ and, under favourable circumstances, assumes a stalactical

form,”—a description perfectly applicable to the margin of Keeling

atoll.[13] Although Chamisso does not state that the masses of

Nulliporæ form points or a mound, higher than the flat, yet I believe

that this is the case; for Kotzebue,[14] in another part, speaks of the

rocks on the edge of the reef “as visible for about two feet at low

water,” and these rocks we may feel quite certain are not formed of

true coral.[15] Whether a smooth convex mound of Nulliporæ, like that

which appears as if artificially constructed to protect the margin of

Keeling Island, is of frequent occurrence round atolls, I know not; but

we shall presently meet with it, under precisely the same form, on the

outer edge of the “barrier-reefs” which encircle the Society Islands.

[13] Kotzebue’s “First Voyage,” volume iii., page 142. Near Porto

Praya, in the Cape de Verde Islands, some basaltic rocks, lashed by no

inconsiderable surf, were completely enveloped with a layer of

Nulliporæ. The entire surface over many square inches, was coloured of

a peach-blossomed red; the layer, however, was of no greater thickness

than paper. Another kind, in the form of projecting knobs, grew in the

same situation. These Nulliporæ are closely related to those described

on the coral-reefs, but I believe are of different species.

[14] Kotzebue, “First Voyage,” volume ii., page 16. Lieutenant Nelson,

in his excellent memoir in the Geological Transactions (volume ii.,

page 105), alludes to the rocky points mentioned by Kotzebue, and

infers that they consist of Serpulæ, which compose incrusting masses

on the reefs of Bermudas, as they likewise do on a sandstone bar off

the coast of Brazil (which I have described in “London Phil. Journal,”

October 1841). These masses of Serpulæ hold the same position,

relatively to the action of the sea, with the Nulliporæ on the

coral-reefs in the Indian and Pacific Oceans.

[15] Captain Moresby, in his valuable paper “on the Northern atolls of

Maldivas” (“Geographical Journal”, volume v.), says that the edges of

the reefs there stand above water at low spring-tides.

There appears to be scarcely a feature in the structure of Keeling

reef, which is not of common, if not of universal occurrence, in other

atolls. Thus Chamisso describes[16] a layer of coarse conglomerate,

outside the islets round the Marshall atolls which “appears on its

upper surface uneven and eaten away.” From drawings, with appended

remarks, of Diego Garcia in the Chagos group and of several of the

Maldiva atolls, shown me by Captain Moresby,[17] it is evident that

their outer coasts are subject to the same round of decay and

renovation as those of Keeling atoll. From the description of the

atolls in the Low Archipelago, given in Captain Beechey’s “Voyage,” it

is not apparent that any conglomerate coral-rock was there observed.

[16] Kotzebue’s “First Voyage,” volume iii., page 144.

[17] See also Moresby on the Northern atolls of the Maldivas,

“Geographical Journal”, volume v., page 400.

The lagoon in Keeling atoll is shallow; in the atolls of the Low

Archipelago the depth varies from 20 to 38 fathoms, and in the Marshall

Group, according to Chamisso, from 30 to 35; in the Caroline atolls it

is only a little less. Within the Maldiva atolls there are large spaces

with 45 fathoms, and some soundings are laid down of 49 fathoms. The

greater part of the bottom in most lagoons, is formed of sediment;

large spaces have exactly the same depth, or the depth varies so

insensibly, that it is evident that no other means, excepting aqueous

deposition, could have leveled the surface so equally. In the Maldiva

atolls this is very conspicuous, and likewise in some of the Caroline

and Marshall Islands. In the former large spaces consist of sand and

_soft clay;_ and Kotzebue speaks of clay having been found within one

of the Marshall atolls. No doubt this clay is calcareous mud, similar

to that at Keeling Island, and to that at Bermuda already referred to,

as undistinguishable from disintegrated chalk, and which Lieutenant

Nelson says is called there pipe-clay.[18]

[18] I may here observe that on the coast of Brazil, where there is

much coral, the soundings near the land are described by Admiral

Roussin, in the _Pilote du Brésil_, as siliceous sand, mingled with

much finely comminuted particles of shells and coral. Further in the

offing, for a space of 1,300 miles along the coast, from the Abrolhos

Islands to Maranham, the bottom in many places is composed of “tuf

blanc, mêlé ou formé de madrépores broyés.” This white substance,

probably, is analogous to that which occurs within the above-mentioned

lagoons; it is sometimes, according to Roussin, firm, and he compares

it to mortar.

Where the waves act with unequal force on the two sides of an atoll,

the islets appear to be first formed, and are generally of greater

continuity on the more exposed shore. The islets, also, which are

placed to leeward, are in most parts of the Pacific liable to be

occasionally swept entirely away by gales, equalling hurricanes in

violence, which blow in an opposite direction to the ordinary

trade-wind. The absence of the islets on the leeward side of atolls, or

when present their lesser dimensions compared with those to windward,

is a comparatively unimportant fact; but in several instances the reef

itself on the leeward side, retaining its usual defined outline, does

not rise to the surface by several fathoms. This is the case with the

southern side of Peros Banhos (Plate I., Figure 9) in the Chagos group,

with Mourileu atoll[19] in the Caroline Archipelago, and with the

barrier-reef (Plate I., Figure 8) of the Gambier Islands. I allude to

the latter reef, although belonging to another class, because Captain

Beechey was first led by it to observe the peculiarity in the question.

At Peros Banhos the submerged part is nine miles in length, and lies at

an average depth of about five fathoms; its surface is nearly level,

and consists of hard stone, with a thin covering of loose sand. There

is scarcely any living coral on it, even on the outer margin, as I have

been particularly assured by Captain Moresby; it is, in fact, a wall of

dead coral-rock, having the same width and transverse section with the

reef in its ordinary state, of which it is a continuous portion. The

living and perfect parts terminate abruptly, and abut on the submerged

portions, in the same manner as on the sides of an ordinary passage

through the reef. The reef to leeward in other cases is nearly or quite

obliterated, and one side of the lagoon is left open; for instance, at

Oulleay (Caroline Archipelago), where a crescent-formed reef is fronted

by an irregular bank, on which the other half of the annular reef

probably once stood. At Namonouïto, in the same Archipelago, both these

modifications of the reef concur; it consists of a great flat bank,

with from twenty to twenty-five fathoms water on it; for a length of

more than forty miles on its southern side it is open and without any

reef, whilst on the other sides it is bounded by a reef, in parts

rising to the surface and perfectly characterised, in parts lying some

fathoms submerged. In the Chagos group there are annular reefs,

entirely submerged, which have the same structure as the submerged and

defined portions just described. The Speaker’s Bank offers an excellent

example of this structure; its central expanse, which is about

twenty-two fathoms deep, is twenty-four miles across; the external rim

is of the usual width of annular reefs, and is well-defined; it lies

between six and eight fathoms beneath the surface, and at the same

depth there are scattered knolls in the lagoon. Captain Moresby

believes the rim consists of dead rock, thinly covered with sand, and

he is certain this is the case with the external rim of the Great

Chagos Bank, which is also essentially a submerged atoll. In both these

cases, as in the submerged portion of the reef at Peros Banhos, Captain

Moresby feels sure that the quantity of living coral, even on the outer

edge overhanging the deep-sea water, is quite insignificant. Lastly, in

several parts of the Pacific and Indian Oceans there are banks, lying

at greater depths than in the cases just mentioned, of the same form

and size with the neighbouring atolls, but with their atoll-like

structure wholly obliterated. It appears from the survey of Freycinet,

that there are banks of this kind in the Caroline Archipelago, and, as

is reported, in the Low Archipelago. When we discuss the origin of the

different classes of coral formations, we shall see that the submerged

state of the whole of some atoll-formed reefs, and of portions of

others, generally but not invariably on the leeward side, and the

existence of more deeply submerged banks now possessing little or no

signs of their original atoll-like structure, are probably the effects

of a uniform cause,—namely, the death of the coral, during the

subsidence of the area, in which the atolls or banks are situated.

[19] Frederick Lutké’s “Voyage autour du Monde,” volume ii., page 291.

See also his account of Namonouïto, below, and the chart of Oulleay in

the Atlas.

There is seldom, with the exception of the Maldiva atolls, more than

two or three channels, and generally only one leading into the lagoon,

of sufficient depth for a ship to enter. in small atolls, there is

usually not even one. Where there is deep water, for instance above

twenty fathoms, in the middle of the lagoon, the channels through the

reef are seldom as deep as the centre,—it may be said that the rim only

of the saucer-shaped hollow forming the lagoon is notched. Mr.

Lyell[20] has observed that the growth of the coral would tend to

obstruct all the channels through a reef, except those kept open by

discharging the water, which during high tide and the greater part of

each ebb is thrown over its circumference. Several facts indicate that

a considerable quantity of sediment is likewise discharged through

these channels; and Captain Moresby informs me that he has observed,

during the change of the monsoon, the sea discoloured to a distance off

the entrances into the Maldiva and Chagos atolls. This, probably, would

check the growth of the coral in them, far more effectually than a mere

current of water. In the many small atolls without any channel, these

causes have not prevented the entire ring attaining the surface. The

channels, like the submerged and effaced parts of the reef, very

generally though not invariably occur on the leeward side of the atoll,

or on that side, according to Beechey,[21] which, from running in the

same direction with the prevalent wind, is not fully exposed to it.

Passages between the islets on the reef, through which boats can pass

at high water, must not be confounded with ship-channels, by which the

annular reef itself is breached. The passages between the islets occur,

of course, on the windward as well as on the leeward side; but they are

more frequent and broader to leeward, owing to the lesser dimensions of

the islets on that side.

[20] “Principles of Geology,” volume iii., page 289.

[21] Beechey’s “Voyage,” 4to edition, volume i., page 189.

At Keeling atoll the shores of the lagoon shelve gradually, where the

bottom is of sediment, and irregularly or abruptly where there are

coral-reefs; but this is by no means the universal structure in other

atolls. Chamisso,[22] speaking in general terms of the lagoons in the

Marshall atolls, says the lead generally sinks “from a depth of two or

three fathoms to twenty or twenty-four, and you may pursue a line in

which on one side of the boat you may see the bottom, and on the other

the azure-blue deep water.” The shores of the lagoon-like channel

within the barrier-reef at Vanikoro have a similar structure. Captain

Beechey has described a modification of this structure (and he believes

it is not uncommon) in two atolls in the Low Archipelago, in which the

shores of the lagoon descend by a few, broad, slightly inclined ledges

or steps: thus at Matilda atoll,[23] the great exterior reef, the

surface of which is gently inclined towards and beneath the surface of

the lagoon, ends abruptly in a little cliff three fathoms deep; at its

foot, a ledge forty yards wide extends, shelving gently inwards like

the surface-reef, and terminated by a second little cliff five fathoms

deep; beyond this, the bottom of the lagoon slopes to twenty fathoms,

which is the average depth of its centre. These ledges seem to be

formed of coral-rock; and Captain Beechey says that the lead often

descended several fathoms through holes in them. In some atolls, all

the coral reefs or knolls in the lagoon come to the surface at low

water; in other cases of rarer occurrence, all lie at nearly the same

depth beneath it, but most frequently they are quite irregular,—some

with perpendicular, some with sloping sides,—some rising to the

surface, and others lying at all intermediate depths from the bottom

upwards. I cannot, therefore, suppose that the union of such reefs

could produce even one uniformly sloping ledge, and much less two or

three, one beneath the other, and each terminated by an abrupt wall. At

Matilda Island, which offers the best example of the step-like

structure, Captain Beechey observes that the coral-knolls within the

lagoon are quite irregular in their height. We shall hereafter see that

the theory which accounts for the ordinary form of atolls, apparently

includes this occasional peculiarity in their structure.

[22] Kotzebue’s “First Voyage,” volume iii., page 142.

[23] Beechey’s “Voyage,” 4to edition, volume i, page 160. At

Whitsunday Island the bottom of the lagoon slopes gradually towards

the centre, and then deepens suddenly, the edge of the bank being

nearly perpendicular. This bank is formed of coral and dead shells.

In the midst of a group of atolls, there sometimes occur small, flat,

very low islands of coral formation, which probably once included a

lagoon, since filled up with sediment and coral-reefs. Captain Beechey

entertains no doubt that this has been the case with the two small

islands, which alone of thirty-one surveyed by him in the Low

Archipelago, did not contain lagoons. Romanzoff Island (in lat. 15° S.)

is described by Chamisso[24]

as formed by a dam of madreporitic rock inclosing a flat space, thinly

covered with trees, into which the sea on the leeward side occasionally

breaks. North Keeling atoll appears to be in a rather less forward

stage of conversion into land; it consists of a horse-shoe shaped strip

of land surrounding a muddy flat, one mile in its longest axis, which

is covered by the sea only at high water. When describing South Keeling

atoll, I endeavoured to show how slow the final process of filling up a

lagoon must be; nevertheless, as all causes do tend to produce this

effect, it is very remarkable that not one instance, as I believe, is

known of a moderately sized lagoon being filled up even to the low

water-line at spring-tides, much less of such a one being converted

into land. It is, likewise, in some degree remarkable, how few atolls,

except small ones, are surrounded by a single linear strip of land,

formed by the union of separate islets. We cannot suppose that the many

atolls in the Pacific and Indian Oceans all have had a late origin, and

yet should they remain at their present level, subjected only to the

action of the sea and to the growing powers of the coral, during as

many centuries as must have elapsed since any of the earlier tertiary

epochs, it cannot, I think, be doubted that their lagoons and the

islets on their reef, would present a totally different appearance from

what they now do. This consideration leads to the suspicion that some

renovating agency (namely subsidence) comes into play at intervals, and

perpetuates their original structure.

[24] Kotzebue’s “First Voyage,” volume iii., page 221.

SECTION III.—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK.

Maldiva Archipelago.—Ring-formed reefs, marginal and central.—Great

depths in the lagoons of the southern atolls.—Reefs in the lagoons all

rising to the surface.—Position of islets and breaches in the reefs,

with respect to the prevalent winds and action of the

waves.—Destruction of islets.—Connection in the position and submarine

foundation of distinct atolls.—The apparent disseverment of large

atolls.—The Great Chagos Bank.—Its submerged condition and

extraordinary structure.

Although occasional references have been made to the Maldiva atolls,

and to the banks in the Chagos group, some points of their structure

deserve further consideration. My description is derived from an

examination of the admirable charts lately published from the survey of

Captain Moresby and Lieutenant Powell, and more especially from

information which Captain Moresby has communicated to me in the kindest

manner.

The Maldiva Archipelago is 470 miles in length, with an average breadth

of about 50 miles. The form and dimensions of the atolls, and their

singular position in a double line, may be seen, but not well, in the

greatly reduced chart (Figure 6) in Plate II. The dimensions of the

longest atoll in the group (called by the double name of

Milla-dou-Madou and Tilla-dou-Matte) have already been given; it is 88

miles in a medial and slightly curved line, and is less than 20 miles

in its broadest part. Suadiva, also, is a noble atoll, being 44 miles

across in one direction, and 34 in another, and the great included

expanse of water has a depth of between 250 and 300 feet. The smaller

atolls in this group differ in no respect from ordinary ones; but the

larger ones are remarkable from being breached by numerous deep-water

channels leading into the lagoon; for instance, there are 42 channels,

through which a ship could enter the lagoon of Suadiva. In the three

southern large atolls, the separate portions of reef between these

channels have the ordinary structure, and are linear; but in the other

atolls, especially the more northern ones, these portions are ring-

formed, like miniature atolls. Other ring-formed reefs rise out of the

lagoons, in the place of those irregular ones which ordinarily occur

there. In the reduction of the chart of Mahlos Mahdoo (Plate II.,

Figure 4), it was not found easy to define the islets and the little

lagoons within each reef, so that the ring-formed structure is very

imperfectly shown; in the large published charts of Tilla-dou-Matte,

the appearance of these rings, from standing further apart from each

other, is very remarkable. The rings on the margin are generally

elongated; many of them are three, and some even five miles, in

diameter; those within the lagoon are usually smaller, few being more

than two miles across, and the greater number rather less than one. The

depth of the little lagoon within these small annular reefs is

generally from five to seven fathoms, but occasionally more; and in Ari

atoll many of the central ones are twelve, and some even more than

twelve fathoms deep. These rings rise abruptly from the platform or

bank, on which they are placed; their outer margin is invariably

bordered by living coral[25] within which there is a flat surface of

coral rock; of this flat, sand and fragments have in many cases

accumulated and been converted into islets, clothed with vegetation. I

can, in fact, point out no essential difference between these little

ring-formed reefs (which, however, are larger, and contain deeper

lagoons than many true atolls that stand in the open sea), and the most

perfectly characterised atolls, excepting that the ring-formed reefs

are based on a shallow foundation, instead of on the floor of the open

sea, and that instead of being scattered irregularly, they are grouped

closely together on one large platform, with the marginal rings

arranged in a rudely formed circle.

[25] Captain Moresby informs me that _Millepora complanata_ is one of

the commonest kinds on the outer margin, as it is at Keeling atoll.

The perfect series which can be traced from portions of simple linear

reef, to others including long linear lagoons, and from these again to

oval or almost circular rings, renders it probable that the latter are

merely modifications of the linear or normal state. It is conformable

with this view, that the ring-formed reefs on the margin, even where

most perfect and standing furthest apart, generally have their longest

axes directed in the line which the reef would have held, if the atoll

had been bounded by an ordinary wall. We may also infer that the

central ring-formed reefs are modifications of those irregular ones,

which are found in the lagoons of all common atolls. It appears from

the charts on a large scale, that the ring-like structure is contingent

on the marginal channels or breaches being wide; and, consequently, on

the whole interior of the atoll being freely exposed to the waters of

the open sea. When the channels are narrow or few in number, although

the lagoon be of great size and depth (as in Suadiva), there are no

ring-formed reefs; where the channels are somewhat broader, the

marginal portions of reef, and especially those close to the larger

channels, are ring-formed, but the central ones are not so; where they

are broadest, almost every reef throughout the atoll is more or less

perfectly ring-formed. Although their presence is thus contingent on

the openness of the marginal channels, the theory of their formation,

as we shall hereafter see, is included in that of the parent atolls, of

which they form the separate portions.

The lagoons of all the atolls in the southern part of the Archipelago

are from ten to twenty fathoms deeper than those in the northern part.

This is well exemplified in the case of Addoo, the southernmost atoll

in the group, for although only nine miles in its longest diameter, it

has a depth of thirty-nine fathoms, whereas all the other small atolls

have comparatively shallow lagoons; I can assign no adequate cause for

this difference in depth. In the central and deepest part of the

lagoons, the bottom consists, as I am informed by Captain Moresby, of

stiff clay (probably a calcareous mud); nearer the border it consists

of sand, and in the channels through the reef, of hard sand-banks,

sandstone, conglomerate rubble, and a little live coral. Close outside

the reef and the line joining its detached portions (where intersected

by many channels), the bottom is sandy, and it slopes abruptly into

unfathomable depths. In most lagoons the depth is considerably greater

in the centre than in the channels; but in Tilla-dou-Matte, where the

marginal ring-formed reefs stand far apart, the same depth is carried

across the entire atoll, from the deep-water line on one side to that

on the other. I cannot refrain from once again remarking on the

singularity of these atolls,—a great sandy and generally concave disc

rises abruptly from the unfathomable ocean, with its central expanse

studded and its border symmetrically fringed with oval basins of

coral-rock, just lipping the surface of the sea, sometimes clothed with

vegetation, and each containing a little lake of clear water!

In the southern Maldiva atolls, of which there are nine large ones, all

the small reefs within the lagoons come to the surface, and are dry at

low water spring-tides; hence in navigating them, there is no danger

from submarine banks. This circumstance is very remarkable, as within

some atolls, for instance those of the neighbouring Chagos group, not a

single reef comes to the surface, and in most other cases a few only

do, and the rest lie at all intermediate depths from the bottom

upwards. When treating of the growth of coral I shall again refer to

this subject.

Although in the neighbourhood of the Maldiva Archipelago the winds,

during the monsoons, blow during nearly an equal time from opposite

quarters, and although, as I am informed by Captain Moresby, the

westerly winds are the strongest, yet the islets are almost all placed

on the eastern side of the northern atolls, and on the south-eastern

side of the southern atolls. That the formation of the islets is due to

detritus thrown up from the outside, as in the ordinary manner, and not

from the interior of the lagoons, may, I think be safely inferred from

several considerations, which it is hardly worth while to detail. As

the easterly winds are not the strongest, their action probably is

aided by some prevailing swell or current.

In groups of atolls, exposed to a trade-wind, the ship-channels into

the lagoons are almost invariably situated on the leeward or less

exposed side of the reef, and the reef itself is sometimes either

wanting there, or is submerged. A strictly analogous, but different

fact, may be observed at the Maldiva atolls—namely, that where two

atolls stand in front of each other, the breaches in the reef are the

most numerous on their near, and therefore less exposed, sides. Thus on

the near sides of Ari and the two Nillandoo atolls, which face S. Māle,

Phaleedoo, and Moloque atolls, there are seventy-three deep-water

channels, and only twenty-five on their outer sides; on the near side

of the three latter named atolls there are fifty- six openings, and

only thirty-seven on their outsides. It is scarcely possible to

attribute this difference to any other cause than the somewhat

different action of the sea on the two sides, which would ensue from

the protection afforded by the two rows of atolls to each other. I may

here remark that in most cases, the conditions favourable to the

greater accumulation of fragments on the reef and to its more perfect

continuity on one side of the atoll than on the other, have concurred,

but this has not been the case with the Maldivas; for we have seen that

the islets are placed on the eastern or south-eastern sides, whilst the

breaches in the reef occur indifferently on any side, where protected

by an opposite atoll. The reef being more continuous on the outer and

more exposed sides of those atolls which stand near each other, accords

with the fact, that the reef of the southern atolls is more continuous

than that of the northern ones; for the former, as I am informed by

Captain Moresby, are more constantly exposed than the northern atolls

to a heavy surf.

The date of the first formation of some of the islets in this

Archipelago is known to the inhabitants; on the other hand, several

islets, and even some of those which are believed to be very old, are

now fast wearing away. The work of destruction has, in some instances,

been completed in ten years. Captain Moresby found on one water-washed

reef the marks of wells and graves, which were excavated when it

supported an islet. In South Nillandoo atoll, the natives say that

three of the islets were formerly larger: in North Nillandoo there is

one now being washed away; and in this latter atoll Lieutenant Prentice

found a reef, about six hundred yards in diameter, which the natives

positively affirmed was lately an island covered with cocoa-nut trees.

It is now only partially dry at low water spring-tides, and is (in

Lieutenant Prentice’s words) “entirely covered with live coral and

madrepore.” In the northern part, also, of the Maldiva Archipelago and

in the Chagos group, it is known that some of the islets are

disappearing. The natives attribute these effects to variations in the

currents of the sea. For my own part I cannot avoid suspecting that

there must be some further cause, which gives rise to such a cycle of

change in the action of the currents of the great and open ocean.

Several of the atolls in this Archipelago are so related to each other

in form and position, that at the first glance one is led to suspect

that they have originated in the disseverment of a single one. Māle

consists of three perfectly characterised atolls, of which the shape

and relative position are such, that a line drawn closely round all

three, gives a symmetrical figure; to see this clearly, a larger chart

is required than that of the Archipelago in Plate II.; the channel

separating the two northern Māle atolls is only little more than a mile

wide, and no bottom was found in it with 100 fathoms. Powell’s Island

is situated at the distance of two miles and a half off the northern

end of Mahlos Mahdoo (see Figure 4, Plate II.), at the exact point

where the two sides of the latter, if prolonged, would meet; no bottom,

however, was found in the channel with 200 fathoms; in the wider

channel between Horsburgh atoll and the southern end of Mahlos Mahdoo,

no bottom was found with 250 fathoms. In these and similar cases, the

relation consists only in the form and position of the atolls. But in

the channel between the two Nillandoo atolls, although three miles and

a quarter wide, soundings were struck at the depth of 200 fathoms; the

channel between Ross and Ari atolls is four miles wide, and only 150

fathoms deep. Here then we have, besides the relation of form, a

submarine connection. The fact of soundings having been obtained

between two separate and perfectly characterised atolls is in itself

interesting, as it has never, I believe, been effected in any of the

many other groups of atolls in the Pacific and Indian seas. In

continuing to trace the connection of adjoining atolls, if a hasty

glance be taken at the chart (Figure 4., Plate II.) of Mahlos Mahdoo,

and the line of unfathomable water be followed, no one will hesitate to

consider it as one atoll. But a second look will show that it is

divided by a bifurcating channel, of which the northern arm is about

one mile and three-quarters in width, with an average depth of 125

fathoms, and the southern one three-quarters of a mile wide, and rather

less deep. These channels resemble in the slope of their sides and

general form, those which separate atolls in every respect distinct;

and the northern arm is wider than that dividing two of the Māle

atolls. The ring-formed reefs on the sides of this bifurcating channel

are elongated, so that the northern and southern portions of Mahlos

Mahdoo may claim, as far as their external outline is concerned, to be

considered as distinct and perfect atolls. But the intermediate

portion, lying in the fork of the channel, is bordered by reefs less

perfect than those which surround any other atoll in the group of

equally small dimensions. Mahlos Mahdoo, therefore, is in every respect

in so intermediate a condition, that it may be considered either as a

single atoll nearly dissevered into three portions, or as three atolls

almost perfect and intimately connected. This is an instance of a very

early stage of the apparent disseverment of an atoll, but a still

earlier one in many respects is exhibited at Tilla-dou- Matte. In one

part of this atoll, the ring-formed reefs stand so far apart from each

other, that the inhabitants have given different names to the northern

and southern halves; nearly all the rings, moreover, are so perfect and

stand so separate, and the space from which they rise is so level and

unlike a true lagoon, that we can easily imagine the conversion of this

one great atoll, not into two or three portions, but into a whole group

of miniature atolls. A perfect series such as we have here traced,

impresses the mind with an idea of actual change; and it will hereafter

be seen, that the theory of subsidence, with the upward growth of the

coral, modified by accidents of probable occurrence, will account for

the occasional disseverment of large atolls.

The Great Chagos bank alone remains to be described. In the Chagos

group there are some ordinary atolls, some annular reefs rising to the

surface but without any islets on them, and some atoll-formed banks,

either quite submerged, or nearly so. Of the latter, the Great Chagos

Bank is much the largest, and differs in its structure from the others:

a plan of it is given in Plate II., Figure 1, in which, for the sake of

clearness, I have had the parts under ten fathoms deep finely shaded:

an east and west vertical section is given in Figure 2, in which the

vertical scale has been necessarily exaggerated. Its longest axis is

ninety nautical miles, and another line drawn at right angles to the

first, across the broadest part, is seventy. The central part consists

of a level muddy flat, between forty and fifty fathoms deep, which is

surrounded on all sides, with the exception of some breaches, by the

steep edges of a set of banks, rudely arranged in a circle. These banks

consist of sand, with a very little live coral; they vary in breadth

from five to twelve miles, and on an average lie about sixteen fathoms

beneath the surface; they are bordered by the steep edges of a third

narrow and upper bank, which forms the rim to the whole. This rim is

about a mile in width, and with the exception of two or three spots

where islets have been formed, is submerged between five and ten

fathoms. It consists of smooth hard rock, covered with a thin layer of

sand, but with scarcely any live coral; it is steep on both sides, and

outwards slopes abruptly into unfathomable depths. At the distance of

less than half a mile from one part, no bottom was found with 190

fathoms; and off another point, at a somewhat greater distance, there

was none with 210 fathoms. Small steep-sided banks or knolls, covered

with luxuriantly growing coral, rise from the interior expanse to the

same level with the external rim, which, as we have seen, is formed

only of dead rock. It is impossible to look at the plan (Figure 1,

Plate II.), although reduced to so small a scale, without at once

perceiving that the Great Chagos Bank is, in the words of Captain

Moresby,[26] “nothing more than a half-drowned atoll.” But of what

great dimensions, and of how extraordinary an internal structure? We

shall hereafter have to consider both the cause of its submerged

condition, a state common to other banks in the group, and the origin

of the singular submarine terraces, which bound the central expanse:

these, I think, it can be shown, have resulted from a cause analogous

to that which has produced the bifurcating channel across Mahlos

Mahdoo.

[26] This officer has had the kindness to lend me an excellent MS.

account of the Chagos Islands; from this paper, from the published

charts, and from verbal information communicated to me by Captain

Moresby, the above account of the Great Chagos Bank is taken.

CHAPTER II.

BARRIER REEFS.

Closely resemble in general form and structure atoll-reefs.—Width and

depth of the lagoon-channels.—Breaches through the reef in front of

valleys, and generally on the leeward side.—Checks to the filling up of

the lagoon-channels.—Size and constitution of the encircled islands.—

Number of islands within the same reef.—Barrier-reefs of New Caledonia

and Australia.—Position of the reef relative to the slope of the

adjoining land.—Probable great thickness of barrier-reefs.

The term “barrier” has been generally applied to that vast reef which

fronts the N.E. shore of Australia, and by most voyagers likewise to

that on the western coast of New Caledonia. At one time I thought it

convenient thus to restrict the term, but as these reefs are similar in

structure, and in position relatively to the land, to those, which,

like a wall with a deep moat within, encircle many smaller islands, I

have classed them together. The reef, also, on the west coast of New

Caledonia, circling round the extremities of the island, is an

intermediate form between a small encircling reef and the Australian

barrier, which stretches for a thousand miles in nearly a straight

line.

The geographer Balbi has in effect described those barrier-reefs, which

encircle moderately sized islands, by calling them atolls with high

land rising from within their central expanse. The general resemblance

between the reefs of the barrier and atoll classes may be seen in the

small, but accurately reduced charts on Plate I.,[1] and this

resemblance can be further shown to extend to every part of the

structure. Beginning with the outside of the reef; many scattered

soundings off Gambier, Oualan, and some other encircled islands, show

that close to the breakers there exists a narrow shelving margin,

beyond which the ocean becomes suddenly unfathomable; but off the west

coast of New Caledonia, Captain Kent[2] found no bottom with 150

fathoms, at two ships’ length from the reef; so that the slope here

must be nearly as precipitous as off the Maldiva atolls.

[1] The authorities from which these charts have been reduced,

together with some remarks on them and descriptive of the Plates, are

given separately.

[2] Dalrymple, “Hydrog. Mem.” volume iii.

I can give little information regarding the kinds of corals which live

on the outer margin. When I visited the reef at Tahiti, although it was

low water, the surf was too violent for me to see the living masses;

but, according to what I heard from some intelligent native chiefs,

they resemble in their rounded and branchless forms, those on the

margin of Keeling atoll. The extreme verge of the reef, which was

visible between the breaking waves at low water, consisted of a

rounded, convex, artificial-like breakwater, entirely coated with

Nulliporæ, and absolutely similar to that which I have described at

Keeling atoll. From what I heard when at Tahiti, and from the writings

of the Revs. W. Ellis and J. Williams, I conclude that this peculiar

structure is common to most of the encircled islands of the Society

Archipelago. The reef within this mound or breakwater, has an extremely

irregular surface, even more so than between the islets on the reef of

Keeling atoll, with which alone (as there are no islets on the reef of

Tahiti) it can properly be compared. At Tahiti, the reef is very

irregular in width; but round many other encircled islands, for

instance, Vanikoro or Gambier Islands (Figures 1 and 8, Plate I.), it

is quite as regular, and of the same average width, as in true atolls.

Most barrier-reefs on the inner side slope irregularly into the

lagoon-channel (as the space of deep water separating the reef from the

included land may be called), but at Vanikoro the reef slopes only for

a short distance, and then terminates abruptly in a submarine wall,

forty feet high,—a structure absolutely similar to that described by

Chamisso in the Marshall atolls.

In the Society Archipelago, Ellis[3] states, that the reefs generally

lie at the distance of from one to one and a half miles, and,

occasionally, even at more than three miles, from the shore. The

central mountains are generally bordered by a fringe of flat, and often

marshy, alluvial land, from one to four miles in width. This fringe

consists of coral-sand and detritus thrown up from the lagoon-channel,

and of soil washed down from the hills; it is an encroachment on the

channel, analogous to that low and inner part of the islets in many

atolls which is formed by the accumulation of matter from the lagoon.

At Hogoleu (Figure 2, Plate I.), in the Caroline Archipelago,[4] the

reef on the south side is no less than twenty miles; on the east side,

five; and on the north side, fourteen miles from the encircled high

islands.

[3] Consult, on this and other points, the “Polynesian Researches,” by

the Rev. W. Ellis, an admirable work, full of curious information.

[4] See “Hydrographical Mem.” and the “Atlas of the Voyage of the

_Astrolabe_,” by Captain Dumont D’Urville, page 428.

The lagoon channels may be compared in every respect with true lagoons.

In some cases they are open, with a level bottom of fine sand; in

others they are choked up with reefs of delicately branched corals,

which have the same general character as those within the Keeling

atoll. These internal reefs either stand separately, or more commonly

skirt the shores of the included high islands. The depth of the

lagoon-channel round the Society Islands varies from two or three to

thirty fathoms; in Cook’s[5] chart of Ulieta, however, there is one

sounding laid down of forty-eight fathoms; at Vanikoro there are

several of fifty-four and one of fifty-six and a half fathoms

(English), a depth which even exceeds by a little that of the interior

of the great Maldiva atolls. Some barrier-reefs have very few islets on

them; whilst others are surmounted by numerous ones; and those round

part of Bolabola (Plate I., Figure 5) form a single linear strip. The

islets first appear either on the angles of the reef, or on the sides

of the breaches through it, and are generally most numerous on the

windward side. The reef to leeward retaining its usual width, sometimes

lies submerged several fathoms beneath the surface; I have already

mentioned Gambier Island as an instance of this structure. Submerged

reefs, having a less defined outline, dead, and covered with sand, have

been observed (see Appendix) off some parts of Huaheine and Tahiti. The

reef is more frequently breached to leeward than to windward; thus I

find in Krusenstern’s “Memoir on the Pacific,” that there are passages

through the encircling reef on the leeward side of each of the seven

Society Islands, which possess ship-harbours; but that there are

openings to windward through the reef of only three of them. The

breaches in the reef are seldom as deep as the interior lagoon-like

channel; they generally occur in front of the main valleys, a

circumstance which can be accounted for, as will be seen in the fourth

chapter, without much difficulty. The breaches being situated in front

of the valleys, which descend indifferently on all sides, explains

their more frequent occurrence through the windward side of

barrier-reefs than through the windward side of atolls,—for in atolls

there is no included land to influence the position of the breaches.

[5] See the chart in volume i. of Hawkesworth’s 4to edition of “Cook’s

First Voyage.”