diff options
Diffstat (limited to '2690-0.txt')
| -rw-r--r-- | 2690-0.txt | 9389 |
1 files changed, 9389 insertions, 0 deletions
diff --git a/2690-0.txt b/2690-0.txt new file mode 100644 index 0000000..6966f83 --- /dev/null +++ b/2690-0.txt @@ -0,0 +1,9389 @@ +The Project Gutenberg eBook of The Structure and Distribution of Coral Reefs, by Charles Darwin + +This eBook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online at +www.gutenberg.org. If you are not located in the United States, you +will have to check the laws of the country where you are located before +using this eBook. + +Title: The Structure and Distribution of Coral Reefs + +Author: Charles Darwin + +Release Date: June, 2000 [eBook #2690] +[Most recently updated: October 10, 2022] + +Language: English + +Character set encoding: UTF-8 + +Produced by: Sue Asscher and David Widger + +*** START OF THE PROJECT GUTENBERG EBOOK CORAL REEFS *** + + + + +CORAL REEFS + + +By Charles Darwin + + + + +CONTENTS + +DETAILED TABLE OF CONTENTS + +DESCRIPTION OF THE PLATES + +THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS + +INTRODUCTION + +CHAPTER I.—ATOLLS OR LAGOON-ISLANDS + +CHAPTER II.—BARRIER REEFS + +CHAPTER III.—FRINGING OR SHORE-REEFS + +CHAPTER IV.—ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS + +CHAPTER V.—THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF +CORAL-REEFS + +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.” + + +It is remarkable, that the lagoon-channels round mountainous islands +have not in every instance been long ago filled up with coral and +sediment; but it is more easily accounted for than appears at first +sight. In cases like that of Hogoleu and the Gambier Islands, where a +few small peaks rise out of a great lagoon, the conditions scarcely +differ from those of an atoll, and I have already shown, at some +length, that the filling up of a true lagoon must be an extremely slow +process. Where the channel is narrow, the agency, which on unprotected +coasts is most productive of sediment, namely the force of the +breakers, is here entirely excluded, and the reef being breached in the +front of the main valleys, much of the finer mud from the rivers must +be transported into the open sea. As a current is formed by the water +thrown over the edge of atoll-formed reefs, which carries sediment with +it through the deep-water breaches, the same thing probably takes place +in barrier-reefs, and this would greatly aid in preventing the +lagoon-channel from being filled up. The low alluvial border, however, +at the foot of the encircled mountains, shows that the work of filling +up is in progress; and at Maura (Plate I., Figure 6), in the Society +group, it has been almost effected, so that there remains only one +harbour for small craft. + +If we look at a set of charts of barrier-reefs, and leave out in +imagination the encircled land, we shall find that, besides the many +points already noticed of resemblance, or rather of identity in +structure with atolls, there is a close general agreement in form, +average dimensions, and grouping. Encircling barrier-reefs, like +atolls, are generally elongated, with an irregularly rounded, though +sometimes angular outline. There are atolls of all sizes, from less +than two miles in diameter to sixty miles (excluding Tilla-dou-Matte, +as it consists of a number of almost independent atoll-formed reefs); +and there are encircling barrier-reefs from three miles and a half to +forty-six miles in diameter,—Turtle Island being an instance of the +former, and Hogoleu of the latter. At Tahiti the encircled island is +thirty-six miles in its longest axis, whilst at Maurua it is only a +little more than two miles. It will be shown, in the last chapter in +this volume, that there is the strictest resemblance in the grouping of +atolls and of common islands, and consequently there must be the same +resemblance in the grouping of atolls and of encircling barrier-reefs. + +The islands lying within reefs of this class, are of very various +heights. Tahiti[6] is 7,000 feet; Maurua about 800; Aitutaki 360, and +Manouai only 50. The geological nature of the included land varies: in +most cases it is of ancient volcanic origin, owing apparently to the +fact that islands of this nature are most frequent within all great +seas; some, however, are of madreporitic limestone, and others of +primary formation, of which latter kind New Caledonia offers the best +example. The central land consists either of one island, or of several: +thus, in the Society group, Eimeo stands by itself; while Taha and +Raiatea (Figure 3, Plate I.), both moderately large islands of nearly +equal size, are included in one reef. Within the reef of the Gambier +group there are four large and some smaller islands (Figure 8, Plate +I.); within that of Hogoleu (Figure 2, Plate I.) nearly a dozen small +islands are scattered over the expanse of one vast lagoon. + + [6] The height of Tahiti is given from Captain Beechey; Maurua from + Mr. F.D. Bennett (“Geograph. Journ.” volume viii., page 220); Aitutaki + from measurements made on board the _Beagle;_ and Manouai or Harvey + Island, from an estimate by the Rev. J. Williams. The two latter + islands, however, are not in some respects well characterised examples + of the encircled class. + + +After the details now given, it may be asserted that there is not one +point of essential difference between encircling barrier-reefs and +atolls: the latter enclose a simple sheet of water, the former encircle +an expanse with one or more islands rising from it. I was much struck +with this fact, when viewing, from the heights of Tahiti, the distant +island of Eimeo standing within smooth water, and encircled by a ring +of snow-white breakers. Remove the central land, and an annular reef +like that of an atoll in an early stage of its formation is left; +remove it from Bolabola, and there remains a circle of linear +coral-islets, crowned with tall cocoa-nut trees, like one of the many +atolls scattered over the Pacific and Indian Oceans. + +The barrier-reefs of Australia and of New Caledonia deserve a separate +notice from their great dimensions. The reef on the west coast of New +Caledonia (Figure 5, Plate II.) is 400 miles in length; and for a +length of many leagues it seldom approaches within eight miles of the +shore; and near the southern end of the island, the space between the +reef and the land is sixteen miles in width. The Australian barrier +extends, with a few interruptions, for nearly a thousand miles; its +average distance from the land is between twenty and thirty miles; and +in some parts from fifty to seventy. The great arm of the sea thus +included, is from ten to twenty-five fathoms deep, with a sandy bottom; +but towards the southern end, where the reef is further from the shore, +the depth gradually increases to forty, and in some parts to more than +sixty fathoms. Flinders[7] has described the surface of this reef as +consisting of a hard white agglomerate of different kinds of coral, +with rough projecting points. The outer edge is the highest part; it is +traversed by narrow gullies, and at rare intervals is breached by +ship-channels. The sea close outside is profoundly deep; but, in front +of the main breaches, soundings can sometimes be obtained. Some low +islets have been formed on the reef. + + [7] Flinders’ “Voyage to Terra Australis,” volume ii., page 88. + + +[Illustration] + +1. VANIKORO, from the “Atlas of the Voyage of the _Astrolabe_,” by D. +D’Urville. + +2. GAMBIER ISLAND, from Beechey. + +3. MAURUA, from the “Atlas of the Voyage of the _Coquille_,” by +Duperrey. + +The horizontal line is the level of the sea, from which on the right +hand a plummet descends, representing a depth of 200 fathoms, or 1,200 +feet. The vertical shading shows the section of the land, and the +horizontal shading that of the encircling barrier-reef: from the +smallness of the scale, the lagoon-channel could not be represented. + +AA.—Outer edge of the coral-reefs, where the sea breaks. + +BB.—The shore of the encircled islands.) + +There is one important point in the structure of barrier-reefs which +must here be considered. The accompanying diagrams represent north and +south vertical sections, taken through the highest points of Vanikoro, +Gambier, and Maurua Islands, and through their encircling reefs. The +scale both in the horizontal and vertical direction is the same, +namely, a quarter of an inch to a nautical mile. The height and width +of these islands is known; and I have attempted to represent the form +of the land from the shading of the hills in the large published +charts. It has long been remarked, even from the time of Dampier, that +considerable degree of relation subsists between the inclination of +that part of the land which is beneath water and that above it; hence +the dotted line in the three sections, probably, does not widely differ +in inclination from the actual submarine prolongation of the land. If +we now look at the outer edge of the reef (AA), and bear in mind that +the plummet on the right hand represents a depth of 1,200 feet, we must +conclude that the vertical thickness of these barrier coral-reefs is +very great. + +I must observe that if the sections had been taken in any other +direction across these islands, or across other encircled islands,[8] +the result would have been the same. In the succeeding chapter it will +be shown that reef-building polypifers cannot flourish at great +depths,—for instance, it is highly improbable that they could exist at +a quarter of the depth represented by the plummet on the right hand of +the woodcut. Here there is a great _apparent_ difficulty—how were the +basal parts of these barrier-reef formed? It will, perhaps, occur to +some, that the actual reefs formed of coral are not of great thickness, +but that before their first growth, the coasts of these encircled +islands were deeply eaten into, and a broad but shallow submarine ledge +thus left, on the edge of which the coral grew; but if this had been +the case, the shore would have been invariably bounded by lofty cliffs, +and not have sloped down to the lagoon-channel, as it does in many +instances. On this view,[9] moreover, the cause of the reef springing +up at such a great distance from the land, leaving a deep and broad +moat within, remains altogether unexplained. A supposition of the same +nature, and appearing at first more probable is, that the reefs sprung +up from banks of sediment, which had accumulated round the shore +previously to the growth of the coral; but the extension of a bank to +the same distance round an unbroken coast, and in front of those deep +arms of the sea (as in Raiatea, see Plate II., Figure 3) which +penetrate nearly to the heart of some encircled islands, is exceedingly +improbable. And why, again, should the reef spring up, in some cases +steep on both sides like a wall, at a distance of two, three or more +miles from the shore, leaving a channel often between two hundred and +three hundred feet deep, and rising from a depth which we have reason +to believe is destructive to the growth of coral? An admission of this +nature cannot possibly be made. The existence, also, of the deep +channel, utterly precludes the idea of the reef having grown outwards, +on a foundation slowly formed on its outside, by the accumulation of +sediment and coral detritus. Nor, again, can it be asserted, that the +reef-building corals will not grow, excepting at a great distance from +the land; for, as we shall soon see, there is a whole class of reefs, +which take their name from growing closely attached (especially where +the sea is deep) to the beach. At New Caledonia (see Plate II., Figure +5) the reefs which run in front of the west coast are prolonged in the +same line 150 miles beyond the northern extremity of the island, and +this shows that some explanation, quite different from any of those +just suggested, is required. The continuation of the reefs on each side +of the submarine prolongation of New Caledonia, is an exceedingly +interesting fact, if this part formerly existed as the northern +extremity of the island, and before the attachment of the coral had +been worn down by the action of the sea, or if it originally existed at +its present height, with or without beds of sediment on each flank, how +can we possibly account for the reefs, not growing on the crest of this +submarine portion, but fronting its sides, in the same line with the +reefs which front the shores of the lofty island? We shall hereafter +see, that there is one, and I believe only one, solution of this +difficulty. + + [8] In the fifth chapter an east and west section across the Island of + Bolabola and its barrier-reefs is given, for the sake of illustrating + another point. The unbroken line in it (woodcut No. 5) is the section + referred to. The scale is .57 of an inch to a mile; it is taken from + the “Atlas of the Voyage of the _Coquille_,” by Duperrey. The depth of + the lagoon-channel is exaggerated. + + + [9] The Rev. D. Tyerman and Mr. Bennett (“Journal of Voyage and + Travels,” volume i., page 215) have briefly suggested this explanation + of the origin of the encircling reefs of the Society Islands. + + +One other supposition to account for the position of encircling +barrier-reefs remains, but it is almost too preposterous to be +mentioned; namely, that they rest on enormous submarine craters, +surrounding the included islands. When the size, height, and form of +the islands in the Society group are considered, together with the fact +that all are thus encircled, such a notion will be rejected by almost +every one. New Caledonia, moreover, besides its size, is composed of +primitive formations, as are some of the Comoro Islands;[10] and +Aitutaki consists of calcareous rock. We must, therefore, reject these +several explanations, and conclude that the vertical thickness of +barrier-reefs, from their outer edges to the foundation on which they +rest (from _AA_ in the section to the dotted lines) is really great; +but in this, there is no difficulty, for it is not necessary to suppose +that the coral has sprung up from an immense depth, as will be evident +when the theory of the upward growth of coral-reefs, during the slow +subsidence of their foundation, is discussed. + + [10] I have been informed that this is the case by Dr. Allan of + Forres, who has visited this group. + + + + +CHAPTER III. + +FRINGING OR SHORE-REEFS. + + +Reefs of Mauritius.—Shallow channel within the reef.—Its slow filling +up.—Currents of water formed within it.—Upraised reefs.—Narrow +fringing-reefs in deep seas.—Reefs on the coast of East Africa and of +Brazil.—Fringing-reefs in very shallow seas, round banks of sediment +and on worn-down islands.—Fringing-reefs affected by currents of the +sea.— Coral coating the bottom of the sea, but not forming reefs. + + +Fringing-reefs, or, as they have been called by some voyagers, +shore-reefs, whether skirting an island or part of a continent, might +at first be thought to differ little, except in generally being of less +breadth, from barrier-reefs. As far as the superficies of the actual +reef is concerned this is the case; but the absence of an interior +deep-water channel, and the close relation in their horizontal +extension with the probable slope beneath the sea of the adjoining +land, present essential points of difference. + +The reefs which fringe the island of Mauritius offer a good example of +this class. They extend round its whole circumference, with the +exception of two or three parts,[1] where the coast is almost +precipitous, and where, if as is probable the bottom of the sea has a +similar inclination, the coral would have no foundation on which to +become attached. A similar fact may sometimes be observed even in reefs +of the barrier class, which follow much less closely the outline of the +adjoining land; as, for instance, on the south-east and precipitous +side of Tahiti, where the encircling reef is interrupted. On the +western side of the Mauritius, which was the only part I visited, the +reef generally lies at the distance of about half a mile from the +shore; but in some parts it is distant from one to two, and even three +miles. But even in this last case, as the coast-land is gently inclined +from the foot of the mountains to the sea-beach, and as the soundings +outside the reef indicate an equally gentle slope beneath the water, +there is no reason for supposing that the basis of the reef, formed by +the prolongation of the strata of the island, lies at a greater depth +than that at which the polypifers could begin constructing the reef. +Some allowance, however, must be made for the outward extension of the +corals on a foundation of sand and detritus, formed from their own +wear, which would give to the reef a somewhat greater vertical +thickness, than would otherwise be possible. + + [1] This fact is stated on the authority of the Officier du Roi, in + his extremely interesting “Voyage à l’Isle de France,” undertaken in + 1768. According to Captain Carmichael (Hooker’s “Bot. Misc.” volume + ii., page 316) on one part of the coast there is a space for sixteen + miles without a reef. + + +The outer edge of the reef on the western or leeward side of the island +is tolerably well defined, and is a little higher than any other part. +It chiefly consists of large strongly branched corals, of the genus +Madrepora, which also form a sloping bed some way out to sea: the kinds +of coral growing in this part will be described in the ensuing chapter. +Between the outer margin and the beach, there is a flat space with a +sandy bottom and a few tufts of living coral; in some parts it is so +shallow, that people, by avoiding the deeper holes and gullies, can +wade across it at low water; in other parts it is deeper, seldom +however exceeding ten or twelve feet, so that it offers a safe coasting +channel for boats. On the eastern and windward side of the island, +which is exposed to a heavy surf, the reef was described to me as +having a hard smooth surface, very slightly inclined inwards, just +covered at low-water, and traversed by gullies; it appears to be quite +similar in structure to the reefs of the barrier and atoll classes. + +The reef of Mauritius, in front of every river and streamlet, is +breached by a straight passage: at Grand Port, however, there is a +channel like that within a barrier-reef; it extends parallel to the +shore for four miles, and has an average depth of ten or twelve +fathoms; its presence may probably be accounted for by two rivers which +enter at each end of the channel, and bend towards each other. The fact +of reefs of the fringing class being always breached in front of +streams, even of those which are dry during the greater part of the +year, will be explained, when the conditions unfavourable to the growth +of coral are considered. Low coral-islets, like those on barrier-reefs +and atolls, are seldom formed on reefs of this class, owing apparently +in some cases to their narrowness, and in others to the gentle slope of +the reef outside not yielding many fragments to the breakers. On the +windward side, however, of the Mauritius, two or three small islets +have been formed. + +It appears, as will be shown in the ensuing chapter, that the action of +the surf is favourable to the vigorous growth of the stronger corals, +and that sand or sediment, if agitated by the waves, is injurious to +them. Hence it is probable that a reef on a shelving shore, like that +of Mauritius, would at first grow up, not attached to the actual beach, +but at some little distance from it; and the corals on the outer margin +would be the most vigorous. A shallow channel would thus be formed +within the reef, and as the breakers are prevented acting on the shores +of the island, and as they do not ordinarily tear up many fragments +from the outside, and as every streamlet has its bed prolonged in a +straight line through the reef, this channel could be filled up only +very slowly with sediment. But a beach of sand and of fragments of the +smaller kinds of coral seems, in the case of Mauritius, to be slowly +encroaching on the shallow channel. On many shelving and sandy coasts, +the breakers tend to form a bar of sand a little way from the beach, +with a slight increase of depth within it; for instance, Captain +Grey[2] states that the west coast of Australia, in latitude 24°, is +fronted by a sand bar about two hundred yards in width, on which there +is only two feet of water; but within it the depth increases to two +fathoms. Similar bars, more or less perfect, occur on other coasts. In +these cases I suspect that the shallow channel (which no doubt during +storms is occasionally obliterated) is scooped out by the flowing away +of the water thrown beyond the line, on which the waves break with the +greatest force. At Pernambuco a bar of hard sandstone,[3] which has the +same external form and height as a coral-reef, extends nearly parallel +to the coast; within this bar currents, apparently caused by the water +thrown over it during the greater part of each tide, run strongly, and +are wearing away its inner wall. From these facts it can hardly be +doubted, that within most fringing-reefs, especially within those lying +some distance from the land, a return stream must carry away the water +thrown over the outer edge; and the current thus produced, would tend +to prevent the channel being filled up with sediment, and might even +deepen it under certain circumstances. To this latter belief I am led, +by finding that channels are almost universally present within the +fringing-reefs of those islands which have undergone recent elevatory +movements; and this could hardly have been the case, if the conversion +of the very shallow channel into land had not been counteracted to a +certain extent. + + [2] Captain Grey’s “Journal of Two Expeditions,” volume i. page 369. + + + [3] I have described this singular structure in the “London and + Edinburgh Phil. Mag.” October 1841. + + +A fringing-reef, if elevated in a perfect condition above the level of +the sea, ought to present the singular appearance of a broad dry moat +within a low mound. The author[4] of an interesting pedestrian tour +round the Mauritius, seems to have met with a structure of this kind: +he says “J’observai que là, où la mer étale, indépendamment des rescifs +du large, il y à terre _une espèce d’effoncement_ ou chemin couvert +naturel. On y pourrait mettre du canon,” etc. In another place he adds, +“Avant de passer le Cap, on remarque un gros banc de corail elévé de +plus de quinze pieds: c’est une espèce de rescif, que la mer abandonné, +il regne au pied une longue flaque d’eau, dont on pourrait faire un +bassin pour de petits vaisseaux.” But the margin of the reef, although +the highest and most perfect part, from being most exposed to the surf, +would generally during a slow rise of the land be either partially or +entirely worn down to that level, at which corals could renew their +growth on its upper edge. On some parts of the coast-land of Mauritius +there are little hillocks of coral-rock, which are either the last +remnants of a continuous reef, or of low islets formed on it. I +observed that two such hillocks between Tamarin Bay and the Great Black +River; they were nearly twenty feet high, about two hundred yards from +the present beach, and about thirty feet above its level. They rose +abruptly from a smooth surface, strewed with worn fragments of coral. +They consisted in their lower part of hard calcareous sandstone, and in +their upper of great blocks of several species of Astræa and Madrepora, +loosely aggregated; they were divided into irregular beds, dipping +seaward, in one hillock at an angle of 8°, and in the other at 18° I +suspect that the superficial parts of the reefs, which have been +upraised together with the islands they fringe, have generally been +much more modified by the wearing action of the sea, than those of +Mauritius. + + [4] “Voyage à l’Isle de France, par un Officier du Roi,” part i., + pages 192, 200. + + +Many islands[5] are fringed by reefs quite similar to those of +Mauritius; but on coasts where the sea deepens very suddenly the reefs +are much narrower, and their limited extension seems evidently to +depend on the high inclination of the submarine slope; a relation, +which, as we have seen, does not exist in reefs of the barrier class. +The fringing-reefs on steep coasts are frequently not more than from +fifty to one hundred yards in width; they have a nearly smooth, hard +surface, scarcely uncovered at low water, and without any interior +shoal channel, like that within those fringing-reefs, which lie at a +greater distance from the land. The fragments torn up during gales from +the outer margin are thrown over the reef on the shores of the island. +I may give as instances, Wateeo, where the reef is described by Cook as +being a hundred yards wide; and Mauti and Elizabeth Islands,[6] where +it is only fifty yards in width: the sea round these islands is very +deep. + + [5] I may give Cuba, as another instance; Mr. Taylor (Loudon’s Mag. of + Nat. Hist. volume ix., page 449) has described a reef several miles in + length between Gibara and Vjaro, which extends parallel to the shore + at the distance of between half and the third part of a mile, and + encloses a space of shallow water, with a sandy bottom and tufts of + coral. Outside the edge of the reef, which is formed of great + branching corals, the depth is six and seven fathoms. This coast has + been upheaved at no very distant geological period.” + + + [6] Mauti is described by Lord Byron in the voyage of H.M.S. _Blonde_, + and Elizabeth Island by Captain Beechey. + + +Fringing-reefs, like barrier-reefs, both surround islands, and front +the shores of continents. In the charts of the eastern coast of Africa, +by Captain Owen, many extensive fringing-reefs are laid down; thus, for +a space of nearly forty miles, from latitude 1° 15′ to 1° 45′ S., a +reef fringes the shore at an average distance of rather more than one +mile, and therefore at a greater distance than is usual in reefs of +this class; but as the coast-land is not lofty, and as the bottom +shoals very gradually (the depth being only from eight to fourteen +fathoms at a mile and a half outside the reef), its extension thus far +from the land offers no difficulty. The external margin of this reef is +described, as formed of projecting points, within which there is a +space, from six to twelve feet deep, with patches of living coral on +it. At Mukdeesha (latitude 2° 1′ N.) “the port is formed,” it is +said,[7] “by a long reef extending eastward, four or five miles, within +which there is a narrow channel, with ten to twelve feet of water at +low spring-tides;” it lies at the distance of a quarter of a mile from +the shore. Again, in the plan of Mombas (latitude 4° S.), a reef +extends for thirty-six miles, at the distance of from half a mile to +one mile and a quarter from the shore; within it, there is a channel +navigable “for canoes and small craft,” between six and fifteen feet +deep: outside the reef the depth is about thirty fathoms at the +distance of nearly half a mile. Part of this reef is very symmetrical, +and has a uniform breadth of two hundred yards. + + [7] Owen’s “Africa,” volume i., page 357, from which work the + foregoing facts are likewise taken. + + +The coast of Brazil is in many parts fringed by reefs. Of these, some +are not of coral formation; for instance, those near Bahia and in front +of Pernambuco; but a few miles south of this latter city, the reef +follows[8] so closely every turn of the shore, that I can hardly doubt +it is of coral; it runs at the distance of three-quarters of a mile +from the land, and within it the depth is from ten to fifteen feet. I +was assured by an intelligent pilot that at Ports Frances and Maceio, +the outer part of the reef consists of living coral, and the inner of a +white stone, full of large irregular cavities, communicating with the +sea. The bottom of the sea off the coast of Brazil shoals gradually to +between thirty and forty fathoms, at the distance of between nine and +ten leagues from the land. + + [8] See Baron Roussin’s “Pilote du Brésil,” and accompanying + hydrographical memoir. + + +From the description now given, we must conclude that the dimensions +and structure of fringing-reefs depend entirely on the greater or less +inclination of the submarine slope, conjoined with the fact that +reef-building polypifers can exist only at limited depths. It follows +from this, that where the sea is very shallow, as in the Persian Gulf +and in parts of the East Indian Archipelago, the reefs lose their +fringing character, and appear as separate and irregularly scattered +patches, often of considerable area. From the more vigorous growth of +the coral on the outside, and from the conditions being less favourable +in several respects within, such reefs are generally higher and more +perfect in their marginal than in their central parts; hence these +reefs sometimes assume (and this circumstance ought not to be +overlooked) the appearance of atolls; but they differ from atolls in +their central expanse being much less deep, in their form being less +defined, and in being based on a shallow foundation. But when in a deep +sea reefs fringe banks of sediment, which have accumulated beneath the +surface, round either islands or submerged rocks, they are +distinguished with difficulty on the one hand from encircling +barrier-reefs, and on the other from atolls. In the West Indies there +are reefs, which I should probably have arranged under both these +classes, had not the existence of large and level banks, lying a little +beneath the surface, ready to serve as the basis for the attachment of +coral, been occasionally brought into view by the entire or partial +absence of reefs on them, and had not the formation of such banks, +through the accumulation of sediment now in progress, been sufficiently +evident. Fringing-reefs sometimes coat, and thus protect the +foundations of islands, which have been worn down by the surf to the +level of the sea. According to Ehrenberg, this has been extensively the +case with the islands in the Red Sea, which formerly ranged parallel to +the shores of the mainland, with deep water within them: hence the +reefs now coating their bases are situated relatively to the land like +barrier-reefs, although not belonging to that class; but there are, as +I believe, in the Red Sea some true barrier-reefs. The reefs of this +sea and of the West Indies will be described in the Appendix. In some +cases, fringing-reefs appear to be considerably modified in outline by +the course of the prevailing currents. Dr. J. Allan informs me that on +the east coast of Madagascar almost every headland and low point of +sand has a coral-reef extending from it in a S.W. and N.E. line, +parallel to the currents on that shore. I should think the influence of +the currents chiefly consisted in causing an extension, in a certain +direction, of a proper foundation for the attachment of the coral. +Round many intertropical islands, for instance the Abrolhos on the +coast of Brazil surveyed by Captain Fitzroy, and, as I am informed by +Mr. Cuming, round the Philippines, the bottom of the sea is entirely +coated by irregular masses of coral, which although often of large +size, do not reach the surface and form proper reefs. This must be +owing, either to insufficient growth, or to the absence of those kinds +of corals which can withstand the breaking of the waves. + +The three classes, atoll-formed, barrier, and fringing-reefs, together +with the modifications just described of the latter, include all the +most remarkable coral formations anywhere existing. At the commencement +of the last chapter in the volume, where I detail the principles on +which the map (Plate III.) is coloured, the exceptional cases will be +enumerated. + + + + +CHAPTER IV. + +ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS. + + +In this chapter I will give all the facts which I have collected, +relating to the distribution of coral-reefs,—to the conditions +favourable to their increase,—to the rate of their growth,—and to the +depth at which they are formed. + +These subjects have an important bearing on the theory of the origin of +the different classes of coral-reefs. + +SECTION I.—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS +FAVOURABLE TO THEIR INCREASE. + +With regard to the limits of latitude, over which coral-reefs extend, I +have nothing new to add. The Bermuda Islands, in 32° 15′ N., is the +point furthest removed from the equator, in which they appear to exist; +and it has been suggested that their extension so far northward in this +instance is owing to the warmth of the Gulf Stream. In the Pacific, the +Loo Choo Islands, in latitude 27° N., have reefs on their shores, and +there is an atoll in 28° 30′, situated N.W. of the Sandwich +Archipelago. In the Red Sea there are coral-reefs in latitude 30°. In +the southern hemisphere coral-reefs do not extend so far from the +equatorial sea. In the Southern Pacific there are only a few reefs +beyond the line of the tropics, but Houtmans Abrolhos, on the western +shores of Australia in latitude 29° S., are of coral formation. + +The proximity of volcanic land, owing to the lime generally evolved +from it, has been thought to be favourable to the increase of +coral-reefs. There is, however, not much foundation for this view; for +nowhere are coral-reefs more extensive than on the shores of New +Caledonia, and of north-eastern Australia, which consist of primary +formations; and in the largest groups of atolls, namely the Maldiva, +Chagos, Marshall, Gilbert, and Low Archipelagoes, there is no volcanic +or other kind of rock, excepting that formed of coral. + +The entire absence of coral-reefs in certain large areas within the +tropical seas, is a remarkable fact. Thus no coral-reefs were observed, +during the surveying voyages of the “Beagle” and her tender on the west +coast of South America south of the equator, or round the Galapagos +Islands. It appears, also, that there are none[1] north of the equator; +Mr. Lloyd, who surveyed the Isthmus of Panama, remarked to me, that +although he had seen corals living in the Bay of Panama, yet he had +never observed any reefs formed by them. I at first attributed this +absence of reefs on the coasts of Peru and of the Galapagos Islands,[2] +to the coldness of the currents from the south, but the Gulf of Panama +is one of the hottest pelagic districts in the world.[3] In the central +parts of the Pacific there are islands entirely free from reefs; in +some few of these cases I have thought that this was owing to recent +volcanic action; but the existence of reefs round the greater part of +Hawaii, one of the Sandwich Islands, shows that recent volcanic action +does not necessarily prevent their growth. + + [1] I have been informed that this is the case, by Lieutenant Ryder, + R.N., and others who have had ample opportunities for observation. + + + [2] The mean temperature of the surface sea from observations made by + the direction of Captain Fitzroy on the shores of the Galapagos + Islands, between the 16th of September and the 20th of October, 1835, + was 68° Fahr. The lowest temperature observed was 58.5° at the + south-west end of Albemarle Island; and on the west coast of this + island, it was several times 62° and 63°. The mean temperature of the + sea in the Low Archipelago of atolls, and near Tahiti, from similar + observations made on board the _Beagle_, was (although further from + the equator) 77.5°, the lowest any day being 76.5°. Therefore we have + here a difference of 9.5° in mean temperature, and 18° in extremes; a + difference doubtless quite sufficient to affect the distribution of + organic beings in the two areas. + + + [3] Humboldt’s “Personal Narrative,” volume vii., page 434. + + +In the last chapter I stated that the bottom of the sea round some +islands is thickly coated with living corals, which nevertheless do not +form reefs, either from insufficient growth, or from the species not +being adapted to contend with the breaking waves. + +I have been assured by several people, that there are no coral-reefs on +the west coast of Africa,[4] or round the islands in the Gulf of +Guinea. This perhaps may be attributed, in part, to the sediment +brought down by the many rivers debouching on that coast, and to the +extensive mud-banks, which line great part of it. But the islands of +St. Helena, Ascension, the Cape Verdes, St. Paul’s, and Fernando +Noronha, are, also, entirely without reefs, although they lie far out +at sea, are composed of the same ancient volcanic rocks, and have the +same general form, with those islands in the Pacific, the shores of +which are surrounded by gigantic walls of coral-rock. With the +exception of Bermuda, there is not a single coral-reef in the central +expanse of the Atlantic Ocean. It will, perhaps, be suggested that the +quantity of carbonate of lime in different parts of the sea, may +regulate the presence of reefs. But this cannot be the case, for at +Ascension, the waves charged to excess precipitate a thick layer of +calcareous matter on the tidal rocks; and at St. Jago, in the Cape +Verdes, carbonate of lime not only is abundant on the shores, but it +forms the chief part of some upraised post-tertiary strata. The +apparently capricious distribution, therefore, of coral-reefs, cannot +be explained by any of these obvious causes; but as the study of the +terrestrial and better known half of the world must convince every one +that no station capable of supporting life is lost,—nay more, that +there is a struggle for each station, between the different orders of +nature,—we may conclude that in those parts of the intertropical sea, +in which there are no coral-reefs, there are other organic bodies +supplying the place of the reef-building polypifers. It has been shown +in the chapter on Keeling atoll that there are some species of large +fish, and the whole tribe of Holothuriæ which prey on the tenderer +parts of the corals. On the other hand, the polypifers in their turn +must prey on some other organic beings; the decrease of which from any +cause would cause a proportionate destruction of the living coral. The +relations, therefore, which determine the formation of reefs on any +shore, by the vigorous growth of the efficient kinds of coral, must be +very complex, and with our imperfect knowledge quite inexplicable. From +these considerations, we may infer that changes in the condition of the +sea, not obvious to our senses, might destroy all the coral-reefs in +one area, and cause them to appear in another: thus, the Pacific or +Indian Ocean might become as barren of coral-reefs as the Atlantic now +is, without our being able to assign any adequate cause for such a +change. + + [4] It might be concluded, from a paper by Captain Owen (“Geographical + Journal”, volume ii., page 89), that the reefs off Cape St. Anne and + the Sherboro’ Islands were of coral, although the author states that + they are not purely coralline. But I have been assured by Lieutenant + Holland, R.N., that these reefs are not of coral, or at least that + they do not at all resemble those in the West Indies. + + +It has been a question with some naturalists, which part of a reef is +most favourable to the growth of coral. The great mounds of living +Porites and of Millepora round Keeling atoll occur exclusively on the +extreme verge of the reef, which is washed by a constant succession of +breakers; and living coral nowhere else forms solid masses. At the +Marshall islands the larger kinds of coral (chiefly species of Astræa, +a genus closely allied to Porites) “which form rocks measuring several +fathoms in thickness,” prefer, according to Chamisso,[5] the most +violent surf. I have stated that the outer margin of the Maldiva atolls +consists of living corals (some of which, if not all, are of the same +species with those at Keeling atoll), and here the surf is so +tremendous, that even large ships have been thrown, by a single heave +of the sea, high and dry on the reef, all on board thus escaping with +their lives. + + [5] Kotzebue’s “First Voyage” (English Translation) volume iii., pages + 142, 143, 331. + + +Ehrenberg[6] remarks, that in the Red Sea the strongest corals live on +the outer reefs, and appear to love the surf; he adds, that the more +branched kinds abound a little way within, but that even these in still +more protected places, become smaller. Many other facts having a +similar tendency might be adduced.[7] It has, however, been doubted by +MM. Quoy and Gaimard, whether any kind of coral can even withstand, +much less flourish in, the breakers of an open sea:[8] they affirm that +the saxigenous lithophytes flourish only where the water is tranquil, +and the heat intense. This statement has passed from one geological +work to another; nevertheless, the protection of the whole reef +undoubtedly is due to those kinds of coral, which cannot exist in the +situations thought by these naturalists to be most favourable to them. +For should the outer and living margin perish, of any one of the many +low coral-islands, round which a line of great breakers is incessantly +foaming, the whole, it is scarcely possible to doubt, would be washed +away and destroyed, in less than half a century. But the vital energies +of the corals conquer the mechanical power of the waves; and the large +fragments of reef torn up by every storm, are replaced by the slow but +steady growth of the innumerable polypifers, which form the living zone +on its outer edge. + + [6] Ehrenberg, “Über die Natür und Bildung der Corallen Bänke im + rothen Meere,” page 49. + + + [7] In the West Indies, as I am informed by Captain Bird Allen, R.N., + it is the common belief of those, who are best acquainted with the + reefs, that the coral flourishes most, where freely exposed to the + swell of the open sea. + + + [8] “Annales des Sciences Naturelles,” tome vi., pages 276, 278.—“Là + où les ondes sont agitées, les Lytophytés ne peuvent travailler, parce + qu’elles détruiraient leurs fragiles édifices,” &c. + + +From these facts, it is certain, that the strongest and most massive +corals flourish, where most exposed. The less perfect state of the reef +of most atolls on the leeward and less exposed side, compared with its +state to windward; and the analogous case of the greater number of +breaches on the near sides of those atolls in the Maldiva Archipelago, +which afford some protection to each other, are obviously explained by +this circumstance. If the question had been, under what conditions the +greater number of species of coral, not regarding their bulk and +strength, were developed, I should answer,—probably in the situations +described by MM. Quoy and Gaimard, where the water is tranquil and the +heat intense. The total number of species of coral in the +circumtropical seas must be very great: in the Red Sea alone, 120 +kinds, according to Ehrenberg,[9] have been observed. + + [9] Ehrenberg, “Über die Natür,” etc., etc., page 46. + + +The same author has observed that the recoil of the sea from a steep +shore is injurious to the growth of coral, although waves breaking over +a bank are not so. Ehrenberg also states, that where there is much +sediment, placed so as to be liable to be moved by the waves there is +little or no coral; and a collection of living specimens placed by him +on a sandy shore died in the course of a few days.[10] An experiment, +however, will presently be related in which some large masses of living +coral increased rapidly in size, after having been secured by stakes on +a sandbank. That loose sediment should be injurious to the living +polypifers, appears, at first sight, probable; and accordingly, in +sounding off Keeling atoll, and (as will hereafter be shown) off +Mauritius, the arming of the lead invariably came up clean, where the +coral was growing vigorously. This same circumstance has probably given +rise to a strange belief, which, according to Captain Owen,[11] is +general amongst the inhabitants of the Maldiva atolls, namely that +corals have roots, and therefore that if merely broken down to the +surface, they grow up again; but, if rooted out, they are permanently +destroyed. By this means the inhabitants keep their harbours clear; and +thus the French Governor of St. Mary’s in Madagascar, “cleared out and +made a beautiful little port at that place.” For it is probable that +sand would accumulate in the hollows formed by tearing out the corals, +but not on the broken and projecting stumps, and therefore, in the +former case, the fresh growth of the coral might be thus prevented. + + [10] Ibid., page 49. + + + [11] Captain Owen on the Geography of the Maldiva Islands, + “Geographical Journal”, volume ii., page 88. + + +In the last chapter I remarked that fringing-reefs are almost +universally breached, where streams enter the sea.[12] Most authors +have attributed this fact to the injurious effects of the fresh water, +even where it enters the sea only in small quantity, and during a part +of the year. No doubt brackish water would prevent or retard the growth +of coral; but I believe that the mud and sand which is deposited, even +by rivulets when flooded, is a much more efficient check. The reef on +each side of the channel leading into Port Louis at Mauritius, ends +abruptly in a wall, at the foot of which I sounded and found a bed of +thick mud. This steepness of the sides appears to be a general +character in such breaches. Cook,[13] speaking of one at Raiatea, says, +“like all the rest, it is very steep on both sides.” Now, if it were +the fresh water mingling with the salt which prevented the growth of +coral, the reef certainly would not terminate abruptly, but as the +polypifers nearest the impure stream would grow less vigorously than +those farther off, so would the reef gradually thin away. On the other +hand, the sediment brought down from the land would only prevent the +growth of the coral in the line of its deposition, but would not check +it on the side, so that the reefs might increase till they overhung the +bed of the channel. The breaches are much fewer in number, and front +only the larger valleys in reefs of the encircling barrier class. They +probably are kept open in the same manner as those into the lagoon of +an atoll, namely, by the force of the currents and the drifting +outwards of fine sediment. Their position in front of valleys, although +often separated from the land by deep water lagoon-channels, which it +might be thought would entirely remove the injurious effects both of +the fresh water and the sediment, will receive a simple explanation +when we discuss the origin of barrier-reefs. + + [12] Lieutenant Wellstead and others have remarked that this is the + case in the Red Sea; Dr. Rüppell (“Reise in Abyss.” Band. i., page + 142) says that there are pear-shaped harbours in the upraised + coral-coast, into which periodical streams enter. From this + circumstance, I presume, we must infer that before the upheaval of the + strata now forming the coast-land, fresh water and sediment entered + the sea at these points; and the coral being thus prevented growing, + the pear-shaped harbours were produced. + + + [13] Cook’s “First Voyage,” volume ii., page 271 (Hawkesworth’s + edition).) + + +In the vegetable kingdom every different station has its peculiar group +of plants, and similar relations appear to prevail with corals. We have +already described the great difference between the corals within the +lagoon of an atoll and those on its outer margin. The corals, also, on +the margin of Keeling Island occurred in zones; thus the _Porites_ and +_Millepora complanata_ grow to a large size only where they are washed +by a heavy sea, and are killed by a short exposure to the air; whereas, +three species of Nullipora also live amidst the breakers, but are able +to survive uncovered for a part of each tide; at greater depths, a +strong Madrepora and _Millepora alcicornis_ are the commonest kinds, +the former appearing to be confined to this part, beneath the zone of +massive corals, minute encrusting corallines and other organic bodies +live. If we compare the external margin of the reef at Keeling atoll +with that on the leeward side of Mauritius, which are very differently +circumstanced, we shall find a corresponding difference in the +appearance of the corals. At the latter place, the genus Madrepora is +preponderant over every other kind, and beneath the zone of massive +corals there are large beds of Seriatopora. There is also a marked +difference, according to Captain Moresby,[14] between the great +branching corals of the Red Sea, and those on the reefs of the Maldiva +atolls. + + [14] Captain Moresby on the Northern Maldiva atolls, “Geographical + Journal”, volume v., page 401. + + +These facts, which in themselves are deserving of notice, bear, +perhaps, not very remotely, on a remarkable circumstance which has been +pointed out to me by Captain Moresby, namely, that with very few +exceptions, none of the coral-knolls within the lagoons of Peros +Banhos, Diego Garcia, and the Great Chagos Bank (all situated in the +Chagos group), rise to the surface of the water; whereas all those, +with equally few exceptions, within Solomon and Egmont atolls in the +same group, and likewise within the large southern Maldiva atolls, +reach the surface. I make these statements, after having examined the +charts of each atoll. In the lagoon of Peros Banhos, which is nearly +twenty miles across, there is only one single reef which rises to the +surface; in Diego Garcia there are seven, but several of these lie +close to the margin of the lagoon, and need scarcely have been +reckoned; in the Great Chagos Bank there is not one. On the other hand, +in the lagoons of some of the great southern Maldiva atolls, although +thickly studded with reefs, every one without exception rises to the +surface; and on an average there are less than two submerged reefs in +each atoll; in the northern atolls, however, the submerged lagoon-reefs +are not quite so rare. The submerged reefs in the Chagos atolls +generally have from one to seven fathoms water on them, but some have +from seven to ten. Most of them are small with very steep sides;[15] at +Peros Banhos they rise from a depth of about thirty fathoms, and some +of them in the Great Chagos Bank from above forty fathoms; they are +covered, Captain Moresby informs me, with living and healthy coral, two +and three feet high, consisting of several species. Why then have not +these lagoon-reefs reached the surface, like the innumerable ones in +the atolls above named? If we attempt to assign any difference in their +external conditions, as the cause of this diversity, we are at once +baffled. The lagoon of Diego Garcia is not deep, and is almost wholly +surrounded by its reef; Peros Banhos is very deep, much larger, with +many wide passages communicating with the open sea. On the other hand, +of those atolls, in which all or nearly all the lagoon-reefs have +reached the surface, some are small, others large, some shallow, others +deep, some well-enclosed, and others open. + + [15] Some of these statements were not communicated to me verbally by + Captain Moresby, but are taken from the MS. account before alluded to, + of the Chagos Group. + + +Captain Moresby informs me that he has seen a French chart of Diego +Garcia made eighty years before his survey, and apparently very +accurate; and from it he infers, that during this interval there has +not been the smallest change in the depth on any of the knolls within +the lagoon. It is also known that during the last fifty-one years, the +eastern channel into the lagoon has neither become narrower, nor +decreased in depth; and as there are numerous small knolls of living +coral within it, some change might have been anticipated. Moreover, as +the whole reef round the lagoon of this atoll has been converted into +land—an unparalleled case, I believe, in an atoll of such large +size,—and as the strip of land is for considerable spaces more than +half a mile wide—also a very unusual circumstance,—we have the best +possible evidence, that Diego Garcia has remained at its present level +for a very long period. With this fact, and with the knowledge that no +sensible change has taken place during eighty years in the +coral-knolls, and considering that every single reef has reached the +surface in other atolls, which do not present the smallest appearance +of being older than Diego Garcia and Peros Banhos, and which are placed +under the same external conditions with them, one is led to conclude +that these submerged reefs, although covered with luxuriant coral, have +no tendency to grow upwards, and that they would remain at their +present levels for an almost indefinite period. + +From the number of these knolls, from their position, size, and form, +many of them being only one or two hundred yards across, with a rounded +outline, and precipitous sides,—it is indisputable that they have been +formed by the growth of coral; and this makes the case much more +remarkable. In Peros Banhos and in the Great Chagos Bank, some of these +almost columnar masses are 200 feet high, and their summits lie only +from two to eight fathoms beneath the surface; therefore, a small +proportional amount more of growth would cause them to attain the +surface, like those numerous knolls, which rise from an equally great +depth within the Maldiva atolls. We can hardly suppose that time has +been wanting for the upward growth of the coral, whilst in Diego +Garcia, the broad annular strip of land, formed by the continued +accumulation of detritus, shows how long this atoll has remained at its +present level. We must look to some other cause than the rate of +growth; and I suspect it will be found in the reefs being formed of +different species of corals, adapted to live at different depths. + +The Great Chagos Bank is situated in the centre of the Chagos Group, +and the Pitt and Speaker Banks at its two extreme points. These banks +resemble atolls, except in their external rim being about eight fathoms +submerged, and in being formed of dead rock, with very little living +coral on it: a portion nine miles long of the annular reef of Peros +Banhos atoll is in the same condition. These facts, as will hereafter +be shown, render it very probable that the whole group at some former +period subsided seven or eight fathoms; and that the coral perished on +the outer margin of those atolls which are now submerged, but that it +continued alive, and grew up to the surface on those which are now +perfect. If these atolls did subside, and if from the suddenness of the +movement or from any other cause, those corals which are better adapted +to live at a certain depth than at the surface, once got possession of +the knolls, supplanting the former occupants, they would exert little +or no tendency to grow upwards. To illustrate this, I may observe, that +if the corals of the upper zone on the outer edge of Keeling atoll were +to perish, it is improbable that those of the lower zone would grow to +the surface, and thus become exposed to conditions for which they do +not appear to be adapted. The conjecture, that the corals on the +submerged knolls within the Chagos atolls have analogous habits with +those of the lower zone outside Keeling atoll, receives some support +from a remark by Captain Moresby, namely, that they have a different +appearance from those on the reefs in the Maldiva atolls, which, as we +have seen, all rise to the surface: he compares the kind of difference +to that of the vegetation under different climates. I have entered at +considerable length into this case, although unable to throw much light +on it, in order to show that an equal tendency to upward growth ought +not to be attributed to all coral-reefs,—to those situated at different +depths,—to those forming the ring of an atoll or those on the knolls +within a lagoon,—to those in one area and those in another. The +inference, therefore, that one reef could not grow up to the surface +within a given time, because another, not known to be covered with the +same species of corals, and not known to be placed under conditions +exactly the same, has not within the same time reached the surface, is +unsound. + +SECTION II.—ON THE RATE OF GROWTH OF CORAL-REEFS. + +The remark made at the close of the last section, naturally leads to +this division of our subject, which has not, I think, hitherto been +considered under a right point of view. Ehrenberg[16] has stated, that +in the Red Sea, the corals only coat other rocks in a layer from one to +two feet in thickness, or at most to a fathom and a half; and he +disbelieves that, in any case, they form, by their own proper growth, +great masses, stratum over stratum. A nearly similar observation has +been made by MM. Quoy and Gaimard,[17] with respect to the thickness of +some upraised beds of coral, which they examined at Timor and some +other places. Ehrenberg[18] saw certain large massive corals in the Red +Sea, which he imagines to be of such vast antiquity, that they might +have been beheld by Pharaoh; and according to Mr. Lyell[19] there are +certain corals at Bermuda, which are known by tradition, to have been +living for centuries. To show how slowly coral-reefs grow upwards, +Captain Beechey[20] has adduced the case of the Dolphin Reef off +Tahiti, which has remained at the same depth beneath the surface, +namely about two fathoms and a half, for a period of sixty-seven years. +There are reefs in the Red Sea, which certainly do not appear[21] to +have increased in dimensions during the last half-century, and from the +comparison of old charts with recent surveys, probably not during the +last two hundred years. These, and other similar facts, have so +strongly impressed many with the belief of the extreme slowness of the +growth of corals, that they have even doubted the possibility of +islands in the great oceans having been formed by their agency. Others, +again, who have not been overwhelmed by this difficulty, have admitted +that it would require thousands, and tens of thousands of years, to +form a mass, even of inconsiderable thickness; but the subject has not, +I believe, been viewed in the proper light. + + [16] Ehrenberg, as before cited, pages 39, 46, and 50. + + + [17] “Annales des Sciences Nat.” tom. vi., page 28. + + + [18] Ehrenberg, ut sup., page 42. + + + [19] Lyell’s “Principles of Geology,” book iii., chapter xviii. + + + [20] Beechey’s “Voyage to the Pacific,” chapter viii. + + + [21] Ehrenberg, ut sup., page 43. + + +That masses of considerable thickness have been formed by the growth of +coral, may be inferred with certainty from the following facts. In the +deep lagoons of Peros Banhos and of the Great Chagos Bank, there are, +as already described, small steep-sided knolls covered with living +coral. There are similar knolls in the southern Maldiva atolls, some of +which, as Captain Moresby assures me, are less than a hundred yards in +diameter, and rise to the surface from a depth of between two hundred +and fifty and three hundred feet. Considering their number, form, and +position, it would be preposterous to suppose that they are based on +pinnacles of any rock, not of coral formation; or that sediment could +have been heaped up into such small and steep isolated cones. As no +kind of living coral grows above the height of a few feet, we are +compelled to suppose that these knolls have been formed by the +successive growth and death of many individuals,—first one being broken +off or killed by some accident, and then another, and one set of +species being replaced by another set with different habits, as the +reef rose nearer the surface, or as other changes supervened. The +spaces between the corals would become filled up with fragments and +sand, and such matter would probably soon be consolidated, for we learn +from Lieutenant Nelson,[22] that at Bermuda a process of this kind +takes place beneath water, without the aid of evaporation. In reefs, +also, of the barrier class, we may feel sure, as I have shown, that +masses of great thickness have been formed by the growth of the coral; +in the case of Vanikoro, judging only from the depth of the moat +between the land and the reef, the wall of coral-rock must be at least +three hundred feet in vertical thickness. + + [22] “Geological Transactions,” volume v., page 113. + + +It is unfortunate that the upraised coral-islands in the Pacific have +not been examined by a geologist. The cliffs of Elizabeth Island, in +the Low Archipelago, are eighty feet high, and appear, from Captain +Beechey’s description, to consist of a homogeneous coral-rock. From the +isolated position of this island, we may safely infer that it is an +upraised atoll, and therefore that it has been formed by masses of +coral, grown together. Savage Island seems, from the description of the +younger Forster,[23] to have a similar structure, and its shores are +about forty feet high: some of the Cook Islands also appear[24] to be +similarly composed. Captain Belcher, R.N., in a letter which Captain +Beaufort showed me at the admiralty, speaking of Bow atoll, says, “I +have succeeded in boring forty-five feet through coral-sand, when the +auger became jammed by the falling in of the surrounding _creamy_ +matter.” On one of the Maldiva atolls, Captain Moresby bored to a depth +of twenty-six feet, when his auger also broke: he has had the kindness +to give me the matter brought up; it is perfectly white, and like +finely triturated coral-rock. + + [23] Forster’s “Voyage round the World with Cook,” volume ii., pages + 163, 167. + + + [24] Williams’s “Narrative of Missionary Enterprise,” page 30. + + +In my description of Keeling atoll, I have given some facts, which show +that the reef probably has grown outwards; and I have found, just +within the outer margin, the great mounds of Porites and of Millepora, +with their summits lately killed, and their sides subsequently +thickened by the growth of the coral: a layer, also, of Nullipora had +already coated the dead surface. As the external slope of the reef is +the same round the whole of this atoll, and round many other atolls, +the angle of inclination must result from an adaption between the +growing powers of the coral, and the force of the breakers, and their +action on the loose sediment. The reef, therefore, could not increase +outwards, without a nearly equal addition to every part of the slope, +so that the original inclination might be preserved, and this would +require a large amount of sediment, all derived from the wear of corals +and shells, to be added to the lower part. Moreover, at Keeling atoll, +and probably in many other cases, the different kinds of corals would +have to encroach on each other; thus the Nulliporæ cannot increase +outwards without encroaching on the Porites and _Millepora complanata_, +as is now taking place; nor these latter without encroaching on the +strongly branched Madreporet, the _Millepora alcicornis_, and some +Astræas; nor these again without a foundation being formed for them +within the requisite depth, by the accumulation of sediment. How slow, +then, must be the ordinary lateral or outward growth of such reefs. But +off Christmas atoll, where the sea is much more shallow than is usual, +we have good reason to believe that, within a period not very remote, +the reef has increased considerably in width. The land has the +extraordinary breadth of three miles; it consists of parallel ridges of +shells and broken corals, which furnish “an incontestable proof,” as +observed by Cook,[25] “that the island has been produced by accessions +from the sea, and is in a state of increase.” The land is fronted by a +coral-reef, and from the manner in which islets are known to be formed, +we may feel confident that the reef was not three miles wide, when the +first, or most backward ridge, was thrown up; and, therefore, we must +conclude that the reef has grown outwards during the accumulation of +the successive ridges. Here then, a wall of coral-rock of very +considerable breadth has been formed by the outward growth of the +living margin, within a period during which ridges of shells and +corals, lying on the bare surface, have not decayed. There can be +little doubt, from the account given by Captain Beechey, that Matilda +atoll, in the Low Archipelago, has been converted in the space of +thirty-four years, from being, as described by the crew of a wrecked +whaling vessel, a “reef of rocks” into a lagoon-island, fourteen miles +in length, with “one of its sides covered nearly the whole way with +high trees.”[26] The islets, also, on Keeling atoll, it has been shown, +have increased in length, and since the construction of an old chart, +several of them have become united into one long islet; but in this +case, and in that of Matilda atoll, we have no proof, and can only +infer as probable, that the reef, that is the foundation of the islets, +has increased as well as the islets themselves. + + [25] Cook’s “Third Voyage,” book III., chapter x. + + + [26] Beechey’s “Voyage to the Pacific,” chapter vii. and viii. + + +After these considerations, I attach little importance, as indicating +the ordinary and still less the possible rate of _outward_ growth of +coral-reefs, to the fact that certain reefs in the Red Sea have not +increased during a long interval of time; or to other such cases, as +that of Ouluthy atoll in the Caroline group, where every islet, +described a thousand years before by Cantova was found in the same +state by Lutké,[27]—without it could be shown that, in these cases, the +conditions were favourable to the vigorous and unopposed growth of the +corals living in the different zones of depth, and that a proper basis +for the extent of the reef was present. The former conditions must +depend on many contingencies, and in the deep oceans where coral +formations most abound, a basis within the requisite depth can rarely +be present. + + [27] F. Lutké’s “Voyage autour du Monde.” In the group Elato, however, + it appears that what is now the islet Falipi, is called in Cantova’s + Chart, the Banc de Falipi. It is not stated whether this has been + caused by the growth of coral, or by the accumulation of sand. + + +Nor do I attach any importance to the fact of certain submerged reefs, +as those off Tahiti, or those within Diego Garcia not now being nearer +the surface than they were many years ago, as an indication of the rate +under favourable circumstances of the _upward_ growth of reefs; after +it has been shown, that all the reefs have grown to the surface in some +of the Chagos atolls, but that in neighbouring atolls which appear to +be of equal antiquity and to be exposed to the same external +conditions, every reef remains submerged; for we are almost driven to +attribute this to a difference, not in the rate of growth, but in the +habits of the corals in the two cases. + +In an old-standing reef, the corals, which are so different in kind on +different parts of it, are probably all adapted to the stations they +occupy, and hold their places, like other organic beings, by a struggle +one with another, and with external nature; hence we may infer that +their growth would generally be slow, except under peculiarly +favourable circumstances. Almost the only natural condition, allowing a +quick upward growth of the whole surface of a reef, would be a slow +subsidence of the area in which it stood; if, for instance, Keeling +atoll were to subside two or three feet, can we doubt that the +projecting margin of live coral, about half an inch in thickness, which +surrounds the dead upper surfaces of the mounds of Porites, would in +this case form a concentric layer over them, and the reef thus increase +upwards, instead of, as at present, outwards? The Nulliporæ are now +encroaching on the Porites and Millepora, but in this case might we not +confidently expect that the latter would, in their turn, encroach on +the Nulliporæ? After a subsidence of this kind, the sea would gain on +the islets, and the great fields of dead but upright corals in the +lagoon, would be covered by a sheet of clear water; and might we not +then expect that these reefs would rise to the surface, as they +anciently did when the lagoon was less confined by islets, and as they +did within a period of ten years in the schooner-channel, cut by the +inhabitants? In one of the Maldiva atolls, a reef, which within a very +few years existed as an islet bearing cocoa-nut trees, was found by +Lieutenant Prentice “_entirely covered with live coral and Madrepore_.” +The natives believe that the islet was washed away by a change in the +currents, but if, instead of this, it had quietly subsided, surely +every part of the island which offered a solid foundation, would in a +like manner have become coated with living coral. + +Through steps such as these, any thickness of rock, composed of a +singular intermixture of various kinds of corals, shells, and +calcareous sediment, might be formed; but without subsidence, the +thickness would necessarily be determined by the depth at which the +reef-building polypifers can exist. If it be asked, at what rate in +years I suppose a reef of coral favourably circumstanced could grow up +from a given depth; I should answer, that we have no precise evidence +on this point, and comparatively little concern with it. We see, in +innumerable points over wide areas, that the rate has been sufficient, +either to bring up the reefs from various depths to the surface, or, as +is more probable, to keep them at the surface, during progressive +subsidences; and this is a much more important standard of comparison +than any cycle of years. + +It may, however, be inferred from the following facts, that the rate in +years under favourable circumstances would be very far from slow. Dr. +Allan, of Forres, has, in his MS. Thesis deposited in the library of +the Edinburgh University (extracts from which I owe to the kindness of +Dr. Malcolmson), the following account of some experiments, which he +tried during his travels in the years 1830 to 1832 on the east coast of +Madagascar. “To ascertain the rise and progress of the coral-family, +and fix the number of species met with at Foul Point (latitude 17° 40′) +twenty species of coral were taken off the reef and planted apart on a +sand-bank _three feet deep at low water_. Each portion weighed ten +pounds, and was kept in its place by stakes. Similar quantities were +placed in a clump and secured as the rest. This was done in December +1830. In July following, each detached mass was nearly level with the +sea at low water, quite immovable, and several feet long, stretching as +the parent reef, with the coast current from north to south. The masses +accumulated in a clump were found equally increased, but some of the +species in such unequal ratios, as to be growing over each other.” The +loss of Dr. Allan’s magnificent collection by shipwreck, unfortunately +prevents its being known to what genera these corals belonged; but from +the numbers experimented on, it is certain that all the more +conspicuous kinds must have been included. Dr. Allan informs me, in a +letter, that he believes it was a Madrepora, which grew most +vigorously. One may be permitted to suspect that the level of the sea +might possibly have been somewhat different at the two stated periods; +nevertheless, it is quite evident that the growth of the ten-pound +masses, during the six or seven months, at the end of which they were +found immovably fixed[28] and several feet in length, must have been +very great. The fact of the different kinds of coral, when placed in +one clump, having increased in extremely unequal ratios, is very +interesting, as it shows the manner in which a reef, supporting many +species of coral, would probably be affected by a change in the +external conditions favouring one kind more than another. The growth of +the masses of coral in N. and S. lines parallel to the prevailing +currents, whether due to the drifting of sediment or to the simple +movement of the water, is, also, a very interesting circumstance. + + [28] It is stated by De la Beche (“Geological Manual,” page 143), on + the authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that + some specimens of Polypifers, placed by him in a sheltered pool of + water, were found in the course of a few days firmly fixed by the + secretion of a stony matter, to the bottom. + + +A fact, communicated to me by Lieutenant Wellstead, I.N., in some +degree corroborates the result of Dr. Allan’s experiments: it is, that +in the Persian Gulf a ship had her copper bottom encrusted in the +course of twenty months with a layer of coral, _two feet_ in thickness, +which it required great force to remove, when the vessel was docked: it +was not ascertained to what order this coral belonged. The case of the +schooner-channel choked up with coral in an interval of less than ten +years, in the lagoon of Keeling atoll, should be here borne in mind. We +may also infer, from the trouble which the inhabitants of the Maldiva +atolls take to root out, as they express it, the coral-knolls from +their harbours, that their growth can hardly be very slow.[29] + + [29] Mr. Stutchbury (“West of England Journal”, No. I., page 50.) has + described a specimen of Agaricia, “weighing 2 lbs. 9 oz., which + surrounds a species of oyster, whose age could not be more than two + years, and yet is completely enveloped by this dense coral.” I presume + that the oyster was living when the specimen was procured; otherwise + the fact tells nothing. Mr. Stutchbury also mentions an anchor, which + had become entirely encrusted with coral in fifty years; other cases, + however, are recorded of anchors which have long remained amidst + coral-reefs without having become coated. The anchor of the _Beagle_, + in 1832, after having been down exactly one month at Rio de Janeiro, + was so thickly coated by two species of Tubularia, that large spaces + of the iron were entirely concealed; the tufts of this horny zoophyte + were between two and three inches in length. It has been attempted to + compute, but I believe erroneously, the rate of growth of a reef, from + the fact mentioned by Captain Beechey, of the _Chama gigas_ being + embedded in coral-rock. But it should be remembered, that some species + of this genus invariably live, both whilst young and old, in cavities, + which the animal has the power of enlarging with its growth. I saw + many of these shells thus embedded in the outer “flat” of Keeling + atoll, which is composed of dead rock; and therefore the cavities in + this case had no relation whatever with the growth of coral. M. + Lesson, also, speaking of this shell (Partie Zoolog. “Voyage de la + ‘Coquille’”), has remarked, “que constamment ses valves étaient + engagés complètement dans la masse des Madrepores.” + + +From the facts given in this section, it may be concluded, first, that +considerable thicknesses of rock have certainly been formed within the +present geological area by the growth of coral and the accumulation of +its detritus; and, secondly, that the increase of individual corals and +of reefs, both outwards or horizontally and upwards or vertically, +under the peculiar conditions favourable to such increase, is not slow, +when referred either to the standard of the average oscillations of +level in the earth’s crust, or to the more precise but less important +one of a cycle of years. + +SECTION III.—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE. + +I have already described in detail, which might have appeared trivial, +the nature of the bottom of the sea immediately surrounding Keeling +atoll; and I will now describe with almost equal care the soundings off +the fringing-reefs of Mauritius. I have preferred this arrangement, for +the sake of grouping together facts of a similar nature. I sounded with +the wide bell-shaped lead which Captain Fitzroy used at Keeling Island, +but my examination of the bottom was confined to a few miles of coast +(between Port Louis and Tomb Bay) on the leeward side of the island. +The edge of the reef is formed of great shapeless masses of branching +Madrepores, which chiefly consist of two species,—apparently _M. +corymbosa_ and _pocillifera_,— mingled with a few other kinds of coral. +These masses are separated from each other by the most irregular +gullies and cavities, into which the lead sinks many feet. Outside this +irregular border of Madrepores, the water deepens gradually to twenty +fathoms, which depth generally is found at the distance of from half to +three-quarters of a mile from the reef. A little further out the depth +is thirty fathoms, and thence the bank slopes rapidly into the depths +of the ocean. This inclination is very gentle compared with that +outside Keeling and other atolls, but compared with most coasts it is +steep. The water was so clear outside the reef, that I could +distinguish every object forming the rugged bottom. In this part, and +to a depth of eight fathoms, I sounded repeatedly, and at each cast +pounded the bottom with the broad lead, nevertheless the arming +invariably came up perfectly clean, but deeply indented. From eight to +fifteen fathoms a little calcareous sand was occasionally brought up, +but more frequently the arming was simply indented. In all this space +the two Madrepores above mentioned, and two species of Astræa, with +rather large[30] stars, seemed the commonest kinds; and it must be +noticed that twice at the depth of fifteen fathoms, the arming was +marked with a clean impression of an Astræa. Besides these lithophytes, +some fragments of the _Millepora alcicornis_, which occurs in the same +relative position at Keeling Island, were brought up; and in the deeper +parts there were large beds of a Seriatopora, different from _S. +subulata_, but closely allied to it. On the beach within the reef, the +rolled fragments consisted chiefly of the corals just mentioned, and of +a massive Porites, like that at Keeling atoll, of a Meandrina, +_Pocillopora verrucosa_, and of numerous fragments of Nullipora. From +fifteen to twenty fathoms the bottom was, with few exceptions, either +formed of sand, or thickly covered with Seriatopora: this delicate +coral seems to form at these depths extensive beds unmingled with any +other kind. At twenty fathoms, one sounding brought up a fragment of +Madrepora apparently _M. pocillifera_, and I believe it is the same +species (for I neglected to bring specimens from both stations) which +mainly forms the upper margin of the reef; if so, it grows in depths +varying from 0 to 20 fathoms. Between 20 and 23 fathoms I obtained +several soundings, and they all showed a sandy bottom, with one +exception at 30 fathoms, when the arming came up scooped out, as if by +the margin of a large Caryophyllia. Beyond 33 fathoms I sounded only +once; and from 86 fathoms, at the distance of one mile and a third from +the edge of the reef, the arming brought up calcareous sand with a +pebble of volcanic rock. The circumstance of the arming having +invariably come up quite clean, when sounding within a certain number +of fathoms off the reefs of Mauritius and Keeling atoll (eight fathoms +in the former case, and twelve in the latter) and of its having always +come up (with one exception) smoothed and covered with sand, when the +depth exceeded twenty fathoms, probably indicates a criterion, by which +the limits of the vigorous growth of coral might in all cases be +readily ascertained. I do not, however, suppose that if a vast number +of soundings were obtained round these islands, the limit above +assigned would be found never to vary, but I conceive the facts are +sufficient to show, that the exceptions would be few. The circumstance +of a _gradual_ change, in the two cases, from a field of clean coral to +a smooth sandy bottom, is far more important in indicating the depth at +which the larger kinds of coral flourish than almost any number of +separate observations on the depth, at which certain species have been +dredged up. For we can understand the gradation, only as a prolonged +struggle against unfavourable conditions. If a person were to find the +soil clothed with turf on the banks of a stream of water, but on going +to some distance on one side of it, he observed the blades of grass +growing thinner and thinner, with intervening patches of sand, until he +entered a desert of sand, he would safely conclude, especially if +changes of the same kind were noticed in other places, that the +presence of the water was absolutely necessary to the formation of a +thick bed of turf: so may we conclude, with the same feeling of +certainty, that thick beds of coral are formed only at small depths +beneath the surface of the sea. + + [30] Since the preceding pages were printed off, I have received from + Mr. Lyell a very interesting pamphlet, entitled “Remarks upon Coral + Formations,” etc., by J. Couthouy, Boston, United States, 1842. There + is a statement (page 6), on the authority of the Rev. J. Williams, + corroborating the remarks made by Ehrenberg and Lyell (page 71 of this + volume), on the antiquity of certain individual corals in the Red Sea + and at Bermuda; namely, that at Upolu, one of the Navigator Islands, + “particular clumps of coral are known to the fishermen by name, + derived from either some particular configuration or tradition + attached to them, and handed down from time immemorial.” With respect + to the thickness of masses of coral-rock, it clearly appears, from the + descriptions given by Mr. Couthouy (pages 34, 58) that Mangaia and + Aurora Islands are upraised atolls, composed of coral rock: the level + summit of the former is about three hundred feet, and that of Aurora + Island is two hundred feet above the sea-level. + + +I have endeavoured to collect every fact, which might either invalidate +or corroborate this conclusion. Captain Moresby, whose opportunities +for observation during his survey of the Maldiva and Chagos +Archipelagoes have been unrivalled, informs me, that the upper part or +zone of the steep-sided reefs, on the inner and outer coasts of the +atolls in both groups, invariably consists of coral, and the lower +parts of sand. At seven or eight fathoms depth, the bottom is formed, +as could be seen through the clear water, of great living masses of +coral, which at about ten fathoms generally stand some way apart from +each other, with patches of white sand between them, and at a little +greater depth these patches become united into a smooth steep slope, +without any coral. Captain Moresby, also, informs me in support of his +statement, that he found only decayed coral on the Padua Bank (northern +part of the Laccadive group) which has an average depth between +twenty-five and thirty-five fathoms, but that on some other banks in +the same group with only ten or twelve fathoms water on them (for +instance, the Tillacapeni bank), the coral was living. + +With regard to the coral-reefs in the Red Sea, Ehrenberg has the +following passage:—“The living corals do not descend there into great +depths. On the edges of islets and near reefs, where the depth was +small, very many lived; but we found no more even at six fathoms. The +pearl-fishers at Yemen and Massaua asserted that there was no coral +near the pearl-banks at nine fathoms depth, but only sand. We were not +able to institute any more special researches.”[31] I am, however, +assured both by Captain Moresby and Lieutenant Wellstead, that in the +more northern parts of the Red Sea, there are extensive beds of living +coral at a depth of twenty-five fathoms, in which the anchors of their +vessels were frequently entangled. Captain Moresby attributes the less +depth, at which the corals are able to live in the places mentioned by +Ehrenberg, to the greater quantity of sediment there; and the +situations, where they were flourishing at the depth of twenty-five +fathoms, were protected, and the water was extraordinarily limpid. On +the leeward side of Mauritius where I found the coral growing at a +somewhat greater depth than at Keeling atoll, the sea, owing apparently +to its tranquil state, was likewise very clear. Within the lagoons of +some of the Marshall atolls, where the water can be but little +agitated, there are, according to Kotzebue, living beds of coral in +twenty-five fathoms. From these facts, and considering the manner in +which the beds of clean coral off Mauritius, Keeling Island, the +Maldiva and Chagos atolls, graduated into a sandy slope, it appears +very probable that the depth, at which reef-building polypifers can +exist, is partly determined by the extent of inclined surface, which +the currents of the sea and the recoiling waves have the power to keep +free from sediment. + + [31] Ehrenberg, “Über die Natür,” &c., page 50. + + +MM. Quoy and Gaimard[32] believe that the growth of coral is confined +within very limited depths; and they state that they never found any +fragment of an Astræa (the genus they consider most efficient in +forming reefs) at a depth above twenty-five or thirty feet. But we have +seen that in several places the bottom of the sea is paved with massive +corals at more than twice this depth; and at fifteen fathoms (or twice +this depth) off the reefs of Mauritius, the arming was marked with the +distinct impression of a living Astræa. _Millepora alcicornis_ lives in +from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0 +to 20 fathoms. Captain Moresby has given me a specimen of _Sideropora +scabra_ (Porites of Lamarck) brought up alive from 17 fathoms. Mr. +Couthouy[33] states that he has dredged up on the Bahama banks +considerable masses of Meandrina from 16 fathoms, and he has seen this +coral growing in 20 fathoms. A Caryophyllia, half an inch in diameter, +was dredged up alive from 80 fathoms off Juan Fernandez (latitude 33° +S.) by Captain P.P. King:[34] this is the most remarkable fact with +which I am acquainted, showing the depth at which a genus of corals +often found on reefs, can exist.[35] We ought, however, to feel less +surprise at this fact, as Caryophyllia alone of the lamelliform genera, +ranges far beyond the tropics; it is found in Zetland (Fleming’s +“British Animals,” genus Caryophyllia.) in Latitude 60° N. in deep +water, and I procured a small species from Tierra del Fuego in Latitude +53° S. Captain Beechey informs me, that branches of pink and yellow +coral were frequently brought up from between twenty and twenty-five +fathoms off the Low atolls; and Lieutenant Stokes, writing to me from +the N.W. coast of Australia, says that a strongly branched coral was +procured there from thirty fathoms; unfortunately it is not known to +what genera these corals belong. + + [32] “Annales des Sci. Nat.” tom. vi. + + + [33] “Remarks on Coral Formations,” page 12. + + + [34] I am indebted to Mr. Stokes for having kindly communicated this + fact to me, together with much other valuable information. + + + [35] I will record in the form of a note all the facts that I have + been able to collect on the depths, both within and without the + tropics, at which those corals and corallines can live, which there is + no reason to suppose ever materially aid in the construction of a + reef. + +NAME OF ZOOPHYTE Depth in fathoms COUNTRY and S. LATITUDE AUTHORITY +SERTULARIA 40 Cape Horn 66° Where none is given, + the observation is my own +CELLARIA ditto Ditto +CELLARIA, A minute 190 Keeling Atoll, 12° +scarlet encrusting +species found living +CELLARIA, An allied, 48 St Cruz Riv. 50° +small stony sub-generic +form +A coral allied to 40 Cape Horn +VINCULARIA, with eight +rows of cells +TUBULIPORA, near to 40 Cape Horn +T. patina +TUBULIPORA, near to 94 East Chiloe 43° +T. patina +CELLEPORA, several 40 Cape Horn +species, and allied +sub-generic forms +CELLEPORA, several 40 and 57 Chonos Archipelago 45° +species, and allied +sub-generic forms +CELLEPORA, several 48 St Cruz 50° +species, and allied +sub-generic forms +ESCHARA 30 Tierra del Fuego 53° +ESCHARA 48 St Cruz R. 50° +RETEPORA 40 Cape Horn +RETEPORA 100 Cape of Good Hope 34° Quoy and Gaimard, “Ann. Scien. Nat.” tome vi., page 284. +MILLEPORA, a strong 94 and 30 E. Chiloe 43° +coral with cylindrical +branches, of a pink +colour, about two +inches high, +resembling in the +form of its orifices +M. aspera of Lamarck +Tierra del Fuego 53° +CORALIUM 120 Barbary 33° N. Peyssonel in paper read to Royal Society May 1752. +ANTIPATHES 16 Chonos 45° +GORGONIA (or an allied 160 Abrolhos on the Captain Beechey informed me of this fact in a letter. +form) coast of Brazil 18° + + +Ellis (“Nat. Hist. of Coralline,” page 96) states that Ombellularia was +procured in latitude 79° N. _sticking_ to a _line_ from the depth of +236 fathoms; hence this coral either must have been floating loose, or +was entangled in stray line at the bottom. Off Keeling atoll a compound +Ascidia (Sigillina) was brought up from 39 fathoms, and a piece of +sponge, apparently living, from 70, and a fragment of Nullipora also +apparently living from 92 fathoms. At a greater depth than 90 fathoms +off this coral island, the bottom was thickly strewed with joints of +Halimeda and small fragments of other Nulliporæ, but all dead. Captain +B. Allen, R.N., informs me that in the survey of the West Indies it was +noticed that between the depth of 10 and 200 fathoms, the sounding lead +very generally came up coated with the dead joints of a Halimeda, of +which he showed me specimens. Off Pernambuco, in Brazil, in about +twelve fathoms, the bottom was covered with fragments dead and alive of +a dull red Nullipora, and I infer from Roussin’s chart, that a bottom +of this kind extends over a wide area. On the beach, within the +coral-reefs of Mauritius, vast quantities of fragments of Nulliporæ +were piled up. From these facts it appears, that these simply organized +bodies are amongst the most abundant productions of the sea.) + + +Although the limit of depth, at which each particular kind of coral +ceases to exist, is far from being accurately known; yet when we bear +in mind the manner in which the clumps of coral gradually became +infrequent at about the same depth, and wholly disappeared at a greater +depth than twenty fathoms, on the slope round Keeling atoll, on the +leeward side of the Mauritius, and at rather less depth, both without +and within the atolls of the Maldiva and Chagos Archipelagoes; and when +we know that the reefs round these islands do not differ from other +coral formations in their form and structure, we may, I think, conclude +that in ordinary cases, reef-building polypifers do not flourish at +greater depths than between twenty and thirty fathoms. + +It has been argued[36] that reefs may possibly rise from very great +depths through the means of small corals, first making a platform for +the growth of the stronger kinds. This, however, is an arbitrary +supposition: it is not always remembered, that in such cases there is +an antagonist power in action, namely, the decay of organic bodies, +when not protected by a covering of sediment, or by their own rapid +growth. We have, moreover, no right to calculate on unlimited time for +the accumulation of small organic bodies into great masses. Every fact +in geology proclaims that neither the land, nor the bed of the sea +retain for indefinite periods the same level. As well might it be +imagined that the British Seas would in time become choked up with beds +of oysters, or that the numerous small corallines off the inhospitable +shores of Tierra del Fuego would in time form a solid and extensive +coral-reef. + + [36] “Journal of the Royal Geographical Society,” 1831, page 218. + + + + +CHAPTER V. + +THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF CORAL-REEFS. + + +The atolls of the larger archipelagoes are not formed on submerged +craters, or on banks of sediment.—Immense areas interspersed with +atolls.—Their subsidence.—The effects of storms and earthquakes on +atolls.—Recent changes in their state.—The origin of barrier-reefs and +of atolls.—Their relative forms.—The step-formed ledges and walls round +the shores of some lagoons.—The ring-formed reefs of the Maldiva +atolls.—The submerged condition of parts or of the whole of some +annular reefs.—The disseverment of large atolls.—The union of atolls by +linear reefs.—The Great Chagos Bank.—Objections from the area and +amount of subsidence required by the theory, considered.—The probable +composition of the lower parts of atolls. + + +The naturalists who have visited the Pacific, seem to have had their +attention riveted by the lagoon-islands, or atolls,—those singular +rings of coral-land which rise abruptly out of the unfathomable +ocean—and have passed over, almost unnoticed, the scarcely less +wonderful encircling barrier-reefs. The theory most generally received +on the formation of atolls, is that they are based on submarine +craters; but where can we find a crater of the shape of Bow atoll, +which is five times as long as it is broad (Plate I., Figure 4); or +like that of Menchikoff Island (Plate II., Figure 3.), with its three +loops, together sixty miles in length; or like Rimsky Korsacoff, +narrow, crooked, and fifty-four miles long; or like the northern +Maldiva atolls, made up of numerous ring-formed reefs, placed on the +margin of a disc,—one of which discs is eighty-eight miles in length, +and only from ten to twenty in breadth? It is, also, not a little +improbable, that there should have existed as many craters of immense +size crowded together beneath the sea, as there are now in some parts +atolls. But this theory lies under a greater difficulty, as will be +evident, when we consider on what foundations the atolls of the larger +archipelagoes rest: nevertheless, if the rim of a crater afforded a +basis at the proper depth, I am far from denying that a reef like a +perfectly characterised atoll might not be formed; some such, perhaps, +now exist; but I cannot believe in the possibility of the greater +number having thus originated. + +An earlier and better theory was proposed by Chamisso;[1] he supposes +that as the more massive kinds of corals prefer the surf, the outer +portions, in a reef rising from a submarine basis, would first reach +the surface and consequently form a ring. But on this view it must be +assumed, that in every case the basis consists of a flat bank; for if +it were conically formed, like a mountainous mass, we can see no reason +why the coral should spring up from the flanks, instead of from the +central and highest parts: considering the number of the atolls in the +Pacific and Indian Oceans, this assumption is very improbable. As the +lagoons of atolls are sometimes even more than forty fathoms deep, it +must, also, be assumed on this view, that at a depth at which the waves +do not break, the coral grows more vigorously on the edges of a bank +than on its central part; and this is an assumption without any +evidence in support of it. I remarked, in the third chapter, that a +reef, growing on a detached bank, would tend to assume an atoll-like +structure; if, therefore, corals were to grow up from a bank, with a +level surface some fathoms submerged, having steep sides and being +situated in a deep sea, a reef not to be distinguished from an atoll, +might be formed: I believe some such exist in the West Indies. But a +difficulty of the same kind with that affecting the crater theory, +runners, as we shall presently see, this view inapplicable to the +greater number of atolls. + + [1] Kotzebue’s “First Voyage,” volume iii., page 331. + + +No theory worthy of notice has been advanced to account for those +barrier-reefs, which encircle islands of moderate dimensions. The great +reef which fronts the coast of Australia has been supposed, but without +any special facts, to rest on the edge of a submarine precipice, +extending parallel to the shore. The origin of the third class or of +fringing-reefs presents, I believe, scarcely any difficulty, and is +simply consequent on the polypifers not growing up from great depths, +and their not flourishing close to gently shelving beaches where the +water is often turbid. + +What cause, then, has given to atolls and barrier-reefs their +characteristic forms? Let us see whether an important deduction will +not follow from the consideration of these two circumstances, first, +the reef-building corals flourishing only at limited depths; and +secondly, the vastness of the areas interspersed with coral-reefs and +coral-islets, none of which rise to a greater height above the level of +the sea, than that attained by matter thrown up by the waves and winds. +I do not make this latter statement vaguely; I have carefully sought +for descriptions of every island in the intertropical seas; and my task +has been in some degree abridged by a map of the Pacific, corrected in +1834 by MM. D’Urville and Lottin, in which the low islands are +distinguished from the high ones (even from those much less than a +hundred feet in height) by being written without a capital letter; I +have detected a few errors in this map, respecting the height of some +of the islands, which will be noticed in the Appendix, where I treat of +coral formations in geographical order. To the Appendix, also, I must +refer for a more particular account of the data on which the statements +on the next page are grounded. I have ascertained, and chiefly from the +writings of Cook, Kotzebue, Bellinghausen, Duperrey, Beechey, and +Lutké, regarding the Pacific; and from Moresby[2] with respect to the +Indian Ocean, that in the following cases the term “low island” +strictly means land of the height commonly attained by matter thrown up +by the winds and the waves of an open sea. If we draw a line (the plan +I have always adopted) joining the external atolls of that part of the +Low Archipelago in which the islands are numerous, the figure will be a +pointed ellipse (reaching from Hood to Lazaref Island), of which the +longer axis is 840 geographical miles, and the shorter 420 miles; in +this space[3] none of the innumerable islets united into great rings +rise above the stated level. The Gilbert group is very narrow, and 300 +miles in length. In a prolonged line from this group, at the distance +of 240 miles, is the Marshall Archipelago, the figure of which is an +irregular square, one end being broader than the other; its length is +520 miles, with an average width of 240; these two groups together are +1,040 miles in length, and all their islets are low. Between the +southern end of the Gilbert and the northern end of Low Archipelago, +the ocean is thinly strewed with islands, all of which, as far as I +have been able to ascertain, are low; so that from nearly the southern +end of the Low Archipelago, to the northern end of the Marshall +Archipelago, there is a narrow band of ocean, more than 4,000 miles in +length, containing a great number of islands, all of which are low. In +the western part of the Caroline Archipelago, there is a space of 480 +miles in length, and about 100 broad, thinly interspersed with low +islands. Lastly, in the Indian Ocean, the archipelago of the Maldivas +is 470 miles in length, and 60 in breadth; that of the Laccadives is +150 by 100 miles; as there is a low island between these two groups, +they may be considered as one group of 1,000 miles in length. To this +may be added the Chagos group of low islands, situated 280 miles +distant, in a line prolonged from the southern extremity of the +Maldivas. This group, including the submerged banks, is 170 miles in +length and 80 in breadth. So striking is the uniformity in direction of +these three archipelagoes, all the islands of which are low, that +Captain Moresby, in one of his papers, speaks of them as parts of one +great chain, nearly 1,500 miles long. I am, then, fully justified in +repeating, that enormous spaces, both in the Pacific and Indian Oceans, +are interspersed with islands, of which not one rises above that +height, to which the waves and winds in an open sea can heap up matter. + + [2] See also Captain Owen’s and Lieutenant Wood’s papers in the + “Geographical Journal”, on the Maldiva and Laccadive Archipelagoes. + These officers particularly refer to the lowness of the islets; but I + chiefly ground my assertion respecting these two groups, and the + Chagos group, from information communicated to me by Captain Moresby. + + + [3] I find from Mr. Couthouy’s pamphlet (page 58) that Aurora Island + is about two hundred feet in height; it consists of coral-rock, and + seems to have been formed by the elevation of an atoll. It lies + north-east of Tahiti, close without the line bounding the space + coloured dark blue in the map appended to this volume. Honden Island, + which is situated in the extreme north-west part of the Low + Archipelago, according to measurements made on board the _Beagle_, + whilst sailing by, is 114 feet from the _summit of the trees_ to the + water’s edge. This island appeared to resemble the other atolls of the + group. + + +On what foundations, then, have these reefs and islets of coral been +constructed? A foundation must originally have been present beneath +each atoll at that limited depth, which is indispensable for the first +growth of the reef-building polypifers. A conjecture will perhaps be +hazarded, that the requisite bases might have been afforded by the +accumulation of great banks of sediment, which owing to the action of +superficial currents (aided possibly by the undulatory movement of the +sea) did not quite reach the surface,—as actually appears to have been +the case in some parts of the West Indian Sea. But in the form and +disposition of the groups of atolls, there is nothing to countenance +this notion; and the assumption without any proof, that a number of +immense piles of sediment have been heaped on the floor of the great +Pacific and Indian Oceans, in their central parts far remote from land, +and where the dark blue colour of the limpid water bespeaks its purity, +cannot for one moment be admitted. + +The many widely-scattered atolls must, therefore, rest on rocky bases. +But we cannot believe that the broad summit of a mountain lies buried +at the depth of a few fathoms beneath every atoll, and nevertheless +throughout the immense areas above-named, with not one point of rock +projecting above the level of the sea; for we may judge with some +accuracy of mountains beneath the sea, by those on the land; and where +can we find a single chain several hundred miles in length and of +considerable breadth, much less several such chains, with their many +broad summits attaining the same height, within from 120 to 180 feet? +If the data be thought insufficient, on which I have grounded my +belief, respecting the depth at which the reef-building polypifers can +exist, and it be assumed that they can flourish at a depth of even one +hundred fathoms, yet the weight of the above argument is but little +diminished, for it is almost equally improbable, that as many submarine +mountains, as there are low islands in the several great and widely +separated areas above specified, should all rise within six hundred +feet of the surface of the sea and not one above it, as that they +should be of the same height within the smaller limit of one or two +hundred feet. So highly improbable is this supposition, that we are +compelled to believe, that the bases of the many atolls did never at +any one period all lie submerged within the depth of a few fathoms +beneath the surface, but that they were brought into the requisite +position or level, some at one period and some at another, through +movements in the earth’s crust. But this could not have been effected +by elevation, for the belief that points so numerous and so widely +separated were successively uplifted to a certain level, but that not +one point was raised above that level, is quite as improbable as the +former supposition, and indeed differs little from it. It will probably +occur to those who have read Ehrenberg’s account of the Reefs of the +Red Sea, that many points in these great areas may have been elevated, +but that as soon as raised, the protuberant parts were cut off by the +destroying action of the waves: a moment’s reflection, however, on the +basin-like form of the atolls, will show that this is impossible; for +the upheaval and subsequent abrasion of an island would leave a flat +disc, which might become coated with coral, but not a deeply concave +surface; moreover, we should expect to see, in some parts at least, the +rock of the foundation brought to the surface. If, then, the +foundations of the many atolls were not uplifted into the requisite +position, they must of necessity have subsided into it; and this at +once solves every difficulty,[4] for we may safely infer, from the +facts given in the last chapter, that during a gradual subsidence the +corals would be favourably circumstanced for building up their solid +frame works and reaching the surface, as island after island slowly +disappeared. Thus areas of immense extent in the central and most +profound parts of the great oceans, might become interspersed with +coral-islets, none of which would rise to a greater height than that +attained by detritus heaped up by the sea, and nevertheless they might +all have been formed by corals, which absolutely required for their +growth a solid foundation within a few fathoms of the surface. + + [4] The additional difficulty on the crater hypothesis before alluded + to, will now be evident; for on this view the volcanic action must be + supposed to have formed within the areas specified a vast number of + craters, all rising within a few fathoms of the surface, and not one + above it. The supposition that the craters were at different times + upraised above the surface, and were there abraded by the surf and + subsequently coated by corals, is subject to nearly the same + objections with those given above in this paragraph; but I consider it + superfluous to detail all the arguments opposed to such a notion. + Chamisso’s theory, from assuming the existence of so many banks, all + lying at the proper depth beneath the water, is also vitally + defective. The same observation applies to an hypothesis of Lieutenant + Nelson’s (“Geolog. Trans.” volume v., page 122), who supposes that the + ring-formed structure is caused by a greater number of germs of corals + becoming attached to the declivity, than to the central plateau of a + submarine bank: it likewise applies to the notion formerly entertained + (Forster’s “Observ.” page 151), that lagoon-islands owe their peculiar + form to the instinctive tendencies of the polypifers. According to + this latter view, the corals on the outer margin of the reef + instinctively expose themselves to the surf in order to afford + protection to corals living in the lagoon, which belong to other + genera, and to other families! + + +It would be out of place here to do more than allude to the many facts, +showing that the supposition of a gradual subsidence over large areas +is by no means improbable. We have the clearest proof that a movement +of this kind is possible, in the upright trees buried under the strata +many thousand feet in thickness; we have also every reason for +believing that there are now large areas gradually sinking, in the same +manner as others are rising. And when we consider how many parts of the +surface of the globe have been elevated within recent geological +periods, we must admit that there have been subsidences on a +corresponding scale, for otherwise the whole globe would have swollen. +It is very remarkable that Mr. Lyell,[5] even in the first edition of +his “Principles of Geology,” inferred that the amount of subsidence in +the Pacific must have exceeded that of elevation, from the area of land +being very small relatively to the agents there tending to form it, +namely, the growth of coral and volcanic action. But it will be asked, +are there any direct proofs of a subsiding movement in those areas, in +which subsidence will explain a phenomenon otherwise inexplicable? +This, however, can hardly be expected, for it must ever be most +difficult, excepting in countries long civilised, to detect a movement, +the tendency of which is to conceal the part affected. In barbarous and +semi-civilised nations how long might not a slow movement, even of +elevation such as that now affecting Scandinavia, have escaped +attention! + + [5] “Principles of Geology,” sixth edition, volume iii., page 386. + + +Mr. Williams[6] insists strongly that the traditions of the natives, +which he has taken much pains in collecting, do not indicate the +appearance of any new islands: but on the theory of a gradual +subsidence, all that would be apparent would be, the water sometimes +encroaching slowly on the land, and the land again recovering by the +accumulation of detritus its former extent, and perhaps sometimes the +conversion of an atoll with coral islets on it, into a bare or into a +sunken annular reef. Such changes would naturally take place at the +periods when the sea rose above its usual limits, during a gale of more +than ordinary strength; and the effects of the two causes would be +hardly distinguishable. In Kotzebue’s “Voyage” there are accounts of +islands, both in the Caroline and Marshall Archipelagoes, which have +been partly washed away during hurricanes; and Kadu, the native who was +on board one of the Russian vessels, said “he saw the sea at Radack +rise to the feet of the cocoa-nut trees; but it was conjured in +time.”[7] A storm lately entirely swept away two of the Caroline +islands, and converted them into shoals; it partly, also, destroyed two +other islands.[8] According to a tradition which was communicated to +Captain Fitzroy, it is believed in the Low Archipelago, that the +arrival of the first ship caused a great inundation, which destroyed +many lives. Mr. Stutchbury relates, that in 1825, the western side of +Chain Atoll, in the same group, was completely devastated by a +hurricane, and not less than 300 lives lost: “in this instance it was +evident, even to the natives, that the hurricane alone was not +sufficient to account for the violent agitation of the ocean.”[9] That +considerable changes have taken place recently in some of the atolls in +the Low Archipelago, appears certain from the case already given of +Matilda Island: with respect to Whitsunday and Gloucester Islands in +this same group, we must either attribute great inaccuracy to their +discoverer, the famous circumnavigator Wallis, or believe that they +have undergone a considerable change in the period of fifty-nine years, +between his voyage and that of Captain Beechey’s. Whitsunday Island is +described by Wallis as “about four miles long, and three wide,” now it +is only one mile and a half long. The appearance of Gloucester Island, +in Captain Beechey’s words,[10] “has been accurately described by its +discoverer, but its present form and extent differ materially.” +Blenheim reef, in the Chagos group, consists of a water-washed annular +reef, thirteen miles in circumference, surrounding a lagoon ten fathoms +deep: on its surface there were a few worn patches of conglomerate +coral-rock, of about the size of hovels; and these Captain Moresby +considered as being, without doubt, the last remnants of islets; so +that here an atoll has been converted into an atoll-formed reef. The +inhabitants of the Maldiva Archipelago, as long ago as 1605, declared, +“that the high tides and violent currents were diminishing the number +of the islands:”[11] and I have already shown, on the authority of +Captain Moresby, that the work of destruction is still in progress; but +that on the other hand the first formation of some islets is known to +the present inhabitants. In such cases, it would be exceedingly +difficult to detect a gradual subsidence of the foundation, on which +these mutable structures rest. + + [6] Williams’s “Narrative of Missionary Enterprise,” page 31. + + + [7] Kotzebue’s “First Voyage,” volume iii., page 168. + + + [8] M. Desmoulins in “Comptes Rendus,” 1840, page 837. + + + [9] “West of England Journal”, No. I., page 35. + + + [10] Beechey’s “Voyage to the Pacific,” chapter vii., and Wallis’s + “Voyage in the ‘Dolphin’,” chapter iv. + + + [11] See an extract from Pyrard’s Voyage in Captain Owen’s paper on + the Maldiva Archipelago, in the “Geographical Journal”, volume ii., + page 84. + + +Some of the archipelagoes of low coral-islands are subject to +earthquakes: Captain Moresby informs me that they are frequent, though +not very strong, in the Chagos group, which occupies a very central +position in the Indian Ocean, and is far from any land not of coral +formation. One of the islands in this group was formerly covered by a +bed of mould, which, after an earthquake, disappeared, and was believed +by the residents to have been washed by the rain through the broken +masses of underlying rock; the island was thus rendered unproductive. +Chamisso[12] states, that earthquakes are felt in the Marshall atolls, +which are far from any high land, and likewise in the islands of the +Caroline Archipelago. On one of the latter, namely Oulleay atoll, +Admiral Lutké, as he had the kindness to inform me, observed several +straight fissures about a foot in width, running for some hundred yards +obliquely across the whole width of the reef. Fissures indicate a +stretching of the earth’s crust, and, therefore, probably changes in +its level; but these coral-islands, which have been shaken and +fissured, certainly have not been elevated, and, therefore, probably +they have subsided. In the chapter on Keeling atoll, I attempted to +show by direct evidence, that the island underwent a movement of +subsidence, during the earthquakes lately felt there. + + [12] See Chamisso, in Kotzebue’s “First Voyage,” volume iii., pages + 182 and 136. + + +The facts stand thus;—there are many large tracts of ocean, without any +high land, interspersed with reefs and islets, formed by the growth of +those kinds of corals, which cannot live at great depths; and the +existence of these reefs and low islets, in such numbers and at such +distant points, is quite inexplicable, excepting on the theory, that +the bases on which the reefs first became attached, slowly and +successively sank beneath the level of the sea, whilst the corals +continued to grow upwards. No positive facts are opposed to this view, +and some general considerations render it probable. There is evidence +of change in form, whether or not from subsidence, on some of these +coral-islands; and there is evidence of subterranean disturbances +beneath them. Will then the theory, to which we have thus been led, +solve the curious problem,—what has given to each class of reef its +peculiar form? + +[Illustration] + +AA—Outer edge of the reef at the level of the sea. + +BB—Shores of the island. + +A′A′—Outer edge of the reef, after its upward growth during a period of +subsidence. + +CC—The lagoon-channel between the reef and the shores of the now +encircled land. + +B′B′—The shores of the encircled island. + +N.B.—In this, and the following woodcut, the subsidence of the land +could only be represented by an apparent rise in the level of the sea. + +[Illustration] + +A′A′—Outer edges of the barrier-reef at the level of the sea. The +cocoa-nut trees represent coral-islets formed on the reef. + +CC—The lagoon-channel. + +B′B′—The shores of the island, generally formed of low alluvial land +and of coral detritus from the lagoon-channel. + +A″A″—The outer edges of the reef now forming an atoll. + +C′—The lagoon of the newly formed atoll. According to the scale, the +depth of the lagoon and of the lagoon-channel is exaggerated. + +Let us in imagination place within one of the subsiding areas, an +island surrounded by a “fringing-reef,”—that kind, which alone offers +no difficulty in the explanation of its origin. Let the unbroken lines +and the oblique shading in the woodcut (No. 4) represent a vertical +section through such an island; and the horizontal shading will +represent the section of the reef. Now, as the island sinks down, +either a few feet at a time or quite insensibly, we may safely infer +from what we know of the conditions favourable to the growth of coral, +that the living masses bathed by the surf on the margin of the reef, +will soon regain the surface. The water, however, will encroach, little +by little, on the shore, the island becoming lower and smaller, and the +space between the edge of the reef and the beach proportionately +broader. A section of the reef and island in this state, after a +subsidence of several hundred feet, is given by the dotted lines: +coral-islets are supposed to have been formed on the new reef, and a +ship is anchored in the lagoon-channel. This section is in every +respect that of an encircling barrier-reef; it is, in fact, a section +taken[13] E. and W. through the highest point of the encircled island +of Bolabola; of which a plan is given in Plate I., Figure 5. The same +section is more clearly shown in the following woodcut (No. 5) by the +unbroken lines. The width of the reef, and its slope, both on the outer +and inner side, will have been determined by the growing powers of the +coral, under the conditions (for instance the force of the breakers and +of the currents) to which it has been exposed; and the lagoon-channel +will be deeper or shallower, in proportion to the growth of the +delicately branched corals within the reef, and to the accumulation of +sediment, relatively, also, to the rate of subsidence and the length of +the intervening stationary periods. + + [13] The section has been made from the chart given in the “Atlas of + the Voyage of the _Coquille_.” The scale is .57 of an inch to a mile. + The height of the island, according to M. Lesson, is 4,026 feet. The + deepest part of the lagoon-channel is 162 feet; its depth is + exaggerated in the woodcut for the sake of clearness. + + +It is evident in this section, that a line drawn perpendicularly down +from the outer edge of the new reef to the foundation of solid rock, +exceeds by as many feet as there have been feet of subsidence, that +small limit of depth at which the effective polypifers can live—the +corals having grown up, as the whole sank down, from a basis formed of +other corals and their consolidated fragments. Thus the difficulty on +this head, which before seemed so great, disappears. + +As the space between the reef and the subsiding shore continued to +increase in breadth and depth, and as the injurious effects of the +sediment and fresh water borne down from the land were consequently +lessened, the greater number of the channels, with which the reef in +its fringing state must have been breached, especially those which +fronted the smaller streams, will have become choked up with the growth +of coral: on the windward side of the reef, where the coral grows most +vigorously, the breaches will probably have first been closed. In +barrier-reefs, therefore, the breaches kept open by draining the tidal +waters of the lagoon-channel, will generally be placed on the leeward +side, and they will still face the mouths of the larger streams, +although removed beyond the influence of their sediment and fresh +water;—and this, it has been shown, is commonly the case. + +Referring to the diagram shown above, in which the newly formed +barrier-reef is represented by unbroken lines, instead of by dots as in +the former woodcut, let the work of subsidence go on, and the doubly +pointed hill will form two small islands (or more, according to the +number of the hills) included within one annular reef. Let the island +continue subsiding, and the coral-reef will continue growing up on its +own foundation, whilst the water gains inch by inch on the land, until +the last and highest pinnacle is covered, and there remains a perfect +atoll. A vertical section of this atoll is shown in the woodcut by the +dotted lines;—a ship is anchored in its lagoon, but islets are not +supposed yet to have been formed on the reef. The depth of the lagoon +and the width and slope of the reef, will depend on the circumstances +just referred to under barrier-reefs. Any further subsidence will +produce no change in the atoll, except perhaps a diminution in its +size, from the reef not growing vertically upwards; but should the +currents of the sea act violently upon it, and should the corals perish +on part or on the whole of its margin, changes would result during +subsidence which will be presently noticed. I may here observe, that a +bank either of rock or of hardened sediment, level with the surface of +the sea, and fringed with living coral, would (if not so small as to +allow the central space to be quickly filled up with detritus) by +subsidence be converted immediately into an atoll, without passing, as +in the case of a reef fringing the shore of an island, through the +intermediate form of a barrier-reef. If such a bank lay a few fathoms +submerged, the simple growth of the coral (as remarked in the third +chapter) without the aid of subsidence, would produce a structure +scarcely to be distinguished from a true atoll; for in all cases the +corals on the outer margin of a reef, from having space and being +freely exposed to the open sea, will grow vigorously and tend to form a +continuous ring whilst the growth of the less massive kinds on the +central expanse, will be checked by the sediment formed there, and by +that washed inwards by the breakers; and as the space becomes +shallower, their growth will, also, be checked by the impurities of the +water, and probably by the small amount of food brought by the +enfeebled currents, in proportion to the surface of living reefs +studded with innumerable craving mouths: the subsidence of a reef based +on a bank of this kind, would give depth to its central expanse or +lagoon, steepness to its flanks, and through the free growth of the +coral, symmetry to its outline:—I may here repeat that the larger +groups of atolls in the Pacific and Indian Oceans cannot be supposed to +be founded on banks of this nature. + +If, instead of the island in the diagram, the shore of a continent +fringed by a reef had subsided, a great barrier-reef, like that on the +north-east coast of Australia, would have necessarily resulted; and it +would have been separated from the main land by a deep-water channel, +broad in proportion to the amount of subsidence, and to the less or +greater inclination of the neighbouring coast-line. The effect of the +continued subsidence of a great barrier-reef of this kind, and its +probable conversion into a chain of separate atolls, will be noticed, +when we discuss the apparent progressive disseverment of the larger +Maldiva atolls. + +We now are able to perceive that the close similarity in form, +dimensions, structure, and relative position (which latter point will +hereafter be more fully noticed) between fringing and encircling +barrier-reefs, and between these latter and atolls, is the necessary +result of the transformation, during subsidence of the one class into +the other. On this view, the three classes of reefs ought to graduate +into each other. Reefs having intermediate character between those of +the fringing and barrier classes do exist; for instance, on the +south-west coast of Madagascar, a reef extends for several miles, +within which there is a broad channel from seven to eight fathoms deep, +but the sea does not deepen abruptly outside the reef. Such cases, +however, are open to some doubts, for an old fringing-reef, which had +extended itself a little on a basis of its own formation, would hardly +be distinguishable from a barrier-reef, produced by a small amount of +subsidence, and with its lagoon-channel nearly filled up with sediment +during a long stationary period. Between barrier-reefs, encircling +either one lofty island or several small low ones, and atolls including +a mere expanse of water, a striking series can be shown: in proof of +this, I need only refer to the first plate in this volume, which speaks +more plainly to the eye, than any description could to the ear. The +authorities from which the charts have been engraved, together with +some remarks on them and descriptive of the plates, are given above. At +New Caledonia (Plate II., Figure 5.) the barrier-reefs extend for 150 +miles on each side of the submarine prolongation of the island; and at +their northern extremity they appear broken up and converted into a +vast atoll-formed reef, supporting a few low coral-islets: we may +imagine that we here see the effects of subsidence actually in +progress, the water always encroaching on the northern end of the +island, towards which the mountains slope down, and the reefs steadily +building up their massive fabrics in the lines of their ancient growth. + +We have as yet only considered the origin of barrier-reefs and atolls +in their simplest form; but there remain some peculiarities in +structure and some special cases, described in the two first chapters, +to be accounted for by our theory. These consist—in the inclined ledge +terminated by a wall, and sometimes succeeded by a second ledge with a +wall, round the shores of certain lagoons and lagoon-channels; a +structure which cannot, as I endeavoured to show, be explained by the +simple growing powers of the corals,—in the ring or basin-like forms of +the central reefs, as well as of the separate marginal portions of the +northern Maldiva atolls,—in the submerged condition of the whole, or of +parts of certain barrier and atoll-formed reefs; where only a part is +submerged, this being generally to leeward,—in the apparent progressive +disseverment of some of the Maldiva atolls,—in the existence of +irregularly formed atolls, some being tied together by linear reefs, +and others with spurs projecting from them,—and, lastly, in the +structure and origin of the Great Chagos bank. + +_Step-formed ledges round certain lagoons_.—If we suppose an atoll to +subside at an extremely slow rate, it is difficult to follow out the +complex results. The living corals would grow up on the outer margin; +and likewise probably in the gullies and deeper parts of the bare +surface of the annular reef; the water would encroach on the islets, +but the accumulation of fresh detritus might possibly prevent their +entire submergence. After a subsidence of this very slow nature, the +surface of the annular reef sloping gently into the lagoon, would +probably become united with the irregular reefs and banks of sand, +which line the shores of most lagoons. Should, however, the atoll be +carried down by a more rapid movement, the whole surface of the annular +reef, where there was a foundation of solid matter, would be favourably +circumstanced for the fresh growth of coral; but as the corals grew +upwards on its exterior margin, and the waves broke heavily on this +part, the increase of the massive polypifers on the inner side would be +checked from the want of water. Consequently, the exterior parts would +first reach the surface, and the new annular reef thus formed on the +old one, would have its summit inclined inwards, and be terminated by a +subaqueous wall, formed by the upward growth of the coral (before being +much checked), from the inner edge of the solid parts of the old reef. +The inner portion of the new reef, from not having grown to the +surface, would be covered by the waters of the lagoon. Should a +subsidence of the same kind be repeated, the corals would again grow up +in a wall, from all the solid parts of the resunken reef, and, +therefore, not from within the sandy shores of the lagoon; and the +inner part of the new annular reef would, from being as before checked +in its upward growth, be of less height than the exterior parts, and +therefore would not reach the surface of the lagoon. In this case the +shores of the lagoon would be surrounded by two inclined ledges, one +beneath the other, and both abruptly terminated by subaqueous +cliffs.[14] + + [14] According to Mr. Couthouy (page 26) the external reef round many + atolls descends by a succession of ledges or terraces. He attempts, I + doubt whether successfully, to explain this structure somewhat in the + same manner as I have attempted, with respect to the internal ledges + round the lagoons of some atolls. More facts are wanted regarding the + nature both of the interior and exterior step-like ledges: are all the + ledges, or only the upper ones, covered with living coral? If they are + all covered, are the kinds different on the ledges according to the + depth? Do the interior and exterior ledges occur together in the same + atolls; if so, what is their total width, and is the intervening + surface-reef narrow, etc.? + + +_The ring or basin-formed reefs of the northern Maldiva atolls_.—I may +first observe, that the reefs within the lagoons of atolls and within +lagoon-channels, would, if favourably circumstanced, grow upwards +during subsidence in the same manner as the annular rim; and, +therefore, we might expect that such lagoon-reefs, when not surrounded +and buried by an accumulation of sediment more rapid than the rate of +subsidence, would rise abruptly from a greater depth than that at which +the efficient polypifers can flourish: we see this well exemplified in +the small abruptly-sided reefs, with which the deep lagoons of the +Chagos and Southern Maldiva atolls are studded. With respect to the +ring or basin-formed reefs of the Northern Maldiva atolls, it is +evident, from the perfectly continuous series which exists that the +marginal rings, although wider than the exterior or bounding reef of +ordinary atolls, are only modified portions of such a reef; it is also +evident that the central rings, although wider than the knolls or reefs +which commonly occur in lagoons, occupy their place. The ring-like +structure has been shown to be contingent on the breaches into the +lagoon being broad and numerous, so that all the reefs which are bathed +by the waters of the lagoon are placed under nearly the same conditions +with the outer coast of an atoll standing in the open sea. Hence the +exterior and living margins of these reefs must have been favourably +circumstanced for growing outwards, and increasing beyond the usual +breadth; and they must likewise have been favourably circumstanced for +growing vigorously upwards, during the subsiding movements, to which by +our theory the whole archipelago has been subjected; and subsidence +with this upward growth of the margins would convert the central space +of each little reef into a small lagoon. This, however, could only take +place with those reefs, which had increased to a breadth sufficient to +prevent their central spaces from being almost immediately filled up +with the sand and detritus driven inwards from all sides: hence it is +that few reefs, which are less than half a mile in diameter, even in +the atolls where the basin-like structure is most strikingly exhibited, +include lagoons. This remark, I may add, applies to all coral-reefs +wherever found. The basin-formed reefs of the Maldiva Archipelago may, +in fact, be briefly described, as small atolls formed during subsidence +over the separate portions of large and broken atolls, in the same +manner as these latter were formed over the barrier-reefs, which +encircled the islands of a large archipelago now wholly submerged. + +_Submerged and Dead Reefs_.—In the second section of the first chapter, +I have shown that there are in the neighbourhood of atolls, some deeply +submerged banks, with level surfaces; that there are others, less +deeply but yet wholly submerged, having all the characters of perfect +atolls, but consisting merely of dead coral-rock; that there are +barrier-reefs and atolls with merely a portion of their reef, generally +on the leeward side, submerged; and that such portions either retain +their perfect outline, or they appear to be quite effaced, their former +place being marked only by a bank, conforming in outline with that part +of the reef which remains perfect. These several cases are, I believe, +intimately related together, and can be explained by the same means. +There, perhaps, exist some submerged reefs, covered with living coral +and growing upwards, but to these I do not here refer. + +As we see that in those parts of the ocean, where coral-reefs are most +abundant, one island is fringed and another neighbouring one is not +fringed; as we see in the same archipelago, that all the reefs are more +perfect in one part of it than in another, for instance, in the +southern half compared with the northern half of the Maldiva +Archipelago, and likewise on the outer coasts compared with the inner +coasts of the atolls in this same group, which are placed in a double +row; as we know that the existence of the innumerable polypifers +forming a reef, depends on their sustenance, and that they are preyed +on by other organic beings; and, lastly, as we know that some inorganic +causes are highly injurious to the growth of coral, it cannot be +expected that during the round of change to which earth, air, and water +are exposed, the reef-building polypifers should keep alive for +perpetuity in any one place; and still less can this be expected, +during the progressive subsidences, perhaps at some periods more rapid +than at others, to which by our theory these reefs and islands have +been subjected and are liable. It is, then, not improbable that the +corals should sometimes perish either on the whole or on part of a +reef; if on part, the dead portion, after a small amount of subsidence, +would still retain its proper outline and position beneath the water. +After a more prolonged subsidence, it would probably form, owing to the +accumulation of sediment, only the margin of a flat bank, marking the +limits of the former lagoon. Such dead portions of reef would generally +lie on the leeward side,[15] for the impure water and fine sediment +would more easily flow out from the lagoon over this side of the reef, +where the force of the breakers is less than to windward; and therefore +the corals would be less vigorous on this side, and be less able to +resist any destroying agent. It is likewise owing to this same cause, +that reefs are more frequently breached to leeward by narrow channels, +serving as by ship-channels, than to windward. If the corals perished +entirely, or on the greater part of the circumference of an atoll, an +atoll-shaped bank of dead rock, more or less entirely submerged, would +be produced; and further subsidence, together with the accumulation of +sediment, would often obliterate its atoll-like structure, and leave +only a bank with a level surface. + + [15] Mr. Lyell, in the first edition of his “Principles of Geology,” + offered a somewhat different explanation of this structure. He + supposes that there has been subsidence; but he was not aware that the + submerged portions of reef were in most cases, if not in all, dead; + and he attributes the difference in height in the two sides of most + atolls, chiefly to the greater accumulation of detritus to windward + than to leeward. But as matter is accumulated only on the backward + part of the reef, the front part would remain of the same height on + both sides. I may here observe that in most cases (for instance, at + Peros Banhos, the Gambier group and the Great Chagos Bank), and I + suspect in all cases, the dead and submerged portions do not blend or + slope into the living and perfect parts, but are separated from them + by an abrupt line. In some instances small patches of living reef rise + to the surface from the middle of the submerged and dead parts. + + +In the Chagos group of atolls, within an area of 160 miles by 60, there +are two atoll-formed banks of dead rock (besides another very imperfect +one), entirely submerged; a third, with merely two or three very small +pieces of living reef rising to the surface; and a fourth, namely, +Peros Banhos (Plate I., Figure 9), with a portion nine miles in length +dead and submerged. As by our theory this area has subsided, and as +there is nothing improbable in the death, either from changes in the +state of the surrounding sea or from the subsidence being great or +sudden, of the corals on the whole, or on portions of some of the +atolls, the case of the Chagos group presents no difficulty. So far +indeed are any of the above-mentioned cases of submerged reefs from +being inexplicable, that their occurrence might have been anticipated +on our theory, and as fresh atolls are supposed to be in progressive +formation by the subsidence of encircling barrier-reefs, a weighty +objection, namely that the number of atolls must be increasing +infinitely, might even have been raised, if proofs of the occasional +destruction and loss of atolls could not have been adduced. + +_The disseverment of the larger Maldiva atolls_.—The apparent +progressive disseverment in the Maldiva Archipelago of large atolls +into smaller ones, is, in many respects, an important consideration, +and requires an explanation. The graduated series which marks, as I +believe, this process, can be observed only in the northern half of the +group, where the atolls have exceedingly imperfect margins, consisting +of detached basin-formed reefs. The currents of the sea flow across +these atolls, as I am informed by Captain Moresby, with considerable +force, and drift the sediment from side to side during the monsoons, +transporting much of it seaward; yet the currents sweep with greater +force round their flanks. It is historically known that these atolls +have long existed in their present state; and we can believe, that even +during a very slow subsidence they might thus remain, the central +expanse being kept at nearly its original depth by the accumulation of +sediment. But in the action of such nicely balanced forces during a +progressive subsidence (like that, to which by our theory this +archipelago has been subjected), it would be strange if the currents of +the sea should never make a direct passage across some one of the +atolls, through the many wide breaches in their margins. If this were +once effected, a deep-water channel would soon be formed by the removal +of the finer sediment, and the check to its further accumulation; and +the sides of the channel would be worn into a slope like that on the +outer coasts, which are exposed to the same force of the currents. In +fact, a channel precisely like that bifurcating one which divides +Mahlos Mahdoo (Plate II., Figure 4.), would almost necessarily be +formed. The scattered reefs situated near the borders of the new +ocean-channel, from being favourably placed for the growth of coral, +would, by their extension, tend to produce fresh margins to the +dissevered portions; such a tendency is very evident (as may be seen in +the large published chart) in the elongated reefs on the borders of the +two channels intersecting Mahlos Mahdoo. Such channels would become +deeper with continued subsidence, and probably from the reefs not +growing up perpendicularly, somewhat broader. In this case, and more +especially if the channels had been formed originally of considerable +breadth, the dissevered portions would become perfect and distinct +atolls, like Ari and Ross atolls (Plate II., Figure 6), or like the two +Nillandoo atolls, which must be considered as distinct, although +related in form and position, and separated from each other by +channels, which though deep have been sounded. Further subsidence would +render such channels unfathomable, and the dissevered portions would +then resemble Phaleedoo and Moluque atolls, or Mahlos Mahdoo and +Horsburgh atolls (Plate II., Figure 4), which are related to each other +in no respect except in proximity and position. Hence, on the theory of +subsidence, the disseverment of large atolls, which have imperfect +margins (for otherwise their disseverment would be scarcely possible), +and which are exposed to strong currents, is far from being an +improbable event; and the several stages, from close relation to entire +isolation in the atolls of the Maldiva Archipelago, are readily +explicable. + +We might go even further, and assert as not improbable, that the first +formation of the Maldiva Archipelago was due to a barrier-reef, of +nearly the same dimensions with that of New Caledonia (Plate II., +Figure 5), for if, in imagination, we complete the subsidence of that +great island, we might anticipate from the present broken condition of +the northern portion of the reef, and from the almost entire absence of +reefs on the eastern coast, that the barrier-reef after repeated +subsidences, would become during its upward growth separated into +distinct portions; and these portions would tend to assume an +atoll-like structure, from the coral growing with vigour round their +entire circumferences, when freely exposed to an open sea. As we have +some large islands partly submerged with barrier-reefs marking their +former limits, such as New Caledonia, so our theory makes it probable +that there should be other large islands wholly submerged; and these, +we may now infer, would be surmounted, not by one enormous atoll, but +by several large elongated ones, like the atolls in the Maldiva group; +and these again, during long periods of subsidence, would sometimes +become dissevered into smaller atolls. I may add, that both in the +Marshall and Caroline Archipelagoes, there are atolls standing close +together, which have an evident relationship in form: we may suppose, +in such cases, either that two or more encircled islands originally +stood close together, and afforded bases for two or more atolls, or +that one atoll has been dissevered. From the position, as well as form, +of three atolls in the Caroline Archipelago (the Namourrek and Elato +group), which are placed in an irregular circle, I am strongly tempted +to believe that they have originated by the process of +disseverment.[16] + + [16] The same remark is, perhaps, applicable to the islands of Ollap, + Fanadik, and Tamatam in the Caroline Archipelago, of which charts are + given in the atlas of Duperrey’s voyage: a line drawn through the + linear reefs and lagoons of these three islands forms a semicircle. + Consult also, the atlas of Lutké’s voyage; and for the Marshall group + that of Kotzebue; for the Gilbert group consult the atlas of + Duperrey’s voyage. Most of the points here referred to may, however, + be seen in Krusenstern’s general Atlas of the Pacific. + + +_Irregularly formed Atolls_.—In the Marshall group, Musquillo atoll +consists of two loops united in one point; and Menchikoff atoll is +formed of three loops, two of which (as may be seen in Figure 3, Plate +II.) are connected by a mere ribbon-shaped reef, and the three together +are sixty miles in length. In the Gilbert group some of the atolls have +narrow strips of reef, like spurs, projecting from them. There occur +also in parts of the open sea, a few linear and straight reefs, +standing by themselves; and likewise some few reefs in the form of +crescents, with their extremities more or less curled inwards. Now, the +upward growth of a barrier-reef which fronted only one side of an +island, or one side of an elongated island with its extremities (of +which cases exist), would produce after the complete subsidence of the +land, mere strips or crescent or hook-formed reefs: if the island thus +partially fronted became divided during subsidence into two or more +islands, these islands would be united together by linear reefs; and +from the further growth of the coral along their shores together with +subsidence, reefs of various forms might ultimately be produced, either +atolls united together by linear reefs, or atolls with spurs projecting +from them. Some, however, of the more simple forms above specified, +might, as we have seen, be equally well produced by the coral perishing +during subsidence on part of the circumference of an atoll, whilst on +the other parts it continued to grow up till it reached the surface. + +_The Great Chagos Bank_.—I have already shown that the submerged +condition of the Great Chagos Bank (Plate II., Figure 1, with its +section Figure 2), and of some other banks in the Chagos group, may in +all probability be attributed to the coral having perished before or +during the movements of subsidence, to which this whole area by our +theory has been subjected. The external rim or upper ledge (shaded in +the chart), consists of dead coral-rock thinly covered with sand; it +lies at an average depth of between five and eight fathoms, and +perfectly resembles in form the annular reef of an atoll. The banks of +the second level, the boundaries of which are marked by dotted lines in +the chart, lie from about fifteen to twenty fathoms beneath the +surface; they are several miles broad, and terminate in a very steep +slope round the central expanse. This central expanse I have already +described, as consisting of a level muddy flat between thirty and forty +fathoms deep. The banks of the second level, might at first sight be +thought analogous to the internal step-like ledge of coral-rock which +borders the lagoons of some atolls, but their much greater width, and +their being formed of sand, are points of essential difference. On the +eastern side of the atoll some of the banks are linear and parallel, +resembling islets in a great river, and pointed directly towards a +great breach on the opposite side of the atoll; these are best seen in +the large published chart. I inferred from this circumstance, that +strong currents sometimes set directly across this vast bank; and I +have since heard from Captain Moresby that this is the case. I +observed, also, that the channels or breaches through the rim, were all +of the same depth as the central lagoon-like space into which they +lead; whereas the channels into the other atolls of the Chagos group, +and as I believe into most other large atolls, are not nearly as deep +as their lagoons: for instance at Peros Banhos, the channels are only +of the same depth, namely between ten and twenty fathoms, as the bottom +of the lagoon for a space about a mile and a half in width round its +shores, whilst the central expanse of the lagoon is from thirty-five to +forty fathoms deep. Now, if an atoll during a gradual subsidence once +became entirely submerged, like the Great Chagos Bank, and therefore no +longer exposed to the surf, very little sediment could be formed from +it; and consequently the channels leading into the lagoon from not +being filled up with drifted sand and coral detritus, would continue +increasing in depth, as the whole sank down. In this case, we might +expect that the currents of the open sea, instead of any longer +sweeping round the submarine flanks, would flow directly through the +breaches across the lagoon, removing in their course the finer +sediment, and preventing its further accumulation. We should then have +the submerged reef forming an external and upper rim of rock, and +beneath this portion of the sandy bottom of the old lagoon, intersected +by deep-water channels or breaches, and thus formed into separate +marginal banks; and these would be cut off by steep slopes, overhanging +the central space, worn down by the passage of the oceanic currents. + +By these means, I have scarcely any doubt that the Great Chagos Bank +has originated,—a structure which at first appeared to me far more +anomalous than any I had met with. The process of formation is nearly +the same with that, by which Mahlos Mahdoo had been trisected; but in +the Chagos Bank the channels of the oceanic currents entering at +several different quarters, have united in a central space. + +This great atoll-formed bank appears to be in an early stage of +disseverment; should the work of subsidence go on, from the submerged +and dead condition of the whole reef, and the imperfection of the +south-east quarter a mere wreck would probably be left. The Pitt’s +Bank, situated not far southward, appears to be precisely in this +state; it consists of a moderately level, oblong bank of sand, lying +from 10 to 20 fathoms beneath the surface, with two sides protected by +a narrow ledge of rock which is submerged between 5 and 8 fathoms. A +little further south, at about the same distance as the southern rim of +the Great Chagos Bank is from the northern rim, there are two other +small banks with from 10 to 20 fathoms on them; and not far eastward +soundings were struck on a sandy bottom, with between 110 and 145 +fathoms. The northern portion with its ledge-like margin, closely +resembles any one segment of the Great Chagos Bank, between two of the +deep-water channels, and the scattered banks, southward appear to be +the last wrecks of less perfect portions. + +I have examined with care the charts of the Indian and Pacific Oceans, +and have now brought before the reader all the examples, which I have +met with, of reefs differing from the type of the class to which they +belong; and I think it has been satisfactorily shown, that they are all +included in our theory, modified by occasional accidents which might +have been anticipated as probable. In this course we have seen, that in +the lapse of ages encircling barrier-reefs are occasionally converted +into atolls, the name of atoll being properly applicable, at the moment +when the last pinnacle of encircled land sinks beneath the surface of +the sea. We have, also, seen that large atolls during the progressive +subsidence of the areas in which they stand, sometimes become +dissevered into smaller ones; at other times, the reef-building +polypifers having entirely perished, atolls are converted into +atoll-formed banks of dead rock; and these again through further +subsidence and the accumulation of sediment modified by the force of +the oceanic currents, pass into level banks with scarcely any +distinguishing character. Thus may the history of an atoll be followed +from its first origin, through the occasional accidents of its +existence, to its destruction and final obliteration. + +_Objections to the theory of the formation of Atolls and +Barrier-reefs_.—The vast amount of subsidence, both horizontally or in +area, and vertically or in depth, necessary to have submerged every +mountain, even the highest, throughout the immense spaces of ocean +interspersed with atolls, will probably strike most people as a +formidable objection to my theory. But as continents, as large as the +spaces supposed to have subsided, have been raised above the level of +the sea,—as whole regions are now rising, for instance, in Scandinavia +and South America,—and as no reason can be assigned, why subsidences +should not have occurred in some parts of the earth’s crust on as great +a scale both in extent and amount as those of elevation, objections of +this nature strike me as of little force. The remarkable point is that +movements to such an extent should have taken place within a period, +during which the polypifers have continued adding matter on and above +the same reefs. Another and less obvious objection to the theory will +perhaps be advanced from the circumstance, of the lagoons within atolls +and within barrier-reefs never having become in any one instance during +prolonged subsidences of a greater depth than sixty fathoms, and seldom +more than forty fathoms; but we already admit, if the theory be worth +considering, that the rate of subsidence has not exceeded that of the +upward growth of the coral on the exterior margin; we are, therefore, +only further required to admit, that the subsidence has not exceeded in +rate the filling up of the interior spaces by the growth of the corals +living there, and by the accumulation of sediment. As this filling up +must take place very slowly within barrier-reefs lying far from the +land, and within atolls which are of large dimensions and which have +open lagoons with very few reefs, we are led to conclude that the +subsidence thus counter-balanced, must have been slow in an +extraordinary degree; a conclusion which accords with our only means, +namely, with what is known of the rate and manner of recent elevatory +movements, of judging by analogy what is the probable rate of +subsidence. + +In this chapter it has, I think, been shown, that the theory of +subsidence, which we were compelled to receive from the necessity of +giving to the corals, in certain large areas, foundations at the +requisite depth, explains both the normal structure and the less +regular forms of those two great classes of reefs, which have justly +excited the astonishment of all persons who have sailed through the +Pacific and Indian Oceans. But further to test the truth of the theory, +a crowd of questions will occur to the reader: Do the different kinds +of reefs, which have been produced by the same kind of movement, +generally lie within the same areas? What is their relation of form and +position,—for instance, do adjoining groups of atolls, and the separate +atolls in these groups, bear the same relation to each other which +islands do in common archipelagoes? Have we reason to believe, that +where there are fringing-reefs, there has not lately been subsidence; +or, for it is almost our only way of ascertaining this point, are there +frequently proofs of recent elevation? Can we by this means account for +the presence of certain classes of reefs in some large areas, and their +entire absence in others? Do the areas which have subsided, as +indicated by the presence of atolls and barrier-reefs, and the areas +which have remained stationary or have been upraised, as shown by +fringing-reefs, bear any determinate relation to each other; and are +the dimensions of these areas such as harmonise with the greatness of +the subterranean changes, which, it must be supposed, have lately taken +place beneath them? Is there any connection between the movements thus +indicated, and recent volcanic action? All these questions ought to +receive answers in accordance with the theory; and if this can be +satisfactorily shown, not only is the theory confirmed, but as +deductions, the answers are in themselves important. Under this latter +point of view, these questions will be chiefly considered in the +following chapter.[17] + + [17] I may take this opportunity of briefly considering the + appearances, which would probably be presented by a vertical and deep + section across a coral formation (referring chiefly to an atoll), + formed by the upward growth of coral during successive subsidences. + This is a subject worthy of attention, as a means of comparison with + ancient coral-strata. The circumferential parts would consist of + massive species, in a vertical position, with their interstices filled + up with detritus; but this would be the part most subject to + subsequent denudation and removal. It is useless to speculate how + large a portion of the exterior annular reef would consist of upright + coral, and how much of fragmentary rock, for this would depend on many + contingencies,—such as on the rate of subsidence, occasionally + allowing a fresh growth of coral to cover the whole surface, and on + the breakers having force sufficient to throw fragments over this same + space. The conglomerate which composes the base of the islets, would + (if not removed by denudation together with the exterior reef on which + it rests) be conspicuous from the size of the fragments,—the different + degrees in which they have been rounded,—the presence of fragments of + conglomerate torn up, rounded, and recemented,—and from the oblique + stratification. The corals which lived in the lagoon-reefs at each + successive level, would be preserved upright, and they would consist + of many kinds, generally much branched. In this part, however, a very + large proportion of the rock (and in some cases nearly all of it) + would be formed of sedimentary matter, either in an excessively fine, + or in a moderately coarse state, and with the particles almost blended + together. The conglomerate which was formed of rounded pieces of the + branched corals, on the shores of the lagoon, would differ from that + formed on the islets and derived from the outer coast; yet both might + have accumulated very near each other. I have seen a conglomerate + limestone from Devonshire like a conglomerate now forming on the + shores of the Maldiva atolls. The stratification taken as a whole, + would be horizontal; but the conglomerate beds resting on the exterior + reef, and the beds of sandstone on the shores of the lagoon (and no + doubt on the external flanks) would probably be divided (as at Keeling + atoll and at Mauritius) by numerous layers dipping at considerable + angles in different directions. The calcareous sandstone and + coral-rock would almost necessarily contain innumerable shells, + echini, and the bones of fish, turtle, and perhaps of birds; possibly, + also, the bones of small saurians, as these animals find their way to + the islands far remote from any continent. The large shells of some + species of Tridacna would be found vertically imbedded in the solid + rock, in the position in which they lived. We might expect also to + find a mixture of the remains of pelagic and littoral animals in the + strata formed in the lagoon, for pumice and the seeds of plants are + floated from distant countries into the lagoons of many atolls: on the + outer coast of Keeling atoll, near the mouth of the lagoon, the case + of a pelagic Pteropodous animal was brought up on the arming of the + sounding lead. All the loose blocks of coral on Keeling atoll were + burrowed by vermiform animals; and as every cavity, no doubt, + ultimately becomes filled with spathose limestone, slabs of the rock + taken from a considerable depth, would, if polished, probably exhibit + the excavations of such burrowing animals. The conglomerate and + fine-grained beds of coral-rock would be hard, sonorous, white and + composed of nearly pure calcareous matter; in some few parts, judging + from the specimens at Keeling atoll, they would probably contain a + small quantity of iron. Floating pumice and scoriae, and occasionally + stones transported in the root of trees (see my “Journal of + Researches,” page 549) appear the only sources, through which foreign + matter is brought to coral-formations standing in the open ocean. The + area over which sediment is transported from coral-reefs must be + considerable: Captain Moresby informs me that during the change of + monsoons the sea is discoloured to a considerable distance off the + Maldiva and Chagos atolls. The sediment of fringing and barrier + coral-reefs must be mingled with the mud, which is brought down from + the land, and is transported seaward through the breaches, which occur + in front of almost every valley. If the atolls of the larger + archipelagoes were upraised, the bed of the ocean being converted into + land, they would form flat-topped mountains, varying in diameter from + a few miles (the smallest atolls being worn away) to sixty miles; and + from being horizontally stratified and of similar composition, they + would, as Mr. Lyell has remarked, falsely appear as if they had + originally been united into one vast continuous mass. Such great + strata of coral-rock would rarely be associated with erupted volcanic + matter, for this could only take place, as may be inferred from what + follows in the next chapter, when the area, in which they were + situated, commenced to rise, or at least ceased to subside. During the + enormous period necessary to effect an elevation of the kind just + alluded to, the surface would necessarily be denuded to a great + thickness; hence it is highly improbable that any fringing-reef, or + even any barrier-reef, at least of those encircling small islands, + would be preserved. From this same cause, the strata which were formed + within the lagoons of atolls and lagoon-channels of barrier-reefs, and + which must consist in a large part of sedimentary matter, would more + often be preserved to future ages, than the exterior solid reef, + composed of massive corals in an upright position; although it is on + this exterior part that the present existence and further growth of + atolls and barrier-reefs entirely depend. + + + + +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. + + +It will be convenient to give here a short account of the appended map +(Plate III.) [Inasmuch as the coloured map would have proved too costly +to be given in this series, the indications of colour have been +replaced by numbers referring to the dotted groups of reefs, etc. The +author’s original wording, however, is retained in full, as it will be +easy to refer to the map by the numbers, and thus the flow of the +narrative is undisturbed.]: a fuller one, with the data for colouring +each spot, is reserved for the Appendix; and every place there referred +to may be found in the Index. A larger chart would have been desirable; +but, small as the adjoined one is, it is the result of many months’ +labour. I have consulted, as far as I was able, every original voyage +and map; and the colours were first laid down on charts on a larger +scale. The same blue colour, with merely a difference in the depth of +tint, is used for atolls or lagoon-islands, and barrier-reefs, for we +have seen, that as far as the actual coral-formation is concerned, they +have no distinguishing character. Fringing-reefs have been coloured +red, for between them on the one hand, and barrier-reefs and atolls on +the other, there is an important distinction with respect to the depth +beneath the surface, at which we are compelled to believe their +foundations lie. The two distinct colours, therefore, mark two great +types of structure. + +The _dark blue colour_ [represented by (3) in our plate] represents +atolls and submerged annular reefs, with deep water in their centres. I +have coloured as atolls, a few low and small coral-islands, without +lagoons; but this has been done only when it clearly appeared that they +originally contained lagoons, since filled up with sediment: when there +were not good grounds for this belief, they have been left uncoloured. + +The _pale blue colour_ [represented by (2)] represents barrier-reefs. +The most obvious character of reefs of this class is the broad and +deep-water moat within the reef: but this, like the lagoons of small +atolls, is liable to become filled up with detritus and with reefs of +delicately branched corals: when, therefore, a reef round the entire +circumference of an island extends very far into a profoundly deep sea, +so that it can hardly be confounded with a fringing-reef which must +rest on a foundation of rock within a small depth, it has been coloured +pale blue, although it does not include a deep-water moat: but this has +only been done rarely, and each case is distinctly mentioned in the +Appendix. + +The _red colour_ (4) represents reefs fringing the land quite closely +where the sea is deep, and where the bottom is gently inclined +extending to a moderate distance from it, but not having a deep-water +moat or lagoon-like space parallel to the shore. It must be remembered +that fringing-reefs are frequently _breached_ in front of rivers and +valleys by deepish channels, where mud has been deposited. A space of +thirty miles in width has been coloured round or in front of the reefs +of each class, in order that the colours might be conspicuous on the +appended map, which is reduced to so small a scale. + +The _vermillion spots_, and streaks (1) represent volcanoes now in +action, or historically known to have been so. They are chiefly laid +down from Von Buch’s work on the Canary Islands; and my reasons for +making a few alterations are given in the note below.[1] + + [1] I have also made considerable use of the geological part of + Berghaus’ “Physical Atlas.” Beginning at the eastern side of the + Pacific, I have added to the number of the volcanoes in the southern + part of the Cordillera, and have coloured Juan Fernandez according to + observations collected during the voyage of the _Beagle_ (“Geological + Transactions,” volume v., page 601.) I have added a volcano to + Albemarle Island, one of the Galapagos Archipelago (the author’s + “Journal of Researches,” page 457). In the Sandwich group there are no + active volcanoes, except at Hawaii; but the Rev. W. Ellis informs me, + there are streams of lava apparently modern on Maui, having a very + recent appearance, which can be traced to the craters whence they + flowed. The same gentleman informs me, that there is no reason to + believe that any active volcano exists in the Society Archipelago; nor + are there any known in the Samoa or Navigator group, although some of + the streams of lava and craters there appear recent. In the Friendly + group, the Rev. J. Williams says (“Narrative of Missionary + Enterprise,” page 29) that Toofoa and Proby Islands are active + volcanoes. I infer from Hamilton’s “Voyage in the ‘Pandora’” (Page + 95), that Proby Island is synonymous with Onouafou, but I have not + ventured to colour it. There can be no doubt respecting Toofoa, and + Captain Edwards (Von Buch, page 386) found the lava of recent eruption + at Amargura still smoking. Berghaus marks four active volcanoes + actually within the Friendly group; but I do not know on what + authority: I may mention that Maurelle describes Latte as having a + burnt-up appearance: I have marked only Toofoa and Amargura. South of + the New Hebrides lies Matthews Rock, which is drawn and described as + an active crater in the “Voyage of the _Astrolabe_.” Between it and + the volcano on the eastern side of New Zealand, lies Brimstone Island, + which from the high temperature of the water in the crater, may be + ranked as active (Berghaus “Vorbemerk,” II Lief. S. 56). Malte Brun, + volume xii., page 231, says that there is a volcano near port St. + Vincent in New Caledonia. I believe this to be an error, arising from + a smoke seen on the _opposite_ coast by Cook (“Second Voyage,” volume + ii., page 23) which smoke went out at night. The Mariana Islands, + especially the northern ones, contain many craters (see Freycinet’s + “Hydrog. Descript.”) which are not active. Von Buch, however, states + (page 462) on the authority of La Peyrouse, that there are no less + than seven volcanoes between these islands and Japan. Gemelli Creri + (Churchill’s “Collect.” volume iv., page 458), says there are two + active volcanoes in latitude 23° 30′, and in latitude 24°: but I have + not coloured them. From the statements in Beechey’s “Voyage” (page + 518, 4to edition) I have coloured one in the northern part of the + Bonin group. M. S. Julien has clearly made out from Chinese + manuscripts not very ancient (“Comptes Rendus,” 1840, page 832), that + there are two active volcanoes on the eastern side of Formosa. In + Torres Straits, on Cap Island (9° 48′ S., 142° 39′ E.) a volcano was + seen burning with great violence in 1793 by Captain Bampton (see + Introduction to Flinders’ “Voyage,” page 41). Mr. M’Clelland (Report + of Committee for investigating Coal in India, page 39) has shown that + the volcanic band passing through Barren Island must be extended + northwards. It appears by an old chart, that Cheduba was once an + active volcano (see also “Silliman’s North American Journal”, volume + xxxviii., page 385). In Berghaus’ “Physical Atlas,” 1840, No. 7 of + Geological Part, a volcano on the coast of Pondicherry is said to have + burst forth in 1757. Ordinaire (“Hist. Nat. des Volcans,” page 218) + says that there is one at the mouth of the Persian Gulf, but I have + not coloured it, as he gives no particulars. A volcano in Amsterdam, + or St. Paul’s, in the southern part of the Indian Ocean, has been seen + (“Naut. Mag.” 1838, page 842) in action. Dr. J. Allan, of Forres, + informs me in a letter, that when he was at Joanna, he saw at night + flames apparently volcanic, issuing from the chief Comoro Island, and + that the Arabs assured him that they were volcanic, adding that the + volcano burned more during the wet season. I have marked this as a + volcano, though with some hesitation, on account of the possibility of + the flame arising from gaseous sources. + + +The _uncoloured coasts_ consist, first and chiefly, of those, where +there are no coral-reefs, or such small portions as to be quite +insignificant. Secondly, of those coasts where there are reefs, but +where the sea is very shallow, for in this case the reefs generally lie +far from the land, and become very irregular, in their forms: where +they have not become irregular, they have been coloured. thirdly, if I +had the means of ascertaining the fact, I should not colour a reef +merely coating the edges of a submarine crater, or of a level submerged +bank; for such superficial formations differ essentially, even when not +in external appearance, from reefs whose foundations as well as +superficies have been wholly formed by the growth of coral. Fourthly, +in the Red Sea, and within some parts of the East Indian Archipelago +(if the imperfect charts of the latter can be trusted), there are many +scattered reefs, of small size, represented in the chart by mere dots, +which rise out of deep water: these cannot be arranged under either of +the three classes: in the Red Sea, however, some of these little reefs, +from their position, seem once to have formed parts of a continuous +barrier. There exist, also, scattered in the open ocean, some linear +and irregularly formed strips of coral-reef, which, as shown in the +last chapter, are probably allied in their origin to atolls; but as +they do not belong to that class, they have not been coloured; they are +very few in number and of insignificant dimensions. Lastly, some reefs +are left uncoloured from the want of information respecting them, and +some because they are of an intermediate structure between the barrier +and fringing classes. The value of the map is lessened, in proportion +to the number of reefs which I have been obliged to leave uncoloured, +although, in a theoretical point of view, few of them present any great +difficulty: but their number is not very great, as will be found by +comparing the map with the statements in the Appendix. I have +experienced more difficulty in colouring fringing-reefs than in +colouring barrier-reefs, as the former, from their much less +dimensions, have less attracted the attention of navigators. As I have +had to seek my information from all kinds of sources, and often from +indirect ones, I do not venture to hope that the map is free from many +errors. Nevertheless, I trust it will give an approximately correct +view of the general distribution of the coral-reefs over the whole +world (with the exception of some fringing-reefs on the coast of +Brazil, not included within the limits of the map), and of their +arrangement into the three great classes, which, though necessarily +very imperfect from the nature of the objects classified, have been +adopted by most voyagers. I may further remark, that the dark blue +colour represents land entirely composed of coral-rock; the pale blue, +land with a wide and thick border of coral-rock; and the red, a mere +narrow fringe of coral-rock. + +Looking now at the map under the theoretical point of view indicated in +the last chapter, the two blue tints signify that the foundations of +the reefs thus coloured have subsided to a considerable amount, at a +slower rate than that of the upward growth of the corals, and that +probably in many cases they are still subsiding. The red signifies that +the shores which support fringing-reefs have not subsided (at least to +any considerable amount, for the effects of a subsidence on a small +scale would in no case be distinguishable); but that they have remained +nearly stationary since the period when they first became fringed by +reefs; or that they are now rising or have been upraised, with new +lines of reefs successively formed on them: these latter alternatives +are obviously implied, as newly formed lines of shore, after elevations +of the land, would be in the same state with respect to the growth of +fringing-reefs, as stationary coasts. If during the prolonged +subsidence of a shore, coral-reefs grew for the first time on it, or if +an old barrier-reef were destroyed and submerged, and new reefs became +attached to the land, these would necessarily at first belong to the +fringing class, and, therefore, be coloured red, although the coast was +sinking: but I have no reason to believe, that from this source of +error, any coast has been coloured wrongly with respect to movement +indicated. Well characterised atolls and encircling barrier-reefs, +where several occur in a group, or a single barrier-reef if of large +dimensions, leave scarcely any doubt on the mind respecting the +movement by which they have been produced; and even a small amount of +subsequent elevation is soon betrayed. The evidence from a single atoll +or a single encircling barrier-reef, must be received with some +caution, for the former may possibly be based upon a submerged crater +or bank, and the latter on a submerged margin of sediment, or of +worn-down rock. From these remarks we may with greater certainty infer +that the spaces, especially the larger ones, tinted blue in the map, +have subsided, than that the red spaces have remained stationary, or +have been upraised. + +_On the grouping of the different classes of reefs_.—Having made these +preliminary remarks, I will consider first how far the grouping of the +different kinds of coral-islands and reefs is corroborative of the +truth of the theory. A glance at the map shows that the reefs, coloured +blue and red, produced under widely different conditions, are not +indiscriminately mixed together. Atolls and barrier-reefs, on the other +hand, as may be seen by the two blue tints, generally lie near each +other; and this would be the natural result of both having been +produced during the subsidence of the areas in which they stand. Thus, +the largest group of encircled islands is that of the Society +Archipelago; and these islands are surrounded by atolls, and only +separated by a narrow space from the large group of Low atolls. In the +midst of the Caroline atolls, there are three fine encircled islands. +The northern point of the barrier-reef of New Caledonia seems itself, +as before remarked, to form a complete large atoll. The great +Australian barrier is described as including both atolls and small +encircled islands. Captain King[2] mentions many atoll-formed and +encircling coral-reefs, some of which lie within the barrier, and +others may be said (for instance between latitude 16° and 13°) to form +part of it. Flinders[3] has described an atoll-formed reef in latitude +10°, seven miles long and from one to three broad, resembling a boot in +shape, with apparently very deep water within. Eight miles westward of +this, and forming part of the barrier, lie the Murray Islands, which +are high and are encircled. In the Corallian Sea, between the two great +barriers of Australia and New Caledonia, there are many low islets and +coral-reefs, some of which are annular, or horse-shoe shaped. Observing +the smallness of the scale of the map, the parallels of latitude being +nine hundred miles apart, we see that none of the large groups of reefs +and islands supposed to have been produced by long-continued +subsidence, lie near extensive lines of coast coloured red, which are +supposed to have remained stationary since the growth of their reefs, +or to have been upraised and new lines of reefs formed on them. Where +the red and blue circles do occur near each other, I am able, in +several instances, to show that there have been oscillations of level, +subsidence having preceded the elevation of the red spots; and +elevation having preceded the subsidence of the blue spots: and in this +case the juxtaposition of reefs belonging to the two great types of +structure is little surprising. We may, therefore, conclude that the +proximity in the same areas of the two classes of reefs, which owe +their origin to the subsidence of the earth’s crust, and their +separation from those formed during its stationary or uprising +condition, holds good to the full extent, which might have been +anticipated by our theory. + + [2] Sailing directions, appended to volume ii. of his “Surveying + Voyage to Australia.” + + + [3] “Voyage to Terra Australis,” volume ii. page 336. + + +As groups of atolls have originated in the upward growth, at each fresh +sinking of the land, of those reefs which primarily fringed the shores +of one great island, or of several smaller ones; so we might expect +that these rings of coral-rock, like so many rude outline charts, will +still retain some traces of the general form, or at least general +range, of the land, round which they were first modelled. That this is +the case with the atolls in the Southern Pacific as far as their range +is concerned, seems highly probable, when we observe that the three +principal groups are directed in north-west and south-east lines, and +that nearly all the land in the S. Pacific ranges in this same +direction; namely, N. Western Australia, New Caledonia, the northern +half of New Zealand, the New Hebrides, Saloman, Navigator, Society, +Marquesas, and Austral archipelagoes: in the Northern Pacific, the +Caroline atolls abut against the north-west line of the Marshall +atolls, much in the same manner as the east and west line of islands +from Ceram to New Britain do on New Ireland: in the Indian Ocean the +Laccadive and Maldiva atolls extend nearly parallel to the western and +mountainous coast of India. In most respects, there is a perfect +resemblance with ordinary islands in the grouping of atolls and in +their form: thus the outline of all the larger groups is elongated; and +the greater number of the individual atolls are elongated in the same +direction with the group, in which they stand. The Chagos group is less +elongated than is usual with other groups, and the individual atolls in +it are likewise but little elongated; this is strikingly seen by +comparing them with the neighbouring Maldiva atolls. In the Marshall +and Maldiva archipelagoes, the atolls are ranged in two parallel lines, +like the mountains in a great double mountain-chain. Some of the +atolls, in the larger archipelagoes, stand so near to each other, and +have such an evident relationship in form, that they compose little +sub-groups: in the Caroline Archipelago, one such sub-group consists of +Pouynipète, a lofty island encircled by a barrier-reef, and separated +by a channel only four miles and a half wide from Andeema atoll, with a +second atoll a little further off. In all these respects an examination +of a series of charts will show how perfectly groups of atolls resemble +groups of common islands. + +_On the direct evidence of the blue spaces in the map having subsided +during the upward growth of the reefs so coloured, and of the red +spaces having remained stationary, or having been upraised._—With +respect to subsidence, I have shown in the last chapter, that we cannot +expect to obtain in countries inhabited only by semi-civilised races, +demonstrative proofs of a movement, which invariably tends to conceal +its own evidence. But on the coral-islands supposed to have been +produced by subsidence, we have proofs of changes in their external +appearance—of a round of decay and renovation—of the last vestiges of +land on some—of its first commencement on others: we hear of storms +desolating them to the astonishment of their inhabitants: we know by +the great fissures with which some of them are traversed, and by the +earthquakes felt under others, that subterranean disturbances of some +kind are in progress. These facts, if not directly connected with +subsidence, as I believe they are, at least show how difficult it would +be to discover proofs of such movement by ordinary means. At Keeling +atoll, however, I have described some appearances, which seem directly +to show that subsidence did take place there during the late +earthquakes. Vanikoro, according to Chevalier Dillon,[4] is often +violently shaken by earthquakes, and there, the unusual depth of the +channel between the shore and the reef,—the almost entire absence of +islets on the reef,— its wall-like structure on the inner side, and the +small quantity of low alluvial land at the foot of the mountains, all +seem to show that this island has not remained long at its present +level, with the lagoon-channel subjected to the accumulation of +sediment, and the reef to the wear and tear of the breakers. At the +Society Archipelago, on the other hand, where a slight tremor is only +rarely felt, the shoaliness of the lagoon-channels round some of the +islands, the number of islets formed on the reefs of others, and the +broad belt of low land at the foot of the mountains, indicate that, +although there must have been great subsidence to have produced the +barrier-reefs, there has since elapsed a long stationary period.[5] + + [4] See Captain Dillon’s “Voyage in search of La Peyrouse.” M. Cordier + in his “Report on the Voyage of the ‘Astrolabe’” (page cxi., volume + i.), speaking of Vanikoro, says the shores are surrounded by reefs of + madrepore, _“qu’on assure être de formation tout-a-fait moderne_.” I + have in vain endeavoured to learn some further particulars about this + remarkable passage. I may here add, that according to our theory, the + island of Pouynipète (Plate I., Figure 7), in the Caroline + Archipelago, being encircled by a barrier-reef, must have subsided. In + the “New S. Wales Lit. Advert.” February 1835 (which I have seen + through the favour of Dr. Lloghtsky), there is an account of this + island (subsequently confirmed by Mr. Campbell), in which it is said, + “At the N.E. end, at a place called Tamen, there are ruins of a town, + _now only_ accessible by boats, the waves _reaching to the steps of + the houses_.” Judging from this passage, one would be tempted to + conclude that the island must have subsided, since these houses were + built. I may, also, here append a statement in Malte Brun (volume ix., + page 775, given without any authority), that the sea gains in an + extraordinary manner on the coast of Cochin China, which lies in front + and near the subsiding coral-reefs in the China Sea: as the coast is + granitic, and not alluvial, it is scarcely possible that the + encroachment of the sea can be owing to the washing away of the land; + and if so, it must be due to subsidence. + + + [5] Mr. Couthouy states (“Remarks,” page 44) that at Tahiti and Eimeo + the space between the reef and the shore has been nearly filled up by + the extension of those coral-reefs, which within most barrier-reefs + merely fringe the land. From this circumstance, he arrives at the same + conclusion as I have done, that the Society Islands since their + subsidence, have remained stationary during a long period; but he + further believes that they have recently commenced rising, as well as + the whole area of the Low Archipelago. He does not give any detailed + proofs regarding the elevation of the Society Islands, but I shall + refer to this subject in another part of this chapter. Before making + some further comments, I may observe how satisfactory it is to me, to + find Mr. Couthouy affirming, that “having personally examined a large + number of coral-islands, and also residing eight months among the + volcanic class, having shore and partially encircling reefs, I may be + permitted to state that my own observations have impressed a + conviction of the correctness of the theory of Mr. Darwin.” + + This gentleman believes, that subsequently to the subsidence by + which the atolls in the Low Archipelago were produced, the whole + area has been elevated to the amount of a few feet; this would + indeed be a remarkable fact; but as far as I am able to judge, the + grounds of his conclusion are not sufficiently strong. He states + that he found in almost every atoll which he visited, the shores of + the lagoon raised from eighteen to thirty inches above the + sea-level, and containing imbedded Tridacnae and corals standing as + they grew; some of the corals were dead in their upper parts, but + below a certain line they continued to flourish. In the lagoons, + also, he frequently met with clusters of Madrepore, with their + extremities standing from one inch to a foot above the surface of + the water. Now, these appearances are exactly what I should have + expected, without any subsequent elevation having taken place; and + I think Mr. Couthouy has not borne in mind the indisputable fact, + that corals, when constantly bathed by the surf, can exist at a + higher level than in quite tranquil water, as in a lagoon. As long, + therefore, as the waves continued at low water to break entirely + over parts of the annular reef of an atoll, submerged to a small + depth, the corals and shells attached on these parts might continue + living at a level above the smooth surface of the lagoon, into + which the waves rolled; but as soon as the outer edge of the reef + grew up to its utmost possible height, or if the reef were very + broad nearly to that height, the force of the breakers would be + checked, and the corals and shells on the inner parts near the + lagoon would occasionally be left dry, and thus be partially or + wholly destroyed. Even in atolls, which have not lately subsided, + if the outer margin of the reef continued to increase in breadth + seaward (each fresh zone of corals rising to the same vertical + height as at Keeling atoll), the line where the waves broke most + heavily would advance outwards, and therefore the corals, which + when living near the margin, were washed by the breaking waves + during the whole of each tide, would cease being so, and would + therefore be left on the backward part of the reef standing exposed + and dead. The case of the madrepores in the lagoons with the tops + of their branches exposed, seems to be an analogous fact, to the + great fields of dead but upright corals in the lagoon of Keeling + atoll; a condition of things which I have endeavoured to show, has + resulted from the lagoon having become more and more enclosed and + choked up with reefs, so that during high winds, the rising of the + tide (as observed by the inhabitants) is checked, and the corals, + which had formerly grown to the greatest possible height, are + occasionally exposed, and thus are killed: and this is a condition + of things, towards which almost every atoll in the intervals of its + subsidence must be tending. Or if we look to the state of an atoll + directly after a subsidence of some fathoms, the waves would roll + heavily over the entire circumference of the reef, and the surface + of the lagoon would, like the ocean, never be quite at rest, and + therefore the corals in the lagoon, from being constantly laved by + the rippling water, might extend their branches to a little greater + height than they could, when the lagoon became enclosed and + protected. Christmas atoll (2° N. latitude) which has a very + shallow lagoon, and differs in several respects from most atolls, + possibly may have been elevated recently; but its highest part + appears (Couthouy, page 46) to be only ten feet above the + sea-level. The facts of a second class, adduced by Mr. Couthouy, in + support of the alleged recent elevation of the Low Archipelago, are + not all (especially those referring to a shelf of rock) quite + intelligible to me; he believes that certain enormous fragments of + rock on the reef, must have been moved into their present position, + when the reef was at a lower level; but here again the force of the + breakers on any inner point of the reef being diminished by its + outward growth without any change in its level, has not, I think, + been borne in mind. We should, also, not overlook the occasional + agency of waves caused by earthquakes and hurricanes. Mr. Couthouy + further argues, that since these great fragments were deposited and + fixed on the reef, they have been elevated; he infers this from the + greatest amount of erosion not being near their bases, where they + are unceasingly washed by the reflux of the tides, but at some + height on their sides, near the line of high-water mark, as shown + in an accompanying diagram. My former remark again applies here, + with this further observation, that as the waves have to roll over + a wide space of reef before they reach the fragments, their force + must be greatly increased with the increasing depth of water as the + tide rises, and therefore I should have expected that the chief + line of present erosion would have coincided with the line of + high-water mark; and if the reef had grown outwards, that there + would have been lines of erosion at greater heights. The + conclusion, to which I am finally led by the interesting + observations of Mr. Couthouy is, that the atolls in the Low + Archipelago have, like the Society Islands, remained at a + stationary level for a long period: and this probably is the + ordinary course of events, subsidence supervening after long + intervals of rest. + + +Turning now to the red colour; as on our map, the areas which have sunk +slowly downwards to great depths are many and large, we might naturally +have been led to conjecture, that with such great changes of level in +progress, the coasts which have been fringed probably for ages (for we +have no reason to believe that coral-reefs are of short duration), +would not have remained all this time stationary, but would frequently +have undergone movements of elevation. This supposition, we shall +immediately see, holds good to a remarkable extent; and although a +stationary condition of the land can hardly ever be open to proof, from +the evidence being only negative, we are, in some degree, enabled to +ascertain the correctness of the parts coloured red on the map, by the +direct testimony of upraised organic remains of a modern date. Before +going into the details on this head (printed in small type), I may +mention, that when reading a memoir on coral formations by MM. Quoy and +Gaimard[6] I was astonished to find, for I knew that they had crossed +both the Pacific and Indian Oceans, that their descriptions were +applicable only to reefs of the fringing class; but my astonishment +ended satisfactorily, when I discovered that, by a strange chance, all +the islands which these eminent naturalists had visited, though several +in number, namely, the Mauritius, Timor, New Guinea, the Mariana, and +Sandwich Archipelagoes, could be shown by their own statements to have +been elevated within a recent geological era. + + [6] “Annales des Sciences Nat.” tom. vi., page 279, etc. + + +In the eastern half of the Pacific, the _Sandwich Islands_ are all +fringed, and almost every naturalist who has visited them, has remarked +on the abundance of elevated corals and shells, apparently identical +with living species. The Rev. W. Ellis informs me, that he has noticed +round several parts of Hawaii, beds of coral-detritus, about twenty +feet above the level of the sea, and where the coast is low they extend +far inland. Upraised coral-rock forms a considerable part of the +borders of Oahu; and at Elizabeth Island[7] it composes three strata, +each about ten feet thick. Nihau, which forms the northern, as Hawaii +does the southern end of the group (350 miles in length), likewise +seems to consist of coral and volcanic rocks. Mr. Couthouy[8] has +lately described with interesting details, several upraised beaches, +ancient reefs with their surfaces perfectly preserved, and beds of +recent shells and corals, at the islands of Maui, Morokai, Oahu, and +Tauai (or Kauai) in this group. Mr. Pierce, an intelligent resident at +Oahu, is convinced, from changes which have taken place within his +memory, during the last sixteen years, “that the elevation is at +present going forward at a very perceptible rate.” The natives at Kauai +state that the land is there gaining rapidly on the sea, and Mr. +Couthouy has no doubt, from the nature of the strata, that this has +been effected by an elevation of the land. + + [7] “Zoology of Captain Beechey’s Voyage,” page 176. See also MM. Quoy + and Gaimard in “Annales de Scien. Nat.” tom. vi. + + + [8] “Remarks on Coral Formations,” page 51. + + +In the southern part of the Low Archipelago, Elizabeth Island is +described by Captain Beechey,[9] as being quite flat, and about eighty +feet in height; it is entirely composed of dead corals, forming a +honeycombed, but compact rock. In cases like this, of an island having +exactly the appearance, which the elevation of any one of the smaller +surrounding atolls with a shallow lagoon would present, one is led to +conclude (with little better reason, however, than the improbability of +such small and low fabrics lasting, for an immense period, exposed to +the many destroying agents of nature), that the elevation has taken +place at an epoch not geologically remote. When merely the surface of +an island of ordinary formation is strewed with marine bodies, and that +continuously, or nearly so, from the beach to a certain height, and not +above that height, it is exceedingly improbable that such organic +remains, although they may not have been specially examined, should +belong to any ancient period. It is necessary to bear these remarks in +mind, in considering the evidence of the elevatory movements in the +Pacific and Indian Oceans, as it does not often rest on specific +determinations, and therefore should be received with caution. Six of +the _Cook and Austral Islands_ (S.W. of the Society group), are +fringed; of these, five were described to me by the Rev. J. Williams, +as formed of coral-rock, associated with some basalt in Mangaia), and +the sixth as lofty and basaltic. Mangaia is nearly three hundred feet +high, with a level summit; and according to Mr. S. Wilson[10] it is an +upraised reef; “and there are in the central hollow, formerly the bed +of the lagoon, many scattered patches of coral-rock, some of them +raised to a height of forty feet.” These knolls of coral-rock were +evidently once separate reefs in the lagoon of an atoll. Mr. Martens, +at Sydney, informed me that this island is surrounded by a terrace-like +plain at about the height of a hundred feet, which probably marks a +pause in its elevation. From these facts we may infer, perhaps, that +the Cook and Austral Islands have been upheaved at a period probably +not very remote. + + [9] Beechey’s “Voyage in the Pacific,” page 46, 4to edition. + + + [10] Couthouy’s “Remarks,” page 34. + + +_Savage Island_ (S.E. of the Friendly group), is about forty feet in +height. Forster[11] describes the plants as _already_ growing out of +the dead, but still upright and spreading trees of coral; and the +younger Forster[12] believes that an ancient lagoon is now represented +by a central plain; here we cannot doubt that the elevatory forces have +recently acted. The same conclusion may be extended, though with +somewhat less certainty, to the islands of the _Friendly Group_, which +have been well described in the second and third voyages of Cook. The +surface of Tongatabou is low and level, but with some parts a hundred +feet high; the whole consists of coral-rock, “which yet shows the +cavities and irregularities worn into it by the action of the +tides.”[13] On Eoua the same appearances were noticed at an elevation +of between two hundred and three hundred feet. Vavao, also, at the +opposite or northern end of the group, consists, according to the Rev. +J. Williams, of coral-rock. Tongatabou, with its northern extensive +reefs, resembles either an upraised atoll with one half originally +imperfect, or one unequally elevated; and Anamouka, an atoll equally +elevated. This latter island contains[14] in its centre a salt-water +lake, about a mile-and-a-half in diameter, without any communication +with the sea, and around it the land rises gradually like a bank; the +highest part is only between twenty and thirty feet; but on this part, +as well as on the rest of the land (which, as Cook observes, rises +above the height of true lagoon-islands), coral-rock, like that on the +beach, was found. In the _Navigator Archipelago_, Mr. Couthouy[15] +found on Manua many and very large fragments of coral at the height of +eighty feet, “on a steep hill-side, rising half a mile inland from a +low sandy plain abounding in marine remains.” The fragments were +embedded in a mixture of decomposed lava and sand. It is not stated +whether they were accompanied by shells, or whether the corals +resembled recent species; as these remains were embedded they possibly +may belong to a remote epoch; but I presume this was not the opinion of +Mr. Couthouy. Earthquakes are very frequent in this archipelago. + + [11] “Observations made during Voyage round the World,” page 147. + + + [12] “Voyage,” volume ii., page 163. + + + [13] Cook’s “Third Voyage” (4to edition), volume i., page 314. + + + [14] Ibid., volume i., page 235. + + + [15] “Remarks on Coral-Formations,” page 50. + + +Still proceeding westward we come to the _New Hebrides;_ on these +islands, Mr. G. Bennett (author of “Wanderings in New South Wales”), +informs me he found much coral at a great altitude, which he considered +of recent origin. Respecting _Santa Cruz_, and the _Salomon +Archipelago_, I have no information; but at New Ireland, which forms +the northern point of the latter chain, both Labillardière and Lesson +have described large beds of an apparently very modern madreporitic +rock, with the form of the corals little altered. The latter author[16] +states that this formation composes a newer line of coast, modelled +round an ancient one. There only remains to be described in the +Pacific, that curved line of fringed islands, of which the _Marianas_ +form the main part. Of these Guam, Rota, Tiniam, Saypan, and some +islets farther north, are described by Quoy and Gaimard,[17] and +Chamisso,[18] as chiefly composed of madreporitic limestone, which +attains a considerable elevation, and is in several cases worn into +successively rising cliffs: the two former naturalists seem to have +compared the corals and shells with the existing ones, and state that +they are of recent species. _Fais_, which lies in the prolonged line of +the Marianas, is the only island in this part of the sea which is +fringed; it is ninety feet high, and consists entirely of madreporitic +rock.[19] + + [16] “Voyage de la ‘Coquille’,” Part. Zoolog. + + + [17] Freycinet’s “Voyage autour du Monde.” See also the + “Hydrographical Memoir,” page 215. + + + [18] Kotzebue’s “First Voyage.” + + + [19] Lutké’s “Voyage,” volume ii., page 304. + + +In the _East Indian Archipelago_, many authors have recorded proofs of +recent elevation. M. Lesson[20] states, that near Port Dory, on the +north coast of New Guinea, the shores are flanked, to the height of 150 +feet, by madreporitic strata of modern date. He mentions similar +formations at Waigiou, Amboina, Bourou, Ceram, Sonda, and Timor: at +this latter place, MM. Quoy and Gaimard[21] have likewise described the +primitive rocks, as coated to a considerable height with coral. Some +small islets eastward of Timor are said in Kolff’s “Voyage,”[22] to +resemble small coral islets upraised some feet above the sea. Dr. +Malcolmson informs me that Dr. Hardie found in _Java_ an extensive +formation, containing an abundance of shells, of which the greater part +appear to be of existing species. Dr. Jack[23] has described some +upraised shells and corals, apparently recent, on Pulo Nias off +_Sumatra;_ and Marsden relates in his history of this great island, +that the names of many promontories, show that they were originally +islands. On part of the west coast of _Borneo_ and at the _Sooloo +Islands_, the form of the land, the nature of the soil, and the +water-washed rocks, present appearances[24] (although it is doubtful +whether such vague evidence is worthy of mention), of having recently +been covered by the sea; and the inhabitants of the Sooloo Islands +believe that this has been the case. Mr. Cuming, who has lately +investigated, with so much success, the natural history of the +_Phillippines_, found near Cabagan, in Luzon, about fifty feet above +the level of the R. Cagayan, and seventy miles from its mouth, a large +bed of fossil shells: these, he informs me, are of the same species +with those now existing on the shores of the neighbouring islands. From +the accounts given us by Captain Basil Hall and Captain Beechey[25] of +the lines of inland reefs, and walls of coral-rock worn into caves, +above the present reach of the waves, at the _Loo Choo_ Islands, there +can be little doubt that they have been upraised at no very remote +period. + + [20] Partie Zoolog., “Voyage de la ‘Coquille’.” + + + [21] “Ann. des Scien. Nat.” tom. vi., page 281. + + + [22] Translated by Windsor Earl, chapters vi., vii. + + + [23] “Geolog. Transact.” 2nd series, volume i., page 403. On the + Peninsula of Malacca, in front of Pinang, 5° 30′ N., Dr. Ward + collected some shells, which Dr. Malcolmson informs me, although not + compared with existing species, had a recent appearance. Dr. Ward + describes in this neighbourhood (“Trans. Asiat. Soc.” volume xviii., + part ii., page 166) a single water-worn rock, with a conglomerate of + sea-shells at its base, situated six miles inland, which, according to + the traditions of the natives, was once surrounded by the sea. Captain + Low has also described (Ibid., part i., page 131) mounds of shells + lying two miles inland on this line of coast. + + + [24] “Notices of the East Indian Arch.” Singapore, 1828, page 6, and + Append., page 43. + + + [25] Captain B. Hall, “Voyage to Loo Choo,” Append., pages xxi. and + xxv. Captain Beechey’s “Voyage,” page 496. + + +Dr. Davy[26] describes the northern province of _Ceylon_ as being very +low, and consisting of a limestone with shells and corals of very +recent origin; he adds, that it does not admit of a doubt that the sea +has retired from this district even within the memory of man. There is +also some reason for believing that the western shores of India, north +of Ceylon, have been upraised within the recent period.[27] _Mauritius_ +has certainly been upraised within the recent period, as I have stated +in the chapter on fringing-reefs. The northern extremity of +_Madagascar_ is described by Captain Owen[28] as formed of madreporitic +rock, as likewise are the shores and outlying islands along an immense +space of _Eastern Africa_, from a little north of the equator for nine +hundred miles southward. Nothing can be more vague than the expression +“madreporitic rock;” but at the same time it is, I think, scarcely +possible to look at the chart of the linear islets, which rise to a +greater height than can be accounted for by the growth of coral, in +front of the coast, from the equator to 2° S., without feeling +convinced that a line of fringing-reefs has been elevated at a period +so recent, that no great changes have since taken place on the surface +of this part of the globe. Some, also, of the higher islands of +madreporitic rock on this coast, for instance Pemba, have very singular +forms, which seem to show the combined effect of the growth of coral +round submerged banks, and their subsequent upheaval. Dr. Allan informs +me that he never observed any elevated organic remains on the +_Seychelles_, which come under our fringed class. + + [26] “Travels in Ceylon,” page 13. This madreporitic formation is + mentioned by M. Cordier in his report to the Institute (May 4th, + 1839), on the voyage of the “Chevrette”, as one of immense extent, and + belonging to the latest tertiary period. + + + [27] Dr. Benza, in his “Journey through the N. Circars” (the “Madras + Lit. and Scient. Journ.” volume v.) has described a formation with + recent fresh-water and marine shells, occurring at the distance of + three or four miles from the present shore. Dr. Benza, in conversation + with me, attributed their position to a rise of the land. Dr. + Malcolmson, however (and there cannot be a higher authority on the + geology of India) informs me that he suspects that these beds may have + been formed by the mere action of the waves and currents accumulating + sediment. From analogy I should much incline to Dr. Benza’s opinion. + + + [28] Owen’s “Africa,” volume ii., page 37, for Madagascar; and for S. + Africa, volume i., pages 412 and 426. Lieutenant Boteler’s narrative + contains fuller particulars regarding the coral-rock, volume i., page + 174, and volume ii., pages 41 and 54. See also Ruschenberger’s “Voyage + round the World,” volume i., page 60. + + +The nature of the formations round the shores of the _Red Sea_, as +described by several authors, shows that the whole of this large area +has been elevated within a very recent tertiary epoch. A part of this +space in the appended map, is coloured blue, indicating the presence of +barrier-reefs: on which circumstance I shall presently make some +remarks. Rüppell[29] states that the tertiary formation, of which he +has examined the organic remains, forms a fringe along the shores with +a uniform height of from thirty and forty feet from the mouth of the +Gulf of Suez to about latitude 26°; but that south of 26°, the beds +attain only the height of from twelve to fifteen feet. This, however, +can hardly be quite accurate; although possibly there may be a decrease +in the elevation of the shores in the middle parts of the Red Sea, for +Dr. Malcolmson (as he informs me) collected from the cliffs of Camaran +Island (latitude 15° 30′ S.) shells and corals, apparently recent, at a +height between thirty and forty feet; and Mr. Salt (“Travels in +Abyssinia”) describes a similar formation a little southward on the +opposite shore at Amphila. Moreover, near the mouth of the Gulf of +Suez, although on the coast opposite to that on which Dr. Rüppell says +that the modern beds attain a height of only thirty to forty feet, Mr. +Burton[30] found a deposit replete with existing species of shells, at +the height of 200 feet. In an admirable series of drawings by Captain +Moresby, I could see how continuously the cliff-bounded low plains of +this formation extended with a nearly equable height, both on the +eastern and western shores. The southern coast of Arabia seems to have +been subjected to the same elevatory movement, for Dr. Malcolmson found +at Sahar low cliffs containing shells and corals, apparently of recent +species. + + [29] Rüppell, “Reise in Abyssinien,” Band i., s. 141. + + + [30] Lyell’s “Principles of Geology,” 5th edition, volume iv., page + 25. + + +The _Persian Gulf_ abounds with coral-reefs; but as it is difficult to +distinguish them from sand-banks in this shallow sea, I have coloured +only some near the mouth; towards the head of the gulf Mr. +Ainsworth[31] says that the land is worn into terraces, and that the +beds contain organic remains of existing forms. The _West Indian +Archipelago_ of “fringed” islands, alone remains to be mentioned; +evidence of an elevation within a late tertiary epoch of nearly the +whole of this great area, may be found in the works of almost all the +naturalists who have visited it. I will give some of the principal +references in a note.[32] + + [31] Ainsworth’s “Assyria and Babylon,” page 217. + + + [32] On Florida and the north shores of the Gulf of Mexico, Rogers’ + “Report to Brit. Assoc.” volume iii., page 14.—On the shores of + Mexico, Humboldt, “Polit. Essay on New Spain,” volume i., page 62. (I + have also some corroborative facts with respect to the shores of + Mexico.)—Honduras and the Antilles, Lyell’s “Principles,” 5th edition, + volume iv., page 22.—Santa Cruz and Barbadoes, Prof. Hovey, + “Silliman’s Journal”, volume xxxv., page 74.—St. Domingo, + Courrojolles, “Journ de Phys.” tom. liv., page 106.—Bahamas, “United + Service Journal”, No. lxxi., pages 218 and 224. Jamaica, De la Beche, + “Geol. Man.” page 142.—Cuba, Taylor in “Lond. and Edin. Mag.” volume + xi., page 17. Dr. Daubeny also, at a meeting of the Geolog. Soc., + orally described some very modern beds lying on the N.W. parts of + Cuba. I might have added many other less important references. + + +It is very remarkable on reviewing these details, to observe in how +many instances fringing-reefs round the shores, have coincided with the +existence on the land of upraised organic remains, which seem, from +evidence more or less satisfactory, to belong to a late tertiary +period. It may, however, be objected, that similar proofs of elevation, +perhaps, occur on the coasts coloured blue in our map: but this +certainly is not the case with the few following and doubtful +exceptions. + +The entire area of the Red Sea appears to have been upraised within a +modern period; nevertheless I have been compelled (though on +unsatisfactory evidence, as given in the Appendix) to class the reefs +in the middle part, as barrier-reefs; should, however, the statements +prove accurate to the less height of the tertiary bed in this middle +part, compared with the northern and southern districts, we might well +suspect that it had subsided subsequently to the general elevation by +which the whole area has been upraised. Several authors[33] have stated +that they have observed shells and corals high up on the mountains of +the Society Islands,—a group encircled by barrier-reefs, and, +therefore, supposed to have subsided: at Tahiti Mr. Stutchbury found on +the apex of one of the highest mountains, between 5,000 and 7,000 feet +above the level of the sea, “a distinct and regular stratum of +semi-fossil coral.” At Tahiti, however, other naturalists, as well as +myself, have searched in vain at a low level near the coast, for +upraised shells or masses of coral-reef, where if present they could +hardly have been overlooked. From this fact, I concluded that probably +the organic remains strewed high up on the surface of the land, had +originally been embedded in the volcanic strata, and had subsequently +been washed out by the rain. I have since heard from the Rev. W. Ellis, +that the remains which he met with, were (as he believes) +interstratified with an argillaceous tuff; this likewise was the case +with the shells observed by the Rev. D. Tyerman at Huaheine. These +remains have not been specifically examined; they may, therefore, and +especially the stratum observed by Mr. Stutchbury at an immense height, +be contemporaneous with the first formation of the Society Islands, and +be of any degree of antiquity; or they may have been deposited at some +subsequent, but probably not very recent, period of elevation; for if +the period had been recent, the entire surface of the coast land of +these islands, where the reefs are so extensive, would have been coated +with upraised coral, which certainly is not the case. Two of the +Harvey, or Cook Islands, namely, Aitutaki and Manouai, are encircled by +reefs, which extend so far from the land, that I have coloured them +blue, although with much hesitation, as the space within the reef is +shallow, and the outline of the land is not abrupt. These two islands +consist of coral-rock; but I have no evidence of their recent +elevation, besides, the improbability of Mangaia, a fringed island in +the same group (but distant 170 miles), having retained its nearly +perfect atoll-like structure, during any immense lapse of time after +its upheaval. The Red Sea, therefore, is the only area in which we have +clear proofs of the recent elevation of a district, which, by our +theory (although the barrier-reefs are there not well characterised), +has lately subsided. But we have no reason to be surprised at +oscillation, of level of this kind having occasionally taken place. +There can be scarcely any doubt that Savage, Aurora,[34] and Mangaia +Islands, and several of the islands in the Friendly group, existed +originally as atolls, and these have undoubtedly since been upraised to +some height above the level of the sea; so that by our theory, there +has here, also, been an oscillation of level, —elevation having +succeeded subsidence, instead of, as in the middle part of the Red Sea +and at the Harvey Islands, subsidence having probably succeeded recent +elevation. + + [33] Ellis, in his “Polynesian Researches,” was the first to call + attention to these remains (volume i., page 38), and the tradition of + the natives concerning them. See also Williams, “Nar. of Missionary + Enterprise,” page 21; also Tyerman and G. Bennett, “Journal of + Voyage,” volume i., page 213; also Mr. Couthouy’s “Remarks,” page 51; + but this principal fact, namely, that there is a mass of upraised + coral on the narrow peninsula of Tiarubu, is from hearsay evidence; + also Mr. Stutchbury, “West of England Journal,” No. i., page 54. There + is a passage in Von Zach, “Corres. Astronom.” volume x., page 266, + inferring an uprising at Tahiti, from a footpath now used, which was + formerly impassable; but I particularly inquired from several native + chiefs, whether they knew of any change of this kind, and they were + unanimous in giving me an answer in the negative. + + + [34] Aurora Island is described by Mr. Couthouy (“Remarks,” page 58); + it lies 120 miles north-east of Tahiti; it is not coloured in the + appended map, because it does not appear to be fringed by living + reefs. Mr. Couthouy describes its summit as “presenting a broad + table-land which declines a few feet towards the centre, where we may + suppose the lagoon to have been placed.” It is about two hundred feet + in height, and consists of reef-rock and conglomerate, with existing + species of coral embedded in it. The island has been elevated at two + successive periods; the cliffs being marked halfway up with a + horizontal water-worn line of deep excavations. Aurora Island seems + closely to resemble in structure Elizabeth Island, at the southern end + of the Low Archipelago. + + +It is an interesting fact, that Fais, which, from its composition, +form, height, and situation at the western end of the Caroline +Archipelago, one is strongly induced to believe existed before its +upheaval as an atoll, lies exactly in the prolongation of the curved +line of the Mariana group, which we know to be a line of recent +elevation. I may add, that Elizabeth Island, in the southern part of +the Low Archipelago, which seems to have had the same kind of origin as +Fais, lies near Pitcairn Island, the only one in this part of the ocean +which is high, and at the same time not surrounded by an encircling +barrier-reef. + +_On the absence of active Volcanoes in the areas of subsidence, and on +their frequent presence in the areas of elevation._—Before making some +concluding remarks on the relations of the spaces coloured blue and +red, it will be convenient to consider the position on our map of the +volcanoes historically known to have been in action. It is impossible +not to be struck, first with the absence of volcanoes in the great +areas of subsidence tinted pale and dark blue,—namely, in the central +parts of the Indian Ocean, in the China Sea, in the sea between the +barriers of Australia and New Caledonia, in the Caroline, Marshall, +Gilbert, and Low Archipelagoes; and, secondly, with the coincidence of +the principal volcanic chains with the parts coloured red, which +indicates the presence of fringing-reefs; and, as we have just seen, +the presence in most cases of upraised organic remains of a modern +date. I may here remark that the reefs were all coloured before the +volcanoes were added to the map, or indeed before I knew of the +existence of several of them. + +The volcano in Torres Strait, at the northern point of Australia, is +that which lies nearest to a large subsiding area, although situated +125 miles within the outer margin of the actual barrier-reef. The Great +Comoro Island, which probably contains a volcano, is only twenty miles +distant from the barrier-reef of Mohila; Ambil volcano, in the +Philippines, is distant only a little more than sixty miles from the +atoll-formed Appoo reef: and there are two other volcanoes in the map +within ninety miles of circles coloured blue. These few cases, which +thus offer partial exceptions to the rule, of volcanoes being placed +remote from the areas of subsidence, lie either near single and +isolated atolls, or near small groups of encircled islands; and these +by our theory can have, in few instances, subsided to the same amount +in depth or area, as groups of atolls. There is not one active volcano +within several hundred miles of an archipelago, or even a small group +of atolls. It is, therefore, a striking fact that in the Friendly +Archipelago, which owes its origin to the elevation of a group of +atolls, two volcanoes, and, perhaps, others[35] are known to be in +action: on the other hand, on several of the encircled islands in the +Pacific, supposed by our theory to have subsided, there are old craters +and streams of lava, which show the effects of past and ancient +eruptions. In these cases, it would appear as if the volcanoes had come +into action, and had become extinguished on the same spots, according +as the elevating or subsiding movements prevailed. + + [35] See the note at p. 121, on the authorities for colouring the + volcanoes in the appended map. + + +There are some other coasts on the map, where volcanoes in a state of +action concur with proofs of recent elevation, besides those coloured +red from being fringed by coral-reefs. Thus I hope to show in a future +volume, that nearly the whole line of the west coast of South America, +which forms the greatest volcanic chain in the world, from near the +equator for a space of between 2,000 and 3,000 miles southward, has +undergone an upward movement during a late geological period. The +islands on the north-western shores of the Pacific, which form the +second greatest volcanic chain, are very imperfectly known; but Luzon, +in the Philippines, and the Loo Choo Islands, have been recently +elevated; and at Kamtschatka[36] there are extensive tertiary beds of +modern date. Evidence of the same nature, but not very satisfactory, +may be detected in Northern New Zealand where there are two volcanoes. +The co-existence in other parts of the world of active volcanoes, with +upraised beds of a modern tertiary origin, will occur to every +geologist.[37] Nevertheless, until it could be shown that volcanoes +were inactive, or did not exist in subsiding areas, the conclusion that +their distribution depended on the nature of the subterranean movements +in progress, would have been hazardous. But now, viewing the appended +map, it may, I think, be considered as almost established, that +volcanoes are often (not necessarily always) present in those areas +where the subterranean motive power has lately forced, or is now +forcing outwards, the crust of the earth, but that they are invariably +absent in those, where the surface has lately subsided or is still +subsiding.[38] + + [36] At Sedanka, in latitude 58° N. (Von Buch’s “Descrip. des Isles + Canaries,” page 455). In a forthcoming part, I shall give the evidence + referred to with respect to the elevation of New Zealand. + + + [37] During the subterranean disturbances which took place in Chile, + in 1835, I have shown (“Geolog. Trans.” 2nd Ser., vol. v., page 606) + that at the same moment that a large district was upraised, volcanic + matter burst forth at widely separated points, through both new and + old vents. + + + [38] We may infer from this rule, that in any old deposit, which + contains interstratified beds of erupted matter, there was at the + period, and in the area of its formation, a _tendency_ to an upward + movement in the earth’s surface, and certainly no movement of + subsidence. + + +_On the relations of the areas of Subsidence and Elevation_.—The +immense surfaces on the map, which, both by our theory and by the plain +evidence of upraised marine remains, have undergone a change of level +either downwards or upwards during a late period, is a most remarkable +fact. The existence of continents shows that the areas have been +immense which at some period have been upraised; in South America we +may feel sure, and on the north-western shores of the Indian Ocean we +may suspect, that this rising is either now actually in progress, or +has taken place quite recently. By our theory, we may conclude that the +areas are likewise immense which have lately subsided, or, judging from +the earthquakes occasionally felt and from other appearances, are now +subsiding. The smallness of the scale of our map should not be +overlooked: each of the squares on it contains (not allowing for the +curvature of the earth) 810,000 square miles. Look at the space of +ocean from near the southern end of the Low Archipelago to the northern +end of the Marshall Archipelago, a length of 4,500 miles, in which, as +far as is known, every island, except Aurora which lies just without +the Low Archipelago, is atoll-formed. The eastern and western +boundaries of our map are continents, and they are rising areas: the +central spaces of the great Indian and Pacific Oceans, are mostly +subsiding; between them, north of Australia, lies the most broken land +on the globe, and there the rising parts are surrounded and penetrated +by areas of subsidence,[39] so that the prevailing movements now in +progress, seem to accord with the actual states of surface of the great +divisions of the world. + + [39] I suspect that the Arru and Timor-laut Islands present an + included small area of subsidence, like that of the China Sea, but I + have not ventured to colour them from my imperfect information, as + given in the Appendix. + + +The blue spaces on the map are nearly all elongated; but it does not +necessarily follow from this (a caution, for which I am indebted to Mr. +Lyell), that the areas of subsidence were likewise elongated; for the +subsidence of a long, narrow space of the bed of the ocean, including +in it a transverse chain of mountains, surmounted by atolls, would only +be marked on the map by a transverse blue band. But where a chain of +atolls and barrier-reefs lies in an elongated area, between spaces +coloured red, which therefore have remained stationary or have been +upraised, this must have resulted either from the area of subsidence +having originally been elongated (owing to some tendency in the earth’s +crust thus to subside), or from the subsiding area having originally +been of an irregular figure, or as broad as long, and having since been +narrowed by the elevation of neighbouring districts. Thus the areas, +which subsided during the formation of the great north and south lines +of atolls in the Indian Ocean,—of the east and west line of the +Caroline atolls,—and of the north-west and south-east line of the +barrier-reefs of New Caledonia and Louisiade, must have originally been +elongated, or if not so, they must have since been made elongated by +elevations, which we know to belong to a recent period. + +I infer from Mr. Hopkins’ researches,[40] that for the formation of a +long chain of mountains, with few lateral spurs, an area elongated in +the same direction with the chain, must have been subjected to an +elevatory movement. Mountain-chains, however, when already formed, +although running in very different directions, it seems,[41] may be +raised together by a widely-acting force: so, perhaps, mountain-chains +may subside together. Hence, we cannot tell, whether the Caroline and +Marshall Archipelagoes, two groups of atolls running in different +directions and meeting each other, have been formed by the subsidence +of two areas, or of one large area, including two distinct lines of +mountains. We have, however, in the southern prolongation of the +Mariana Islands, probable evidence of a line of recent elevation having +intersected one of recent subsidence. A view of the map will show that, +generally, there is a tendency to alternation in the parallel areas +undergoing opposite kinds of movement; as if the sinking of one area +balanced the rising of another. + + [40] “Researches in Physical Geology,” Transact. Cambridge Phil. Soc., + volume vi, part i. + + + [41] For instance in S. America from latitude 34°, for very many + degrees southward there are upraised beds containing recent species of + shells, on both the Atlantic and Pacific side of the continent, and + from the gradual ascent of the land, although with very unequal + slopes, on both sides towards the Cordillera, I think it can hardly be + doubted that the entire width has been upraised in mass within the + recent period. In this case the two W.N.W. and E.S.E. mountain-lines, + namely the Sierra Ventana and the S. Tapalguen, and the great north + and south line of the Cordillera have been together raised. In the + West Indies the N. and S. line of the Eastern Antilles, and the E. and + W. line of Jamaica, appear both to have been upraised within the + latest geological period. + + +The existence in many parts of the world of high table-land, proves +that large surfaces have been upraised in mass to considerable heights +above the level of the ocean; although the highest points in almost +every country consist of upturned strata, or erupted matter: and from +the immense spaces scattered with atolls, which indicate that land +originally existed there, although not one pinnacle now remains above +the level of the sea, we may conclude that wide areas have subsided to +an amount, sufficient to bury not only any formerly existing +table-land, but even the heights formed by fractured strata, and +erupted matter. The effects produced on the land by the later elevatory +movements, namely, successively rising cliffs, lines of erosion, and +beds of literal shells and pebbles, all requiring time for their +production, prove that these movements have been very slow; we can, +however, infer this with safety, only with respect to the few last +hundred feet of rise. But with reference to the whole vast amount of +subsidence, necessary to have produced the many atolls widely scattered +over immense spaces, it has already been shown (and it is, perhaps, the +most interesting conclusion in this volume), that the movements must +either have been uniform and exceedingly slow, or have been effected by +small steps, separated from each other by long intervals of time, +during which the reef-constructing polypifers were able to bring up +their solid frameworks to the surface. We have little means of judging +whether many considerable oscillations of level have generally occurred +during the elevation of large tracts; but we know, from clear +geological evidence, that this has frequently taken place; and we have +seen on our map, that some of the same islands have both subsided and +been upraised. I conclude, however, that most of the large blue spaces, +have subsided without many and great elevatory oscillations, because +only a few upraised atolls have been observed: the supposition that +such elevations have taken place, but that the upraised parts have been +worn down by the surf, and thus have escaped observation, is overruled +by the very considerable depth of the lagoons of all the larger atolls; +for this could not have been the case, if they had suffered repeated +elevations and abrasion. From the comparative observations made in +these latter pages, we may finally conclude, that the subterranean +changes which have caused some large areas to rise, and others to +subside, have acted in a very similar manner. + +_Recapitulation_.—In the three first chapters, the principal kinds of +coral-reefs were described in detail, and they were found to differ +little, as far as relates to the actual surface of the reef. An atoll +differs from an encircling barrier-reef only in the absence of land +within its central expanse; and a barrier-reef differs from a +fringing-reef, in being placed at a much greater distance from the land +with reference to the probable inclination of its submarine foundation, +and in the presence of a deep-water lagoon-like space or moat within +the reef. In the fourth chapter the growing powers of the +reef-constructing polypifers were discussed; and it was shown, that +they cannot flourish beneath a very limited depth. In accordance with +this limit, there is no difficulty respecting the foundations on which +fringing-reefs are based; whereas, with barrier-reefs and atolls, there +is a great apparent difficulty on this head; in barrier-reefs from the +improbability of the rock of the coast or of banks of sediment +extending, in every instance, so far seaward within the required +depth;—and in atolls, from the immensity of the spaces over which they +are interspersed, and the apparent necessity for believing that they +are all supported on mountain-summits, which although rising very near +to the surface-level of the sea, in no one instance emerge above it. To +escape this latter most improbable admission, which implies the +existence of submarine chains of mountains of almost the same height, +extending over areas of many thousand square miles, there is but one +alternative; namely, the prolonged subsidence of the foundations, on +which the atolls were primarily based, together with the upward growth +of the reef-constructing corals. On this view every difficulty +vanishes; fringing reefs are thus converted into barrier-reefs; and +barrier-reefs, when encircling islands, are thus converted into atolls, +the instant the last pinnacle of land sinks beneath the surface of the +ocean. + +Thus the ordinary forms and certain peculiarities in the structure of +atolls and barrier-reefs can be explained;—namely, the wall-like +structure on their inner sides, the basin or ring-like shape both of +the marginal and central reefs in the Maldiva atolls—the union of some +atolls as if by a ribbon—the apparent disseverment of others—and the +occurrence, in atolls as well as in barrier-reefs, of portions of reef, +and of the whole of some reefs, in a dead and submerged state, but +retaining the outline of living reefs. Thus can be explained the +existence of breaches through barrier-reefs in front of valleys, though +separated from them by a wide space of deep water; thus, also, the +ordinary outline of groups of atolls and the relative forms of the +separate atolls one to another; thus can be explained the proximity of +the two kinds of reefs formed during subsidence, and their separation +from the spaces where fringing-reefs abound. On searching for other +evidence of the movements supposed by our theory, we find marks of +change in atolls and in barrier-reefs, and of subterranean disturbances +under them; but from the nature of things, it is scarcely possible to +detect any direct proofs of subsidence, although some appearances are +strongly in favour of it. On the fringed coasts, however, the presence +of upraised marine bodies of a recent epoch, plainly show, that these +coasts, instead of having remained stationary, which is all that can be +directly inferred from our theory, have generally been elevated. + +Finally, when the two great types of structure, namely barrier-reefs +and atolls on the one hand, and fringing-reefs on the other, were laid +down in colours on our map, a magnificent and harmonious picture of the +movements, which the crust of the earth has within a late period +undergone, is presented to us. We there see vast areas rising, with +volcanic matter every now and then bursting forth through the vents or +fissures with which they are traversed. We see other wide spaces slowly +sinking without any volcanic outburst, and we may feel sure, that this +sinking must have been immense in amount as well as in area, thus to +have buried over the broad face of the ocean every one of those +mountains, above which atolls now stand like monuments, marking the +place of their former existence. Reflecting how powerful an agent with +respect to denudation, and consequently to the nature and thickness of +the deposits in accumulation, the sea must ever be, when acting for +prolonged periods on the land, during either its slow emergence or +subsidence; reflecting, also, on the final effects of these movements +in the interchange of land and ocean-water on the climate of the earth, +and on the distribution of organic beings, I may be permitted to hope, +that the conclusions derived from the study of coral-formations, +originally attempted merely to explain their peculiar forms, may be +thought worthy of the attention of geologists. + + + + +APPENDIX. + +CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS IN PLATE +III. + +In the beginning of the last chapter I stated the principles on which +the map is coloured. There only remains to be said, that it is an exact +copy of one by M. C. Gressier, published by the Dépôt général de la +Marine, in 1835. The names have been altered into English, and the +longitude has been reduced to that of Greenwich. The colours were first +laid down on accurate charts, on a large scale. The data, on which the +volcanoes historically known to have been in action, have been marked +with vermillion, were given in a note to the last chapter. I will +commence my description on the eastern side of the map, and will +describe each group of islands consecutively, proceeding westward +across the Pacific and Indian Oceans, but ending with the West Indies. + +The WESTERN SHORES OF AMERICA appear to be entirely without +coral-reefs; south of the equator the survey of the “Beagle”, and north +of it, the published charts show that this is the case. Even in the Bay +of _Panama_, where corals flourish, there are no true coral-reefs, as I +have been informed by Mr. Lloyd. There are no coral-reefs in the +_Galapagos_ Archipelago, as I know from personal inspection; and I +believe there are none on the _Cocos, Revilla-gigedo_, and other +neighbouring islands. _Clipperton_ rock, 10° N., 109° W., has lately +been surveyed by Captain Belcher; in form it is like the crater of a +volcano. From a drawing appended to the MS. plan in the Admiralty, it +evidently is not an atoll. The eastern parts of the Pacific present an +enormous area, without any islands, except _Easter_, and _Sala_, and +_Gomez_ Islands, which do not appear to be surrounded by reefs. + +The LOW ARCHIPELAGO.—This group consists of about eighty atolls: it +will be quite superfluous to refer to descriptions of each. In +D’Urville and Lottin’s chart, one island (_Wolchonsky_) is written with +a capital letter, signifying, as explained in a former chapter, that it +is a high island; but this must be a mistake, as the original chart by +Bellinghausen shows that it is a true atoll. Captain Beechey says of +the thirty-two groups which he examined (of the greater number of which +I have seen beautiful MS. charts in the Admiralty), that twenty-nine +now contain lagoons, and he believes the other three originally did. +Bellinghausen (see an account of his Russian voyage, in the “Biblioth. +des Voyages,” 1834, page 443) says, that the seventeen islands which he +discovered resembled each other in structure, and he has given charts +on a large scale of all of them. Kotzebue has given plans of several; +Cook and Bligh mention others; a few were seen during the voyage of the +“Beagle”; and notices of other atolls are scattered through several +publications. The _Actæon_ group in this archipelago has lately been +discovered (“Geographical Journal”, volume vii., page 454); it consists +of three small and low islets, one of which has a lagoon. Another +lagoon-island has been discovered (“Naut. Mag.” 1839, page 770), in 22° +4′ S., and 136° 20′ W. Towards the S.E. part of the group, there are +some islands of different formation: _Elizabeth_ Island is described by +Beechey (page 46, 4to edition) as fringed by reefs, at the distance of +between two and three hundred yards; coloured red. _Pitcairn_ Island, +in the immediate neighbourhood, according to the same authority, has no +reefs of any kind, although numerous pieces of coral are thrown up on +the beach; the sea close to its shore is very deep (see “Zool. of +Beechey’s Voyage,” page 164); it is left uncoloured. _Gambier_ Islands +(see Plate I., Figure 8), are encircled by a barrier-reef; the greatest +depth within is thirty-eight fathoms; coloured pale blue. _Aurora_ +Island, which lies N.E. of Tahiti close to the large space coloured +dark blue in the map, has been already described in a note (page 71), +on the authority of Mr. Couthouy; it is an upraised atoll, but as it +does not appear to be fringed by living reefs, it is left uncoloured. + +The SOCIETY Archipelago is separated by a narrow space from the Low +Archipelago; and in their parallel direction they manifest some +relation to each other. I have already described the general character +of the reefs of these fine encircled islands. In the “Atlas of the +‘Coquille’s’ Voyage” there is a good general chart of the group, and +separate plans of some of the islands. _Tahiti_, the largest island in +the group, is almost surrounded, as seen in Cook’s chart, by a reef +from half a mile to a mile and a half from the shore, with from ten to +thirty fathoms within it. Some considerable submerged reefs lying +parallel to the shore, with a broad and deep space within, have lately +been discovered (“Naut. Mag.” 1836, page 264) on the N.E. coast of the +island, where none are laid down by Cook. At _Eimeo_ the reef “which +like a ring surrounds it, is in some places one or two miles distant +from the shore, in others united to the beach” (Ellis, “Polynesian +Researches,” volume i., page 18, 12mo edition). Cook found deep water +(twenty fathoms) in some of the harbours within the reef. Mr. Couthouy, +however, states (“Remarks,” page 45) that both at Tahiti and Eimeo, the +space between the barrier-reef and the shore, has been almost filled +up,—“a nearly continuous fringing-reef surrounding the island, and +varying from a few yards to rather more than a mile in width, the +lagoons merely forming canals between this and the sea-reef,” that is +the barrier-reef. _Tapamanoa_ is surrounded by a reef at a considerable +distance from the shore; from the island being small it is breached, as +I am informed by the Rev. W. Ellis, only by a narrow and crooked boat +channel. This is the lowest island in the group, its height probably +not exceeding 500 feet. A little way north of Tahiti, the low +coral-islets of _Teturoa_ are situated; from the description of them +given me by the Rev. J. Williams (the author of the “Narrative of +Missionary Enterprise”), I should have thought they had formed a small +atoll, and likewise from the description given by the Rev. D. Tyerman +and G. Bennett (“Journal of Voyage and Travels,” volume i., page 183), +who say that ten low coral-islets “are comprehended within one general +reef, and separated from each other by interjacent lagoons;” but as Mr. +Stutchbury (“West of England Journal,” volume i., page 54) describes it +as consisting of a mere narrow ridge, I have left it uncoloured. +_Maitea_, eastward of the group, is classed by Forster as a high +encircled island; but from the account given by the Rev. D. Tyerman and +G. Bennett (volume i., page 57) it appears to be an exceedingly abrupt +cone, rising from the sea without any reef; I have left it uncoloured. +It would be superfluous to describe the northern islands in this group, +as they may be well seen in the chart accompanying the 4to edition of +Cook’s “Voyages,” and in the “Atlas of the ‘Coquille’s’ Voyage.” +_Maurua_ is the only one of the northern islands, in which the water +within the reef is not deep, being only four and a half fathoms; but +the great width of the reef, stretching three miles and a half +southward of the land (which is represented in the drawing in the +“Atlas of the ‘Coquille’s’ Voyage” as descending abruptly to the water) +shows, on the principle explained in the beginning of the last chapter, +that it belongs to the barrier class. I may here mention, from +information communicated to me by the Rev. W. Ellis, that on the N.E. +side of _Huaheine_ there is a bank of sand, about a quarter of a mile +wide, extending parallel to the shore, and separated from it by an +extensive and deep lagoon; this bank of sand rests on coral-rock, and +undoubtedly was originally a living reef. North of Bolabola lies the +atoll of _Toubai_ (Motou-iti of the “‘Coquille’s’ Atlas”) which is +coloured dark blue; the other islands, surrounded by barrier-reefs, are +pale blue; three of them are represented in Figures 3, 4, and 5, in +Plate I. There are three low coral-groups lying a little E. of the +Society Archipelago, and almost forming part of it, namely +_Bellinghausen_, which is said by Kotzebue (“Second Voyage,” volume +ii., page 255), to be a lagoon-island; _Mopeha_, which, from Cook’s +description (“Second Voyage,” book iii., chapter i.), no doubt is an +atoll; and the _Scilly_ Islands, which are said by Wallis (“Voyage,” +chapter ix.) to form a _group_ of _low_ islets and shoals, and, +therefore, probably, they compose an atoll: the two former have been +coloured blue, but not the latter. + +MENDANA or MARQUESAS Group.—These islands are entirely without reefs, +as may be seen in Krusenstern’s Atlas, making a remarkable contrast +with the adjacent group of the Society Islands. Mr. F.D. Bennett has +given some account of this group, in the seventh volume of the +“Geographical Journal”. He informs me that all the islands have the +same general character, and that the water is very deep close to their +shores. He visited three of them, namely, _Dominicana_, _Christiana_, +and _Roapoa;_ their beaches are strewed with rounded masses of coral, +and although no regular reefs exist, yet the shore is in many places +lined by coral-rock, so that a boat grounds on this formation. Hence +these islands ought probably to come within the class of fringed +islands and be coloured red; but as I am determined to err on the +cautious side, I have left them uncoloured. + +COOK or HARVEY and AUSTRAL ISLAND.—_Palmerston_ Island is minutely +described as an atoll by Captain Cook during his voyage in 1774; +coloured blue. _Aitutaki_ was partially surveyed by the “Beagle” (see +map accompanying “Voyages of ‘Adventure’ and ‘Beagle’”); the land is +hilly, sloping gently to the beach; the highest point is 360 feet; on +the southern side the reef projects five miles from the land: off this +point the “Beagle” found no bottom with 270 fathoms: the reef is +surmounted by many low coral-islets. Although within the reef the water +is exceedingly shallow, not being more than a few feet deep, as I am +informed by the Rev. J. Williams, nevertheless, from the great +extension of this reef into a profoundly deep ocean, this island +probably belongs, on the principle lately adverted to, to the barrier +class, and I have coloured it pale blue; although with much +hesitation.—_Manouai_ or _Harvey_ Island. The highest point is about +fifty feet: the Rev. J. Williams informs me that the reef here, +although it lies far from the shore, is less distant than at Aitutaki, +but the water within the reef is rather deeper: I have also coloured +this pale blue with many doubts.—Round _Mitiaro_ Island, as I am +informed by Mr. Williams, the reef is attached to the shore; coloured +red. —_Mauki_ or Maouti; the reef round this island (under the name of +Parry Island, in the “Voyage of H.M.S. ‘Blonde’,” page 209) is +described as a coral-flat, only fifty yards wide, and two feet under +water. This statement has been corroborated by Mr. Williams, who calls +the reef attached; coloured red.—_Aitu_, or Wateeo; a moderately +elevated hilly island, like the others of this group. The reef is +described in Cook’s “Voyage,” as attached to the shore, and about one +hundred yards wide; coloured red.—_Fenoua-iti;_ Cook describes this +island as very low, not more than six or seven feet high (volume i., +book ii., chapter iii, 1777); in the chart published in the +“‘Coquille’s’ Atlas,” a reef is engraved close to the shore: this +island is not mentioned in the list given by Mr. Williams (page 16) in +the “Narrative of Missionary Enterprise;” nature doubtful. As it is so +near Atiu, it has been unavoidably coloured red.— _Rarotonga;_ Mr. +Williams informs me that it is a lofty basaltic island with an attached +reef; coloured red.—There are three islands, _Rourouti_, _Roxburgh_, +and _Hull_, of which I have not been able to obtain any account, and +have left them uncoloured. Hull Island, in the French chart, is written +with small letters as being low.—_Mangaia;_ height about three hundred +feet; “the surrounding reef joins the shore” (Williams, “Narrative,” +page 18); coloured red.—_Rimetara;_ Mr. Williams informs me that the +reef is rather close to the shore; but, from information given me by +Mr. Ellis, the reef does not appear to be quite so closely attached to +it as in the foregoing cases: the island is about three hundred feet +high (“Naut. Mag.” 1839, page 738); coloured red.—_Rurutu;_ Mr. +Williams and Mr. Ellis inform me that this island has an attached reef; +coloured red. It is described by Cook under the name of Oheteroa: he +says it is not surrounded, like the neighbouring islands by a reef; he +must have meant a distant reef.—_Toubouai;_ in Cook’s chart (“Second +Voyage,” volume ii., page 2) the reef is laid down in part one mile, +and in part two miles from the shore. Mr. Ellis (“Polynes. Res.” volume +iii., page 381) says the low land round the base of the island is very +extensive; and this gentleman informs me that the water within the reef +appears deep; coloured blue.—_Raivaivai_, or Vivitao; Mr. Williams +informs me that the reef is here distant: Mr. Ellis, however, says that +this is certainly not the case on one side of the island; and he +believes that the water within the reef is not deep; hence I have left +it uncoloured.—_Lancaster_ Reef, described in “Naut. Mag.” 1833 (page +693), as an extensive crescent-formed coral-reef. I have not coloured +it.—_Rapa_, or Oparree; from the accounts given of it by Ellis and +Vancouver, there does not appear to be any reef.—_I. de Bass_ is an +adjoining island, of which I cannot find any account.—_Kemin_ Island; +Krusenstern seems hardly to know its position, and gives no further +particulars. + +ISLANDS BETWEEN _the Low and Gilbert Archipelagoes_. + + +_Caroline_ Island (10° S., 150° W.) is described by Mr. F.D. Bennett +(“Geographical Journal”, volume vii., page 225) as containing a fine +lagoon; coloured blue.—_Flint_ Island (11° S., 151° W.); Krusenstern +believes that it is the same with _Peregrino_, which is described by +Quiros (Burney’s “Chron. Hist.” volume ii., page 283) as “a cluster of +small islands connected by a reef, and forming a lagoon in the middle;” +coloured blue.—_Wostock_ is an island a little more than half a mile in +diameter, and apparently quite flat and low, and was discovered by +Bellinghausen; it is situated a little west of Caroline Island, but it +is not placed on the French charts; I have not coloured it, although I +entertain little doubt from the chart of Bellinghausen, that it +originally contained a small lagoon.—_Penrhyn_ Island (9° S., 158° W.); +a plan of it in the “Atlas of the First Voyage” of Kotzebue, shows that +it is an atoll; blue.— _Starbuck_ Island (5° S., 156° W.) is described +in Byron’s “Voyage in the ‘Blonde’” (page 206) as formed of a flat +coral-rock, with no trees; the height not given; not coloured.—_Malden_ +Island (4° S., 154° W.); in the same voyage (page 205) this island is +said to be of coral formation, and no part above forty feet high; I +have not ventured to colour it, although, from being of +coral-formation, it is probably fringed; in which case it should be +red.—_Jarvis_, or _Bunker_ Island (0° 20′ S., 160° W.) is described by +Mr. F.D. Bennett (“Geographical Journal”, volume vii., page 227) as a +narrow, low strip of coral-formation; not coloured.—_Brook_, is a small +low island between the two latter; the position, and perhaps even the +existence of it is doubtful; not coloured.—_Pescado_ and _Humphrey_ +Islands; I can find out nothing about these islands, except that the +latter appears to be small and low; not coloured.—_Rearson_, or Grand +Duke Alexander’s (10 S., 161° W.); an atoll, of which a plan is given +by Bellinghausen; blue.— _Souvoroff_ Islands (13° S., 163° W.); Admiral +Krusenstern, in the most obliging manner, obtained for me an account of +these islands from Admiral Lazareff, who discovered them. They consist +of five very low islands of coral-formation, two of which are connected +by a reef, with deep water close to it. They do not surround a lagoon, +but are so placed that a line drawn through them includes an oval +space, part of which is shallow; these islets, therefore, probably once +(as is the case with some of the islands in the Caroline Archipelago) +formed a single atoll; but I have not coloured them.—_Danger_ Island +(10° S., 166° W.); described as low by Commodore Byron, and more lately +surveyed by Bellinghausen; it is a small atoll with three islets on it; +blue.—_Clarence_ Island (9° S., 172° W.); discovered in the “Pandora” +(G. Hamilton’s “Voyage,” page 75): it is said, “in running along the +land, we saw several canoes crossing the _lagoons;_” as this island is +in the close vicinity of other low islands, and as it is said, that the +natives make reservoirs of water in old cocoa-nut trees (which shows +the nature of the land), I have no doubt it is an atoll, and have +coloured it blue. _York_ Island (8° S., 172° W.) is described by +Commodore Byron (chapter x. of his “Voyage”) as an atoll; +blue.—_Sydney_ Island (4° S., 172° W.) is about three miles in +diameter, with its interior occupied by a lagoon (Captain Tromelin, +“Annal. Marit.” 1829, page 297); blue.—_Phœnix_ Island (4° S., 171° W.) +is nearly circular, low, sandy, not more than two miles in diameter, +and very steep outside (Tromelin, “Annal. Marit.” 1829, page 297); it +may be inferred that this island originally contained a lagoon, but I +have not coloured it.—_New Nantucket_ (0° 15′ N., 174° W.). From the +French chart it must be a low island; I can find nothing more about it +or about _Mary_ Island; both uncoloured.—_Gardner_ Island (5° S., 174° +W.) from its position is certainly the same as _Kemin_ Island described +(Krusenstern, page 435, Appen. to Mem., published 1827) as having a +lagoon in its centre; blue. + +ISLANDS SOUTH _of the Sandwich Archipelago_. + + +_Christmas_ Island (2° N., 157° W.). Captain Cook, in his “Third +Voyage” (Volume ii., chapter x.), has given a detailed account of this +atoll. The breadth of the islets on the reef is unusually great, and +the sea near it does not deepen so suddenly as is generally the case. +It has more lately been visited by Mr. F.D. Bennett (“Geographical +Journal,” volume vii., page 226); and he assures me that it is low and +of coral-formation: I particularly mention this, because it is engraved +with a capital letter, signifying a high island, in D’Urville and +Lottin’s chart. Mr. Couthouy, also, has given some account of it +(“Remarks,” page 46) from the Hawaiian “Spectator”; he believes it has +lately undergone a small elevation, but his evidence does not appear to +me satisfactory; the deepest part of the lagoon is said to be only ten +feet; nevertheless, I have coloured it blue.—_Fanning_ Island (4° N., +158° W.) according to Captain Tromelin (“Ann. Maritim.” 1829, page +283), is an atoll: his account as observed by Krusenstern, differs from +that given in Fanning’s “Voyage” (page 224), which, however, is far +from clear; coloured blue.— _Washington_ Island (4° N., 159° W.) is +engraved as a low island in D’Urville’s chart, but is described by +Fanning (page 226) as having a much greater elevation than Fanning +Island, and hence I presume it is not an atoll; not coloured.—_Palmyra_ +Island (6° N., 162° W.) is an atoll divided into two parts +(Krusenstern’s “Mem. Suppl.” page 50, also Fanning’s “Voyage,” page +233); blue.—_Smyth’s_ or Johnston’s Islands (17° N., 170° W.). Captain +Smyth, R.N., has had the kindness to inform me that they consist of two +very low, small islands, with a dangerous reef off the east end of +them. Captain Smyth does not recollect whether these islets, together +with the reef, surrounded a lagoon; uncoloured. + +SANDWICH ARCHIPELAGO.—_Hawaii;_ in the chart in Freycinet’s “Atlas,” +small portions of the coast are fringed by reefs; and in the +accompanying “Hydrog. Memoir,” reefs are mentioned in several places, +and the coral is said to injure the cables. On one side of the islet of +Kohaihai there is a bank of sand and coral with five feet water on it, +running parallel to the shore, and leaving a channel of about fifteen +feet deep within. I have coloured this island red, but it is very much +less perfectly fringed than others of the group.—_Maui;_ in Freycinet’s +chart of the anchorage of Raheina, two or three miles of coast are seen +to be fringed; and in the “Hydrog. Memoir,” “banks of coral along +shore” are spoken of. Mr. F.D. Bennett informs me that the reefs, on an +average, extend about a quarter of a mile from the beach; the land is +not very steep, and outside the reefs the sea does not become deep very +suddenly; coloured red.—_Morotoi_, I presume, is fringed: Freycinet +speaks of the breakers extending along the shore at a little distance +from it. From the chart, I believe it is fringed; coloured red.—_Oahu;_ +Freycinet, in his “Hydrog. Memoir,” mentions some of the reefs. Mr. +F.D. Bennett informs me that the shore is skirted for forty or fifty +miles in length. There is even a harbour for ships formed by the reefs, +but it is at the mouth of a valley; red.—_Atooi_, in La Peyrouse’s +charts, is represented as fringed by a reef, in the same manner as Oahu +and Morotoi; and this, as I have been informed by Mr. Ellis, on part at +least of the shore, is of coral-formation: the reef does not leave a +deep channel within; red.—_Oneehow;_ Mr. Ellis believes that this +island is also fringed by a coral-reef: considering its close proximity +to the other islands, I have ventured to colour it red. I have in vain +consulted the works of Cook, Vancouver, La Peyrouse, and Lisiansky, for +any satisfactory account of the small islands and reefs, which lie +scattered in a N.W. line prolonged from the Sandwich group, and hence +have left them uncoloured, with one exception; for I am indebted to Mr. +F.D. Bennett for informing me of an atoll-formed reef, in latitude 28° +22′, longitude 178° 30′ W., on which the “Gledstanes” was wrecked in +1837. It is apparently of large size, and extends in a N.W. and S.E. +line: very few islets have been formed on it. The lagoon seems to be +shallow; at least, the deepest part which was surveyed was only three +fathoms. Mr. Couthouy (“Remarks,” page 38) describes this island under +the name of _Ocean_ island. Considerable doubts should be entertained +regarding the nature of a reef of this kind, with a very shallow +lagoon, and standing far from any other atoll, on account of the +possibility of a crater or flat bank of rock lying at the proper depth +beneath the surface of the water, thus affording a foundation for a +ring-formed coral-reef. I have, however, thought myself compelled, from +its large size and symmetrical outline, to colour it blue. + +SAMOA or NAVIGATOR GROUP.—Kotzebue, in his “Second Voyage,” contrasts +the structure of these islands with many others in the Pacific, in not +being furnished with harbours for ships, formed by distant coral-reefs. +The Rev. J. Williams, however, informs me, that coral-reefs do occur in +irregular patches on the shores of these islands; but that they do not +form a continuous band, as round Mangaia, and other such perfect cases +of fringed islands. From the charts accompanying La Peyrouse’s +“Voyage,” it appears that the north shore of _Savaii_, _Maouna_, +_Orosenga_, and _Mannua_, are fringed by reefs. La Peyrouse, speaking +of Maouna (page 126), says that the coral-reef surrounding its shores, +almost touches the beach; and is breached in front of the little coves +and streams, forming passages for canoes, and probably even for boats. +Further on (page 159), he extends the same observation to all the +islands which he visited. Mr. Williams in his “Narrative,” speaks of a +reef going round a small island attached to _Oyolava_, and returning +again to it: all these islands have been coloured red.—A chart of +_Rose_ Island, at the extreme west end of the group, is given by +Freycinet, from which I should have thought that it had been an atoll; +but according to Mr. Couthouy (“Remarks,” page 43), it consists of a +reef, only a league in circuit, surmounted by a very few low islets; +the lagoon is very shallow, and is strewed with numerous large boulders +of volcanic rock. This island, therefore, probably consists of a bank +of rock, a few feet submerged, with the outer margin of its upper +surface fringed with reefs; hence it cannot be properly classed with +atolls, in which the foundations are always supposed to lie at a depth, +greater than that at which the reef-constructing polypifers can live; +not coloured. + +_Beveridge Reef_, 20° S., 167° W., is described in the “Naut. Mag.” +(May 1833, page 442) as ten miles long in a N. and S. line, and eight +wide; “in the inside of the reef there appears deep water;” there is a +passage near the S.W. corner: this therefore seems to be a submerged +atoll, and is coloured blue. + +_Savage_ Island, 19° S., 170° W., has been described by Cook and +Forster. The younger Forster (volume ii., page 163) says it is about +forty feet high: he suspects that it contains a low plain, which +formerly was the lagoon. The Rev. J. Williams informs me that the reef +fringing its shores, resembles that round Mangaia; coloured red. + +FRIENDLY ARCHIPELAGO. _Pylstaart_ Island. Judging from the chart in +Freycinet’s “Atlas,” I should have supposed that it had been regularly +fringed; but as nothing is said in the “Hydrog. Memoir” (or in the +“Voyage” of Tasman, the discoverer) about coral-reefs, I have left it +uncoloured.—_Tongatabou:_ In the “Atlas of the Voyage of the +‘Astrolabe’,” the whole south side of the island is represented as +narrowly fringed by the same reef which forms an extensive platform on +the northern side. The origin of this latter reef, which might have +been mistaken for a barrier-reef, has already been attempted to be +explained, when giving the proofs of the recent elevation of this +island.— In Cook’s charts the little outlying island also of _Eoaigee_, +is represented as fringed; coloured red.—_Eoua_. I cannot make out from +Captain Cook’s charts and descriptions, that this island has any reef, +although the bottom of the neighbouring sea seems to be corally, and +the island itself is formed of coral-rock. Forster, however, distinctly +(“Observations,” page 14) classes it with high islands having reefs, +but it certainly is not encircled by a barrier-reef and the younger +Forster (“Voyage,” volume i., page 426) says, that “a bed of +coral-rocks surrounded the coast towards the landing-place.” I have +therefore classed it with the fringed islands and coloured it red. The +several islands lying N.W. of Tongatabou, namely _Anamouka_, _Komango_, +_Kotou_, _Lefouga_, _Foa_, &c., are seen in Captain Cook’s chart to be +fringed by reefs, in several of them are connected together. From the +various statements in the first volume of Cook’s “Third Voyage,” and +especially in the fourth and sixth chapters, it appears that these +reefs are of coral-formation, and certainly do not belong to the +barrier class; coloured red.—_Toufoua_ & _Kao_, forming the western +part of the group, according to Forster have no reefs; the former is an +active volcano.—_Vavao_. There is a chart of this singularly formed +island, by Espinoza: according to Mr. Williams it consists of +coral-rock: the Chevalier Dillon informs me that it is not fringed; not +coloured. Nor are the islands of _Latte_ and _Amargura_, for I have not +seen plans on a large scale of them, and do not know whether they are +fringed. + +_Niouha_, 16° S., 174° W., or _Keppel_ Island of Wallis, or _Cocos_ +Island. From a view and chart of this island given in Wallis’s “Voyage” +(4to edition) it is evidently encircled by a reef; coloured blue: it is +however remarkable that _Boscawen_ Island, immediately adjoining, has +no reef of any kind; uncoloured. + +_Wallis Island_, 13° S., 176° W., a chart and view of this island in +Wallis’s “Voyage” (4to edition) shows that it is encircled. A view of +it in the “Naut. Mag.” July 1833, page 376, shows the same fact; blue. + +_Alloufatou_, or _Horn_ Island, _Onouafu_, or _Proby_ Island, and +_Hunter_ Islands, lie between the Navigator and Fidji groups. I can +find no distinct accounts of them. + +FIDJI or VITI GROUP.—The best chart of the numerous islands of this +group, will be found in the “Atlas of the ‘Astrolabe’s’ Voyage.” From +this, and from the description given in the “Hydrog. Memoir,” +accompanying it, it appears that many of these islands are bold and +mountainous, rising to the height of between 3,000 and 4,000 feet. Most +of the islands are surrounded by reefs, lying far from the land, and +outside of which the ocean appears very deep. The “Astrolabe” sounded +with ninety fathoms in several places about a mile from the reefs, and +found no bottom. Although the depth within the reef is not laid down, +it is evident from several expressions, that Captain D’Urville believes +that ships could anchor within, if passages existed through the outer +barriers. The Chevallier Dillon informs me that this is the case: hence +I have coloured this group blue. In the S.E. part lies _Batoa_, or +_Turtle_ Island of Cook (“Second Voyage,” volume ii., page 23, and +chart, 4to edition) surrounded by a coral-reef, “which in some places +extends two miles from the shore;” within the reef the water appears to +be deep, and outside it is unfathomable; coloured pale blue. At the +distance of a few miles, Captain Cook (Ibid., page 24) found a circular +coral-reef, four or five leagues in circuit, with deep water within; +“in short, the bank wants only a few little islets to make it exactly +like one of the half-drowned isles so often mentioned,”—namely, atolls. +South of Batoa, lies the high island of _Ono_, which appears in +Bellinghausen’s “Atlas” to be encircled; as do some other small islands +to the south; coloured pale blue; near Ono, there is an annular reef, +quite similar to the one just described in the words of Captain Cook; +coloured dark blue. + +_Rotoumah_, 13° S., 179° E.—From the chart in Duperrey’s “Atlas,” I +thought this island was encircled, and had coloured it blue, but the +Chevallier Dillon assures me that the reef is only a shore or fringing +one; red. + +_Independence_ Island, 10° S., 179° E., is described by Mr. G. Bennett, +(“United Service Journal,” 1831, part ii., page 197) as a low island of +coral-formation, it is small, and does not appear to contain a lagoon, +although an opening through the reef is referred to. A lagoon probably +once existed, and has since been filled up; left uncoloured. + +ELLICE GROUP.—_Oscar_, _Peyster_, and _Ellice_ Islands are figured in +Arrowsmith’s “Chart of the Pacific” (corrected to 1832) as atolls, and +are said to be very low; blue.—_Nederlandisch_ Island. I am greatly +indebted to the kindness of Admiral Krusenstern, for sending me the +original documents concerning this island. From the plans given by +Captains Eeg and Khremtshenko, and from the detailed account given by +the former, it appears that it is a narrow coral-island, about two +miles long, containing a small lagoon. The sea is very deep close to +the shore, which is fronted by sharp coral-rocks. Captain Eeg compares +the lagoon with that of other coral-islands; and he distinctly says, +the land is “very low.” I have therefore coloured it blue. Admiral +Krusenstern (“Memoir on the Pacific,” Append., 1835) states that its +shores are eighty feet high; this probably arose from the height of the +cocoa-nut trees, with which it is covered, being mistaken for land. +—_Gran Cocal_ is said in Krusenstern’s “Memoir,” to be low, and to be +surrounded by a reef; it is small, and therefore probably once +contained a lagoon; uncoloured.—_St. Augustin_. From a chart and view +of it, given in the “Atlas of the ‘Coquille’s’ Voyage,” it appears to +be a small atoll, with its lagoon partly filled up; coloured blue. + +GILBERT GROUP.—The chart of this group, given in the “Atlas of the +‘Coquille’s’ Voyage,” at once shows that it is composed of ten well +characterised atolls. In D’Urville and Lottin’s chart, _Sydenham_ is +written with a capital letter, signifying that it is high; but this +certainly is not the case, for it is a perfectly characterised atoll, +and a sketch, showing how low it is, is given in the “‘Coquille’s’ +Atlas.” Some narrow strip-like reefs project from the southern side of +_Drummond_ atoll, and render it irregular. The southern island of the +group is called _Chase_ (in some charts, _Rotches_); of this I can find +no account, but Mr. F.D. Bennett discovered (“Geographical Journal”, +volume vii., page 229), a low extensive island in nearly the same +latitude, about three degrees westward of the longitude assigned to +Rotches, but very probably it is the same island. Mr. Bennett informs +me that the man at the masthead reported an appearance of lagoon-water +in the centre; and, therefore, considering its position, I have +coloured it blue. —_Pitt_ Island, at the extreme northern point of the +group, is left uncoloured, as its exact position and nature is not +known.—_Byron_ Island, which lies a little to the eastward, does not +appear to have been visited since Commodore Byron’s voyage, and it was +then seen only from a distance of eighteen miles; it is said to be low; +uncoloured. + +_Ocean_, _Pleasant_, and _Atlantic_ Islands all lie considerably to the +west of the Gilbert group: I have been unable to find any distinct +account of them. Ocean Island is written with small letters in the +French chart, but in Krusenstern’s “Memoir” it is said to be high. + +MARSHALL GROUP.—We are well acquainted with this group from the +excellent charts of the separate islands, made during the two voyages +of Kotzebue: a reduced one of the whole group may be easily seen in +Krusenstern’s “Atlas,” and in Kotzebue’s “Second Voyage.” The group +consists (with the exception of two _little_ islands which probably +have had their lagoon filled up) of a double row of twenty-three large +and well-characterised atolls, from the examination of which Chamisso +has given us his well-known account of coral-formations. I include +_Gaspar-Rico_, or _Cornwallis_ Island in this group, which is described +by Chamisso (Kotzebue’s “First Voyage,” volume iii., page 179) “as a +low sickle-formed group, with mould only on the windward side.” Gaspard +Island is considered by some geographers as a distinct island lying +N.E. of the group, but it is not entered in the chart by Krusenstern; +left uncoloured. In the S.W. part of this group lies _Baring_ Island, +of which little is known (see Krusenstern’s “Appendix,” 1835, page +149). I have left it uncoloured; but _Boston_ Island I have coloured +blue, as it is described (Ibid.) as consisting of fourteen small +islands, which, no doubt, enclose a lagoon, as represented in a chart +in the “‘Coquille’s’ Atlas.”—Two islands, _Aur Kawen_ and _Gaspar +Rico_, are written in the French chart with capital letters; but this +is an error, for from the account given by Chamisso in Kotzebue’s +“First Voyage,” they are certainly low. The nature, position, and even +existence, of the shoals and small islands north of the Marshall group, +are doubtful. + +NEW HEBRIDES.—Any chart, on even a small scale, of these islands, will +show that their shores are almost without reefs, presenting a +remarkable contrast with those of New Caledonia on the one hand, and +the Fidji group on the other. Nevertheless, I have been assured by Mr. +G. Bennett, that coral grows vigorously on their shores; as indeed, +will be further shown in some of the following notices. As, therefore, +these islands are not encircled, and as coral grows vigorously on their +shores, we might almost conclude, without further evidence, that they +were fringed, and hence I have applied the red colour with rather +greater freedom than in other instances.—_Matthew’s Rock_, an active +volcano, some way south of the group (of which a plan is given in the +“Atlas of the ‘Astrolabe’s’ Voyage”) does not appear to have reefs of +any kind about it.—_Annatom_, the southernmost of the Hebrides; from a +rough woodcut given in the “United Service Journal” (1831, part iii., +page 190), accompanying a paper by Mr. Bennett, it appears that the +shore is fringed; coloured red.—_Tanna_. Forster, in his “Observations” +(page 22), says Tanna has on its shores coral-rock and madrepores; and +the younger Forster, in his account (volume ii., page 269) speaking of +the harbour says, the whole S.E. side consists of coral-reefs, which +are overflowed at high-water; part of the southern shore in Cook’s +chart is represented as fringed; coloured red.—_Immer_ is described +(“United Service Journal,” 1831, part iii., page 192) by Mr. Bennett as +being of moderate elevation, with cliffs appearing like sandstone: +coral grows in patches on its shore, but I have not coloured it; and I +mention these facts, because Immer might have been thought from +Forster’s classification (“Observations,” page 14), to have been a low +island or even an atoll.— _Erromango_ Island; Cook (“Second Voyage,” +volume ii., page 45, 4to edition) speaks of rocks everywhere _lining_ +the coast, and the natives offered to haul his boat over the breakers +to the sandy beach: Mr. Bennett, in a letter to the Editor of the +“Singapore Chron.,” alludes to the _reefs_ on its shores. It may, I +think, be safely inferred from these passages that the shore is fringed +in parts by coral-reefs; coloured red.—_Sandwich_ Island. The east +coast is said (Cook’s “Second Voyage,” volume ii., page 41) to be low, +and to be guarded by a chain of breakers. In the accompanying chart it +is seen to be fringed by a reef; coloured red.—_Mallicollo_; Forster +speaks of the reef-bounded shore: the reef is about thirty yards wide, +and so shallow that a boat cannot pass over it. Forster also +(“Observations,” page 23) says, that the rocks of the sea-shore consist +of madrepore. In the plan of Sandwich harbour, the headlands are +represented as fringed; coloured red.—_Aurora_ and _Pentecost_ Islands, +according to Bougainville, apparently have no reefs; nor has the large +island of _S. Espiritu_, nor _Bligh_ Island or _Banks’_ Islands, which +latter lie to the N.E. of the Hebrides. But in none of these cases, +have I met with any detailed account of their shores, or seen plans on +a large scale; and it will be evident, that a fringing-reef of only +thirty or even a few hundred yards in width, is of so little importance +to navigation, that it will seldom be noticed, excepting by chance; and +hence I do not doubt that several of these islands, now left +uncoloured, ought to be red. + +SANTA-CRUZ GROUP.—_Vanikoro_ (Figure 1, Plate I.) offers a striking +example of a barrier-reef: it was first described by the Chevalier +Dillon, in his voyage, and was surveyed in the “Astrolabe”; coloured +pale blue.—_Tikopia_ and _Fataka_ Islands appear, from the descriptions +of Dillon and D’Urville, to have no reefs; _Anouda_ is a low, flat +island, surrounded by cliffs (“‘Astrolabe’ Hydrog.” and Krusenstern, +“Mem.” volume ii., page 432); these are uncoloured. _Toupoua_ +(_Otooboa_ of Dillon) is stated by Captain Tromelin (“Annales Marit.” +1829, page 289) to be almost entirely included in a reef, lying at the +distance of two miles from the shore. There is a space of three miles +without any reef, which, although indented with bays, offers no +anchorage from the extreme depth of the water close to the shore: +Captain Dillon also speaks of the reefs fronting this island; coloured +blue.— _Santa-Cruz_. I have carefully examined the works of Carteret, +D’Entrecasteaux, Wilson, and Tromelin, and I cannot discover any +mention of reefs on its shores; left uncoloured.—_Tinakoro_ is a +constantly active volcano without reefs.—_Mendana Isles_ (mentioned by +Dillon under the name of _Mammee_, etc.); said by Krusenstern to be +low, and intertwined with reefs. I do not believe they include a +lagoon; I have left them uncoloured.—_Duff’s_ Islands compose a small +group directed in a N.W. and S.E. band; they are described by Wilson +(page 296, “Miss. Voy.” 4to edition), as formed by bold-peaked land, +with the islands surrounded by coral-reefs, extending about half a mile +from the shore; at a distance of a mile from the reefs he found only +seven fathoms. As I have no reason for supposing there is deep water +within these reefs, I have coloured them red. _Kennedy_ Island, N.E. of +Duff’s. I have been unable to find any account of it. + +NEW CALEDONIA.—The great barrier-reefs on the shores of this island +have already been described (Figure 5, Plate II.). They have been +visited by Labillardière, Cook, and the northern point by D’Urville; +this latter part so closely resembles an atoll that I have coloured it +dark blue. The _Loyalty_ group is situated eastward of this island; +from the chart and description given in the “Voyage of the +‘Astrolabe’,” they do not appear to have any reefs; north of this +group, there are some extensive low reefs (called _Astrolabe_ and +_Beaupré_,) which do not seem to be atoll-formed; these are left +uncoloured. + +AUSTRALIAN BARRIER-REEF.—The limits of this great reef, which has +already been described, have been coloured from the charts of Flinders +and King. In the northern parts, an atoll-formed reef, lying outside +the barrier, has been described by Bligh, and is coloured dark blue. In +the space between Australia and New Caledonia, called by Flinders the +Corallian Sea, there are numerous reefs. Of these, some are represented +in Krusenstern’s “Atlas” as having an atoll-like structure; namely, +_Bampton_ shoal, _Frederic_, _Vine_ or Horse-shoe, and _Alert_ reefs; +these have been coloured dark blue. + +LOUISIADE: the dangerous reefs which front and surround the western, +southern, and northern coasts of this so-called peninsula and +archipelago, seem evidently to belong to the barrier class. The land is +lofty, with a low fringe on the coast; the reefs are distant, and the +sea outside them profoundly deep. Nearly all that is known of this +group is derived from the labours of D’Entrecasteaux and Bougainville: +the latter has represented one continuous reef ninety miles long, +parallel to the shore, and in places as much as ten miles from it; +coloured pale blue. A little distance northward we have the _Laughlan_ +Islands, the reefs round which are engraved in the “Atlas of the Voyage +of the ‘Astrolabe’,” in the same manner as in the encircled islands of +the Caroline Archipelago, the reef is, in parts, a mile and a half from +the shore, to which it does not appear to be attached; coloured blue. +At some little distance from the extremity of the Louisiade lies the +_Wells_ reef, described in G. Hamilton’s “Voyage in H.M.S. ‘Pandora’” +(page 100): it is said, “We found we had got embayed in a double reef, +which will soon be an island.” As this statement is only intelligible +on the supposition of the reef being crescent or horse-shoe formed, +like so many other submerged annular reefs, I have ventured to colour +it blue. + +SALOMON ARCHIPELAGO: the chart in Krusenstern’s “Atlas” shows that +these islands are not encircled, and as coral appears from the works of +Surville, Bougainville, and Labillardière, to grow on their shores, +this circumstance, as in the case of the New Hebrides, is a presumption +that they are fringed. I cannot find out anything from +D’Entrecasteaux’s “Voyage,” regarding the southern islands of the +group, so have left them uncoloured.—_Malayta_ Island in a rough MS. +chart in the Admiralty has its northern shore fringed.—_Ysabel_ Island, +the N.E. part of this island, in the same chart, is also fringed: +Mendana, speaking (Burney, volume i., page 280) of an islet adjoining +the northern coast, says it is surrounded by reefs; the shores, also of +Port Praslin appear regularly fringed.—_Choiseul_ Island. In +Bougainville’s “Chart of Choiseul Bay,” parts of the shores are fringed +by coral-reefs.— _Bougainville_ Island. According to D’Entrecasteaux +the western shore abounds with coral-reefs, and the smaller islands are +said to be attached to the larger ones by reefs; all the +before-mentioned islands have been coloured red.—_Bouka_ Islands. +Captain Duperrey has kindly informed me in a letter that he passed +close round the northern side of this island (of which a plan is given +in his “Atlas of the ‘Coquille’s’ Voyage”), and that it was “garnie +d’une bande de récifs à fleur d’eau adherentes au rivage;” and he +infers, from the abundance of coral on the islands north and south of +Bouka, that the reef probably is of coral; coloured red. + +Off the north coast of the Solomon Archipelago there are several small +groups which are little known; they appear to be low, and of +coral-formation; and some of them probably have an atoll-like +structure; the Chevallier Dillon, however, informs me that this is not +the case with the B. de _Candelaria_.—_Outong Java_, according to the +Spanish navigator, Maurelle, is thus characterised; but this is the +only one which I have ventured to colour blue. + +NEW IRELAND.—The shores of the S.W. point of this island and some +adjoining islets, are fringed by reefs, as may be seen in the “Atlases +of the Voyages of the ‘Coquille’ and ‘Astrolabe’.” M. Lesson observes +that the reefs are open in front of each streamlet. The _Duke of +York’s_ Island is also fringed; but with regard to the other parts of +_New Ireland_, _New Hanover_, and the small islands lying northward, I +have been unable to obtain any information. I will only add that no +part of New Ireland appears to be fronted by distant reefs. I have +coloured red only the above specified portions. + +NEW BRITAIN and the NORTHERN SHORE of NEW GUINEA.—From the charts in +the “Voyage of the ‘Astrolabe’,” and from the “Hydrog. Memoir,” it +appears that these coasts are entirely without reefs, as are the +_Schouten Islands_, lying close to the northern shore of New Guinea. +The western and south-western parts of New Guinea, will be treated of +when we come to the islands of the East Indian Archipelago. + +ADMIRALTY GROUP.—From the accounts by Bougainville, Maurelle, +D’Entrecasteaux, and the scattered notices collected by Horsburgh, it +appears, that some of the many islands composing it, are high, with a +bold outline; and others are very low, small and interlaced with reefs. +All the high islands appear to be fronted by distant reefs rising +abruptly from the sea, and within some of which there is reason to +believe that the water is deep. I have therefore little doubt they are +of the barrier class.—In the southern part of the group we have +_Elizabeth Island_, which is surrounded by a reef at the distance of a +mile; and two miles eastward of it (Krusenstern, “Append.” 1835, page +42) there is a little island containing a lagoon.—Near here, also lies +_Circular-reef_ (Horsburgh, “Direct.” volume i., page 691, 4th +edition), “three or four miles in diameter having deep water inside +with an opening at the N.N.W. part, and on the outside steep to.” I +have from these data, coloured the group pale blue, and _circular-reef_ +dark blue.—the _Anachorites_, _Echequier_, and _Hermites_, consist of +innumerable low islands of coral-formation, which probably have +atoll-like forms; but not being able to ascertain this, I have not +coloured them, nor _Durour_ Island, which is described by Carteret as +low. + +The CAROLINE ARCHIPELAGO is now well-known, chiefly from the +hydrographical labours of Lutké; it contains about forty groups of +atolls, and three encircled islands, two of which are engraved in +Figures 2 and 7, Plate I. Commencing with the eastern part; the +encircling reef round _Ualen_ appears to be only about half a mile from +the shore; but as the land is low and covered with mangroves (“Voyage +autour du Monde,” par F. Lutké, volume i., page 339), the real margin +has not probably been ascertained. The extreme depth in one of the +harbours within the reef is thirty-three fathoms (see charts in “Atlas +of ‘Coquille’s’ Voyage”), and outside at half a mile distant from the +reef, no bottom was obtained with two hundred and fifty fathoms. The +reef is surmounted by many islets, and the lagoon-like channel within +is mostly shallow, and appears to have been much encroached on by the +low land surrounding the central mountains; these facts show that time +has allowed much detritus to accumulate; coloured pale blue.— +_Pouynipète_, or _Seniavine_. In the greater part of the circumference +of this island, the reef is about one mile and three quarters distant; +on the north side it is five miles off the included high islets. The +reef is broken in several places; and just within it, the depth in one +place is thirty fathoms, and in another, twenty-eight, beyond which, to +all appearance, there was “un porte vaste et sur” (Lutké, volume ii., +page 4); coloured pale blue.—_Hogoleu_ or _Roug_. This wonderful group +contains at least sixty-two islands, and its reef is one hundred and +thirty-five miles in circuit. Of the islands, only a few, about six or +eight (see “Hydrog. Descrip.” page 428, of the “Voyage of the +‘Astrolabe’,” and the large accompanying chart taken chiefly from that +given by Duperrey) are high, and the rest are all small, low, and +formed on the reef. The depth of the great interior lake has not been +ascertained; but Captain D’Urville appears to have entertained no doubt +about the possibility of taking in a frigate. The reef lies no less +than fourteen miles distant from the northern coasts of the interior +high islands, seven from their western sides, and twenty from the +southern; the sea is deep outside. This island is a likeness on a grand +scale to the Gambier group in the Low Archipelago. Of the groups of +low[1] islands forming the chief part of the Caroline Archipelago, all +those of larger size, have the true atoll-structure (as may be seen in +the “Atlas” by Captain Lutké), and some even of the very small ones, as +_Macaskill_ and _Duperrey_, of which plans are given in the “Atlas of +the ‘Coquille’s’ Voyage.” There are, however, some low small islands of +coral-formation, namely _Ollap_, _Tamatam_, _Bigali_, _Satahoual_, +which do not contain lagoons; but it is probable that lagoons +originally existed, but have since filled up: Lutké (volume ii., page +304) seems to have thought that all the low islands, with only one +exception, contained lagoons. From the sketches, and from the manner in +which the margins of these islands are engraved in the “Atlas of the +Voyage of the ‘Coquille’,” it might have been thought that they were +not low; but by a comparison with the remarks of Lutké (volume ii., +page 107, regarding Bigali) and of Freycinet (“Hydrog. Memoir +‘L’Uranie’ Voyage,” page 188, regarding Tamatam, Ollap, etc.), it will +be seen that the artist must have represented the land incorrectly. The +most southern island in the group, namely _Piguiram_, is not coloured, +because I have found no account of it. _Nougouor_, or _Monte Verdison_, +which was not visited by Lutké, is described and figured by Mr. Bennett +(“United Service Journal,” January 1832) as an atoll. All the +above-mentioned islands have been coloured blue. + + [1] In D’Urville and Lottin’s chart, Peserare is written with capital + letters; but this evidently is an error, for it is one of the low + islets on the reef of Namonouyto (see Lutké’s charts)—a regular atoll. + + +WESTERN PART OF THE CAROLINE ARCHIPELAGO.—_Fais_ Island is ninety feet +high, and is surrounded, as I have been informed by Admiral Lutké, by a +narrow reef of living coral, of which the broadest part, as represented +in the charts, is only 150 yards; coloured red.— _Philip_ Island., I +believe, is low; but Hunter, in his “Historical Journal,” gives no +clear account of it; uncoloured.—_Elivi;_ from the manner in which the +islets on the reefs are engraved, in the “Atlas of the ‘Astrolabe’s’ +Voyage,” I should have thought they were above the ordinary height, but +Admiral Lutké assures me this is not the case: they form a regular +atoll; coloured blue.—_Gouap_ (_Eap_ of Chamisso), is a high island +with a reef (see chart in “Voyage of the ‘Astrolabe’”), more than a +mile distant in most parts from the shore, and two miles in one part. +Captain D’Urville thinks that there would be anchorage (“Hydrog. +Descript. ‘Astrolabe’ Voyage,” page 436) for ships within the reef, if +a passage could be found; coloured pale blue.—_Goulou_, from the chart +in the “‘Astrolabe’s’ Atlas,” appears to be an atoll. D’Urville +(“Hydrog. Descript.” page 437) speaks of the low islets on the reef; +coloured dark blue. + +PELEW ISLANDS.—Krusenstern speaks of some of the islands being +mountainous; the reefs are distant from the shore, and there are spaces +within them, and not opposite valleys, with from ten to fifteen +fathoms. According to a MS. chart of the group by Lieutenant Elmer in +the Admiralty, there is a large space within the reef with deepish +water; although the high land does not hold a central position with +respect to the reefs, as is generally the case, I have little doubt +that the reefs of the Pelew Islands ought to be ranked with the barrier +class, and I have coloured them pale blue. In Lieutenant Elmer’s chart +there is a horseshoe-formed shoal, laid down thirteen miles N.W. of +Pelew, with fifteen fathoms within the reef, and some dry banks on it; +coloured dark blue.—_Spanish_, _Martires_, _Sanserot_, _Pulo Anna_ and +_Mariere_ Islands are not coloured, because I know nothing about them, +excepting that according to Krusenstern, the second, third, and fourth +mentioned, are low, placed on coral-reefs, and therefore, perhaps, +contain lagoons; but Pulo Mariere is a little higher. + +MARIANA ARCHIPELAGO, or LADRONES.—_Guahan_. Almost the whole of this +island is fringed by reefs, which extend in most parts about a third of +a mile from the land. Even where the reefs are most extensive, the +water within them is shallow. In several parts there is a navigable +channel for boats and canoes within the reefs. In Freycinet’s “Hydrog. +Mem.” there is an account of these reefs, and in the “Atlas,” a map on +a large scale; coloured red.—_Rota_. “L’ile est presque entièrement +entourée des récifs” (page 212, Freycinet’s “Hydrog. Mem.”). These +reefs project about a quarter of a mile from the shore; coloured +red.—_Tinian_. The eastern coast is precipitous, and is without reefs; +but the western side is fringed like the last island; coloured +red.—_Saypan_. The N.E. coast, and likewise the western shores appear +to be fringed; but there is a great, irregular, horn-like reef +projecting far from this side; coloured red.—_Farallon de Medinilla_, +appears so regularly and closely fringed in Freycinet’s charts, that I +have ventured to colour it red, although nothing is said about reefs in +the “Hydrographical Memoir.” The several islands which form the +northern part of the group are volcanic (with the exception perhaps of +Torres, which resembles in form the madreporitic island of Medinilla), +and appear to be without reefs.—_Mangs_, however, is described (by +Freycinet, page 219, “Hydrog.”) from some Spanish charts, as formed of +small islands placed “au milieu des nombreux récifs;” and as these +reefs in the general chart of the group do not project so much as a +mile; and as there is no appearance from a double line, of the +existence of deep water within, I have ventured, although with much +hesitation, to colour them red. Respecting _Folger_ and _Marshall_ +Islands which lie some way east of the Marianas, I can find out +nothing, excepting that they are probably low. Krusenstern says this of +Marshall Island; and Folger Island is written with small letters in +D’Urville’s chart; uncoloured. + +BONIN or ARZOBISPO GROUP.—_Peel Island_ has been examined by Captain +Beechey, to whose kindness I am much indebted for giving me information +regarding it: “At Port Lloyd there is a great deal of coral; and the +inner harbour is entirely formed by coral-reefs, which extend outside +the port along the coast.” Captain Beechey, in another part of his +letter to me, alludes to the reefs fringing the island in all +directions; but at the same time it must be observed that the surf +washes the volcanic rocks of the coast in the greater part of its +circumference. I do not know whether the other islands of the +Archipelago are fringed; I have coloured Peel Island red.—_Grampus +Island_ to the eastward, does not appear (Meare’s “Voyage,” page 95) to +have any reefs, nor does _Rosario Island_ (from Lutké’s chart), which +lies to the westward. Respecting the few other islands in this part of +the sea, namely the _Sulphur Islands_, with an active volcano, and +those lying between Bonin and Japan (which are situated near the +extreme limit in latitude, at which reefs are formed), I have not been +able to find any clear account. + +WEST END OF NEW GUINEA.—_Port Dory_. From the charts in the “Voyage of +the ‘Coquille’,” it would appear that the coast in this part is fringed +by coral-reefs; M. Lesson, however, remarks that the coral is sickly; +coloured red.—_Waigiou_. A considerable portion of the northern shores +of these islands is seen in the charts (on a large scale) in +Freycinet’s “Atlas” to be fringed by coral-reefs. Forrest (page 21, +“Voyage to New Guinea”) alludes to the coral-reefs lining the heads of +Piapis Bay; and Horsburgh (volume ii., page 599, 4th edition), speaking +of the islands in Dampier Strait, says “sharp coral-rocks line their +shores;” coloured red.—In the sea north of these islands, we have +_Guedes_ (or _Freewill_, or _St. David’s_), which from the chart given +in the 4to edition of Carteret’s “Voyage,” must be an atoll. +Krusenstern says the islets are very low; coloured blue.—_Carteret’s +Shoals_, in 2° 53′ N., are described as circular, with stony points +showing all round, with deeper water in the middle; coloured +blue.—_Aiou;_ the plan of this group, given in the “Atlas of the Voyage +of the ‘Astrolabe’,” shows that it is an atoll; and, from a chart in +Forrest’s “Voyage,” it appears that there is twelve fathoms within the +circular reef; coloured blue.—The S.W. coast of New Guinea appears to +be low, muddy, and devoid of reefs. The _Arru_, _Timor laut_, and +_Tenimber_ groups have lately been examined by Captain Kolff, the MS. +translation of which, by Mr. W. Earl, I have been permitted to read, +through the kindness of Captain Washington, R.N. These islands are +mostly rather low, and are surrounded by distant reefs (the Ki Islands, +however, are lofty, and, from Mr. Stanley’s survey, appear without +reefs); the sea in some parts is shallow, in others profoundly deep (as +near Larrat). From the imperfection of the published charts, I have +been unable to decide to which class these reefs belong. From the +distance to which they extend from the land, where the sea is very +deep, I am strongly inclined to believe they ought to come within the +barrier class, and be coloured blue; but I have been forced to leave +them uncoloured.—The last-mentioned groups are connected with the east +end of Ceram by a chain of small islands, of which the small groups of +_Ceram-laut_, _Goram_ and _Keffing_ are surrounded by very extensive +reefs, projecting into deep water, which, as in the last case, I +strongly suspect belong to the barrier class; but I have not coloured +them. From the south side of Keffing, the reefs project five miles +(Windsor Earl’s “Sailing Direct. for the Arafura Sea,” page 9). + +CERAM.—In various charts which I have examined, several parts of the +coast are represented as fringed by reefs.—_Manipa_ Island, between +Ceram and Bourou, in an old MS. chart in the Admiralty, is fringed by a +very irregular reef, partly dry at low water, which I do not doubt is +of coral-formation; both islands coloured red.—_Bourou;_ parts of this +island appear fringed by coral-reefs, namely, the eastern coast, as +seen in Freycinet’s chart; and _Cajeli Bay_, which is said by Horsburgh +(volume ii., page 630) to be lined by coral-reefs, that stretch out a +little way, and have only a few feet water on them. In several charts, +portions of the islands forming the AMBOINA GROUP are fringed by reefs; +for instance, _Noessa_, _Harenca_, and _Ucaster_, in Freycinet’s +charts. The above-mentioned islands have been coloured red, although +the evidence is not very satisfactory.—North of Bourou the parallel +line of the _Xulla Isles_ extends: I have not been able to find out +anything about them, excepting that Horsburgh (volume ii., page 543) +says that the northern shore is surrounded by a reef at the distance of +two or three miles; uncoloured.—_Mysol Group;_ the Kanary Islands are +said by Forrest (“Voyage,” page 130) to be divided from each other by +deep straits, and are lined with coral-rocks; coloured red.—_Guebe_, +lying between Waigiou and Gilolo, is engraved as if fringed; and it is +said by Freycinet, that all the soundings under five fathoms were on +coral; coloured red.—_Gilolo_. In a chart published by Dalrymple, the +numerous islands on the western, southern (_Batchian_ and the _Strait +of Patientia_), and eastern sides appear fringed by narrow reefs; these +reefs, I suppose, are of coral, for it is said in “Malte Brun” (volume +xii., page 156), “Sur les cotes (of Batchian) comme _dans le plupart_ +des iles de cet archipel, il y a de rocs de medrepores d’une beauté et +d’une variété infinies.” Forrest, also (page 50), says Seland, near +Batchian, is a little island with reefs of coral; coloured red.—_Morty_ +Island (north of Gilolo). Horsburgh (volume ii., page 506) says the +northern coast is lined by reefs, projecting one or two miles, and +having no soundings close to them; I have left it uncoloured, although, +as in some former cases, it ought probably to be pale blue.—_Celebes_. +The western and northern coasts appear in the charts to be bold and +without reefs. Near the extreme northern point, however, an islet in +the _Straits of Limbe_, and parts of the adjoining shore, appear to be +fringed: the east side of the bay of _Manado_, has deep water, and is +fringed by sand and coral (“‘Astrol.’ Voyage,” Hydrog. Part, pages +453-4); this extreme point, therefore, I have coloured red.—Of the +islands leading from this point to Magindanao, I have not been able to +find any account, except of _Serangani_, which appears surrounded by +narrow reefs; and Forrest (“Voyage,” page 164) speaks of coral on its +shores; I have, therefore, coloured this island red. To the eastward of +this chain lie several islands; of which I cannot find any account, +except of _Karkalang_, which is said by Horsburgh (volume ii., page +504) to be lined by a dangerous reef, projecting several miles from the +northern shore; not coloured. + +ISLANDS NEAR TIMOR.—The account of the following islands is taken from +Captain D. Kolff’s “Voyage,” in 1825, translated by Mr. W. Earl, from +the Dutch.—_Lette_ has “reefs extending along shore at the distance of +half a mile from the land.”—_Moa_ has reefs on the S.W. part.—_Lakor_ +has a reef lining its shore; these islands are coloured red.—Still more +eastward, _Luan_ has, differently from the last-mentioned islands, an +extensive reef; it is steep outside, and within there is a depth of +twelve feet; from these facts, it is impossible to decide to which +class this island belongs.—_Kissa_, off the point of Timor, has its +“shore fronted by a reef, steep too on the outer side, over which small +proahs can go at the time of high water;” coloured red.—_Timor;_ most +of the points, and some considerable spaces of the northern shore, are +seen in Freycinet’s chart to be fringed by coral-reefs; and mention is +made of them in the accompanying “Hydrog. Memoir;” coloured +red.—_Savu_, S.E. of Timor, appears in Flinders’ chart to be fringed; +but I have not coloured it, as I do not know that the reefs are of +coral.— _Sandalwood_ Island has, according to Horsburgh (volume ii., +page 607), a reef on its southern shore, four miles distant from the +land; as the neighbouring sea is deep, and generally bold, this +probably is a barrier- reef, but I have not ventured to colour it. + +N.W. COAST OF AUSTRALIA.—It appears, in Captain King’s Sailing +Directions (“Narrative of Survey,” volume ii, pages 325-369), that +there are many extensive coral-reefs skirting, often at considerable +distances, the N.W. shores, and encompassing the small adjoining +islets. Deep water, in no instance, is represented in the charts +between these reefs and the land; and, therefore, they probably belong +to the fringing class. But as they extend far into the sea, which is +generally shallow, even in places where the land seems to be somewhat +precipitous; I have not coloured them. Houtman’s Abrolhos (latitude 28° +S. on west coast) have lately been surveyed by Captain Wickham (as +described in “Naut. Mag.” 1841, page 511): they lie on the edge of a +steeply shelving bank, which extends about thirty miles seaward, along +the whole line of coast. The two southern reefs, or islands, enclose a +lagoon-like space of water, varying in depth from five to fifteen +fathoms, and in one spot with twenty-three fathoms. The greater part of +the island has been formed on their inland sides, by the accumulation +of fragments of coral; the seaward face consisting of nearly bare +ledges of rock. Some of the specimens, brought home by Captain Wickham, +contained fragments of marine shells, but others did not; and these +closely resembled a formation at King George’s Sound, principally due +to the action of the wind on calcareous dust, which I shall describe in +a forthcoming part. From the extreme irregularity of these reefs with +their lagoons, and from their position on a bank, the usual depth of +which is only thirty fathoms, I have not ventured to class them with +atolls, and hence have left them uncoloured.—_Rowley Shoals_. These lie +some way from the N.W. coast of Australia: according to Captain King +(“Narrative of Survey,” volume i., page 60), they are of +coral-formation. They rise abruptly from the sea, and Captain King had +no bottom with 170 fathoms close to them. Three of them are +crescent-shaped; they are mentioned by Mr. Lyell, on the authority of +Captain King, with reference to the direction of their open sides. “A +third oval reef of the same group is entirely submerged” (“Principles +of Geology,” book iii. chapter xviii.); coloured blue.—_Scott’s Reefs_, +lying north of Rowley Shoals, are briefly described by Captain Wickham +(“Naut. Mag.” 1841, page 440): they appear to be of great size, of a +circular form, and “with smooth water within, forming probably a lagoon +of great extent.” There is a break on the western side, where there +probably is an entrance: the water is very deep off these reefs; +coloured blue. + +Proceeding westward along the great volcanic chain of the East Indian +Archipelago, _Solor Strait_ is represented in a chart published by +Dalrymple from a Dutch MS., as fringed; as are parts of _Flores_, of +_Adenara_, and of _Solor_. Horsburgh speaks of coral growing on these +shores; and therefore I have no doubt that the reefs are of coral, and +accordingly have coloured them red. We hear from Horsburgh (volume ii., +page 602) that a coral-flat bounds the shores of _Sapy_ Bay. From the +same authority it appears (page 610) that reefs fringe the island of +_Timor-Young_, on the N. shore of Sumbawa; and, likewise (page 600), +that _Bally_ town in _Lombock_, is fronted by a reef, stretching along +the shore at a distance of a hundred fathoms, with channels through it +for boats; these places, therefore, have been coloured red.—_Bally_ +Island. In a Dutch MS. chart on a large scale of Java, which was +brought from that island by Dr. Horsfield, who had the kindness to show +it me at the India House, its western, northern, and southern shores +appear very regularly fringed by a reef (see also Horsburgh, volume +ii., page 593); and as coral is found abundantly there, I have not the +least doubt that the reef is of coral, and therefore have coloured it +red. + +JAVA.—My information regarding the reefs of this great island is +derived from the chart just mentioned. The greater part of _Madura_ is +represented in it as regularly fringed, and likewise portions of the +coast of Java immediately south of it. Dr. Horsfield informs me that +coral is very abundant near _Sourabaya_. The islets and parts of the N. +coast of Java, west of _Point Buang_, or _Japara_, are fringed by +reefs, said to be of coral. _Lubeck_, or _Bavian_ Islands, lying at +some distance from the shore of Java, are regularly fringed by +coral-reefs. _Carimon Java_ appears equally so, though it is not +directly said that the reefs are of coral; there is a depth between +thirty and forty fathoms round these islands. Parts of the shores of +_Sunda_ Strait, where the water is from forty to eighty fathoms deep, +and the islets near _Batavia_ appear in several charts to be fringed. +In the Dutch chart the southern shore, in the narrowest part of the +island, is in two places fringed by reefs of coral. West of +_Segorrowodee_ Bay, and the extreme S.E. and E. portions are likewise +fringed by coral-reefs; all the above-mentioned places coloured red. + +_Macassar Strait_; the east coast of _Borneo_ appears, in most parts, +free from reefs, and where they occur, as on the east coast of +_Pamaroong_, the sea is very shallow; hence no part is coloured. In +_Macassar_ Strait itself, in about latitude 2° S., there are many small +islands with coral-shoals projecting far from them. There are also (old +charts by Dalrymple) numerous little flats of coral, not rising to the +surface of the water, and shelving suddenly from five fathoms to no +bottom with fifty fathoms; they do not appear to have a lagoon-like +structure. There are similar coral-shoals a little farther south; and +in latitude 4° 55′ there are two, which are engraved from modern +surveys, in a manner which might represent an annular reef with deep +water inside: Captain Moresby, however, who was formerly in this sea, +doubts this fact, so that I have left them uncoloured: at the same time +I may remark, that these two shoals make a nearer approach to the +atoll-like structure than any other within the E. Indian Archipelago. +Southward of these shoals there are other low islands and irregular +coral-reefs; and in the space of sea, north of the great volcanic +chain, from Timor to Java, we have also other islands, such as the +_Postillions_, _Kalatoa_, _Tokan-Bessees_, &c., which are chiefly low, +and are surrounded by very irregular and distant reefs. From the +imperfect charts I have seen, I have not been able to decide whether +they belong to the atoll or barrier-classes, or whether they merely +fringe submarine banks, and gently sloping land. In the Bay of _Bonin_, +between the two southern arms of Celebes, there are numerous coral- +reefs; but none of them seem to have an atoll-like structure. I have, +therefore, not coloured any of the islands in this part of the sea; I +think it, however, exceedingly probable that some of them ought to be +blue. I may add that there is a harbour on the S.E. coast of _Bouton_ +which, according to an old chart, is formed by a reef, parallel to the +shore, with deep water within; and in the “Voyage of the ‘Coquille’,” +some neighbouring islands are represented with reefs a good way +distant, but I do not know whether with deep water within. I have not +thought the evidence sufficient to permit me to colour them. + +SUMATRA.—Commencing with the west coast and outlying islands, _Engano +Island_ is represented in the published chart as surrounded by a narrow +reef, and Napier, in his “Sailing Directions,” speaks of the reef being +of coral (also Horsburgh, volume ii., page 115); coloured red.—_Rat +Island_ (3° 51′ S.) is surrounded by reefs of coral, partly dry at low +water, (Horsburgh, volume ii., page 96).—_Trieste Island_ (4° 2′ S.). +The shore is represented in a chart which I saw at the India House, as +fringed in such a manner, that I feel sure the fringe consists of +coral; but as the island is so low, that the sea sometimes flows quite +over it (Dampier, “Voyage,” volume i., page 474), I have not coloured +it.—_Pulo Dooa_ (latitude 3°). In an old chart it is said there are +chasms in the reefs round the island, admitting boats to the +watering-place, and that the southern islet consists of a mass of sand +and coral.—_Pulo Pisang;_ Horsburgh (volume ii., page 86) says that the +rocky coral-bank, which stretches about forty yards from the shore, is +steep to all round: in a chart, also, which I have seen, the island is +represented as regularly fringed.—_Pulo Mintao_ is lined with reefs on +its west side (Horsburgh, volume ii., page 107).—_Pulo Baniak;_ the +same authority (volume ii., page 105), speaking of a part, says it is +faced with coral-rocks.—_Minguin_ (3° 36′ N.). A coral-reef fronts this +place, and projects into the sea nearly a quarter of a mile (“Notices +of the Indian Arch.” published at Singapore, page 105).—_Pulo Brassa_ +(5° 46′ N.). A reef surrounds it at a cable’s length (Horsburgh, volume +ii., page 60). I have coloured all the above-specified points red. I +may here add, that both Horsburgh and Mr. Moor (in the “Notices” just +alluded to) frequently speak of the numerous reefs and banks of coral +on the west coast of Sumatra; but these nowhere have the structure of a +barrier-reef, and Marsden (“History of Sumatra”) states, that where the +coast is flat, the fringing-reefs extend furthest from it. The northern +and southern points, and the greater part of the east coast, are low, +and faced with mud banks, and therefore without coral. + +NICOBAR ISLANDS.—The chart represents the islands of this group as +fringed by reefs. With regard to _Great Nicobar_, Captain Moresby +informs me, that it is fringed by reefs of coral, extending between two +and three hundred yards from the shore. The _Northern Nicobars_ appear +so regularly fringed in the published charts, that I have no doubt the +reefs are of coral. This group, therefore, is coloured red. + +ANDAMAN ISLANDS.—From an examination of the MS. chart, on a large +scale, of this island, by Captain Arch. Blair, in the Admiralty, +several portions of the coast appear fringed; and as Horsburgh speaks +of coral-reefs being numerous in the vicinity of these islands, I +should have coloured them red, had not some expressions in a paper in +the “Asiatic Researches” (volume iv., page 402) led me to doubt the +existence of reefs; uncoloured. + +The coast of _Malacca_, _Tenasserim_ and the coasts northward, appear +in the greater part to be low and muddy: where reefs occur, as in parts +of _Malacca Straits_, and near _Singapore_, they are of the fringing +kind; but the water is so shoal, that I have not coloured them. In the +sea, however, between Malacca and the west coast of Borneo, where there +is a greater depth from forty to fifty fathoms, I have coloured red +some of the groups, which are regularly fringed. The northern _Natunas_ +and the _Anambas_ Islands are represented in the charts on a large +scale, published in the “Atlas of the Voyage of the ‘Favourite’,” as +fringed by reefs of coral, with very shoal water within +them.—_Tumbelan_ and _Bunoa_ Islands (1° N.) are represented in the +English charts as surrounded by a very regular fringe.— _St. Barbes_ +(0° 15′ N.) is said by Horsburgh (volume ii., page 279) to be fronted +by a reef, over which boats can land only at high water.—The shore of +_Borneo_ at _Tunjong Apee_ is also fronted by a reef, extending not far +from the land (Horsburgh, volume ii., page 468). These places I have +coloured red; although with some hesitation, as the water is shallow. I +might perhaps have added _Pulo Leat_, in Gaspar Strait, _Lucepara_, and +_Carimata;_ but as the sea is confined and shallow, and the reefs not +very regular, I have left them uncoloured. + +The water shoals gradually towards the whole west coast of _Borneo:_ I +cannot make out that it has any reefs of coral. The islands, however, +off the northern extremity, and near the S.W. end of _Palawan_, are +fringed by very distant coral-reefs; thus the reefs in the case of +_Balabac_ are no less than five miles from the land; but the sea, in +the whole of this district, is so shallow, that the reefs might be +expected to extend very far from the land. I have not, therefore, +thought myself authorised to colour them. The N.E. point of Borneo, +where the water is very shoal, is connected with Magindanao by a chain +of islands called the _Sooloo Archipelago_, about which I have been +able to obtain very little information; _Pangootaran_, although ten +miles long, entirely consists of a bed of coral-rock (“Notices of E. +Indian Arch.” page 58): I believe from Horsburgh that the island is +low; not coloured.—_Tahow Bank_, in some old charts, appears like a +submerged atoll; not coloured. Forrest (“Voyage,” page 21) states that +one of the islands near Sooloo is surrounded by coral-rocks; but there +is no distant reef. Near the S. end of _Basselan_, some of the islets +in the chart accompanying Forrest’s “Voyage,” appear fringed with +reefs; hence I have coloured, though unwillingly, parts of the Sooloo +group red. The sea between Sooloo and Palawan, near the shoal coast of +Borneo, is interspersed with irregular reefs and shoal patches; not +coloured: but in the northern part of this sea, there are two low +islets, _Cagayanes_ and _Cavilli_, surrounded by extensive coral-reefs; +the breakers round the latter (Horsburgh, volume ii., page 513) extend +five or six miles from a sandbank, which forms the only dry part; these +breakers are steep to outside; there appears to be an opening through +them on one side, with four or five fathoms within: from this +description, I strongly suspect that Cavilli ought to be considered an +atoll; but, as I have not seen any chart of it, on even a moderately +large scale, I have not coloured it. The islets off the northern end of +_Palawan_, are in the same case as those off the southern end, namely +they are fringed by reefs, some way distant from the shore, but the +water is exceedingly shallow; uncoloured. The western shore of Palawan +will be treated of under the head of China Sea. + +PHILIPPINE ARCHIPELAGO.—A chart on a large scale of _Appoo shoal_, +which lies near the S.E. coast of Mindoro, has been executed by Captain +D. Ross: it appears atoll-formed, but with rather an irregular outline; +its diameter is about ten miles; there are two well-defined passages +leading into the interior lagoon, which appears open; close outside the +reef all round, there is no bottom with seventy fathoms; coloured +blue.—_Mindoro:_ the N.W. coast is represented in several charts, as +fringed by a reef, and _Luban Island_ is said, by Horsburgh (volume +ii., page 436), to be “lined by a reef.”—_Luzon:_ Mr. Cuming, who has +lately investigated with so much success the Natural History of the +Philippines, informs me, that about three miles of the shore north of +Point St. Jago, is fringed by a reef; as are (Horsburgh, volume ii., +page 437) the Three Friars off Silanguin Bay. Between Point Capones and +Playa Honda, the coast is “lined by a coral-reef, stretching out nearly +a mile in some places,” (Horsburgh); and Mr. Cuming visited some +fringing- reefs on parts of this coast, namely, near Puebla, Iba, and +Mansinglor. In the neighbourhood of Solon-solon Bay, the shore is lined +(Horsburgh ii., page 439) by coral-reefs, stretching out a great way: +there are also reefs about the islets off Solamague; and as I am +informed by Mr. Cuming, near St. Catalina, and a little north of it. +The same gentleman informs me there are reefs on the S.E. point of this +island in front of Samar, extending from Malalabon to Bulusan. These +appear to be the principal fringing-reefs on the coasts of Luzon; and +they have all been coloured red. Mr. Cuming informs me that none of +them have deep water within; although it appears from Horsburgh that +some few extend to a considerable distance from the shore. Within the +Philippine Archipelago, the shores of the islands do not appear to be +commonly fringed, with the exception of the S. shore of _Masbate_, and +nearly the whole of _Bohol;_ which are both coloured red. On the S. +shore of _Magindanao_, Bunwoot Island is surrounded (according to +Forrest, “Voyage,” page 253), by a coral-reef, which in the chart +appears one of the fringing class. With respect to the eastern coasts +of the whole Archipelago, I have not been able to obtain any account. + +BABUYAN ISLANDS.—Horsburgh says (volume ii., page 442), coral-reefs +line the shores of the harbour in Fuga; and the charts show there are +other reefs about these islands. _Camiguin_ has its shore in parts +lined by coral-rock (Horsburgh, page 443); about a mile off shore there +is between thirty and thirty-five fathoms. The plan of Port San Pio +Quinto shows that its shores are fringed with coral; coloured +red.—BABUYAN ISLANDS: Horsburgh, speaking of the southern part of the +group (volume ii., page 445) says the shores of both islands are +fortified by a reef, and through some of the gaps in it, the natives +can pass in their boats in fine weather; the bottom near the land is +coral-rock. From the published charts, it is evident that several of +these islands are most regularly fringed; coloured red. The northern +islands are left uncoloured, as I have been unable to find any account +of them.—FORMOSA. The shores, especially the western one, seem chiefly +composed of mud and sand, and I cannot make out that they are anywhere +lined by reefs; except in a harbour (Horsburgh, volume ii., page 449) +at the extreme northern point: hence, of course, the whole of this +island is left uncoloured. The small adjoining islands are in the same +case.— PATCHOW, or MADJIKO-SIMA GROUPS. _Patchuson_ has been described +by Captain Broughton (“Voy. to the N. Pacific,” page 191); he says, the +boats, with some difficulty, found a passage through the coral-reefs, +which extend along the coast, nearly half a mile off it. The boats were +well sheltered within the reef; but it does not appear that the water +is deep there. Outside the reef the depth is very irregular, varying +from five to fifty fathoms; the form of the land is not very abrupt; +coloured red.—_Taypin-san;_ from the description given (page 195) by +the same author, it appears that a very irregular reef extends, to the +distance of several miles, from the southern island; but whether it +encircles a space of deep water is not evident; nor, indeed, whether +these outlying reefs are connected with those more immediately +adjoining the land; left uncoloured. I may here just add that the shore +of _Kumi_ (lying west of Patchow), has a narrow reef attached to it in +the plan of it, in La Peyrouse’s “Atlas;” but it does not appear in the +account of the voyage that it is of coral; uncoloured.—LOO CHOO. The +greater part of the coast of this moderately hilly island, is skirted +by reefs, which do not extend far from the shore, and which do not +leave a channel of deep water within them, as may be seen in the charts +accompanying Captain B. Hall’s voyage to Loo Choo (see also remarks in +Appendix, pages xxi. and xxv.). There are, however, some ports with +deep water, formed by reefs in front of valleys, in the same manner as +happens at Mauritius. Captain Beechey, in a letter to me, compares +these reefs with those encircling the Society Islands; but there +appears to me a marked difference between them, in the less distance at +which the Loo Choo reefs lie from the land with relation to the +probable submarine inclination, and in the absence of an interior deep +water-moat or channel, parallel to the land. Hence, I have classed +these reefs with fringing-reefs, and coloured them red.—PESCADORES +(west of Formosa). Dampier (volume i., page 416), has compared the +appearance of the land to the southern parts of England. The islands +are interlaced with coral-reefs; but as the water is very shoal, and as +spits of sand and gravel (Horsburgh, volume ii., page 450) extend far +out from them, it is impossible to draw any inferences regarding the +nature of the reefs. + +CHINA SEA.—Proceeding from north to south, we first meet the _Pratas +Shoal_ (latitude 20° N.) which, according to Horsburgh (volume ii., +page 335), is composed of coral, is of a circular form, and has a low +islet on it. The reef is on a level with the water’s edge, and when the +sea runs high, there are breakers mostly all round, “but the water +within seems pretty deep in some places; although steep-to in most +parts outside, there appear to be several parts where a ship might find +anchorage outside the breakers;” coloured blue.—The _Paracells_ have +been accurately surveyed by Captain D. Ross, and charts on a large +scale published: but few low islets have been formed on these shoals, +and this seems to be a general circumstance in the China Sea; the sea +close outside the reefs is very deep; several of them have a +lagoon-like structure; or separate islets (_Prattle_, _Robert_, +_Drummond_, etc.) are so arranged round a moderately shallow space, as +to appear as if they had once formed one large atoll.— _Bombay Shoal_ +(one of the Paracells) has the form of an annular reef, and is +“apparently deep within;” it seems to have an entrance (Horsburgh, +volume ii., page 332) on its west side; it is very steep +outside.—_Discovery Shoal_, also is of an oval form, with a lagoon-like +space within, and three openings leading into it, in which there is a +depth from two to twenty fathoms. Outside, at the distance (Horsburgh, +volume ii., page 333) of only twenty yards from the reef, soundings +could not be obtained. The Paracells are coloured blue.—_Macclesfield +Bank:_ this is a coral-bank of great size, lying east of the Paracells; +some parts of the bank are level, with a sandy bottom, but, generally, +the depth is very irregular. It is intersected by deep cuts or +channels. I am not able to perceive in the published charts (its +limits, however, are not very accurately known) whether the central +part is deeper, which I suspect is the case, as in the Great Chagos +Bank, in the Indian Ocean; not coloured.—_Scarborough Shoal:_ this +coral-shoal is engraved with a double row of crosses, forming a circle, +as if there was deep water within the reef: close outside there was no +bottom, with a hundred fathoms; coloured blue.—The sea off the west +coast of Palawan and the northern part of Borneo is strewed with +shoals: _Swallow Shoal_, according to Horsburgh (volume ii., page 431) +“is formed, _like most_ of the shoals hereabouts, of a belt of +coral-rocks, “with a basin of deep water within.”—_Half-Moon Shoal_ has +a similar structure; Captain D. Ross describes it, as a narrow belt of +coral-rock, “with a basin of deep water in the centre,” and deep sea +close outside.—_Bombay Shoal_ appears (Horsburgh, volume ii., page 432) +“to be a basin of smooth water surrounded by breakers.” These three +shoals I have coloured blue.—The _Paraquas Shoals_ are of a circular +form, with deep gaps running through them; not coloured.—A bank +gradually shoaling to the depth of thirty fathoms, extends to a +distance of about twenty miles from the northern part of _Borneo_, and +to thirty miles from the northern part of _Palawan_. Near the land this +bank appears tolerably free from danger, but a little further out it is +thickly studded with coral-shoals, which do not generally rise quite to +the surface; some of them are very steep-to, and others have a fringe +of shoal-water round them. I should have thought that these shoals had +level surfaces, had it not been for the statement made by Horsburgh +“that most of the shoals hereabouts are formed of a belt of coral.” +But, perhaps that expression was more particularly applied to the +shoals further in the offing. If these reefs of coral have a +lagoon-like structure, they should have been coloured blue, and they +would have formed an imperfect barrier in front of Palawan and the +northern part of Borneo. But, as the water is not very deep, these +reefs may have grown up from inequalities on the bank: I have not +coloured them.—The coast of _China_, _Tonquin_, and _Cochin-China_, +forming the western boundary of the China Sea, appear to be without +reefs: with regard to the two last-mentioned coasts, I speak after +examining the charts on a large scale in the “Atlas of the Voyage of +the ‘Favourite’.” + +INDIAN OCEAN.—_South Keeling_ has been specially described: nine miles +north of it lies North Keeling, a very small atoll, surveyed by the +“Beagle,” the lagoon of which is dry at low water.—_Christmas Island_, +lying to the east, is a high island, without, as I have been informed +by a person who passed it, any reefs at all.—CEYLON: a space about +eighty miles in length of the south-western and southern shores of +these islands has been described by Mr. Twynam (“Naut. Mag.” 1836, +pages 365 and 518); parts of this space appear to be very regularly +fringed by coral-reefs, which extend from a quarter to half a mile from +the shore. These reefs are in places breached, and afford safe +anchorage for the small trading craft. Outside, the sea gradually +deepens; there is forty fathoms about six miles off shore: this part I +have coloured red. In the published charts of Ceylon there appear to be +fringing-reefs in several parts of the south-eastern shores, which I +have also coloured red.—At Venloos Bay the shore is likewise fringed. +North of Trincomalee there are also reefs of the same kind. The sea off +the northern part of Ceylon is exceedingly shallow; and therefore I +have not coloured the reefs which fringe portions of its shores, and +the adjoining islets, as well as the Indian promontory of _Madura_. + +CHAGOS, MALDIVA, and LACCADIVE ARCHIPELAGOES.—These three great groups +which have already been often noticed, are now well-known from the +admirable surveys of Captain Moresby and Lieutenant Powell. The +published charts, which are worthy of the most attentive examination, +at once show that the _Chagos_ and _Maldiva_ groups are entirely formed +of great atolls, or lagoon-formed reefs, surmounted by islets. In the +_Laccadive_ group, this structure is less evident; the islets are low, +not exceeding the usual height of coral-formations (see Lieutenant +Wood’s account, “Geographical Journal”, volume vi., page 29), and most +of the reefs are circular, as may be seen in the published charts; and +within several of them, as I am informed by Captain Moresby, there is +deepish water; these, therefore, have been coloured blue. Directly +north, and almost forming part of this group, there is a long, narrow, +slightly curved bank, rising out of the depths of the ocean, composed +of sand, shells, and decayed coral, with from twenty-three to thirty +fathoms on it. I have no doubt that it has had the same origin with the +other Laccadive banks; but as it does not deepen towards the centre I +have not coloured it. I might have referred to other authorities +regarding these three archipelagoes; but after the publication of the +charts by Captain Moresby, to whose personal kindness in giving me much +information I am exceedingly indebted, it would have been superfluous. + +_Sahia de Malha_bank consists of a series of narrow banks, with from +eight to sixteen fathoms on them; they are arranged in a semicircular +manner, round a space about forty fathoms deep, which slopes on the +S.E. quarter to unfathomable depths; they are steep-to on both sides, +but more especially on the ocean-side. Hence this bank closely +resembles in structure, and I may add from Captain Moresby’s +information in composition, the Pitt’s Bank in the Chagos group; and +the Pitt’s Bank, must, after what has been shown of the Great Chagos +Bank, be considered as a sunken, half-destroyed atoll; hence coloured +blue.—_Cargados Carajos Bank_. Its southern portion consists of a +large, curved, coral-shoal, with some low islets on its eastern edge, +and likewise some on the western side, between which there is a depth +of about twelve fathoms. Northward, a great bank extends. I cannot +(probably owing to the want of perfect charts) refer this reef and bank +to any class;—therefore not coloured.—_Ile de Sable_ is a little +island, lying west of C. Carajos, only some toises in height (“Voyage +of the ‘Favourite’,” volume i., page 130); it is surrounded by reefs; +but its structure is unintelligible to me. There are some small banks +north of it, of which I can find no clear account.—_Mauritius_. The +reefs round this island have been described in the chapter on +fringing-reefs; coloured red. —_Rodriguez_. The coral-reefs here are +exceedingly extensive; in one part they project even five miles from +the shore. As far as I can make out, there is no deep-water moat within +them; and the sea outside does not deepen very suddenly. The outline, +however, of the land appears to be (“Life of Sir J. Makintosh,” volume +ii., page 165) hilly and rugged. I am unable to decide whether these +reefs belong to the barrier class; as seems probable from their great +extension, or to the fringing class; uncoloured. —_Bourbon_. The +greater part of the shores of this island are without reefs; but +Captain Carmichael (Hooker’s “Bot. Misc.”) states that a portion, +fifteen miles in length, on the S.E. side, is imperfectly fringed with +coral reefs: I have not thought this sufficient to colour the island. + +SEYCHELLES.—The rocky islands of primary formation, composing this +group, rise from a very extensive and tolerably level bank, having a +depth between twenty and forty fathoms. In Captain Owen’s chart, and in +that in the “Atlas of the Voyage of the ‘Favourite’,” it appears that +the east side of _Mahé_ and the adjoining islands of _St. Anne_ and +_Cerf_, are regularly fringed by coral-reefs. A portion of the S.E. +part of _Curieuse Island_, the N., and part of the S.W. shore of +_Praslin Island_, and the whole west side of _Digue Island_, appear +fringed. From a MS. account of these islands by Captain F. Moresby, in +the Admiralty, it appears that _Silhouette_ is also fringed; he states +that all these islands are formed of granite and quartz, that they rise +abruptly from the sea, and that “coral-reefs have grown round them, and +project for some distance.” Dr. Allan, of Forres, who visited these +islands, informs me that there is no deep water between the reefs and +the shore. The above specified points have been coloured red. +_Amirantes Islands:_ The small islands of this neighbouring group, +according to the MS. account of them by Captain F. Moresby, are +situated on an extensive bank; they consist of the debris of corals and +shells; are only about twenty feet in height, and are environed by +reefs, some attached to the shore, and some rather distant from it.—I +have taken great pains to procure plans and information regarding the +several islands lying between S.E. and S.W. of the Amirantes, and the +Seychelles; relying chiefly on Captain F. Moresby and Dr. Allan, it +appears that the greater number, namely—_Platte_, _Alphonse_, +_Coetivi_, _Galega_, _Providence_, _St. Pierre_, _Astova_, +_Assomption_, and _Glorioso_, are low, formed of sand or coral-rock, +and irregularly shaped; they are situated on very extensive banks, and +are connected with great coral-reefs. Galega is said by Dr. Allan, to +be rather higher than the other islands; and St. Pierre is described by +Captain F. Moresby, as being cavernous throughout, and as not +consisting of either limestone or granite. These islands, as well as +the Amirantes, certainly are not atoll-formed, and they differ as a +group from every other group with which I am acquainted; I have not +coloured them; but probably the reefs belong to the fringing class. +Their formation is attributed, both by Dr. Allan and Captain F. +Moresby, to the action of the currents, here exceedingly violent, on +banks, which no doubt have had an independent geological origin. They +resemble in many respects some islands and banks in the West Indies, +which owe their origin to a similar agency, in conjunction with an +elevation of the entire area. In close vicinity to the several islands, +there are three others of an apparently different nature: first, _Juan +de Nova_, which appears from some plans and accounts to be an atoll; +but from others does not appear to be so; not coloured. Secondly +_Cosmoledo;_ “this group consists of a ring of coral, ten leagues in +circumference, and a quarter of a mile broad in some places, enclosing +a magnificent lagoon, into which there did not appear a single opening” +(Horsburgh, volume i., page 151); coloured blue. Thirdly, _Aldabra;_ it +consists of three islets, about twenty-five feet in height, with red +cliffs (Horsburgh, volume i., page 176) surrounding a very shallow +basin or lagoon. The sea is profoundly deep close to the shore. Viewing +this island in a chart, it would be thought an atoll; but the foregoing +description shows that there is something different in its nature; Dr. +Allan also states that it is cavernous, and that the coral-rock has a +vitrified appearance. Is it an upheaved atoll, or the crater of a +volcano?—uncoloured. + +COMORO GROUP.—_Mayotta_, according to Horsburgh (volume i., page 216, +4th edition), is completely surrounded by a reef, which runs at the +distance of three, four, and in some places even five miles from the +land; in an old chart, published by Dalrymple, a depth in many places +of thirty-six and thirty-eight fathoms is laid down within the reef. In +the same chart, the space of open water within the reef in some parts +is even more than three miles wide: the land is bold and peaked; this +island, therefore, is encircled by a well-characterised barrier-reef, +and is coloured pale blue.—_Johanna;_ Horsburgh says (volume I. page +217) this island from the N.W. to the S.W. point, is bounded by a reef, +at the distance of two miles from the shore; in some parts, however, +the reef must be attached, since Lieutenant Boteler (“Narr.” volume i., +page 161) describes a passage through it, within which there is room +only for a few boats. Its height, as I am informed by Dr. Allan, is +about 3,500 feet; it is very precipitous, and is composed of granite, +greenstone, and quartz; coloured blue.—_Mohilla;_ on the S. side of +this island there is anchorage, in from thirty to forty-five fathoms, +between a reef and the shore (Horsburgh, volume i., page 214); in +Captain Owen’s chart of Madagascar, this island is represented as +encircled; coloured blue.—_Great Comoro Island_ is, as I am informed by +Dr. Allan, about 8,000 feet high, and apparently volcanic; it is not +regularly encircled; but reefs of various shapes and dimensions, jut +out from every headland on the W., S., and S.E. coasts, inside of which +reefs there are channels, often parallel with the shore, with deep +water. On the north-western coasts the reefs appear attached to the +shores. The land near the coast is in some places bold, but generally +speaking it is flat; Horsburgh says (volume i., page 214) the water is +profoundly deep close to the _shore_, from which expression I presume +some parts are without reefs. From this description I apprehend the +reef belongs to the barrier class; but I have not coloured it, as most +of the charts which I have seen, represent the reefs round it as very +much less extensive than round the other islands in the group. + +MADAGASCAR.—My information is chiefly derived from the published charts +by Captain Owen, and the accounts given by him and by Lieutenant +Boteler. Commencing at the S.W. extremity of the island; towards the +northern part of the _Star Bank_ (in latitude 25° S.) the coast for ten +miles is fringed by a reef; coloured red. The shore immediately S. of +_St. Augustine’s Bay_ appears fringed; but _Tullear_ Harbour, directly +N. of it, is formed by a narrow reef ten miles long, extending parallel +to the shore, with from four to ten fathoms within it. If this reef had +been more extensive, it must have been classed as a barrier-reef; but +as the line of coast falls inwards here, a submarine bank perhaps +extends parallel to the shore, which has offered a foundation for the +growth of the coral; I have left this part uncoloured. From _lat_. 22° +16′ _to_ 21° 37′, the shore is fringed by coral-reefs (see Lieutenant +Boteler’s “Narrative,” volume ii., page 106), less than a mile in +width, and with shallow water within. There are outlying coral-shoals +in several parts of the offing, with about ten fathoms between them and +the shore, and the depth of the sea one mile and a half seaward, is +about thirty fathoms. The part above specified is engraved on a large +scale; and as in the charts on rather a smaller scale the same fringe +of reef extends as far as latitude 33° 15′; I have coloured the whole +of this part of the coast red. The islands of _Juan de Nova_ (in +latitude 17° S.) appear in the charts on a large scale to be fringed, +but I have not been able to ascertain whether the reefs are of coral; +uncoloured. The main part of the west coast appears to be low, with +outlying sandbanks, which, Lieutenant Boteler (volume ii., page 106) +says, “are faced on the edge of deep water by a line of sharp-pointed +coral-rocks.” Nevertheless I have not coloured this part, as I cannot +make out by the charts that the coast itself is fringed. The headlands +of _Narrenda_ and _Passandava Bays_ (14° 40′) and the islands in front +of _Radama harbour_ are represented in the plans as regularly fringed, +and have accordingly been coloured red. With respect to the _East coast +of Madagascar_, Dr. Allan informs me in a letter, that the whole line +of coast, from _Tamatave_, in 18° 12′, to _C. Amber_, at the extreme +northern point of the island, is bordered by coral-reefs. The land is +low, uneven, and gradually rising from the coast. From Captain Owen’s +charts, also, the existence of these reefs, which evidently belong to +the fringing class, on some parts, namely N. of _British Sound_, and +near _Ngoncy_, of the above line of coast might have been inferred. +Lieutenant Boteler (volume i., page 155) speaks of “the reef +surrounding the island of _St. Mary’s_ at a small distance from the +shore.” In a previous chapter I have described, from the information of +Dr. Allan, the manner in which the reefs extend in N.E. lines from the +headlands on this coast, thus sometimes forming rather deep channels +within them, this seems caused by the action of the currents, and the +reefs spring up from the submarine prolongations of the sandy +headlands. The above specified portion of the coast is coloured red. +The remaining S.E. portions do not appear in any published chart to +possess reefs of any kind; and the Rev. W. Ellis, whose means of +information regarding this side of Madagascar have been extensive, +informs me he believes there are none. + +EAST COAST OF AFRICA.—Proceeding from the northern part, the coast +appears, for a considerable space, without reefs. My information, I may +here observe, is derived from the survey by Captain Owen, together with +his narrative; and that by Lieutenant Boteler. At _Mukdeesha_ (10° 1′ +N.) there is a coral-reef extending four or five miles along the shore +(Owen’s “Narr.” volume i, page 357) which in the chart lies at the +distance of a quarter of a mile from the shore, and has within it from +six to ten feet water: this then is a fringing-reef, and is coloured +red. From _Juba_, a little S. of the equator, to _Lamoo_ (in 2° 20′ S.) +“the coast and islands are formed of madrepore” (Owen’s “Narrative,” +volume i., page 363). The chart of this part (entitled _Dundas +Islands_), presents an extraordinary appearance; the coast of the +mainland is quite straight and it is fronted at the average distance of +two miles, by exceedingly narrow, straight islets, fringed with reefs. +Within the chain of islets, there are extensive tidal flats and muddy +bays, into which many rivers enter; the depths of these spaces varies +from one to four fathoms—the latter depth not being common, and about +twelve feet the average. Outside the chain of islets, the sea, at the +distance of a mile, varies in depth from eight to fifteen fathoms. +Lieutenant Boteler (“Narr.” volume i., page 369) describes the muddy +bay of _Patta_, which seems to resemble other parts of this coast, as +fronted by small, narrow, level islets formed of decomposing coral, the +margin of which is seldom of greater height than twelve feet, +overhanging the rocky surface from which the islets rise. Knowing that +the islets are formed of coral, it is, I think, scarcely possible to +view the coast, and not at once conclude that we here see a +fringing-reef, which has been upraised a few feet: the unusual depth of +from two to four fathoms within some of these islets, is probably due +to muddy rivers having prevented the growth of coral near the shore. +There is, however, one difficulty on this view, namely, that before the +elevation took place, which converted the reef into a chain of islets, +the water must apparently have been still deeper; on the other hand it +may be supposed that the formation of a nearly perfect barrier in +front, of so large an extent of coast, would cause the currents +(especially in front of the rivers), to deepen their muddy beds. When +describing in the chapter on fringing-reefs, those of Mauritius, I have +given my reasons for believing that the shoal spaces within reefs of +this kind, must, in many instances, have been deepened. However this +may be, as several parts of this line of coast are undoubtedly fringed +by living reefs, I have coloured it red.— _Maleenda_ (3° 20′ S.). In +the plan of the harbour, the south headland appears fringed; and in +Owen’s chart on a larger scale, the reefs are seen to extend nearly +thirty miles southward; coloured red.—_Mombas_ (4° 5′ S.). The island +which forms the harbour, “is surrounded by cliffs of madrepore, capable +of being rendered almost impregnable” (Owen’s “Narr.” volume i., page +412). The shore of the mainland N. and S. of the harbour, is most +regularly fringed by a coral-reef at a distance from half a mile to one +mile and a quarter from the land; within the reef the depth is from +nine to fifteen feet; outside the reef the depth at rather less than +half a mile is thirty fathoms. From the charts it appears that a space +about thirty-six miles in length, is here fringed; coloured +red.—_Pemba_ (5° S.) is an island of coral-formation, level, and about +two hundred feet in height (Owen’s “Narr.” volume i., page 425); it is +thirty-five miles long, and is separated from the mainland by a deep +sea. The outer coast is represented in the chart as regularly fringed; +coloured red. The mainland in front of Pemba is likewise fringed; but +there also appear to be some outlying reefs with deep water between +them and the shore. I do not understand their structure, either from +the charts or the description, therefore have not coloured +them.—_Zanzibar_ resembles Pemba in most respects; its southern half on +the western side and the neighbouring islets are fringed; coloured red. +On the mainland, a little S. of Zanzibar, there are some banks parallel +to the coast, which I should have thought had been formed of coral, had +it not been said (Boteler’s “Narr.” volume ii., page 39) that they were +composed of sand; not coloured.—_Latham’s Bank_ is a small island, +fringed by coral-reefs; but being only ten feet high, it has not been +coloured.—_Monfeea_ is an island of the same character as Pemba; its +outer shore is fringed, and its southern extremity is connected with +Keelwa Point on the mainland by a chain of islands fringed by reefs; +coloured red. The four last-mentioned islands resemble in many respects +some of the islands in the Red Sea, which will presently be described.— +_Keelwa_. In a plan of the shore, a space of twenty miles N. and S. of +this place is fringed by reefs, apparently of coral: these reefs are +prolonged still further southward in Owen’s general chart. The coast in +the plans of the rivers _Lindy_ and _Monghow_ (9° 59′ and 10° 7′ S.) +has the same structure; coloured red.—_Querimba Islands_ (from 10° 40′ +to 13° S.). A chart on a large scale is given of these islands; they +are low, and of coral-formation (Boteler’s “Narr.” volume ii., page +54); and generally have extensive reefs projecting from them which are +dry at low water, and which on the outside rise abruptly from a deep +sea: on their insides they are separated from the continent by a +channel, or rather a succession of bays, with an average depth of ten +fathoms. The small headlands on the continent also have coral-banks +attached to them; and the Querimba islands and banks are placed on the +lines of prolongation of these headlands, and are separated from them +by very shallow channels. It is evident that whatever cause, whether +the drifting of sediment or subterranean movements, produced the +headlands, likewise produced, as might have been expected, submarine +prolongations to them; and these towards their outer extremities, have +since afforded a favourable basis for the growth of coral-reefs, and +subsequently for the formation of islets. As these reefs clearly belong +to the fringing class, the Querimba islands have been coloured +red.—_Monabila_ (13° 32′ S.). In the plan of this harbour, the +headlands outside are fringed by reefs apparently of coral; coloured +red.—_Mozambique_ (150° S.) The outer part of the island on which the +city is built, and the neighbouring islands, are fringed by +coral-reefs; coloured red. From the description given in Owen’s “Narr.” +(volume i., page 162), the shore from _Mozambique_ to _Delagoa Bay_ +appears to be low and sandy; many of the shoals and islets off this +line of coast are of coral-formation; but from their small size and +lowness, it is not possible, from the charts, to know whether they are +truly fringed. Hence this portion of coast is left uncoloured, as are +likewise those parts more northward, of which no mention has been made +in the foregoing pages from the want of information. + +PERSIAN GULF.—From the charts lately published on a large scale by the +East India Company, it appears that several parts, especially the +southern shores of this gulf, are fringed by coral-reefs; but as the +water is very shallow, and as there are numerous sandbanks, which are +difficult to distinguish on the chart from reefs, I have not coloured +the upper part red. Towards the mouth, however, where the water is +rather deeper, the islands of _Ormuz_ and _Larrack_, appear so +regularly fringed, that I have coloured them red. There are certainly +no atolls in the Persian Gulf. The shores of _Immaum_, and of the +promontory forming the southern headland of the Persian Gulf, seem to +be without reefs. The whole S.W. part (except one or two small patches) +of _Arabia Felix_, and the shores of _Socotra_ appear from the charts +and memoir of Captain Haines (“Geographical Journal,” 1839, page 125) +to be without any reefs. I believe there are no extensive coral-reefs +on any part of the coasts of _India_, except on the low promontory of +_Madura_ (as already mentioned) in front of Ceylon. + +RED SEA.—My information is chiefly derived from the admirable charts +published by the East India Company in 1836, from personal +communication with Captain Moresby, one of the surveyors, and from the +excellent memoir, “Über die Natur der Corallen-Bänken des Rothen +Meeres,” by Ehrenberg. The plains immediately bordering the Red Sea +seem chiefly to consist of a sedimentary formation of the newer +tertiary period. The shore is, with the exception of a few parts, +fringed by coral-reefs. The water is generally profoundly deep close to +the shore; but this fact, which has attracted the attention of most +voyagers, seems to have no necessary connection with the presence of +reefs; for Captain Moresby particularly observed to me, that, in +latitude 24° 10′ on the eastern side, there is a piece of coast, with +very deep water close to it, without any reefs, but not differing in +other respects from the usual nature of the coast-line. The most +remarkable feature in the Red Sea is the chain of submerged banks, +reefs, and islands, lying some way from the shore, chiefly on the +eastern side; the space within being deep enough to admit a safe +navigation in small vessels. The banks are generally of an oval form, +and some miles in width; but some of them are very long in proportion +to their width. Captain Moresby informs me that any one, who had not +made actual plans of them, would be apt to think that they were much +more elongated than they really are. Many of them rise to the surface, +but the greater number lie from five to thirty fathoms beneath it, with +irregular soundings on them. They consist of sand and living coral; +coral on most of them, according to Captain Moresby, covering the +greater part of their surface. They extend parallel to the shore, and +they are not unfrequently connected in their middle parts by short +transverse banks with the mainland. The sea is generally profoundly +deep quite close to them, as it is near most parts of the coast of the +mainland; but this is not universally the case, for between latitude +15° and 17° the water deepens quite gradually from the banks, both on +the eastern and western shores, towards the middle of the sea. Islands +in many parts arise from these banks; they are low, flat-topped, and +consist of the same horizontally stratified formation with that forming +the plain-like margin of the mainland. Some of the smaller and lower +islands consist of mere sand. Captain Moresby informs me, that small +masses of rock, the remnants of islands, are left on many banks where +there is now no dry land. Ehrenberg also asserts that most of the +islets, even the lowest, have a flat abraded basis, composed of the +same tertiary formation: he believes that as soon as the surf wears +down the protuberant parts of a bank, just beneath the level of the +sea, the surface becomes protected from further abrasion by the growth +of coral, and he thus accounts for the existence of so many banks +standing on a level with the surface of this sea. It appears that most +of the islands are certainly decreasing in size. + +The form of the banks and islands is most singular in the part just +referred to, namely, from latitude 15° to 17°, where the sea deepens +quite gradually: the _Dhalac_ group, on the western coast, is +surrounded by an intricate archipelago of islets and shoals; the main +island is very irregularly shaped, and it includes a bay seven miles +long, by four across, in which no bottom was found with 252 feet: there +is only one entrance into this bay, half a mile wide, and with an +island in front of it. The submerged banks on the eastern coast, within +the same latitudes, round _Farsan_ Island, are, likewise, penetrated by +many narrow creeks of deep water; one is twelve miles long, in the form +of a hatchet, in which, close to its broad upper end, soundings were +not struck with 360 feet, and its entrance is only half a mile wide: in +another creek of the same nature, but even with a more irregular +outline, there was no bottom with 480 feet. The island of Farsan, +itself, has as singular a form as any of its surrounding banks. The +bottom of the sea round the Dhalac and Farsan Islands consists chiefly +of sand and agglutinated fragments, but, in the deep and narrow creeks, +it consists of mud; the islands themselves consist of thin, +horizontally stratified, modern tertiary beds, containing but little +broken coral;[2] their shores are fringed by living coral-reefs. + + [2] Rüppell, “Reise in Abyssinie,” Band. i., S. 247. + + +From the account given by Rüppell[3] of the manner in which Dhalac has +been rent by fissures, the opposite sides of which have been unequally +elevated (in one instance to the amount of fifty feet), it seems +probable that its irregular form, as well as probably that of Farsan, +may have been partly caused by unequal elevations; but, considering the +general form of the banks, and of the deep-water creeks, together with +the composition of the land, I think their configuration is more +probably due in great part to strong currents having drifted sediment +over an uneven bottom: it is almost certain that their form cannot be +attributed to the growth of coral. Whatever may have been the precise +origin of the Dhalac and Farsan Archipelagoes, the greater number of +the banks on the eastern side of the Red Sea seem to have originated +through nearly similar means. I judge of this from their similarity in +configuration (in proof of which I may instance a bank on the east +coast in latitude 22°; and although it is true that the northern banks +generally have a less complicated outline), and from their similarity +in composition, as may be observed in their upraised portions. The +depth within the banks northward of latitude 17°, is usually greater, +and their outer sides shelve more abruptly (circumstances which seem to +go together) than in the Dhalac and Farsan Archipelagoes; but this +might easily have been caused by a difference in the action of the +currents during their formation: moreover, the greater quantity of +living coral, which, according to Captain Moresby, exists on the +northern banks, would tend to give them steeper margins. + + [3] Rüppell, “Reise in Abyssinie,” Band. i., S. 245. + + +From this account, brief and imperfect as it is, we can see that the +great chain of banks on the eastern coast, and on the western side in +the southern portion, differ greatly from true barrier-reefs wholly +formed by the growth of coral. It is indeed the direct conclusion of +Ehrenberg (“Uber die,” etc., pages 45 and 51), that they are connected +in their origin quite secondarily with the growth of coral; and he +remarks that the islands off the coast of Norway, if worn down level +with the sea, and merely coated with living coral, would present a +nearly similar appearance. I cannot, however, avoid suspecting, from +information given me by Dr. Malcolmson and Captain Moresby, that +Ehrenberg has rather under-rated the influence of corals, in some +places at least, on the formation of the tertiary deposits of the Red +Sea. + +_The West Coast of the Red Sea between lat._ 19° _and_ 22°.—There are, +in this space, reefs, which, if I had known nothing of those in other +parts of the Red Sea, I should unhesitatingly have considered as +barrier-reefs; and, after deliberation, I have come to the same +conclusion. One of these reefs, in 20° 15′, is twenty miles long, less +than a mile in width (but expanding at the northern end into a disc), +slightly sinuous, and extending parallel to the mainland at the +distance of five miles from it, with very deep water within; in one +spot soundings were not obtained with 205 fathoms. Some leagues further +south, there is another linear reef, very narrow, ten miles long, with +other small portions of reef, north and south, almost connected with +it; and within this line of reefs (as well as outside) the water is +profoundly deep. There are also some small linear and sickle-formed +reefs, lying a little way out at sea. All these reefs are covered, as I +am informed by Captain Moresby, by living corals. Here, then, we have +all the characters of reefs of the barrier class; and in some outlying +reefs we have an approach to the structure of atolls. The source of my +doubts about the classification of these reefs, arises from having +observed in the Dhalac and Farsan groups the narrowness and +straightness of several spits of sand and rock: one of these spits in +the Dhalac group is nearly fifteen miles long, only two broad, and it +is bordered on each side with deep water; so that, if worn down by the +surf, and coated with living corals, it would form a reef nearly +similar to those within the space under consideration. There is, also, +in this space (latitude 21°) a peninsula, bordered by cliffs, with its +extremity worn down to the level of the sea, and its basis fringed with +reefs: in the line of prolongation of this peninsula, there lies the +island of _Macowa_ (formed, according to Captain Moresby, of the usual +tertiary deposit), and some smaller islands, large parts of which +likewise appear to have been worn down, and are now coated with living +corals. If the removal of the strata in these several cases had been +more complete, the reefs thus formed would have nearly resembled those +barrier-like ones now under discussion. Notwithstanding these facts, I +cannot persuade myself that the many very small, isolated, and +sickle-formed reefs and others, long, nearly straight, and very narrow, +with the water unfathomably deep close round them, could possibly have +been formed by corals merely coating banks of sediment, or the abraded +surfaces of irregularly shaped islands. I feel compelled to believe +that the foundations of these reefs have subsided, and that the corals, +during their upward growth, have given to these reefs their present +forms: I may remark that the subsidence of narrow and +irregularly-shaped peninsulas and islands, such as those existing on +the coasts of the Red Sea, would afford the requisite foundations for +the reefs in question. + +_The West Coast from lat._ 22° _to_ 24°.—This part of the coast (north +of the space coloured blue on the map) is fronted by an irregularly +shelving bank, from about ten to thirty fathoms deep; numerous little +reefs, some of which have the most singular shapes, rise from this +bank. It may be observed, respecting one of them, in latitude 23° 10′, +that if the promontory in latitude 24° were worn down to the level of +the sea, and coated with corals, a very similar and grotesquely formed +reef would be produced. Many of the reefs on this part of the coast may +thus have originated; but there are some sickle, and almost +atoll-formed reefs lying in deep water off the promontory in latitude +24°, which lead me to suppose that all these reefs are more probably +allied to the barrier or atoll classes. I have not, however, ventured +to colour this portion of coast.—_On the west coast from lat._ 19° _to_ +17° (south of space coloured blue on the map), there are many low +islets of very small dimensions, not much elongated, and rising out of +great depths at a distance from the coast; these cannot be classed +either with atolls, or barrier- or fringing-reefs. I may here remark +that the outlying reefs on the west coast, between latitude 19° and +24°, are the only ones in the Red Sea, which approach in structure to +the true atolls of the Indian and Pacific Oceans, but they present only +imperfect miniature likenesses of them. + +_Eastern Coast_.—I have felt the greatest doubt about colouring any +portion of this coast, north of the fringing-reefs round the Farsan +Islands in 16° 10′. There are many small outlying coral-reefs along the +whole line of coast; but as the greater number rise from banks not very +deeply submerged (the formation of which has been shown to be only +secondarily connected with the growth of coral), their origin may be +due simply to the growth of knolls of corals, from an irregular +foundation situated within a limited depth. But between latitude 18° +and 20°, there are so many linear, elliptic, and extremely small reefs, +rising abruptly out of profound depths, that the same reasons, which +led me to colour blue a portion of the west coast, have induced me to +do the same in this part. There exist some small outlying reefs rising +from deep water, north of latitude 20° (the northern limit coloured +blue), on the east coast; but as they are not very numerous and +scarcely any of them linear, I have thought it right to leave them +uncoloured. + +In the _southern parts_ of the Red Sea, considerable spaces of the +mainland, and of some of the Dhalac islands, are skirted by reefs, +which, as I am informed by Captain Moresby, are of living coral, and +have all the characters of the fringing class. As in these latitudes, +there are no outlying linear or sickle-formed reefs, rising out of +unfathomable depths, I have coloured these parts of the coast red. On +similar grounds, I have coloured red the _northern parts of the western +coast_ (north of latitude 24° 30′), and likewise the shores of the +chief part of the _Gulf of Suez_. In the _Gulf of Acaba_, as I am +informed by Captain Moresby there are no coral-reefs, and the water is +profoundly deep. + +WEST INDIES.—My information regarding the reefs of this area, is +derived from various sources, and from an examination of numerous +charts; especially of those lately executed during the survey under +Captain Owen, R.N. I lay under particular obligation to Captain Bird +Allen, R.N., one of the members of the late survey, for many personal +communications on this subject. As in the case of the Red Sea, it is +necessary to make some preliminary remarks on the submerged banks of +the West Indies, which are in some degree connected with coral-reefs, +and cause considerable doubts in their classification. That large +accumulations of sediment are in progress on the West Indian shores, +will be evident to any one who examines the charts of that sea, +especially of the portion north of a line joining Yucutan and Florida. +The area of deposition seems less intimately connected with the +debouchement of the great rivers, than with the course of the +sea-currents; as is evident from the vast extension of the banks from +the promontories of Yucutan and Mosquito. + +Besides the coast-banks, there are many of various dimensions which +stand quite isolated; these closely resemble each other, they lie from +two or three to twenty or thirty fathoms under water, and are composed +of sand, sometimes firmly agglutinated, with little or no coral; their +surfaces are smooth and nearly level, shelving only to the amount of a +few fathoms, very gradually all round towards their edges, where they +plunge abruptly into the unfathomable sea. This steep inclination of +their sides, which is likewise characteristic of the coast-banks, is +very remarkable: I may give as an instance, the Misteriosa Bank, on the +edges of which the soundings change in 250 fathoms horizontal distance, +from 11 to 210 fathoms; off the northern point of the bank of Old +Providence, in 200 fathoms horizontal distance, the change is from 19 +to 152 fathoms; off the Great Bahama Bank, in 160 fathoms horizontal +distance, the inclination is in many places from 10 fathoms to no +bottom with 190 fathoms. On coasts in all parts of the world, where +sediment is accumulating, something of this kind may be observed; the +banks shelve very gently far out to sea, and then terminate abruptly. +The form and composition of the banks standing in the middle parts of +the W. Indian Sea, clearly show that their origin must be chiefly +attributed to the accumulation of sediment; and the only obvious +explanation of their isolated position is the presence of a nucleus, +round which the currents have collected fine drift matter. Any one who +will compare the character of the bank surrounding the hilly island of +Old Providence, with those banks in its neighbourhood which stand +isolated, will scarcely doubt that they surround submerged mountains. +We are led to the same conclusion by examining the bank called Thunder +Knoll, which is separated from the Great Mosquito Bank by a channel +only seven miles wide, and 145 fathoms deep. There cannot be any doubt +that the Mosquito Bank has been formed by the accumulation of sediment +round the promontory of the same name; and Thunder Knoll resembles the +Mosquito Bank, in the state of its surface submerged twenty fathoms, in +the inclinations of its sides, in composition, and in every other +respect. I may observe, although the remark is here irrelevant, that +geologists should be cautious in concluding that all the outlyers of +any formation have once been connected together, for we here see that +deposits, doubtless of exactly the same nature, may be deposited with +large valley-like spaces between them. + +Linear strips of coral-reefs and small knolls project from many of the +isolated, as well as coast-banks; sometimes they occur quite +irregularly placed, as on the Mosquito Bank, but more generally they +form crescents on the windward side, situated some little distance +within the outer edge of the banks:—thus on the Serranilla Bank they +form an interrupted chain which ranges between two and three miles +within the windward margin: generally they occur, as on Roncador, +Courtown, and Anegada Banks, nearer the line of deep water. Their +occurrence on the windward side is conformable to the general rule, of +the efficient kinds of corals flourishing best where most exposed; but +their position some way within the line of deep water I cannot explain, +without it be, that a depth somewhat less than that close to the outer +margin of the banks, is most favourable to their growth. Where the +corals have formed a nearly continuous rim, close to the windward edge +of a bank some fathoms submerged, the reef closely resembles an atoll; +but if the bank surrounds an island (as in the case of Old Providence), +the reef resembles an encircling barrier-reef. I should undoubtedly +have classed some of these fringed banks as imperfect atolls, or +barrier-reefs, if the sedimentary nature of their foundations had not +been evident from the presence of other neighbouring banks, of similar +forms and of similar composition, but without the crescent-like +marginal reef: in the third chapter, I observed that probably some +atoll-like reefs did exist, which had originated in the manner here +supposed. + +Proofs of elevation within recent tertiary periods abound, as referred +to in the sixth chapter, over nearly the whole area of the West Indies. +Hence it is easy to understand the origin of the low land on the +coasts, where sediment is now accumulating; for instance on the +northern part of Yucutan, and on the N.E. part of Mosquito, where the +land is low, and where extensive banks appear to be in progressive +formation. Hence, also, the origin of the Great Bahama Banks, which are +bordered on their western and southern edges by very narrow, long, +singularly shaped islands, formed of sand, shells, and coral-rock, and +some of them about a hundred feet in height, is easily explained by the +elevation of banks fringed on their windward (western and southern) +sides by coral-reefs. On this view, however, we must suppose either +that the chief part of the surfaces of the great Bahama sandbanks were +all originally deeply submerged, and were brought up to their present +level by the same elevatory action, which formed the linear islands; or +that during the elevation of the banks, the superficial currents and +swell of the waves continued wearing them down and keeping them at a +nearly uniform level: the level is not quite uniform; for, in +proceeding from the N.W. end of the Bahama group towards the S.E. end, +the depth of the banks increases, and the area of land decreases, in a +very gradual and remarkable manner. The latter view, namely, that these +banks have been worn down by the currents and swell during their +elevation, seems to me the most probable one. It is, also, I believe, +applicable to many banks, situated in widely distant parts of the West +Indian Sea, which are wholly submerged; for, on any other view, we must +suppose, that the elevatory forces have acted with astonishing +uniformity. + +The shore of the Gulf of Mexico, for the space of many hundred miles, +is formed by a chain of lagoons, from one to twenty miles in breadth +(“Columbian Navigator,” page 178, etc.), containing either fresh or +salt water, and separated from the sea by linear strips of sand. Great +spaces of the shores of Southern Brazil.[4] and of the United States +from Long Island (as observed by Professor Rogers) to Florida have the +same character. Professor Rogers, in his “Report to the British +Association” (volume iii., page 13), speculates on the origin of these +low, sandy, linear islets; he states that the layers of which they are +composed are too homogeneous, and contain too large a proportion of +shells, to permit the common supposition of their formation being +simply due to matter thrown up, where it now lies, by the surf: he +considers these islands as upheaved bars or shoals, which were +deposited in lines where opposed currents met. It is evident that these +islands and spits of sand parallel to the coast, and separated from it +by shallow lagoons, have no necessary connection with coral-formations. +But in Southern Florida, from the accounts I have received from persons +who have resided there, the upraised islands seem to be formed of +strata, containing a good deal of coral, and they are extensively +fringed by living reefs; the channels within these islands are in some +places between two and three miles wide, and five or six fathoms deep, +though generally[5] they are less in depth than width. After having +seen how frequently banks of sediment in the West Indian Sea are +fringed by reefs, we can readily conceive that bars of sediment might +be greatly aided in their formation along a line of coast, by the +growth of corals; and such bars would, in that case, have a deceptive +resemblance with true barrier-reefs. + + [4] In the “London and Edinburgh Philosophical Journal,” 1841, page + 257, I have described a singular bar of sandstone lying parallel to + the coast off Pernambuco in Brazil, which probably is an analogous + formation. + + + [5] In the ordinary sea-charts, no lagoons appear on the coast of + Florida, north of 26°; but Major Whiting (“Silliman’s Journal,” volume + xxxv., page 54) says that many are formed by sand thrown up along the + whole line of coast from St. Augustine’s to Jupiter Inlet. + + +Having now endeavoured to remove some sources of doubt in classifying +the reefs of the West Indies, I will give my authorities for colouring +such portions of the coast as I have thought myself warranted in doing. +Captain Bird Allen informs me, that most of the islands on the _Bahama +Banks_ are fringed, especially on their windward sides, with living +reefs; and hence I have coloured those, which are thus represented in +Captain Owen’s late chart, red. The same officer informs me, that the +islands along the southern part of _Florida_ are similarly fringed; +coloured red. CUBA: Proceeding along the northern coast, at the +distance of forty miles from the extreme S.E. point, the shores are +fringed by reefs, which extend westward for a space of 160 miles, with +only a few breaks. Parts of these reefs are represented in the plans of +the harbours on this coast by Captain Owen; and an excellent +description is given of them by Mr. Taylor (Loudon’s “Mag. of Nat. +Hist.” volume ix., page 449); he states that they enclosed a space +called the “_baxo_,” from half to three-quarters of a mile in width, +with a sandy bottom, and a little coral. In most parts people can wade, +at low water, to the reef; but in some parts the depth is between two +and three fathoms. Close outside the reef, the depth is between six and +seven fathoms; these well-characterised fringing-reefs are coloured +red. Westward of longitude 77° 30′, on the northern side of Cuba, a +great bank commences, which extends along the coast for nearly four +degrees of longitude. In the place of its commencement, in its +structure, and in the “_cays_,” or low islands on its edge, there is a +marked correspondence (as observed by Humboldt, “Pers. Narr.” volume +vii., page 88) between it and the Great Bahama and Sal Banks, which lie +directly in front. Hence one is led to attribute the same origin to +both these sets of banks; namely, the accumulation of sediment, +conjoined with an elevatory movement, and the growth of coral on their +outward edges; those parts which appear fringed by living reefs are +coloured red. Westward of these banks, there is a portion of coast +apparently without reefs, except in the harbours, the shores of which +seem in the published plans to be fringed. The _Colorado Shoals_ (see +Captain Owen’s charts), and the low land at the western end of Cuba, +correspond as closely in relative position and structure to the banks +at the extreme point of Florida, as the banks above described on the +north side of Cuba, do to the Bahamas, the depth within the islets and +reefs on the outer edge of the _Colorados_, is generally between two +and three fathoms, increasing to twelve fathoms in the southern part, +where the bank becomes nearly open, without islets or coral-reefs; the +portions which are fringed are coloured red. The southern shore of Cuba +is deeply concave, and the included space is filled up with mud and +sandbanks, low islands and coral-reefs. Between the mountainous _Isle +of Pines_ and the southern shore of Cuba, the general depth is only +between two and three fathoms; and in this part small islands, formed +of fragmentary rock and broken madrepores (Humboldt, “Pers. Narr.” +volume vii. pages 51, 86 to 90, 291, 309, 320), rise abruptly, and just +reach the surface of the sea. From some expressions used in the +“Columbian Navigator” (volume i., part ii., page 94), it appears that +considerable spaces along the outer coast of Southern Cuba are bounded +by cliffs of coral-rock, formed probably by the upheaval of coral-reefs +and sandbanks. The charts represent the southern part of the Isle of +Pines as fringed by reefs, which the “Columb. Navig.” says extend some +way from the coast, but have only from nine to twelve feet water on +them; these are coloured red.—I have not been able to procure any +detailed description of the large groups of banks and “cays” further +eastward on the southern side of Cuba; within them there is a large +expanse, with a muddy bottom, from eight to twelve fathoms deep; +although some parts of this line of coast are represented in the +general charts of the West Indies, as fringed, I have not thought it +prudent to colour them. The remaining portion of the south coast of +Cuba appears to be without coral-reefs. + +YUCUTAN.—The N.E. part of the promontory appears in Captain Owen’s +charts to be fringed; coloured red. The eastern coast, from 20° to 18° +is fringed. South of latitude 18°, there commences the most remarkable +reef in the West Indies: it is about one hundred and thirty miles in +length, ranging in a N. and S. line, at an average distance of fifteen +miles from the coast. The islets on it are all low, as I have been +informed by Captain B. Allen; the water deepens suddenly on the outside +of the reef, but not more abruptly than off many of the sedimentary +banks: within its southern extremity (off _Honduras_) the depth is +twenty-five fathoms; but in the more northern parts, the depth soon +increases to ten fathoms, and within the northernmost part, for a space +of twenty miles, the depth is only from one to two fathoms. In most of +these respects we have the characteristics of a barrier-reef; +nevertheless, from observing, first, that the channel within the reef +is a continuation of a great irregular bay, which penetrates the +mainland to the depth of fifty miles; and secondly, that considerable +spaces of this barrier-like reef are described in the charts (for +instance, in latitude 16° 45′ and 16° 12′) as formed of pure sand; and +thirdly, from knowing that sediment is accumulating in many parts of +the West Indies in banks parallel to the shore; I have not ventured to +colour this reef as a barrier, without further evidence that it has +really been formed by the growth of corals, and that it is not merely +in parts a spit of sand, and in other parts a worn down promontory, +partially coated and fringed by reefs; I lean, however, to the +probability of its being a barrier-reef, produced by subsidence. To add +to my doubts, immediately on the outside of this barrier-like reef, +_Turneffe_, _Lighthouse_, and _Glover_ reefs are situated, and these +reefs have so completely the form of atolls, that if they had occurred +in the Pacific, I should not have hesitated about colouring them blue. +_turneffe Reef_ seems almost entirely filled up with low mud islets; +and the depth within the other two reefs is only from one to three +fathoms. From this circumstance and from their similarity in form, +structure, and relative position, both to the bank called _Northern +Triangles_, on which there is an islet between seventy and eighty feet, +and to _Cozumel_ Island, the level surface of which is likewise between +seventy and eighty feet in height, I consider it more probable that the +three foregoing banks are the worn down bases of upheaved shoals, +fringed with corals, than that they are true atolls, wholly produced by +the growth of coral during subsidence; left uncoloured. + +In front of the eastern _Mosquito_ coast, there are between latitude +12° and 16° some extensive banks (already mentioned, page 148), with +high islands rising from their centres; and there are other banks +wholly submerged, both of which kinds of banks are bordered, near their +windward margins, by crescent-shaped coral-reefs. But it can hardly be +doubted, as was observed in the preliminary remarks, that these banks +owe their origin, like the great bank extending from the Mosquito +promontory, almost entirely to the accumulation of sediment, and not to +the growth of corals; hence I have not coloured them. + +_Cayman Island_: this island appears in the charts to be fringed; and +Captain B. Allen informs me that the reefs extend about a mile from the +shore, and have only from five to twelve feet water within them; +coloured red.—_Jamaica_: judging from the charts, about fifteen miles +of the S.E. extremity, and about twice that length on the S.W. +extremity, and some portions on the S. side near Kingston and Port +Royal, are regularly fringed, and therefore are coloured red. From the +plans of some harbours on the N. side of Jamaica, parts of the coast +appear to be fringed; but as these are not represented in the charts of +the whole island, I have not coloured them.—_St. Domingo_: I have not +been able to obtain sufficient information, either from plans of the +harbours, or from general charts, to enable me to colour any part of +the coast, except sixty miles from Port de Plata westward, which seems +very regularly fringed; many other parts, however, of the coast are +probably fringed, especially towards the eastern end of the +island.—_Puerto Rico_: considerable portions of the southern, western, +and eastern coasts, and some parts of the northern coast, appear in the +charts to be fringed; coloured red.—Some miles in length of the +southern side of the Island of _St, Thomas_ is fringed; most of the +_Virgin Gorda_ Islands, as I am informed by Mr. Schomburgk, are +fringed; the shores of _Anegada_, as well as the bank on which it +stands, are likewise fringed; these islands have been coloured red. The +greater part of the southern side of _Santa Cruz_ appears in the Danish +survey to be fringed (see also Prof. Hovey’s account of this island, in +“Silliman’s Journal,” volume xxxv., page 74); the reefs extend along +the shore for a considerable space, and project rather more than a +mile; the depth within the reef is three fathoms; coloured red.—The +_Antilles_, as remarked by Von Buch (“Descrip. Iles Canaries,” page +494), may be divided into two linear groups, the western row being +volcanic, and the eastern of modern calcareous origin; my information +is very defective on the whole group. Of the eastern islands, _Barbuda_ +and the western coasts of _Antigua_ and _Mariagalante_ appear to be +fringed: this is also the case with _Barbadoes_, as I have been +informed by a resident; these islands are coloured red. On the shores +of the Western Antilles, of volcanic origin, very few coral-reefs +appear to exist. The island of _Martinique_, of which there are +beautifully executed French charts, on a very large scale, alone +presents any appearance worthy of special notice. The south-western, +southern, and eastern coasts, together forming about half the +circumference of the island, are skirted by very irregular banks, +projecting generally rather less than a mile from the shore, and lying +from two to five fathoms submerged. In front of almost every valley, +they are breached by narrow, crooked, steep-sided passages. The French +engineers ascertained by boring, that these submerged banks consisted +of madreporitic rocks, which were covered in many parts by thin layers +of mud or sand. From this fact, and especially from the structure of +the narrow breaches, I think there can be little doubt that these banks +once formed living reefs, which fringed the shores of the island, and +like other reefs probably reached the surface. From some of these +submerged banks reefs of living coral rise abruptly, either in small +detached patches, or in lines parallel to, but some way within the +outer edges of the banks on which they are based. Besides the above +banks which skirt the shores of the island, there is on the eastern +side a range of linear banks, similarly constituted, twenty miles in +length, extending parallel to the coast line, and separated from it by +a space between two and four miles in width, and from five to fifteen +fathoms in depth. From this range of detached banks, some linear reefs +of living coral likewise rise abruptly; and if they had been of greater +length (for they do not front more than a sixth part of the +circumference of the island), they would necessarily from their +position have been coloured as barrier-reefs; as the case stands they +are left uncoloured. I suspect that after a small amount of subsidence, +the corals were killed by sand and mud being deposited on them, and the +reefs being thus prevented from growing upwards, the banks of +madreporitic rock were left in their present submerged condition. + +THE BERMUDA ISLANDS have been carefully described by Lieutenant Nelson, +in an excellent Memoir in the “Geological Transactions” (volume v., +part i., page 103). In the form of the bank or reef, on one side of +which the islands stand, there is a close general resemblance to an +atoll; but in the following respects there is a considerable +difference,—first, in the margin of the reef not forming (as I have +been informed by Mr. Chaffers, R.N.) a flat, solid surface, laid bare +at low water, and regularly bounding the internal space of shallow +water or lagoon; secondly, in the border of gradually shoaling water, +nearly a mile and a half in width, which surrounds the entire outside +of the reef (as is laid down in Captain Hurd’s chart); and thirdly, in +the size, height, and extraordinary form of the islands, which present +little resemblance to the long, narrow, simple islets, seldom exceeding +half a mile in breadth, which surmount the annular reefs of almost all +the atolls in the Indian and Pacific Oceans. Moreover, there are +evident proofs (Nelson, Ibid., page 118), that islands similar to the +existing ones, formerly extended over other parts of the reef. It +would, I believe, be difficult to find a true atoll with land exceeding +thirty feet in height; whereas, Mr. Nelson estimates the highest point +of the Bermuda Islands to be 260 feet; if, however, Mr. Nelson’s view, +that the whole of the land consists of sand drifted by the winds, and +agglutinated together, were proved correct, this difference would be +immaterial; but, from his own account (page 118), there occur in one +place, five or six layers of red earth, interstratified with the +ordinary calcareous rock, and including stones too heavy for the wind +to have moved, without having at the same time utterly dispersed every +grain of the accompanying drifted matter. Mr. Nelson attributes the +origin of these several layers, with their embedded stones, to as many +violent catastrophes; but further investigation in such cases has +generally succeeded in explaining phenomena of this kind by ordinary +and simpler means. Finally, I may remark, that these islands have a +considerable resemblance in shape to Barbuda in the West Indies, and to +Pemba on the eastern coast of Africa, which latter island is about two +hundred feet in height, and consists of coral-rock. I believe that the +Bermuda Islands, from being fringed by living reefs, ought to have been +coloured red; but I have left them uncoloured, on account of their +general resemblance in external form to a lagoon-island or atoll. + + + + +INDEX. + + +The names not in capitals are all names of places, and refer +exclusively to the Appendix: in well-defined archipelagoes, or groups +of islands, the name of each separate island is not given. + +ABROLHOS, Brazil, coated by corals. + +Abrolhos (Australia). + +ABSENCE of coral-reefs from certain coasts. + +Acaba, gulf of. + +Admiralty group. + +AFRICA, east coast, fringing-reef of. +Madreporitic rock of. + +Africa, east coast. + +AGE of individual corals. + +Aiou. + +Aitutaki. + +Aldabra. + +Alert reef. + +Alexander, Grand Duke, island. + +ALLAN, Dr., on Holuthuriæ feeding on corals. On quick growth of corals +at Madagascar. On reefs affected by currents. + +Alloufatou. + +Alphonse. + +Amargoura. (Amargura.) + +Amboina. + +America, west coast. + +Amirantes. + +Anachorites. + +Anambas. + +ANAMOUKA, description of. + +Anamouka. + +Anadaman islands. + +Antilles. + +Appoo reef. + +Arabia Felix. + +AREAS, great extent of, interspersed with low islands. Of subsidence +and of elevation. Of subsidence appear to be elongated. Of subsidence +alternating with areas of elevation. + +Arru group. + +Arzobispo. + +ASCIDIA, depth at which found. + +Assomption. + +Astova. + +Atlantic islands. + +ATOLLS, breaches in their reefs. Dimensions of. Dimensions of groups +of. Not based on craters or on banks of sediment, or of rock. Of +irregular forms. Steepness of their flanks. Width of their reef and +islets. Their lowness. Lagoons. General range. With part of their reef +submerged, and theory of. + +Augustine, St. + +AURORA island, an upraised atoll. + +Aurora. + +AUSTRAL islands, recently elevated. + +Austral islands. + +Australia, N.W. coast. + +AUSTRALIAN barrier-reef. + +Australian barrier. + +Babuyan group. + +Bahama banks. + +Balahac. + +Bally. + +Baring. + +BARRIER-REEF of Australia. Of New Caledonia. + +BARRIER-REEFS, breaches through. Not based on worn down margin of rock. +On banks of sediment. On submarine craters. Steepness of their flanks. +Their probable vertical thickness. Theory of their formation. + +Bampton shoal. + +Banks islands. + +Banks in the West Indies. + +Bashee islands. + +Bass island. + +Batoa. + +Beaupre reef. + +BEECHEY, Captain, obligations of the author to. On submerged reefs. +Account of Matilda island. + +BELCHER, Captain, on boring through coral-reef. + +Belize reef, off. + +Bellinghausen. + +Bermuda islands. + +Beveridge reef. + +Bligh. + +BOLABOLA, view of. + +Bombay shoal. + +Bonin Bay. + +Bonin group. + +BORINGS through coral-reefs. + +BORNEO, W. coast, recently elevated. + +Borneo, E. coast. S.W. and W. coast N. coast. Western bank. + +Boscawen. + +Boston. + +Bouka. + +Bourbon. + +Bourou. + +Bouton. + +BRAZIL, fringing-reefs on coast of. + +BREACHES through barrier-reefs. + +Brook. + +Bunker. + +Bunoa. + +BYRON. + +Cagayanes. + +Candelaria. + +Cargados Carajos. + +Caroline archipelago. + +Caroline island. + +Carteret shoal. + +CARYOPHYLLIA, depth at which it lives. + +Cavilli. + +Cayman island. + +Celebes. + +Ceram. + +CEYLON, recently elevated. + +Ceylon. + +CHAGOS Great Bank, description and theory of. + +CHAGOS group. + +Chagos group. + +CHAMA-SHELLS embedded in coral-rock. + +CHAMISSO, on corals preferring the surf. + +CHANGES in the state of Keeling atoll. Of atolls. + +CHANNELS leading into the lagoons of atolls. Into the Maldiva atolls. +Through barrier-reefs. + +Chase. + +China sea. + +CHRISTMAS atoll. + +Christmas atoll. + +Christmas island (Indian Ocean). + +Clarence. + +Clipperton rock. + +COCOS, or Keeling atoll. + +Cocos (or Keeling). + +Cocos island (Pacific). + +COCHIN China, encroachments of the sea on the coast. + +Cochin China. + +Coetivi. + +Comoro group. + +COMPOSITION of coral-formations. + +CONGLOMERATE coral-rock on Keeling atoll. On other atolls. Coral-rock. + +COOK islands, recently elevated. + +Cook islands. + +CORAL-BLOCKS bored by vermiform animals. + +CORAL-REEFS, their distribution and absence from certain areas. +Destroyed by loose sediment. + +CORAL-ROCK at Keeling atoll. Mauritius. Organic remains of. + +CORALS dead but upright in Keeling lagoon. Depths at which they live. +Off Keeling atoll. Killed by a short exposure. Living in the lagoon of +Keeling atoll. Quick growth of, in Keeling lagoon. Merely coating the +bottom of the sea. Standing exposed in the Low archipelago. + +CORALLIAN sea. + +Corallian sea. + +Cornwallis. + +Cosmoledo. + +COUTHOUY, Mr., alleged proofs of recent elevation of the Low +archipelago. On coral-rock at Mangaia and Aurora islands. On external +ledges round coral-islands. Remarks confirmatory of the author’s +theory. + +CRESCENT-FORMED reefs. + +Cuba. + +CUMING, Mr., on the recent elevation of the Philippines. + +Dangerous, or Low archipelago. + +Danger islands. + +DEPTHS at which reef-building corals live. At Mauritius, the Red Sea, +and in the Maldiva archipelago. At which other corals and corallines +can live. + +Dhalac group. + +DIEGO GARCIA, slow growth of reef. + +DIMENSIONS of the larger groups of atolls. + +DISSEVERMENT of the Maldiva atolls, and theory of. + +DISTRIBUTION of coral-reefs. + +Domingo, St. + +DORY, Port, recently elevated. + +Dory, Port. + +Duff islands. + +Durour. + +Eap. + +EARTHQUAKES at Keeling atoll. In groups of atolls. In Navigator +archipelago. + +EAST INDIAN ARCHIPELAGO, recently elevated. + +Easter. + +Echequier. + +EHRENBERG, on the banks of the Red Sea. On depths at which corals live +in the Red Sea. On corals preferring the surf. On the antiquity of +certain corals. + +Eimeo. + +ELEVATED reef of Mauritius. + +ELEVATIONS, recent proofs of. Immense areas of. + +Elivi. + +ELIZABETH island. Recently elevated. + +Elizabeth island. + +Ellice group. + +ENCIRCLED ISLANDS, their height. Geological composition. + +EOUA, description of. + +Eoua. + +ERUPTED MATTER probably not associated with thick masses of coral-rock. + +FAIS, recently elevated. + +Fais. + +Fanning. + +Farallon de Medinilla. + +Farson group. + +Fataka. + +FIJI archipelago. + +FISH, feeding on corals. Killed in Keeling lagoon by heavy rain. + +FISSURES across coral-islands. + +FITZROY, Captain, on a submerged shed at Keeling atoll. On an +inundation in the Low archipelago. + +Flint. + +Flores. + +Florida. + +Folger. + +Formosa. + +FORSTER, theory of coral-formations. + +Frederick reef. + +Freewill. + +FRIENDLY group recently elevated. + +Friendly archipelago. + +FRINGING-REEFS, absent where coast precipitous. Breached in front of +streams. Described by MM. Quoy and Gaimard. Not closely attached to +shelving coasts. Of east coast of Africa. Of Cuba. Of Mauritius. On +worn down banks of rock. On banks of sediment. Their appearance when +elevated. Their growth influenced by currents. By shallowness of sea. + +Galapagos archipelago. + +Galega. + +GAMBIER islands, section of. + +Gambier islands. + +Gardner. + +Gaspar rico. + +GEOLOGICAL COMPOSITION of coral-formations. + +Gilbert archipelago. + +Gilolo. + +Glorioso. + +GLOUCESTER Island. + +Glover reef. + +Gomez. + +Gouap. + +Goulou. + +Grampus. + +Gran Cocal. + +GREAT CHAGOS BANK, description and theory of. + +GREY, Captain, on sandbars. + +GROUPING of the different classes of reefs. + +Guedes. + +HALL, Captain B., on Loo Choo. + +HARVEY islands, recently elevated. + +HEIGHT of encircled islands. + +Hermites. + +Hervey or Cook islands. + +Hogoleu. + +HOLOTHURIAE (Holuthuriæ) feeding on coral. + +HOUDEN island, height of. + +Honduras, reef off. + +Horn. + +Houtman Abrolhos. + +HUAHEINE; alleged proofs of its recent elevation. + +Huaheine. + +Humphrey. + +Hunter. + +HURRICANES, effects of, on coral-islands. + +Immaum. + +Independence. + +INDIA, west coast, recently elevated. + +India. + +IRREGULAR REEFS in shallow seas. + +ISLETS of coral-rock, their formation. Their destruction in the Maldiva +atolls. + +Jamaica. + +Jarvis. + +JAVA, recently elevated. + +Java. + +Johnston island. + +Juan de Nova. + +Juan de Nova (Madagascar). + +Kalatoa. + +KAMTSCHATKA, proofs of its recent elevation. + +Karkalang. + +KEELING atoll, section of reef. + +Keeling, south atoll. North atoll. + +Keffing. + +Kemin. + +Kennedy. + +Keppel. + +Kumi. + +Laccadive group. + +LADRONES, or Marianas, recently elevated. + +Ladrones archipelago. + +LAGOON of Keeling atoll. + +LAGOONS bordered by inclined ledges and walls, and theory of their +formation. Of small atolls filled up with sediment. + +LAGOON-CHANNELS within barrier-reefs. + +LAGOON-REEFS, all submerged in some atolls, and rising to the surface +in others. + +Lancaster reef. + +Latte. + +Lauglan islands. + +LEDGES round certain lagoons. + +Lette. + +Lighthouse reef. + +LLOYD, Mr., on corals refixing themselves. + +LOO CHOO, recently elevated. + +Loo Choo. + +Louisiade. + +LOW ARCHIPELAGO, alleged proofs of its recent elevation. + +Low archipelago. + +LOWNESS of coral-islands. + +Loyalty group. + +Lucepara. + +LUTKE, Admiral, on fissures across coral-islands. + +LUZON, recently elevated. + +Luzon. + +LYELL, Mr., on channels into the lagoons of atolls. On the lowness of +their leeward sides. On the antiquity of certain corals. On the +apparent continuity of distinct coral-islands. On the recently elevated +beds of the Red Sea. On the outline of the areas of subsidence. + +Macassar strait. + +Macclesfield bank. + +MADAGASCAR, quick growth of corals at. Madreporitic rock of. + +Madagascar. + +Madjiko-sima. + +Madura (Java). + +Madura (India). + +MAHLOS MAHDOO, theory of formation. + +MALACCA, recently elevated. + +Malacca. + +MALCOLMSON, Dr., on recent elevation of W. coast of India. On recent +elevation of Camaran island. + +Malden. + +MALDIVA atolls, and theory of their formation. Steepness of their +flanks. Growth of coral at. + +Maldiva archipelago. + +MANGAIA island. Recently elevated. + +Mangaia. + +Mangs. + +MARIANAS, recently elevated. + +Mariana archipelago. + +Mariere. + +Marquesas archipelago. + +Marshall archipelago. + +Marshall island. + +Martinique. + +Martires. + +MARY’S ST., in Madagascar, harbour made in reefs. + +Mary island. + +Matia, or Aurora. + +MATILDA atoll. + +MAURITIUS, fringing-reefs of. Depths at which corals live there. +Recently elevated. + +Mauritius. + +MAURUA, section of. + +Maurua. + +MENCHIKOFF atoll. + +Mendana archipelago. + +Mendana isles. + +Mexico, gulf of. + +MILLEPORA COMPLANATA at Keeling atoll. + +Mindoro. + +Mohilla. (Mohila.) + +MOLUCCA islands, recently elevated. + +Mopeha. + +MORESBY, Captain, on boring through coral-reefs. + +Morty. + +Mosquito coast. + +MUSQUILLO atoll. + +Mysol. + +NAMOURREK group. + +Natunas. + +NAVIGATOR archipelago, elevation of. + +Navigator archipelago. + +Nederlandisch. + +NELSON, Lieutenant, on the consolidation of coral-rocks under water. +Theory of coral-formations. On the Bermuda islands. + +New Britain. + +NEW CALEDONIA, steepness of its reefs. Barrier-reef of. + +New Caledonia. + +New Guinea (E. end). + +New Guinea (W. end). + +New Hanover. + +NEW HEBRIDES, recently elevated. + +New Hebrides. + +NEW IRELAND, recently elevated. + +New Ireland. + +New Nantucket. + +Nicobar islands. + +Niouha. + +NULLIPORAE at Keeling atoll. On the reefs of atolls. On barrier-reefs. +Their wide distribution and abundance. + +OBJECTIONS to the theory of subsidence. + +Ocean islands. + +Ono. + +Onouafu. (Onouafou.) + +Ormuz. + +Oscar group. + +OSCILLATIONS of level. + +Ouallan, or Ualan. (Oualan.) + +OULUTHY atoll. + +Outong Java. + +Palawan, S.W. coast. N.W. coast. Western bank. + +Palmerston. + +Palmyra. + +Paracells. + +Paraquas. + +Patchow. + +Pelew islands. + +PEMBA island, singular form of. + +Pemba. + +Penrhyn. + +Peregrino. + +PERNAMBUCO, bar of sandstone at. + +PERSIAN gulf, recently elevated. + +Persian gulf. + +PESCADO. + +Pescadores. + +Peyster group. + +Philip. + +PHILIPPINE archipelago, recently elevated. + +Philippine archipelago. + +Phœnix. + +Piguiram. + +Pitcairn. + +PITT’S bank. + +Pitt island. + +Platte. + +Pleasant. + +PORITES, chief coral on margin of Keeling atoll. + +Postillions. + +POUYNIPETE. Its probable subsidence. + +Pouynipète. + +Pratas shoal. + +Proby. + +Providence. + +Puerto Rico. + +Pulo Anna. + +PUMICE floated to coral-islands. + +Pylstaart. + +PYRARD DE LAVAL, astonishment at the atolls in the Indian Ocean. + +QUOY AND GAIMARD, depths at which corals live. Description of reefs +applicable only to fringing-reefs. + +RANGE of atolls. + +Rapa. + +Rearson. + +RED SEA, banks of rock coated by reefs. Proofs of its recent elevation. +Supposed subsidence of. + +Red Sea. + +REEFS, irregular in shallow seas. Rising to the surface in some lagoons +and all submerged in others. Their distribution. Their absence from +some coasts. + +Revilla-gigedo. + +RING-FORMED REEFS of the Maldiva atolls, and theory of. + +Rodriguez. + +Rosario. + +Rose island. + +Rotches. + +Rotoumah. + +Roug. + +Rowley shoals. + +RUPPELL, Dr., on the recent deposits of Red Sea. + +Sable, ile de. + +Sahia de Malha. + +St. Pierre. + +Sala. + +Salomon archipelago. (Solomon.) + +SAMOA, or Navigator archipelago, elevation of. + +Samoa archipelago. + +SAND-BARS parallel to coasts. + +Sandal-wood. + +SANDWICH archipelago, recently elevated. + +Sandwich archipelago. + +Sanserot. + +Santa-Cruz group. + +SAVAGE island, recently elevated. + +Savage. + +Savu. + +Saya, or Sahia de Malha. + +Scarborough shoal. + +SCARUS feeding on corals. + +Schouten. + +Scilly. + +SCORIAE floated to coral-islands. + +Scott’s reef. + +SECTIONS of islands encircled by barrier-reefs. Of Bolabola. + +SEDIMENT in Keeling lagoon. In other atolls. Injurious to corals. +Transported from coral-islands far seaward. + +Seniavine. + +Serangani. + +Seychelles. + +SHIP-BOTTOM quickly coated with coral. + +SMYTH island. + +SOCIETY archipelago, stationary condition of. Alleged proofs of recent +elevation. + +Society archipelago. + +Socotra. + +Solor. + +SOOLOO islands, recently elevated. + +Sooloo islands. + +Souvaroff. + +Spanish. + +SPONGE, depths at which found. + +Starbuck. (Slarbuck.) + +STONES transported in roots of trees. + +STORMS, effects of, on coral-islands. + +STUTCHBURY, Mr., on the growth of an Agaricia. On upraised corals in +Society archipelago. + +SUBSIDENCE of Keeling atoll. Extreme slowness of. Areas of, apparently +elongated. Areas of immense. Great amount of. + +Suez, gulf of. + +Sulphur islands. + +SUMATRA, recently elevated. + +Sumatra. + +Sumbawa. + +SURF favourable to the growth of massive corals. + +Swallow shoal. + +Sydney island. + +TAHITI, alleged proofs of its recent elevation. + +Tahiti. + +TEMPERATURE of the sea at the Galapagos archipelago. + +Tenasserim. + +Tenimber island. + +Teturoa. + +THEORIES on coral-formations. + +THEORY OF subsidence, and objections to. + +THICKNESS, vertical, of barrier-reefs. + +Thomas, St. + +Tikopia. + +TIMOR, recently elevated. + +Timor. + +Timor-laut. + +Tokan-Bessees. + +Tongatabou. + +Tonquin. + +Toubai. + +Toufoa. (Toofoa.) + +Toupoua. + +TRADITIONS OF CHANGE in coral-islands. + +TRIDACNAE embedded in coral-rock. Left exposed in the Low archipelago. + +TUBULARIA, quick growth of. + +Tumbelan. + +Turneffe reef. + +Turtle. + +Ualan. + +VANIKORO, section of. Its state and changes in its reefs. + +Vanikoro. + +Vine reef. + +Virgin Gorda. + +Viti archipelago. + +VOLCANIC islands, with living corals on their shores. Matter, probably +not associated with thick masses of coral-rock. + +VOLCANOES, authorities for their position on the map. Their presence +determined by the movements in progress. Absent or extinct in the areas +of subsidence. + +Waigiou. + +Wallis island. + +Washington. + +Well’s reef. + +WELLSTEAD, Lieutenant, account of a ship coated with corals. + +WEST INDIES, banks of sediment fringed by reefs. Recently elevated. + +West Indies. + +WHITSUNDAY island, view of. Changes in its state. + +WILLIAMS, Rev. J., on traditions of the natives regarding +coral-islands. On antiquity of certain corals. + +Wolchonsky. + +Wostock. + +Xulla islands. + +York island. + +Yucutan, coast of. + +ZONES of different kinds of corals outside the same reefs. + + + + +*** END OF THE PROJECT GUTENBERG EBOOK CORAL REEFS *** + +Updated editions will replace the previous one--the old editions will +be renamed. + +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the +United States without permission and without paying copyright +royalties. Special rules, set forth in the General Terms of Use part +of this license, apply to copying and distributing Project +Gutenberg-tm electronic works to protect the PROJECT GUTENBERG-tm +concept and trademark. Project Gutenberg is a registered trademark, +and may not be used if you charge for an eBook, except by following +the terms of the trademark license, including paying royalties for use +of the Project Gutenberg trademark. If you do not charge anything for +copies of this eBook, complying with the trademark license is very +easy. You may use this eBook for nearly any purpose such as creation +of derivative works, reports, performances and research. Project +Gutenberg eBooks may be modified and printed and given away--you may +do practically ANYTHING in the United States with eBooks not protected +by U.S. copyright law. Redistribution is subject to the trademark +license, especially commercial redistribution. + +START: FULL LICENSE + +THE FULL PROJECT GUTENBERG LICENSE +PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK + +To protect the Project Gutenberg-tm mission of promoting the free +distribution of electronic works, by using or distributing this work +(or any other work associated in any way with the phrase "Project +Gutenberg"), you agree to comply with all the terms of the Full +Project Gutenberg-tm License available with this file or online at +www.gutenberg.org/license. + +Section 1. General Terms of Use and Redistributing Project +Gutenberg-tm electronic works + +1.A. By reading or using any part of this Project Gutenberg-tm +electronic work, you indicate that you have read, understand, agree to +and accept all the terms of this license and intellectual property +(trademark/copyright) agreement. If you do not agree to abide by all +the terms of this agreement, you must cease using and return or +destroy all copies of Project Gutenberg-tm electronic works in your +possession. If you paid a fee for obtaining a copy of or access to a +Project Gutenberg-tm electronic work and you do not agree to be bound +by the terms of this agreement, you may obtain a refund from the +person or entity to whom you paid the fee as set forth in paragraph +1.E.8. + +1.B. "Project Gutenberg" is a registered trademark. It may only be +used on or associated in any way with an electronic work by people who +agree to be bound by the terms of this agreement. There are a few +things that you can do with most Project Gutenberg-tm electronic works +even without complying with the full terms of this agreement. See +paragraph 1.C below. There are a lot of things you can do with Project +Gutenberg-tm electronic works if you follow the terms of this +agreement and help preserve free future access to Project Gutenberg-tm +electronic works. See paragraph 1.E below. + +1.C. The Project Gutenberg Literary Archive Foundation ("the +Foundation" or PGLAF), owns a compilation copyright in the collection +of Project Gutenberg-tm electronic works. Nearly all the individual +works in the collection are in the public domain in the United +States. If an individual work is unprotected by copyright law in the +United States and you are located in the United States, we do not +claim a right to prevent you from copying, distributing, performing, +displaying or creating derivative works based on the work as long as +all references to Project Gutenberg are removed. Of course, we hope +that you will support the Project Gutenberg-tm mission of promoting +free access to electronic works by freely sharing Project Gutenberg-tm +works in compliance with the terms of this agreement for keeping the +Project Gutenberg-tm name associated with the work. You can easily +comply with the terms of this agreement by keeping this work in the +same format with its attached full Project Gutenberg-tm License when +you share it without charge with others. + +1.D. The copyright laws of the place where you are located also govern +what you can do with this work. Copyright laws in most countries are +in a constant state of change. If you are outside the United States, +check the laws of your country in addition to the terms of this +agreement before downloading, copying, displaying, performing, +distributing or creating derivative works based on this work or any +other Project Gutenberg-tm work. The Foundation makes no +representations concerning the copyright status of any work in any +country other than the United States. + +1.E. Unless you have removed all references to Project Gutenberg: + +1.E.1. The following sentence, with active links to, or other +immediate access to, the full Project Gutenberg-tm License must appear +prominently whenever any copy of a Project Gutenberg-tm work (any work +on which the phrase "Project Gutenberg" appears, or with which the +phrase "Project Gutenberg" is associated) is accessed, displayed, +performed, viewed, copied or distributed: + + This eBook is for the use of anyone anywhere in the United States and + most other parts of the world at no cost and with almost no + restrictions whatsoever. You may copy it, give it away or re-use it + under the terms of the Project Gutenberg License included with this + eBook or online at www.gutenberg.org. If you are not located in the + United States, you will have to check the laws of the country where + you are located before using this eBook. + +1.E.2. If an individual Project Gutenberg-tm electronic work is +derived from texts not protected by U.S. copyright law (does not +contain a notice indicating that it is posted with permission of the +copyright holder), the work can be copied and distributed to anyone in +the United States without paying any fees or charges. If you are +redistributing or providing access to a work with the phrase "Project +Gutenberg" associated with or appearing on the work, you must comply +either with the requirements of paragraphs 1.E.1 through 1.E.7 or +obtain permission for the use of the work and the Project Gutenberg-tm +trademark as set forth in paragraphs 1.E.8 or 1.E.9. + +1.E.3. If an individual Project Gutenberg-tm electronic work is posted +with the permission of the copyright holder, your use and distribution +must comply with both paragraphs 1.E.1 through 1.E.7 and any +additional terms imposed by the copyright holder. Additional terms +will be linked to the Project Gutenberg-tm License for all works +posted with the permission of the copyright holder found at the +beginning of this work. + +1.E.4. Do not unlink or detach or remove the full Project Gutenberg-tm +License terms from this work, or any files containing a part of this +work or any other work associated with Project Gutenberg-tm. + +1.E.5. Do not copy, display, perform, distribute or redistribute this +electronic work, or any part of this electronic work, without +prominently displaying the sentence set forth in paragraph 1.E.1 with +active links or immediate access to the full terms of the Project +Gutenberg-tm License. + +1.E.6. You may convert to and distribute this work in any binary, +compressed, marked up, nonproprietary or proprietary form, including +any word processing or hypertext form. However, if you provide access +to or distribute copies of a Project Gutenberg-tm work in a format +other than "Plain Vanilla ASCII" or other format used in the official +version posted on the official Project Gutenberg-tm website +(www.gutenberg.org), you must, at no additional cost, fee or expense +to the user, provide a copy, a means of exporting a copy, or a means +of obtaining a copy upon request, of the work in its original "Plain +Vanilla ASCII" or other form. Any alternate format must include the +full Project Gutenberg-tm License as specified in paragraph 1.E.1. + +1.E.7. Do not charge a fee for access to, viewing, displaying, +performing, copying or distributing any Project Gutenberg-tm works +unless you comply with paragraph 1.E.8 or 1.E.9. + +1.E.8. You may charge a reasonable fee for copies of or providing +access to or distributing Project Gutenberg-tm electronic works +provided that: + +* You pay a royalty fee of 20% of the gross profits you derive from + the use of Project Gutenberg-tm works calculated using the method + you already use to calculate your applicable taxes. The fee is owed + to the owner of the Project Gutenberg-tm trademark, but he has + agreed to donate royalties under this paragraph to the Project + Gutenberg Literary Archive Foundation. Royalty payments must be paid + within 60 days following each date on which you prepare (or are + legally required to prepare) your periodic tax returns. Royalty + payments should be clearly marked as such and sent to the Project + Gutenberg Literary Archive Foundation at the address specified in + Section 4, "Information about donations to the Project Gutenberg + Literary Archive Foundation." + +* You provide a full refund of any money paid by a user who notifies + you in writing (or by e-mail) within 30 days of receipt that s/he + does not agree to the terms of the full Project Gutenberg-tm + License. You must require such a user to return or destroy all + copies of the works possessed in a physical medium and discontinue + all use of and all access to other copies of Project Gutenberg-tm + works. + +* You provide, in accordance with paragraph 1.F.3, a full refund of + any money paid for a work or a replacement copy, if a defect in the + electronic work is discovered and reported to you within 90 days of + receipt of the work. + +* You comply with all other terms of this agreement for free + distribution of Project Gutenberg-tm works. + +1.E.9. If you wish to charge a fee or distribute a Project +Gutenberg-tm electronic work or group of works on different terms than +are set forth in this agreement, you must obtain permission in writing +from the Project Gutenberg Literary Archive Foundation, the manager of +the Project Gutenberg-tm trademark. Contact the Foundation as set +forth in Section 3 below. + +1.F. + +1.F.1. Project Gutenberg volunteers and employees expend considerable +effort to identify, do copyright research on, transcribe and proofread +works not protected by U.S. copyright law in creating the Project +Gutenberg-tm collection. Despite these efforts, Project Gutenberg-tm +electronic works, and the medium on which they may be stored, may +contain "Defects," such as, but not limited to, incomplete, inaccurate +or corrupt data, transcription errors, a copyright or other +intellectual property infringement, a defective or damaged disk or +other medium, a computer virus, or computer codes that damage or +cannot be read by your equipment. + +1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right +of Replacement or Refund" described in paragraph 1.F.3, the Project +Gutenberg Literary Archive Foundation, the owner of the Project +Gutenberg-tm trademark, and any other party distributing a Project +Gutenberg-tm electronic work under this agreement, disclaim all +liability to you for damages, costs and expenses, including legal +fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT +LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE +PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE +TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE +LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR +INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH +DAMAGE. + +1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a +defect in this electronic work within 90 days of receiving it, you can +receive a refund of the money (if any) you paid for it by sending a +written explanation to the person you received the work from. If you +received the work on a physical medium, you must return the medium +with your written explanation. The person or entity that provided you +with the defective work may elect to provide a replacement copy in +lieu of a refund. If you received the work electronically, the person +or entity providing it to you may choose to give you a second +opportunity to receive the work electronically in lieu of a refund. If +the second copy is also defective, you may demand a refund in writing +without further opportunities to fix the problem. + +1.F.4. Except for the limited right of replacement or refund set forth +in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO +OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT +LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE. + +1.F.5. Some states do not allow disclaimers of certain implied +warranties or the exclusion or limitation of certain types of +damages. If any disclaimer or limitation set forth in this agreement +violates the law of the state applicable to this agreement, the +agreement shall be interpreted to make the maximum disclaimer or +limitation permitted by the applicable state law. The invalidity or +unenforceability of any provision of this agreement shall not void the +remaining provisions. + +1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the +trademark owner, any agent or employee of the Foundation, anyone +providing copies of Project Gutenberg-tm electronic works in +accordance with this agreement, and any volunteers associated with the +production, promotion and distribution of Project Gutenberg-tm +electronic works, harmless from all liability, costs and expenses, +including legal fees, that arise directly or indirectly from any of +the following which you do or cause to occur: (a) distribution of this +or any Project Gutenberg-tm work, (b) alteration, modification, or +additions or deletions to any Project Gutenberg-tm work, and (c) any +Defect you cause. + +Section 2. Information about the Mission of Project Gutenberg-tm + +Project Gutenberg-tm is synonymous with the free distribution of +electronic works in formats readable by the widest variety of +computers including obsolete, old, middle-aged and new computers. It +exists because of the efforts of hundreds of volunteers and donations +from people in all walks of life. + +Volunteers and financial support to provide volunteers with the +assistance they need are critical to reaching Project Gutenberg-tm's +goals and ensuring that the Project Gutenberg-tm collection will +remain freely available for generations to come. In 2001, the Project +Gutenberg Literary Archive Foundation was created to provide a secure +and permanent future for Project Gutenberg-tm and future +generations. To learn more about the Project Gutenberg Literary +Archive Foundation and how your efforts and donations can help, see +Sections 3 and 4 and the Foundation information page at +www.gutenberg.org + +Section 3. Information about the Project Gutenberg Literary +Archive Foundation + +The Project Gutenberg Literary Archive Foundation is a non-profit +501(c)(3) educational corporation organized under the laws of the +state of Mississippi and granted tax exempt status by the Internal +Revenue Service. The Foundation's EIN or federal tax identification +number is 64-6221541. Contributions to the Project Gutenberg Literary +Archive Foundation are tax deductible to the full extent permitted by +U.S. federal laws and your state's laws. + +The Foundation's business office is located at 809 North 1500 West, +Salt Lake City, UT 84116, (801) 596-1887. Email contact links and up +to date contact information can be found at the Foundation's website +and official page at www.gutenberg.org/contact + +Section 4. Information about Donations to the Project Gutenberg +Literary Archive Foundation + +Project Gutenberg-tm depends upon and cannot survive without +widespread public support and donations to carry out its mission of +increasing the number of public domain and licensed works that can be +freely distributed in machine-readable form accessible by the widest +array of equipment including outdated equipment. Many small donations +($1 to $5,000) are particularly important to maintaining tax exempt +status with the IRS. + +The Foundation is committed to complying with the laws regulating +charities and charitable donations in all 50 states of the United +States. Compliance requirements are not uniform and it takes a +considerable effort, much paperwork and many fees to meet and keep up +with these requirements. We do not solicit donations in locations +where we have not received written confirmation of compliance. To SEND +DONATIONS or determine the status of compliance for any particular +state visit www.gutenberg.org/donate + +While we cannot and do not solicit contributions from states where we +have not met the solicitation requirements, we know of no prohibition +against accepting unsolicited donations from donors in such states who +approach us with offers to donate. + +International donations are gratefully accepted, but we cannot make +any statements concerning tax treatment of donations received from +outside the United States. U.S. laws alone swamp our small staff. + +Please check the Project Gutenberg web pages for current donation +methods and addresses. Donations are accepted in a number of other +ways including checks, online payments and credit card donations. To +donate, please visit: www.gutenberg.org/donate + +Section 5. General Information About Project Gutenberg-tm electronic works + +Professor Michael S. Hart was the originator of the Project +Gutenberg-tm concept of a library of electronic works that could be +freely shared with anyone. For forty years, he produced and +distributed Project Gutenberg-tm eBooks with only a loose network of +volunteer support. + +Project Gutenberg-tm eBooks are often created from several printed +editions, all of which are confirmed as not protected by copyright in +the U.S. unless a copyright notice is included. Thus, we do not +necessarily keep eBooks in compliance with any particular paper +edition. + +Most people start at our website which has the main PG search +facility: www.gutenberg.org + +This website includes information about Project Gutenberg-tm, +including how to make donations to the Project Gutenberg Literary +Archive Foundation, how to help produce our new eBooks, and how to +subscribe to our email newsletter to hear about new eBooks. + |