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+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 ***
+
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diff --git a/2690-0.zip b/2690-0.zip
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+<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
+<meta http-equiv="Content-Style-Type" content="text/css" />
+<title>The Project Gutenberg eBook of The Structure and Distribution of Coral Reefs, by Charles Darwin</title>
+
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+<div style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of The Structure and Distribution of Coral Reefs, by Charles Darwin</div>
+<div style='display:block; margin:1em 0'>
+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 <a href="https://www.gutenberg.org">www.gutenberg.org</a>. 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.
+</div>
+<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: The Structure and Distribution of Coral Reefs</div>
+<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Charles Darwin</div>
+<div style='display:block; margin:1em 0'>Release Date: June, 2000 [eBook #2690]<br />
+[Most recently updated: October 10, 2022]</div>
+<div style='display:block; margin:1em 0'>Language: English</div>
+<div style='display:block; margin:1em 0'>Character set encoding: UTF-8</div>
+<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Sue Asscher and David Widger</div>
+<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK CORAL REEFS ***</div>
+
+<h1>CORAL REEFS</h1>
+
+<h2 class="no-break">
+
+By Charles Darwin</h2>
+
+<hr />
+
+<h2>CONTENTS</h2>
+
+<p>
+<a href="#pre01">DETAILED TABLE OF CONTENTS</a>
+</p>
+
+<p>
+<a href="#pre02">DESCRIPTION OF THE PLATES</a>
+</p>
+
+<p>
+<a href="#pre03">THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS</a>
+</p>
+
+<p>
+<a href="#pre04">INTRODUCTION</a>
+</p>
+
+<p>
+<a href="#chap01">CHAPTER I.&mdash;ATOLLS OR LAGOON-ISLANDS</a>
+</p>
+
+<p>
+<a href="#chap02">CHAPTER II.&mdash;BARRIER REEFS</a>
+</p>
+
+<p>
+<a href="#chap03">CHAPTER III.&mdash;FRINGING OR SHORE-REEFS</a>
+</p>
+
+<p>
+<a href="#chap04">CHAPTER IV.&mdash;ON THE DISTRIBUTION AND GROWTH OF
+CORAL-REEFS</a>
+</p>
+
+<p>
+<a href="#chap05">CHAPTER V.&mdash;THEORY OF THE FORMATION OF THE DIFFERENT
+CLASSES OF CORAL-REEFS</a>
+</p>
+
+<p>
+<a href="#chap06">CHAPTER VI.&mdash;ON THE DISTRIBUTION OF CORAL-REEFS WITH
+REFERENCE TO THE THEORY OF THEIR FORMATION</a>
+</p>
+
+<p>
+<a href="#chap07">APPENDIX</a>
+</p>
+
+<p>
+<a href="#chap08">INDEX</a>
+</p>
+
+<hr />
+
+<div class="chapter">
+
+<h2><a name="pre01"></a>
+DETAILED TABLE OF CONTENTS</h2>
+
+<p class="noindent">
+<a href="#pre02">DESCRIPTION OF THE PLATES</a><br/><br/>
+</p>
+
+<p class="noindent">
+<a href="#pre04">INTRODUCTION</a><br/><br/>
+</p>
+
+<p class="noindent">
+<a href="#chap01">CHAPTER I.&mdash;ATOLLS OR LAGOON-ISLANDS</a>
+</p>
+
+<p>
+SECTION I.&mdash;DESCRIPTION OF KEELING ATOLL. Corals on the outer
+margin.&mdash;Zone of Nulliporæ.&mdash;Exterior
+reef.&mdash;Islets.&mdash;Coral-conglomerate.&mdash;Lagoon.&mdash;Calcareous
+sediment.&mdash;Scari and Holuthuriæ subsisting on corals.&mdash;Changes in the
+condition of the reefs and islets.&mdash;Probable subsidence of the
+atoll.&mdash;Future state of the lagoon.
+</p>
+
+<p>
+SECTION II.&mdash;GENERAL DESCRIPTION OF ATOLLS. General form and size of
+atolls, their reefs and islets.&mdash;External slope.&mdash; Zone of
+Nulliporæ.&mdash;Conglomerate.&mdash;Depth of
+lagoons.&mdash;Sediment.&mdash;Reefs submerged wholly or in
+part.&mdash;Breaches in the reef.&mdash;Ledge-formed shores round certain
+lagoons.&mdash;Conversion of lagoons into land.
+</p>
+
+<p>
+SECTION III.&mdash;ATOLLS OF THE MALDIVA ARCHIPELAGO&mdash;GREAT CHAGOS BANK.
+Maldiva Archipelago.&mdash;Ring-formed reefs, marginal and central.&mdash;Great
+depths in the lagoons of the southern atolls.&mdash;Reefs in the lagoons all
+rising to the surface.&mdash;Position of islets and breaches in the reefs, with
+respect to the prevalent winds and action of the waves.&mdash;Destruction of
+islets.&mdash;Connection in the position and submarine foundation of distinct
+atolls.&mdash;The apparent disseverment of large atolls.&mdash;The Great Chagos
+Bank.&mdash;Its submerged condition and extraordinary structure.<br/><br/>
+</p>
+
+<p class="noindent">
+<a href="#chap02">CHAPTER II.&mdash;BARRIER REEFS</a><br/>
+Closely resemble in general form and structure atoll-reefs.&mdash;Width and
+depth of the lagoon-channels.&mdash;Breaches through the reef in front of
+valleys, and generally on the leeward side.&mdash;Checks to the filling up of
+the lagoon-channels.&mdash;Size and constitution of the encircled
+islands.&mdash; Number of islands within the same reef.&mdash;Barrier-reefs of
+New Caledonia and Australia.&mdash;Position of the reef relative to the slope
+of the adjoining land.&mdash;Probable great thickness of
+barrier-reefs.<br/><br/>
+</p>
+
+<p class="noindent">
+<a href="#chap03">CHAPTER III.&mdash;FRINGING OR SHORE-REEFS</a><br/>
+Reefs of Mauritius.&mdash;Shallow channel within the reef.&mdash;Its slow
+filling up.&mdash;Currents of water formed within it.&mdash;Upraised
+reefs.&mdash;Narrow fringing-reefs in deep seas.&mdash;Reefs on the coast of E.
+Africa and of Brazil.&mdash;Fringing-reefs in very shallow seas, round banks of
+sediment and on worn-down islands.&mdash;Fringing-reefs affected by currents of
+the sea. &mdash;Coral coating the bottom of the sea, but not forming
+reefs.<br/><br/>
+</p>
+
+<p class="noindent">
+<a href="#chap04">CHAPTER IV.&mdash;ON THE DISTRIBUTION AND GROWTH OF
+CORAL-REEFS</a>
+</p>
+
+<p>
+SECTION I.&mdash;ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
+FAVOURABLE TO THEIR INCREASE.
+</p>
+
+<p>
+SECTION II.&mdash;ON THE RATE OF GROWTH OF CORAL-REEFS.
+</p>
+
+<p>
+SECTION III.&mdash;ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.<br/><br/>
+</p>
+
+<p class="noindent">
+<a href="#chap05">CHAPTER V.&mdash;THEORY OF THE FORMATION OF THE DIFFERENT
+CLASSES OF CORAL-REEFS</a><br/>
+The atolls of the larger archipelagoes are not formed on submerged craters, or
+on banks of sediment.&mdash;Immense areas interspersed with
+atolls.&mdash;Recent changes in their state.&mdash;The origin of barrier-reefs
+and of atolls.&mdash;Their relative forms.&mdash;The step-formed ledges and
+walls round the shores of some lagoons.&mdash;The ring-formed reefs of the
+Maldiva atolls.&mdash;The submerged condition of parts or of the whole of some
+annular reefs.&mdash;The disseverment of large atolls.&mdash;The union of
+atolls by linear reefs.&mdash;The Great Chagos Bank.&mdash;Objections, from the
+area and amount of subsidence required by the theory, considered.&mdash;The
+probable composition of the lower parts of atolls.<br/><br/>
+</p>
+
+<p class="noindent">
+<a href="#chap06">CHAPTER VI.&mdash;ON THE DISTRIBUTION OF CORAL-REEFS WITH
+REFERENCE TO THE THEORY OF THEIR FORMATION</a><br/>
+Description of the coloured map.&mdash;Proximity of atolls and
+barrier-reefs.&mdash; Relation in form and position of atolls with ordinary
+islands.&mdash;Direct evidence of subsidence difficult to be
+detected.&mdash;Proofs of recent elevation where fringing-reefs
+occur.&mdash;Oscillations of level.&mdash;Absence of active volcanoes in the
+areas of subsidence.&mdash;Immensity of the areas which have been elevated and
+have subsided.&mdash;Their relation to the present distribution of the
+land.&mdash;Areas of subsidence elongated, their intersection and alternation
+with those of elevation.&mdash;Amount and slow rate of the
+subsidence.&mdash;Recapitulation.<br/><br/>
+</p>
+
+<p class="noindent">
+<a href="#chap07">APPENDIX</a><br/><br/>
+</p>
+
+<p class="noindent">
+<a href="#chap08">INDEX</a>
+</p>
+
+<hr />
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="pre02"></a>DESCRIPTION OF THE PLATES.</h2>
+
+<p class="center">
+PLATE I.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus01"></a>
+<a href="images/plate1.jpg">
+<img src="images/plate1.jpg" width="750" height="410" alt="Illustration:" /></a>
+<p class="caption">PLATE I.&mdash;MAP SHOWING THE RESEMBLANCE IN FORM BETWEEN
+BARRIER CORAL-REEFS SURROUNDING MOUNTAINOUS ISLANDS, AND ATOLLS OR LAGOON
+ISLANDS.
+</p>
+</div>
+
+<p>
+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 <i>annular reef</i>, which when surrounding an open expanse of water,
+forms an &ldquo;atoll,&rdquo; and when surrounding one or more high islands,
+forms an encircling &ldquo;barrier-reef,&rdquo; 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.
+</p>
+
+<p>
+Figure 1.&mdash;V<small>ANIKORO</small>, situated in the western part of the
+South Pacific; taken from the survey by Captain D&rsquo;Urville in the
+<i>Astrolabe;</i> 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&rsquo;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.
+</p>
+
+<p>
+Figure 2.&mdash;H<small>OGOLEU</small>, or R<small>OUG</small>, in the Caroline
+Archipelago; taken from the atlas of the voyage of the <i>Astrolabe</i>,
+compiled from the surveys of Captains Duperrey and D&rsquo;Urville; the depth
+of the immense lagoon-like space within the reef is not known.
+</p>
+
+<p>
+Figure 3.&mdash;R<small>AIATEA</small>, in the Society Archipelago; from the
+map given in the quarto edition of &ldquo;Cook&rsquo;s First Voyage;&rdquo; it
+is probably not accurate.
+</p>
+
+<p>
+Figure 4.&mdash;B<small>OW</small>, or H<small>EYOU ATOLL</small> (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.
+</p>
+
+<p>
+Figure 5.&mdash;B<small>OLABOLA</small>, in the Society Archipelago, from the
+survey of Captain Duperrey in the <i>Coquille:</i> 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.
+</p>
+
+<p>
+Figure 6.&mdash;M<small>AURUA</small>, in the Society Archipelago; from the
+survey by Captain Duperrey in the <i>Coquille:</i> height of land about eight
+hundred feet.
+</p>
+
+<p>
+Figure 7.&mdash;P<small>OUYNIPÈTE</small>, or S<small>ENIAVINE</small>, in the
+Caroline Archipelago; from the survey by Admiral Lutké.
+</p>
+
+<p>
+Figure 8.&mdash;G<small>AMBIER</small> I<small>SLANDS</small>, 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.
+</p>
+
+<p>
+Figure 9.&mdash;P<small>EROS</small> B<small>ANHOS ATOLL</small> (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.
+</p>
+
+<p>
+Figure 10.&mdash;K<small>EELING</small>, or C<small>OCOS ATOLL</small> (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.
+</p>
+
+<p class="center">
+PLATE II.&mdash;GREAT CHAGOS BANK, NEW CALEDONIA, MENCHIKOFF ATOLL, ETC.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus02"></a>
+<a href="images/plate2.jpg">
+<img src="images/plate2.jpg" width="750" height="557" alt="Illustration:" /></a>
+<p class="caption">Plate 2
+</p>
+</div>
+
+<p>
+Figure 1.&mdash;G<small>REAT</small> C<small>HAGOS</small> B<small>ANK</small>,
+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.
+</p>
+
+<p>
+Figure 2.&mdash;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.
+</p>
+
+<p>
+Figure 3.&mdash;M<small>ENCHIKOFF ATOLL</small> (or lagoon-island), in the
+Marshall Archipelago, Northern Pacific Ocean; from Krusenstern&rsquo;s
+&ldquo;Atlas of the Pacific;&rdquo; originally surveyed by Captain Hagemeister;
+the depth within the lagoons is unknown.
+</p>
+
+<p>
+Figure 4.&mdash;M<small>AHLOS</small> M<small>AHDOO ATOLL</small>, 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&rsquo;s island, signifies that soundings were not obtained at these
+depths.
+</p>
+
+<p>
+Figure 5.&mdash;N<small>EW</small> C<small>ALEDONIA</small>, in the western
+part of the Pacific; from Krusenstern&rsquo;s &ldquo;Atlas,&rdquo; compiled
+from several surveys; I have slightly altered the northern point of the reef,
+in accordance with the &ldquo;Atlas of the Voyage of the
+<i>Astrolabe</i>.&rdquo; In Krusenstern&rsquo;s &ldquo;Atlas,&rdquo; the reef
+is represented by a single line with crosses; I have for the sake of uniformity
+added an interior line.
+</p>
+
+<p>
+Figure 6.&mdash;M<small>ALDIVA</small> A<small>RCHIPELAGO</small>, in the
+Indian Ocean; from the survey by Captain Moresby and Lieutenant Powell.
+</p>
+
+<p class="center">
+PLATE III.&mdash;MAP SHOWING THE DISTRIBUTION OF CORAL-REEFS AND ACTIVE
+VOLCANOES.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus03"></a>
+<a href="images/plate3.jpg">
+<img src="images/plate3.jpg" width="750" height="273" alt="Illustration:" /></a>
+<p class="caption">Plate 3 Shewing the Distribution of the Different Kinds of
+Coral Reefs, Together with the Position of the Active Volcanoes in the Map.
+</p>
+</div>
+
+<p>
+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 &ldquo;Coral Reefs.&rdquo; The names not printed in upper case
+in the Index refer to the Appendix.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="pre03"></a>
+
+THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.</h2>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="pre04"></a>
+
+INTRODUCTION.</h2>
+
+<p>
+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:
+&ldquo;lagoon-islands,&rdquo; or &ldquo;atolls,&rdquo; &ldquo;barrier&rdquo; or
+&ldquo;encircling reefs,&rdquo; and &ldquo;fringing&rdquo; or
+&ldquo;shore-reefs.&rdquo; 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, &ldquo;C&rsquo;est une merueille de
+voir chacun de ces atollons, enuironné d&rsquo;un grand banc de pierre tout
+autour, n&rsquo;y ayant point d&rsquo;artifice humain.&rdquo; The accompanying
+sketch of Whitsunday island, in the South Pacific, taken from Captain
+Beechey&rsquo;s admirable &ldquo;Voyage,&rdquo; although excellent of its kind,
+gives but a faint idea of the singular aspect of one of these lagoon-islands.
+</p>
+
+<div class="fig" style="width:100%;">
+<img src="images/img01.jpg" width="500" height="222" alt="[Illustration]" />
+</div>
+
+<p>
+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
+&ldquo;island,&rdquo; applied to the whole, is often the cause of confusion;
+hence I have invariably used in this volume the term &ldquo;atoll,&rdquo; which
+is the name given to these circular groups of coral-islets by their inhabitants
+in the Indian Ocean, and is synonymous with &ldquo;lagoon-island.&rdquo;
+</p>
+
+<div class="fig" style="width:100%;">
+<img src="images/img02.jpg" width="500" height="231" alt="[Illustration]" />
+</div>
+
+<p>
+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 &ldquo;Voyage of the
+&lsquo;Coquille&rsquo;,&rdquo; the reef is seen from within, from one of the
+high peaks of the island of Bolabola.<a href="#fn1"
+name="fnref1"><sup>[1]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn1"></a> <a href="#fnref1">[1]</a> I have taken the liberty of
+simplifying the foreground, and leaving out a mountainous island in the far
+distance.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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
+(<a href="#illus03">Plate III.</a>), 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.
+</p>
+
+<p>
+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.<a
+href="#fn2" name="fnref2"><sup>[2]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn2"></a> <a href="#fnref2">[2]</a> 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.
+</p>
+
+<hr />
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap01"></a>
+
+CHAPTER I.<br/>
+
+ATOLLS OR LAGOON-ISLANDS.</h2>
+
+<h3>SECTION I.&mdash;KEELING ATOLL.</h3>
+
+<p class="letter">
+Corals on the outer margin.&mdash;Zone of Nulliporæ.&mdash;Exterior
+reef.&mdash;Islets.&mdash;Coral-conglomerate.&mdash;Lagoon.&mdash;Calcareous
+sediment.&mdash;Scari and Holuthuriæ subsisting on corals.&mdash;Changes in the
+condition of the reefs and islets.&mdash;Probable subsidence of the
+atoll.&mdash;Future state of the lagoon.
+</p>
+
+<div class="fig" style="width:100%;">
+<img src="images/img03.jpg" width="500" height="109" alt="[Illustration]" />
+</div>
+
+<p>
+A.&mdash;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.
+</p>
+
+<p>
+B.&mdash;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.
+</p>
+
+<p>
+C.&mdash;A flat of coral-rock, covered at high water.
+</p>
+
+<p>
+D.&mdash;A low projecting ledge of brecciated coral-rock washed by the waves at
+high water.
+</p>
+
+<p>
+E.&mdash;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.
+</p>
+
+<p>
+F.&mdash;Level of the lagoon at low water.
+</p>
+
+<p class="p2">
+Keeling or Cocos atoll is situated in the Indian Ocean, in 12° 5&#x2032; S.,
+and longitude 90° 55&#x2032; E.: a reduced chart of it was made from the survey
+of Captain Fitzroy and the Officers of H.M.S. &ldquo;Beagle,&rdquo; 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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+Next, but much inferior in importance to the Porites, is the <i>Millepora
+complanata</i>.<a href="#fn1.1" name="fnref1.1"><sup>[1]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.1"></a> <a href="#fnref1.1">[1]</a> This Millepora (Palmipora of
+Blainville), as well as the <i>M. alcicornis</i>, possesses the singular
+property of stinging the skin where it is delicate, as on the face and arm.
+</p>
+
+<p>
+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 (<i>A</i> in section), beyond which the sides plunge into the
+unfathomable ocean at an angle of 45°.<a href="#fn1.2"
+name="fnref1.2"><sup>[2]</sup></a> 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 <i>Millepora alcicornis</i> 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 <i>M. corymbosa</i>, 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 <i>Millepora
+alcicornis</i> 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.2"></a> <a href="#fnref1.2">[2]</a> 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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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&rsquo;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.<a href="#fn1.3" name="fnref1.3"><sup>[3]</sup></a> 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 (<i>B</i> 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.3"></a> <a href="#fnref1.3">[3]</a> 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.
+</p>
+
+<p>
+The woodcut represents a section across one of the islets on the reef, but if
+all that part which is above the level of <i>C</i> 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 <i>Pocillopora verrucosa</i>,
+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 <i>M.
+pocillifera</i>, 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 (<i>C</i> 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 &ldquo;flat&rdquo; 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
+&ldquo;flat&rdquo; is shown in the foregoing woodcut of Whitsunday atoll.
+</p>
+
+<p>
+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 (<i>E</i> 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 (<i>D</i> 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 &ldquo;flat&rdquo; 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.
+</p>
+
+<p>
+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 <i>Seriatapora subulata;</i> two species of
+Porites<a href="#fn1.4" name="fnref1.4"><sup>[4]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.4"></a> <a href="#fnref1.4">[4]</a> This Porites has somewhat the
+habit of <i>P. clavaria</i>, 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.
+</p>
+
+<p>
+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 &ldquo;consumed and be consumed,&rdquo; holds good even with
+the polypifers forming those massive bulwarks, which are able to withstand the
+force of the open ocean.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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 &ldquo;flat&rdquo; 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 &ldquo;flat,&rdquo; 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
+&ldquo;flat,&rdquo; 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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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.
+</p>
+
+<h3>SECTION II.&mdash;GENERAL DESCRIPTION OF ATOLLS.</h3>
+
+<p class="letter">
+General form and size of atolls, their reefs and islets.&mdash;External
+slope.&mdash; Zone of Nulliporæ.&mdash;Conglomerate.&mdash;Depth of
+lagoons.&mdash;Sediment.&mdash;Reefs submerged wholly or in
+part.&mdash;Breaches in the reef.&mdash;Ledge-formed shores round certain
+lagoons.&mdash;Conversion of lagoons into land.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+The average width of the annular wreath may be taken as about a quarter of a
+mile. Captain Beechey<a href="#fn1.5" name="fnref1.5"><sup>[5]</sup></a> 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&mdash;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&rsquo;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.
+</p>
+
+<p class="footnote">
+<a name="fn1.5"></a> <a href="#fnref1.5">[5]</a> Beechey&rsquo;s &ldquo;Voyage
+to the Pacific and Beering&rsquo;s Straits,&rdquo; chapter viii.
+</p>
+
+<p>
+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<a href="#fn1.6" name="fnref1.6"><sup>[6]</sup></a> 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<a
+href="#fn1.7" name="fnref1.7"><sup>[7]</sup></a> 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&#x2032; N., 157°
+45&#x2032; W.), described by Cook,<a href="#fn1.8"
+name="fnref1.8"><sup>[8]</sup></a> 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<a href="#fn1.9"
+name="fnref1.9"><sup>[9]</sup></a> 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.<a href="#fn1.10" name="fnref1.10"><sup>[10]</sup></a> 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<a href="#fn1.11"
+name="fnref1.11"><sup>[11]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.6"></a> <a href="#fnref1.6">[6]</a> The form of the bottom round
+the Marshall atolls in the Northern Pacific is probably similar: Kotzebue
+(&ldquo;First Voyage,&rdquo; volume ii., page 16.) says: &ldquo;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.&rdquo;)
+</p>
+
+<p class="footnote">
+<a name="fn1.7"></a> <a href="#fnref1.7">[7]</a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.8"></a> <a href="#fnref1.8">[8]</a> Cook&rsquo;s &ldquo;Third
+Voyage,&rdquo; volume ii., chapter 10.
+</p>
+
+<p class="footnote">
+<a name="fn1.9"></a> <a href="#fnref1.9">[9]</a> This fact is taken from a MS.
+account of these groups lent me by Captain Moresby. See also Captain
+Moresby&rsquo;s paper on the Maldiva atolls in the &ldquo;Geographical
+Journal&rdquo;, volume v., page 401.
+</p>
+
+<p class="footnote">
+<a name="fn1.10"></a> <a href="#fnref1.10">[10]</a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.11"></a> <a href="#fnref1.11">[11]</a> &ldquo;Mémoires pour servir
+à une description Géolog. de France,&rdquo; tome iv., page 216.
+</p>
+
+<p>
+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&rsquo;s Island he says<a href="#fn1.12"
+name="fnref1.12"><sup>[12]</sup></a> the buttress, as it may be called, which
+&ldquo;has the most powerful enemy (the S.W. swell) to oppose, is carried out
+much further, and with less abruptness than the other.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.12"></a> <a href="#fnref1.12">[12]</a> Beechey&rsquo;s
+&ldquo;Voyage,&rdquo; 4to edition, page 44.
+</p>
+
+<p>
+Where islets have been formed on the reef, that part which I have sometimes
+called the &ldquo;flat&rdquo; 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&rsquo;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 &ldquo;the red colour of the reef (at the Marshall atolls) under
+the breakers is caused by a Nullipora, which covers the stone <i>wherever the
+waves beat;</i> and, under favourable circumstances, assumes a stalactical
+form,&rdquo;&mdash;a description perfectly applicable to the margin of Keeling
+atoll.<a href="#fn1.13" name="fnref1.13"><sup>[13]</sup></a> 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,<a href="#fn1.14"
+name="fnref1.14"><sup>[14]</sup></a> in another part, speaks of the rocks on
+the edge of the reef &ldquo;as visible for about two feet at low water,&rdquo;
+and these rocks we may feel quite certain are not formed of true coral.<a
+href="#fn1.15" name="fnref1.15"><sup>[15]</sup></a> 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 &ldquo;barrier-reefs&rdquo; which encircle the Society
+Islands.
+</p>
+
+<p class="footnote">
+<a name="fn1.13"></a> <a href="#fnref1.13">[13]</a> Kotzebue&rsquo;s
+&ldquo;First Voyage,&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.14"></a> <a href="#fnref1.14">[14]</a> Kotzebue, &ldquo;First
+Voyage,&rdquo; 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 &ldquo;London
+Phil. Journal,&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.15"></a> <a href="#fnref1.15">[15]</a> Captain Moresby, in his
+valuable paper &ldquo;on the Northern atolls of Maldivas&rdquo;
+(&ldquo;Geographical Journal&rdquo;, volume v.), says that the edges of the
+reefs there stand above water at low spring-tides.
+</p>
+
+<p>
+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<a href="#fn1.16" name="fnref1.16"><sup>[16]</sup></a> a
+layer of coarse conglomerate, outside the islets round the Marshall atolls
+which &ldquo;appears on its upper surface uneven and eaten away.&rdquo; From
+drawings, with appended remarks, of Diego Garcia in the Chagos group and of
+several of the Maldiva atolls, shown me by Captain Moresby,<a href="#fn1.17"
+name="fnref1.17"><sup>[17]</sup></a> 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&rsquo;s &ldquo;Voyage,&rdquo; it is not apparent that any conglomerate
+coral-rock was there observed.
+</p>
+
+<p class="footnote">
+<a name="fn1.16"></a> <a href="#fnref1.16">[16]</a> Kotzebue&rsquo;s
+&ldquo;First Voyage,&rdquo; volume iii., page 144.
+</p>
+
+<p class="footnote">
+<a name="fn1.17"></a> <a href="#fnref1.17">[17]</a> See also Moresby on the
+Northern atolls of the Maldivas, &ldquo;Geographical Journal&rdquo;, volume v.,
+page 400.
+</p>
+
+<p>
+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 <i>soft
+clay;</i> 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.<a href="#fn1.18" name="fnref1.18"><sup>[18]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn1.18"></a> <a href="#fnref1.18">[18]</a> 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 <i>Pilote du Brésil</i>, 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
+&ldquo;tuf blanc, mêlé ou formé de madrépores broyés.&rdquo; 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.
+</p>
+
+<p>
+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 (<a
+href="#illus01">Plate I., Figure 9</a>) in the Chagos group, with Mourileu
+atoll<a href="#fn1.19" name="fnref1.19"><sup>[19]</sup></a> in the Caroline
+Archipelago, and with the barrier-reef (<a href="#illus01">Plate I., Figure
+8</a>) 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&rsquo;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,&mdash;namely, the death of the coral,
+during the subsidence of the area, in which the atolls or banks are situated.
+</p>
+
+<p class="footnote">
+<a name="fn1.19"></a> <a href="#fnref1.19">[19]</a> Frederick Lutké&rsquo;s
+&ldquo;Voyage autour du Monde,&rdquo; volume ii., page 291. See also his
+account of Namonouïto, below, and the chart of Oulleay in the Atlas.
+</p>
+
+<p>
+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,&mdash;it may be said that the rim only of the saucer-shaped hollow
+forming the lagoon is notched. Mr. Lyell<a href="#fn1.20"
+name="fnref1.20"><sup>[20]</sup></a> 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,<a href="#fn1.21"
+name="fnref1.21"><sup>[21]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.20"></a> <a href="#fnref1.20">[20]</a> &ldquo;Principles of
+Geology,&rdquo; volume iii., page 289.
+</p>
+
+<p class="footnote">
+<a name="fn1.21"></a> <a href="#fnref1.21">[21]</a> Beechey&rsquo;s
+&ldquo;Voyage,&rdquo; 4to edition, volume i., page 189.
+</p>
+
+<p>
+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,<a
+href="#fn1.22" name="fnref1.22"><sup>[22]</sup></a> speaking in general terms
+of the lagoons in the Marshall atolls, says the lead generally sinks
+&ldquo;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.&rdquo; 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,<a href="#fn1.23" name="fnref1.23"><sup>[23]</sup></a> 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,&mdash;some
+with perpendicular, some with sloping sides,&mdash;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.
+</p>
+
+<p class="footnote">
+<a name="fn1.22"></a> <a href="#fnref1.22">[22]</a> Kotzebue&rsquo;s
+&ldquo;First Voyage,&rdquo; volume iii., page 142.
+</p>
+
+<p class="footnote">
+<a name="fn1.23"></a> <a href="#fnref1.23">[23]</a> Beechey&rsquo;s
+&ldquo;Voyage,&rdquo; 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.
+</p>
+
+<p>
+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<a href="#fn1.24"
+name="fnref1.24"><sup>[24]</sup></a>
+
+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.
+</p>
+
+<p class="footnote">
+<a name="fn1.24"></a> <a href="#fnref1.24">[24]</a> Kotzebue&rsquo;s
+&ldquo;First Voyage,&rdquo; volume iii., page 221.
+</p>
+
+<h3>SECTION III.&mdash;ATOLLS OF THE MALDIVA ARCHIPELAGO&mdash;GREAT CHAGOS
+BANK.</h3>
+
+<p class="letter">
+Maldiva Archipelago.&mdash;Ring-formed reefs, marginal and central.&mdash;Great
+depths in the lagoons of the southern atolls.&mdash;Reefs in the lagoons all
+rising to the surface.&mdash;Position of islets and breaches in the reefs, with
+respect to the prevalent winds and action of the waves.&mdash;Destruction of
+islets.&mdash;Connection in the position and submarine foundation of distinct
+atolls.&mdash;The apparent disseverment of large atolls.&mdash;The Great Chagos
+Bank.&mdash;Its submerged condition and extraordinary structure.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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 (<a href="#illus02">Plate
+II., Figure 4</a>), 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<a
+href="#fn1.25" name="fnref1.25"><sup>[25]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.25"></a> <a href="#fnref1.25">[25]</a> Captain Moresby informs me
+that <i>Millepora complanata</i> is one of the commonest kinds on the outer
+margin, as it is at Keeling atoll.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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,&mdash;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!
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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&mdash;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.
+</p>
+
+<p>
+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&rsquo;s words) &ldquo;entirely covered with live coral and
+madrepore.&rdquo; 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.
+</p>
+
+<p>
+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&rsquo;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.
+</p>
+
+<p>
+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,<a
+href="#fn1.26" name="fnref1.26"><sup>[26]</sup></a> &ldquo;nothing more than a
+half-drowned atoll.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn1.26"></a> <a href="#fnref1.26">[26]</a> 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.
+</p>
+
+<hr />
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap02"></a>
+
+CHAPTER II.<br/>
+
+BARRIER REEFS.</h2>
+
+<p class="letter">
+Closely resemble in general form and structure atoll-reefs.&mdash;Width and
+depth of the lagoon-channels.&mdash;Breaches through the reef in front of
+valleys, and generally on the leeward side.&mdash;Checks to the filling up of
+the lagoon-channels.&mdash;Size and constitution of the encircled
+islands.&mdash; Number of islands within the same reef.&mdash;Barrier-reefs of
+New Caledonia and Australia.&mdash;Position of the reef relative to the slope
+of the adjoining land.&mdash;Probable great thickness of barrier-reefs.
+</p>
+
+<p>
+The term &ldquo;barrier&rdquo; 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.
+</p>
+
+<p>
+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.,<a href="#fn2.1" name="fnref2.1"><sup>[1]</sup></a> 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<a
+href="#fn2.2" name="fnref2.2"><sup>[2]</sup></a> found no bottom with 150
+fathoms, at two ships&rsquo; length from the reef; so that the slope here must
+be nearly as precipitous as off the Maldiva atolls.
+</p>
+
+<p class="footnote">
+<a name="fn2.1"></a> <a href="#fnref2.1">[1]</a> The authorities from which
+these charts have been reduced, together with some remarks on them and
+descriptive of the Plates, are given separately.
+</p>
+
+<p class="footnote">
+<a name="fn2.2"></a> <a href="#fnref2.2">[2]</a> Dalrymple, &ldquo;Hydrog.
+Mem.&rdquo; volume iii.
+</p>
+
+<p>
+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,&mdash;a structure
+absolutely similar to that described by Chamisso in the Marshall atolls.
+</p>
+
+<p>
+In the Society Archipelago, Ellis<a href="#fn2.3"
+name="fnref2.3"><sup>[3]</sup></a> 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,<a href="#fn2.4"
+name="fnref2.4"><sup>[4]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn2.3"></a> <a href="#fnref2.3">[3]</a> Consult, on this and other
+points, the &ldquo;Polynesian Researches,&rdquo; by the Rev. W. Ellis, an
+admirable work, full of curious information.
+</p>
+
+<p class="footnote">
+<a name="fn2.4"></a> <a href="#fnref2.4">[4]</a> See &ldquo;Hydrographical
+Mem.&rdquo; and the &ldquo;Atlas of the Voyage of the <i>Astrolabe</i>,&rdquo;
+by Captain Dumont D&rsquo;Urville, page 428.
+</p>
+
+<p>
+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&rsquo;s<a href="#fn2.5"
+name="fnref2.5"><sup>[5]</sup></a> 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 (<a href="#illus01">Plate I.,
+Figure 5</a>) 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&rsquo;s &ldquo;Memoir on the Pacific,&rdquo; 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,&mdash;for in atolls
+there is no included land to influence the position of the breaches.
+</p>
+
+<p class="footnote">
+<a name="fn2.5"></a> <a href="#fnref2.5">[5]</a> See the chart in volume i. of
+Hawkesworth&rsquo;s 4to edition of &ldquo;Cook&rsquo;s First Voyage.&rdquo;
+</p>
+
+<p>
+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 (<a href="#illus01">Plate
+I., Figure 6</a>), in the Society group, it has been almost effected, so that
+there remains only one harbour for small craft.
+</p>
+
+<p>
+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,&mdash;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.
+</p>
+
+<p>
+The islands lying within reefs of this class, are of very various heights.
+Tahiti<a href="#fn2.6" name="fnref2.6"><sup>[6]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn2.6"></a> <a href="#fnref2.6">[6]</a> The height of Tahiti is given
+from Captain Beechey; Maurua from Mr. F.D. Bennett (&ldquo;Geograph.
+Journ.&rdquo; volume viii., page 220); Aitutaki from measurements made on board
+the <i>Beagle;</i> 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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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<a href="#fn2.7" name="fnref2.7"><sup>[7]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn2.7"></a> <a href="#fnref2.7">[7]</a> Flinders&rsquo; &ldquo;Voyage
+to Terra Australis,&rdquo; volume ii., page 88.
+</p>
+
+<div class="fig" style="width:100%;">
+<img src="images/img04.jpg" width="500" height="254" alt="[Illustration]" />
+</div>
+
+<p>
+1. VANIKORO, from the &ldquo;Atlas of the Voyage of the
+<i>Astrolabe</i>,&rdquo; by D. D&rsquo;Urville.
+</p>
+
+<p>
+2. GAMBIER ISLAND, from Beechey.
+</p>
+
+<p>
+3. MAURUA, from the &ldquo;Atlas of the Voyage of the <i>Coquille</i>,&rdquo;
+by Duperrey.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+AA.&mdash;Outer edge of the coral-reefs, where the sea breaks.
+</p>
+
+<p>
+BB.&mdash;The shore of the encircled islands.)
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+I must observe that if the sections had been taken in any other direction
+across these islands, or across other encircled islands,<a href="#fn2.8"
+name="fnref2.8"><sup>[8]</sup></a> the result would have been the same. In the
+succeeding chapter it will be shown that reef-building polypifers cannot
+flourish at great depths,&mdash;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 <i>apparent</i> difficulty&mdash;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,<a href="#fn2.9"
+name="fnref2.9"><sup>[9]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn2.8"></a> <a href="#fnref2.8">[8]</a> 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 &ldquo;Atlas of the Voyage of the <i>Coquille</i>,&rdquo; by
+Duperrey. The depth of the lagoon-channel is exaggerated.
+</p>
+
+<p class="footnote">
+<a name="fn2.9"></a> <a href="#fnref2.9">[9]</a> The Rev. D. Tyerman and Mr.
+Bennett (&ldquo;Journal of Voyage and Travels,&rdquo; volume i., page 215) have
+briefly suggested this explanation of the origin of the encircling reefs of the
+Society Islands.
+</p>
+
+<p>
+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;<a
+href="#fn2.10" name="fnref2.10"><sup>[10]</sup></a> 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 <i>AA</i> 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.
+</p>
+
+<p class="footnote">
+<a name="fn2.10"></a> <a href="#fnref2.10">[10]</a> I have been informed that
+this is the case by Dr. Allan of Forres, who has visited this group.
+</p>
+
+<hr />
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap03"></a>
+
+CHAPTER III.<br/>
+
+FRINGING OR SHORE-REEFS.</h2>
+
+<p class="letter">
+Reefs of Mauritius.&mdash;Shallow channel within the reef.&mdash;Its slow
+filling up.&mdash;Currents of water formed within it.&mdash;Upraised
+reefs.&mdash;Narrow fringing-reefs in deep seas.&mdash;Reefs on the coast of
+East Africa and of Brazil.&mdash;Fringing-reefs in very shallow seas, round
+banks of sediment and on worn-down islands.&mdash;Fringing-reefs affected by
+currents of the sea.&mdash; Coral coating the bottom of the sea, but not
+forming reefs.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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,<a href="#fn3.1" name="fnref3.1"><sup>[1]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn3.1"></a> <a href="#fnref3.1">[1]</a> This fact is stated on the
+authority of the Officier du Roi, in his extremely interesting &ldquo;Voyage à
+l&rsquo;Isle de France,&rdquo; undertaken in 1768. According to Captain
+Carmichael (Hooker&rsquo;s &ldquo;Bot. Misc.&rdquo; volume ii., page 316) on
+one part of the coast there is a space for sixteen miles without a reef.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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<a href="#fn3.2"
+name="fnref3.2"><sup>[2]</sup></a> 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,<a href="#fn3.3"
+name="fnref3.3"><sup>[3]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn3.2"></a> <a href="#fnref3.2">[2]</a> Captain Grey&rsquo;s
+&ldquo;Journal of Two Expeditions,&rdquo; volume i. page 369.
+</p>
+
+<p class="footnote">
+<a name="fn3.3"></a> <a href="#fnref3.3">[3]</a> I have described this singular
+structure in the &ldquo;London and Edinburgh Phil. Mag.&rdquo; October 1841.
+</p>
+
+<p>
+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<a href="#fn3.4" name="fnref3.4"><sup>[4]</sup></a> of an
+interesting pedestrian tour round the Mauritius, seems to have met with a
+structure of this kind: he says &ldquo;J&rsquo;observai que là, où la mer
+étale, indépendamment des rescifs du large, il y à terre <i>une espèce
+d&rsquo;effoncement</i> ou chemin couvert naturel. On y pourrait mettre du
+canon,&rdquo; etc. In another place he adds, &ldquo;Avant de passer le Cap, on
+remarque un gros banc de corail elévé de plus de quinze pieds: c&rsquo;est une
+espèce de rescif, que la mer abandonné, il regne au pied une longue flaque
+d&rsquo;eau, dont on pourrait faire un bassin pour de petits vaisseaux.&rdquo;
+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.
+</p>
+
+<p class="footnote">
+<a name="fn3.4"></a> <a href="#fnref3.4">[4]</a> &ldquo;Voyage à l&rsquo;Isle
+de France, par un Officier du Roi,&rdquo; part i., pages 192, 200.
+</p>
+
+<p>
+Many islands<a href="#fn3.5" name="fnref3.5"><sup>[5]</sup></a> 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,<a href="#fn3.6" name="fnref3.6"><sup>[6]</sup></a> where it is only
+fifty yards in width: the sea round these islands is very deep.
+</p>
+
+<p class="footnote">
+<a name="fn3.5"></a> <a href="#fnref3.5">[5]</a> I may give Cuba, as another
+instance; Mr. Taylor (Loudon&rsquo;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.&rdquo;
+</p>
+
+<p class="footnote">
+<a name="fn3.6"></a> <a href="#fnref3.6">[6]</a> Mauti is described by Lord
+Byron in the voyage of H.M.S. <i>Blonde</i>, and Elizabeth Island by Captain
+Beechey.
+</p>
+
+<p>
+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&#x2032; to 1° 45&#x2032; 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&#x2032; N.) &ldquo;the port is formed,&rdquo; it is said,<a
+href="#fn3.7" name="fnref3.7"><sup>[7]</sup></a> &ldquo;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;&rdquo; 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 &ldquo;for canoes and small craft,&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn3.7"></a> <a href="#fnref3.7">[7]</a> Owen&rsquo;s
+&ldquo;Africa,&rdquo; volume i., page 357, from which work the foregoing facts
+are likewise taken.
+</p>
+
+<p>
+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<a href="#fn3.8"
+name="fnref3.8"><sup>[8]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn3.8"></a> <a href="#fnref3.8">[8]</a> See Baron Roussin&rsquo;s
+&ldquo;Pilote du Brésil,&rdquo; and accompanying hydrographical memoir.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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 (<a
+href="#illus03">Plate III.</a>) is coloured, the exceptional cases will be
+enumerated.
+</p>
+
+<hr />
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap04"></a>
+
+CHAPTER IV.<br/>
+
+ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.</h2>
+
+<p>
+In this chapter I will give all the facts which I have collected, relating to
+the distribution of coral-reefs,&mdash;to the conditions favourable to their
+increase,&mdash;to the rate of their growth,&mdash;and to the depth at which
+they are formed.
+</p>
+
+<p>
+These subjects have an important bearing on the theory of the origin of the
+different classes of coral-reefs.
+</p>
+
+<p>
+SECTION I.&mdash;ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
+FAVOURABLE TO THEIR INCREASE.
+</p>
+
+<p>
+With regard to the limits of latitude, over which coral-reefs extend, I have
+nothing new to add. The Bermuda Islands, in 32° 15&#x2032; 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&#x2032;, 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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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 &ldquo;Beagle&rdquo; 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<a href="#fn4.1"
+name="fnref4.1"><sup>[1]</sup></a> 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,<a href="#fn4.2" name="fnref4.2"><sup>[2]</sup></a> to
+the coldness of the currents from the south, but the Gulf of Panama is one of
+the hottest pelagic districts in the world.<a href="#fn4.3"
+name="fnref4.3"><sup>[3]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.1"></a> <a href="#fnref4.1">[1]</a> I have been informed that this
+is the case, by Lieutenant Ryder, R.N., and others who have had ample
+opportunities for observation.
+</p>
+
+<p class="footnote">
+<a name="fn4.2"></a> <a href="#fnref4.2">[2]</a> 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 <i>Beagle</i>, 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.3"></a> <a href="#fnref4.3">[3]</a> Humboldt&rsquo;s
+&ldquo;Personal Narrative,&rdquo; volume vii., page 434.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+I have been assured by several people, that there are no coral-reefs on the
+west coast of Africa,<a href="#fn4.4" name="fnref4.4"><sup>[4]</sup></a> 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&rsquo;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,&mdash;nay more, that there is a struggle for each station, between the
+different orders of nature,&mdash;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.
+</p>
+
+<p class="footnote">
+<a name="fn4.4"></a> <a href="#fnref4.4">[4]</a> It might be concluded, from a
+paper by Captain Owen (&ldquo;Geographical Journal&rdquo;, volume ii., page
+89), that the reefs off Cape St. Anne and the Sherboro&rsquo; 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.
+</p>
+
+<p>
+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)
+&ldquo;which form rocks measuring several fathoms in thickness,&rdquo; prefer,
+according to Chamisso,<a href="#fn4.5" name="fnref4.5"><sup>[5]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.5"></a> <a href="#fnref4.5">[5]</a> Kotzebue&rsquo;s &ldquo;First
+Voyage&rdquo; (English Translation) volume iii., pages 142, 143, 331.
+</p>
+
+<p>
+Ehrenberg<a href="#fn4.6" name="fnref4.6"><sup>[6]</sup></a> 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.<a href="#fn4.7"
+name="fnref4.7"><sup>[7]</sup></a> 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:<a href="#fn4.8"
+name="fnref4.8"><sup>[8]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.6"></a> <a href="#fnref4.6">[6]</a> Ehrenberg, &ldquo;Über die
+Natür und Bildung der Corallen Bänke im rothen Meere,&rdquo; page 49.
+</p>
+
+<p class="footnote">
+<a name="fn4.7"></a> <a href="#fnref4.7">[7]</a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.8"></a> <a href="#fnref4.8">[8]</a> &ldquo;Annales des Sciences
+Naturelles,&rdquo; tome vi., pages 276, 278.&mdash;&ldquo;Là où les ondes sont
+agitées, les Lytophytés ne peuvent travailler, parce qu&rsquo;elles
+détruiraient leurs fragiles édifices,&rdquo; &amp;c.
+</p>
+
+<p>
+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,&mdash;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,<a href="#fn4.9" name="fnref4.9"><sup>[9]</sup></a> have been
+observed.
+</p>
+
+<p class="footnote">
+<a name="fn4.9"></a> <a href="#fnref4.9">[9]</a> Ehrenberg, &ldquo;Über die
+Natür,&rdquo; etc., etc., page 46.
+</p>
+
+<p>
+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.<a href="#fn4.10" name="fnref4.10"><sup>[10]</sup></a> 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,<a href="#fn4.11" name="fnref4.11"><sup>[11]</sup></a> 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&rsquo;s in Madagascar, &ldquo;cleared out and made a beautiful little port
+at that place.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.10"></a> <a href="#fnref4.10">[10]</a> Ibid., page 49.
+</p>
+
+<p class="footnote">
+<a name="fn4.11"></a> <a href="#fnref4.11">[11]</a> Captain Owen on the
+Geography of the Maldiva Islands, &ldquo;Geographical Journal&rdquo;, volume
+ii., page 88.
+</p>
+
+<p>
+In the last chapter I remarked that fringing-reefs are almost universally
+breached, where streams enter the sea.<a href="#fn4.12"
+name="fnref4.12"><sup>[12]</sup></a> 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,<a href="#fn4.13" name="fnref4.13"><sup>[13]</sup></a>
+speaking of one at Raiatea, says, &ldquo;like all the rest, it is very steep on
+both sides.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.12"></a> <a href="#fnref4.12">[12]</a> Lieutenant Wellstead and
+others have remarked that this is the case in the Red Sea; Dr. Rüppell
+(&ldquo;Reise in Abyss.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.13"></a> <a href="#fnref4.13">[13]</a> Cook&rsquo;s &ldquo;First
+Voyage,&rdquo; volume ii., page 271 (Hawkesworth&rsquo;s edition).)
+</p>
+
+<p>
+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 <i>Porites</i> and <i>Millepora
+complanata</i> 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 <i>Millepora
+alcicornis</i> 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,<a href="#fn4.14"
+name="fnref4.14"><sup>[14]</sup></a> between the great branching corals of the
+Red Sea, and those on the reefs of the Maldiva atolls.
+</p>
+
+<p class="footnote">
+<a name="fn4.14"></a> <a href="#fnref4.14">[14]</a> Captain Moresby on the
+Northern Maldiva atolls, &ldquo;Geographical Journal&rdquo;, volume v., page
+401.
+</p>
+
+<p>
+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;<a href="#fn4.15"
+name="fnref4.15"><sup>[15]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.15"></a> <a href="#fnref4.15">[15]</a> 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.
+</p>
+
+<p>
+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&mdash;an unparalleled case, I believe, in an atoll of such large
+size,&mdash;and as the strip of land is for considerable spaces more than half
+a mile wide&mdash;also a very unusual circumstance,&mdash;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.
+</p>
+
+<p>
+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,&mdash;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.
+</p>
+
+<p>
+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,&mdash;to those situated at different depths,&mdash;to those
+forming the ring of an atoll or those on the knolls within a lagoon,&mdash;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.
+</p>
+
+<h3>SECTION II.&mdash;ON THE RATE OF GROWTH OF CORAL-REEFS.</h3>
+
+<p>
+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<a href="#fn4.16"
+name="fnref4.16"><sup>[16]</sup></a> 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,<a href="#fn4.17"
+name="fnref4.17"><sup>[17]</sup></a> with respect to the thickness of some
+upraised beds of coral, which they examined at Timor and some other places.
+Ehrenberg<a href="#fn4.18" name="fnref4.18"><sup>[18]</sup></a> 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<a href="#fn4.19" name="fnref4.19"><sup>[19]</sup></a> 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<a
+href="#fn4.20" name="fnref4.20"><sup>[20]</sup></a> 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<a
+href="#fn4.21" name="fnref4.21"><sup>[21]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.16"></a> <a href="#fnref4.16">[16]</a> Ehrenberg, as before cited,
+pages 39, 46, and 50.
+</p>
+
+<p class="footnote">
+<a name="fn4.17"></a> <a href="#fnref4.17">[17]</a> &ldquo;Annales des Sciences
+Nat.&rdquo; tom. vi., page 28.
+</p>
+
+<p class="footnote">
+<a name="fn4.18"></a> <a href="#fnref4.18">[18]</a> Ehrenberg, ut sup., page
+42.
+</p>
+
+<p class="footnote">
+<a name="fn4.19"></a> <a href="#fnref4.19">[19]</a> Lyell&rsquo;s
+&ldquo;Principles of Geology,&rdquo; book iii., chapter xviii.
+</p>
+
+<p class="footnote">
+<a name="fn4.20"></a> <a href="#fnref4.20">[20]</a> Beechey&rsquo;s
+&ldquo;Voyage to the Pacific,&rdquo; chapter viii.
+</p>
+
+<p class="footnote">
+<a name="fn4.21"></a> <a href="#fnref4.21">[21]</a> Ehrenberg, ut sup., page
+43.
+</p>
+
+<p>
+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,&mdash;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,<a href="#fn4.22" name="fnref4.22"><sup>[22]</sup></a>
+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.
+</p>
+
+<p class="footnote">
+<a name="fn4.22"></a> <a href="#fnref4.22">[22]</a> &ldquo;Geological
+Transactions,&rdquo; volume v., page 113.
+</p>
+
+<p>
+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&rsquo;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,<a href="#fn4.23"
+name="fnref4.23"><sup>[23]</sup></a> to have a similar structure, and its
+shores are about forty feet high: some of the Cook Islands also appear<a
+href="#fn4.24" name="fnref4.24"><sup>[24]</sup></a> to be similarly composed.
+Captain Belcher, R.N., in a letter which Captain Beaufort showed me at the
+admiralty, speaking of Bow atoll, says, &ldquo;I have succeeded in boring
+forty-five feet through coral-sand, when the auger became jammed by the falling
+in of the surrounding <i>creamy</i> matter.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.23"></a> <a href="#fnref4.23">[23]</a> Forster&rsquo;s
+&ldquo;Voyage round the World with Cook,&rdquo; volume ii., pages 163, 167.
+</p>
+
+<p class="footnote">
+<a name="fn4.24"></a> <a href="#fnref4.24">[24]</a> Williams&rsquo;s
+&ldquo;Narrative of Missionary Enterprise,&rdquo; page 30.
+</p>
+
+<p>
+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
+<i>Millepora complanata</i>, as is now taking place; nor these latter without
+encroaching on the strongly branched Madreporet, the <i>Millepora
+alcicornis</i>, 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 &ldquo;an incontestable proof,&rdquo; as observed by Cook,<a
+href="#fn4.25" name="fnref4.25"><sup>[25]</sup></a> &ldquo;that the island has
+been produced by accessions from the sea, and is in a state of increase.&rdquo;
+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 &ldquo;reef of rocks&rdquo; into a lagoon-island, fourteen miles in
+length, with &ldquo;one of its sides covered nearly the whole way with high
+trees.&rdquo;<a href="#fn4.26" name="fnref4.26"><sup>[26]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.25"></a> <a href="#fnref4.25">[25]</a> Cook&rsquo;s &ldquo;Third
+Voyage,&rdquo; book III., chapter x.
+</p>
+
+<p class="footnote">
+<a name="fn4.26"></a> <a href="#fnref4.26">[26]</a> Beechey&rsquo;s
+&ldquo;Voyage to the Pacific,&rdquo; chapter vii. and viii.
+</p>
+
+<p>
+After these considerations, I attach little importance, as indicating the
+ordinary and still less the possible rate of <i>outward</i> 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,<a href="#fn4.27"
+name="fnref4.27"><sup>[27]</sup></a>&mdash;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.
+</p>
+
+<p class="footnote">
+<a name="fn4.27"></a> <a href="#fnref4.27">[27]</a> F. Lutké&rsquo;s
+&ldquo;Voyage autour du Monde.&rdquo; In the group Elato, however, it appears
+that what is now the islet Falipi, is called in Cantova&rsquo;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.
+</p>
+
+<p>
+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 <i>upward</i> 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.
+</p>
+
+<p>
+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
+&ldquo;<i>entirely covered with live coral and Madrepore</i>.&rdquo; 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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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. &ldquo;To ascertain the
+rise and progress of the coral-family, and fix the number of species met with
+at Foul Point (latitude 17° 40&#x2032;) twenty species of coral were taken off
+the reef and planted apart on a sand-bank <i>three feet deep at low water</i>.
+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.&rdquo; The loss of Dr.
+Allan&rsquo;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<a href="#fn4.28" name="fnref4.28"><sup>[28]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.28"></a> <a href="#fnref4.28">[28]</a> It is stated by De la Beche
+(&ldquo;Geological Manual,&rdquo; 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.
+</p>
+
+<p>
+A fact, communicated to me by Lieutenant Wellstead, I.N., in some degree
+corroborates the result of Dr. Allan&rsquo;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, <i>two feet</i> 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.<a
+href="#fn4.29" name="fnref4.29"><sup>[29]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn4.29"></a> <a href="#fnref4.29">[29]</a> Mr. Stutchbury (&ldquo;West
+of England Journal&rdquo;, No. I., page 50.) has described a specimen of
+Agaricia, &ldquo;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.&rdquo; 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 <i>Beagle</i>, 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 <i>Chama gigas</i> 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
+&ldquo;flat&rdquo; 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. &ldquo;Voyage de
+la &lsquo;Coquille&rsquo;&rdquo;), has remarked, &ldquo;que constamment ses
+valves étaient engagés complètement dans la masse des Madrepores.&rdquo;
+</p>
+
+<p>
+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&rsquo;s crust,
+or to the more precise but less important one of a cycle of years.
+</p>
+
+<h3>SECTION III.&mdash;ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN
+LIVE.</h3>
+
+<p>
+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,&mdash;apparently <i>M. corymbosa</i> and <i>pocillifera</i>,&mdash;
+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<a href="#fn4.30" name="fnref4.30"><sup>[30]</sup></a> 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 <i>Millepora alcicornis</i>,
+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
+<i>S. subulata</i>, 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, <i>Pocillopora
+verrucosa</i>, 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 <i>M. pocillifera</i>, 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 <i>gradual</i> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.30"></a> <a href="#fnref4.30">[30]</a> Since the preceding pages
+were printed off, I have received from Mr. Lyell a very interesting pamphlet,
+entitled &ldquo;Remarks upon Coral Formations,&rdquo; 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,
+&ldquo;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.&rdquo; 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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+With regard to the coral-reefs in the Red Sea, Ehrenberg has the following
+passage:&mdash;&ldquo;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.&rdquo;<a href="#fn4.31" name="fnref4.31"><sup>[31]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.31"></a> <a href="#fnref4.31">[31]</a> Ehrenberg, &ldquo;Über die
+Natür,&rdquo; &amp;c., page 50.
+</p>
+
+<p>
+MM. Quoy and Gaimard<a href="#fn4.32" name="fnref4.32"><sup>[32]</sup></a>
+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. <i>Millepora alcicornis</i> 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 <i>Sideropora scabra</i>
+(Porites of Lamarck) brought up alive from 17 fathoms. Mr. Couthouy<a
+href="#fn4.33" name="fnref4.33"><sup>[33]</sup></a> 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:<a href="#fn4.34" name="fnref4.34"><sup>[34]</sup></a>
+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.<a href="#fn4.35"
+name="fnref4.35"><sup>[35]</sup></a> 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&rsquo;s &ldquo;British
+Animals,&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.32"></a> <a href="#fnref4.32">[32]</a> &ldquo;Annales des Sci.
+Nat.&rdquo; tom. vi.
+</p>
+
+<p class="footnote">
+<a name="fn4.33"></a> <a href="#fnref4.33">[33]</a> &ldquo;Remarks on Coral
+Formations,&rdquo; page 12.
+</p>
+
+<p class="footnote">
+<a name="fn4.34"></a> <a href="#fnref4.34">[34]</a> I am indebted to Mr. Stokes
+for having kindly communicated this fact to me, together with much other
+valuable information.
+</p>
+
+<p class="footnote">
+<a name="fn4.35"></a> <a href="#fnref4.35">[35]</a> 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.
+</p>
+
+<table summary="" style="margin-top: 1em; margin-bottom: 1em" border="1">
+
+<tr>
+<td>NAME OF ZOOPHYTE</td><td>Depth in fathoms</td><td>COUNTRY and S. LATITUDE</td><td>AUTHORITY</td>
+</tr>
+
+<tr>
+<td>SERTULARIA</td><td>40</td><td>Cape Horn 66°</td><td>Where none is given, the observation is my own</td>
+</tr>
+
+<tr>
+<td>CELLARIA</td><td>ditto</td><td>Ditto</td><td></td>
+</tr>
+
+<tr>
+<td>CELLARIA, A minute scarlet encrusting species found living</td><td>190</td><td>Keeling Atoll, 12°</td><td></td>
+</tr>
+
+<tr>
+<td>CELLARIA, An allied, small stony sub-generic form</td><td>48</td><td>St Cruz Riv. 50°</td><td></td>
+</tr>
+
+<tr>
+<td>A coral allied to VINCULARIA, with eight rows of cells</td><td>40</td><td>Cape Horn</td><td></td>
+</tr>
+
+<tr>
+<td>TUBULIPORA, near to T. patina</td><td>40</td><td>Cape Horn</td><td></td>
+</tr>
+
+<tr>
+<td>TUBULIPORA, near to T. patina</td><td>94</td><td>East Chiloe 43°</td><td></td>
+</tr>
+
+<tr>
+<td>CELLEPORA, several species, and allied sub-generic forms</td><td>40</td><td>Cape Horn</td><td></td>
+</tr>
+
+<tr>
+<td>CELLEPORA, several species, and allied sub-generic forms</td><td>40 and 57</td><td>Chonos Archipelago 45°</td><td></td>
+</tr>
+
+<tr>
+<td>CELLEPORA, several species, and allied sub-generic forms</td><td>48</td><td>St Cruz 50°</td><td></td>
+</tr>
+
+<tr>
+<td>ESCHARA</td><td>30</td><td>Tierra del Fuego 53°</td><td></td>
+</tr>
+
+<tr>
+<td>ESCHARA</td><td>48</td><td>St Cruz R. 50°</td><td></td>
+</tr>
+
+<tr>
+<td>RETEPORA</td><td>40</td><td>Cape Horn</td><td></td>
+</tr>
+
+<tr>
+<td>RETEPORA</td><td>100</td><td>Cape of Good Hope 34°</td><td>Quoy and Gaimard, &ldquo;Ann. Scien. Nat.&rdquo; tome vi., page 284.</td>
+</tr>
+
+<tr>
+<td>MILLEPORA, a strong coral with cylindrical branches, of a pink colour, about
+two inches high, resembling in the form of its orifices M. aspera of Lamarck</td><td>94 and 30</td><td>E. Chiloe 43°<br/>
+
+Tierra del Fuego 53°</td><td></td>
+</tr>
+
+<tr>
+<td>CORALIUM</td><td>120</td><td>Barbary 33° N.</td><td>Peyssonel in paper read to Royal Society May 1752.</td>
+</tr>
+
+<tr>
+<td>ANTIPATHES</td><td>16</td><td>Chonos 45°</td><td></td>
+</tr>
+
+<tr>
+<td>GORGONIA (or an allied form)</td><td>160</td><td>Abrolhos on the coast of Brazil 18°</td><td>Captain Beechey informed me of this fact in a letter.</td>
+</tr>
+
+</table>
+
+<p class="footnote">
+Ellis (&ldquo;Nat. Hist. of Coralline,&rdquo; page 96) states that Ombellularia
+was procured in latitude 79° N. <i>sticking</i> to a <i>line</i> 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&rsquo;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.)
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+It has been argued<a href="#fn4.36" name="fnref4.36"><sup>[36]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn4.36"></a> <a href="#fnref4.36">[36]</a> &ldquo;Journal of the Royal
+Geographical Society,&rdquo; 1831, page 218.
+</p>
+
+<hr />
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap05"></a>
+
+CHAPTER V.<br/>
+
+THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF CORAL-REEFS.</h2>
+
+<p class="letter">
+The atolls of the larger archipelagoes are not formed on submerged craters, or
+on banks of sediment.&mdash;Immense areas interspersed with atolls.&mdash;Their
+subsidence.&mdash;The effects of storms and earthquakes on atolls.&mdash;Recent
+changes in their state.&mdash;The origin of barrier-reefs and of
+atolls.&mdash;Their relative forms.&mdash;The step-formed ledges and walls
+round the shores of some lagoons.&mdash;The ring-formed reefs of the Maldiva
+atolls.&mdash;The submerged condition of parts or of the whole of some annular
+reefs.&mdash;The disseverment of large atolls.&mdash;The union of atolls by
+linear reefs.&mdash;The Great Chagos Bank.&mdash;Objections from the area and
+amount of subsidence required by the theory, considered.&mdash;The probable
+composition of the lower parts of atolls.
+</p>
+
+<p>
+The naturalists who have visited the Pacific, seem to have had their attention
+riveted by the lagoon-islands, or atolls,&mdash;those singular rings of
+coral-land which rise abruptly out of the unfathomable ocean&mdash;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 (<a
+href="#illus01">Plate I., Figure 4</a>); or like that of Menchikoff Island (<a
+href="#illus02">Plate II., Figure 3.</a>), 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,&mdash;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.
+</p>
+
+<p>
+An earlier and better theory was proposed by Chamisso;<a href="#fn5.1"
+name="fnref5.1"><sup>[1]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn5.1"></a> <a href="#fnref5.1">[1]</a> Kotzebue&rsquo;s &ldquo;First
+Voyage,&rdquo; volume iii., page 331.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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&rsquo;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<a href="#fn5.2"
+name="fnref5.2"><sup>[2]</sup></a> with respect to the Indian Ocean, that in
+the following cases the term &ldquo;low island&rdquo; 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<a href="#fn5.3" name="fnref5.3"><sup>[3]</sup></a>
+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.
+</p>
+
+<p class="footnote">
+<a name="fn5.2"></a> <a href="#fnref5.2">[2]</a> See also Captain Owen&rsquo;s
+and Lieutenant Wood&rsquo;s papers in the &ldquo;Geographical Journal&rdquo;,
+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.
+</p>
+
+<p class="footnote">
+<a name="fn5.3"></a> <a href="#fnref5.3">[3]</a> I find from Mr.
+Couthouy&rsquo;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 <i>Beagle</i>, whilst
+sailing by, is 114 feet from the <i>summit of the trees</i> to the
+water&rsquo;s edge. This island appeared to resemble the other atolls of the
+group.
+</p>
+
+<p>
+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,&mdash;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.
+</p>
+
+<p>
+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&rsquo;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&rsquo;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&rsquo;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,<a href="#fn5.4" name="fnref5.4"><sup>[4]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn5.4"></a> <a href="#fnref5.4">[4]</a> 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&rsquo;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&rsquo;s (&ldquo;Geolog. Trans.&rdquo; 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&rsquo;s &ldquo;Observ.&rdquo; 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!
+</p>
+
+<p>
+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,<a href="#fn5.5"
+name="fnref5.5"><sup>[5]</sup></a> even in the first edition of his
+&ldquo;Principles of Geology,&rdquo; 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!
+</p>
+
+<p class="footnote">
+<a name="fn5.5"></a> <a href="#fnref5.5">[5]</a> &ldquo;Principles of
+Geology,&rdquo; sixth edition, volume iii., page 386.
+</p>
+
+<p>
+Mr. Williams<a href="#fn5.6" name="fnref5.6"><sup>[6]</sup></a> 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&rsquo;s &ldquo;Voyage&rdquo; 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 &ldquo;he saw the sea at Radack rise to the feet of the cocoa-nut
+trees; but it was conjured in time.&rdquo;<a href="#fn5.7"
+name="fnref5.7"><sup>[7]</sup></a> A storm lately entirely swept away two of
+the Caroline islands, and converted them into shoals; it partly, also,
+destroyed two other islands.<a href="#fn5.8" name="fnref5.8"><sup>[8]</sup></a>
+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: &ldquo;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.&rdquo;<a
+href="#fn5.9" name="fnref5.9"><sup>[9]</sup></a> 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&rsquo;s.
+Whitsunday Island is described by Wallis as &ldquo;about four miles long, and
+three wide,&rdquo; now it is only one mile and a half long. The appearance of
+Gloucester Island, in Captain Beechey&rsquo;s words,<a href="#fn5.10"
+name="fnref5.10"><sup>[10]</sup></a> &ldquo;has been accurately described by
+its discoverer, but its present form and extent differ materially.&rdquo;
+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, &ldquo;that the high tides and violent currents were
+diminishing the number of the islands:&rdquo;<a href="#fn5.11"
+name="fnref5.11"><sup>[11]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn5.6"></a> <a href="#fnref5.6">[6]</a> Williams&rsquo;s
+&ldquo;Narrative of Missionary Enterprise,&rdquo; page 31.
+</p>
+
+<p class="footnote">
+<a name="fn5.7"></a> <a href="#fnref5.7">[7]</a> Kotzebue&rsquo;s &ldquo;First
+Voyage,&rdquo; volume iii., page 168.
+</p>
+
+<p class="footnote">
+<a name="fn5.8"></a> <a href="#fnref5.8">[8]</a> M. Desmoulins in
+&ldquo;Comptes Rendus,&rdquo; 1840, page 837.
+</p>
+
+<p class="footnote">
+<a name="fn5.9"></a> <a href="#fnref5.9">[9]</a> &ldquo;West of England
+Journal&rdquo;, No. I., page 35.
+</p>
+
+<p class="footnote">
+<a name="fn5.10"></a> <a href="#fnref5.10">[10]</a> Beechey&rsquo;s
+&ldquo;Voyage to the Pacific,&rdquo; chapter vii., and Wallis&rsquo;s
+&ldquo;Voyage in the &lsquo;Dolphin&rsquo;,&rdquo; chapter iv.
+</p>
+
+<p class="footnote">
+<a name="fn5.11"></a> <a href="#fnref5.11">[11]</a> See an extract from
+Pyrard&rsquo;s Voyage in Captain Owen&rsquo;s paper on the Maldiva Archipelago,
+in the &ldquo;Geographical Journal&rdquo;, volume ii., page 84.
+</p>
+
+<p>
+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<a href="#fn5.12" name="fnref5.12"><sup>[12]</sup></a>
+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&rsquo;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.
+</p>
+
+<p class="footnote">
+<a name="fn5.12"></a> <a href="#fnref5.12">[12]</a> See Chamisso, in
+Kotzebue&rsquo;s &ldquo;First Voyage,&rdquo; volume iii., pages 182 and 136.
+</p>
+
+<p>
+The facts stand thus;&mdash;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,&mdash;what has given to each class of reef its peculiar form?
+</p>
+
+<div class="fig" style="width:100%;">
+<img src="images/img05.jpg" width="700" height="202" alt="[Illustration]" />
+</div>
+
+<p>
+AA&mdash;Outer edge of the reef at the level of the sea.
+</p>
+
+<p>
+BB&mdash;Shores of the island.
+</p>
+
+<p>
+A&#x2032;A&#x2032;&mdash;Outer edge of the reef, after its upward growth during
+a period of subsidence.
+</p>
+
+<p>
+CC&mdash;The lagoon-channel between the reef and the shores of the now
+encircled land.
+</p>
+
+<p>
+B&#x2032;B&#x2032;&mdash;The shores of the encircled island.
+</p>
+
+<p>
+N.B.&mdash;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.
+</p>
+
+<div class="fig" style="width:100%;">
+<img src="images/img06.jpg" width="700" height="267" alt="[Illustration]" />
+</div>
+
+<p>
+A&#x2032;A&#x2032;&mdash;Outer edges of the barrier-reef at the level of the
+sea. The cocoa-nut trees represent coral-islets formed on the reef.
+</p>
+
+<p>
+CC&mdash;The lagoon-channel.
+</p>
+
+<p>
+B&#x2032;B&#x2032;&mdash;The shores of the island, generally formed of low
+alluvial land and of coral detritus from the lagoon-channel.
+</p>
+
+<p>
+A&#x2033;A&#x2033;&mdash;The outer edges of the reef now forming an atoll.
+</p>
+
+<p>
+C&#x2032;&mdash;The lagoon of the newly formed atoll. According to the scale,
+the depth of the lagoon and of the lagoon-channel is exaggerated.
+</p>
+
+<p>
+Let us in imagination place within one of the subsiding areas, an island
+surrounded by a &ldquo;fringing-reef,&rdquo;&mdash;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<a href="#fn5.13"
+name="fnref5.13"><sup>[13]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn5.13"></a> <a href="#fnref5.13">[13]</a> The section has been made
+from the chart given in the &ldquo;Atlas of the Voyage of the
+<i>Coquille</i>.&rdquo; 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.
+</p>
+
+<p>
+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&mdash;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.
+</p>
+
+<p>
+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;&mdash;and this, it has been shown,
+is commonly the case.
+</p>
+
+<p>
+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;&mdash;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:&mdash;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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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 (<a href="#illus02">Plate II., Figure 5.</a>) 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.
+</p>
+
+<p>
+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&mdash;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,&mdash;in the
+ring or basin-like forms of the central reefs, as well as of the separate
+marginal portions of the northern Maldiva atolls,&mdash;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,&mdash;in the
+apparent progressive disseverment of some of the Maldiva atolls,&mdash;in the
+existence of irregularly formed atolls, some being tied together by linear
+reefs, and others with spurs projecting from them,&mdash;and, lastly, in the
+structure and origin of the Great Chagos bank.
+</p>
+
+<p>
+<i>Step-formed ledges round certain lagoons</i>.&mdash;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.<a
+href="#fn5.14" name="fnref5.14"><sup>[14]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn5.14"></a> <a href="#fnref5.14">[14]</a> 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.?
+</p>
+
+<p>
+<i>The ring or basin-formed reefs of the northern Maldiva atolls</i>.&mdash;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.
+</p>
+
+<p>
+<i>Submerged and Dead Reefs</i>.&mdash;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.
+</p>
+
+<p>
+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,<a
+href="#fn5.15" name="fnref5.15"><sup>[15]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn5.15"></a> <a href="#fnref5.15">[15]</a> Mr. Lyell, in the first
+edition of his &ldquo;Principles of Geology,&rdquo; 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.
+</p>
+
+<p>
+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 (<a
+href="#illus01">Plate I., Figure 9</a>), 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.
+</p>
+
+<p>
+<i>The disseverment of the larger Maldiva atolls</i>.&mdash;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 (<a
+href="#illus02">Plate II., Figure 4.</a>), 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 (<a href="#illus02">Plate II., Figure 6</a>), 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 (<a href="#illus02">Plate II., Figure
+4</a>), 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.
+</p>
+
+<p>
+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 (<a href="#illus02">Plate II.,
+Figure 5</a>), 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.<a href="#fn5.16"
+name="fnref5.16"><sup>[16]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn5.16"></a> <a href="#fnref5.16">[16]</a> 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&rsquo;s voyage: a line drawn through the linear reefs and lagoons of
+these three islands forms a semicircle. Consult also, the atlas of
+Lutké&rsquo;s voyage; and for the Marshall group that of Kotzebue; for the
+Gilbert group consult the atlas of Duperrey&rsquo;s voyage. Most of the points
+here referred to may, however, be seen in Krusenstern&rsquo;s general Atlas of
+the Pacific.
+</p>
+
+<p>
+<i>Irregularly formed Atolls</i>.&mdash;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.
+</p>
+
+<p>
+<i>The Great Chagos Bank</i>.&mdash;I have already shown that the submerged
+condition of the Great Chagos Bank (<a href="#illus02">Plate II., Figure 1,
+with its section Figure 2</a>), 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.
+</p>
+
+<p>
+By these means, I have scarcely any doubt that the Great Chagos Bank has
+originated,&mdash;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.
+</p>
+
+<p>
+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&rsquo;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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+<i>Objections to the theory of the formation of Atolls and
+Barrier-reefs</i>.&mdash;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,&mdash;as whole regions are now
+rising, for instance, in Scandinavia and South America,&mdash;and as no reason
+can be assigned, why subsidences should not have occurred in some parts of the
+earth&rsquo;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.
+</p>
+
+<p>
+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,&mdash;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.<a href="#fn5.17"
+name="fnref5.17"><sup>[17]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn5.17"></a> <a href="#fnref5.17">[17]</a> 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,&mdash;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,&mdash;the different degrees in
+which they have been rounded,&mdash;the presence of fragments of conglomerate
+torn up, rounded, and recemented,&mdash;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
+&ldquo;Journal of Researches,&rdquo; 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.
+</p>
+
+<hr />
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap06"></a>
+
+CHAPTER VI.<br/>
+
+ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE THEORY OF THEIR
+FORMATION.</h2>
+
+<p class="letter">
+Description of the coloured map.&mdash;Proximity of atolls and
+barrier-reefs.&mdash; Relation in form and position of atolls with ordinary
+islands.&mdash;Direct evidence of subsidence difficult to be
+detected.&mdash;Proofs of recent elevation where fringing-reefs
+occur.&mdash;Oscillations of level.&mdash;Absence of active volcanoes in the
+areas of subsidence.&mdash;Immensity of the areas which have been elevated and
+have subsided.&mdash;Their relation to the present distribution of the
+land.&mdash;Areas of subsidence elongated, their intersection and alternation
+with those of elevation.&mdash;Amount and slow rate of the
+subsidence.&mdash;Recapitulation.
+</p>
+
+<p>
+It will be convenient to give here a short account of the appended map (<a
+href="#illus03">Plate III.</a>) [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&rsquo;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&rsquo; 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.
+</p>
+
+<p>
+The <i>dark blue colour</i> [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.
+</p>
+
+<p>
+The <i>pale blue colour</i> [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.
+</p>
+
+<p>
+The <i>red colour</i> (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 <i>breached</i> 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.
+</p>
+
+<p>
+The <i>vermillion spots</i>, and streaks (1) represent volcanoes now in action,
+or historically known to have been so. They are chiefly laid down from Von
+Buch&rsquo;s work on the Canary Islands; and my reasons for making a few
+alterations are given in the note below.<a href="#fn6.1"
+name="fnref6.1"><sup>[1]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn6.1"></a> <a href="#fnref6.1">[1]</a> I have also made considerable
+use of the geological part of Berghaus&rsquo; &ldquo;Physical Atlas.&rdquo;
+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
+<i>Beagle</i> (&ldquo;Geological Transactions,&rdquo; volume v., page 601.) I
+have added a volcano to Albemarle Island, one of the Galapagos Archipelago (the
+author&rsquo;s &ldquo;Journal of Researches,&rdquo; 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
+(&ldquo;Narrative of Missionary Enterprise,&rdquo; page 29) that Toofoa and
+Proby Islands are active volcanoes. I infer from Hamilton&rsquo;s &ldquo;Voyage
+in the &lsquo;Pandora&rsquo;&rdquo; (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 &ldquo;Voyage of the
+<i>Astrolabe</i>.&rdquo; 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 &ldquo;Vorbemerk,&rdquo; 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 <i>opposite</i> coast by Cook (&ldquo;Second Voyage,&rdquo;
+volume ii., page 23) which smoke went out at night. The Mariana Islands,
+especially the northern ones, contain many craters (see Freycinet&rsquo;s
+&ldquo;Hydrog. Descript.&rdquo;) 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&rsquo;s &ldquo;Collect.&rdquo; volume iv., page 458), says there are
+two active volcanoes in latitude 23° 30&#x2032;, and in latitude 24°: but I
+have not coloured them. From the statements in Beechey&rsquo;s
+&ldquo;Voyage&rdquo; (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 (&ldquo;Comptes Rendus,&rdquo; 1840, page
+832), that there are two active volcanoes on the eastern side of Formosa. In
+Torres Straits, on Cap Island (9° 48&#x2032; S., 142° 39&#x2032; E.) a volcano
+was seen burning with great violence in 1793 by Captain Bampton (see
+Introduction to Flinders&rsquo; &ldquo;Voyage,&rdquo; page 41). Mr.
+M&rsquo;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 &ldquo;Silliman&rsquo;s North American Journal&rdquo;, volume
+xxxviii., page 385). In Berghaus&rsquo; &ldquo;Physical Atlas,&rdquo; 1840, No.
+7 of Geological Part, a volcano on the coast of Pondicherry is said to have
+burst forth in 1757. Ordinaire (&ldquo;Hist. Nat. des Volcans,&rdquo; 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&rsquo;s, in the southern part of the Indian Ocean, has been seen
+(&ldquo;Naut. Mag.&rdquo; 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.
+</p>
+
+<p>
+The <i>uncoloured coasts</i> 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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+<i>On the grouping of the different classes of reefs</i>.&mdash;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<a href="#fn6.2"
+name="fnref6.2"><sup>[2]</sup></a> 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<a
+href="#fn6.3" name="fnref6.3"><sup>[3]</sup></a> 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&rsquo;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.
+</p>
+
+<p class="footnote">
+<a name="fn6.2"></a> <a href="#fnref6.2">[2]</a> Sailing directions, appended
+to volume ii. of his &ldquo;Surveying Voyage to Australia.&rdquo;
+</p>
+
+<p class="footnote">
+<a name="fn6.3"></a> <a href="#fnref6.3">[3]</a> &ldquo;Voyage to Terra
+Australis,&rdquo; volume ii. page 336.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+<i>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.</i>&mdash;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&mdash;of a round of decay and
+renovation&mdash;of the last vestiges of land on some&mdash;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,<a href="#fn6.4"
+name="fnref6.4"><sup>[4]</sup></a> is often violently shaken by earthquakes,
+and there, the unusual depth of the channel between the shore and the
+reef,&mdash;the almost entire absence of islets on the reef,&mdash; 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.<a href="#fn6.5" name="fnref6.5"><sup>[5]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn6.4"></a> <a href="#fnref6.4">[4]</a> See Captain Dillon&rsquo;s
+&ldquo;Voyage in search of La Peyrouse.&rdquo; M. Cordier in his &ldquo;Report
+on the Voyage of the &lsquo;Astrolabe&rsquo;&rdquo; (page cxi., volume i.),
+speaking of Vanikoro, says the shores are surrounded by reefs of madrepore,
+<i>&ldquo;qu&rsquo;on assure être de formation tout-a-fait moderne</i>.&rdquo;
+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 (<a href="#illus01">Plate I., Figure 7</a>), in the Caroline
+Archipelago, being encircled by a barrier-reef, must have subsided. In the
+&ldquo;New S. Wales Lit. Advert.&rdquo; 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, &ldquo;At the
+N.E. end, at a place called Tamen, there are ruins of a town, <i>now only</i>
+accessible by boats, the waves <i>reaching to the steps of the
+houses</i>.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn6.5"></a> <a href="#fnref6.5">[5]</a> Mr. Couthouy states
+(&ldquo;Remarks,&rdquo; 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 &ldquo;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.&rdquo;<br/>
+
+    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.
+</p>
+
+<p>
+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<a href="#fn6.6"
+name="fnref6.6"><sup>[6]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn6.6"></a> <a href="#fnref6.6">[6]</a> &ldquo;Annales des Sciences
+Nat.&rdquo; tom. vi., page 279, etc.
+</p>
+
+<p>
+In the eastern half of the Pacific, the <i>Sandwich Islands</i> 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<a
+href="#fn6.7" name="fnref6.7"><sup>[7]</sup></a> 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<a href="#fn6.8"
+name="fnref6.8"><sup>[8]</sup></a> 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, &ldquo;that the elevation is at present
+going forward at a very perceptible rate.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn6.7"></a> <a href="#fnref6.7">[7]</a> &ldquo;Zoology of Captain
+Beechey&rsquo;s Voyage,&rdquo; page 176. See also MM. Quoy and Gaimard in
+&ldquo;Annales de Scien. Nat.&rdquo; tom. vi.
+</p>
+
+<p class="footnote">
+<a name="fn6.8"></a> <a href="#fnref6.8">[8]</a> &ldquo;Remarks on Coral
+Formations,&rdquo; page 51.
+</p>
+
+<p>
+In the southern part of the Low Archipelago, Elizabeth Island is described by
+Captain Beechey,<a href="#fn6.9" name="fnref6.9"><sup>[9]</sup></a> 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 <i>Cook and Austral Islands</i> (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<a href="#fn6.10"
+name="fnref6.10"><sup>[10]</sup></a> it is an upraised reef; &ldquo;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.&rdquo;
+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.
+</p>
+
+<p class="footnote">
+<a name="fn6.9"></a> <a href="#fnref6.9">[9]</a> Beechey&rsquo;s &ldquo;Voyage
+in the Pacific,&rdquo; page 46, 4to edition.
+</p>
+
+<p class="footnote">
+<a name="fn6.10"></a> <a href="#fnref6.10">[10]</a> Couthouy&rsquo;s
+&ldquo;Remarks,&rdquo; page 34.
+</p>
+
+<p>
+<i>Savage Island</i> (S.E. of the Friendly group), is about forty feet in
+height. Forster<a href="#fn6.11" name="fnref6.11"><sup>[11]</sup></a> describes
+the plants as <i>already</i> growing out of the dead, but still upright and
+spreading trees of coral; and the younger Forster<a href="#fn6.12"
+name="fnref6.12"><sup>[12]</sup></a> 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 <i>Friendly Group</i>, 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, &ldquo;which yet shows the cavities and irregularities
+worn into it by the action of the tides.&rdquo;<a href="#fn6.13"
+name="fnref6.13"><sup>[13]</sup></a> 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<a href="#fn6.14" name="fnref6.14"><sup>[14]</sup></a> 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
+<i>Navigator Archipelago</i>, Mr. Couthouy<a href="#fn6.15"
+name="fnref6.15"><sup>[15]</sup></a> found on Manua many and very large
+fragments of coral at the height of eighty feet, &ldquo;on a steep hill-side,
+rising half a mile inland from a low sandy plain abounding in marine
+remains.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn6.11"></a> <a href="#fnref6.11">[11]</a> &ldquo;Observations made
+during Voyage round the World,&rdquo; page 147.
+</p>
+
+<p class="footnote">
+<a name="fn6.12"></a> <a href="#fnref6.12">[12]</a> &ldquo;Voyage,&rdquo;
+volume ii., page 163.
+</p>
+
+<p class="footnote">
+<a name="fn6.13"></a> <a href="#fnref6.13">[13]</a> Cook&rsquo;s &ldquo;Third
+Voyage&rdquo; (4to edition), volume i., page 314.
+</p>
+
+<p class="footnote">
+<a name="fn6.14"></a> <a href="#fnref6.14">[14]</a> Ibid., volume i., page 235.
+</p>
+
+<p class="footnote">
+<a name="fn6.15"></a> <a href="#fnref6.15">[15]</a> &ldquo;Remarks on
+Coral-Formations,&rdquo; page 50.
+</p>
+
+<p>
+Still proceeding westward we come to the <i>New Hebrides;</i> on these islands,
+Mr. G. Bennett (author of &ldquo;Wanderings in New South Wales&rdquo;), informs
+me he found much coral at a great altitude, which he considered of recent
+origin. Respecting <i>Santa Cruz</i>, and the <i>Salomon Archipelago</i>, 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<a href="#fn6.16"
+name="fnref6.16"><sup>[16]</sup></a> 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
+<i>Marianas</i> form the main part. Of these Guam, Rota, Tiniam, Saypan, and
+some islets farther north, are described by Quoy and Gaimard,<a href="#fn6.17"
+name="fnref6.17"><sup>[17]</sup></a> and Chamisso,<a href="#fn6.18"
+name="fnref6.18"><sup>[18]</sup></a> 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. <i>Fais</i>, 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.<a href="#fn6.19"
+name="fnref6.19"><sup>[19]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn6.16"></a> <a href="#fnref6.16">[16]</a> &ldquo;Voyage de la
+&lsquo;Coquille&rsquo;,&rdquo; Part. Zoolog.
+</p>
+
+<p class="footnote">
+<a name="fn6.17"></a> <a href="#fnref6.17">[17]</a> Freycinet&rsquo;s
+&ldquo;Voyage autour du Monde.&rdquo; See also the &ldquo;Hydrographical
+Memoir,&rdquo; page 215.
+</p>
+
+<p class="footnote">
+<a name="fn6.18"></a> <a href="#fnref6.18">[18]</a> Kotzebue&rsquo;s
+&ldquo;First Voyage.&rdquo;
+</p>
+
+<p class="footnote">
+<a name="fn6.19"></a> <a href="#fnref6.19">[19]</a> Lutké&rsquo;s
+&ldquo;Voyage,&rdquo; volume ii., page 304.
+</p>
+
+<p>
+In the <i>East Indian Archipelago</i>, many authors have recorded proofs of
+recent elevation. M. Lesson<a href="#fn6.20"
+name="fnref6.20"><sup>[20]</sup></a> 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<a href="#fn6.21" name="fnref6.21"><sup>[21]</sup></a> have likewise
+described the primitive rocks, as coated to a considerable height with coral.
+Some small islets eastward of Timor are said in Kolff&rsquo;s
+&ldquo;Voyage,&rdquo;<a href="#fn6.22" name="fnref6.22"><sup>[22]</sup></a> to
+resemble small coral islets upraised some feet above the sea. Dr. Malcolmson
+informs me that Dr. Hardie found in <i>Java</i> an extensive formation,
+containing an abundance of shells, of which the greater part appear to be of
+existing species. Dr. Jack<a href="#fn6.23"
+name="fnref6.23"><sup>[23]</sup></a> has described some upraised shells and
+corals, apparently recent, on Pulo Nias off <i>Sumatra;</i> 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 <i>Borneo</i>
+and at the <i>Sooloo Islands</i>, the form of the land, the nature of the soil,
+and the water-washed rocks, present appearances<a href="#fn6.24"
+name="fnref6.24"><sup>[24]</sup></a> (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 <i>Phillippines</i>, 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<a href="#fn6.25"
+name="fnref6.25"><sup>[25]</sup></a> of the lines of inland reefs, and walls of
+coral-rock worn into caves, above the present reach of the waves, at the <i>Loo
+Choo</i> Islands, there can be little doubt that they have been upraised at no
+very remote period.
+</p>
+
+<p class="footnote">
+<a name="fn6.20"></a> <a href="#fnref6.20">[20]</a> Partie Zoolog.,
+&ldquo;Voyage de la &lsquo;Coquille&rsquo;.&rdquo;
+</p>
+
+<p class="footnote">
+<a name="fn6.21"></a> <a href="#fnref6.21">[21]</a> &ldquo;Ann. des Scien.
+Nat.&rdquo; tom. vi., page 281.
+</p>
+
+<p class="footnote">
+<a name="fn6.22"></a> <a href="#fnref6.22">[22]</a> Translated by Windsor Earl,
+chapters vi., vii.
+</p>
+
+<p class="footnote">
+<a name="fn6.23"></a> <a href="#fnref6.23">[23]</a> &ldquo;Geolog.
+Transact.&rdquo; 2nd series, volume i., page 403. On the Peninsula of Malacca,
+in front of Pinang, 5° 30&#x2032; 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 (&ldquo;Trans.
+Asiat. Soc.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn6.24"></a> <a href="#fnref6.24">[24]</a> &ldquo;Notices of the East
+Indian Arch.&rdquo; Singapore, 1828, page 6, and Append., page 43.
+</p>
+
+<p class="footnote">
+<a name="fn6.25"></a> <a href="#fnref6.25">[25]</a> Captain B. Hall,
+&ldquo;Voyage to Loo Choo,&rdquo; Append., pages xxi. and xxv. Captain
+Beechey&rsquo;s &ldquo;Voyage,&rdquo; page 496.
+</p>
+
+<p>
+Dr. Davy<a href="#fn6.26" name="fnref6.26"><sup>[26]</sup></a> describes the
+northern province of <i>Ceylon</i> 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.<a href="#fn6.27" name="fnref6.27"><sup>[27]</sup></a> <i>Mauritius</i>
+has certainly been upraised within the recent period, as I have stated in the
+chapter on fringing-reefs. The northern extremity of <i>Madagascar</i> is
+described by Captain Owen<a href="#fn6.28" name="fnref6.28"><sup>[28]</sup></a>
+as formed of madreporitic rock, as likewise are the shores and outlying islands
+along an immense space of <i>Eastern Africa</i>, from a little north of the
+equator for nine hundred miles southward. Nothing can be more vague than the
+expression &ldquo;madreporitic rock;&rdquo; 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
+<i>Seychelles</i>, which come under our fringed class.
+</p>
+
+<p class="footnote">
+<a name="fn6.26"></a> <a href="#fnref6.26">[26]</a> &ldquo;Travels in
+Ceylon,&rdquo; page 13. This madreporitic formation is mentioned by M. Cordier
+in his report to the Institute (May 4th, 1839), on the voyage of the
+&ldquo;Chevrette&rdquo;, as one of immense extent, and belonging to the latest
+tertiary period.
+</p>
+
+<p class="footnote">
+<a name="fn6.27"></a> <a href="#fnref6.27">[27]</a> Dr. Benza, in his
+&ldquo;Journey through the N. Circars&rdquo; (the &ldquo;Madras Lit. and
+Scient. Journ.&rdquo; 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&rsquo;s
+opinion.
+</p>
+
+<p class="footnote">
+<a name="fn6.28"></a> <a href="#fnref6.28">[28]</a> Owen&rsquo;s
+&ldquo;Africa,&rdquo; volume ii., page 37, for Madagascar; and for S. Africa,
+volume i., pages 412 and 426. Lieutenant Boteler&rsquo;s narrative contains
+fuller particulars regarding the coral-rock, volume i., page 174, and volume
+ii., pages 41 and 54. See also Ruschenberger&rsquo;s &ldquo;Voyage round the
+World,&rdquo; volume i., page 60.
+</p>
+
+<p>
+The nature of the formations round the shores of the <i>Red Sea</i>, 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<a
+href="#fn6.29" name="fnref6.29"><sup>[29]</sup></a> 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&#x2032; S.) shells and corals, apparently recent, at a height between thirty
+and forty feet; and Mr. Salt (&ldquo;Travels in Abyssinia&rdquo;) 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<a href="#fn6.30"
+name="fnref6.30"><sup>[30]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn6.29"></a> <a href="#fnref6.29">[29]</a> Rüppell, &ldquo;Reise in
+Abyssinien,&rdquo; Band i., s. 141.
+</p>
+
+<p class="footnote">
+<a name="fn6.30"></a> <a href="#fnref6.30">[30]</a> Lyell&rsquo;s
+&ldquo;Principles of Geology,&rdquo; 5th edition, volume iv., page 25.
+</p>
+
+<p>
+The <i>Persian Gulf</i> 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<a href="#fn6.31"
+name="fnref6.31"><sup>[31]</sup></a> says that the land is worn into terraces,
+and that the beds contain organic remains of existing forms. The <i>West Indian
+Archipelago</i> of &ldquo;fringed&rdquo; 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.<a href="#fn6.32" name="fnref6.32"><sup>[32]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn6.31"></a> <a href="#fnref6.31">[31]</a> Ainsworth&rsquo;s
+&ldquo;Assyria and Babylon,&rdquo; page 217.
+</p>
+
+<p class="footnote">
+<a name="fn6.32"></a> <a href="#fnref6.32">[32]</a> On Florida and the north
+shores of the Gulf of Mexico, Rogers&rsquo; &ldquo;Report to Brit.
+Assoc.&rdquo; volume iii., page 14.&mdash;On the shores of Mexico, Humboldt,
+&ldquo;Polit. Essay on New Spain,&rdquo; volume i., page 62. (I have also some
+corroborative facts with respect to the shores of Mexico.)&mdash;Honduras and
+the Antilles, Lyell&rsquo;s &ldquo;Principles,&rdquo; 5th edition, volume iv.,
+page 22.&mdash;Santa Cruz and Barbadoes, Prof. Hovey, &ldquo;Silliman&rsquo;s
+Journal&rdquo;, volume xxxv., page 74.&mdash;St. Domingo, Courrojolles,
+&ldquo;Journ de Phys.&rdquo; tom. liv., page 106.&mdash;Bahamas, &ldquo;United
+Service Journal&rdquo;, No. lxxi., pages 218 and 224. Jamaica, De la Beche,
+&ldquo;Geol. Man.&rdquo; page 142.&mdash;Cuba, Taylor in &ldquo;Lond. and Edin.
+Mag.&rdquo; 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.
+</p>
+
+<p class="p2">
+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.
+</p>
+
+<p>
+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<a href="#fn6.33" name="fnref6.33"><sup>[33]</sup></a> have stated that
+they have observed shells and corals high up on the mountains of the Society
+Islands,&mdash;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, &ldquo;a
+distinct and regular stratum of semi-fossil coral.&rdquo; 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,<a href="#fn6.34"
+name="fnref6.34"><sup>[34]</sup></a> 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,
+&mdash;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.
+</p>
+
+<p class="footnote">
+<a name="fn6.33"></a> <a href="#fnref6.33">[33]</a> Ellis, in his
+&ldquo;Polynesian Researches,&rdquo; was the first to call attention to these
+remains (volume i., page 38), and the tradition of the natives concerning them.
+See also Williams, &ldquo;Nar. of Missionary Enterprise,&rdquo; page 21; also
+Tyerman and G. Bennett, &ldquo;Journal of Voyage,&rdquo; volume i., page 213;
+also Mr. Couthouy&rsquo;s &ldquo;Remarks,&rdquo; 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, &ldquo;West of England
+Journal,&rdquo; No. i., page 54. There is a passage in Von Zach, &ldquo;Corres.
+Astronom.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn6.34"></a> <a href="#fnref6.34">[34]</a> Aurora Island is described
+by Mr. Couthouy (&ldquo;Remarks,&rdquo; 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
+&ldquo;presenting a broad table-land which declines a few feet towards the
+centre, where we may suppose the lagoon to have been placed.&rdquo; 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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+<i>On the absence of active Volcanoes in the areas of subsidence, and on their
+frequent presence in the areas of elevation.</i>&mdash;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,&mdash;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.
+</p>
+
+<p>
+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<a href="#fn6.35"
+name="fnref6.35"><sup>[35]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn6.35"></a> <a href="#fnref6.35">[35]</a> See the note at p. 121, on
+the authorities for colouring the volcanoes in the appended map.
+</p>
+
+<p>
+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<a href="#fn6.36"
+name="fnref6.36"><sup>[36]</sup></a> 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.<a
+href="#fn6.37" name="fnref6.37"><sup>[37]</sup></a> 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.<a href="#fn6.38"
+name="fnref6.38"><sup>[38]</sup></a>
+</p>
+
+<p class="footnote">
+<a name="fn6.36"></a> <a href="#fnref6.36">[36]</a> At Sedanka, in latitude 58°
+N. (Von Buch&rsquo;s &ldquo;Descrip. des Isles Canaries,&rdquo; page 455). In a
+forthcoming part, I shall give the evidence referred to with respect to the
+elevation of New Zealand.
+</p>
+
+<p class="footnote">
+<a name="fn6.37"></a> <a href="#fnref6.37">[37]</a> During the subterranean
+disturbances which took place in Chile, in 1835, I have shown (&ldquo;Geolog.
+Trans.&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn6.38"></a> <a href="#fnref6.38">[38]</a> 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
+<i>tendency</i> to an upward movement in the earth&rsquo;s surface, and
+certainly no movement of subsidence.
+</p>
+
+<p>
+<i>On the relations of the areas of Subsidence and Elevation</i>.&mdash;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,<a href="#fn6.39"
+name="fnref6.39"><sup>[39]</sup></a> so that the prevailing movements now in
+progress, seem to accord with the actual states of surface of the great
+divisions of the world.
+</p>
+
+<p class="footnote">
+<a name="fn6.39"></a> <a href="#fnref6.39">[39]</a> 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.
+</p>
+
+<p>
+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&rsquo;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,&mdash;of the east and west line of the Caroline
+atolls,&mdash;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.
+</p>
+
+<p>
+I infer from Mr. Hopkins&rsquo; researches,<a href="#fn6.40"
+name="fnref6.40"><sup>[40]</sup></a> 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,<a href="#fn6.41" name="fnref6.41"><sup>[41]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn6.40"></a> <a href="#fnref6.40">[40]</a> &ldquo;Researches in
+Physical Geology,&rdquo; Transact. Cambridge Phil. Soc., volume vi, part i.
+</p>
+
+<p class="footnote">
+<a name="fn6.41"></a> <a href="#fnref6.41">[41]</a> 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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+<i>Recapitulation</i>.&mdash;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;&mdash;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.
+</p>
+
+<p>
+Thus the ordinary forms and certain peculiarities in the structure of atolls
+and barrier-reefs can be explained;&mdash;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&mdash;the union of some atolls as if by a
+ribbon&mdash;the apparent disseverment of others&mdash;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.
+</p>
+
+<p>
+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.
+</p>
+
+<hr />
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap07"></a>
+
+APPENDIX.</h2>
+
+<h3>CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS IN PLATE
+III.</h3>
+
+<p>
+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.
+</p>
+
+<p>
+The W<small>ESTERN</small> S<small>HORES OF</small> A<small>MERICA</small>
+appear to be entirely without coral-reefs; south of the equator the survey of
+the &ldquo;Beagle&rdquo;, and north of it, the published charts show that this
+is the case. Even in the Bay of <i>Panama</i>, where corals flourish, there are
+no true coral-reefs, as I have been informed by Mr. Lloyd. There are no
+coral-reefs in the <i>Galapagos</i> Archipelago, as I know from personal
+inspection; and I believe there are none on the <i>Cocos, Revilla-gigedo</i>,
+and other neighbouring islands. <i>Clipperton</i> 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 <i>Easter</i>, and <i>Sala</i>, and <i>Gomez</i>
+Islands, which do not appear to be surrounded by reefs.
+</p>
+
+<p>
+The L<small>OW</small> A<small>RCHIPELAGO</small>.&mdash;This group consists of
+about eighty atolls: it will be quite superfluous to refer to descriptions of
+each. In D&rsquo;Urville and Lottin&rsquo;s chart, one island
+(<i>Wolchonsky</i>) 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 &ldquo;Biblioth.
+des Voyages,&rdquo; 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 &ldquo;Beagle&rdquo;;
+and notices of other atolls are scattered through several publications. The
+<i>Actæon</i> group in this archipelago has lately been discovered
+(&ldquo;Geographical Journal&rdquo;, volume vii., page 454); it consists of
+three small and low islets, one of which has a lagoon. Another lagoon-island
+has been discovered (&ldquo;Naut. Mag.&rdquo; 1839, page 770), in 22° 4&#x2032;
+S., and 136° 20&#x2032; W. Towards the S.E. part of the group, there are some
+islands of different formation: <i>Elizabeth</i> 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. <i>Pitcairn</i> 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 &ldquo;Zool. of Beechey&rsquo;s Voyage,&rdquo; page
+164); it is left uncoloured. <i>Gambier</i> Islands (see Plate I., Figure 8),
+are encircled by a barrier-reef; the greatest depth within is thirty-eight
+fathoms; coloured pale blue. <i>Aurora</i> 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.
+</p>
+
+<p>
+The S<small>OCIETY</small> 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 &ldquo;Atlas of the
+&lsquo;Coquille&rsquo;s&rsquo; Voyage&rdquo; there is a good general chart of
+the group, and separate plans of some of the islands. <i>Tahiti</i>, the
+largest island in the group, is almost surrounded, as seen in Cook&rsquo;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 (&ldquo;Naut. Mag.&rdquo; 1836, page 264) on the N.E. coast of the
+island, where none are laid down by Cook. At <i>Eimeo</i> the reef &ldquo;which
+like a ring surrounds it, is in some places one or two miles distant from the
+shore, in others united to the beach&rdquo; (Ellis, &ldquo;Polynesian
+Researches,&rdquo; volume i., page 18, 12mo edition). Cook found deep water
+(twenty fathoms) in some of the harbours within the reef. Mr. Couthouy,
+however, states (&ldquo;Remarks,&rdquo; page 45) that both at Tahiti and Eimeo,
+the space between the barrier-reef and the shore, has been almost filled
+up,&mdash;&ldquo;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,&rdquo; that is the
+barrier-reef. <i>Tapamanoa</i> 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 <i>Teturoa</i> are
+situated; from the description of them given me by the Rev. J. Williams (the
+author of the &ldquo;Narrative of Missionary Enterprise&rdquo;), I should have
+thought they had formed a small atoll, and likewise from the description given
+by the Rev. D. Tyerman and G. Bennett (&ldquo;Journal of Voyage and
+Travels,&rdquo; volume i., page 183), who say that ten low coral-islets
+&ldquo;are comprehended within one general reef, and separated from each other
+by interjacent lagoons;&rdquo; but as Mr. Stutchbury (&ldquo;West of England
+Journal,&rdquo; volume i., page 54) describes it as consisting of a mere narrow
+ridge, I have left it uncoloured. <i>Maitea</i>, 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&rsquo;s &ldquo;Voyages,&rdquo; and in the &ldquo;Atlas of the
+&lsquo;Coquille&rsquo;s&rsquo; Voyage.&rdquo; <i>Maurua</i> 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 &ldquo;Atlas of the &lsquo;Coquille&rsquo;s&rsquo; Voyage&rdquo; 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 <i>Huaheine</i> 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 <i>Toubai</i>
+(Motou-iti of the &ldquo;&lsquo;Coquille&rsquo;s&rsquo; Atlas&rdquo;) 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 <i>Bellinghausen</i>, which is said by
+Kotzebue (&ldquo;Second Voyage,&rdquo; volume ii., page 255), to be a
+lagoon-island; <i>Mopeha</i>, which, from Cook&rsquo;s description
+(&ldquo;Second Voyage,&rdquo; book iii., chapter i.), no doubt is an atoll; and
+the <i>Scilly</i> Islands, which are said by Wallis (&ldquo;Voyage,&rdquo;
+chapter ix.) to form a <i>group</i> of <i>low</i> islets and shoals, and,
+therefore, probably, they compose an atoll: the two former have been coloured
+blue, but not the latter.
+</p>
+
+<p>
+M<small>ENDANA</small> or M<small>ARQUESAS</small> Group.&mdash;These islands
+are entirely without reefs, as may be seen in Krusenstern&rsquo;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
+&ldquo;Geographical Journal&rdquo;. 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, <i>Dominicana</i>, <i>Christiana</i>, and
+<i>Roapoa;</i> 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.
+</p>
+
+<p>
+C<small>OOK</small> or H<small>ARVEY</small> and A<small>USTRAL</small>
+I<small>SLAND</small>.&mdash;<i>Palmerston</i> Island is minutely described as
+an atoll by Captain Cook during his voyage in 1774; coloured blue.
+<i>Aitutaki</i> was partially surveyed by the &ldquo;Beagle&rdquo; (see map
+accompanying &ldquo;Voyages of &lsquo;Adventure&rsquo; and
+&lsquo;Beagle&rsquo;&rdquo;); 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 &ldquo;Beagle&rdquo; 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.&mdash;<i>Manouai</i> or
+<i>Harvey</i> 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.&mdash;Round
+<i>Mitiaro</i> Island, as I am informed by Mr. Williams, the reef is attached
+to the shore; coloured red. &mdash;<i>Mauki</i> or Maouti; the reef round this
+island (under the name of Parry Island, in the &ldquo;Voyage of H.M.S.
+&lsquo;Blonde&rsquo;,&rdquo; 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.&mdash;<i>Aitu</i>, or
+Wateeo; a moderately elevated hilly island, like the others of this group. The
+reef is described in Cook&rsquo;s &ldquo;Voyage,&rdquo; as attached to the
+shore, and about one hundred yards wide; coloured red.&mdash;<i>Fenoua-iti;</i>
+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
+&ldquo;&lsquo;Coquille&rsquo;s&rsquo; Atlas,&rdquo; a reef is engraved close to
+the shore: this island is not mentioned in the list given by Mr. Williams (page
+16) in the &ldquo;Narrative of Missionary Enterprise;&rdquo; nature doubtful.
+As it is so near Atiu, it has been unavoidably coloured red.&mdash;
+<i>Rarotonga;</i> Mr. Williams informs me that it is a lofty basaltic island
+with an attached reef; coloured red.&mdash;There are three islands,
+<i>Rourouti</i>, <i>Roxburgh</i>, and <i>Hull</i>, 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.&mdash;<i>Mangaia;</i>
+height about three hundred feet; &ldquo;the surrounding reef joins the
+shore&rdquo; (Williams, &ldquo;Narrative,&rdquo; page 18); coloured
+red.&mdash;<i>Rimetara;</i> 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 (&ldquo;Naut. Mag.&rdquo; 1839, page
+738); coloured red.&mdash;<i>Rurutu;</i> 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.&mdash;<i>Toubouai;</i> in
+Cook&rsquo;s chart (&ldquo;Second Voyage,&rdquo; volume ii., page 2) the reef
+is laid down in part one mile, and in part two miles from the shore. Mr. Ellis
+(&ldquo;Polynes. Res.&rdquo; 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.&mdash;<i>Raivaivai</i>, 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.&mdash;<i>Lancaster</i> Reef, described in &ldquo;Naut.
+Mag.&rdquo; 1833 (page 693), as an extensive crescent-formed coral-reef. I have
+not coloured it.&mdash;<i>Rapa</i>, or Oparree; from the accounts given of it
+by Ellis and Vancouver, there does not appear to be any reef.&mdash;<i>I. de
+Bass</i> is an adjoining island, of which I cannot find any
+account.&mdash;<i>Kemin</i> Island; Krusenstern seems hardly to know its
+position, and gives no further particulars.
+</p>
+
+<p class="center">
+I<small>SLANDS BETWEEN</small> <i>the Low and Gilbert Archipelagoes</i>.
+</p>
+
+<p>
+<i>Caroline</i> Island (10° S., 150° W.) is described by Mr. F.D. Bennett
+(&ldquo;Geographical Journal&rdquo;, volume vii., page 225) as containing a
+fine lagoon; coloured blue.&mdash;<i>Flint</i> Island (11° S., 151° W.);
+Krusenstern believes that it is the same with <i>Peregrino</i>, which is
+described by Quiros (Burney&rsquo;s &ldquo;Chron. Hist.&rdquo; volume ii., page
+283) as &ldquo;a cluster of small islands connected by a reef, and forming a
+lagoon in the middle;&rdquo; coloured blue.&mdash;<i>Wostock</i> 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.&mdash;<i>Penrhyn</i> Island (9° S., 158°
+W.); a plan of it in the &ldquo;Atlas of the First Voyage&rdquo; of Kotzebue,
+shows that it is an atoll; blue.&mdash; <i>Starbuck</i> Island (5° S., 156° W.)
+is described in Byron&rsquo;s &ldquo;Voyage in the &lsquo;Blonde&rsquo;&rdquo;
+(page 206) as formed of a flat coral-rock, with no trees; the height not given;
+not coloured.&mdash;<i>Malden</i> 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.&mdash;<i>Jarvis</i>, or <i>Bunker</i> Island (0° 20&#x2032; S., 160° W.)
+is described by Mr. F.D. Bennett (&ldquo;Geographical Journal&rdquo;, volume
+vii., page 227) as a narrow, low strip of coral-formation; not
+coloured.&mdash;<i>Brook</i>, is a small low island between the two latter; the
+position, and perhaps even the existence of it is doubtful; not
+coloured.&mdash;<i>Pescado</i> and <i>Humphrey</i> Islands; I can find out
+nothing about these islands, except that the latter appears to be small and
+low; not coloured.&mdash;<i>Rearson</i>, or Grand Duke Alexander&rsquo;s (10
+S., 161° W.); an atoll, of which a plan is given by Bellinghausen; blue.&mdash;
+<i>Souvoroff</i> 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.&mdash;<i>Danger</i> 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.&mdash;<i>Clarence</i> Island (9° S., 172° W.);
+discovered in the &ldquo;Pandora&rdquo; (G. Hamilton&rsquo;s
+&ldquo;Voyage,&rdquo; page 75): it is said, &ldquo;in running along the land,
+we saw several canoes crossing the <i>lagoons;</i>&rdquo; 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. <i>York</i>
+Island (8° S., 172° W.) is described by Commodore Byron (chapter x. of his
+&ldquo;Voyage&rdquo;) as an atoll; blue.&mdash;<i>Sydney</i> Island (4° S.,
+172° W.) is about three miles in diameter, with its interior occupied by a
+lagoon (Captain Tromelin, &ldquo;Annal. Marit.&rdquo; 1829, page 297);
+blue.&mdash;<i>Phœnix</i> Island (4° S., 171° W.) is nearly circular, low,
+sandy, not more than two miles in diameter, and very steep outside (Tromelin,
+&ldquo;Annal. Marit.&rdquo; 1829, page 297); it may be inferred that this
+island originally contained a lagoon, but I have not coloured it.&mdash;<i>New
+Nantucket</i> (0° 15&#x2032; N., 174° W.). From the French chart it must be a
+low island; I can find nothing more about it or about <i>Mary</i> Island; both
+uncoloured.&mdash;<i>Gardner</i> Island (5° S., 174° W.) from its position is
+certainly the same as <i>Kemin</i> Island described (Krusenstern, page 435,
+Appen. to Mem., published 1827) as having a lagoon in its centre; blue.
+</p>
+
+<p class="center">
+I<small>SLANDS SOUTH</small> <i>of the Sandwich Archipelago</i>.
+</p>
+
+<p>
+<i>Christmas</i> Island (2° N., 157° W.). Captain Cook, in his &ldquo;Third
+Voyage&rdquo; (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 (&ldquo;Geographical Journal,&rdquo;
+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&rsquo;Urville and Lottin&rsquo;s
+chart. Mr. Couthouy, also, has given some account of it (&ldquo;Remarks,&rdquo;
+page 46) from the Hawaiian &ldquo;Spectator&rdquo;; 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.&mdash;<i>Fanning</i> Island (4° N., 158°
+W.) according to Captain Tromelin (&ldquo;Ann. Maritim.&rdquo; 1829, page 283),
+is an atoll: his account as observed by Krusenstern, differs from that given in
+Fanning&rsquo;s &ldquo;Voyage&rdquo; (page 224), which, however, is far from
+clear; coloured blue.&mdash; <i>Washington</i> Island (4° N., 159° W.) is
+engraved as a low island in D&rsquo;Urville&rsquo;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.&mdash;<i>Palmyra</i> Island
+(6° N., 162° W.) is an atoll divided into two parts (Krusenstern&rsquo;s
+&ldquo;Mem. Suppl.&rdquo; page 50, also Fanning&rsquo;s &ldquo;Voyage,&rdquo;
+page 233); blue.&mdash;<i>Smyth&rsquo;s</i> or Johnston&rsquo;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.
+</p>
+
+<p>
+S<small>ANDWICH</small> A<small>RCHIPELAGO</small>.&mdash;<i>Hawaii;</i> in the
+chart in Freycinet&rsquo;s &ldquo;Atlas,&rdquo; small portions of the coast are
+fringed by reefs; and in the accompanying &ldquo;Hydrog. Memoir,&rdquo; 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.&mdash;<i>Maui;</i> in
+Freycinet&rsquo;s chart of the anchorage of Raheina, two or three miles of
+coast are seen to be fringed; and in the &ldquo;Hydrog. Memoir,&rdquo;
+&ldquo;banks of coral along shore&rdquo; 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.&mdash;<i>Morotoi</i>, 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.&mdash;<i>Oahu;</i> Freycinet, in his &ldquo;Hydrog. Memoir,&rdquo;
+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.&mdash;<i>Atooi</i>, in La Peyrouse&rsquo;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.&mdash;<i>Oneehow;</i> 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&#x2032;, longitude 178° 30&#x2032; W., on which the
+&ldquo;Gledstanes&rdquo; 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 (&ldquo;Remarks,&rdquo; page 38) describes
+this island under the name of <i>Ocean</i> 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.
+</p>
+
+<p>
+S<small>AMOA</small> or N<small>AVIGATOR</small>
+G<small>ROUP</small>.&mdash;Kotzebue, in his &ldquo;Second Voyage,&rdquo;
+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&rsquo;s &ldquo;Voyage,&rdquo; it appears
+that the north shore of <i>Savaii</i>, <i>Maouna</i>, <i>Orosenga</i>, and
+<i>Mannua</i>, 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 &ldquo;Narrative,&rdquo; speaks of a reef going round a small island
+attached to <i>Oyolava</i>, and returning again to it: all these islands have
+been coloured red.&mdash;A chart of <i>Rose</i> 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 (&ldquo;Remarks,&rdquo; 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.
+</p>
+
+<p>
+<i>Beveridge Reef</i>, 20° S., 167° W., is described in the &ldquo;Naut.
+Mag.&rdquo; (May 1833, page 442) as ten miles long in a N. and S. line, and
+eight wide; &ldquo;in the inside of the reef there appears deep water;&rdquo;
+there is a passage near the S.W. corner: this therefore seems to be a submerged
+atoll, and is coloured blue.
+</p>
+
+<p>
+<i>Savage</i> 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.
+</p>
+
+<p>
+F<small>RIENDLY</small> A<small>RCHIPELAGO</small>. <i>Pylstaart</i> Island.
+Judging from the chart in Freycinet&rsquo;s &ldquo;Atlas,&rdquo; I should have
+supposed that it had been regularly fringed; but as nothing is said in the
+&ldquo;Hydrog. Memoir&rdquo; (or in the &ldquo;Voyage&rdquo; of Tasman, the
+discoverer) about coral-reefs, I have left it
+uncoloured.&mdash;<i>Tongatabou:</i> In the &ldquo;Atlas of the Voyage of the
+&lsquo;Astrolabe&rsquo;,&rdquo; 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.&mdash; In
+Cook&rsquo;s charts the little outlying island also of <i>Eoaigee</i>, is
+represented as fringed; coloured red.&mdash;<i>Eoua</i>. I cannot make out from
+Captain Cook&rsquo;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
+(&ldquo;Observations,&rdquo; page 14) classes it with high islands having
+reefs, but it certainly is not encircled by a barrier-reef and the younger
+Forster (&ldquo;Voyage,&rdquo; volume i., page 426) says, that &ldquo;a bed of
+coral-rocks surrounded the coast towards the landing-place.&rdquo; I have
+therefore classed it with the fringed islands and coloured it red. The several
+islands lying N.W. of Tongatabou, namely <i>Anamouka</i>, <i>Komango</i>,
+<i>Kotou</i>, <i>Lefouga</i>, <i>Foa</i>, &amp;c., are seen in Captain
+Cook&rsquo;s chart to be fringed by reefs, in several of them are connected
+together. From the various statements in the first volume of Cook&rsquo;s
+&ldquo;Third Voyage,&rdquo; 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.&mdash;<i>Toufoua</i> &amp; <i>Kao</i>, forming
+the western part of the group, according to Forster have no reefs; the former
+is an active volcano.&mdash;<i>Vavao</i>. 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 <i>Latte</i> and <i>Amargura</i>, for I have
+not seen plans on a large scale of them, and do not know whether they are
+fringed.
+</p>
+
+<p>
+<i>Niouha</i>, 16° S., 174° W., or <i>Keppel</i> Island of Wallis, or
+<i>Cocos</i> Island. From a view and chart of this island given in
+Wallis&rsquo;s &ldquo;Voyage&rdquo; (4to edition) it is evidently encircled by
+a reef; coloured blue: it is however remarkable that <i>Boscawen</i> Island,
+immediately adjoining, has no reef of any kind; uncoloured.
+</p>
+
+<p>
+<i>Wallis Island</i>, 13° S., 176° W., a chart and view of this island in
+Wallis&rsquo;s &ldquo;Voyage&rdquo; (4to edition) shows that it is encircled. A
+view of it in the &ldquo;Naut. Mag.&rdquo; July 1833, page 376, shows the same
+fact; blue.
+</p>
+
+<p>
+<i>Alloufatou</i>, or <i>Horn</i> Island, <i>Onouafu</i>, or <i>Proby</i>
+Island, and <i>Hunter</i> Islands, lie between the Navigator and Fidji groups.
+I can find no distinct accounts of them.
+</p>
+
+<p>
+F<small>IDJI</small> or V<small>ITI</small> G<small>ROUP</small>.&mdash;The
+best chart of the numerous islands of this group, will be found in the
+&ldquo;Atlas of the &lsquo;Astrolabe&rsquo;s&rsquo; Voyage.&rdquo; From this,
+and from the description given in the &ldquo;Hydrog. Memoir,&rdquo;
+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 &ldquo;Astrolabe&rdquo; 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&rsquo;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 <i>Batoa</i>, or <i>Turtle</i> Island of Cook (&ldquo;Second
+Voyage,&rdquo; volume ii., page 23, and chart, 4to edition) surrounded by a
+coral-reef, &ldquo;which in some places extends two miles from the
+shore;&rdquo; 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; &ldquo;in short, the bank wants only a few little
+islets to make it exactly like one of the half-drowned isles so often
+mentioned,&rdquo;&mdash;namely, atolls. South of Batoa, lies the high island of
+<i>Ono</i>, which appears in Bellinghausen&rsquo;s &ldquo;Atlas&rdquo; 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.
+</p>
+
+<p>
+<i>Rotoumah</i>, 13° S., 179° E.&mdash;From the chart in Duperrey&rsquo;s
+&ldquo;Atlas,&rdquo; 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.
+</p>
+
+<p>
+<i>Independence</i> Island, 10° S., 179° E., is described by Mr. G. Bennett,
+(&ldquo;United Service Journal,&rdquo; 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.
+</p>
+
+<p>
+E<small>LLICE</small> G<small>ROUP</small>.&mdash;<i>Oscar</i>, <i>Peyster</i>,
+and <i>Ellice</i> Islands are figured in Arrowsmith&rsquo;s &ldquo;Chart of the
+Pacific&rdquo; (corrected to 1832) as atolls, and are said to be very low;
+blue.&mdash;<i>Nederlandisch</i> 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 &ldquo;very low.&rdquo; I have therefore coloured it blue. Admiral
+Krusenstern (&ldquo;Memoir on the Pacific,&rdquo; 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.
+&mdash;<i>Gran Cocal</i> is said in Krusenstern&rsquo;s &ldquo;Memoir,&rdquo;
+to be low, and to be surrounded by a reef; it is small, and therefore probably
+once contained a lagoon; uncoloured.&mdash;<i>St. Augustin</i>. From a chart
+and view of it, given in the &ldquo;Atlas of the &lsquo;Coquille&rsquo;s&rsquo;
+Voyage,&rdquo; it appears to be a small atoll, with its lagoon partly filled
+up; coloured blue.
+</p>
+
+<p>
+G<small>ILBERT</small> G<small>ROUP</small>.&mdash;The chart of this group,
+given in the &ldquo;Atlas of the &lsquo;Coquille&rsquo;s&rsquo; Voyage,&rdquo;
+at once shows that it is composed of ten well characterised atolls. In
+D&rsquo;Urville and Lottin&rsquo;s chart, <i>Sydenham</i> 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 &ldquo;&lsquo;Coquille&rsquo;s&rsquo; Atlas.&rdquo; Some narrow
+strip-like reefs project from the southern side of <i>Drummond</i> atoll, and
+render it irregular. The southern island of the group is called <i>Chase</i>
+(in some charts, <i>Rotches</i>); of this I can find no account, but Mr. F.D.
+Bennett discovered (&ldquo;Geographical Journal&rdquo;, 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. &mdash;<i>Pitt</i> Island, at the extreme
+northern point of the group, is left uncoloured, as its exact position and
+nature is not known.&mdash;<i>Byron</i> Island, which lies a little to the
+eastward, does not appear to have been visited since Commodore Byron&rsquo;s
+voyage, and it was then seen only from a distance of eighteen miles; it is said
+to be low; uncoloured.
+</p>
+
+<p>
+<i>Ocean</i>, <i>Pleasant</i>, and <i>Atlantic</i> 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&rsquo;s &ldquo;Memoir&rdquo; it is said to be high.
+</p>
+
+<p>
+M<small>ARSHALL</small> G<small>ROUP</small>.&mdash;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&rsquo;s &ldquo;Atlas,&rdquo; and in Kotzebue&rsquo;s &ldquo;Second
+Voyage.&rdquo; The group consists (with the exception of two <i>little</i>
+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
+<i>Gaspar-Rico</i>, or <i>Cornwallis</i> Island in this group, which is
+described by Chamisso (Kotzebue&rsquo;s &ldquo;First Voyage,&rdquo; volume
+iii., page 179) &ldquo;as a low sickle-formed group, with mould only on the
+windward side.&rdquo; 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 <i>Baring</i>
+Island, of which little is known (see Krusenstern&rsquo;s
+&ldquo;Appendix,&rdquo; 1835, page 149). I have left it uncoloured; but
+<i>Boston</i> 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 &ldquo;&lsquo;Coquille&rsquo;s&rsquo;
+Atlas.&rdquo;&mdash;Two islands, <i>Aur Kawen</i> and <i>Gaspar Rico</i>, are
+written in the French chart with capital letters; but this is an error, for
+from the account given by Chamisso in Kotzebue&rsquo;s &ldquo;First
+Voyage,&rdquo; they are certainly low. The nature, position, and even
+existence, of the shoals and small islands north of the Marshall group, are
+doubtful.
+</p>
+
+<p>
+N<small>EW</small> H<small>EBRIDES</small>.&mdash;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.&mdash;<i>Matthew&rsquo;s Rock</i>, an active volcano, some way south
+of the group (of which a plan is given in the &ldquo;Atlas of the
+&lsquo;Astrolabe&rsquo;s&rsquo; Voyage&rdquo;) does not appear to have reefs of
+any kind about it.&mdash;<i>Annatom</i>, the southernmost of the Hebrides; from
+a rough woodcut given in the &ldquo;United Service Journal&rdquo; (1831, part
+iii., page 190), accompanying a paper by Mr. Bennett, it appears that the shore
+is fringed; coloured red.&mdash;<i>Tanna</i>. Forster, in his
+&ldquo;Observations&rdquo; (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&rsquo;s
+chart is represented as fringed; coloured red.&mdash;<i>Immer</i> is described
+(&ldquo;United Service Journal,&rdquo; 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&rsquo;s
+classification (&ldquo;Observations,&rdquo; page 14), to have been a low island
+or even an atoll.&mdash; <i>Erromango</i> Island; Cook (&ldquo;Second
+Voyage,&rdquo; volume ii., page 45, 4to edition) speaks of rocks everywhere
+<i>lining</i> 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
+&ldquo;Singapore Chron.,&rdquo; alludes to the <i>reefs</i> 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.&mdash;<i>Sandwich</i> Island. The east
+coast is said (Cook&rsquo;s &ldquo;Second Voyage,&rdquo; 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.&mdash;<i>Mallicollo</i>;
+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
+(&ldquo;Observations,&rdquo; 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.&mdash;<i>Aurora</i> and <i>Pentecost</i>
+Islands, according to Bougainville, apparently have no reefs; nor has the large
+island of <i>S. Espiritu</i>, nor <i>Bligh</i> Island or <i>Banks&rsquo;</i>
+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.
+</p>
+
+<p>
+S<small>ANTA</small>-C<small>RUZ</small>
+G<small>ROUP</small>.&mdash;<i>Vanikoro</i> (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 &ldquo;Astrolabe&rdquo;;
+coloured pale blue.&mdash;<i>Tikopia</i> and <i>Fataka</i> Islands appear, from
+the descriptions of Dillon and D&rsquo;Urville, to have no reefs; <i>Anouda</i>
+is a low, flat island, surrounded by cliffs (&ldquo;&lsquo;Astrolabe&rsquo;
+Hydrog.&rdquo; and Krusenstern, &ldquo;Mem.&rdquo; volume ii., page 432); these
+are uncoloured. <i>Toupoua</i> (<i>Otooboa</i> of Dillon) is stated by Captain
+Tromelin (&ldquo;Annales Marit.&rdquo; 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.&mdash; <i>Santa-Cruz</i>. I have carefully examined the works of
+Carteret, D&rsquo;Entrecasteaux, Wilson, and Tromelin, and I cannot discover
+any mention of reefs on its shores; left uncoloured.&mdash;<i>Tinakoro</i> is a
+constantly active volcano without reefs.&mdash;<i>Mendana Isles</i> (mentioned
+by Dillon under the name of <i>Mammee</i>, etc.); said by Krusenstern to be
+low, and intertwined with reefs. I do not believe they include a lagoon; I have
+left them uncoloured.&mdash;<i>Duff&rsquo;s</i> Islands compose a small group
+directed in a N.W. and S.E. band; they are described by Wilson (page 296,
+&ldquo;Miss. Voy.&rdquo; 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. <i>Kennedy</i> Island, N.E. of Duff&rsquo;s. I have been unable to
+find any account of it.
+</p>
+
+<p>
+N<small>EW</small> C<small>ALEDONIA</small>.&mdash;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&rsquo;Urville; this latter part so closely resembles an atoll that I have
+coloured it dark blue. The <i>Loyalty</i> group is situated eastward of this
+island; from the chart and description given in the &ldquo;Voyage of the
+&lsquo;Astrolabe&rsquo;,&rdquo; they do not appear to have any reefs; north of
+this group, there are some extensive low reefs (called <i>Astrolabe</i> and
+<i>Beaupré</i>,) which do not seem to be atoll-formed; these are left
+uncoloured.
+</p>
+
+<p>
+A<small>USTRALIAN</small> B<small>ARRIER-REEF</small>.&mdash;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&rsquo;s &ldquo;Atlas&rdquo; as having an atoll-like structure;
+namely, <i>Bampton</i> shoal, <i>Frederic</i>, <i>Vine</i> or Horse-shoe, and
+<i>Alert</i> reefs; these have been coloured dark blue.
+</p>
+
+<p>
+L<small>OUISIADE</small>: 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&rsquo;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 <i>Laughlan</i> Islands, the reefs round which are engraved in the
+&ldquo;Atlas of the Voyage of the &lsquo;Astrolabe&rsquo;,&rdquo; 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 <i>Wells</i> reef, described in G. Hamilton&rsquo;s
+&ldquo;Voyage in H.M.S. &lsquo;Pandora&rsquo;&rdquo; (page 100): it is said,
+&ldquo;We found we had got embayed in a double reef, which will soon be an
+island.&rdquo; 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.
+</p>
+
+<p>
+S<small>ALOMON</small> A<small>RCHIPELAGO</small>: the chart in
+Krusenstern&rsquo;s &ldquo;Atlas&rdquo; 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&rsquo;Entrecasteaux&rsquo;s &ldquo;Voyage,&rdquo; regarding the
+southern islands of the group, so have left them
+uncoloured.&mdash;<i>Malayta</i> Island in a rough MS. chart in the Admiralty
+has its northern shore fringed.&mdash;<i>Ysabel</i> 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.&mdash;<i>Choiseul</i> Island. In Bougainville&rsquo;s &ldquo;Chart of
+Choiseul Bay,&rdquo; parts of the shores are fringed by coral-reefs.&mdash;
+<i>Bougainville</i> Island. According to D&rsquo;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.&mdash;<i>Bouka</i> 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 &ldquo;Atlas of the &lsquo;Coquille&rsquo;s&rsquo;
+Voyage&rdquo;), and that it was &ldquo;garnie d&rsquo;une bande de récifs à
+fleur d&rsquo;eau adherentes au rivage;&rdquo; 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.
+</p>
+
+<p>
+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
+<i>Candelaria</i>.&mdash;<i>Outong Java</i>, according to the Spanish
+navigator, Maurelle, is thus characterised; but this is the only one which I
+have ventured to colour blue.
+</p>
+
+<p>
+N<small>EW</small> I<small>RELAND</small>.&mdash;The shores of the S.W. point
+of this island and some adjoining islets, are fringed by reefs, as may be seen
+in the &ldquo;Atlases of the Voyages of the &lsquo;Coquille&rsquo; and
+&lsquo;Astrolabe&rsquo;.&rdquo; M. Lesson observes that the reefs are open in
+front of each streamlet. The <i>Duke of York&rsquo;s</i> Island is also
+fringed; but with regard to the other parts of <i>New Ireland</i>, <i>New
+Hanover</i>, 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.
+</p>
+
+<p>
+N<small>EW</small> B<small>RITAIN</small> and the N<small>ORTHERN</small>
+S<small>HORE</small> of N<small>EW</small> G<small>UINEA</small>.&mdash;From
+the charts in the &ldquo;Voyage of the &lsquo;Astrolabe&rsquo;,&rdquo; and from
+the &ldquo;Hydrog. Memoir,&rdquo; it appears that these coasts are entirely
+without reefs, as are the <i>Schouten Islands</i>, 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.
+</p>
+
+<p>
+A<small>DMIRALTY</small> G<small>ROUP</small>.&mdash;From the accounts by
+Bougainville, Maurelle, D&rsquo;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.&mdash;In the southern part of the group we have <i>Elizabeth Island</i>,
+which is surrounded by a reef at the distance of a mile; and two miles eastward
+of it (Krusenstern, &ldquo;Append.&rdquo; 1835, page 42) there is a little
+island containing a lagoon.&mdash;Near here, also lies <i>Circular-reef</i>
+(Horsburgh, &ldquo;Direct.&rdquo; volume i., page 691, 4th edition),
+&ldquo;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.&rdquo; I have from these data,
+coloured the group pale blue, and <i>circular-reef</i> dark blue.&mdash;the
+<i>Anachorites</i>, <i>Echequier</i>, and <i>Hermites</i>, 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
+<i>Durour</i> Island, which is described by Carteret as low.
+</p>
+
+<p>
+The C<small>AROLINE</small> A<small>RCHIPELAGO</small> 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 <i>Ualen</i> appears to be only about half a mile from the shore; but as
+the land is low and covered with mangroves (&ldquo;Voyage autour du
+Monde,&rdquo; 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 &ldquo;Atlas of
+&lsquo;Coquille&rsquo;s&rsquo; Voyage&rdquo;), 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.&mdash;
+<i>Pouynipète</i>, or <i>Seniavine</i>. 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 &ldquo;un porte vaste et sur&rdquo; (Lutké, volume ii., page 4);
+coloured pale blue.&mdash;<i>Hogoleu</i> or <i>Roug</i>. 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 &ldquo;Hydrog. Descrip.&rdquo; page 428, of the &ldquo;Voyage of the
+&lsquo;Astrolabe&rsquo;,&rdquo; 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&rsquo;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<a href="#fn7.1"
+name="fnref7.1"><sup>[1]</sup></a> 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 &ldquo;Atlas&rdquo; by Captain Lutké), and some even of
+the very small ones, as <i>Macaskill</i> and <i>Duperrey</i>, of which plans
+are given in the &ldquo;Atlas of the &lsquo;Coquille&rsquo;s&rsquo;
+Voyage.&rdquo; There are, however, some low small islands of coral-formation,
+namely <i>Ollap</i>, <i>Tamatam</i>, <i>Bigali</i>, <i>Satahoual</i>, 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 &ldquo;Atlas of the Voyage of the
+&lsquo;Coquille&rsquo;,&rdquo; 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 (&ldquo;Hydrog. Memoir
+&lsquo;L&rsquo;Uranie&rsquo; Voyage,&rdquo; 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 <i>Piguiram</i>, is
+not coloured, because I have found no account of it. <i>Nougouor</i>, or
+<i>Monte Verdison</i>, which was not visited by Lutké, is described and figured
+by Mr. Bennett (&ldquo;United Service Journal,&rdquo; January 1832) as an
+atoll. All the above-mentioned islands have been coloured blue.
+</p>
+
+<p class="footnote">
+<a name="fn7.1"></a> <a href="#fnref7.1">[1]</a> In D&rsquo;Urville and
+Lottin&rsquo;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&rsquo;s charts)&mdash;a regular atoll.
+</p>
+
+<p>
+W<small>ESTERN PART OF THE</small> C<small>AROLINE</small>
+A<small>RCHIPELAGO</small>.&mdash;<i>Fais</i> 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.&mdash; <i>Philip</i> Island., I believe, is low; but
+Hunter, in his &ldquo;Historical Journal,&rdquo; gives no clear account of it;
+uncoloured.&mdash;<i>Elivi;</i> from the manner in which the islets on the
+reefs are engraved, in the &ldquo;Atlas of the &lsquo;Astrolabe&rsquo;s&rsquo;
+Voyage,&rdquo; 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.&mdash;<i>Gouap</i> (<i>Eap</i> of Chamisso), is a high island
+with a reef (see chart in &ldquo;Voyage of the &lsquo;Astrolabe&rsquo;&rdquo;),
+more than a mile distant in most parts from the shore, and two miles in one
+part. Captain D&rsquo;Urville thinks that there would be anchorage
+(&ldquo;Hydrog. Descript. &lsquo;Astrolabe&rsquo; Voyage,&rdquo; page 436) for
+ships within the reef, if a passage could be found; coloured pale
+blue.&mdash;<i>Goulou</i>, from the chart in the
+&ldquo;&lsquo;Astrolabe&rsquo;s&rsquo; Atlas,&rdquo; appears to be an atoll.
+D&rsquo;Urville (&ldquo;Hydrog. Descript.&rdquo; page 437) speaks of the low
+islets on the reef; coloured dark blue.
+</p>
+
+<p>
+P<small>ELEW</small> I<small>SLANDS</small>.&mdash;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&rsquo;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.&mdash;<i>Spanish</i>,
+<i>Martires</i>, <i>Sanserot</i>, <i>Pulo Anna</i> and <i>Mariere</i> 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.
+</p>
+
+<p>
+M<small>ARIANA</small> A<small>RCHIPELAGO</small>, or
+L<small>ADRONES</small>.&mdash;<i>Guahan</i>. 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&rsquo;s &ldquo;Hydrog. Mem.&rdquo; there is an
+account of these reefs, and in the &ldquo;Atlas,&rdquo; a map on a large scale;
+coloured red.&mdash;<i>Rota</i>. &ldquo;L&rsquo;ile est presque entièrement
+entourée des récifs&rdquo; (page 212, Freycinet&rsquo;s &ldquo;Hydrog.
+Mem.&rdquo;). These reefs project about a quarter of a mile from the shore;
+coloured red.&mdash;<i>Tinian</i>. The eastern coast is precipitous, and is
+without reefs; but the western side is fringed like the last island; coloured
+red.&mdash;<i>Saypan</i>. 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.&mdash;<i>Farallon de
+Medinilla</i>, appears so regularly and closely fringed in Freycinet&rsquo;s
+charts, that I have ventured to colour it red, although nothing is said about
+reefs in the &ldquo;Hydrographical Memoir.&rdquo; 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.&mdash;<i>Mangs</i>, however, is described (by
+Freycinet, page 219, &ldquo;Hydrog.&rdquo;) from some Spanish charts, as formed
+of small islands placed &ldquo;au milieu des nombreux récifs;&rdquo; 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 <i>Folger</i> and <i>Marshall</i> 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&rsquo;Urville&rsquo;s chart; uncoloured.
+</p>
+
+<p>
+B<small>ONIN</small> or A<small>RZOBISPO</small>
+G<small>ROUP</small>.&mdash;<i>Peel Island</i>  has been examined by Captain
+Beechey, to whose kindness I am much indebted for giving me information
+regarding it: &ldquo;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.&rdquo; 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.&mdash;<i>Grampus Island</i> to the eastward, does not appear
+(Meare&rsquo;s &ldquo;Voyage,&rdquo; page 95) to have any reefs, nor does
+<i>Rosario Island</i> (from Lutké&rsquo;s chart), which lies to the westward.
+Respecting the few other islands in this part of the sea, namely the <i>Sulphur
+Islands</i>, 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.
+</p>
+
+<p>
+W<small>EST</small> E<small>ND OF</small> N<small>EW</small>
+G<small>UINEA</small>.&mdash;<i>Port Dory</i>. From the charts in the
+&ldquo;Voyage of the &lsquo;Coquille&rsquo;,&rdquo; 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.&mdash;<i>Waigiou</i>. A considerable portion
+of the northern shores of these islands is seen in the charts (on a large
+scale) in Freycinet&rsquo;s &ldquo;Atlas&rdquo; to be fringed by coral-reefs.
+Forrest (page 21, &ldquo;Voyage to New Guinea&rdquo;) 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 &ldquo;sharp
+coral-rocks line their shores;&rdquo; coloured red.&mdash;In the sea north of
+these islands, we have <i>Guedes</i> (or <i>Freewill</i>, or <i>St.
+David&rsquo;s</i>), which from the chart given in the 4to edition of
+Carteret&rsquo;s &ldquo;Voyage,&rdquo; must be an atoll. Krusenstern says the
+islets are very low; coloured blue.&mdash;<i>Carteret&rsquo;s Shoals</i>, in 2°
+53&#x2032; N., are described as circular, with stony points showing all round,
+with deeper water in the middle; coloured blue.&mdash;<i>Aiou;</i> the plan of
+this group, given in the &ldquo;Atlas of the Voyage of the
+&lsquo;Astrolabe&rsquo;,&rdquo; shows that it is an atoll; and, from a chart in
+Forrest&rsquo;s &ldquo;Voyage,&rdquo; it appears that there is twelve fathoms
+within the circular reef; coloured blue.&mdash;The S.W. coast of New Guinea
+appears to be low, muddy, and devoid of reefs. The <i>Arru</i>, <i>Timor
+laut</i>, and <i>Tenimber</i> 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&rsquo;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.&mdash;The last-mentioned groups are connected with the
+east end of Ceram by a chain of small islands, of which the small groups of
+<i>Ceram-laut</i>, <i>Goram</i> and <i>Keffing</i> 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&rsquo;s &ldquo;Sailing Direct. for the Arafura Sea,&rdquo; page 9).
+</p>
+
+<p>
+C<small>ERAM</small>.&mdash;In various charts which I have examined, several
+parts of the coast are represented as fringed by reefs.&mdash;<i>Manipa</i>
+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.&mdash;<i>Bourou;</i> parts of
+this island appear fringed by coral-reefs, namely, the eastern coast, as seen
+in Freycinet&rsquo;s chart; and <i>Cajeli Bay</i>, 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 A<small>MBOINA</small> G<small>ROUP</small> are fringed by
+reefs; for instance, <i>Noessa</i>, <i>Harenca</i>, and <i>Ucaster</i>, in
+Freycinet&rsquo;s charts. The above-mentioned islands have been coloured red,
+although the evidence is not very satisfactory.&mdash;North of Bourou the
+parallel line of the <i>Xulla Isles</i> 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.&mdash;<i>Mysol Group;</i> the Kanary Islands are said by
+Forrest (&ldquo;Voyage,&rdquo; page 130) to be divided from each other by deep
+straits, and are lined with coral-rocks; coloured red.&mdash;<i>Guebe</i>,
+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.&mdash;<i>Gilolo</i>. In a chart published by Dalrymple, the numerous
+islands on the western, southern (<i>Batchian</i> and the <i>Strait of
+Patientia</i>), and eastern sides appear fringed by narrow reefs; these reefs,
+I suppose, are of coral, for it is said in &ldquo;Malte Brun&rdquo; (volume
+xii., page 156), &ldquo;Sur les cotes (of Batchian) comme <i>dans le
+plupart</i> des iles de cet archipel, il y a de rocs de medrepores d&rsquo;une
+beauté et d&rsquo;une variété infinies.&rdquo; Forrest, also (page 50), says
+Seland, near Batchian, is a little island with reefs of coral; coloured
+red.&mdash;<i>Morty</i> 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.&mdash;<i>Celebes</i>.
+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 <i>Straits of
+Limbe</i>, and parts of the adjoining shore, appear to be fringed: the east
+side of the bay of <i>Manado</i>, has deep water, and is fringed by sand and
+coral (&ldquo;&lsquo;Astrol.&rsquo; Voyage,&rdquo; Hydrog. Part, pages 453-4);
+this extreme point, therefore, I have coloured red.&mdash;Of the islands
+leading from this point to Magindanao, I have not been able to find any
+account, except of <i>Serangani</i>, which appears surrounded by narrow reefs;
+and Forrest (&ldquo;Voyage,&rdquo; 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
+<i>Karkalang</i>, 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.
+</p>
+
+<p>
+I<small>SLANDS NEAR</small> T<small>IMOR</small>.&mdash;The account of the
+following islands is taken from Captain D. Kolff&rsquo;s &ldquo;Voyage,&rdquo;
+in 1825, translated by Mr. W. Earl, from the Dutch.&mdash;<i>Lette</i> has
+&ldquo;reefs extending along shore at the distance of half a mile from the
+land.&rdquo;&mdash;<i>Moa</i> has reefs on the S.W. part.&mdash;<i>Lakor</i>
+has a reef lining its shore; these islands are coloured red.&mdash;Still more
+eastward, <i>Luan</i> 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.&mdash;<i>Kissa</i>, off the point of Timor, has its &ldquo;shore
+fronted by a reef, steep too on the outer side, over which small proahs can go
+at the time of high water;&rdquo; coloured red.&mdash;<i>Timor;</i> most of the
+points, and some considerable spaces of the northern shore, are seen in
+Freycinet&rsquo;s chart to be fringed by coral-reefs; and mention is made of
+them in the accompanying &ldquo;Hydrog. Memoir;&rdquo; coloured
+red.&mdash;<i>Savu</i>, S.E. of Timor, appears in Flinders&rsquo; chart to be
+fringed; but I have not coloured it, as I do not know that the reefs are of
+coral.&mdash; <i>Sandalwood</i> 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.
+</p>
+
+<p>
+N.W. C<small>OAST OF</small> A<small>USTRALIA</small>.&mdash;It appears, in
+Captain King&rsquo;s Sailing Directions (&ldquo;Narrative of Survey,&rdquo;
+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&rsquo;s Abrolhos (latitude 28° S. on west coast)
+have lately been surveyed by Captain Wickham (as described in &ldquo;Naut.
+Mag.&rdquo; 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&rsquo;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.&mdash;<i>Rowley Shoals</i>. These lie some way from the
+N.W. coast of Australia: according to Captain King (&ldquo;Narrative of
+Survey,&rdquo; 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. &ldquo;A third oval reef of the same group is entirely submerged&rdquo;
+(&ldquo;Principles of Geology,&rdquo; book iii. chapter xviii.); coloured
+blue.&mdash;<i>Scott&rsquo;s Reefs</i>, lying north of Rowley Shoals, are
+briefly described by Captain Wickham (&ldquo;Naut. Mag.&rdquo; 1841, page 440):
+they appear to be of great size, of a circular form, and &ldquo;with smooth
+water within, forming probably a lagoon of great extent.&rdquo; There is a
+break on the western side, where there probably is an entrance: the water is
+very deep off these reefs; coloured blue.
+</p>
+
+<p>
+Proceeding westward along the great volcanic chain of the East Indian
+Archipelago, <i>Solor Strait</i> is represented in a chart published by
+Dalrymple from a Dutch MS., as fringed; as are parts of <i>Flores</i>, of
+<i>Adenara</i>, and of <i>Solor</i>. 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 <i>Sapy</i> Bay. From the same
+authority it appears (page 610) that reefs fringe the island of
+<i>Timor-Young</i>, on the N. shore of Sumbawa; and, likewise (page 600), that
+<i>Bally</i> town in <i>Lombock</i>, 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.&mdash;<i>Bally</i> 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.
+</p>
+
+<p>
+J<small>AVA</small>.&mdash;My information regarding the reefs of this great
+island is derived from the chart just mentioned. The greater part of
+<i>Madura</i> 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 <i>Sourabaya</i>. The islets and parts of the N.
+coast of Java, west of <i>Point Buang</i>, or <i>Japara</i>, are fringed by
+reefs, said to be of coral. <i>Lubeck</i>, or <i>Bavian</i> Islands, lying at
+some distance from the shore of Java, are regularly fringed by coral-reefs.
+<i>Carimon Java</i> 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 <i>Sunda</i> Strait, where the water is
+from forty to eighty fathoms deep, and the islets near <i>Batavia</i> 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 <i>Segorrowodee</i> Bay, and the extreme S.E. and E. portions are likewise
+fringed by coral-reefs; all the above-mentioned places coloured red.
+</p>
+
+<p>
+<i>Macassar Strait</i>; the east coast of <i>Borneo</i> appears, in most parts,
+free from reefs, and where they occur, as on the east coast of
+<i>Pamaroong</i>, the sea is very shallow; hence no part is coloured. In
+<i>Macassar</i> 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&#x2032; 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 <i>Postillions</i>, <i>Kalatoa</i>,
+<i>Tokan-Bessees</i>, &amp;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 <i>Bonin</i>, 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 <i>Bouton</i> which, according to
+an old chart, is formed by a reef, parallel to the shore, with deep water
+within; and in the &ldquo;Voyage of the &lsquo;Coquille&rsquo;,&rdquo; 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.
+</p>
+
+<p>
+S<small>UMATRA</small>.&mdash;Commencing with the west coast and outlying
+islands, <i>Engano Island</i> is represented in the published chart as
+surrounded by a narrow reef, and Napier, in his &ldquo;Sailing
+Directions,&rdquo; speaks of the reef being of coral (also Horsburgh, volume
+ii., page 115); coloured red.&mdash;<i>Rat Island</i> (3° 51&#x2032; S.) is
+surrounded by reefs of coral, partly dry at low water, (Horsburgh, volume ii.,
+page 96).&mdash;<i>Trieste Island</i> (4° 2&#x2032; 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, &ldquo;Voyage,&rdquo;
+volume i., page 474), I have not coloured it.&mdash;<i>Pulo Dooa</i> (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.&mdash;<i>Pulo Pisang;</i> 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.&mdash;<i>Pulo Mintao</i> is lined with
+reefs on its west side (Horsburgh, volume ii., page 107).&mdash;<i>Pulo
+Baniak;</i> the same authority (volume ii., page 105), speaking of a part, says
+it is faced with coral-rocks.&mdash;<i>Minguin</i> (3° 36&#x2032; N.). A
+coral-reef fronts this place, and projects into the sea nearly a quarter of a
+mile (&ldquo;Notices of the Indian Arch.&rdquo; published at Singapore, page
+105).&mdash;<i>Pulo Brassa</i> (5° 46&#x2032; N.). A reef surrounds it at a
+cable&rsquo;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 &ldquo;Notices&rdquo; 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 (&ldquo;History of Sumatra&rdquo;)
+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.
+</p>
+
+<p>
+N<small>ICOBAR</small> I<small>SLANDS</small>.&mdash;The chart represents the
+islands of this group as fringed by reefs. With regard to <i>Great Nicobar</i>,
+Captain Moresby informs me, that it is fringed by reefs of coral, extending
+between two and three hundred yards from the shore. The <i>Northern
+Nicobars</i> appear so regularly fringed in the published charts, that I have
+no doubt the reefs are of coral. This group, therefore, is coloured red.
+</p>
+
+<p>
+A<small>NDAMAN</small> I<small>SLANDS</small>.&mdash;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
+&ldquo;Asiatic Researches&rdquo; (volume iv., page 402) led me to doubt the
+existence of reefs; uncoloured.
+</p>
+
+<p>
+The coast of <i>Malacca</i>, <i>Tenasserim</i> and the coasts northward, appear
+in the greater part to be low and muddy: where reefs occur, as in parts of
+<i>Malacca Straits</i>, and near <i>Singapore</i>, 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 <i>Natunas</i> and the <i>Anambas</i>
+Islands are represented in the charts on a large scale, published in the
+&ldquo;Atlas of the Voyage of the &lsquo;Favourite&rsquo;,&rdquo; as fringed by
+reefs of coral, with very shoal water within them.&mdash;<i>Tumbelan</i> and
+<i>Bunoa</i> Islands (1° N.) are represented in the English charts as
+surrounded by a very regular fringe.&mdash; <i>St. Barbes</i> (0° 15&#x2032;
+N.) is said by Horsburgh (volume ii., page 279) to be fronted by a reef, over
+which boats can land only at high water.&mdash;The shore of <i>Borneo</i> at
+<i>Tunjong Apee</i> 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
+<i>Pulo Leat</i>, in Gaspar Strait, <i>Lucepara</i>, and <i>Carimata;</i> but
+as the sea is confined and shallow, and the reefs not very regular, I have left
+them uncoloured.
+</p>
+
+<p>
+The water shoals gradually towards the whole west coast of <i>Borneo:</i> 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 <i>Palawan</i>, are fringed by
+very distant coral-reefs; thus the reefs in the case of <i>Balabac</i> 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 <i>Sooloo Archipelago</i>, about which I have
+been able to obtain very little information; <i>Pangootaran</i>, although ten
+miles long, entirely consists of a bed of coral-rock (&ldquo;Notices of E.
+Indian Arch.&rdquo; page 58): I believe from Horsburgh that the island is low;
+not coloured.&mdash;<i>Tahow Bank</i>, in some old charts, appears like a
+submerged atoll; not coloured. Forrest (&ldquo;Voyage,&rdquo; 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 <i>Basselan</i>, some of the islets in the
+chart accompanying Forrest&rsquo;s &ldquo;Voyage,&rdquo; 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, <i>Cagayanes</i> and
+<i>Cavilli</i>, 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 <i>Palawan</i>, 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.
+</p>
+
+<p>
+P<small>HILIPPINE</small> A<small>RCHIPELAGO</small>.&mdash;A chart on a large
+scale of <i>Appoo shoal</i>, 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.&mdash;<i>Mindoro:</i> the N.W. coast is represented in several charts, as
+fringed by a reef, and <i>Luban Island</i> is said, by Horsburgh (volume ii.,
+page 436), to be &ldquo;lined by a reef.&rdquo;&mdash;<i>Luzon:</i> 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
+&ldquo;lined by a coral-reef, stretching out nearly a mile in some
+places,&rdquo; (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 <i>Masbate</i>, and nearly the whole of
+<i>Bohol;</i> which are both coloured red. On the S. shore of
+<i>Magindanao</i>, Bunwoot Island is surrounded (according to Forrest,
+&ldquo;Voyage,&rdquo; 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.
+</p>
+
+<p>
+B<small>ABUYAN</small> I<small>SLANDS</small>.&mdash;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. <i>Camiguin</i> 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.&mdash;B<small>ABUYAN</small> I<small>SLANDS</small>: 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.&mdash;F<small>ORMOSA</small>. 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.&mdash;
+P<small>ATCHOW</small>, or M<small>ADJIKO-SIMA</small> G<small>ROUPS</small>.
+<i>Patchuson</i> has been described by Captain Broughton (&ldquo;Voy. to the N.
+Pacific,&rdquo; 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.&mdash;<i>Taypin-san;</i> 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 <i>Kumi</i> (lying west of Patchow), has
+a narrow reef attached to it in the plan of it, in La Peyrouse&rsquo;s
+&ldquo;Atlas;&rdquo; but it does not appear in the account of the voyage that
+it is of coral; uncoloured.&mdash;L<small>OO</small> C<small>HOO</small>. 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&rsquo;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.&mdash;P<small>ESCADORES</small> (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.
+</p>
+
+<p>
+C<small>HINA</small> S<small>EA</small>.&mdash;Proceeding from north to south,
+we first meet the <i>Pratas Shoal</i> (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&rsquo;s edge,
+and when the sea runs high, there are breakers mostly all round, &ldquo;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;&rdquo; coloured blue.&mdash;The <i>Paracells</i> 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
+(<i>Prattle</i>, <i>Robert</i>, <i>Drummond</i>, etc.) are so arranged round a
+moderately shallow space, as to appear as if they had once formed one large
+atoll.&mdash; <i>Bombay Shoal</i> (one of the Paracells) has the form of an
+annular reef, and is &ldquo;apparently deep within;&rdquo; it seems to have an
+entrance (Horsburgh, volume ii., page 332) on its west side; it is very steep
+outside.&mdash;<i>Discovery Shoal</i>, 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.&mdash;<i>Macclesfield Bank:</i> 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.&mdash;<i>Scarborough
+Shoal:</i> 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.&mdash;The sea off the west coast
+of Palawan and the northern part of Borneo is strewed with shoals: <i>Swallow
+Shoal</i>, according to Horsburgh (volume ii., page 431) &ldquo;is formed,
+<i>like most</i> of the shoals hereabouts, of a belt of coral-rocks,
+&ldquo;with a basin of deep water within.&rdquo;&mdash;<i>Half-Moon Shoal</i>
+has a similar structure; Captain D. Ross describes it, as a narrow belt of
+coral-rock, &ldquo;with a basin of deep water in the centre,&rdquo; and deep
+sea close outside.&mdash;<i>Bombay Shoal</i> appears (Horsburgh, volume ii.,
+page 432) &ldquo;to be a basin of smooth water surrounded by breakers.&rdquo;
+These three shoals I have coloured blue.&mdash;The <i>Paraquas Shoals</i> are
+of a circular form, with deep gaps running through them; not coloured.&mdash;A
+bank gradually shoaling to the depth of thirty fathoms, extends to a distance
+of about twenty miles from the northern part of <i>Borneo</i>, and to thirty
+miles from the northern part of <i>Palawan</i>. 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 &ldquo;that most of the shoals hereabouts are
+formed of a belt of coral.&rdquo; 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.&mdash;The coast of <i>China</i>, <i>Tonquin</i>, and <i>Cochin-China</i>,
+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 &ldquo;Atlas of the Voyage of the
+&lsquo;Favourite&rsquo;.&rdquo;
+</p>
+
+<p>
+I<small>NDIAN</small> O<small>CEAN</small>.&mdash;<i>South Keeling</i> has been
+specially described: nine miles north of it lies North Keeling, a very small
+atoll, surveyed by the &ldquo;Beagle,&rdquo; the lagoon of which is dry at low
+water.&mdash;<i>Christmas Island</i>, lying to the east, is a high island,
+without, as I have been informed by a person who passed it, any reefs at
+all.&mdash;C<small>EYLON</small>: a space about eighty miles in length of the
+south-western and southern shores of these islands has been described by Mr.
+Twynam (&ldquo;Naut. Mag.&rdquo; 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.&mdash;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 <i>Madura</i>.
+</p>
+
+<p>
+C<small>HAGOS</small>, M<small>ALDIVA</small>, and L<small>ACCADIVE</small>
+A<small>RCHIPELAGOES</small>.&mdash;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 <i>Chagos</i> and
+<i>Maldiva</i> groups are entirely formed of great atolls, or lagoon-formed
+reefs, surmounted by islets. In the <i>Laccadive</i> group, this structure is
+less evident; the islets are low, not exceeding the usual height of
+coral-formations (see Lieutenant Wood&rsquo;s account, &ldquo;Geographical
+Journal&rdquo;, 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.
+</p>
+
+<p>
+<i>Sahia de Malha</i>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&rsquo;s information in composition, the Pitt&rsquo;s Bank
+in the Chagos group; and the Pitt&rsquo;s Bank, must, after what has been shown
+of the Great Chagos Bank, be considered as a sunken, half-destroyed atoll;
+hence coloured blue.&mdash;<i>Cargados Carajos Bank</i>. 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;&mdash;therefore not coloured.&mdash;<i>Ile de Sable</i> is a little
+island, lying west of C. Carajos, only some toises in height (&ldquo;Voyage of
+the &lsquo;Favourite&rsquo;,&rdquo; 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.&mdash;<i>Mauritius</i>. The
+reefs round this island have been described in the chapter on fringing-reefs;
+coloured red. &mdash;<i>Rodriguez</i>. 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
+(&ldquo;Life of Sir J. Makintosh,&rdquo; 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. &mdash;<i>Bourbon</i>. The greater part of the shores of this
+island are without reefs; but Captain Carmichael (Hooker&rsquo;s &ldquo;Bot.
+Misc.&rdquo;) 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.
+</p>
+
+<p>
+S<small>EYCHELLES</small>.&mdash;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&rsquo;s chart,
+and in that in the &ldquo;Atlas of the Voyage of the
+&lsquo;Favourite&rsquo;,&rdquo; it appears that the east side of <i>Mahé</i>
+and the adjoining islands of <i>St. Anne</i> and <i>Cerf</i>, are regularly
+fringed by coral-reefs. A portion of the S.E. part of <i>Curieuse Island</i>,
+the N., and part of the S.W. shore of <i>Praslin Island</i>, and the whole west
+side of <i>Digue Island</i>, appear fringed. From a MS. account of these
+islands by Captain F. Moresby, in the Admiralty, it appears that
+<i>Silhouette</i> is also fringed; he states that all these islands are formed
+of granite and quartz, that they rise abruptly from the sea, and that
+&ldquo;coral-reefs have grown round them, and project for some distance.&rdquo;
+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. <i>Amirantes Islands:</i> 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.&mdash;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&mdash;<i>Platte</i>, <i>Alphonse</i>, <i>Coetivi</i>,
+<i>Galega</i>, <i>Providence</i>, <i>St. Pierre</i>, <i>Astova</i>,
+<i>Assomption</i>, and <i>Glorioso</i>, 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, <i>Juan de Nova</i>, which appears from some plans and
+accounts to be an atoll; but from others does not appear to be so; not
+coloured. Secondly <i>Cosmoledo;</i> &ldquo;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&rdquo; (Horsburgh, volume i., page 151); coloured blue. Thirdly,
+<i>Aldabra;</i> 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?&mdash;uncoloured.
+</p>
+
+<p>
+C<small>OMORO</small> G<small>ROUP</small>.&mdash;<i>Mayotta</i>, 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.&mdash;<i>Johanna;</i> 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 (&ldquo;Narr.&rdquo; 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.&mdash;<i>Mohilla;</i> 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&rsquo;s chart of Madagascar, this island
+is represented as encircled; coloured blue.&mdash;<i>Great Comoro Island</i>
+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
+<i>shore</i>, 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.
+</p>
+
+<p>
+M<small>ADAGASCAR</small>.&mdash;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 <i>Star Bank</i> (in latitude 25° S.) the coast for ten
+miles is fringed by a reef; coloured red. The shore immediately S. of <i>St.
+Augustine&rsquo;s Bay</i> appears fringed; but <i>Tullear</i> 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 <i>lat</i>. 22° 16&#x2032; <i>to</i> 21° 37&#x2032;,
+the shore is fringed by coral-reefs (see Lieutenant Boteler&rsquo;s
+&ldquo;Narrative,&rdquo; 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&#x2032;; I have coloured the whole of this part of the coast red. The
+islands of <i>Juan de Nova</i> (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, &ldquo;are faced on the edge of deep water by a line of sharp-pointed
+coral-rocks.&rdquo; Nevertheless I have not coloured this part, as I cannot
+make out by the charts that the coast itself is fringed. The headlands of
+<i>Narrenda</i> and <i>Passandava Bays</i> (14° 40&#x2032;) and the islands in
+front of <i>Radama harbour</i> are represented in the plans as regularly
+fringed, and have accordingly been coloured red. With respect to the <i>East
+coast of Madagascar</i>, Dr. Allan informs me in a letter, that the whole line
+of coast, from <i>Tamatave</i>, in 18° 12&#x2032;, to <i>C. Amber</i>, 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&rsquo;s
+charts, also, the existence of these reefs, which evidently belong to the
+fringing class, on some parts, namely N. of <i>British Sound</i>, and near
+<i>Ngoncy</i>, of the above line of coast might have been inferred. Lieutenant
+Boteler (volume i., page 155) speaks of &ldquo;the reef surrounding the island
+of <i>St. Mary&rsquo;s</i> at a small distance from the shore.&rdquo; 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.
+</p>
+
+<p>
+E<small>AST</small> C<small>OAST OF</small>
+A<small>FRICA</small>.&mdash;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 <i>Mukdeesha</i> (10° 1&#x2032;
+N.) there is a coral-reef extending four or five miles along the shore
+(Owen&rsquo;s &ldquo;Narr.&rdquo; 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
+<i>Juba</i>, a little S. of the equator, to <i>Lamoo</i> (in 2° 20&#x2032; S.)
+&ldquo;the coast and islands are formed of madrepore&rdquo; (Owen&rsquo;s
+&ldquo;Narrative,&rdquo; volume i., page 363). The chart of this part (entitled
+<i>Dundas Islands</i>), 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&mdash;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
+(&ldquo;Narr.&rdquo; volume i., page 369) describes the muddy bay of
+<i>Patta</i>, 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.&mdash; <i>Maleenda</i> (3° 20&#x2032;
+S.). In the plan of the harbour, the south headland appears fringed; and in
+Owen&rsquo;s chart on a larger scale, the reefs are seen to extend nearly
+thirty miles southward; coloured red.&mdash;<i>Mombas</i> (4° 5&#x2032; S.).
+The island which forms the harbour, &ldquo;is surrounded by cliffs of
+madrepore, capable of being rendered almost impregnable&rdquo; (Owen&rsquo;s
+&ldquo;Narr.&rdquo; 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.&mdash;<i>Pemba</i>
+(5° S.) is an island of coral-formation, level, and about two hundred feet in
+height (Owen&rsquo;s &ldquo;Narr.&rdquo; 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.&mdash;<i>Zanzibar</i> 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&rsquo;s &ldquo;Narr.&rdquo; volume ii.,
+page 39) that they were composed of sand; not coloured.&mdash;<i>Latham&rsquo;s
+Bank</i> is a small island, fringed by coral-reefs; but being only ten feet
+high, it has not been coloured.&mdash;<i>Monfeea</i> 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.&mdash;
+<i>Keelwa</i>. 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&rsquo;s general chart. The coast in the plans
+of the rivers <i>Lindy</i> and <i>Monghow</i> (9° 59&#x2032; and 10° 7&#x2032;
+S.) has the same structure; coloured red.&mdash;<i>Querimba Islands</i> (from
+10° 40&#x2032; to 13° S.). A chart on a large scale is given of these islands;
+they are low, and of coral-formation (Boteler&rsquo;s &ldquo;Narr.&rdquo;
+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.&mdash;<i>Monabila</i> (13° 32&#x2032; S.). In the plan of this harbour,
+the headlands outside are fringed by reefs apparently of coral; coloured
+red.&mdash;<i>Mozambique</i> (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&rsquo;s &ldquo;Narr.&rdquo;
+(volume i., page 162), the shore from <i>Mozambique</i> to <i>Delagoa Bay</i>
+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.
+</p>
+
+<p>
+P<small>ERSIAN</small> G<small>ULF</small>.&mdash;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 <i>Ormuz</i> and <i>Larrack</i>, appear so regularly fringed,
+that I have coloured them red. There are certainly no atolls in the Persian
+Gulf. The shores of <i>Immaum</i>, 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 <i>Arabia Felix</i>, and the shores of
+<i>Socotra</i> appear from the charts and memoir of Captain Haines
+(&ldquo;Geographical Journal,&rdquo; 1839, page 125) to be without any reefs. I
+believe there are no extensive coral-reefs on any part of the coasts of
+<i>India</i>, except on the low promontory of <i>Madura</i> (as already
+mentioned) in front of Ceylon.
+</p>
+
+<p>
+R<small>ED</small> S<small>EA</small>.&mdash;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, &ldquo;Über die Natur der Corallen-Bänken des Rothen
+Meeres,&rdquo; 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&#x2032; 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.
+</p>
+
+<p>
+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 <i>Dhalac</i> 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 <i>Farsan</i> 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;<a href="#fn7.2" name="fnref7.2"><sup>[2]</sup></a> their shores are
+fringed by living coral-reefs.
+</p>
+
+<p class="footnote">
+<a name="fn7.2"></a> <a href="#fnref7.2">[2]</a> Rüppell, &ldquo;Reise in
+Abyssinie,&rdquo; Band. i., S. 247.
+</p>
+
+<p>
+From the account given by Rüppell<a href="#fn7.3"
+name="fnref7.3"><sup>[3]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn7.3"></a> <a href="#fnref7.3">[3]</a> Rüppell, &ldquo;Reise in
+Abyssinie,&rdquo; Band. i., S. 245.
+</p>
+
+<p>
+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 (&ldquo;Uber die,&rdquo;
+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.
+</p>
+
+<p>
+<i>The West Coast of the Red Sea between lat.</i> 19° <i>and</i>
+22°.&mdash;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&#x2032;, 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 <i>Macowa</i> (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.
+</p>
+
+<p>
+<i>The West Coast from lat.</i> 22° <i>to</i> 24°.&mdash;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&#x2032;, 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.&mdash;<i>On the west coast from
+lat.</i> 19° <i>to</i> 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.
+</p>
+
+<p>
+<i>Eastern Coast</i>.&mdash;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&#x2032;. 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.
+</p>
+
+<p>
+In the <i>southern parts</i> 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 <i>northern parts of the western coast</i> (north of latitude 24°
+30&#x2032;), and likewise the shores of the chief part of the <i>Gulf of
+Suez</i>. In the <i>Gulf of Acaba</i>, as I am informed by Captain Moresby
+there are no coral-reefs, and the water is profoundly deep.
+</p>
+
+<p>
+W<small>EST</small> I<small>NDIES</small>.&mdash;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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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:&mdash;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.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+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 (&ldquo;Columbian
+Navigator,&rdquo; 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.<a href="#fn7.4" name="fnref7.4"><sup>[4]</sup></a> and of the
+United States from Long Island (as observed by Professor Rogers) to Florida
+have the same character. Professor Rogers, in his &ldquo;Report to the British
+Association&rdquo; (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<a href="#fn7.5"
+name="fnref7.5"><sup>[5]</sup></a> 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.
+</p>
+
+<p class="footnote">
+<a name="fn7.4"></a> <a href="#fnref7.4">[4]</a> In the &ldquo;London and
+Edinburgh Philosophical Journal,&rdquo; 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.
+</p>
+
+<p class="footnote">
+<a name="fn7.5"></a> <a href="#fnref7.5">[5]</a> In the ordinary sea-charts, no
+lagoons appear on the coast of Florida, north of 26°; but Major Whiting
+(&ldquo;Silliman&rsquo;s Journal,&rdquo; volume xxxv., page 54) says that many
+are formed by sand thrown up along the whole line of coast from St.
+Augustine&rsquo;s to Jupiter Inlet.
+</p>
+
+<p>
+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 <i>Bahama Banks</i> are fringed,
+especially on their windward sides, with living reefs; and hence I have
+coloured those, which are thus represented in Captain Owen&rsquo;s late chart,
+red. The same officer informs me, that the islands along the southern part of
+<i>Florida</i> are similarly fringed; coloured red. C<small>UBA</small>:
+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&rsquo;s
+&ldquo;Mag. of Nat. Hist.&rdquo; volume ix., page 449); he states that they
+enclosed a space called the &ldquo;<i>baxo</i>,&rdquo; 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&#x2032;, 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 &ldquo;<i>cays</i>,&rdquo; or low islands on its edge, there is a marked
+correspondence (as observed by Humboldt, &ldquo;Pers. Narr.&rdquo; 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 <i>Colorado
+Shoals</i> (see Captain Owen&rsquo;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 <i>Colorados</i>, 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 <i>Isle of Pines</i> 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, &ldquo;Pers. Narr.&rdquo; 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 &ldquo;Columbian Navigator&rdquo; (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 &ldquo;Columb. Navig.&rdquo; says
+extend some way from the coast, but have only from nine to twelve feet water on
+them; these are coloured red.&mdash;I have not been able to procure any
+detailed description of the large groups of banks and &ldquo;cays&rdquo;
+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.
+</p>
+
+<p>
+Y<small>UCUTAN</small>.&mdash;The N.E. part of the promontory appears in
+Captain Owen&rsquo;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 <i>Honduras</i>) 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&#x2032; and 16° 12&#x2032;) 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,
+<i>Turneffe</i>, <i>Lighthouse</i>, and <i>Glover</i> 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. <i>turneffe
+Reef</i> 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 <i>Northern Triangles</i>, on which there is
+an islet between seventy and eighty feet, and to <i>Cozumel</i> 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.
+</p>
+
+<p>
+In front of the eastern <i>Mosquito</i> 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.
+</p>
+
+<p>
+<i>Cayman Island</i>: 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.&mdash;<i>Jamaica</i>: 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.&mdash;<i>St. Domingo</i>: 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.&mdash;<i>Puerto Rico</i>: 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.&mdash;Some miles in length of the
+southern side of the Island of <i>St, Thomas</i> is fringed; most of the
+<i>Virgin Gorda</i> Islands, as I am informed by Mr. Schomburgk, are fringed;
+the shores of <i>Anegada</i>, 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 <i>Santa Cruz</i> appears in the Danish survey to be fringed
+(see also Prof. Hovey&rsquo;s account of this island, in
+&ldquo;Silliman&rsquo;s Journal,&rdquo; 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.&mdash;The
+<i>Antilles</i>, as remarked by Von Buch (&ldquo;Descrip. Iles Canaries,&rdquo;
+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, <i>Barbuda</i> and the
+western coasts of <i>Antigua</i> and <i>Mariagalante</i> appear to be fringed:
+this is also the case with <i>Barbadoes</i>, 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 <i>Martinique</i>, 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.
+</p>
+
+<p>
+T<small>HE</small> B<small>ERMUDA</small> I<small>SLANDS</small> have been
+carefully described by Lieutenant Nelson, in an excellent Memoir in the
+&ldquo;Geological Transactions&rdquo; (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,&mdash;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&rsquo;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&rsquo;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.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap08"></a>INDEX.</h2>
+
+<p>
+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.
+</p>
+
+<p>
+ABROLHOS, Brazil, coated by corals.
+</p>
+
+<p>
+Abrolhos (Australia).
+</p>
+
+<p>
+ABSENCE of coral-reefs from certain coasts.
+</p>
+
+<p>
+Acaba, gulf of.
+</p>
+
+<p>
+Admiralty group.
+</p>
+
+<p>
+AFRICA, east coast, fringing-reef of.<br/>
+Madreporitic rock of.
+</p>
+
+<p>
+Africa, east coast.
+</p>
+
+<p>
+AGE of individual corals.
+</p>
+
+<p>
+Aiou.
+</p>
+
+<p>
+Aitutaki.
+</p>
+
+<p>
+Aldabra.
+</p>
+
+<p>
+Alert reef.
+</p>
+
+<p>
+Alexander, Grand Duke, island.
+</p>
+
+<p>
+ALLAN, Dr., on Holuthuriæ feeding on corals. On quick growth of corals at
+Madagascar. On reefs affected by currents.
+</p>
+
+<p>
+Alloufatou.
+</p>
+
+<p>
+Alphonse.
+</p>
+
+<p>
+Amargoura. (Amargura.)
+</p>
+
+<p>
+Amboina.
+</p>
+
+<p>
+America, west coast.
+</p>
+
+<p>
+Amirantes.
+</p>
+
+<p>
+Anachorites.
+</p>
+
+<p>
+Anambas.
+</p>
+
+<p>
+ANAMOUKA, description of.
+</p>
+
+<p>
+Anamouka.
+</p>
+
+<p>
+Anadaman islands.
+</p>
+
+<p>
+Antilles.
+</p>
+
+<p>
+Appoo reef.
+</p>
+
+<p>
+Arabia Felix.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+Arru group.
+</p>
+
+<p>
+Arzobispo.
+</p>
+
+<p>
+ASCIDIA, depth at which found.
+</p>
+
+<p>
+Assomption.
+</p>
+
+<p>
+Astova.
+</p>
+
+<p>
+Atlantic islands.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+Augustine, St.
+</p>
+
+<p>
+AURORA island, an upraised atoll.
+</p>
+
+<p>
+Aurora.
+</p>
+
+<p>
+AUSTRAL islands, recently elevated.
+</p>
+
+<p>
+Austral islands.
+</p>
+
+<p>
+Australia, N.W. coast.
+</p>
+
+<p>
+AUSTRALIAN barrier-reef.
+</p>
+
+<p>
+Australian barrier.
+</p>
+
+<p>
+Babuyan group.
+</p>
+
+<p>
+Bahama banks.
+</p>
+
+<p>
+Balahac.
+</p>
+
+<p>
+Bally.
+</p>
+
+<p>
+Baring.
+</p>
+
+<p>
+BARRIER-REEF of Australia. Of New Caledonia.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+Bampton shoal.
+</p>
+
+<p>
+Banks islands.
+</p>
+
+<p>
+Banks in the West Indies.
+</p>
+
+<p>
+Bashee islands.
+</p>
+
+<p>
+Bass island.
+</p>
+
+<p>
+Batoa.
+</p>
+
+<p>
+Beaupre reef.
+</p>
+
+<p>
+BEECHEY, Captain, obligations of the author to. On submerged reefs. Account of
+Matilda island.
+</p>
+
+<p>
+BELCHER, Captain, on boring through coral-reef.
+</p>
+
+<p>
+Belize reef, off.
+</p>
+
+<p>
+Bellinghausen.
+</p>
+
+<p>
+Bermuda islands.
+</p>
+
+<p>
+Beveridge reef.
+</p>
+
+<p>
+Bligh.
+</p>
+
+<p>
+BOLABOLA, view of.
+</p>
+
+<p>
+Bombay shoal.
+</p>
+
+<p>
+Bonin Bay.
+</p>
+
+<p>
+Bonin group.
+</p>
+
+<p>
+BORINGS through coral-reefs.
+</p>
+
+<p>
+BORNEO, W. coast, recently elevated.
+</p>
+
+<p>
+Borneo, E. coast. S.W. and W. coast N. coast. Western bank.
+</p>
+
+<p>
+Boscawen.
+</p>
+
+<p>
+Boston.
+</p>
+
+<p>
+Bouka.
+</p>
+
+<p>
+Bourbon.
+</p>
+
+<p>
+Bourou.
+</p>
+
+<p>
+Bouton.
+</p>
+
+<p>
+BRAZIL, fringing-reefs on coast of.
+</p>
+
+<p>
+BREACHES through barrier-reefs.
+</p>
+
+<p>
+Brook.
+</p>
+
+<p>
+Bunker.
+</p>
+
+<p>
+Bunoa.
+</p>
+
+<p>
+BYRON.
+</p>
+
+<p>
+Cagayanes.
+</p>
+
+<p>
+Candelaria.
+</p>
+
+<p>
+Cargados Carajos.
+</p>
+
+<p>
+Caroline archipelago.
+</p>
+
+<p>
+Caroline island.
+</p>
+
+<p>
+Carteret shoal.
+</p>
+
+<p>
+CARYOPHYLLIA, depth at which it lives.
+</p>
+
+<p>
+Cavilli.
+</p>
+
+<p>
+Cayman island.
+</p>
+
+<p>
+Celebes.
+</p>
+
+<p>
+Ceram.
+</p>
+
+<p>
+CEYLON, recently elevated.
+</p>
+
+<p>
+Ceylon.
+</p>
+
+<p>
+CHAGOS Great Bank, description and theory of.
+</p>
+
+<p>
+CHAGOS group.
+</p>
+
+<p>
+Chagos group.
+</p>
+
+<p>
+CHAMA-SHELLS embedded in coral-rock.
+</p>
+
+<p>
+CHAMISSO, on corals preferring the surf.
+</p>
+
+<p>
+CHANGES in the state of Keeling atoll. Of atolls.
+</p>
+
+<p>
+CHANNELS leading into the lagoons of atolls. Into the Maldiva atolls. Through
+barrier-reefs.
+</p>
+
+<p>
+Chase.
+</p>
+
+<p>
+China sea.
+</p>
+
+<p>
+CHRISTMAS atoll.
+</p>
+
+<p>
+Christmas atoll.
+</p>
+
+<p>
+Christmas island (Indian Ocean).
+</p>
+
+<p>
+Clarence.
+</p>
+
+<p>
+Clipperton rock.
+</p>
+
+<p>
+COCOS, or Keeling atoll.
+</p>
+
+<p>
+Cocos (or Keeling).
+</p>
+
+<p>
+Cocos island (Pacific).
+</p>
+
+<p>
+COCHIN China, encroachments of the sea on the coast.
+</p>
+
+<p>
+Cochin China.
+</p>
+
+<p>
+Coetivi.
+</p>
+
+<p>
+Comoro group.
+</p>
+
+<p>
+COMPOSITION of coral-formations.
+</p>
+
+<p>
+CONGLOMERATE coral-rock on Keeling atoll. On other atolls. Coral-rock.
+</p>
+
+<p>
+COOK islands, recently elevated.
+</p>
+
+<p>
+Cook islands.
+</p>
+
+<p>
+CORAL-BLOCKS bored by vermiform animals.
+</p>
+
+<p>
+CORAL-REEFS, their distribution and absence from certain areas. Destroyed by
+loose sediment.
+</p>
+
+<p>
+CORAL-ROCK at Keeling atoll. Mauritius. Organic remains of.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+CORALLIAN sea.
+</p>
+
+<p>
+Corallian sea.
+</p>
+
+<p>
+Cornwallis.
+</p>
+
+<p>
+Cosmoledo.
+</p>
+
+<p>
+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&rsquo;s theory.
+</p>
+
+<p>
+CRESCENT-FORMED reefs.
+</p>
+
+<p>
+Cuba.
+</p>
+
+<p>
+CUMING, Mr., on the recent elevation of the Philippines.
+</p>
+
+<p>
+Dangerous, or Low archipelago.
+</p>
+
+<p>
+Danger islands.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+Dhalac group.
+</p>
+
+<p>
+DIEGO GARCIA, slow growth of reef.
+</p>
+
+<p>
+DIMENSIONS of the larger groups of atolls.
+</p>
+
+<p>
+DISSEVERMENT of the Maldiva atolls, and theory of.
+</p>
+
+<p>
+DISTRIBUTION of coral-reefs.
+</p>
+
+<p>
+Domingo, St.
+</p>
+
+<p>
+DORY, Port, recently elevated.
+</p>
+
+<p>
+Dory, Port.
+</p>
+
+<p>
+Duff islands.
+</p>
+
+<p>
+Durour.
+</p>
+
+<p>
+Eap.
+</p>
+
+<p>
+EARTHQUAKES at Keeling atoll. In groups of atolls. In Navigator archipelago.
+</p>
+
+<p>
+EAST INDIAN ARCHIPELAGO, recently elevated.
+</p>
+
+<p>
+Easter.
+</p>
+
+<p>
+Echequier.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+Eimeo.
+</p>
+
+<p>
+ELEVATED reef of Mauritius.
+</p>
+
+<p>
+ELEVATIONS, recent proofs of. Immense areas of.
+</p>
+
+<p>
+Elivi.
+</p>
+
+<p>
+ELIZABETH island. Recently elevated.
+</p>
+
+<p>
+Elizabeth island.
+</p>
+
+<p>
+Ellice group.
+</p>
+
+<p>
+ENCIRCLED ISLANDS, their height. Geological composition.
+</p>
+
+<p>
+EOUA, description of.
+</p>
+
+<p>
+Eoua.
+</p>
+
+<p>
+ERUPTED MATTER probably not associated with thick masses of coral-rock.
+</p>
+
+<p>
+FAIS, recently elevated.
+</p>
+
+<p>
+Fais.
+</p>
+
+<p>
+Fanning.
+</p>
+
+<p>
+Farallon de Medinilla.
+</p>
+
+<p>
+Farson group.
+</p>
+
+<p>
+Fataka.
+</p>
+
+<p>
+FIJI archipelago.
+</p>
+
+<p>
+FISH, feeding on corals. Killed in Keeling lagoon by heavy rain.
+</p>
+
+<p>
+FISSURES across coral-islands.
+</p>
+
+<p>
+FITZROY, Captain, on a submerged shed at Keeling atoll. On an inundation in the
+Low archipelago.
+</p>
+
+<p>
+Flint.
+</p>
+
+<p>
+Flores.
+</p>
+
+<p>
+Florida.
+</p>
+
+<p>
+Folger.
+</p>
+
+<p>
+Formosa.
+</p>
+
+<p>
+FORSTER, theory of coral-formations.
+</p>
+
+<p>
+Frederick reef.
+</p>
+
+<p>
+Freewill.
+</p>
+
+<p>
+FRIENDLY group recently elevated.
+</p>
+
+<p>
+Friendly archipelago.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+Galapagos archipelago.
+</p>
+
+<p>
+Galega.
+</p>
+
+<p>
+GAMBIER islands, section of.
+</p>
+
+<p>
+Gambier islands.
+</p>
+
+<p>
+Gardner.
+</p>
+
+<p>
+Gaspar rico.
+</p>
+
+<p>
+GEOLOGICAL COMPOSITION of coral-formations.
+</p>
+
+<p>
+Gilbert archipelago.
+</p>
+
+<p>
+Gilolo.
+</p>
+
+<p>
+Glorioso.
+</p>
+
+<p>
+GLOUCESTER Island.
+</p>
+
+<p>
+Glover reef.
+</p>
+
+<p>
+Gomez.
+</p>
+
+<p>
+Gouap.
+</p>
+
+<p>
+Goulou.
+</p>
+
+<p>
+Grampus.
+</p>
+
+<p>
+Gran Cocal.
+</p>
+
+<p>
+GREAT CHAGOS BANK, description and theory of.
+</p>
+
+<p>
+GREY, Captain, on sandbars.
+</p>
+
+<p>
+GROUPING of the different classes of reefs.
+</p>
+
+<p>
+Guedes.
+</p>
+
+<p>
+HALL, Captain B., on Loo Choo.
+</p>
+
+<p>
+HARVEY islands, recently elevated.
+</p>
+
+<p>
+HEIGHT of encircled islands.
+</p>
+
+<p>
+Hermites.
+</p>
+
+<p>
+Hervey or Cook islands.
+</p>
+
+<p>
+Hogoleu.
+</p>
+
+<p>
+HOLOTHURIAE (Holuthuriæ) feeding on coral.
+</p>
+
+<p>
+HOUDEN island, height of.
+</p>
+
+<p>
+Honduras, reef off.
+</p>
+
+<p>
+Horn.
+</p>
+
+<p>
+Houtman Abrolhos.
+</p>
+
+<p>
+HUAHEINE; alleged proofs of its recent elevation.
+</p>
+
+<p>
+Huaheine.
+</p>
+
+<p>
+Humphrey.
+</p>
+
+<p>
+Hunter.
+</p>
+
+<p>
+HURRICANES, effects of, on coral-islands.
+</p>
+
+<p>
+Immaum.
+</p>
+
+<p>
+Independence.
+</p>
+
+<p>
+INDIA, west coast, recently elevated.
+</p>
+
+<p>
+India.
+</p>
+
+<p>
+IRREGULAR REEFS in shallow seas.
+</p>
+
+<p>
+ISLETS of coral-rock, their formation. Their destruction in the Maldiva atolls.
+</p>
+
+<p>
+Jamaica.
+</p>
+
+<p>
+Jarvis.
+</p>
+
+<p>
+JAVA, recently elevated.
+</p>
+
+<p>
+Java.
+</p>
+
+<p>
+Johnston island.
+</p>
+
+<p>
+Juan de Nova.
+</p>
+
+<p>
+Juan de Nova (Madagascar).
+</p>
+
+<p>
+Kalatoa.
+</p>
+
+<p>
+KAMTSCHATKA, proofs of its recent elevation.
+</p>
+
+<p>
+Karkalang.
+</p>
+
+<p>
+KEELING atoll, section of reef.
+</p>
+
+<p>
+Keeling, south atoll. North atoll.
+</p>
+
+<p>
+Keffing.
+</p>
+
+<p>
+Kemin.
+</p>
+
+<p>
+Kennedy.
+</p>
+
+<p>
+Keppel.
+</p>
+
+<p>
+Kumi.
+</p>
+
+<p>
+Laccadive group.
+</p>
+
+<p>
+LADRONES, or Marianas, recently elevated.
+</p>
+
+<p>
+Ladrones archipelago.
+</p>
+
+<p>
+LAGOON of Keeling atoll.
+</p>
+
+<p>
+LAGOONS bordered by inclined ledges and walls, and theory of their formation.
+Of small atolls filled up with sediment.
+</p>
+
+<p>
+LAGOON-CHANNELS within barrier-reefs.
+</p>
+
+<p>
+LAGOON-REEFS, all submerged in some atolls, and rising to the surface in
+others.
+</p>
+
+<p>
+Lancaster reef.
+</p>
+
+<p>
+Latte.
+</p>
+
+<p>
+Lauglan islands.
+</p>
+
+<p>
+LEDGES round certain lagoons.
+</p>
+
+<p>
+Lette.
+</p>
+
+<p>
+Lighthouse reef.
+</p>
+
+<p>
+LLOYD, Mr., on corals refixing themselves.
+</p>
+
+<p>
+LOO CHOO, recently elevated.
+</p>
+
+<p>
+Loo Choo.
+</p>
+
+<p>
+Louisiade.
+</p>
+
+<p>
+LOW ARCHIPELAGO, alleged proofs of its recent elevation.
+</p>
+
+<p>
+Low archipelago.
+</p>
+
+<p>
+LOWNESS of coral-islands.
+</p>
+
+<p>
+Loyalty group.
+</p>
+
+<p>
+Lucepara.
+</p>
+
+<p>
+LUTKE, Admiral, on fissures across coral-islands.
+</p>
+
+<p>
+LUZON, recently elevated.
+</p>
+
+<p>
+Luzon.
+</p>
+
+<p>
+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.
+</p>
+
+<p>
+Macassar strait.
+</p>
+
+<p>
+Macclesfield bank.
+</p>
+
+<p>
+MADAGASCAR, quick growth of corals at. Madreporitic rock of.
+</p>
+
+<p>
+Madagascar.
+</p>
+
+<p>
+Madjiko-sima.
+</p>
+
+<p>
+Madura (Java).
+</p>
+
+<p>
+Madura (India).
+</p>
+
+<p>
+MAHLOS MAHDOO, theory of formation.
+</p>
+
+<p>
+MALACCA, recently elevated.
+</p>
+
+<p>
+Malacca.
+</p>
+
+<p>
+MALCOLMSON, Dr., on recent elevation of W. coast of India. On recent elevation
+of Camaran island.
+</p>
+
+<p>
+Malden.
+</p>
+
+<p>
+MALDIVA atolls, and theory of their formation. Steepness of their flanks.
+Growth of coral at.
+</p>
+
+<p>
+Maldiva archipelago.
+</p>
+
+<p>
+MANGAIA island. Recently elevated.
+</p>
+
+<p>
+Mangaia.
+</p>
+
+<p>
+Mangs.
+</p>
+
+<p>
+MARIANAS, recently elevated.
+</p>
+
+<p>
+Mariana archipelago.
+</p>
+
+<p>
+Mariere.
+</p>
+
+<p>
+Marquesas archipelago.
+</p>
+
+<p>
+Marshall archipelago.
+</p>
+
+<p>
+Marshall island.
+</p>
+
+<p>
+Martinique.
+</p>
+
+<p>
+Martires.
+</p>
+
+<p>
+MARY&rsquo;S ST., in Madagascar, harbour made in reefs.
+</p>
+
+<p>
+Mary island.
+</p>
+
+<p>
+Matia, or Aurora.
+</p>
+
+<p>
+MATILDA atoll.
+</p>
+
+<p>
+MAURITIUS, fringing-reefs of. Depths at which corals live there. Recently
+elevated.
+</p>
+
+<p>
+Mauritius.
+</p>
+
+<p>
+MAURUA, section of.
+</p>
+
+<p>
+Maurua.
+</p>
+
+<p>
+MENCHIKOFF atoll.
+</p>
+
+<p>
+Mendana archipelago.
+</p>
+
+<p>
+Mendana isles.
+</p>
+
+<p>
+Mexico, gulf of.
+</p>
+
+<p>
+MILLEPORA COMPLANATA at Keeling atoll.
+</p>
+
+<p>
+Mindoro.
+</p>
+
+<p>
+Mohilla. (Mohila.)
+</p>
+
+<p>
+MOLUCCA islands, recently elevated.
+</p>
+
+<p>
+Mopeha.
+</p>
+
+<p>
+MORESBY, Captain, on boring through coral-reefs.
+</p>
+
+<p>
+Morty.
+</p>
+
+<p>
+Mosquito coast.
+</p>
+
+<p>
+MUSQUILLO atoll.
+</p>
+
+<p>
+Mysol.
+</p>
+
+<p>
+NAMOURREK group.
+</p>
+
+<p>
+Natunas.
+</p>
+
+<p>
+NAVIGATOR archipelago, elevation of.
+</p>
+
+<p>
+Navigator archipelago.
+</p>
+
+<p>
+Nederlandisch.
+</p>
+
+<p>
+NELSON, Lieutenant, on the consolidation of coral-rocks under water. Theory of
+coral-formations. On the Bermuda islands.
+</p>
+
+<p>
+New Britain.
+</p>
+
+<p>
+NEW CALEDONIA, steepness of its reefs. Barrier-reef of.
+</p>
+
+<p>
+New Caledonia.
+</p>
+
+<p>
+New Guinea (E. end).
+</p>
+
+<p>
+New Guinea (W. end).
+</p>
+
+<p>
+New Hanover.
+</p>
+
+<p>
+NEW HEBRIDES, recently elevated.
+</p>
+
+<p>
+New Hebrides.
+</p>
+
+<p>
+NEW IRELAND, recently elevated.
+</p>
+
+<p>
+New Ireland.
+</p>
+
+<p>
+New Nantucket.
+</p>
+
+<p>
+Nicobar islands.
+</p>
+
+<p>
+Niouha.
+</p>
+
+<p>
+NULLIPORAE at Keeling atoll. On the reefs of atolls. On barrier-reefs. Their
+wide distribution and abundance.
+</p>
+
+<p>
+OBJECTIONS to the theory of subsidence.
+</p>
+
+<p>
+Ocean islands.
+</p>
+
+<p>
+Ono.
+</p>
+
+<p>
+Onouafu. (Onouafou.)
+</p>
+
+<p>
+Ormuz.
+</p>
+
+<p>
+Oscar group.
+</p>
+
+<p>
+OSCILLATIONS of level.
+</p>
+
+<p>
+Ouallan, or Ualan. (Oualan.)
+</p>
+
+<p>
+OULUTHY atoll.
+</p>
+
+<p>
+Outong Java.
+</p>
+
+<p>
+Palawan, S.W. coast. N.W. coast. Western bank.
+</p>
+
+<p>
+Palmerston.
+</p>
+
+<p>
+Palmyra.
+</p>
+
+<p>
+Paracells.
+</p>
+
+<p>
+Paraquas.
+</p>
+
+<p>
+Patchow.
+</p>
+
+<p>
+Pelew islands.
+</p>
+
+<p>
+PEMBA island, singular form of.
+</p>
+
+<p>
+Pemba.
+</p>
+
+<p>
+Penrhyn.
+</p>
+
+<p>
+Peregrino.
+</p>
+
+<p>
+PERNAMBUCO, bar of sandstone at.
+</p>
+
+<p>
+PERSIAN gulf, recently elevated.
+</p>
+
+<p>
+Persian gulf.
+</p>
+
+<p>
+PESCADO.
+</p>
+
+<p>
+Pescadores.
+</p>
+
+<p>
+Peyster group.
+</p>
+
+<p>
+Philip.
+</p>
+
+<p>
+PHILIPPINE archipelago, recently elevated.
+</p>
+
+<p>
+Philippine archipelago.
+</p>
+
+<p>
+Phœnix.
+</p>
+
+<p>
+Piguiram.
+</p>
+
+<p>
+Pitcairn.
+</p>
+
+<p>
+PITT&rsquo;S bank.
+</p>
+
+<p>
+Pitt island.
+</p>
+
+<p>
+Platte.
+</p>
+
+<p>
+Pleasant.
+</p>
+
+<p>
+PORITES, chief coral on margin of Keeling atoll.
+</p>
+
+<p>
+Postillions.
+</p>
+
+<p>
+POUYNIPETE. Its probable subsidence.
+</p>
+
+<p>
+Pouynipète.
+</p>
+
+<p>
+Pratas shoal.
+</p>
+
+<p>
+Proby.
+</p>
+
+<p>
+Providence.
+</p>
+
+<p>
+Puerto Rico.
+</p>
+
+<p>
+Pulo Anna.
+</p>
+
+<p>
+PUMICE floated to coral-islands.
+</p>
+
+<p>
+Pylstaart.
+</p>
+
+<p>
+PYRARD DE LAVAL, astonishment at the atolls in the Indian Ocean.
+</p>
+
+<p>
+QUOY AND GAIMARD, depths at which corals live. Description of reefs applicable
+only to fringing-reefs.
+</p>
+
+<p>
+RANGE of atolls.
+</p>
+
+<p>
+Rapa.
+</p>
+
+<p>
+Rearson.
+</p>
+
+<p>
+RED SEA, banks of rock coated by reefs. Proofs of its recent elevation.
+Supposed subsidence of.
+</p>
+
+<p>
+Red Sea.
+</p>
+
+<p>
+REEFS, irregular in shallow seas. Rising to the surface in some lagoons and all
+submerged in others. Their distribution. Their absence from some coasts.
+</p>
+
+<p>
+Revilla-gigedo.
+</p>
+
+<p>
+RING-FORMED REEFS of the Maldiva atolls, and theory of.
+</p>
+
+<p>
+Rodriguez.
+</p>
+
+<p>
+Rosario.
+</p>
+
+<p>
+Rose island.
+</p>
+
+<p>
+Rotches.
+</p>
+
+<p>
+Rotoumah.
+</p>
+
+<p>
+Roug.
+</p>
+
+<p>
+Rowley shoals.
+</p>
+
+<p>
+RUPPELL, Dr., on the recent deposits of Red Sea.
+</p>
+
+<p>
+Sable, ile de.
+</p>
+
+<p>
+Sahia de Malha.
+</p>
+
+<p>
+St. Pierre.
+</p>
+
+<p>
+Sala.
+</p>
+
+<p>
+Salomon archipelago. (Solomon.)
+</p>
+
+<p>
+SAMOA, or Navigator archipelago, elevation of.
+</p>
+
+<p>
+Samoa archipelago.
+</p>
+
+<p>
+SAND-BARS parallel to coasts.
+</p>
+
+<p>
+Sandal-wood.
+</p>
+
+<p>
+SANDWICH archipelago, recently elevated.
+</p>
+
+<p>
+Sandwich archipelago.
+</p>
+
+<p>
+Sanserot.
+</p>
+
+<p>
+Santa-Cruz group.
+</p>
+
+<p>
+SAVAGE island, recently elevated.
+</p>
+
+<p>
+Savage.
+</p>
+
+<p>
+Savu.
+</p>
+
+<p>
+Saya, or Sahia de Malha.
+</p>
+
+<p>
+Scarborough shoal.
+</p>
+
+<p>
+SCARUS feeding on corals.
+</p>
+
+<p>
+Schouten.
+</p>
+
+<p>
+Scilly.
+</p>
+
+<p>
+SCORIAE floated to coral-islands.
+</p>
+
+<p>
+Scott&rsquo;s reef.
+</p>
+
+<p>
+SECTIONS of islands encircled by barrier-reefs. Of Bolabola.
+</p>
+
+<p>
+SEDIMENT in Keeling lagoon. In other atolls. Injurious to corals. Transported
+from coral-islands far seaward.
+</p>
+
+<p>
+Seniavine.
+</p>
+
+<p>
+Serangani.
+</p>
+
+<p>
+Seychelles.
+</p>
+
+<p>
+SHIP-BOTTOM quickly coated with coral.
+</p>
+
+<p>
+SMYTH island.
+</p>
+
+<p>
+SOCIETY archipelago, stationary condition of. Alleged proofs of recent
+elevation.
+</p>
+
+<p>
+Society archipelago.
+</p>
+
+<p>
+Socotra.
+</p>
+
+<p>
+Solor.
+</p>
+
+<p>
+SOOLOO islands, recently elevated.
+</p>
+
+<p>
+Sooloo islands.
+</p>
+
+<p>
+Souvaroff.
+</p>
+
+<p>
+Spanish.
+</p>
+
+<p>
+SPONGE, depths at which found.
+</p>
+
+<p>
+Starbuck. (Slarbuck.)
+</p>
+
+<p>
+STONES transported in roots of trees.
+</p>
+
+<p>
+STORMS, effects of, on coral-islands.
+</p>
+
+<p>
+STUTCHBURY, Mr., on the growth of an Agaricia. On upraised corals in Society
+archipelago.
+</p>
+
+<p>
+SUBSIDENCE of Keeling atoll. Extreme slowness of. Areas of, apparently
+elongated. Areas of immense. Great amount of.
+</p>
+
+<p>
+Suez, gulf of.
+</p>
+
+<p>
+Sulphur islands.
+</p>
+
+<p>
+SUMATRA, recently elevated.
+</p>
+
+<p>
+Sumatra.
+</p>
+
+<p>
+Sumbawa.
+</p>
+
+<p>
+SURF favourable to the growth of massive corals.
+</p>
+
+<p>
+Swallow shoal.
+</p>
+
+<p>
+Sydney island.
+</p>
+
+<p>
+TAHITI, alleged proofs of its recent elevation.
+</p>
+
+<p>
+Tahiti.
+</p>
+
+<p>
+TEMPERATURE of the sea at the Galapagos archipelago.
+</p>
+
+<p>
+Tenasserim.
+</p>
+
+<p>
+Tenimber island.
+</p>
+
+<p>
+Teturoa.
+</p>
+
+<p>
+THEORIES on coral-formations.
+</p>
+
+<p>
+THEORY OF subsidence, and objections to.
+</p>
+
+<p>
+THICKNESS, vertical, of barrier-reefs.
+</p>
+
+<p>
+Thomas, St.
+</p>
+
+<p>
+Tikopia.
+</p>
+
+<p>
+TIMOR, recently elevated.
+</p>
+
+<p>
+Timor.
+</p>
+
+<p>
+Timor-laut.
+</p>
+
+<p>
+Tokan-Bessees.
+</p>
+
+<p>
+Tongatabou.
+</p>
+
+<p>
+Tonquin.
+</p>
+
+<p>
+Toubai.
+</p>
+
+<p>
+Toufoa. (Toofoa.)
+</p>
+
+<p>
+Toupoua.
+</p>
+
+<p>
+TRADITIONS OF CHANGE in coral-islands.
+</p>
+
+<p>
+TRIDACNAE embedded in coral-rock. Left exposed in the Low archipelago.
+</p>
+
+<p>
+TUBULARIA, quick growth of.
+</p>
+
+<p>
+Tumbelan.
+</p>
+
+<p>
+Turneffe reef.
+</p>
+
+<p>
+Turtle.
+</p>
+
+<p>
+Ualan.
+</p>
+
+<p>
+VANIKORO, section of. Its state and changes in its reefs.
+</p>
+
+<p>
+Vanikoro.
+</p>
+
+<p>
+Vine reef.
+</p>
+
+<p>
+Virgin Gorda.
+</p>
+
+<p>
+Viti archipelago.
+</p>
+
+<p>
+VOLCANIC islands, with living corals on their shores. Matter, probably not
+associated with thick masses of coral-rock.
+</p>
+
+<p>
+VOLCANOES, authorities for their position on the map. Their presence determined
+by the movements in progress. Absent or extinct in the areas of subsidence.
+</p>
+
+<p>
+Waigiou.
+</p>
+
+<p>
+Wallis island.
+</p>
+
+<p>
+Washington.
+</p>
+
+<p>
+Well&rsquo;s reef.
+</p>
+
+<p>
+WELLSTEAD, Lieutenant, account of a ship coated with corals.
+</p>
+
+<p>
+WEST INDIES, banks of sediment fringed by reefs. Recently elevated.
+</p>
+
+<p>
+West Indies.
+</p>
+
+<p>
+WHITSUNDAY island, view of. Changes in its state.
+</p>
+
+<p>
+WILLIAMS, Rev. J., on traditions of the natives regarding coral-islands. On
+antiquity of certain corals.
+</p>
+
+<p>
+Wolchonsky.
+</p>
+
+<p>
+Wostock.
+</p>
+
+<p>
+Xulla islands.
+</p>
+
+<p>
+York island.
+</p>
+
+<p>
+Yucutan, coast of.
+</p>
+
+<p>
+ZONES of different kinds of corals outside the same reefs.
+</p>
+
+</div><!--end chapter-->
+
+<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK CORAL REEFS ***</div>
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+
+
+CORAL REEFS
+
+by CHARLES DARWIN
+
+
+
+
+EDITORIAL NOTE.
+
+Although in some respects more technical in their subjects and style than
+Darwin's "Journal," the books here reprinted will never lose their value
+and interest for the originality of the observations they contain.  Many
+parts of them are admirably adapted for giving an insight into problems
+regarding the structure and changes of the earth's surface, and in fact
+they form a charming introduction to physical geology and physiography in
+their application to special domains.  The books themselves cannot be
+obtained for many times the price of the present volume, and both the
+general reader, who desires to know more of Darwin's work, and the student
+of geology, who naturally wishes to know how a master mind reasoned on most
+important geological subjects, will be glad of the opportunity of
+possessing them in a convenient and cheap form.
+
+The three introductions, which my friend Professor Judd has kindly
+furnished, give critical and historical information which makes this
+edition of special value.
+
+G.T.B.
+
+
+
+CORAL REEFS.
+
+
+TABLE OF CONTENTS.
+
+
+CRITICAL INTRODUCTION.
+
+
+INTRODUCTION.
+
+
+CHAPTER I.--ATOLLS OR LAGOON-ISLANDS.
+
+SECTION 1.I.--DESCRIPTION OF KEELING ATOLL.
+Corals on the outer margin.--Zone of Nulliporae.--Exterior reef.--Islets.--
+Coral-conglomerate.--Lagoon.--Calcareous sediment.--Scari and Holuthuriae
+subsisting on corals.--Changes in the condition of the reefs and islets.--
+Probable subsidence of the atoll.--Future state of the lagoon.
+
+SECTION 1.II.--GENERAL DESCRIPTION OF ATOLLS.
+General form and size of atolls, their reefs and islets.--External slope.--
+Zone of Nulliporae.--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 1.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 4.I.--ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
+FAVOURABLE TO THEIR INCREASE.
+
+SECTION 4.II.--ON THE RATE OF GROWTH OF CORAL-REEFS.
+
+SECTION 4.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.
+Containing a detailed description of the reefs and islands in Plate III.
+
+
+INDEX.
+
+
+
+THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.
+
+
+CRITICAL INTRODUCTION.
+
+A scientific discovery is the outcome of an interesting process of
+evolution in the mind of its author.  When we are able to detect the germs
+of thought in which such a discovery has originated, and to trace the
+successive stages of the reasoning by which the crude idea has developed
+into an epoch-making book, we have the materials for reconstructing an
+important chapter of scientific history.  Such a contribution to the story
+of the "making of science" may be furnished in respect to Darwin's famous
+theory of coral-reefs, and the clearly reasoned treatise in which it was
+first fully set forth.
+
+The subject of corals and coral-reefs is one concerning which much popular
+misconception has always prevailed.  The misleading comparison of coral-rock
+with the combs of bees and the nests of wasps is perhaps responsible
+for much of this misunderstanding; one writer has indeed described a
+coral-reef as being "built by fishes by means of their teeth."  Scarcely
+less misleading, however, are the references we so frequently meet with,
+both in prose and verse, to the "skill," "industry," and "perseverance" of
+the "coral-insect" in "building" his "home."  As well might we praise men
+for their cleverness in making their own skeletons, and laud their assiduity
+in filling churchyards with the same.  The polyps and other organisms, whose
+remains accumulate to form a coral-reef, simply live and perform their
+natural functions, and then die, leaving behind them, in the natural course
+of events, the hard calcareous portions of their structures to add to the
+growing reef.
+
+While the forms of coral-reefs and coral-islands are sometimes very
+remarkable and worthy of attentive study, there is no ground, it need
+scarcely be added, for the suggestion that they afford proofs of design on
+the part of the living builders, or that, in the words of Flinders, they
+constitute breastworks, defending the workshops from whence "infant
+colonies might be safely sent forth."
+
+It was not till the beginning of the present century that travellers like
+Beechey, Chamisso, Quoy and Gaimard, Moresby, Nelson, and others, began to
+collect accurate details concerning the forms and structure of coral-masses,
+and to make such observations on the habits of reef-forming polyps,
+as might serve as a basis for safe reasoning concerning the origin of
+coral-reefs and islands.  In the second volume of Lyell's "Principles of
+Geology," published in 1832, the final chapter gives an admirable summary
+of all that was then known on the subject.  At that time, the ring-form of
+the atolls was almost universally regarded as a proof that they had grown
+up on submerged volcanic craters; and Lyell gave his powerful support to
+that theory.
+
+Charles Darwin was never tired of acknowledging his indebtedness to Lyell.
+In dedicating to his friend the second edition of his "Naturalist's Voyage
+Round the World," Darwin writes that he does so "with grateful pleasure, as
+an acknowledgment that the chief part of whatever scientific merit this
+journal and the other works of the author may possess, has been derived
+from studying the well-known and admirable 'Principles of Geology.'"
+
+The second volume of Lyell's "Principles" appeared after Darwin had left
+England; but it was doubtless sent on to him without delay by his faithful
+friend and correspondent, Professor Henslow.  It appears to have reached
+Darwin at a most opportune moment, while, in fact, he was studying the
+striking evidences of slow and long-continued, but often interrupted
+movement on the west coast of South America.  Darwin's acute mind could not
+fail to detect the weakness of the then prevalent theory concerning the
+origin of the ring-shaped atolls--and the difficulty which he found in
+accepting the volcanic theory, as an explanation of the phenomena of
+coral-reefs, is well set forth in his book.
+
+In an interesting fragment of autobiography, Darwin has given us a very
+clear account of the way in which the leading idea of the theory of
+coral-reefs originated in his mind; he writes, "No other work of mine was
+begun in so deductive a spirit as this, for the whole theory was thought
+out on the west coast of South America, before I had seen a true
+coral-reef.  I had therefore only to verify and extend my views by a
+careful examination of living reefs.  But it should be observed that I had
+during the two previous years been incessantly attending to the effects on
+the shores of South America of the intermittent elevation of the land,
+together with the denudation and deposition of sediment.  This necessarily
+led me to reflect much on the effects of subsidence, and it was easy to
+replace in imagination the continued deposition of sediment by the upward
+growth of corals.  To do this was to form my theory of the formation of
+barrier-reefs and atolls."
+
+On her homeward voyage, the "Beagle" visited Tahiti, Australia, and some of
+the coral-islands in the Indian Ocean, and Darwin had an opportunity of
+testing and verifying the conclusion at which he had arrived by studying
+the statements of other observers.
+
+I well recollect a remarkable conversation I had with Darwin, shortly after
+the death of Lyell.  With characteristic modesty, he told me that he never
+fully realised the importance of his theory of coral-reefs till he had an
+opportunity of discussing it with Lyell, shortly after the return of the
+"Beagle".  Lyell, on receiving from the lips of its author a sketch of the
+new theory, was so overcome with delight that he danced about and threw
+himself into the wildest contortions, as was his manner when excessively
+pleased.  He wrote shortly afterwards to Darwin as follows:--"I could think
+of nothing for days after your lesson on coral-reefs, but of the tops of
+submerged continents.  It is all true, but do not flatter yourself that you
+will be believed till you are growing bald like me, with hard work and
+vexation at the incredulity of the world."  On May 24th, 1837, Lyell wrote
+to Sir John Herschel as follows:--"I am very full of Darwin's new theory of
+coral-islands, and have urged Whewell to make him read it at our next
+meeting.  I must give up my volcanic crater forever, though it cost me a
+pang at first, for it accounted for so much."  Dr. Whewell was president of
+the Geological Society at the time, and on May 31st, 1837, Darwin read a
+paper entitled "On Certain Areas of Elevation and Subsidence in the Pacific
+and Indian oceans, as deduced from the Study of Coral Formations," an
+abstract of which appeared in the second volume of the Society's
+proceedings.
+
+It was about this time that Darwin, having settled himself in lodgings at
+Great Marlborough Street, commenced the writing of his book on "Coral-Reefs."
+Many delays from ill-health and the interruption of other work,
+caused the progress to be slow, and his journal speaks of "recommencing"
+the subject in February 1839, shortly after his marriage, and again in
+October of the same year.  In July 1841, he states that he began once more
+"after more than thirteen month's interval," and the last proof-sheet of
+the book was not corrected till May 6th, 1842.  Darwin writes in his
+autobiography, "This book, though a small one, cost me twenty months of
+hard work, as I had to read every work on the islands of the Pacific, and
+to consult many charts."  The task of elaborating and writing out his books
+was, with Darwin, always a very slow and laborious one; but it is clear
+that in accomplishing the work now under consideration, there was a long
+and constant struggle with the lethargy and weakness resulting from the sad
+condition of his health at that time.
+
+Lyell's anticipation that the theory of coral-reefs would be slow in
+meeting with general acceptance was certainly not justified by the actual
+facts.  On the contrary the new book was at once received with general
+assent among both geologists and zoologists, and even attracted a
+considerable amount of attention from the general public.
+
+It was not long before the coral-reef theory of Darwin found an able
+exponent and sturdy champion in the person of the great American
+naturalist, Professor James D. Dana.  Two years after the return of the
+"Beagle" to England, the ships of the United States Exploring Expedition
+set sail upon their four years' cruise, under the command of Captain
+Wilkes, and Dana was a member of the scientific staff.  When, in 1839, the
+expedition arrived at Sydney, a newspaper paragraph was found which gave
+the American naturalist the first intimation of Darwin's new theory of the
+origin of atolls and barrier-reefs.  Writing in 1872, Dana describes the
+effect produced on his mind by reading this passage:--"The paragraph threw
+a flood of light over the subject, and called forth feelings of peculiar
+satisfaction, and of gratefulness to Mr. Darwin, which still come up afresh
+whenever the subject of coral islands is mentioned.  The Gambier Islands in
+the Paumotus, which gave him the key to the theory, I had not seen; but on
+reaching the Feejees, six months later, in 1840, I found there similar
+facts on a still grander scale and of a more diversified character, so that
+I was afterward enabled to speak of his theory as established with more
+positiveness than he himself, in his philosophic caution, had been ready to
+adopt.  His work on coral-reefs appeared in 1842, when my report on the
+subject was already in manuscript.  It showed that the conclusions on other
+points, which we had independently reached, were for the most part the
+same.  The principal points of difference relate to the reason for the
+absence of corals from some coasts, and the evidence therefrom as to
+changes of level, and the distribution of the oceanic regions of elevation
+and subsidence--topics which a wide range of travel over the Pacific
+brought directly and constantly to my attention."
+
+Among the Reports of the United States Exploring Expedition, two important
+works from the pen of Professor Dana made their appearance;--one on
+"Zoophytes," which treats at length on "Corals and Coral-Animals," and the
+other on "Coral-Reefs and Islands."  In 1872, Dana prepared a work of a
+more popular character in which some of the chief results of his studies
+are described; it bore the title of "Corals and Coral-Islands."  Of this
+work, new and enlarged editions appeared in 1874 and 1890 in America, while
+two editions were published in this country in 1872 and 1875.  In all these
+works their author, while maintaining an independent judgment on certain
+matters of detail, warmly defends the views of Darwin on all points
+essential to the theory.
+
+Another able exponent and illustrator of the theory of coral-reefs was
+found in Professor J.B. Jukes, who accompanied H.M.S. "Fly", as naturalist,
+during the survey of the Great Barrier-Reef--in the years 1842 to 1846.
+Jukes, who was a man of great acuteness as well as independence of mind,
+concludes his account of the great Australian reefs with the following
+words:--"After seeing much of the Great Barrier-Reefs, and reflecting much
+upon them, and trying if it were possible by any means to evade the
+conclusions to which Mr. Darwin has come, I cannot help adding that his
+hypothesis is perfectly satisfactory to my mind, and rises beyond a mere
+hypothesis into the true theory of coral-reefs."
+
+As the result of the clear exposition of the subject by Darwin, Lyell,
+Dana, and Jukes, the theory of coral-reefs had, by the middle of the
+present century, commanded the almost universal assent of both biologists
+and geologists.  In 1859 Baron von Richthofen brought forward new facts in
+its support, by showing that the existence of the thick masses of dolomitic
+limestone in the Tyrol could be best accounted for if they were regarded as
+of coralline origin and as being formed during a period of long continued
+subsidence.  The same views were maintained by Professor Mojsisovics in his
+"Dolomit-riffe von Sudtirol und Venetien," which appeared in 1879.
+
+The first serious note of dissent to the generally accepted theory was
+heard in 1863, when a distinguished German naturalist, Dr. Karl Semper,
+declared that his study of the Pelew Islands showed that uninterrupted
+subsidence could not have been going on in that region.  Dr. Semper's
+objections were very carefully considered by Mr. Darwin, and a reply to
+them appeared in the second and revised edition of his "Coral-Reefs," which
+was published in 1874.  With characteristic frankness and freedom from
+prejudice, Darwin admitted that the facts brought forward by Dr. Semper
+proved that in certain specified cases, subsidence could not have played
+the chief part in originating the peculiar forms of the coral-islands.  But
+while making this admission, he firmly maintained that exceptional cases,
+like those described in the Pelew Islands, were not sufficient to
+invalidate the theory of subsidence as applied to the widely spread atolls,
+encircling reefs, and barrier-reefs of the Pacific and Indian Oceans.  It
+is worthy of note that to the end of his life Darwin maintained a friendly
+correspondence with Semper concerning the points on which they were at
+issue.
+
+After the appearance of Semper's work, Dr. J.J. Rein published an account
+of the Bermudas, in which he opposed the interpretation of the structure of
+the islands given by Nelson and other authors, and maintained that the
+facts observed in them are opposed to the views of Darwin.  Although, so
+far as I am aware, Darwin had no opportunity of studying and considering
+these particular objections, it may be mentioned that two American
+geologists have since carefully re-examined the district--Professor W.N.
+Rice in 1884 and Professor A. Heilprin in 1889--and they have independently
+arrived at the conclusion that Dr. Rein's objections cannot be maintained.
+
+The most serious opposition to Darwin's coral-reef theory, however, was
+that which developed itself after the return of H.M.S. "Challenger" from
+her famous voyage.  Mr. John Murray, one of the staff of naturalists on
+board that vessel, propounded a new theory of coral-reefs, and maintained
+that the view that they were formed by subsidence was one that was no
+longer tenable; these objections have been supported by Professor Alexander
+Agassiz in the United States, and by Dr. A. Geikie, and Dr. H.B. Guppy in
+this country.
+
+Although Mr. Darwin did not live to bring out a third edition of his
+"Coral-Reefs," I know from several conversations with him that he had given
+the most patient and thoughtful consideration to Mr. Murray's paper on the
+subject.  He admitted to me that had he known, when he wrote his work, of
+the abundant deposition of the remains of calcareous organisms on the sea
+floor, he might have regarded this cause as sufficient in a few cases to
+raise the summits of submerged volcanoes or other mountains to a level at
+which reef-forming corals can commence to flourish.  But he did not think
+that the admission that under certain favourable conditions, atolls might
+be thus formed without subsidence, necessitated an abandonment of his
+theory in the case of the innumerable examples of the kind which stud the
+Indian and Pacific Oceans.
+
+A letter written by Darwin to Professor Alexander Agassiz in May 1881 shows
+exactly the attitude which careful consideration of the subject led him to
+maintain towards the theory propounded by Mr. Murray:--"You will have
+seen," he writes, "Mr. Murray's views on the formation of atolls and
+barrier-reefs.  Before publishing my book, I thought long over the same
+view, but only as far as ordinary marine organisms are concerned, for at
+that time little was known of the multitude of minute oceanic organisms.  I
+rejected this view, as from the few dredgings made in the "Beagle", in the
+south temperate regions, I concluded that shells, the smaller corals, etc.,
+decayed and were dissolved when not protected by the deposition of
+sediment, and sediment could not accumulate in the open ocean.  Certainly,
+shells, etc., were in several cases completely rotten, and crumbled into
+mud between my fingers; but you will know whether this is in any degree
+common.  I have expressly said that a bank at the proper depth would give
+rise to an atoll, which could not be distinguished from one formed during
+subsidence.  I can, however, hardly believe in the existence of as many
+banks (there having been no subsidence) as there are atolls in the great
+oceans, within a reasonable depth, on which minute oceanic organisms could
+have accumulated to the depth of many hundred feet."
+
+Darwin's concluding words in the same letter written within a year of his
+death, are a striking proof of the candour and openness of mind which he
+preserved so well to the end, in this as in other controversies.
+
+"If I am wrong, the sooner I am knocked on the head and annihilated so much
+the better.  It still seems to me a marvellous thing that there should not
+have been much, and long-continued, subsidence in the beds of the great
+oceans.  I wish some doubly rich millionaire would take it into his head to
+have borings made in some of the Pacific and Indian atolls, and bring home
+cores for slicing from a depth of 500 or 600 feet."
+
+It is noteworthy that the objections to Darwin's theory have for the most
+part proceeded from zoologists, while those who have fully appreciated the
+geological aspect of the question, have been the staunchest supporters of
+the theory of subsidence.  The desirability of such boring operations in
+atolls has been insisted upon by several geologists, and it may be hoped
+that before many years have passed away, Darwin's hopes may be realised,
+either with or without the intervention of the "doubly rich millionaire."
+
+Three years after the death of Darwin, the veteran Professor Dana
+re-entered the lists and contributed a powerful defence of the theory of
+subsidence in the form of a reply to an essay written by the ablest
+exponent of the anti-Darwinian views on this subject, Dr. A. Geikie.  While
+pointing out that the Darwinian position had been to a great extent
+misunderstood by its opponents, he showed that the rival theory presented
+even greater difficulties than those which it professed to remove.
+
+During the last five years, the whole question of the origin of coral-reefs
+and islands has been re-opened, and a controversy has arisen, into which,
+unfortunately, acrimonious elements have been very unnecessarily
+introduced.  Those who desire it, will find clear and impartial statements
+of the varied and often mutually destructive views put forward by different
+authors, in three works which have made their appearance within the last
+year,--"The Bermuda Islands," by Professor Angelo Heilprin; "Corals and
+Coral-Islands," new edition by Professor J.D. Dana; and the third edition
+of Darwin's "Coral-Reefs," with Notes and Appendix by Professor T.G.
+Bonney.
+
+Most readers will, I think, rise from the perusal of these works with the
+conviction that, while on certain points of detail it is clear that,
+through the want of knowledge concerning the action of marine organisms in
+the open ocean, Darwin was betrayed into some grave errors, yet the main
+foundations of his argument have not been seriously impaired by the new
+facts observed in the deep-sea researches, or by the severe criticism to
+which his theory has been subjected during the last ten years.  On the
+other hand, I think it will appear that much misapprehension has been
+exhibited by some of Darwin's critics, as to what his views and arguments
+really were; so that the reprint and wide circulation of the book in its
+original form is greatly to be desired, and cannot but be attended with
+advantage to all those who will have the fairness to acquaint themselves
+with Darwin's views at first hand, before attempting to reply to them.
+
+JOHN W. JUDD.
+
+
+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 Francois Pyrard de Laval, in
+the year 1605, exclaim, "C'est une merueille de voir chacun de ces
+atollons, enuironne 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.
+
+(PLATE: UNTITLED WOODCUT, WHITSUNDAY ATOLL.)
+
+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."
+
+(PLATE: UNTITLED WOODCUT, REEF AT BOLABOLA ISLAND.)
+
+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.  (I have taken the liberty of simplifying the
+foreground, and leaving out a mountainous island in the far distance.)
+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.
+
+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.  (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.)
+
+
+(DESCRIPTION OF THE PLATES.
+
+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.--POUYNIPETE, or SENIAVINE, in the Caroline Archipelago; from the
+survey by Admiral Lutke.
+
+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.
+
+
+
+CHAPTER I.--ATOLLS OR LAGOON-ISLANDS.
+
+SECTION 1.I.--KEELING ATOLL.
+
+Corals on the outer margin.--Zone of Nulliporae.--Exterior reef.--Islets.--
+Coral-conglomerate.--Lagoon.--Calcareous sediment.--Scari and Holuthuriae
+subsisting on corals.--Changes in the condition of the reefs and islets.--
+Probable subsidence of the atoll.--Future state of the lagoon.
+
+(PLATE: UNTITLED WOODCUT, VERTICAL SECTION THROUGH KEELING ATOLL.)
+
+A.--Level of the sea at low water: where the letter A is placed, the depth
+is twenty-five fathoms, and the distance rather more than one hundred and
+fifty yards from the edge of the reef.
+
+B.--Outer edge of that flat part of the reef, which dries at low water:
+the edge either consists of a convex mound, as represented, or of rugged
+points, like those a little farther seaward, beneath the water.
+
+C.--A flat of coral-rock, covered at high water.
+
+D.--A low projecting ledge of brecciated coral-rock washed by the waves at
+high water.
+
+E.--A slope of loose fragments, reached by the sea only during gales: the
+upper part, which is from six to twelve feet high, is clothed with
+vegetation.  The surface of the islet gently slopes to the lagoon.
+
+F.--Level of the lagoon at low water.
+
+KEELING or COCOS atoll is situated in the Indian Ocean, in 12 deg 5' S.,
+and longitude 90 deg 55' E.: a reduced chart of it was made from the
+survey of Captain Fitzroy and the Officers of H.M.S. "Beagle," is given in
+Plate I., Figure 10.  The greatest width of this atoll is nine miles and a
+half.  Its structure is in most respects characteristic of the class to
+which it belongs, with the exception of the shallowness of the lagoon.  The
+accompanying woodcut represents a vertical section, supposed to be drawn at
+low water from the outer coast across one of the low islets (one being
+taken of average dimensions) to within the lagoon.
+
+The section is true to the scale in a horizontal line, but it could not be
+made so in a vertical one, as the average greatest height of the land is
+only between six and twelve feet above high-water mark.
+
+I will describe the section, commencing with the outer margin.  I must
+first observe that the reef-building polypifers, not being tidal animals,
+require to be constantly submerged or washed by the breakers.  I was
+assured by Mr. Liesk, a very intelligent resident on these islands, as well
+as by some chiefs at Tahiti (Otaheite), that an exposure to the rays of the
+sun for a very short time invariably causes their destruction.  Hence it is
+possible only under the most favourable circumstances, afforded by an
+unusually low tide and smooth water, to reach the outer margin, where the
+coral is alive.  I succeeded only twice in gaining this part, and found it
+almost entirely composed of a living Porites, which forms great irregularly
+rounded masses (like those of an Astraea, but larger) from four to eight
+feet broad, and little less in thickness.  These mounds are separated from
+each other by narrow crooked channels, about six feet deep, most of which
+intersect the line of reef at right angles.  On the furthest mound, which I
+was able to reach by the aid of a leaping-pole, and over which the sea
+broke with some violence, although the day was quite calm and the tide low,
+the polypifers in the uppermost cells were all dead, but between three and
+four inches lower down on its side they were living, and formed a
+projecting border round the upper and dead surface.  The coral being thus
+checked in its upward growth, extends laterally, and hence most of the
+masses, especially those a little further inwards, had broad flat dead
+summits.  On the other hand I could see, during the recoil of the breakers,
+that a few yards further seaward, the whole convex surface of the Porites
+was alive; so that the point where we were standing was almost on the exact
+upward and shoreward limit of existence of those corals which form the
+outer margin of the reef.  We shall presently see that there are other
+organic productions, fitted to bear a somewhat longer exposure to the air
+and sun.
+
+Next, but much inferior in importance to the Porites, is the Millepora
+complanata.  (This Millepora (Palmipora of Blainville), as well as the M.
+alcicornis, possesses the singular property of stinging the skin where it
+is delicate, as on the face and arm.)
+
+It grows in thick vertical plates, intersecting each other at various
+angles, and forms an exceedingly strong honeycombed mass, which generally
+affects a circular form, the marginal plates alone being alive.  Between
+these plates and in the protected crevices on the reef, a multitude of
+branching zoophytes and other productions flourish, but the Porites and
+Millepora alone seem able to resist the fury of the breakers on its upper
+and outer edge: at the depth of a few fathoms other kinds of stony corals
+live.  Mr. Liesk, who was intimately acquainted with every part of this
+reef, and likewise with that of North Keeling atoll, assured me that these
+corals invariably compose the outer margin.  The lagoon is inhabited by
+quite a distinct set of corals, generally brittle and thinly branched; but
+a Porites, apparently of the same species with that on the outside, is
+found there, although it does not seem to thrive, and certainly does not
+attain the thousandth part in bulk of the masses opposed to the breakers.
+
+The woodcut shows the form of the bottom off the reef: the water deepens
+for a space between one and two hundred yards wide, very gradually to
+twenty-five fathoms (A in section), beyond which the sides plunge into the
+unfathomable ocean at an angle of 45 deg.  (The soundings from which this
+section is laid down were taken with great care by Captain Fitzroy himself.
+He used a bell-shaped lead, having a diameter of four inches, and the
+armings each time were cut off and brought on board for me to examine.  The
+arming is a preparation of tallow, placed in the concavity at the bottom of
+the lead.  Sand, and even small fragments of rock, will adhere to it; and
+if the bottom be of rock it brings up an exact impression of its surface.)
+To the depth of ten or twelve fathoms the bottom is exceedingly rugged, and
+seems formed of great masses of living coral, similar to those on the
+margin.  The arming of the lead here invariably came up quite clean, but
+deeply indented, and chains and anchors which were lowered, in the hopes of
+tearing up the coral, were broken.  Many small fragments, however, of
+Millepora alcicornis were brought up; and on the arming from an eight-fathom
+cast, there was a perfect impression of an Astraea, apparently
+alive.  I examined the rolled fragments cast on the beach during gales, in
+order further to ascertain what corals grew outside the reef.  The
+fragments consisted of many kinds, of which the Porites already mentioned
+and a Madrepora, apparently the M. corymbosa, were the most abundant.  As I
+searched in vain in the hollows on the reef and in the lagoon, for a living
+specimen of this Madrepore, I conclude that it is confined to a zone
+outside, and beneath the surface, where it must be very abundant.
+Fragments of the Millepora alcicornis and of an Astraea were also numerous;
+the former is found, but not in proportionate numbers, in the hollows on
+the reef; but the Astraea I did not see living.  Hence we may infer, that
+these are the kinds of coral which form the rugged sloping surface
+(represented in the woodcut by an uneven line), round and beneath the
+external margin.  Between twelve and twenty fathoms the arming came up an
+equal number of times smoothed with sand, and indented with coral: an
+anchor and lead were lost at the respective depths of thirteen and sixteen
+fathoms.  Out of twenty-five soundings taken at a greater depth than twenty
+fathoms, every one showed that the bottom was covered with sand; whereas,
+at a less depth than twelve fathoms, every sounding showed that it was
+exceedingly rugged, and free from all extraneous particles.  Two soundings
+were obtained at the depth of 360 fathoms, and several between two hundred
+and three hundred fathoms.  The sand brought up from these depths consisted
+of finely triturated fragments of stony zoophytes, but not, as far as I
+could distinguish, of a particle of any lamelliform genus: fragments of
+shells were rare.
+
+At a distance of 2,200 yards from the breakers, Captain Fitzroy found no
+bottom with a line of 7,200 feet in length; hence the submarine slope of
+this coral formation is steeper than that of any volcanic cone.  Off the
+mouth of the lagoon, and likewise off the northern point of the atoll,
+where the currents act violently, the inclination, owing to the
+accumulation of sediment, is less.  As the arming of the lead from all the
+greater depths showed a smooth sandy bottom, I at first concluded that the
+whole consisted of a vast conical pile of calcareous sand, but the sudden
+increase of depth at some points, and the circumstance of the line having
+been cut, as if rubbed, when between five hundred and six hundred fathoms
+were out, indicate the probable existence of submarine cliffs.
+
+On the margin of the reef, close within the line where the upper surface of
+the Porites and of the Millepora is dead, three species of Nullipora
+flourish.  One grows in thin sheets, like a lichen on old trees; the second
+in stony knobs, as thick as a man's finger, radiating from a common centre;
+and the third, which is less common, in a moss-like reticulation of thin,
+but perfectly rigid branches.  (This last species is of a beautiful bright
+peach-blossom colour.  Its branches are about as thick as crow-quills; they
+are slightly flattened and knobbed at the extremities.  The extremities
+only are alive and brightly coloured.  The two other species are of a dirty
+purplish-white.  The second species is extremely hard; its short knob-like
+branches are cylindrical, and do not grow thicker at their extremities.)
+The three species occur either separately or mingled together; and they
+form by their successive growth a layer two or three feet in thickness,
+which in some cases is hard, but where formed of the lichen-like kind,
+readily yields an impression to the hammer: the surface is of a reddish
+colour.  These Nulliporae, although able to exist above the limit of true
+corals, seem to require to be bathed during the greater part of each tide
+by breaking water, for they are not found in any abundance in the protected
+hollows on the back part of the reef, where they might be immersed either
+during the whole or an equal proportional time of each tide.  It is
+remarkable that organic productions of such extreme simplicity, for the
+Nulliporae undoubtedly belong to one of the lowest classes of the vegetable
+kingdom, should be limited to a zone so peculiarly circumstanced.  Hence
+the layer composed by their growth merely fringes the reef for a space of
+about twenty yards in width, either under the form of separate mammillated
+projections, where the outer masses of coral are separate, or, more
+commonly, where the corals are united into a solid margin, as a continuous
+smooth convex mound (B in woodcut), like an artificial breakwater.  Both
+the mound and mammillated projections stand about three feet higher than
+any other part of the reef, by which term I do not include the islets,
+formed by the accumulation of rolled fragments.  We shall hereafter see
+that other coral reefs are protected by a similar thick growth of
+Nulliporae on the outer margin, the part most exposed to the breakers, and
+this must effectually aid in preserving it from being worn down.
+
+The woodcut represents a section across one of the islets on the reef, but
+if all that part which is above the level of C were removed, the section
+would be that of a simple reef, as it occurs where no islet has been
+formed.  It is this reef which essentially forms the atoll.  It is a ring,
+enclosing the lagoon on all sides except at the northern end, where there
+are two open spaces, through one of which ships can enter.  The reef varies
+in width from two hundred and fifty to five hundred yards, its surface is
+level, or very slightly inclined towards the lagoon, and at high tide the
+sea breaks entirely over it: the water at low tide thrown by the breakers
+on the reef, is carried by the many narrow and shoal gullies or channels on
+its surface, into the lagoon: a return stream sets out of the lagoon
+through the main entrance.  The most frequent coral in the hollows on the
+reef is Pocillopora verrucosa, which grows in short sinuous plates, or
+branches, and when alive is of a beautiful pale lake-red: a Madrepora,
+closely allied or identical with M. pocillifera, is also common.  As soon
+as an islet is formed, and the waves are prevented breaking entirely over
+the reef, the channels and hollows in it become filled up with cemented
+fragments, and its surface is converted into a hard smooth floor (C of
+woodcut), like an artificial one of freestone.  This flat surface varies in
+width from one hundred to two hundred, or even three hundred yards, and is
+strewed with a few large fragments of coral torn up during gales: it is
+uncovered only at low water.  I could with difficulty, and only by the aid
+of a chisel, procure chips of rock from its surface, and therefore could
+not ascertain how much of it is formed by the aggregation of detritus, and
+how much by the outward growth of mounds of corals, similar to those now
+living on the margin.  Nothing can be more singular than the appearance at
+low tide of this "flat" of naked stone, especially where it is externally
+bounded by the smooth convex mound of Nulliporae, appearing like a
+breakwater built to resist the waves, which are constantly throwing over it
+sheets of foaming water.  The characteristic appearance of this "flat" is
+shown in the foregoing woodcut of Whitsunday atoll.
+
+The islets on the reef are first formed between two hundred and three
+hundred yards from its outer edge, through the accumulation of a pile of
+fragments, thrown together by some unusually strong gale.  Their ordinary
+width is under a quarter of a mile, and their length varies from a few
+yards to several miles.  Those on the south-east and windward side of the
+atoll, increase solely by the addition of fragments on their outer side;
+hence the loose blocks of coral, of which their surface is composed, as
+well as the shells mingled with them, almost exclusively consist of those
+kinds which live on the outer coast.  The highest part of the islets
+(excepting hillocks of blown sand, some of which are thirty feet high), is
+close to the outer beach (E of the woodcut), and averages from six to ten
+feet above ordinary high-water mark.  From the outer beach the surface
+slopes gently to the shores of the lagoon, which no doubt has been caused
+by the breakers the further they have rolled over the reef, having had less
+power to throw up fragments.  The little waves of the lagoon heap up sand
+and fragments of thinly-branched corals on the inner side of the islets on
+the leeward side of the atoll; and these islets are broader than those to
+windward, some being even eight hundred yards in width; but the land thus
+added is very low.  The fragments beneath the surface are cemented into a
+solid mass, which is exposed as a ledge (D of the woodcut), projecting some
+yards in front of the outer shore and from two to four feet high.  This
+ledge is just reached by the waves at ordinary high-water: it extends in
+front of all the islets, and everywhere has a water-worn and scooped
+appearance.  The fragments of coral which are occasionally cast on the
+"flat" are during gales of unusual violence swept together on the beach,
+where the waves each day at high-water tend to remove and gradually wear
+them down; but the lower fragments having become firmly cemented together
+by the percolation of calcareous matter, resist the daily tides longer, and
+hence project as a ledge.  The cemented mass is generally of a white
+colour, but in some few parts reddish from ferruginous matter; it is very
+hard, and is sonorous under the hammer; it is obscurely divided by seams,
+dipping at a small angle seaward; it consists of fragments of the corals
+which grow on the outer margin, some quite and others partially rounded,
+some small and others between two and three feet across; and of masses of
+previously formed conglomerate, torn up, rounded, and re-cemented; or it
+consists of a calcareous sandstone, entirely composed of rounded particles,
+generally almost blended together, of shells, corals, the spines of echini,
+and other such organic bodies; rocks, of this latter kind, occur on many
+shores, where there are no coral reefs.  The structure of the coral in the
+conglomerate has generally been much obscured by the infiltration of
+spathose calcareous matter; and I collected a very interesting series,
+beginning with fragments of unaltered coral, and ending with others, where
+it was impossible to discover with the naked eye any trace of organic
+structure.  In some specimens I was unable, even with the aid of a lens,
+and by wetting them, to distinguish the boundaries of the altered coral and
+spathose limestone.  Many even of the blocks of coral lying loose on the
+beach, had their central parts altered and infiltrated.
+
+The lagoon alone remains to be described; it is much shallower than that of
+most atolls of considerable size.  The southern part is almost filled up
+with banks of mud and fields of coral, both dead and alive, but there are
+considerable spaces, between three and four fathoms, and smaller basins,
+from eight to ten fathoms deep.  Probably about half its area consists of
+sediment, and half of coral-reefs.  The corals composing these reefs have a
+very different aspect from those on the outside; they are very numerous in
+kind, and most of them are thinly branched.  Meandrina, however, lives in
+the lagoon, and great rounded masses of this coral are numerous, lying
+quite or almost loose on the bottom.  The other commonest kinds consist of
+three closely allied species of true Madrepora in thin branches; of
+Seriatapora subulata; two species of Porites (This Porites has somewhat the
+habit of P. clavaria, but the branches are not knobbed at their ends.  When
+alive it is of a yellow colour, but after having been washed in fresh water
+and placed to dry, a jet-black slimy substance exuded from the entire
+surface, so that the specimen now appears as if it had been dipped in ink.)
+with cylindrical branches, one of which forms circular clumps, with the
+exterior branches only alive; and lastly, a coral something like an
+Explanaria, but with stars on both surfaces, growing in thin, brittle,
+stony, foliaceous expansions, especially in the deeper basins of the
+lagoon.  The reefs on which these corals grow are very irregular in form,
+are full of cavities, and have not a solid flat surface of dead rock, like
+that surrounding the lagoon; nor can they be nearly so hard, for the
+inhabitants made with crowbars a channel of considerable length through
+these reefs, in which a schooner, built on the S.E. islet, was floated out.
+It is a very interesting circumstance, pointed out to us by Mr. Liesk, that
+this channel, although made less than ten years before our visit, was then,
+as we saw, almost choked up with living coral, so that fresh excavations
+would be absolutely necessary to allow another vessel to pass through it.
+
+The sediment from the deepest parts in the lagoon, when wet, appeared
+chalky, but when dry, like very fine sand.  Large soft banks of similar,
+but even finer grained mud, occur on the S.E. shore of the lagoon,
+affording a thick growth of a Fucus, on which turtle feed: this mud,
+although discoloured by vegetable matter, appears from its entire solution
+in acids to be purely calcareous.  I have seen in the Museum of the
+Geological Society, a similar but more remarkable substance, brought by
+Lieutenant Nelson from the reefs of Bermuda, which, when shown to several
+experienced geologists, was mistaken by them for true chalk.  On the
+outside of the reef much sediment must be formed by the action of the surf
+on the rolled fragments of coral; but in the calm waters of the lagoon,
+this can take place only in a small degree.  There are, however, other and
+unexpected agents at work here: large shoals of two species of Scarus, one
+inhabiting the surf outside the reef and the other the lagoon, subsist
+entirely, as I was assured by Mr. Liesk, the intelligent resident before
+referred to, by browsing on the living polypifers.  I opened several of
+these fish, which are very numerous and of considerable size, and I found
+their intestines distended by small pieces of coral, and finely ground
+calcareous matter.  This must daily pass from them as the finest sediment;
+much also must be produced by the infinitely numerous vermiform and
+molluscous animals, which make cavities in almost every block of coral.
+Dr. J. Allan, of Forres, who has enjoyed the best means of observation,
+informs me in a letter that the Holothuriae (a family of Radiata) subsist
+on living coral; and the singular structure of bone within the anterior
+extremity of their bodies, certainly appears well adapted for this purpose.
+The number of the species of Holothuria, and of the individuals which swarm
+on every part of these coral-reefs, is extraordinarily great; and many
+shiploads are annually freighted, as is well-known, for China with the
+trepang, which is a species of this genus.  The amount of coral yearly
+consumed, and ground down into the finest mud, by these several creatures,
+and probably by many other kinds, must be immense.  These facts are,
+however, of more importance in another point of view, as showing us that
+there are living checks to the growth of coral-reefs, and that the almost
+universal law of "consumed and be consumed," holds good even with the
+polypifers forming those massive bulwarks, which are able to withstand the
+force of the open ocean.
+
+Considering that Keeling atoll, like other coral formations, has been
+entirely formed by the growth of organic beings, and the accumulation of
+their detritus, one is naturally led to inquire how long it has continued,
+and how long it is likely to continue, in its present state.  Mr. Liesk
+informed me that he had seen an old chart in which the present long island
+on the S.E. side was divided by several channels into as many islets; and
+he assures me that the channels can still be distinguished by the smaller
+size of the trees on them.  On several islets, also, I observed that only
+young cocoa-nut trees were growing on the extremities; and that older and
+taller trees rose in regular succession behind them; which shows that these
+islets have very lately increased in length.  In the upper and south-eastern
+part of the lagoon, I was much surprised by finding an irregular
+field of at least a mile square of branching corals, still upright, but
+entirely dead.  They consisted of the species already mentioned; they were
+of a brown colour, and so rotten, that in trying to stand on them I sank
+halfway up the leg, as if through decayed brushwood.  The tops of the
+branches were barely covered by water at the time of lowest tide.  Several
+facts having led me to disbelieve in any elevation of the whole atoll, I
+was at first unable to imagine what cause could have killed so large a
+field of coral.  Upon reflection, however, it appeared to me that the
+closing up of the above-mentioned channels would be a sufficient cause; for
+before this, a strong breeze by forcing water through them into the head of
+the lagoon, would tend to raise its level.  But now this cannot happen, and
+the inhabitants observe that the tide rises to a less height, during a high
+S.E. wind, at the head than at the mouth of the lagoon.  The corals, which,
+under the former condition of things, had attained the utmost possible
+limit of upward growth, would thus occasionally be exposed for a short time
+to the sun, and be killed.
+
+Besides the increase of dry land, indicated by the foregoing facts, the
+exterior solid reef appears to have grown outwards.  On the western side of
+the atoll, the "flat" lying between the margin of the reef and the beach,
+is very wide; and in front of the regular beach with its conglomerate
+basis, there is, in most parts, a bed of sand and loose fragments with
+trees growing out of it, which apparently is not reached even by the spray
+at high water.  It is evident some change has taken place since the waves
+formed the inner beach; that they formerly beat against it with violence
+was evident, from a remarkably thick and water-worn point of conglomerate
+at one spot, now protected by vegetation and a bank of sand; that they beat
+against it in the same peculiar manner in which the swell from windward now
+obliquely curls round the margin of the reef, was evident from the
+conglomerate having been worn into a point projecting from the beach in a
+similarly oblique manner.  This retreat in the line of action of the
+breakers might result, either from the surface of the reef in front of the
+islets having been submerged at one time, and afterward having grown
+upwards, or from the mounds of coral on the margin having continued to grow
+outwards.  That an outward growth of this part is in process, can hardly be
+doubted from the fact already mentioned of the mounds of Porites with their
+summits apparently lately killed, and their sides only three or four inches
+lower down thickened by a fresh layer of living coral.  But there is a
+difficulty on this supposition which I must not pass over.  If the whole,
+or a large part of the "flat," had been formed by the outward growth of the
+margin, each successive margin would naturally have been coated by the
+Nulliporae, and so much of the surface would have been of equal height with
+the existing zone of living Nulliporae: this is not the case, as may be
+seen in the woodcut.  It is, however, evident from the abraded state of the
+"flat," with its original inequalities filled up, that its surface has been
+much modified; and it is possible that the hinder portions of the zone of
+Nulliporae, perishing as the reef grows outwards, might be worn down by the
+surf.  If this has not taken place, the reef can in no part have increased
+outwards in breadth since its formation, or at least since the Nulliporae
+formed the convex mound on its margin; for the zone thus formed, and which
+stands between two and three feet above the other parts of the reef, is
+nowhere much above twenty yards in width.
+
+Thus far we have considered facts, which indicate, with more or less
+probability, the increase of the atoll in its different parts: there are
+others having an opposite tendency.  On the south-east side, Lieutenant
+Sulivan, to whose kindness I am indebted for many interesting observations,
+found the conglomerate projecting on the reef nearly fifty yards in front
+of the beach: we may infer from what we see in all other parts of the
+atoll, that the conglomerate was not originally so much exposed, but formed
+the base of an islet, the front and upper part of which has since been
+swept away.  The degree to which the conglomerate, round nearly the whole
+atoll, has been scooped, broken up, and the fragments cast on the beach, is
+certainly very surprising, even on the view that it is the office of
+occasional gales to pile up fragments, and of the daily tides to wear them
+away.  On the western side, also, of the atoll, where I have described a
+bed of sand and fragments with trees growing out of it, in front of an old
+beach, it struck both Lieutenant Sulivan and myself, from the manner in
+which the trees were being washed down, that the surf had lately
+recommenced an attack on this line of coast.  Appearances indicating a
+slight encroachment of the water on the land, are plainer within the
+lagoon: I noticed in several places, both on its windward and leeward
+shores, old cocoa-nut trees falling with their roots undermined, and the
+rotten stumps of others on the beach, where the inhabitants assured us the
+cocoa-nut could not now grow.  Captain Fitzroy pointed out to me, near the
+settlement, the foundation posts of a shed, now washed by every tide, but
+which the inhabitants stated, had seven years before stood above high
+watermark.  In the calm waters of the lagoon, directly connected with a
+great, and therefore stable ocean, it seems very improbable that a change
+in the currents, sufficiently great to cause the water to eat into the land
+on all sides, should have taken place within a limited period.  From these
+considerations I inferred, that probably the atoll had lately subsided to a
+small amount; and this inference was strengthened by the circumstance, that
+in 1834, two years before our visit, the island had been shaken by a severe
+earthquake, and by two slighter ones during the ten previous years.  If,
+during these subterranean disturbances, the atoll did subside, the downward
+movement must have been very small, as we must conclude from the fields of
+dead coral still lipping the surface of the lagoon, and from the breakers
+on the western shore not having yet regained the line of their former
+action.  The subsidence must, also, have been preceded by a long period of
+rest, during which the islets extended to their present size, and the
+living margin of the reef grew either upwards, or as I believe outwards, to
+its present distance from the beach.
+
+Whether this view be correct or not, the above facts are worthy of
+attention, as showing how severe a struggle is in progress on these low
+coral formations between the two nicely balanced powers of land and water.
+With respect to the future state of Keeling atoll, if left undisturbed, we
+can see that the islets may still extend in length; but as they cannot
+resist the surf until broken by rolling over a wide space, their increase
+in breadth must depend on the increasing breadth of the reef; and this must
+be limited by the steepness of the submarine flanks, which can be added to
+only by sediment derived from the wear and tear of the coral.  From the
+rapid growth of the coral in the channel cut for the schooner, and from the
+several agents at work in producing fine sediment, it might be thought that
+the lagoon would necessarily become quickly filled up.  Some of this
+sediment, however, is transported into the open sea, as appears from the
+soundings off the mouth of the lagoon, instead of being deposited within
+it.  The deposition, moreover, of sediment, checks the growth of coral-reefs,
+so that these two agencies cannot act together with full effect in
+filling it up.  We know so little of the habits of the many different
+species of corals, which form the lagoon-reefs, that we have no more
+reasons for supposing that their whole surface would grow up as quickly as
+the coral did in the schooner-channel, than for supposing that the whole
+surface of a peat-moss would increase as quickly as parts are known to do
+in holes, where the peat has been cut away.  These agencies, nevertheless,
+tend to fill up the lagoon; but in proportion as it becomes shallower, so
+must the polypifers be subject to many injurious agencies, such as impure
+water and loss of food.  For instance, Mr. Liesk informed me, that some
+years before our visit unusually heavy rain killed nearly all the fish in
+the lagoon, and probably the same cause would likewise injure the corals.
+The reefs also, it must be remembered, cannot possibly rise above the level
+of the lowest spring-tide, so that the final conversion of the lagoon into
+land must be due to the accumulation of sediment; and in the midst of the
+clear water of the ocean, and with no surrounding high land, this process
+must be exceedingly slow.
+
+
+SECTION 1.II.--GENERAL DESCRIPTION OF ATOLLS.
+
+General form and size of atolls, their reefs and islets.--External slope.--
+Zone of Nulliporae.--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 (Beechey's "Voyage to the Pacific and Beering's
+Straits," chapter viii.) 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 Lutke'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 Lutke,
+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.
+
+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 deg into
+unfathomable depths, is exactly the same (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.") 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 (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.)
+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 deg 4' N., 157 deg 45' W.), described by Cook (Cook's "Third Voyage,"
+volume ii., chapter 10.), 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
+(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.) 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.  (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
+(page 45, 4to 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.)  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. Elie de Beaumont
+("Memoires pour servir a une description Geolog. de France," tome iv., page
+216.) 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. Elie 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.
+
+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 (Beechey's "Voyage," 4to
+edition, page 44.) 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.
+
+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.  (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
+Nulliporae.  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 Nulliporae are closely related to those described on the
+coral-reefs, but I believe are of different species.)  Although Chamisso
+does not state that the masses of Nulliporae form points or a mound, higher
+than the flat, yet I believe that this is the case; for Kotzebue (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
+Serpulae, 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
+Serpulae hold the same position, relatively to the action of the sea, with
+the Nulliporae on the coral-reefs in the Indian and Pacific Oceans.), 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 (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.)
+Whether a smooth convex mound of Nulliporae, 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.
+
+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 (Kotzebue's "First Voyage," volume iii., page 144.)
+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 (see also
+Moresby on the Northern atolls of the Maldivas, "Geographical Journal",
+volume v., page 400.), 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.
+
+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.  (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
+Bresil", 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, mele ou forme de madrepores broyes."
+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 (Frederick Lutke's "Voyage autour du Monde,"
+volume ii., page 291.  See also his account of Namonouito, below, and the
+chart of Oulleay in the Atlas.) 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 Namonouito, 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.
+
+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 ("Principles
+of Geology," volume iii., page 289.) 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 (Beechey's "Voyage," 4to edition, volume i., page
+189.), 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.
+
+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 (Kotzebue's "First Voyage," volume iii., page 142.), 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 (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.), 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.
+
+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 deg S.) is described by Chamisso
+(Kotzebue's "First Voyage," volume iii., page 221.) 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.
+
+(DESCRIPTION OF THE PLATES.
+
+PLATE II.--GREAT CHAGOS BANK, NEW CALEDONIA,MENCHIKOFF ATOLL, ETC.
+
+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.)
+
+
+SECTION 1.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 (Captain Moresby informs me that Millepora complanata is one of the
+commonest kinds on the outer margin, as it is at Keeling atoll.) 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.
+
+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. Male, 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.  Male 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 Male 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 Male
+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 (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.),
+"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.
+
+
+CHAPTER II.--BARRIER REEFS.
+
+Closely resemble in general form and structure atoll-reefs.--Width and
+depth of the lagoon-channels.--Breaches through the reef in front of
+valleys, and generally on the leeward side.--Checks to the filling up of
+the lagoon-channels.--Size and constitution of the encircled islands.--
+Number of islands within the same reef.--Barrier-reefs of New Caledonia and
+Australia.--Position of the reef relative to the slope of the adjoining
+land.--Probable great thickness of barrier-reefs.
+
+The term "barrier" has been generally applied to that vast reef which
+fronts the N.E. shore of Australia, and by most voyagers likewise to that
+on the western coast of New Caledonia.  At one time I thought it convenient
+thus to restrict the term, but as these reefs are similar in structure, and
+in position relatively to the land, to those, which, like a wall with a
+deep moat within, encircle many smaller islands, I have classed them
+together.  The reef, also, on the west coast of New Caledonia, circling
+round the extremities of the island, is an intermediate form between a
+small encircling reef and the Australian barrier, which stretches for a
+thousand miles in nearly a straight line.
+
+The geographer Balbi has in effect described those barrier-reefs, which
+encircle moderately sized islands, by calling them atolls with high land
+rising from within their central expanse.  The general resemblance between
+the reefs of the barrier and atoll classes may be seen in the small, but
+accurately reduced charts on Plate I. (The authorities from which these
+charts have been reduced, together with some remarks on them and
+descriptive of the Plates, are given separately.), and this resemblance can
+be further shown to extend to every part of the structure.  Beginning with
+the outside of the reef; many scattered soundings off Gambier, Oualan, and
+some other encircled islands, show that close to the breakers there exists
+a narrow shelving margin, beyond which the ocean becomes suddenly
+unfathomable; but off the west coast of New Caledonia, Captain Kent
+(Dalrymple, "Hydrog. Mem." volume iii.) found no bottom with 150 fathoms,
+at two ships' length from the reef; so that the slope here must be nearly
+as precipitous as off the Maldiva atolls.
+
+I can give little information regarding the kinds of corals which live on
+the outer margin.  When I visited the reef at Tahiti, although it was low
+water, the surf was too violent for me to see the living masses; but,
+according to what I heard from some intelligent native chiefs, they
+resemble in their rounded and branchless forms, those on the margin of
+Keeling atoll.  The extreme verge of the reef, which was visible between
+the breaking waves at low water, consisted of a rounded, convex,
+artificial-like breakwater, entirely coated with Nulliporae, and absolutely
+similar to that which I have described at Keeling atoll.  From what I heard
+when at Tahiti, and from the writings of the Revs. W. Ellis and J.
+Williams, I conclude that this peculiar structure is common to most of the
+encircled islands of the Society Archipelago.  The reef within this mound
+or breakwater, has an extremely irregular surface, even more so than
+between the islets on the reef of Keeling atoll, with which alone (as there
+are no islets on the reef of Tahiti) it can properly be compared.  At
+Tahiti, the reef is very irregular in width; but round many other encircled
+islands, for instance, Vanikoro or Gambier Islands (Figures 1 and 8, Plate
+I.), it is quite as regular, and of the same average width, as in true
+atolls.  Most barrier-reefs on the inner side slope irregularly into the
+lagoon-channel (as the space of deep water separating the reef from the
+included land may be called), but at Vanikoro the reef slopes only for a
+short distance, and then terminates abruptly in a submarine wall, forty
+feet high,--a structure absolutely similar to that described by Chamisso in
+the Marshall atolls.
+
+In the Society Archipelago, Ellis (Consult, on this and other points, the
+"Polynesian Researches," by the Rev. W. Ellis, an admirable work, full of
+curious information.) states, that the reefs generally lie at the distance
+of from one to one and a half miles, and, occasionally, even at more than
+three miles, from the shore.  The central mountains are generally bordered
+by a fringe of flat, and often marshy, alluvial land, from one to four
+miles in width.  This fringe consists of coral-sand and detritus thrown up
+from the lagoon-channel, and of soil washed down from the hills; it is an
+encroachment on the channel, analogous to that low and inner part of the
+islets in many atolls which is formed by the accumulation of matter from
+the lagoon.  At Hogoleu (Figure 2, Plate I.), in the Caroline Archipelago
+(See "Hydrographical Mem." and the "Atlas of the Voyage of the
+'Astrolabe'," by Captain Dumont D'Urville, page 428.), the reef on the
+south side is no less than twenty miles; on the east side, five; and on the
+north side, fourteen miles from the encircled high islands.
+
+The lagoon channels may be compared in every respect with true lagoons.  In
+some cases they are open, with a level bottom of fine sand; in others they
+are choked up with reefs of delicately branched corals, which have the same
+general character as those within the Keeling atoll.  These internal reefs
+either stand separately, or more commonly skirt the shores of the included
+high islands.  The depth of the lagoon-channel round the Society Islands
+varies from two or three to thirty fathoms; in Cook's (See the chart in
+volume i. of Hawkesworth's 4to edition of "Cook's First Voyage.") chart of
+Ulieta, however, there is one sounding laid down of forty-eight fathoms; at
+Vanikoro there are several of fifty-four and one of fifty-six and a half
+fathoms (English), a depth which even exceeds by a little that of the
+interior of the great Maldiva atolls.  Some barrier-reefs have very few
+islets on them; whilst others are surmounted by numerous ones; and those
+round part of Bolabola (Plate I., Figure 5) form a single linear strip.
+The islets first appear either on the angles of the reef, or on the sides
+of the breaches through it, and are generally most numerous on the windward
+side.  The reef to leeward retaining its usual width, sometimes lies
+submerged several fathoms beneath the surface; I have already mentioned
+Gambier Island as an instance of this structure.  Submerged reefs, having a
+less defined outline, dead, and covered with sand, have been observed (see
+Appendix) off some parts of Huaheine and Tahiti.  The reef is more
+frequently breached to leeward than to windward; thus I find in
+Krusenstern's "Memoir on the Pacific," that there are passages through the
+encircling reef on the leeward side of each of the seven Society Islands,
+which possess ship-harbours; but that there are openings to windward
+through the reef of only three of them.  The breaches in the reef are
+seldom as deep as the interior lagoon-like channel; they generally occur in
+front of the main valleys, a circumstance which can be accounted for, as
+will be seen in the fourth chapter, without much difficulty.  The breaches
+being situated in front of the valleys, which descend indifferently on all
+sides, explains their more frequent occurrence through the windward side of
+barrier-reefs than through the windward side of atolls,--for in atolls
+there is no included land to influence the position of the breaches.
+
+It is remarkable, that the lagoon-channels round mountainous islands have
+not in every instance been long ago filled up with coral and sediment; but
+it is more easily accounted for than appears at first sight.  In cases like
+that of Hogoleu and the Gambier Islands, where a few small peaks rise out
+of a great lagoon, the conditions scarcely differ from those of an atoll,
+and I have already shown, at some length, that the filling up of a true
+lagoon must be an extremely slow process.  Where the channel is narrow, the
+agency, which on unprotected coasts is most productive of sediment, namely
+the force of the breakers, is here entirely excluded, and the reef being
+breached in the front of the main valleys, much of the finer mud from the
+rivers must be transported into the open sea.  As a current is formed by
+the water thrown over the edge of atoll-formed reefs, which carries
+sediment with it through the deep-water breaches, the same thing probably
+takes place in barrier-reefs, and this would greatly aid in preventing the
+lagoon-channel from being filled up.  The low alluvial border, however, at
+the foot of the encircled mountains, shows that the work of filling up is
+in progress; and at Maura (Plate I., Figure 6), in the Society group, it
+has been almost effected, so that there remains only one harbour for small
+craft.
+
+If we look at a set of charts of barrier-reefs, and leave out in
+imagination the encircled land, we shall find that, besides the many points
+already noticed of resemblance, or rather of identity in structure with
+atolls, there is a close general agreement in form, average dimensions, and
+grouping.  Encircling barrier-reefs, like atolls, are generally elongated,
+with an irregularly rounded, though sometimes angular outline.  There are
+atolls of all sizes, from less than two miles in diameter to sixty miles
+(excluding Tilla-dou-Matte, as it consists of a number of almost
+independent atoll-formed reefs); and there are encircling barrier-reefs
+from three miles and a half to forty-six miles in diameter,--Turtle Island
+being an instance of the former, and Hogoleu of the latter.  At Tahiti the
+encircled island is thirty-six miles in its longest axis, whilst at Maurua
+it is only a little more than two miles.  It will be shown, in the last
+chapter in this volume, that there is the strictest resemblance in the
+grouping of atolls and of common islands, and consequently there must be
+the same resemblance in the grouping of atolls and of encircling
+barrier-reefs.
+
+The islands lying within reefs of this class, are of very various heights.
+Tahiti is 7,000 feet (The height of Tahiti is given from Captain Beechey;
+Maurua from Mr. F.D. Bennett ("Geograph. Journ." volume viii., page 220);
+Aitutaki from measurements made on board the "Beagle"; and Manouai or
+Harvey Island, from an estimate by the Rev. J. Williams.  The two latter
+islands, however, are not in some respects well characterised examples of
+the encircled class.); Maurua about 800; Aitutaki 360, and Manouai only 50.
+The geological nature of the included land varies: in most cases it is of
+ancient volcanic origin, owing apparently to the fact that islands of this
+nature are most frequent within all great seas; some, however, are of
+madreporitic limestone, and others of primary formation, of which latter
+kind New Caledonia offers the best example.  The central land consists
+either of one island, or of several: thus, in the Society group, Eimeo
+stands by itself; while Taha and Raiatea (Figure 3, Plate I.), both
+moderately large islands of nearly equal size, are included in one reef.
+Within the reef of the Gambier group there are four large and some smaller
+islands (Figure 8, Plate I.); within that of Hogoleu (Figure 2, Plate I.)
+nearly a dozen small islands are scattered over the expanse of one vast
+lagoon.
+
+After the details now given, it may be asserted that there is not one point
+of essential difference between encircling barrier-reefs and atolls: the
+latter enclose a simple sheet of water, the former encircle an expanse with
+one or more islands rising from it.  I was much struck with this fact, when
+viewing, from the heights of Tahiti, the distant island of Eimeo standing
+within smooth water, and encircled by a ring of snow-white breakers.
+Remove the central land, and an annular reef like that of an atoll in an
+early stage of its formation is left; remove it from Bolabola, and there
+remains a circle of linear coral-islets, crowned with tall cocoa-nut trees,
+like one of the many atolls scattered over the Pacific and Indian Oceans.
+
+The barrier-reefs of Australia and of New Caledonia deserve a separate
+notice from their great dimensions.  The reef on the west coast of New
+Caledonia (Figure 5, Plate II.) is 400 miles in length; and for a length of
+many leagues it seldom approaches within eight miles of the shore; and near
+the southern end of the island, the space between the reef and the land is
+sixteen miles in width.  The Australian barrier extends, with a few
+interruptions, for nearly a thousand miles; its average distance from the
+land is between twenty and thirty miles; and in some parts from fifty to
+seventy.  The great arm of the sea thus included, is from ten to twenty-five
+fathoms deep, with a sandy bottom; but towards the southern end, where
+the reef is further from the shore, the depth gradually increases to forty,
+and in some parts to more than sixty fathoms.  Flinders (Flinders' "Voyage
+to Terra Australis," volume ii., page 88.) has described the surface of
+this reef as consisting of a hard white agglomerate of different kinds of
+coral, with rough projecting points.  The outer edge is the highest part;
+it is traversed by narrow gullies, and at rare intervals is breached by
+ship-channels.  The sea close outside is profoundly deep; but, in front of
+the main breaches, soundings can sometimes be obtained.  Some low islets
+have been formed on the reef.
+
+(PLATE: UNNAMED, THREE VERTICAL SECTIONS (WOODCUT DIAGRAMS):
+
+1. VANIKORO, from the "Atlas of the Voyage of the 'Astrolabe'," by D.
+D'Urville.
+
+2. GAMBIER ISLAND, from Beechey.
+
+3. MAURUA, from the "Atlas of the Voyage of the 'Coquille'," by Duperrey.
+
+The horizontal line is the level of the sea, from which on the right hand a
+plummet descends, representing a depth of 200 fathoms, or 1,200 feet.  The
+vertical shading shows the section of the land, and the horizontal shading
+that of the encircling barrier-reef: from the smallness of the scale, the
+lagoon-channel could not be represented.
+
+AA.--Outer edge of the coral-reefs, where the sea breaks.
+
+BB.--The shore of the encircled islands.)
+
+There is one important point in the structure of barrier-reefs which must
+here be considered.  The accompanying diagrams represent north and south
+vertical sections, taken through the highest points of Vanikoro, Gambier,
+and Maurua Islands, and through their encircling reefs.  The scale both in
+the horizontal and vertical direction is the same, namely, a quarter of an
+inch to a nautical mile.  The height and width of these islands is known;
+and I have attempted to represent the form of the land from the shading of
+the hills in the large published charts.  It has long been remarked, even
+from the time of Dampier, that considerable degree of relation subsists
+between the inclination of that part of the land which is beneath water and
+that above it; hence the dotted line in the three sections, probably, does
+not widely differ in inclination from the actual submarine prolongation of
+the land.  If we now look at the outer edge of the reef (AA), and bear in
+mind that the plummet on the right hand represents a depth of 1,200 feet,
+we must conclude that the vertical thickness of these barrier coral-reefs
+is very great.
+
+I must observe that if the sections had been taken in any other direction
+across these islands, or across other encircled islands (In the fifth
+chapter an east and west section across the Island of Bolabola and its
+barrier-reefs is given, for the sake of illustrating another point.  The
+unbroken line in it (woodcut No. 5) is the section referred to.  The scale
+is .57 of an inch to a mile; it is taken from the "Atlas of the Voyage of
+the 'Coquille'," by Duperrey.  The depth of the lagoon-channel is
+exaggerated.), the result would have been the same.  In the succeeding
+chapter it will be shown that reef-building polypifers cannot flourish at
+great depths,--for instance, it is highly improbable that they could exist
+at a quarter of the depth represented by the plummet on the right hand of
+the woodcut.  Here there is a great APPARENT difficulty--how were the basal
+parts of these barrier-reef formed?  It will, perhaps, occur to some, that
+the actual reefs formed of coral are not of great thickness, but that
+before their first growth, the coasts of these encircled islands were
+deeply eaten into, and a broad but shallow submarine ledge thus left, on
+the edge of which the coral grew; but if this had been the case, the shore
+would have been invariably bounded by lofty cliffs, and not have sloped
+down to the lagoon-channel, as it does in many instances.  On this view
+(The Rev. D. Tyerman and Mr. Bennett ("Journal of Voyage and Travels,"
+volume i., page 215) have briefly suggested this explanation of the origin
+of the encircling reefs of the Society Islands.), moreover, the cause of
+the reef springing up at such a great distance from the land, leaving a
+deep and broad moat within, remains altogether unexplained.  A supposition
+of the same nature, and appearing at first more probable is, that the reefs
+sprung up from banks of sediment, which had accumulated round the shore
+previously to the growth of the coral; but the extension of a bank to the
+same distance round an unbroken coast, and in front of those deep arms of
+the sea (as in Raiatea, see Plate II., Figure 3) which penetrate nearly to
+the heart of some encircled islands, is exceedingly improbable.  And why,
+again, should the reef spring up, in some cases steep on both sides like a
+wall, at a distance of two, three or more miles from the shore, leaving a
+channel often between two hundred and three hundred feet deep, and rising
+from a depth which we have reason to believe is destructive to the growth
+of coral?  An admission of this nature cannot possibly be made.  The
+existence, also, of the deep channel, utterly precludes the idea of the
+reef having grown outwards, on a foundation slowly formed on its outside,
+by the accumulation of sediment and coral detritus.  Nor, again, can it be
+asserted, that the reef-building corals will not grow, excepting at a great
+distance from the land; for, as we shall soon see, there is a whole class
+of reefs, which take their name from growing closely attached (especially
+where the sea is deep) to the beach.  At New Caledonia (see Plate II.,
+Figure 5) the reefs which run in front of the west coast are prolonged in
+the same line 150 miles beyond the northern extremity of the island, and
+this shows that some explanation, quite different from any of those just
+suggested, is required.  The continuation of the reefs on each side of the
+submarine prolongation of New Caledonia, is an exceedingly interesting
+fact, if this part formerly existed as the northern extremity of the
+island, and before the attachment of the coral had been worn down by the
+action of the sea, or if it originally existed at its present height, with
+or without beds of sediment on each flank, how can we possibly account for
+the reefs, not growing on the crest of this submarine portion, but fronting
+its sides, in the same line with the reefs which front the shores of the
+lofty island?  We shall hereafter see, that there is one, and I believe
+only one, solution of this difficulty.
+
+One other supposition to account for the position of encircling barrier-reefs
+remains, but it is almost too preposterous to be mentioned; namely,
+that they rest on enormous submarine craters, surrounding the included
+islands.  When the size, height, and form of the islands in the Society
+group are considered, together with the fact that all are thus encircled,
+such a notion will be rejected by almost every one.  New Caledonia,
+moreover, besides its size, is composed of primitive formations, as are
+some of the Comoro Islands (I have been informed that this is the case by
+Dr. Allan of Forres, who has visited this group.); and Aitutaki consists of
+calcareous rock.  We must, therefore, reject these several explanations,
+and conclude that the vertical thickness of barrier-reefs, from their outer
+edges to the foundation on which they rest (from AA in the section to the
+dotted lines) is really great; but in this, there is no difficulty, for it
+is not necessary to suppose that the coral has sprung up from an immense
+depth, as will be evident when the theory of the upward growth of
+coral-reefs, during the slow subsidence of their foundation, is discussed.
+
+
+CHAPTER III.--FRINGING OR SHORE-REEFS.
+
+Reefs of Mauritius.--Shallow channel within the reef.--Its slow filling
+up.--Currents of water formed within it.--Upraised reefs.--Narrow
+fringing-reefs in deep seas.--Reefs on the coast of East Africa and of
+Brazil.--Fringing-reefs in very shallow seas, round banks of sediment and
+on worn-down islands.--Fringing-reefs affected by currents of the sea.--
+Coral coating the bottom of the sea, but not forming reefs.
+
+Fringing-reefs, or, as they have been called by some voyagers, shore-reefs,
+whether skirting an island or part of a continent, might at first be
+thought to differ little, except in generally being of less breadth, from
+barrier-reefs.  As far as the superficies of the actual reef is concerned
+this is the case; but the absence of an interior deep-water channel, and
+the close relation in their horizontal extension with the probable slope
+beneath the sea of the adjoining land, present essential points of
+difference.
+
+The reefs which fringe the island of Mauritius offer a good example of this
+class.  They extend round its whole circumference, with the exception of
+two or three parts (This fact is stated on the authority of the Officier du
+Roi, in his extremely interesting "Voyage a l'Isle de France," undertaken
+in 1768.  According to Captain Carmichael (Hooker's "Bot. Misc." volume
+ii., page 316) on one part of the coast there is a space for sixteen miles
+without a reef.), where the coast is almost precipitous, and where, if as
+is probable the bottom of the sea has a similar inclination, the coral
+would have no foundation on which to become attached.  A similar fact may
+sometimes be observed even in reefs of the barrier class, which follow much
+less closely the outline of the adjoining land; as, for instance, on the
+south-east and precipitous side of Tahiti, where the encircling reef is
+interrupted.  On the western side of the Mauritius, which was the only part
+I visited, the reef generally lies at the distance of about half a mile
+from the shore; but in some parts it is distant from one to two, and even
+three miles.  But even in this last case, as the coast-land is gently
+inclined from the foot of the mountains to the sea-beach, and as the
+soundings outside the reef indicate an equally gentle slope beneath the
+water, there is no reason for supposing that the basis of the reef, formed
+by the prolongation of the strata of the island, lies at a greater depth
+than that at which the polypifers could begin constructing the reef.  Some
+allowance, however, must be made for the outward extension of the corals on
+a foundation of sand and detritus, formed from their own wear, which would
+give to the reef a somewhat greater vertical thickness, than would
+otherwise be possible.
+
+The outer edge of the reef on the western or leeward side of the island is
+tolerably well defined, and is a little higher than any other part.  It
+chiefly consists of large strongly branched corals, of the genus Madrepora,
+which also form a sloping bed some way out to sea: the kinds of coral
+growing in this part will be described in the ensuing chapter.  Between the
+outer margin and the beach, there is a flat space with a sandy bottom and a
+few tufts of living coral; in some parts it is so shallow, that people, by
+avoiding the deeper holes and gullies, can wade across it at low water; in
+other parts it is deeper, seldom however exceeding ten or twelve feet, so
+that it offers a safe coasting channel for boats.  On the eastern and
+windward side of the island, which is exposed to a heavy surf, the reef was
+described to me as having a hard smooth surface, very slightly inclined
+inwards, just covered at low-water, and traversed by gullies; it appears to
+be quite similar in structure to the reefs of the barrier and atoll
+classes.
+
+The reef of Mauritius, in front of every river and streamlet, is breached
+by a straight passage: at Grand Port, however, there is a channel like
+that within a barrier-reef; it extends parallel to the shore for four
+miles, and has an average depth of ten or twelve fathoms; its presence may
+probably be accounted for by two rivers which enter at each end of the
+channel, and bend towards each other.  The fact of reefs of the fringing
+class being always breached in front of streams, even of those which are
+dry during the greater part of the year, will be explained, when the
+conditions unfavourable to the growth of coral are considered.  Low
+coral-islets, like those on barrier-reefs and atolls, are seldom formed on
+reefs of this class, owing apparently in some cases to their narrowness, and
+in others to the gentle slope of the reef outside not yielding many fragments
+to the breakers.  On the windward side, however, of the Mauritius, two or
+three small islets have been formed.
+
+It appears, as will be shown in the ensuing chapter, that the action of the
+surf is favourable to the vigorous growth of the stronger corals, and that
+sand or sediment, if agitated by the waves, is injurious to them.  Hence it
+is probable that a reef on a shelving shore, like that of Mauritius, would
+at first grow up, not attached to the actual beach, but at some little
+distance from it; and the corals on the outer margin would be the most
+vigorous.  A shallow channel would thus be formed within the reef, and as
+the breakers are prevented acting on the shores of the island, and as they
+do not ordinarily tear up many fragments from the outside, and as every
+streamlet has its bed prolonged in a straight line through the reef, this
+channel could be filled up only very slowly with sediment.  But a beach of
+sand and of fragments of the smaller kinds of coral seems, in the case of
+Mauritius, to be slowly encroaching on the shallow channel.  On many
+shelving and sandy coasts, the breakers tend to form a bar of sand a little
+way from the beach, with a slight increase of depth within it; for
+instance, Captain Grey (Captain Grey's "Journal of Two Expeditions," volume
+i. page 369.) states that the west coast of Australia, in latitude 24 deg.,
+is fronted by a sand bar about two hundred yards in width, on which there
+is only two feet of water; but within it the depth increases to two
+fathoms.  Similar bars, more or less perfect, occur on other coasts.  In
+these cases I suspect that the shallow channel (which no doubt during
+storms is occasionally obliterated) is scooped out by the flowing away of
+the water thrown beyond the line, on which the waves break with the
+greatest force.  At Pernambuco a bar of hard sandstone (I have described
+this singular structure in the "London and Edinburgh Phil. Mag." October
+1841.), which has the same external form and height as a coral-reef,
+extends nearly parallel to the coast; within this bar currents, apparently
+caused by the water thrown over it during the greater part of each tide,
+run strongly, and are wearing away its inner wall.  From these facts it can
+hardly be doubted, that within most fringing-reefs, especially within those
+lying some distance from the land, a return stream must carry away the
+water thrown over the outer edge; and the current thus produced, would tend
+to prevent the channel being filled up with sediment, and might even deepen
+it under certain circumstances.  To this latter belief I am led, by finding
+that channels are almost universally present within the fringing-reefs of
+those islands which have undergone recent elevatory movements; and this
+could hardly have been the case, if the conversion of the very shallow
+channel into land had not been counteracted to a certain extent.
+
+A fringing-reef, if elevated in a perfect condition above the level of the
+sea, ought to present the singular appearance of a broad dry moat within a
+low mound.  The author ("Voyage a l'Isle de France, par un Officier du
+Roi," part i., pages 192, 200.) of an interesting pedestrian tour round the
+Mauritius, seems to have met with a structure of this kind: he says
+"J'observai que la, ou la mer etale, independamment des rescifs du large,
+il y a terre UNE ESPECE D'EFFONCEMENT ou chemin couvert naturel.  On y
+pourrait mettre du canon," etc.  In another place he adds, "Avant de passer
+le Cap, on remarque un gros banc de corail eleve de plus de quinze pieds:
+c'est une espece de rescif, que la mer abandonne, il regne au pied une
+longue flaque d'eau, dont on pourrait faire un bassin pour de petits
+vaisseaux."  But the margin of the reef, although the highest and most
+perfect part, from being most exposed to the surf, would generally during a
+slow rise of the land be either partially or entirely worn down to that
+level, at which corals could renew their growth on its upper edge.  On some
+parts of the coast-land of Mauritius there are little hillocks of coral-rock,
+which are either the last remnants of a continuous reef, or of low
+islets formed on it.  I observed that two such hillocks between Tamarin Bay
+and the Great Black River; they were nearly twenty feet high, about two
+hundred yards from the present beach, and about thirty feet above its
+level.  They rose abruptly from a smooth surface, strewed with worn
+fragments of coral.  They consisted in their lower part of hard calcareous
+sandstone, and in their upper of great blocks of several species of Astraea
+and Madrepora, loosely aggregated; they were divided into irregular beds,
+dipping seaward, in one hillock at an angle of 8 deg., and in the other at
+18 deg.  I suspect that the superficial parts of the reefs, which have been
+upraised together with the islands they fringe, have generally been much
+more modified by the wearing action of the sea, than those of Mauritius.
+
+Many islands are fringed by reefs quite similar to those of Mauritius (I
+may give Cuba, as another instance; Mr. Taylor ("Loudon's Mag. of Nat.
+Hist." volume ix., page 449) has described a reef several miles in length
+between Gibara and Vjaro, which extends parallel to the shore at the
+distance of between half and the third part of a mile, and encloses a space
+of shallow water, with a sandy bottom and tufts of coral.  Outside the edge
+of the reef, which is formed of great branching corals, the depth is six
+and seven fathoms.  This coast has been upheaved at no very distant
+geological period."); but on coasts where the sea deepens very suddenly the
+reefs are much narrower, and their limited extension seems evidently to
+depend on the high inclination of the submarine slope; a relation, which,
+as we have seen, does not exist in reefs of the barrier class.  The
+fringing-reefs on steep coasts are frequently not more than from fifty to
+one hundred yards in width; they have a nearly smooth, hard surface,
+scarcely uncovered at low water, and without any interior shoal channel,
+like that within those fringing-reefs, which lie at a greater distance from
+the land.  The fragments torn up during gales from the outer margin are
+thrown over the reef on the shores of the island.  I may give as instances,
+Wateeo, where the reef is described by Cook as being a hundred yards wide;
+and Mauti and Elizabeth Islands (Mauti is described by Lord Byron in the
+voyage of H.M.S. "Blonde", and Elizabeth Island by Captain Beechey.), where
+it is only fifty yards in width: the sea round these islands is very deep.
+
+Fringing-reefs, like barrier-reefs, both surround islands, and front the
+shores of continents.  In the charts of the eastern coast of Africa, by
+Captain Owen, many extensive fringing-reefs are laid down; thus, for a
+space of nearly forty miles, from latitude 1 deg 15' to 1 deg 45' S., a
+reef fringes the shore at an average distance of rather more than one mile,
+and therefore at a greater distance than is usual in reefs of this class;
+but as the coast-land is not lofty, and as the bottom shoals very gradually
+(the depth being only from eight to fourteen fathoms at a mile and a half
+outside the reef), its extension thus far from the land offers no
+difficulty.  The external margin of this reef is described, as formed of
+projecting points, within which there is a space, from six to twelve feet
+deep, with patches of living coral on it.  At Mukdeesha (latitude 2 deg 1'
+N.) "the port is formed," it is said (Owen's "Africa," volume i., page 357,
+from which work the foregoing facts are likewise taken.) "by a long reef
+extending eastward, four or five miles, within which there is a narrow
+channel, with ten to twelve feet of water at low spring-tides;" it lies at
+the distance of a quarter of a mile from the shore.  Again, in the plan of
+Mombas (latitude 4 deg S.), a reef extends for thirty-six miles, at the
+distance of from half a mile to one mile and a quarter from the shore;
+within it, there is a channel navigable "for canoes and small craft,"
+between six and fifteen feet deep: outside the reef the depth is about
+thirty fathoms at the distance of nearly half a mile.  Part of this reef is
+very symmetrical, and has a uniform breadth of two hundred yards.
+
+The coast of Brazil is in many parts fringed by reefs.  Of these, some are
+not of coral formation; for instance, those near Bahia and in front of
+Pernambuco; but a few miles south of this latter city, the reef follows
+(See Baron Roussin's "Pilote du Bresil," and accompanying hydrographical
+memoir.) so closely every turn of the shore, that I can hardly doubt it is
+of coral; it runs at the distance of three-quarters of a mile from the
+land, and within it the depth is from ten to fifteen feet.  I was assured
+by an intelligent pilot that at Ports Frances and Maceio, the outer part of
+the reef consists of living coral, and the inner of a white stone, full of
+large irregular cavities, communicating with the sea.  The bottom of the
+sea off the coast of Brazil shoals gradually to between thirty and forty
+fathoms, at the distance of between nine and ten leagues from the land.
+
+From the description now given, we must conclude that the dimensions and
+structure of fringing-reefs depend entirely on the greater or less
+inclination of the submarine slope, conjoined with the fact that
+reef-building polypifers can exist only at limited depths.  It follows from
+this, that where the sea is very shallow, as in the Persian Gulf and in
+parts of the East Indian Archipelago, the reefs lose their fringing
+character, and appear as separate and irregularly scattered patches, often
+of considerable area.  From the more vigorous growth of the coral on the
+outside, and from the conditions being less favourable in several respects
+within, such reefs are generally higher and more perfect in their marginal
+than in their central parts; hence these reefs sometimes assume (and this
+circumstance ought not to be overlooked) the appearance of atolls; but they
+differ from atolls in their central expanse being much less deep, in their
+form being less defined, and in being based on a shallow foundation.  But
+when in a deep sea reefs fringe banks of sediment, which have accumulated
+beneath the surface, round either islands or submerged rocks, they are
+distinguished with difficulty on the one hand from encircling barrier-reefs,
+and on the other from atolls.  In the West Indies there are reefs,
+which I should probably have arranged under both these classes, had not the
+existence of large and level banks, lying a little beneath the surface,
+ready to serve as the basis for the attachment of coral, been occasionally
+brought into view by the entire or partial absence of reefs on them, and
+had not the formation of such banks, through the accumulation of sediment
+now in progress, been sufficiently evident.  Fringing-reefs sometimes coat,
+and thus protect the foundations of islands, which have been worn down by
+the surf to the level of the sea.  According to Ehrenberg, this has been
+extensively the case with the islands in the Red Sea, which formerly ranged
+parallel to the shores of the mainland, with deep water within them: hence
+the reefs now coating their bases are situated relatively to the land like
+barrier-reefs, although not belonging to that class; but there are, as I
+believe, in the Red Sea some true barrier-reefs.  The reefs of this sea and
+of the West Indies will be described in the Appendix.  In some cases,
+fringing-reefs appear to be considerably modified in outline by the course
+of the prevailing currents.  Dr. J. Allan informs me that on the east coast
+of Madagascar almost every headland and low point of sand has a coral-reef
+extending from it in a S.W. and N.E. line, parallel to the currents on that
+shore.  I should think the influence of the currents chiefly consisted in
+causing an extension, in a certain direction, of a proper foundation for
+the attachment of the coral.  Round many intertropical islands, for
+instance the Abrolhos on the coast of Brazil surveyed by Captain Fitzroy,
+and, as I am informed by Mr. Cuming, round the Philippines, the bottom of
+the sea is entirely coated by irregular masses of coral, which although
+often of large size, do not reach the surface and form proper reefs.  This
+must be owing, either to insufficient growth, or to the absence of those
+kinds of corals which can withstand the breaking of the waves.
+
+The three classes, atoll-formed, barrier, and fringing-reefs, together with
+the modifications just described of the latter, include all the most
+remarkable coral formations anywhere existing.  At the commencement of the
+last chapter in the volume, where I detail the principles on which the map
+(Plate III.) is coloured, the exceptional cases will be enumerated.
+
+
+CHAPTER IV.--ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.
+
+In this chapter I will give all the facts which I have collected, relating
+to the distribution of coral-reefs,--to the conditions favourable to their
+increase,--to the rate of their growth,--and to the depth at which they are
+formed.
+
+These subjects have an important bearing on the theory of the origin of the
+different classes of coral-reefs.
+
+
+SECTION 4.I.--ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
+FAVOURABLE TO THEIR INCREASE.
+
+With regard to the limits of latitude, over which coral-reefs extend, I
+have nothing new to add.  The Bermuda Islands, in 32 deg 15' N., is the
+point furthest removed from the equator, in which they appear to exist; and
+it has been suggested that their extension so far northward in this
+instance is owing to the warmth of the Gulf Stream.  In the Pacific, the
+Loo Choo Islands, in latitude 27 deg N., have reefs on their shores, and
+there is an atoll in 28 deg 30', situated N.W. of the Sandwich Archipelago.
+In the Red Sea there are coral-reefs in latitude 30 deg.  In the southern
+hemisphere coral-reefs do not extend so far from the equatorial sea.  In
+the Southern Pacific there are only a few reefs beyond the line of the
+tropics, but Houtmans Abrolhos, on the western shores of Australia in
+latitude 29 deg S., are of coral formation.
+
+The proximity of volcanic land, owing to the lime generally evolved from
+it, has been thought to be favourable to the increase of coral-reefs.
+There is, however, not much foundation for this view; for nowhere are
+coral-reefs more extensive than on the shores of New Caledonia, and of
+north-eastern Australia, which consist of primary formations; and in the
+largest groups of atolls, namely the Maldiva, Chagos, Marshall, Gilbert,
+and Low Archipelagoes, there is no volcanic or other kind of rock,
+excepting that formed of coral.
+
+The entire absence of coral-reefs in certain large areas within the
+tropical seas, is a remarkable fact.  Thus no coral-reefs were observed,
+during the surveying voyages of the "Beagle" and her tender on the west
+coast of South America south of the equator, or round the Galapagos
+Islands.  It appears, also, that there are none (I have been informed that
+this is the case, by Lieutenant Ryder, R.N., and others who have had ample
+opportunities for observation.) north of the equator; Mr. Lloyd, who
+surveyed the Isthmus of Panama, remarked to me, that although he had seen
+corals living in the Bay of Panama, yet he had never observed any reefs
+formed by them.  I at first attributed this absence of reefs on the coasts
+of Peru and of the Galapagos Islands (The mean temperature of the surface
+sea from observations made by the direction of Captain Fitzroy on the
+shores of the Galapagos Islands, between the 16th of September and the 20th
+of October, 1835, was 68 deg Fahr.  The lowest temperature observed was
+58.5 deg at the south-west end of Albemarle Island; and on the west coast
+of this island, it was several times 62 deg and 63 deg.  The mean
+temperature of the sea in the Low Archipelago of atolls, and near Tahiti,
+from similar observations made on board the "Beagle", was (although further
+from the equator) 77.5 deg, the lowest any day being 76.5 deg.  Therefore
+we have here a difference of 9.5 deg in mean temperature, and 18 deg in
+extremes; a difference doubtless quite sufficient to affect the
+distribution of organic beings in the two areas.), to the coldness of the
+currents from the south, but the Gulf of Panama is one of the hottest
+pelagic districts in the world.  (Humboldt's "Personal Narrative," volume
+vii., page 434.)  In the central parts of the Pacific there are islands
+entirely free from reefs; in some few of these cases I have thought that
+this was owing to recent volcanic action; but the existence of reefs round
+the greater part of Hawaii, one of the Sandwich Islands, shows that recent
+volcanic action does not necessarily prevent their growth.
+
+In the last chapter I stated that the bottom of the sea round some islands
+is thickly coated with living corals, which nevertheless do not form reefs,
+either from insufficient growth, or from the species not being adapted to
+contend with the breaking waves.
+
+I have been assured by several people, that there are no coral-reefs on the
+west coast of Africa (It might be concluded, from a paper by Captain Owen
+("Geographical Journal", volume ii., page 89), that the reefs off Cape St.
+Anne and the Sherboro' Islands were of coral, although the author states
+that they are not purely coralline.  But I have been assured by Lieutenant
+Holland, R.N., that these reefs are not of coral, or at least that they do
+not at all resemble those in the West Indies.), or round the islands in the
+Gulf of Guinea.  This perhaps may be attributed, in part, to the sediment
+brought down by the many rivers debouching on that coast, and to the
+extensive mud-banks, which line great part of it.  But the islands of St.
+Helena, Ascension, the Cape Verdes, St. Paul's, and Fernando Noronha, are,
+also, entirely without reefs, although they lie far out at sea, are
+composed of the same ancient volcanic rocks, and have the same general
+form, with those islands in the Pacific, the shores of which are surrounded
+by gigantic walls of coral-rock.  With the exception of Bermuda, there is
+not a single coral-reef in the central expanse of the Atlantic Ocean.  It
+will, perhaps, be suggested that the quantity of carbonate of lime in
+different parts of the sea, may regulate the presence of reefs.  But this
+cannot be the case, for at Ascension, the waves charged to excess
+precipitate a thick layer of calcareous matter on the tidal rocks; and at
+St. Jago, in the Cape Verdes, carbonate of lime not only is abundant on the
+shores, but it forms the chief part of some upraised post-tertiary strata.
+The apparently capricious distribution, therefore, of coral-reefs, cannot
+be explained by any of these obvious causes; but as the study of the
+terrestrial and better known half of the world must convince every one that
+no station capable of supporting life is lost,--nay more, that there is a
+struggle for each station, between the different orders of nature,--we may
+conclude that in those parts of the intertropical sea, in which there are
+no coral-reefs, there are other organic bodies supplying the place of the
+reef-building polypifers.  It has been shown in the chapter on Keeling
+atoll that there are some species of large fish, and the whole tribe of
+Holothuriae which prey on the tenderer parts of the corals.  On the other
+hand, the polypifers in their turn must prey on some other organic beings;
+the decrease of which from any cause would cause a proportionate
+destruction of the living coral.  The relations, therefore, which determine
+the formation of reefs on any shore, by the vigorous growth of the
+efficient kinds of coral, must be very complex, and with our imperfect
+knowledge quite inexplicable.  From these considerations, we may infer that
+changes in the condition of the sea, not obvious to our senses, might
+destroy all the coral-reefs in one area, and cause them to appear in
+another: thus, the Pacific or Indian Ocean might become as barren of
+coral-reefs as the Atlantic now is, without our being able to assign any
+adequate cause for such a change.
+
+It has been a question with some naturalists, which part of a reef is most
+favourable to the growth of coral.  The great mounds of living Porites and
+of Millepora round Keeling atoll occur exclusively on the extreme verge of
+the reef, which is washed by a constant succession of breakers; and living
+coral nowhere else forms solid masses.  At the Marshall islands the larger
+kinds of coral (chiefly species of Astraea, a genus closely allied to
+Porites) "which form rocks measuring several fathoms in thickness," prefer,
+according to Chamisso (Kotzebue's "First Voyage" (English Translation),
+volume iii., pages 142, 143, 331.), the most violent surf.  I have stated
+that the outer margin of the Maldiva atolls consists of living corals (some
+of which, if not all, are of the same species with those at Keeling atoll),
+and here the surf is so tremendous, that even large ships have been thrown,
+by a single heave of the sea, high and dry on the reef, all on board thus
+escaping with their lives.
+
+Ehrenberg (Ehrenberg, "Uber die Natur und Bildung der Corallen Banke im
+rothen Meere," page 49.) remarks, that in the Red Sea the strongest corals
+live on the outer reefs, and appear to love the surf; he adds, that the
+more branched kinds abound a little way within, but that even these in
+still more protected places, become smaller.  Many other facts having a
+similar tendency might be adduced.  (In the West Indies, as I am informed
+by Captain Bird Allen, R.N., it is the common belief of those, who are best
+acquainted with the reefs, that the coral flourishes most, where freely
+exposed to the swell of the open sea.)  It has, however, been doubted by
+MM. Quoy and Gaimard, whether any kind of coral can even withstand, much
+less flourish in, the breakers of an open sea ("Annales des Sciences
+Naturelles," tome vi., pages 276, 278.--"La ou les ondes sont agitees, les
+Lytophytes ne peuvent travailler, parce qu'elles detruiraient leurs
+fragiles edifices," etc.): they affirm that the saxigenous lithophytes
+flourish only where the water is tranquil, and the heat intense.  This
+statement has passed from one geological work to another; nevertheless, the
+protection of the whole reef undoubtedly is due to those kinds of coral,
+which cannot exist in the situations thought by these naturalists to be
+most favourable to them.  For should the outer and living margin perish, of
+any one of the many low coral-islands, round which a line of great breakers
+is incessantly foaming, the whole, it is scarcely possible to doubt, would
+be washed away and destroyed, in less than half a century.  But the vital
+energies of the corals conquer the mechanical power of the waves; and the
+large fragments of reef torn up by every storm, are replaced by the slow
+but steady growth of the innumerable polypifers, which form the living zone
+on its outer edge.
+
+From these facts, it is certain, that the strongest and most massive corals
+flourish, where most exposed.  The less perfect state of the reef of most
+atolls on the leeward and less exposed side, compared with its state to
+windward; and the analogous case of the greater number of breaches on the
+near sides of those atolls in the Maldiva Archipelago, which afford some
+protection to each other, are obviously explained by this circumstance.  If
+the question had been, under what conditions the greater number of species
+of coral, not regarding their bulk and strength, were developed, I should
+answer,--probably in the situations described by MM. Quoy and Gaimard,
+where the water is tranquil and the heat intense.  The total number of
+species of coral in the circumtropical seas must be very great: in the Red
+Sea alone, 120 kinds, according to Ehrenberg (Ehrenberg, "Uber die Natur,"
+etc., etc., page 46.), have been observed.
+
+The same author has observed that the recoil of the sea from a steep shore
+is injurious to the growth of coral, although waves breaking over a bank
+are not so.  Ehrenberg also states, that where there is much sediment,
+placed so as to be liable to be moved by the waves there is little or no
+coral; and a collection of living specimens placed by him on a sandy shore
+died in the course of a few days.  (Ibid., page 49.)  An experiment,
+however, will presently be related in which some large masses of living
+coral increased rapidly in size, after having been secured by stakes on a
+sandbank.  That loose sediment should be injurious to the living
+polypifers, appears, at first sight, probable; and accordingly, in sounding
+off Keeling atoll, and (as will hereafter be shown) off Mauritius, the
+arming of the lead invariably came up clean, where the coral was growing
+vigorously.  This same circumstance has probably given rise to a strange
+belief, which, according to Captain Owen (Captain Owen on the Geography of
+the Maldiva Islands, "Geographical Journal", volume ii., page 88.), is
+general amongst the inhabitants of the Maldiva atolls, namely that corals
+have roots, and therefore that if merely broken down to the surface, they
+grow up again; but, if rooted out, they are permanently destroyed.  By this
+means the inhabitants keep their harbours clear; and thus the French
+Governor of St. Mary's in Madagascar, "cleared out and made a beautiful
+little port at that place."  For it is probable that sand would accumulate
+in the hollows formed by tearing out the corals, but not on the broken and
+projecting stumps, and therefore, in the former case, the fresh growth of
+the coral might be thus prevented.
+
+In the last chapter I remarked that fringing-reefs are almost universally
+breached, where streams enter the sea.  (Lieutenant Wellstead and others
+have remarked that this is the case in the Red Sea; Dr. Ruppell ("Reise in
+Abyss." Band. i., page 142) says that there are pear-shaped harbours in the
+upraised coral-coast, into which periodical streams enter.  From this
+circumstance, I presume, we must infer that before the upheaval of the
+strata now forming the coast-land, fresh water and sediment entered the sea
+at these points; and the coral being thus prevented growing, the pear-shaped
+harbours were produced.)  Most authors have attributed this fact to
+the injurious effects of the fresh water, even where it enters the sea only
+in small quantity, and during a part of the year.  No doubt brackish water
+would prevent or retard the growth of coral; but I believe that the mud and
+sand which is deposited, even by rivulets when flooded, is a much more
+efficient check.  The reef on each side of the channel leading into Port
+Louis at Mauritius, ends abruptly in a wall, at the foot of which I sounded
+and found a bed of thick mud.  This steepness of the sides appears to be a
+general character in such breaches.  Cook (Cook's "First Voyage," volume
+ii., page 271 (Hawkesworth's edition).), speaking of one at Raiatea, says,
+"like all the rest, it is very steep on both sides."  Now, if it were the
+fresh water mingling with the salt which prevented the growth of coral, the
+reef certainly would not terminate abruptly, but as the polypifers nearest
+the impure stream would grow less vigorously than those farther off, so
+would the reef gradually thin away.  On the other hand, the sediment
+brought down from the land would only prevent the growth of the coral in
+the line of its deposition, but would not check it on the side, so that the
+reefs might increase till they overhung the bed of the channel.  The
+breaches are much fewer in number, and front only the larger valleys in
+reefs of the encircling barrier class.  They probably are kept open in the
+same manner as those into the lagoon of an atoll, namely, by the force of
+the currents and the drifting outwards of fine sediment.  Their position in
+front of valleys, although often separated from the land by deep water
+lagoon-channels, which it might be thought would entirely remove the
+injurious effects both of the fresh water and the sediment, will receive a
+simple explanation when we discuss the origin of barrier-reefs.
+
+In the vegetable kingdom every different station has its peculiar group of
+plants, and similar relations appear to prevail with corals.  We have
+already described the great difference between the corals within the lagoon
+of an atoll and those on its outer margin.  The corals, also, on the margin
+of Keeling Island occurred in zones; thus the Porites and Millepora
+complanata grow to a large size only where they are washed by a heavy sea,
+and are killed by a short exposure to the air; whereas, three species of
+Nullipora also live amidst the breakers, but are able to survive uncovered
+for a part of each tide; at greater depths, a strong Madrepora and
+Millepora alcicornis are the commonest kinds, the former appearing to be
+confined to this part, beneath the zone of massive corals, minute
+encrusting corallines and other organic bodies live.  If we compare the
+external margin of the reef at Keeling atoll with that on the leeward side
+of Mauritius, which are very differently circumstanced, we shall find a
+corresponding difference in the appearance of the corals.  At the latter
+place, the genus Madrepora is preponderant over every other kind, and
+beneath the zone of massive corals there are large beds of Seriatopora.
+There is also a marked difference, according to Captain Moresby (Captain
+Moresby on the Northern Maldiva atolls, "Geographical Journal", volume v.,
+page 401.), between the great branching corals of the Red Sea, and those on
+the reefs of the Maldiva atolls.
+
+These facts, which in themselves are deserving of notice, bear, perhaps,
+not very remotely, on a remarkable circumstance which has been pointed out
+to me by Captain Moresby, namely, that with very few exceptions, none of
+the coral-knolls within the lagoons of Peros Banhos, Diego Garcia, and the
+Great Chagos Bank (all situated in the Chagos group), rise to the surface
+of the water; whereas all those, with equally few exceptions, within
+Solomon and Egmont atolls in the same group, and likewise within the large
+southern Maldiva atolls, reach the surface.  I make these statements, after
+having examined the charts of each atoll.  In the lagoon of Peros Banhos,
+which is nearly twenty miles across, there is only one single reef which
+rises to the surface; in Diego Garcia there are seven, but several of these
+lie close to the margin of the lagoon, and need scarcely have been
+reckoned; in the Great Chagos Bank there is not one.  On the other hand, in
+the lagoons of some of the great southern Maldiva atolls, although thickly
+studded with reefs, every one without exception rises to the surface; and
+on an average there are less than two submerged reefs in each atoll; in the
+northern atolls, however, the submerged lagoon-reefs are not quite so rare.
+The submerged reefs in the Chagos atolls generally have from one to seven
+fathoms water on them, but some have from seven to ten.  Most of them are
+small with very steep sides (Some of these statements were not communicated
+to me verbally by Captain Moresby, but are taken from the MS. account
+before alluded to, of the Chagos Group.); at Peros Banhos they rise from a
+depth of about thirty fathoms, and some of them in the Great Chagos Bank
+from above forty fathoms; they are covered, Captain Moresby informs me,
+with living and healthy coral, two and three feet high, consisting of
+several species.  Why then have not these lagoon-reefs reached the surface,
+like the innumerable ones in the atolls above named?  If we attempt to
+assign any difference in their external conditions, as the cause of this
+diversity, we are at once baffled.  The lagoon of Diego Garcia is not deep,
+and is almost wholly surrounded by its reef; Peros Banhos is very deep,
+much larger, with many wide passages communicating with the open sea.  On
+the other hand, of those atolls, in which all or nearly all the lagoon-reefs
+have reached the surface, some are small, others large, some shallow,
+others deep, some well-enclosed, and others open.
+
+Captain Moresby informs me that he has seen a French chart of Diego Garcia
+made eighty years before his survey, and apparently very accurate; and from
+it he infers, that during this interval there has not been the smallest
+change in the depth on any of the knolls within the lagoon.  It is also
+known that during the last fifty-one years, the eastern channel into the
+lagoon has neither become narrower, nor decreased in depth; and as there
+are numerous small knolls of living coral within it, some change might have
+been anticipated.  Moreover, as the whole reef round the lagoon of this
+atoll has been converted into land--an unparalleled case, I believe, in an
+atoll of such large size,--and as the strip of land is for considerable
+spaces more than half a mile wide--also a very unusual circumstance,--we
+have the best possible evidence, that Diego Garcia has remained at its
+present level for a very long period.  With this fact, and with the
+knowledge that no sensible change has taken place during eighty years in
+the coral-knolls, and considering that every single reef has reached the
+surface in other atolls, which do not present the smallest appearance of
+being older than Diego Garcia and Peros Banhos, and which are placed under
+the same external conditions with them, one is led to conclude that these
+submerged reefs, although covered with luxuriant coral, have no tendency to
+grow upwards, and that they would remain at their present levels for an
+almost indefinite period.
+
+From the number of these knolls, from their position, size, and form, many
+of them being only one or two hundred yards across, with a rounded outline,
+and precipitous sides,--it is indisputable that they have been formed by
+the growth of coral; and this makes the case much more remarkable.  In
+Peros Banhos and in the Great Chagos Bank, some of these almost columnar
+masses are 200 feet high, and their summits lie only from two to eight
+fathoms beneath the surface; therefore, a small proportional amount more of
+growth would cause them to attain the surface, like those numerous knolls,
+which rise from an equally great depth within the Maldiva atolls.  We can
+hardly suppose that time has been wanting for the upward growth of the
+coral, whilst in Diego Garcia, the broad annular strip of land, formed by
+the continued accumulation of detritus, shows how long this atoll has
+remained at its present level.  We must look to some other cause than the
+rate of growth; and I suspect it will be found in the reefs being formed of
+different species of corals, adapted to live at different depths.
+
+The Great Chagos Bank is situated in the centre of the Chagos Group, and
+the Pitt and Speaker Banks at its two extreme points.  These banks resemble
+atolls, except in their external rim being about eight fathoms submerged,
+and in being formed of dead rock, with very little living coral on it: a
+portion nine miles long of the annular reef of Peros Banhos atoll is in the
+same condition.  These facts, as will hereafter be shown, render it very
+probable that the whole group at some former period subsided seven or eight
+fathoms; and that the coral perished on the outer margin of those atolls
+which are now submerged, but that it continued alive, and grew up to the
+surface on those which are now perfect.  If these atolls did subside, and
+if from the suddenness of the movement or from any other cause, those
+corals which are better adapted to live at a certain depth than at the
+surface, once got possession of the knolls, supplanting the former
+occupants, they would exert little or no tendency to grow upwards.  To
+illustrate this, I may observe, that if the corals of the upper zone on the
+outer edge of Keeling atoll were to perish, it is improbable that those of
+the lower zone would grow to the surface, and thus become exposed to
+conditions for which they do not appear to be adapted.  The conjecture,
+that the corals on the submerged knolls within the Chagos atolls have
+analogous habits with those of the lower zone outside Keeling atoll,
+receives some support from a remark by Captain Moresby, namely, that they
+have a different appearance from those on the reefs in the Maldiva atolls,
+which, as we have seen, all rise to the surface: he compares the kind of
+difference to that of the vegetation under different climates.  I have
+entered at considerable length into this case, although unable to throw
+much light on it, in order to show that an equal tendency to upward growth
+ought not to be attributed to all coral-reefs,--to those situated at
+different depths,--to those forming the ring of an atoll or those on the
+knolls within a lagoon,--to those in one area and those in another.  The
+inference, therefore, that one reef could not grow up to the surface within
+a given time, because another, not known to be covered with the same
+species of corals, and not known to be placed under conditions exactly the
+same, has not within the same time reached the surface, is unsound.
+
+
+SECTION 4.II.--ON THE RATE OF GROWTH OF CORAL-REEFS.
+
+The remark made at the close of the last section, naturally leads to this
+division of our subject, which has not, I think, hitherto been considered
+under a right point of view.  Ehrenberg (Ehrenberg, as before cited, pages
+39, 46, and 50.) has stated, that in the Red Sea, the corals only coat
+other rocks in a layer from one to two feet in thickness, or at most to a
+fathom and a half; and he disbelieves that, in any case, they form, by
+their own proper growth, great masses, stratum over stratum.  A nearly
+similar observation has been made by MM. Quoy and Gaimard ("Annales des
+Sciences Nat." tom. vi., page 28.), with respect to the thickness of some
+upraised beds of coral, which they examined at Timor and some other places.
+Ehrenberg (Ehrenberg, ut sup., page 42.) saw certain large massive corals
+in the Red Sea, which he imagines to be of such vast antiquity, that they
+might have been beheld by Pharaoh; and according to Mr. Lyell (Lyell's
+"Principles of Geology," book iii., chapter xviii.) there are certain
+corals at Bermuda, which are known by tradition, to have been living for
+centuries.  To show how slowly coral-reefs grow upwards, Captain Beechey
+(Beechey's "Voyage to the Pacific," chapter viii.) has adduced the case of
+the Dolphin Reef off Tahiti, which has remained at the same depth beneath
+the surface, namely about two fathoms and a half, for a period of
+sixty-seven years.  There are reefs in the Red Sea, which certainly do not
+appear (Ehrenberg, ut sup., page 43.) to have increased in dimensions during
+the last half-century, and from the comparison of old charts with recent
+surveys, probably not during the last two hundred years.  These, and other
+similar facts, have so strongly impressed many with the belief of the
+extreme slowness of the growth of corals, that they have even doubted the
+possibility of islands in the great oceans having been formed by their
+agency.  Others, again, who have not been overwhelmed by this difficulty,
+have admitted that it would require thousands, and tens of thousands of
+years, to form a mass, even of inconsiderable thickness; but the subject
+has not, I believe, been viewed in the proper light.
+
+That masses of considerable thickness have been formed by the growth of
+coral, may be inferred with certainty from the following facts.  In the
+deep lagoons of Peros Banhos and of the Great Chagos Bank, there are, as
+already described, small steep-sided knolls covered with living coral.
+There are similar knolls in the southern Maldiva atolls, some of which, as
+Captain Moresby assures me, are less than a hundred yards in diameter, and
+rise to the surface from a depth of between two hundred and fifty and three
+hundred feet.  Considering their number, form, and position, it would be
+preposterous to suppose that they are based on pinnacles of any rock, not
+of coral formation; or that sediment could have been heaped up into such
+small and steep isolated cones.  As no kind of living coral grows above the
+height of a few feet, we are compelled to suppose that these knolls have
+been formed by the successive growth and death of many individuals,--first
+one being broken off or killed by some accident, and then another, and one
+set of species being replaced by another set with different habits, as the
+reef rose nearer the surface, or as other changes supervened.  The spaces
+between the corals would become filled up with fragments and sand, and such
+matter would probably soon be consolidated, for we learn from Lieutenant
+Nelson ("Geological Transactions," volume v., page 113.), that at Bermuda a
+process of this kind takes place beneath water, without the aid of
+evaporation.  In reefs, also, of the barrier class, we may feel sure, as I
+have shown, that masses of great thickness have been formed by the growth
+of the coral; in the case of Vanikoro, judging only from the depth of the
+moat between the land and the reef, the wall of coral-rock must be at least
+three hundred feet in vertical thickness.
+
+It is unfortunate that the upraised coral-islands in the Pacific have not
+been examined by a geologist.  The cliffs of Elizabeth Island, in the Low
+Archipelago, are eighty feet high, and appear, from Captain Beechey's
+description, to consist of a homogeneous coral-rock.  From the isolated
+position of this island, we may safely infer that it is an upraised atoll,
+and therefore that it has been formed by masses of coral, grown together.
+Savage Island seems, from the description of the younger Forster (Forster's
+"Voyage round the World with Cook," volume ii., pages 163, 167.), to have a
+similar structure, and its shores are about forty feet high: some of the
+Cook Islands also appear (Williams's "Narrative of Missionary Enterprise,"
+page 30.) to be similarly composed.  Captain Belcher, R.N., in a letter
+which Captain Beaufort showed me at the admiralty, speaking of Bow atoll,
+says, "I have succeeded in boring forty-five feet through coral-sand, when
+the auger became jammed by the falling in of the surrounding CREAMY
+matter."  On one of the Maldiva atolls, Captain Moresby bored to a depth of
+twenty-six feet, when his auger also broke: he has had the kindness to
+give me the matter brought up; it is perfectly white, and like finely
+triturated coral-rock.
+
+In my description of Keeling atoll, I have given some facts, which show
+that the reef probably has grown outwards; and I have found, just within
+the outer margin, the great mounds of Porites and of Millepora, with their
+summits lately killed, and their sides subsequently thickened by the growth
+of the coral: a layer, also, of Nullipora had already coated the dead
+surface.  As the external slope of the reef is the same round the whole of
+this atoll, and round many other atolls, the angle of inclination must
+result from an adaption between the growing powers of the coral, and the
+force of the breakers, and their action on the loose sediment.  The reef,
+therefore, could not increase outwards, without a nearly equal addition to
+every part of the slope, so that the original inclination might be
+preserved, and this would require a large amount of sediment, all derived
+from the wear of corals and shells, to be added to the lower part.
+Moreover, at Keeling atoll, and probably in many other cases, the different
+kinds of corals would have to encroach on each other; thus the Nulliporae
+cannot increase outwards without encroaching on the Porites and Millepora
+complanata, as is now taking place; nor these latter without encroaching on
+the strongly branched Madreporet, the Millepora alcicornis, and some
+Astraeas; nor these again without a foundation being formed for them within
+the requisite depth, by the accumulation of sediment.  How slow, then, must
+be the ordinary lateral or outward growth of such reefs.  But off Christmas
+atoll, where the sea is much more shallow than is usual, we have good
+reason to believe that, within a period not very remote, the reef has
+increased considerably in width.  The land has the extraordinary breadth of
+three miles; it consists of parallel ridges of shells and broken corals,
+which furnish "an incontestable proof," as observed by Cook (Cook's "Third
+Voyage," book III., chapter x.), "that the island has been produced by
+accessions from the sea, and is in a state of increase."  The land is
+fronted by a coral-reef, and from the manner in which islets are known to
+be formed, we may feel confident that the reef was not three miles wide,
+when the first, or most backward ridge, was thrown up; and, therefore, we
+must conclude that the reef has grown outwards during the accumulation of
+the successive ridges.  Here then, a wall of coral-rock of very
+considerable breadth has been formed by the outward growth of the living
+margin, within a period during which ridges of shells and corals, lying on
+the bare surface, have not decayed.  There can be little doubt, from the
+account given by Captain Beechey, that Matilda atoll, in the Low
+Archipelago, has been converted in the space of thirty-four years, from
+being, as described by the crew of a wrecked whaling vessel, a "reef of
+rocks" into a lagoon-island, fourteen miles in length, with "one of its
+sides covered nearly the whole way with high trees."  (Beechey's "Voyage to
+the Pacific," chapter vii. and viii.)  The islets, also, on Keeling atoll,
+it has been shown, have increased in length, and since the construction of
+an old chart, several of them have become united into one long islet; but
+in this case, and in that of Matilda atoll, we have no proof, and can only
+infer as probable, that the reef, that is the foundation of the islets, has
+increased as well as the islets themselves.
+
+After these considerations, I attach little importance, as indicating the
+ordinary and still less the possible rate of OUTWARD growth of coral-reefs,
+to the fact that certain reefs in the Red Sea have not increased during a
+long interval of time; or to other such cases, as that of Ouluthy atoll in
+the Caroline group, where every islet, described a thousand years before by
+Cantova was found in the same state by Lutke (F. Lutke's "Voyage autour du
+Monde."  In the group Elato, however, it appears that what is now the islet
+Falipi, is called in Cantova's Chart, the Banc de Falipi.  It is not stated
+whether this has been caused by the growth of coral, or by the accumulation
+of sand.),--without it could be shown that, in these cases, the conditions
+were favourable to the vigorous and unopposed growth of the corals living
+in the different zones of depth, and that a proper basis for the extent of
+the reef was present.  The former conditions must depend on many
+contingencies, and in the deep oceans where coral formations most abound, a
+basis within the requisite depth can rarely be present.
+
+Nor do I attach any importance to the fact of certain submerged reefs, as
+those off Tahiti, or those within Diego Garcia not now being nearer the
+surface than they were many years ago, as an indication of the rate under
+favourable circumstances of the UPWARD growth of reefs; after it has been
+shown, that all the reefs have grown to the surface in some of the Chagos
+atolls, but that in neighbouring atolls which appear to be of equal
+antiquity and to be exposed to the same external conditions, every reef
+remains submerged; for we are almost driven to attribute this to a
+difference, not in the rate of growth, but in the habits of the corals in
+the two cases.
+
+In an old-standing reef, the corals, which are so different in kind on
+different parts of it, are probably all adapted to the stations they
+occupy, and hold their places, like other organic beings, by a struggle one
+with another, and with external nature; hence we may infer that their
+growth would generally be slow, except under peculiarly favourable
+circumstances.  Almost the only natural condition, allowing a quick upward
+growth of the whole surface of a reef, would be a slow subsidence of the
+area in which it stood; if, for instance, Keeling atoll were to subside two
+or three feet, can we doubt that the projecting margin of live coral, about
+half an inch in thickness, which surrounds the dead upper surfaces of the
+mounds of Porites, would in this case form a concentric layer over them,
+and the reef thus increase upwards, instead of, as at present, outwards?
+The Nulliporae are now encroaching on the Porites and Millepora, but in
+this case might we not confidently expect that the latter would, in their
+turn, encroach on the Nulliporae?  After a subsidence of this kind, the sea
+would gain on the islets, and the great fields of dead but upright corals
+in the lagoon, would be covered by a sheet of clear water; and might we not
+then expect that these reefs would rise to the surface, as they anciently
+did when the lagoon was less confined by islets, and as they did within a
+period of ten years in the schooner-channel, cut by the inhabitants?  In
+one of the Maldiva atolls, a reef, which within a very few years existed as
+an islet bearing cocoa-nut trees, was found by Lieutenant Prentice
+"ENTIRELY COVERED WITH LIVE CORAL AND MADREPORE."  The natives believe that
+the islet was washed away by a change in the currents, but if, instead of
+this, it had quietly subsided, surely every part of the island which
+offered a solid foundation, would in a like manner have become coated with
+living coral.
+
+Through steps such as these, any thickness of rock, composed of a singular
+intermixture of various kinds of corals, shells, and calcareous sediment,
+might be formed; but without subsidence, the thickness would necessarily be
+determined by the depth at which the reef-building polypifers can exist.
+If it be asked, at what rate in years I suppose a reef of coral favourably
+circumstanced could grow up from a given depth; I should answer, that we
+have no precise evidence on this point, and comparatively little concern
+with it.  We see, in innumerable points over wide areas, that the rate has
+been sufficient, either to bring up the reefs from various depths to the
+surface, or, as is more probable, to keep them at the surface, during
+progressive subsidences; and this is a much more important standard of
+comparison than any cycle of years.
+
+It may, however, be inferred from the following facts, that the rate in
+years under favourable circumstances would be very far from slow.  Dr.
+Allan, of Forres, has, in his MS. Thesis deposited in the library of the
+Edinburgh University (extracts from which I owe to the kindness of Dr.
+Malcolmson), the following account of some experiments, which he tried
+during his travels in the years 1830 to 1832 on the east coast of
+Madagascar.  "To ascertain the rise and progress of the coral-family, and
+fix the number of species met with at Foul Point (latitude 17 deg 40')
+twenty species of coral were taken off the reef and planted apart on a
+sand-bank THREE FEET DEEP AT LOW WATER.  Each portion weighed ten pounds,
+and was kept in its place by stakes.  Similar quantities were placed in a
+clump and secured as the rest.  This was done in December 1830.  In July
+following, each detached mass was nearly level with the sea at low water,
+quite immovable, and several feet long, stretching as the parent reef, with
+the coast current from north to south.  The masses accumulated in a clump
+were found equally increased, but some of the species in such unequal
+ratios, as to be growing over each other."  The loss of Dr. Allan's
+magnificent collection by shipwreck, unfortunately prevents its being known
+to what genera these corals belonged; but from the numbers experimented on,
+it is certain that all the more conspicuous kinds must have been included.
+Dr. Allan informs me, in a letter, that he believes it was a Madrepora,
+which grew most vigorously.  One may be permitted to suspect that the level
+of the sea might possibly have been somewhat different at the two stated
+periods; nevertheless, it is quite evident that the growth of the ten-pound
+masses, during the six or seven months, at the end of which they were found
+immovably fixed (It is stated by De la Beche ("Geological Manual," page
+143), on the authority of Mr. Lloyd, who surveyed the Isthmus of Panama,
+that some specimens of Polypifers, placed by him in a sheltered pool of
+water, were found in the course of a few days firmly fixed by the secretion
+of a stony matter, to the bottom) and several feet in length, must have
+been very great.  The fact of the different kinds of coral, when placed in
+one clump, having increased in extremely unequal ratios, is very
+interesting, as it shows the manner in which a reef, supporting many
+species of coral, would probably be affected by a change in the external
+conditions favouring one kind more than another.  The growth of the masses
+of coral in N. and S. lines parallel to the prevailing currents, whether
+due to the drifting of sediment or to the simple movement of the water, is,
+also, a very interesting circumstance.
+
+A fact, communicated to me by Lieutenant Wellstead, I.N., in some degree
+corroborates the result of Dr. Allan's experiments: it is, that in the
+Persian Gulf a ship had her copper bottom encrusted in the course of twenty
+months with a layer of coral, TWO FEET in thickness, which it required
+great force to remove, when the vessel was docked: it was not ascertained
+to what order this coral belonged.  The case of the schooner-channel choked
+up with coral in an interval of less than ten years, in the lagoon of
+Keeling atoll, should be here borne in mind.  We may also infer, from the
+trouble which the inhabitants of the Maldiva atolls take to root out, as
+they express it, the coral-knolls from their harbours, that their growth
+can hardly be very slow.  (Mr. Stutchbury ("West of England Journal", No.
+I., page 50.) has described a specimen of Agaricia, "weighing 2 lbs. 9 oz.,
+which surrounds a species of oyster, whose age could not be more than two
+years, and yet is completely enveloped by this dense coral."  I presume
+that the oyster was living when the specimen was procured; otherwise the
+fact tells nothing.  Mr. Stutchbury also mentions an anchor, which had
+become entirely encrusted with coral in fifty years; other cases, however,
+are recorded of anchors which have long remained amidst coral-reefs without
+having become coated.  The anchor of the "Beagle", in 1832, after having
+been down exactly one month at Rio de Janeiro, was so thickly coated by two
+species of Tubularia, that large spaces of the iron were entirely
+concealed; the tufts of this horny zoophyte were between two and three
+inches in length.  It has been attempted to compute, but I believe
+erroneously, the rate of growth of a reef, from the fact mentioned by
+Captain Beechey, of the Chama gigas being embedded in coral-rock.  But it
+should be remembered, that some species of this genus invariably live, both
+whilst young and old, in cavities, which the animal has the power of
+enlarging with its growth.  I saw many of these shells thus embedded in the
+outer "flat" of Keeling atoll, which is composed of dead rock; and
+therefore the cavities in this case had no relation whatever with the
+growth of coral.  M. Lesson, also, speaking of this shell (Partie Zoolog.
+"Voyage de la 'Coquille'"), has remarked, "que constamment ses valves
+etaient engages completement dans la masse des Madrepores.")
+
+From the facts given in this section, it may be concluded, first, that
+considerable thicknesses of rock have certainly been formed within the
+present geological area by the growth of coral and the accumulation of its
+detritus; and, secondly, that the increase of individual corals and of
+reefs, both outwards or horizontally and upwards or vertically, under the
+peculiar conditions favourable to such increase, is not slow, when referred
+either to the standard of the average oscillations of level in the earth's
+crust, or to the more precise but less important one of a cycle of years.
+
+
+SECTION 4.III.--ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.
+
+I have already described in detail, which might have appeared trivial, the
+nature of the bottom of the sea immediately surrounding Keeling atoll; and
+I will now describe with almost equal care the soundings off the
+fringing-reefs of Mauritius.  I have preferred this arrangement, for the sake
+of grouping together facts of a similar nature.  I sounded with the wide
+bell-shaped lead which Captain Fitzroy used at Keeling Island, but my
+examination of the bottom was confined to a few miles of coast (between
+Port Louis and Tomb Bay) on the leeward side of the island.  The edge of
+the reef is formed of great shapeless masses of branching Madrepores, which
+chiefly consist of two species,--apparently M. corymbosa and pocillifera,--
+mingled with a few other kinds of coral.  These masses are separated from
+each other by the most irregular gullies and cavities, into which the lead
+sinks many feet.  Outside this irregular border of Madrepores, the water
+deepens gradually to twenty fathoms, which depth generally is found at the
+distance of from half to three-quarters of a mile from the reef.  A little
+further out the depth is thirty fathoms, and thence the bank slopes rapidly
+into the depths of the ocean.  This inclination is very gentle compared
+with that outside Keeling and other atolls, but compared with most coasts
+it is steep.  The water was so clear outside the reef, that I could
+distinguish every object forming the rugged bottom.  In this part, and to a
+depth of eight fathoms, I sounded repeatedly, and at each cast pounded the
+bottom with the broad lead, nevertheless the arming invariably came up
+perfectly clean, but deeply indented.  From eight to fifteen fathoms a
+little calcareous sand was occasionally brought up, but more frequently the
+arming was simply indented.  In all this space the two Madrepores above
+mentioned, and two species of Astraea, with rather large stars, seemed the
+commonest kinds (Since the preceding pages were printed off, I have
+received from Mr. Lyell a very interesting pamphlet, entitled "Remarks upon
+Coral Formations," etc., by J. Couthouy, Boston, United States, 1842.
+There is a statement (page 6), on the authority of the Rev. J. Williams,
+corroborating the remarks made by Ehrenberg and Lyell (page 71 of this
+volume), on the antiquity of certain individual corals in the Red Sea and
+at Bermuda; namely, that at Upolu, one of the Navigator Islands,
+"particular clumps of coral are known to the fishermen by name, derived
+from either some particular configuration or tradition attached to them,
+and handed down from time immemorial."  With respect to the thickness of
+masses of coral-rock, it clearly appears, from the descriptions given by
+Mr. Couthouy (pages 34, 58) that Mangaia and Aurora Islands are upraised
+atolls, composed of coral rock: the level summit of the former is about
+three hundred feet, and that of Aurora Island is two hundred feet above the
+sea-level.); and it must be noticed that twice at the depth of fifteen
+fathoms, the arming was marked with a clean impression of an Astraea.
+Besides these lithophytes, some fragments of the Millepora alcicornis,
+which occurs in the same relative position at Keeling Island, were brought
+up; and in the deeper parts there were large beds of a Seriatopora,
+different from S. subulata, but closely allied to it.  On the beach within
+the reef, the rolled fragments consisted chiefly of the corals just
+mentioned, and of a massive Porites, like that at Keeling atoll, of a
+Meandrina, Pocillopora verrucosa, and of numerous fragments of Nullipora.
+From fifteen to twenty fathoms the bottom was, with few exceptions, either
+formed of sand, or thickly covered with Seriatopora: this delicate coral
+seems to form at these depths extensive beds unmingled with any other kind.
+At twenty fathoms, one sounding brought up a fragment of Madrepora
+apparently M. pocillifera, and I believe it is the same species (for I
+neglected to bring specimens from both stations) which mainly forms the
+upper margin of the reef; if so, it grows in depths varying from 0 to 20
+fathoms.  Between 20 and 23 fathoms I obtained several soundings, and they
+all showed a sandy bottom, with one exception at 30 fathoms, when the
+arming came up scooped out, as if by the margin of a large Caryophyllia.
+Beyond 33 fathoms I sounded only once; and from 86 fathoms, at the distance
+of one mile and a third from the edge of the reef, the arming brought up
+calcareous sand with a pebble of volcanic rock.  The circumstance of the
+arming having invariably come up quite clean, when sounding within a
+certain number of fathoms off the reefs of Mauritius and Keeling atoll
+(eight fathoms in the former case, and twelve in the latter) and of its
+having always come up (with one exception) smoothed and covered with sand,
+when the depth exceeded twenty fathoms, probably indicates a criterion, by
+which the limits of the vigorous growth of coral might in all cases be
+readily ascertained.  I do not, however, suppose that if a vast number of
+soundings were obtained round these islands, the limit above assigned would
+be found never to vary, but I conceive the facts are sufficient to show,
+that the exceptions would be few.  The circumstance of a GRADUAL change, in
+the two cases, from a field of clean coral to a smooth sandy bottom, is far
+more important in indicating the depth at which the larger kinds of coral
+flourish than almost any number of separate observations on the depth, at
+which certain species have been dredged up.  For we can understand the
+gradation, only as a prolonged struggle against unfavourable conditions.
+If a person were to find the soil clothed with turf on the banks of a
+stream of water, but on going to some distance on one side of it, he
+observed the blades of grass growing thinner and thinner, with intervening
+patches of sand, until he entered a desert of sand, he would safely
+conclude, especially if changes of the same kind were noticed in other
+places, that the presence of the water was absolutely necessary to the
+formation of a thick bed of turf: so may we conclude, with the same
+feeling of certainty, that thick beds of coral are formed only at small
+depths beneath the surface of the sea.
+
+I have endeavoured to collect every fact, which might either invalidate or
+corroborate this conclusion.  Captain Moresby, whose opportunities for
+observation during his survey of the Maldiva and Chagos Archipelagoes have
+been unrivalled, informs me, that the upper part or zone of the steep-sided
+reefs, on the inner and outer coasts of the atolls in both groups,
+invariably consists of coral, and the lower parts of sand.  At seven or
+eight fathoms depth, the bottom is formed, as could be seen through the
+clear water, of great living masses of coral, which at about ten fathoms
+generally stand some way apart from each other, with patches of white sand
+between them, and at a little greater depth these patches become united
+into a smooth steep slope, without any coral.  Captain Moresby, also,
+informs me in support of his statement, that he found only decayed coral on
+the Padua Bank (northern part of the Laccadive group) which has an average
+depth between twenty-five and thirty-five fathoms, but that on some other
+banks in the same group with only ten or twelve fathoms water on them (for
+instance, the Tillacapeni bank), the coral was living.
+
+With regard to the coral-reefs in the Red Sea, Ehrenberg has the following
+passage:--"The living corals do not descend there into great depths.  On
+the edges of islets and near reefs, where the depth was small, very many
+lived; but we found no more even at six fathoms.  The pearl-fishers at
+Yemen and Massaua asserted that there was no coral near the pearl-banks at
+nine fathoms depth, but only sand.  We were not able to institute any more
+special researches."  (Ehrenberg, "Uber die Natur," etc., page 50.)  I am,
+however, assured both by Captain Moresby and Lieutenant Wellstead, that in
+the more northern parts of the Red Sea, there are extensive beds of living
+coral at a depth of twenty-five fathoms, in which the anchors of their
+vessels were frequently entangled.  Captain Moresby attributes the less
+depth, at which the corals are able to live in the places mentioned by
+Ehrenberg, to the greater quantity of sediment there; and the situations,
+where they were flourishing at the depth of twenty-five fathoms, were
+protected, and the water was extraordinarily limpid.  On the leeward side
+of Mauritius where I found the coral growing at a somewhat greater depth
+than at Keeling atoll, the sea, owing apparently to its tranquil state, was
+likewise very clear.  Within the lagoons of some of the Marshall atolls,
+where the water can be but little agitated, there are, according to
+Kotzebue, living beds of coral in twenty-five fathoms.  From these facts,
+and considering the manner in which the beds of clean coral off Mauritius,
+Keeling Island, the Maldiva and Chagos atolls, graduated into a sandy
+slope, it appears very probable that the depth, at which reef-building
+polypifers can exist, is partly determined by the extent of inclined
+surface, which the currents of the sea and the recoiling waves have the
+power to keep free from sediment.
+
+MM. Quoy and Gaimard ("Annales des Sci. Nat." tom. vi.) believe that the
+growth of coral is confined within very limited depths; and they state that
+they never found any fragment of an Astraea (the genus they consider most
+efficient in forming reefs) at a depth above twenty-five or thirty feet.
+But we have seen that in several places the bottom of the sea is paved with
+massive corals at more than twice this depth; and at fifteen fathoms (or
+twice this depth) off the reefs of Mauritius, the arming was marked with
+the distinct impression of a living Astraea.  Millepora alcicornis lives in
+from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0 to 20
+fathoms.  Captain Moresby has given me a specimen of Sideropora scabra
+(Porites of Lamarck) brought up alive from 17 fathoms.  Mr. Couthouy
+("Remarks on Coral Formations," page 12.) states that he has dredged up on
+the Bahama banks considerable masses of Meandrina from 16 fathoms, and he
+has seen this coral growing in 20 fathoms.  A Caryophyllia, half an inch in
+diameter, was dredged up alive from 80 fathoms off Juan Fernandez (latitude
+33 deg S.) by Captain P.P. King (I am indebted to Mr. Stokes for having
+kindly communicated this fact to me, together with much other valuable
+information.): this is the most remarkable fact with which I am
+acquainted, showing the depth at which a genus of corals often found on
+reefs, can exist.
+
+We ought, however, to feel less surprise at this fact, as Caryophyllia
+alone of the lamelliform genera, ranges far beyond the tropics; it is found
+in Zetland (Fleming's "British Animals," genus Caryophyllia.) in Latitude
+60 deg N. in deep water, and I procured a small species from Tierra del
+Fuego in Latitude 53 deg S.  Captain Beechey informs me, that branches of
+pink and yellow coral were frequently brought up from between twenty and
+twenty-five fathoms off the Low atolls; and Lieutenant Stokes, writing to
+me from the N.W. coast of Australia, says that a strongly branched coral
+was procured there from thirty fathoms; unfortunately it is not known to
+what genera these corals belong.
+
+(I will record in the form of a note all the facts that I have been able to
+collect on the depths, both within and without the tropics, at which those
+corals and corallines can live, which there is no reason to suppose ever
+materially aid in the construction of a reef.
+
+(In the following list the name of the Zoophyte is followed by the depth in
+fathoms, the country and degrees S. latitude, and the authority.  Where no
+authority is given, the observation is Darwin's own.)
+
+SERTULARIA, 40, Cape Horn 66.
+
+CELLARIA, 40, Cape Horn 66.
+
+CELLARIA, A minute scarlet encrusting species, found living, 190, Keeling
+Atoll, 12.
+
+CELLARIA, An allied, small stony sub-generic form, 48, St Cruz Riv. 50.
+
+A coral allied to VINCULARIA, with eight rows of cells, 40, Cape Horn.
+
+TUBULIPORA, near to T. patima, 40, Cape Horn.
+
+TUBULIPORA, near to T. patima, 94, East Chiloe 43.
+
+CELLEPORA, several species, and allied sub-generic forms, 40, Cape Horn.
+
+CELLEPORA, several species, and allied sub-generic forms, 40 and 57, Chonos
+Archipelago 45.
+
+CELLEPORA, several species, and allied sub-generic forms, 48, St Cruz 50.
+
+ESCHARA, 30, Tierra del Fuego 53.
+
+ESCHARA, 48, St Cruz R. 50.
+
+RETEPORA, 40, Cape Horn.
+
+RETEPORA, 100, Cape of Good Hope 34, Quoy and Gaimard, "Ann. Scien. Nat."
+tome vi., page 284.
+
+MILLEPORA, a strong coral with cylindrical branches, of a pink colour,
+about two inches high, resembling in the form of its orifices M. aspera of
+Lamarck, 94 and 30, E. Chiloe 43, Tierra del Fuego 53.
+
+CORALIUM, 120, Barbary 33 N., Peyssonel in paper read to Royal Society May
+1752.
+
+ANTIPATHES, 16, Chonos 45.
+
+GORGONIA (or an allied form), 160, Abrolhos on the coast of Brazil 18,
+Captain Beechey informed me of this fact in a letter.
+
+Ellis ("Nat. Hist. of Coralline," page 96) states that Ombellularia was
+procured in latitude 79 deg N. STICKING to a LINE from the depth of 236
+fathoms; hence this coral either must have been floating loose, or was
+entangled in stray line at the bottom.  Off Keeling atoll a compound
+Ascidia (Sigillina) was brought up from 39 fathoms, and a piece of sponge,
+apparently living, from 70, and a fragment of Nullipora also apparently
+living from 92 fathoms.  At a greater depth than 90 fathoms off this coral
+island, the bottom was thickly strewed with joints of Halimeda and small
+fragments of other Nulliporae, but all dead.  Captain B. Allen, R.N.,
+informs me that in the survey of the West Indies it was noticed that
+between the depth of 10 and 200 fathoms, the sounding lead very generally
+came up coated with the dead joints of a Halimeda, of which he showed me
+specimens.  Off Pernambuco, in Brazil, in about twelve fathoms, the bottom
+was covered with fragments dead and alive of a dull red Nullipora, and I
+infer from Roussin's chart, that a bottom of this kind extends over a wide
+area.  On the beach, within the coral-reefs of Mauritius, vast quantities
+of fragments of Nulliporae were piled up.  From these facts it appears,
+that these simply organized bodies are amongst the most abundant
+productions of the sea.)
+
+Although the limit of depth, at which each particular kind of coral ceases
+to exist, is far from being accurately known; yet when we bear in mind the
+manner in which the clumps of coral gradually became infrequent at about
+the same depth, and wholly disappeared at a greater depth than twenty
+fathoms, on the slope round Keeling atoll, on the leeward side of the
+Mauritius, and at rather less depth, both without and within the atolls of
+the Maldiva and Chagos Archipelagoes; and when we know that the reefs round
+these islands do not differ from other coral formations in their form and
+structure, we may, I think, conclude that in ordinary cases, reef-building
+polypifers do not flourish at greater depths than between twenty and thirty
+fathoms.
+
+It has been argued ("Journal of the Royal Geographical Society," 1831, page
+218.) that reefs may possibly rise from very great depths through the means
+of small corals, first making a platform for the growth of the stronger
+kinds.  This, however, is an arbitrary supposition: it is not always
+remembered, that in such cases there is an antagonist power in action,
+namely, the decay of organic bodies, when not protected by a covering of
+sediment, or by their own rapid growth.  We have, moreover, no right to
+calculate on unlimited time for the accumulation of small organic bodies
+into great masses.  Every fact in geology proclaims that neither the land,
+nor the bed of the sea retain for indefinite periods the same level.  As
+well might it be imagined that the British Seas would in time become choked
+up with beds of oysters, or that the numerous small corallines off the
+inhospitable shores of Tierra del Fuego would in time form a solid and
+extensive coral-reef.
+
+
+CHAPTER V.--THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
+CORAL-REEFS.
+
+The atolls of the larger archipelagoes are not formed on submerged craters,
+or on banks of sediment.--Immense areas interspersed with atolls.--Their
+subsidence.--The effects of storms and earthquakes on atolls.--Recent
+changes in their state.--The origin of barrier-reefs and of atolls.--Their
+relative forms.--The step-formed ledges and walls round the shores of some
+lagoons.--The ring-formed reefs of the Maldiva atolls.--The submerged
+condition of parts or of the whole of some annular reefs.--The disseverment
+of large atolls.--The union of atolls by linear reefs.--The Great Chagos
+Bank.--Objections from the area and amount of subsidence required by the
+theory, considered.--The probable composition of the lower parts of atolls.
+
+The naturalists who have visited the Pacific, seem to have had their
+attention riveted by the lagoon-islands, or atolls,--those singular rings
+of coral-land which rise abruptly out of the unfathomable ocean--and have
+passed over, almost unnoticed, the scarcely less wonderful encircling
+barrier-reefs.  The theory most generally received on the formation of
+atolls, is that they are based on submarine craters; but where can we find
+a crater of the shape of Bow atoll, which is five times as long as it is
+broad (Plate I., Figure 4); or like that of Menchikoff Island (Plate II.,
+Figure 3.), with its three loops, together sixty miles in length; or like
+Rimsky Korsacoff, narrow, crooked, and fifty-four miles long; or like the
+northern Maldiva atolls, made up of numerous ring-formed reefs, placed on
+the margin of a disc,--one of which discs is eighty-eight miles in length,
+and only from ten to twenty in breadth?  It is, also, not a little
+improbable, that there should have existed as many craters of immense size
+crowded together beneath the sea, as there are now in some parts atolls.
+But this theory lies under a greater difficulty, as will be evident, when
+we consider on what foundations the atolls of the larger archipelagoes
+rest: nevertheless, if the rim of a crater afforded a basis at the proper
+depth, I am far from denying that a reef like a perfectly characterised
+atoll might not be formed; some such, perhaps, now exist; but I cannot
+believe in the possibility of the greater number having thus originated.
+
+An earlier and better theory was proposed by Chamisso (Kotzebue's "First
+Voyage," volume iii., page 331.); he supposes that as the more massive
+kinds of corals prefer the surf, the outer portions, in a reef rising from
+a submarine basis, would first reach the surface and consequently form a
+ring.  But on this view it must be assumed, that in every case the basis
+consists of a flat bank; for if it were conically formed, like a
+mountainous mass, we can see no reason why the coral should spring up from
+the flanks, instead of from the central and highest parts: considering the
+number of the atolls in the Pacific and Indian Oceans, this assumption is
+very improbable.  As the lagoons of atolls are sometimes even more than
+forty fathoms deep, it must, also, be assumed on this view, that at a depth
+at which the waves do not break, the coral grows more vigorously on the
+edges of a bank than on its central part; and this is an assumption without
+any evidence in support of it.  I remarked, in the third chapter, that a
+reef, growing on a detached bank, would tend to assume an atoll-like
+structure; if, therefore, corals were to grow up from a bank, with a level
+surface some fathoms submerged, having steep sides and being situated in a
+deep sea, a reef not to be distinguished from an atoll, might be formed: I
+believe some such exist in the West Indies.  But a difficulty of the same
+kind with that affecting the crater theory, runners, as we shall presently
+see, this view inapplicable to the greater number of atolls.
+
+No theory worthy of notice has been advanced to account for those
+barrier-reefs, which encircle islands of moderate dimensions.  The great
+reef which fronts the coast of Australia has been supposed, but without any
+special facts, to rest on the edge of a submarine precipice, extending
+parallel to the shore.  The origin of the third class or of fringing-reefs
+presents, I believe, scarcely any difficulty, and is simply consequent on
+the polypifers not growing up from great depths, and their not flourishing
+close to gently shelving beaches where the water is often turbid.
+
+What cause, then, has given to atolls and barrier-reefs their
+characteristic forms?  Let us see whether an important deduction will not
+follow from the consideration of these two circumstances, first, the
+reef-building corals flourishing only at limited depths; and secondly, the
+vastness of the areas interspersed with coral-reefs and coral-islets, none
+of which rise to a greater height above the level of the sea, than that
+attained by matter thrown up by the waves and winds.  I do not make this
+latter statement vaguely; I have carefully sought for descriptions of every
+island in the intertropical seas; and my task has been in some degree
+abridged by a map of the Pacific, corrected in 1834 by MM. D'Urville and
+Lottin, in which the low islands are distinguished from the high ones (even
+from those much less than a hundred feet in height) by being written
+without a capital letter; I have detected a few errors in this map,
+respecting the height of some of the islands, which will be noticed in the
+Appendix, where I treat of coral formations in geographical order.  To the
+Appendix, also, I must refer for a more particular account of the data on
+which the statements on the next page are grounded.  I have ascertained,
+and chiefly from the writings of Cook, Kotzebue, Bellinghausen, Duperrey,
+Beechey, and Lutke, regarding the Pacific; and from Moresby (See also
+Captain Owen's and Lieutenant Wood's papers in the "Geographical Journal",
+on the Maldiva and Laccadive Archipelagoes.  These officers particularly
+refer to the lowness of the islets; but I chiefly ground my assertion
+respecting these two groups, and the Chagos group, from information
+communicated to me by Captain Moresby.) with respect to the Indian Ocean,
+that in the following cases the term "low island" strictly means land of
+the height commonly attained by matter thrown up by the winds and the waves
+of an open sea.  If we draw a line (the plan I have always adopted) joining
+the external atolls of that part of the Low Archipelago in which the
+islands are numerous, the figure will be a pointed ellipse (reaching from
+Hood to Lazaref Island), of which the longer axis is 840 geographical
+miles, and the shorter 420 miles; in this space (I find from Mr. Couthouy's
+pamphlet (page 58) that Aurora Island is about two hundred feet in height;
+it consists of coral-rock, and seems to have been formed by the elevation
+of an atoll.  It lies north-east of Tahiti, close without the line bounding
+the space coloured dark blue in the map appended to this volume.  Honden
+Island, which is situated in the extreme north-west part of the Low
+Archipelago, according to measurements made on board the "Beagle", whilst
+sailing by, is 114 feet from the SUMMIT OF THE TREES to the water's edge.
+This island appeared to resemble the other atolls of the group.) none of
+the innumerable islets united into great rings rise above the stated level.
+The Gilbert group is very narrow, and 300 miles in length.  In a prolonged
+line from this group, at the distance of 240 miles, is the Marshall
+Archipelago, the figure of which is an irregular square, one end being
+broader than the other; its length is 520 miles, with an average width of
+240; these two groups together are 1,040 miles in length, and all their
+islets are low.  Between the southern end of the Gilbert and the northern
+end of Low Archipelago, the ocean is thinly strewed with islands, all of
+which, as far as I have been able to ascertain, are low; so that from
+nearly the southern end of the Low Archipelago, to the northern end of the
+Marshall Archipelago, there is a narrow band of ocean, more than 4,000
+miles in length, containing a great number of islands, all of which are
+low.  In the western part of the Caroline Archipelago, there is a space of
+480 miles in length, and about 100 broad, thinly interspersed with low
+islands.  Lastly, in the Indian Ocean, the archipelago of the Maldivas is
+470 miles in length, and 60 in breadth; that of the Laccadives is 150 by
+100 miles; as there is a low island between these two groups, they may be
+considered as one group of 1,000 miles in length.  To this may be added the
+Chagos group of low islands, situated 280 miles distant, in a line
+prolonged from the southern extremity of the Maldivas.  This group,
+including the submerged banks, is 170 miles in length and 80 in breadth.
+So striking is the uniformity in direction of these three archipelagoes,
+all the islands of which are low, that Captain Moresby, in one of his
+papers, speaks of them as parts of one great chain, nearly 1,500 miles
+long.  I am, then, fully justified in repeating, that enormous spaces, both
+in the Pacific and Indian Oceans, are interspersed with islands, of which
+not one rises above that height, to which the waves and winds in an open
+sea can heap up matter.
+
+On what foundations, then, have these reefs and islets of coral been
+constructed?  A foundation must originally have been present beneath each
+atoll at that limited depth, which is indispensable for the first growth of
+the reef-building polypifers.  A conjecture will perhaps be hazarded, that
+the requisite bases might have been afforded by the accumulation of great
+banks of sediment, which owing to the action of superficial currents (aided
+possibly by the undulatory movement of the sea) did not quite reach the
+surface,--as actually appears to have been the case in some parts of the
+West Indian Sea.  But in the form and disposition of the groups of atolls,
+there is nothing to countenance this notion; and the assumption without any
+proof, that a number of immense piles of sediment have been heaped on the
+floor of the great Pacific and Indian Oceans, in their central parts far
+remote from land, and where the dark blue colour of the limpid water
+bespeaks its purity, cannot for one moment be admitted.
+
+The many widely-scattered atolls must, therefore, rest on rocky bases.  But
+we cannot believe that the broad summit of a mountain lies buried at the
+depth of a few fathoms beneath every atoll, and nevertheless throughout the
+immense areas above-named, with not one point of rock projecting above the
+level of the sea; for we may judge with some accuracy of mountains beneath
+the sea, by those on the land; and where can we find a single chain several
+hundred miles in length and of considerable breadth, much less several such
+chains, with their many broad summits attaining the same height, within
+from 120 to 180 feet?  If the data be thought insufficient, on which I have
+grounded my belief, respecting the depth at which the reef-building
+polypifers can exist, and it be assumed that they can flourish at a depth
+of even one hundred fathoms, yet the weight of the above argument is but
+little diminished, for it is almost equally improbable, that as many
+submarine mountains, as there are low islands in the several great and
+widely separated areas above specified, should all rise within six hundred
+feet of the surface of the sea and not one above it, as that they should be
+of the same height within the smaller limit of one or two hundred feet.  So
+highly improbable is this supposition, that we are compelled to believe,
+that the bases of the many atolls did never at any one period all lie
+submerged within the depth of a few fathoms beneath the surface, but that
+they were brought into the requisite position or level, some at one period
+and some at another, through movements in the earth's crust.  But this
+could not have been effected by elevation, for the belief that points so
+numerous and so widely separated were successively uplifted to a certain
+level, but that not one point was raised above that level, is quite as
+improbable as the former supposition, and indeed differs little from it.
+It will probably occur to those who have read Ehrenberg's account of the
+Reefs of the Red Sea, that many points in these great areas may have been
+elevated, but that as soon as raised, the protuberant parts were cut off by
+the destroying action of the waves: a moment's reflection, however, on the
+basin-like form of the atolls, will show that this is impossible; for the
+upheaval and subsequent abrasion of an island would leave a flat disc,
+which might become coated with coral, but not a deeply concave surface;
+moreover, we should expect to see, in some parts at least, the rock of the
+foundation brought to the surface.  If, then, the foundations of the many
+atolls were not uplifted into the requisite position, they must of
+necessity have subsided into it; and this at once solves every difficulty
+(The additional difficulty on the crater hypothesis before alluded to, will
+now be evident; for on this view the volcanic action must be supposed to
+have formed within the areas specified a vast number of craters, all rising
+within a few fathoms of the surface, and not one above it.  The supposition
+that the craters were at different times upraised above the surface, and
+were there abraded by the surf and subsequently coated by corals, is
+subject to nearly the same objections with those given above in this
+paragraph; but I consider it superfluous to detail all the arguments
+opposed to such a notion.  Chamisso's theory, from assuming the existence
+of so many banks, all lying at the proper depth beneath the water, is also
+vitally defective.  The same observation applies to an hypothesis of
+Lieutenant Nelson's ("Geolog. Trans." volume v., page 122), who supposes
+that the ring-formed structure is caused by a greater number of germs of
+corals becoming attached to the declivity, than to the central plateau of a
+submarine bank: it likewise applies to the notion formerly entertained
+(Forster's "Observ." page 151), that lagoon-islands owe their peculiar form
+to the instinctive tendencies of the polypifers.  According to this latter
+view, the corals on the outer margin of the reef instinctively expose
+themselves to the surf in order to afford protection to corals living in
+the lagoon, which belong to other genera, and to other families!), for we
+may safely infer, from the facts given in the last chapter, that during a
+gradual subsidence the corals would be favourably circumstanced for
+building up their solid frame works and reaching the surface, as island
+after island slowly disappeared.  Thus areas of immense extent in the
+central and most profound parts of the great oceans, might become
+interspersed with coral-islets, none of which would rise to a greater
+height than that attained by detritus heaped up by the sea, and
+nevertheless they might all have been formed by corals, which absolutely
+required for their growth a solid foundation within a few fathoms of the
+surface.
+
+It would be out of place here to do more than allude to the many facts,
+showing that the supposition of a gradual subsidence over large areas is by
+no means improbable.  We have the clearest proof that a movement of this
+kind is possible, in the upright trees buried under the strata many
+thousand feet in thickness; we have also every reason for believing that
+there are now large areas gradually sinking, in the same manner as others
+are rising.  And when we consider how many parts of the surface of the
+globe have been elevated within recent geological periods, we must admit
+that there have been subsidences on a corresponding scale, for otherwise
+the whole globe would have swollen.  It is very remarkable that Mr. Lyell
+("Principles of Geology," sixth edition, volume iii., page 386.), even in
+the first edition of his "Principles of Geology," inferred that the amount
+of subsidence in the Pacific must have exceeded that of elevation, from the
+area of land being very small relatively to the agents there tending to
+form it, namely, the growth of coral and volcanic action.  But it will be
+asked, are there any direct proofs of a subsiding movement in those areas,
+in which subsidence will explain a phenomenon otherwise inexplicable?
+This, however, can hardly be expected, for it must ever be most difficult,
+excepting in countries long civilised, to detect a movement, the tendency
+of which is to conceal the part affected.  In barbarous and semi-civilised
+nations how long might not a slow movement, even of elevation such as that
+now affecting Scandinavia, have escaped attention!
+
+Mr. Williams (Williams's "Narrative of Missionary Enterprise," page 31.)
+insists strongly that the traditions of the natives, which he has taken
+much pains in collecting, do not indicate the appearance of any new
+islands: but on the theory of a gradual subsidence, all that would be
+apparent would be, the water sometimes encroaching slowly on the land, and
+the land again recovering by the accumulation of detritus its former
+extent, and perhaps sometimes the conversion of an atoll with coral islets
+on it, into a bare or into a sunken annular reef.  Such changes would
+naturally take place at the periods when the sea rose above its usual
+limits, during a gale of more than ordinary strength; and the effects of
+the two causes would be hardly distinguishable.  In Kotzebue's "Voyage"
+there are accounts of islands, both in the Caroline and Marshall
+Archipelagoes, which have been partly washed away during hurricanes; and
+Kadu, the native who was on board one of the Russian vessels, said "he saw
+the sea at Radack rise to the feet of the cocoa-nut trees; but it was
+conjured in time."  (Kotzebue's "First Voyage," volume iii., page 168.)  A
+storm lately entirely swept away two of the Caroline islands, and converted
+them into shoals; it partly, also, destroyed two other islands.  (M.
+Desmoulins in "Comptes Rendus," 1840, page 837.)  According to a tradition
+which was communicated to Captain Fitzroy, it is believed in the Low
+Archipelago, that the arrival of the first ship caused a great inundation,
+which destroyed many lives.  Mr. Stutchbury relates, that in 1825, the
+western side of Chain Atoll, in the same group, was completely devastated
+by a hurricane, and not less than 300 lives lost: "in this instance it was
+evident, even to the natives, that the hurricane alone was not sufficient
+to account for the violent agitation of the ocean."  ("West of England
+Journal", No. I., page 35.)  That considerable changes have taken place
+recently in some of the atolls in the Low Archipelago, appears certain from
+the case already given of Matilda Island: with respect to Whitsunday and
+Gloucester Islands in this same group, we must either attribute great
+inaccuracy to their discoverer, the famous circumnavigator Wallis, or
+believe that they have undergone a considerable change in the period of
+fifty-nine years, between his voyage and that of Captain Beechey's.
+Whitsunday Island is described by Wallis as "about four miles long, and
+three wide," now it is only one mile and a half long.  The appearance of
+Gloucester Island, in Captain Beechey's words (Beechey's "Voyage to the
+Pacific," chapter vii., and Wallis's "Voyage in the 'Dolphin'," chapter
+iv.), has been accurately described by its discoverer, but its present form
+and extent differ materially."  Blenheim reef, in the Chagos group,
+consists of a water-washed annular reef, thirteen miles in circumference,
+surrounding a lagoon ten fathoms deep: on its surface there were a few
+worn patches of conglomerate coral-rock, of about the size of hovels; and
+these Captain Moresby considered as being, without doubt, the last remnants
+of islets; so that here an atoll has been converted into an atoll-formed
+reef.  The inhabitants of the Maldiva Archipelago, as long ago as 1605,
+declared, "that the high tides and violent currents were diminishing the
+number of the islands"  (See an extract from Pyrard's Voyage in Captain
+Owen's paper on the Maldiva Archipelago, in the "Geographical Journal",
+volume ii., page 84.): and I have already shown, on the authority of
+Captain Moresby, that the work of destruction is still in progress; but
+that on the other hand the first formation of some islets is known to the
+present inhabitants.  In such cases, it would be exceedingly difficult to
+detect a gradual subsidence of the foundation, on which these mutable
+structures rest.
+
+Some of the archipelagoes of low coral-islands are subject to earthquakes:
+Captain Moresby informs me that they are frequent, though not very strong,
+in the Chagos group, which occupies a very central position in the Indian
+Ocean, and is far from any land not of coral formation.  One of the islands
+in this group was formerly covered by a bed of mould, which, after an
+earthquake, disappeared, and was believed by the residents to have been
+washed by the rain through the broken masses of underlying rock; the island
+was thus rendered unproductive.  Chamisso (See Chamisso, in Kotzebue's
+"First Voyage," volume iii., pages 182 and 136.) states, that earthquakes
+are felt in the Marshall atolls, which are far from any high land, and
+likewise in the islands of the Caroline Archipelago.  On one of the latter,
+namely Oulleay atoll, Admiral Lutke, as he had the kindness to inform me,
+observed several straight fissures about a foot in width, running for some
+hundred yards obliquely across the whole width of the reef.  Fissures
+indicate a stretching of the earth's crust, and, therefore, probably
+changes in its level; but these coral-islands, which have been shaken and
+fissured, certainly have not been elevated, and, therefore, probably they
+have subsided.  In the chapter on Keeling atoll, I attempted to show by
+direct evidence, that the island underwent a movement of subsidence, during
+the earthquakes lately felt there.
+
+The facts stand thus;--there are many large tracts of ocean, without any
+high land, interspersed with reefs and islets, formed by the growth of
+those kinds of corals, which cannot live at great depths; and the existence
+of these reefs and low islets, in such numbers and at such distant points,
+is quite inexplicable, excepting on the theory, that the bases on which the
+reefs first became attached, slowly and successively sank beneath the level
+of the sea, whilst the corals continued to grow upwards.  No positive facts
+are opposed to this view, and some general considerations render it
+probable.  There is evidence of change in form, whether or not from
+subsidence, on some of these coral-islands; and there is evidence of
+subterranean disturbances beneath them.  Will then the theory, to which we
+have thus been led, solve the curious problem,--what has given to each
+class of reef its peculiar form?
+
+(PLATE: WOODCUT NO. 4.
+
+AA--Outer edge of the reef at the level of the sea.
+
+BB--Shores of the island.
+
+A'A'--Outer edge of the reef, after its upward growth during a period of
+subsidence.
+
+CC--The lagoon-channel between the reef and the shores of the now encircled
+land.
+
+B'B'--The shores of the encircled island.
+
+N.B.--In this, and the following woodcut, the subsidence of the land could
+only be represented by an apparent rise in the level of the sea.
+
+PLATE: WOODCUT NO. 5.
+
+A'A'--Outer edges of the barrier-reef at the level of the sea.  The
+cocoa-nut trees represent coral-islets formed on the reef.
+
+CC--The lagoon-channel.
+
+B'B'--The shores of the island, generally formed of low alluvial land and
+of coral detritus from the lagoon-channel.
+
+A"A"--The outer edges of the reef now forming an atoll.
+
+C'--The lagoon of the newly formed atoll.  According to the scale, the
+depth of the lagoon and of the lagoon-channel is exaggerated.)
+
+Let us in imagination place within one of the subsiding areas, an island
+surrounded by a "fringing-reef,"--that kind, which alone offers no
+difficulty in the explanation of its origin.  Let the unbroken lines and
+the oblique shading in the woodcut (No. 4) represent a vertical section
+through such an island; and the horizontal shading will represent the
+section of the reef.  Now, as the island sinks down, either a few feet at a
+time or quite insensibly, we may safely infer from what we know of the
+conditions favourable to the growth of coral, that the living masses bathed
+by the surf on the margin of the reef, will soon regain the surface.  The
+water, however, will encroach, little by little, on the shore, the island
+becoming lower and smaller, and the space between the edge of the reef and
+the beach proportionately broader.  A section of the reef and island in
+this state, after a subsidence of several hundred feet, is given by the
+dotted lines: coral-islets are supposed to have been formed on the new
+reef, and a ship is anchored in the lagoon-channel.  This section is in
+every respect that of an encircling barrier-reef; it is, in fact, a section
+taken (The section has been made from the chart given in the "Atlas of the
+Voyage of the 'Coquille'."  The scale is .57 of an inch to a mile.  The
+height of the island, according to M. Lesson, is 4,026 feet.  The deepest
+part of the lagoon-channel is 162 feet; its depth is exaggerated in the
+woodcut for the sake of clearness.) east and west through the highest point
+of the encircled island of Bolabola; of which a plan is given in Plate I.,
+Figure 5.  The same section is more clearly shown in the following woodcut
+(No. 5) by the unbroken lines.  The width of the reef, and its slope, both
+on the outer and inner side, will have been determined by the growing
+powers of the coral, under the conditions (for instance the force of the
+breakers and of the currents) to which it has been exposed; and the
+lagoon-channel will be deeper or shallower, in proportion to the growth of
+the delicately branched corals within the reef, and to the accumulation of
+sediment, relatively, also, to the rate of subsidence and the length of the
+intervening stationary periods.
+
+It is evident in this section, that a line drawn perpendicularly down from
+the outer edge of the new reef to the foundation of solid rock, exceeds by
+as many feet as there have been feet of subsidence, that small limit of
+depth at which the effective polypifers can live--the corals having grown
+up, as the whole sank down, from a basis formed of other corals and their
+consolidated fragments.  Thus the difficulty on this head, which before
+seemed so great, disappears.
+
+As the space between the reef and the subsiding shore continued to increase
+in breadth and depth, and as the injurious effects of the sediment and
+fresh water borne down from the land were consequently lessened, the
+greater number of the channels, with which the reef in its fringing state
+must have been breached, especially those which fronted the smaller
+streams, will have become choked up with the growth of coral: on the
+windward side of the reef, where the coral grows most vigorously, the
+breaches will probably have first been closed.  In barrier-reefs,
+therefore, the breaches kept open by draining the tidal waters of the
+lagoon-channel, will generally be placed on the leeward side, and they will
+still face the mouths of the larger streams, although removed beyond the
+influence of their sediment and fresh water;--and this, it has been shown,
+is commonly the case.
+
+Referring to the diagram shown above, in which the newly formed barrier-reef
+is represented by unbroken lines, instead of by dots as in the former
+woodcut, let the work of subsidence go on, and the doubly pointed hill will
+form two small islands (or more, according to the number of the hills)
+included within one annular reef.  Let the island continue subsiding, and
+the coral-reef will continue growing up on its own foundation, whilst the
+water gains inch by inch on the land, until the last and highest pinnacle
+is covered, and there remains a perfect atoll.  A vertical section of this
+atoll is shown in the woodcut by the dotted lines;--a ship is anchored in
+its lagoon, but islets are not supposed yet to have been formed on the
+reef.  The depth of the lagoon and the width and slope of the reef, will
+depend on the circumstances just referred to under barrier-reefs.  Any
+further subsidence will produce no change in the atoll, except perhaps a
+diminution in its size, from the reef not growing vertically upwards; but
+should the currents of the sea act violently upon it, and should the corals
+perish on part or on the whole of its margin, changes would result during
+subsidence which will be presently noticed.  I may here observe, that a
+bank either of rock or of hardened sediment, level with the surface of the
+sea, and fringed with living coral, would (if not so small as to allow the
+central space to be quickly filled up with detritus) by subsidence be
+converted immediately into an atoll, without passing, as in the case of a
+reef fringing the shore of an island, through the intermediate form of a
+barrier-reef.  If such a bank lay a few fathoms submerged, the simple
+growth of the coral (as remarked in the third chapter) without the aid of
+subsidence, would produce a structure scarcely to be distinguished from a
+true atoll; for in all cases the corals on the outer margin of a reef, from
+having space and being freely exposed to the open sea, will grow vigorously
+and tend to form a continuous ring whilst the growth of the less massive
+kinds on the central expanse, will be checked by the sediment formed there,
+and by that washed inwards by the breakers; and as the space becomes
+shallower, their growth will, also, be checked by the impurities of the
+water, and probably by the small amount of food brought by the enfeebled
+currents, in proportion to the surface of living reefs studded with
+innumerable craving mouths: the subsidence of a reef based on a bank of
+this kind, would give depth to its central expanse or lagoon, steepness to
+its flanks, and through the free growth of the coral, symmetry to its
+outline:--I may here repeat that the larger groups of atolls in the Pacific
+and Indian Oceans cannot be supposed to be founded on banks of this nature.
+
+If, instead of the island in the diagram, the shore of a continent fringed
+by a reef had subsided, a great barrier-reef, like that on the north-east
+coast of Australia, would have necessarily resulted; and it would have been
+separated from the main land by a deep-water channel, broad in proportion
+to the amount of subsidence, and to the less or greater inclination of the
+neighbouring coast-line.  The effect of the continued subsidence of a great
+barrier-reef of this kind, and its probable conversion into a chain of
+separate atolls, will be noticed, when we discuss the apparent progressive
+disseverment of the larger Maldiva atolls.
+
+We now are able to perceive that the close similarity in form, dimensions,
+structure, and relative position (which latter point will hereafter be more
+fully noticed) between fringing and encircling barrier-reefs, and between
+these latter and atolls, is the necessary result of the transformation,
+during subsidence of the one class into the other.  On this view, the three
+classes of reefs ought to graduate into each other.  Reefs having
+intermediate character between those of the fringing and barrier classes do
+exist; for instance, on the south-west coast of Madagascar, a reef extends
+for several miles, within which there is a broad channel from seven to
+eight fathoms deep, but the sea does not deepen abruptly outside the reef.
+Such cases, however, are open to some doubts, for an old fringing-reef,
+which had extended itself a little on a basis of its own formation, would
+hardly be distinguishable from a barrier-reef, produced by a small amount
+of subsidence, and with its lagoon-channel nearly filled up with sediment
+during a long stationary period.  Between barrier-reefs, encircling either
+one lofty island or several small low ones, and atolls including a mere
+expanse of water, a striking series can be shown: in proof of this, I need
+only refer to the first plate in this volume, which speaks more plainly to
+the eye, than any description could to the ear.  The authorities from which
+the charts have been engraved, together with some remarks on them and
+descriptive of the plates, are given above.  At New Caledonia (Plate II.,
+Figure 5.) the barrier-reefs extend for 150 miles on each side of the
+submarine prolongation of the island; and at their northern extremity they
+appear broken up and converted into a vast atoll-formed reef, supporting a
+few low coral-islets: we may imagine that we here see the effects of
+subsidence actually in progress, the water always encroaching on the
+northern end of the island, towards which the mountains slope down, and the
+reefs steadily building up their massive fabrics in the lines of their
+ancient growth.
+
+We have as yet only considered the origin of barrier-reefs and atolls in
+their simplest form; but there remain some peculiarities in structure and
+some special cases, described in the two first chapters, to be accounted
+for by our theory.  These consist--in the inclined ledge terminated by a
+wall, and sometimes succeeded by a second ledge with a wall, round the
+shores of certain lagoons and lagoon-channels; a structure which cannot, as
+I endeavoured to show, be explained by the simple growing powers of the
+corals,--in the ring or basin-like forms of the central reefs, as well as
+of the separate marginal portions of the northern Maldiva atolls,--in the
+submerged condition of the whole, or of parts of certain barrier and
+atoll-formed reefs; where only a part is submerged, this being generally to
+leeward,--in the apparent progressive disseverment of some of the Maldiva
+atolls,--in the existence of irregularly formed atolls, some being tied
+together by linear reefs, and others with spurs projecting from them,--and,
+lastly, in the structure and origin of the Great Chagos Bank.
+
+STEP-FORMED LEDGES ROUND CERTAIN LAGOONS.
+
+If we suppose an atoll to subside at an extremely slow rate, it is
+difficult to follow out the complex results.  The living corals would grow
+up on the outer margin; and likewise probably in the gullies and deeper
+parts of the bare surface of the annular reef; the water would encroach on
+the islets, but the accumulation of fresh detritus might possibly prevent
+their entire submergence.  After a subsidence of this very slow nature, the
+surface of the annular reef sloping gently into the lagoon, would probably
+become united with the irregular reefs and banks of sand, which line the
+shores of most lagoons.  Should, however, the atoll be carried down by a
+more rapid movement, the whole surface of the annular reef, where there was
+a foundation of solid matter, would be favourably circumstanced for the
+fresh growth of coral; but as the corals grew upwards on its exterior
+margin, and the waves broke heavily on this part, the increase of the
+massive polypifers on the inner side would be checked from the want of
+water.  Consequently, the exterior parts would first reach the surface, and
+the new annular reef thus formed on the old one, would have its summit
+inclined inwards, and be terminated by a subaqueous wall, formed by the
+upward growth of the coral (before being much checked), from the inner edge
+of the solid parts of the old reef.  The inner portion of the new reef,
+from not having grown to the surface, would be covered by the waters of the
+lagoon.  Should a subsidence of the same kind be repeated, the corals would
+again grow up in a wall, from all the solid parts of the resunken reef,
+and, therefore, not from within the sandy shores of the lagoon; and the
+inner part of the new annular reef would, from being as before checked in
+its upward growth, be of less height than the exterior parts, and therefore
+would not reach the surface of the lagoon.  In this case the shores of the
+lagoon would be surrounded by two inclined ledges, one beneath the other,
+and both abruptly terminated by subaqueous cliffs.  (According to Mr.
+Couthouy (page 26) the external reef round many atolls descends by a
+succession of ledges or terraces.  He attempts, I doubt whether
+successfully, to explain this structure somewhat in the same manner as I
+have attempted, with respect to the internal ledges round the lagoons of
+some atolls.  More facts are wanted regarding the nature both of the
+interior and exterior step-like ledges: are all the ledges, or only the
+upper ones, covered with living coral?  If they are all covered, are the
+kinds different on the ledges according to the depth?  Do the interior and
+exterior ledges occur together in the same atolls; if so, what is their
+total width, and is the intervening surface-reef narrow, etc.?)
+
+THE RING OR BASIN-FORMED REEFS OF THE NORTHERN MALDIVA ATOLLS.
+
+I may first observe, that the reefs within the lagoons of atolls and within
+lagoon-channels, would, if favourably circumstanced, grow upwards during
+subsidence in the same manner as the annular rim; and, therefore, we might
+expect that such lagoon-reefs, when not surrounded and buried by an
+accumulation of sediment more rapid than the rate of subsidence, would rise
+abruptly from a greater depth than that at which the efficient polypifers
+can flourish: we see this well exemplified in the small abruptly-sided
+reefs, with which the deep lagoons of the Chagos and Southern Maldiva
+atolls are studded.  With respect to the ring or basin-formed reefs of the
+Northern Maldiva atolls, it is evident, from the perfectly continuous
+series which exists that the marginal rings, although wider than the
+exterior or bounding reef of ordinary atolls, are only modified portions of
+such a reef; it is also evident that the central rings, although wider than
+the knolls or reefs which commonly occur in lagoons, occupy their place.
+The ring-like structure has been shown to be contingent on the breaches
+into the lagoon being broad and numerous, so that all the reefs which are
+bathed by the waters of the lagoon are placed under nearly the same
+conditions with the outer coast of an atoll standing in the open sea.
+Hence the exterior and living margins of these reefs must have been
+favourably circumstanced for growing outwards, and increasing beyond the
+usual breadth; and they must likewise have been favourably circumstanced
+for growing vigorously upwards, during the subsiding movements, to which by
+our theory the whole archipelago has been subjected; and subsidence with
+this upward growth of the margins would convert the central space of each
+little reef into a small lagoon.  This, however, could only take place with
+those reefs, which had increased to a breadth sufficient to prevent their
+central spaces from being almost immediately filled up with the sand and
+detritus driven inwards from all sides: hence it is that few reefs, which
+are less than half a mile in diameter, even in the atolls where the
+basin-like structure is most strikingly exhibited, include lagoons.  This
+remark, I may add, applies to all coral-reefs wherever found.  The
+basin-formed reefs of the Maldiva Archipelago may, in fact, be briefly
+described, as small atolls formed during subsidence over the separate
+portions of large and broken atolls, in the same manner as these latter were
+formed over the barrier-reefs, which encircled the islands of a large
+archipelago now wholly submerged.
+
+SUBMERGED AND DEAD REEFS.
+
+In the second section of the first chapter, I have shown that there are in
+the neighbourhood of atolls, some deeply submerged banks, with level
+surfaces; that there are others, less deeply but yet wholly submerged,
+having all the characters of perfect atolls, but consisting merely of dead
+coral-rock; that there are barrier-reefs and atolls with merely a portion
+of their reef, generally on the leeward side, submerged; and that such
+portions either retain their perfect outline, or they appear to be quite
+effaced, their former place being marked only by a bank, conforming in
+outline with that part of the reef which remains perfect.  These several
+cases are, I believe, intimately related together, and can be explained by
+the same means.  There, perhaps, exist some submerged reefs, covered with
+living coral and growing upwards, but to these I do not here refer.
+
+As we see that in those parts of the ocean, where coral-reefs are most
+abundant, one island is fringed and another neighbouring one is not
+fringed; as we see in the same archipelago, that all the reefs are more
+perfect in one part of it than in another, for instance, in the southern
+half compared with the northern half of the Maldiva Archipelago, and
+likewise on the outer coasts compared with the inner coasts of the atolls
+in this same group, which are placed in a double row; as we know that the
+existence of the innumerable polypifers forming a reef, depends on their
+sustenance, and that they are preyed on by other organic beings; and,
+lastly, as we know that some inorganic causes are highly injurious to the
+growth of coral, it cannot be expected that during the round of change to
+which earth, air, and water are exposed, the reef-building polypifers
+should keep alive for perpetuity in any one place; and still less can this
+be expected, during the progressive subsidences, perhaps at some periods
+more rapid than at others, to which by our theory these reefs and islands
+have been subjected and are liable.  It is, then, not improbable that the
+corals should sometimes perish either on the whole or on part of a reef; if
+on part, the dead portion, after a small amount of subsidence, would still
+retain its proper outline and position beneath the water.  After a more
+prolonged subsidence, it would probably form, owing to the accumulation of
+sediment, only the margin of a flat bank, marking the limits of the former
+lagoon.  Such dead portions of reef would generally lie on the leeward side
+(Mr. Lyell, in the first edition of his "Principles of Geology," offered a
+somewhat different explanation of this structure.  He supposes that there
+has been subsidence; but he was not aware that the submerged portions of
+reef were in most cases, if not in all, dead; and he attributes the
+difference in height in the two sides of most atolls, chiefly to the
+greater accumulation of detritus to windward than to leeward.  But as
+matter is accumulated only on the backward part of the reef, the front part
+would remain of the same height on both sides.  I may here observe that in
+most cases (for instance, at Peros Banhos, the Gambier group and the Great
+Chagos Bank), and I suspect in all cases, the dead and submerged portions
+do not blend or slope into the living and perfect parts, but are separated
+from them by an abrupt line.  In some instances small patches of living
+reef rise to the surface from the middle of the submerged and dead parts.),
+for the impure water and fine sediment would more easily flow out from the
+lagoon over this side of the reef, where the force of the breakers is less
+than to windward; and therefore the corals would be less vigorous on this
+side, and be less able to resist any destroying agent.  It is likewise
+owing to this same cause, that reefs are more frequently breached to
+leeward by narrow channels, serving as by ship-channels, than to windward.
+If the corals perished entirely, or on the greater part of the
+circumference of an atoll, an atoll-shaped bank of dead rock, more or less
+entirely submerged, would be produced; and further subsidence, together
+with the accumulation of sediment, would often obliterate its atoll-like
+structure, and leave only a bank with a level surface.
+
+In the Chagos group of atolls, within an area of 160 miles by 60, there are
+two atoll-formed banks of dead rock (besides another very imperfect one),
+entirely submerged; a third, with merely two or three very small pieces of
+living reef rising to the surface; and a fourth, namely, Peros Banhos
+(Plate I., Figure 9), with a portion nine miles in length dead and
+submerged.  As by our theory this area has subsided, and as there is
+nothing improbable in the death, either from changes in the state of the
+surrounding sea or from the subsidence being great or sudden, of the corals
+on the whole, or on portions of some of the atolls, the case of the Chagos
+group presents no difficulty.  So far indeed are any of the above-mentioned
+cases of submerged reefs from being inexplicable, that their occurrence
+might have been anticipated on our theory, and as fresh atolls are supposed
+to be in progressive formation by the subsidence of encircling barrier-reefs,
+a weighty objection, namely that the number of atolls must be
+increasing infinitely, might even have been raised, if proofs of the
+occasional destruction and loss of atolls could not have been adduced.
+
+THE DISSEVERMENT OF THE LARGER MALDIVA ATOLLS.
+
+The apparent progressive disseverment in the Maldiva Archipelago of large
+atolls into smaller ones, is, in many respects, an important consideration,
+and requires an explanation.  The graduated series which marks, as I
+believe, this process, can be observed only in the northern half of the
+group, where the atolls have exceedingly imperfect margins, consisting of
+detached basin-formed reefs.  The currents of the sea flow across these
+atolls, as I am informed by Captain Moresby, with considerable force, and
+drift the sediment from side to side during the monsoons, transporting much
+of it seaward; yet the currents sweep with greater force round their
+flanks.  It is historically known that these atolls have long existed in
+their present state; and we can believe, that even during a very slow
+subsidence they might thus remain, the central expanse being kept at nearly
+its original depth by the accumulation of sediment.  But in the action of
+such nicely balanced forces during a progressive subsidence (like that, to
+which by our theory this archipelago has been subjected), it would be
+strange if the currents of the sea should never make a direct passage
+across some one of the atolls, through the many wide breaches in their
+margins.  If this were once effected, a deep-water channel would soon be
+formed by the removal of the finer sediment, and the check to its further
+accumulation; and the sides of the channel would be worn into a slope like
+that on the outer coasts, which are exposed to the same force of the
+currents.  In fact, a channel precisely like that bifurcating one which
+divides Mahlos Mahdoo (Plate II., Figure 4.), would almost necessarily be
+formed.  The scattered reefs situated near the borders of the new
+ocean-channel, from being favourably placed for the growth of coral, would,
+by their extension, tend to produce fresh margins to the dissevered portions;
+such a tendency is very evident (as may be seen in the large published
+chart) in the elongated reefs on the borders of the two channels
+intersecting Mahlos Mahdoo.  Such channels would become deeper with
+continued subsidence, and probably from the reefs not growing up
+perpendicularly, somewhat broader.  In this case, and more especially if
+the channels had been formed originally of considerable breadth, the
+dissevered portions would become perfect and distinct atolls, like Ari and
+Ross atolls (Plate II., Figure 6), or like the two Nillandoo atolls, which
+must be considered as distinct, although related in form and position, and
+separated from each other by channels, which though deep have been sounded.
+Further subsidence would render such channels unfathomable, and the
+dissevered portions would then resemble Phaleedoo and Moluque atolls, or
+Mahlos Mahdoo and Horsburgh atolls (Plate II., Figure 4), which are related
+to each other in no respect except in proximity and position.  Hence, on
+the theory of subsidence, the disseverment of large atolls, which have
+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.  (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 Lutke'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.
+
+(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 PLATES.
+
+PLATE III.--MAP SHOWING THE DISTRIBUTION OF CORAL-REEFS AND ACTIVE
+VOLCANOES.
+
+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.)
+
+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.
+
+(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 deg 30', and in latitude
+24 deg: 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 deg 48' S., 142 deg 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 (Sailing directions, appended to
+volume ii. of his "Surveying Voyage to Australia.") 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 deg and
+13 deg) to form part of it.  Flinders ("Voyage to Terra Australis," volume
+ii. page 336.) has described an atoll-formed reef in latitude 10 deg, 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.
+
+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 Pouynipete, 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 (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 etre 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 Pouynipete (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.), 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.
+
+(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 deg 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 ("Annales des Sciences Nat." tom.
+vi., page 279, etc.) 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.
+
+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 ("Zoology of Captain Beechey's Voyage," page 176.  See
+also MM. Quoy and Gaimard in "Annales de Scien. Nat." tom. vi.) 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
+("Remarks on Coral Formations," page 51.) 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.
+
+In the southern part of the Low Archipelago, Elizabeth Island is described
+by Captain Beechey (Beechey's "Voyage in the Pacific," page 46, 4to
+edition.), 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 (Couthouy's "Remarks,"
+page 34.) 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.
+
+SAVAGE Island (S.E. of the Friendly group), is about forty feet in height.
+Forster ("Observations made during Voyage round the World," page 147.)
+describes the plants as already growing out of the dead, but still upright
+and spreading trees of coral; and the younger Forster ("Voyage," volume
+ii., page 163.) 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."  (Cook's "Third
+Voyage" (4to edition), volume i., page 314.)  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 (Ibid., volume i., page
+235.) 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
+("Remarks on Coral-Formations," page 50.) 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.
+
+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 SOLOMON ARCHIPELAGO, I have no information;
+but at New Ireland, which forms the northern point of the latter chain,
+both Labillardiere and Lesson have described large beds of an apparently
+very modern madreporitic rock, with the form of the corals little altered.
+The latter author ("Voyage de la 'Coquille'," Part. Zoolog.) 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 (Freycinet's "Voyage autour du Monde."  See also the
+"Hydrographical Memoir," page 215.), and Chamisso (Kotzebue's "First
+Voyage."), 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.  (Lutke's "Voyage,"
+volume ii., page 304.)
+
+In the EAST INDIAN ARCHIPELAGO, many authors have recorded proofs of recent
+elevation.  M. Lesson (Partie Zoolog., "Voyage de la 'Coquille'.") 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 ("Ann. des Scien.
+Nat." tom. vi., page 281.) 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," (translated by Windsor Earl, chapters
+vi., vii.) 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 ("Geolog. Transact." 2nd
+series, volume i., page 403.  On the Peninsula of Malacca, in front of
+Pinang, 5 deg 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.) 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 ("Notices of
+the East Indian Arch." Singapore, 1828, page 6, and Append., page 43.)
+(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
+PHILIPPINES, 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 (Captain B. Hall,
+"Voyage to Loo Choo," Append., pages xxi. and xxv.  Captain Beechey's
+"Voyage," page 496.) 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.
+
+Dr. Davy describes the northern province of CEYLON ("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.) 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.  (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.)  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 (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.) 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 deg 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.
+
+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.  Ruppell
+(Ruppell, "Reise in Abyssinien," Band i., s. 141.) 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 deg; but that south of
+26 deg, 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 deg 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. Ruppell says that the
+modern beds attain a height of only thirty to forty feet, Mr. Burton
+(Lyell's "Principles of Geology," 5th edition, volume iv., page 25.) 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.
+
+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
+(Ainsworth's "Assyria and Babylon," page 217.) 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.  (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 (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.) 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 (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.), 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.
+
+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 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.
+
+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 (At Sedanka, in
+latitude 58 deg 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.) 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.  (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.)  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.  (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 (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.), so that the prevailing movements
+now in progress, seem to accord with the actual states of surface of the
+great divisions of the world.
+
+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 ("Researches in Physical Geology,"
+Transact. Cambridge Phil. Soc., volume vi, part i.), 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 (For instance in S. America from
+latitude 34 deg, 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.) 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.
+
+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 Depot General 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 deg N., 109 deg 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 ACTAEON 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 deg 4' S.,
+and 136 deg 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 deg S., 150 deg W.) is described by Mr. F.D. Bennett
+("Geographical Journal", volume vii., page 225) as containing a fine
+lagoon; coloured blue.--FLINT Island (11 deg S., 151 deg 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 deg S., 158 deg W.); a plan of it in the "Atlas
+of the First Voyage" of Kotzebue, shows that it is an atoll; blue.--
+SLARBUCK Island (5 deg S., 156 deg 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 deg S., 154 deg 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 deg 20' S., 160 deg 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 deg W.); an atoll, of which a plan is given by Bellinghausen; blue.--
+SOUVOROFF Islands (13 deg S., 163 deg 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 deg S., 166 deg 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 deg S., 172 deg 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 deg S., 172 deg W.) is described by
+Commodore Byron (chapter x. of his "Voyage") as an atoll; blue.--SYDNEY
+Island (4 deg S., 172 deg W.) is about three miles in diameter, with its
+interior occupied by a lagoon (Captain Tromelin, "Annal. Marit." 1829, page
+297); blue.--PHOENIX Island (4 deg S., 171 deg 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 deg 15' N., 174 deg 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 deg S., 174 deg 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 deg N., 157 deg 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 deg N., 158 deg 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 deg N., 159 deg 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 deg N., 162 deg 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 deg N., 170
+deg 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 deg 22', longitude 178 deg 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 MANUA, 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 deg S., 167 deg 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 deg S., 170 deg 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, etc., 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.--TOUFOA AND 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 deg S., 174 deg 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 deg S., 176 deg 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 deg S., 179 deg 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 deg S., 179 deg 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 Labillardiere,
+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 BEAUPRE,) 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.
+
+SOLOMON 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 Labillardiere, 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 recifs a 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 Lutke; 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. Lutke, 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.--
+POUYNIPETE, 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" (Lutke, 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 (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 Lutke's charts)--a regular atoll.) 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 Lutke), 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:
+Lutke (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 Lutke
+(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 Lutke, is described and figured by Mr. Bennett ("United Service
+Journal," January 1832) as an atoll.  All the above-mentioned islands have
+been coloured blue.
+
+WESTERN PART OF THE CAROLINE ARCHIPELAGO.
+
+FAIS Island is ninety feet high, and is surrounded, as I have been informed
+by Admiral Lutke, 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 Lutke 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
+entierement entouree des recifs" (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 recifs;"
+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 Lutke'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 deg 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 LES PLUPART des iles de cet archipel, il
+y a de rocs de medrepores d'une beaute et d'une variete infimies."
+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 deg 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 MADUARA 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
+deg 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 deg 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,
+etc., 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 deg
+51' S.) is surrounded by reefs of coral, partly dry at low water,
+(Horsburgh, volume ii., page 96).--TRIESTE Island (4 deg 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 deg).  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
+deg 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 deg 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 deg N.) are
+represented in the English charts as surrounded by a very regular fringe.--
+ST. BARBES (0 deg 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.--BASHEE 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 deg 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 atoll 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 MAHE 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 deg 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 latitude 22 deg 16' to 21 deg 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 deg 15'; I have coloured the whole of
+this part of the coast red.  The islands of JUAN DE NOVA (in latitude 17
+deg 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
+deg 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 deg 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 deg 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 deg 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 deg 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 deg 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 deg
+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 deg 59' and 10 deg 7' S.) has the same
+structure; coloured red.--QUERIMBA Islands (from 10 deg 40' to 13 deg 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 deg 32' S.).  In the plan of this harbour, the headlands outside are
+fringed by reefs apparently of coral; coloured red.--MOZAMBIQUE (150 deg
+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, "Uber die
+Natur der Corallen-Banken 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 deg 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 deg
+and 17 deg 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 deg to 17 deg, 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 (Ruppell, "Reise in Abyssinie," Band. i., S. 247.),
+their shores are fringed by living coral-reefs.
+
+From the account given by Ruppell (Ibid., S. 245.) 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 deg; 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
+deg, 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.
+
+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 LATITUDE 19 DEG AND 22 DEG.
+
+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 deg 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 deg) 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 LATITUDE 22 DEG TO 24 DEG.
+
+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 deg 10', that if the promontory in latitude 24 deg 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 deg, 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 LATITUDE
+19 DEG TO 17 DEG (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 deg and 24 deg, 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 deg 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 deg and 20 deg, 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 deg (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
+deg 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 shores 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 (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.), 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 (In the ordinary sea-charts, no lagoons appear on
+the coast of Florida, north of 26 deg; 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.)
+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.
+
+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 deg 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 deg to 18 deg is
+fringed.  South of latitude 18 deg, 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 deg 45' and 16 deg 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 deg
+and 16 deg 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 Holuthuriae 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 (Holuthuriae) 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.
+
+Phoenix.
+
+Piguiram.
+
+Pitcairn.
+
+PITT'S bank.
+
+Pitt island.
+
+Platte.
+
+Pleasant.
+
+PORITES, chief coral on margin of Keeling atoll.
+
+Postillions.
+
+POUYNIPETE.
+Its probable subsidence.
+
+Pouynipete.
+
+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 Gutenburg Etext of Coral Reefs, by Charles Darwin
+
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