ON CORAL REEFS AND ISLANDS. JAMES D. DANA, GEOLOGIST OF THE EXPLORING EXPEDITION AROUND THE WORLD DURING THE TEARS 1838-1842, C. WILKES, U.S.N., COMMAND R. "We wandered where the dreamy palm Murmured above the sleeping wave; And through the waters clear and calm Looked down into the coral cave, Whose echoes never had been stirred By breath of man or song of bird." THULIA, J. C. PALMER, U. S. N., EXPL. EXP. PROM THE AUTHOR S EXPLORING EXPEDITION REPORT ON GEOLOGY, WITH ADDITIONS, NEW YORK: G. P. PUTNAM & Co. 1853. Entered according to Act of Congress, in the year 1853, By JAMES D. DANA, In the Clerk's office of the District Court of Connecticut. CONTENTS. CHAPTER I. STRUCTURE OF CORAL REEFS AND ISLANDS. Page. 1. General Features of Coral Reefs and Islands,............................ 5 Coral Reefs,..................................................... 5 Coral Islands,.............................................. 6 2. Characters of Fringing and Barrier Reefs. General Features.................................................. Structure of Reef Formations,...................................... 9 Outer Reefs,................................................. 9 Inner Reefs,................................................... 11 Channels among Reefs...................................... 13 Shore Accumulations,...................................... 15 Beach and Drift Sandrocks,........................... 16 3. Coral Islands. Forms and Features of Coral Islands,............................. 20 Soundings about Coral Islands.................................... 24 Structure of Coral Islands,........................................ 26 The Completed Coral Island,................................... 43 CHAPTER II. STRUCTURE GROWTH AND HABITS OF CORAL ZOOPHYTES. Structure and Growth of Zoophytes,................................ 47 Texture and Composition of Corals.....5...............................55 Causes influencing the Growth of Coral Zoophytes,........................ 59 Rate of Growth of Zoophytes,.............................. 66 iv CONTENTS. CHAPTER III. FORMATION OF REEFS AND CAUSES OF THEIR FEATURES AND GEOGRAPHICAL DISTRIBUTION. Page. Formation of Reefs,..................................................... 69 Causes Modifying the Forms and Growth of Reefs......................... 79 Rate of Growth of Reefs,............................................. 85 Origin of the Channels within Barriers and of the Atoll Form of Coral Islands, 87 Geographical Distribution of Coral Reefs and Islands,...................... 97 CHAPTER IV. Geological Conclusions from the Structure and Composition of Coral Reefs and Islands,....................................................... 110 CHAPTER V. ON CHANGES OF LEVEL IN THE PACIFIC OCEAN. Evidences of Change of Level......................................... 118 Subsidence indicated by Atolls and Barrier Reefs......................... 120 Extent and Effect of the Subsidence,......................... 124 Period of the Subsidence,.1...................................... 125 Elevation of Modern Eras in the Pacific,................................ 126 Changes of Level in the Pacific preceding the Coral Reefs,................. 136 NOTE.-By a recent letter from Rev. C. F. Winslow, we are informed that he has examined with acids the supposed coral material on Maui, referred to on p. 132, and found that it does not effervesce and is not calcareous. It is therefore no evidence of an elevation of the island. ON CORAL REEFS ANPD ISLANDS, CHAPTER I. STRUCTURE OF CORAL REEFS ANED ISLANDS. 1. GENERAL FEATURES OF CORAL REEFS AND ISLANDS. THE general features of coral reefs and islands have often been delineated by travellers, and are probably almost as familiar to the reader as the scenes of the land around us. Yet a few brief remarks on this point form a necessary introduction to the more minute descriptions of structure which follow. Coral reefs.-A wide platform of rock covered by the sea except at low tide, borders most of the high islands of the Pacific. It is a vast accumulation of coral, based upon the bottom in the shallow waters of the' shores. This bank or table of coral rock, is of varying width, from a few hundred feet to a mile or more; and although the surface is usually nearly flat, it is often intersected by irregular boat channels, or occasionally encloses large bays, affording harbor protection to scores of ships. In very many instances the reef stands at a distance from the shores like an artificial niole, leaving a wide and deep channel between it and the land; and within this channel are other coral reefs, some in scattered patches and others attached to the shore. The inner reef in these cases, is distinguished as the fringing reef, and the outer as the barrier reef. The sea rolls in heavy surges against the outer margin of the barrier; but the still waters of a lake prevail within, affording safe navigation for the tottling canoe sometimes through the whole circuit of an island: and not unfrequently, ships may pass, as by an internal canal, from harbor to harbor around the land. The reef is covered by the sea at high tide, yet the smoother waters indicate its extent, and a line of breakers its outline. Occasionally a green island rises from the reef, and in some instances, a grove of palms stretches alone the barrier for miles, where the action of the sea has raised the coral structure above the waves. 6 ON CORAL REEFS AND ISLANDS. The annexed sketch conveys some idea of the peculiar features presented by a Pacific island and its encircling reefs, though in order to fill out the scene, the jagged heights and deep gorges of the island should be covered with forests and the shores with groves and native villages. The coral platform which borders the shore is represented with its usual uneven line, its broad harbors with a narrow entrance, and to the left, an irregular ship channel running between the inner or fringing reef, and the outer or barrier. At a single place, the sea is faced by a cliff; and here, owing to the boldness of the shores and depth of waters, the reef is wanting. To the right there is only a fringing reef. Co ral islands.-Coral islands resemble the reefs just described, except that a lake or lagoon is encircled instead of a mountainous island. A narrow rim of coral reef, generally but a few hundred yards wide, stretches around the enclosed waters. In some parts the reef is so low that the waves are still dashing over it into the lagoon; in others it is verdant with the rich foliage of the tropics. The coral-made land when highest is seldom more than eight or ten feet above high tide. When first seen from the deck of a vessel, only a series of dark points is descried just above the horizon. Shortly after, the points enlarge into the plumed tops of cocoa-nut trees, and a line of green, interrupted at intervals, is traced along the water's surface. Approaching still nearer, the lake and its belt of verdure are spread out before the eye, and a scene of more interest can scarcely be imagined. The surf beating loud and heavy along the margin of the reef, presents a strange contrast to the prospect beyond,-the white coral beach, the massy foliage of the grove, and the ernbosomned lake with its tiny islets. The color of the lagoon water is often as blue as the ocean, although but fifteen or twenty fathoms deep; yet shades of green and yellow are intermingled, where patches of sand or coral-knolls are near the surface; and the green is a delicate apple-shade, quite unlike the ordinary muddy tint of shallow waters. The belt of verdure, though sometimes continuous around the lagoon, is usually broken into islets separated by varying intervals STRUCTURE OF CORAL REEFS. 7 of bare reef; and through one or more of these intervals, a shipchannel occasionally opens into the lagoon. The larger coral islands are thus a string of islets along a line of reef. The king of the Maldives bears the high-sounding title of "' Ibrahim Sultan King of the thirteen Atollons and Twelve Thousand Isles;" which Capt. W. F. W. Owen, R. N., remarks, is no exaggeration. A few small coral islands are simple reefs without lagoons. In some cases they are bare banks of coral; but generally, the usual vegetation of the islands has obtained a foothold, and affords some protection against the glare of the coral sand. With these general remarks we may enter upon the more particular consideration of the characters of reefs and islands. 2. CHARACTERS OF FRINGING AND BARRIER REEFS. a. General features. —Fringing reefs have been described as those that directly adjoin the shores of an island; and the barrier, as the exterior reefs, separated from the fringing reef, or from the shores when there is no inner reef, by an open channel. While there are only narrow shore-reefs to many islands, around others, a distant barrier extends like an artificial mole, sometimes ten or even fifteen miles from the land, and enclosing not only one, but at times several islands. Between the narrow fringing platform and these remote barriers, there is every possible variation as to extent and relative position. The inner channel is sometimes barely deep enough at low tide for canoes, or for long distances may be wanting entirely. Then again, it is a narrow intricate passage, obstructed by knolls or patches of coral, rendering the navigation dangerous. Again, it is for miles in length an open sea, in which ships find room to beat against a head wind with a depth of twenty, thirty, or even fifty fathoms. Yet hidden reefs make caution necessary. Patches from a few square feet to many square miles in extent are met with over the broad area enclosed by these distant barriers. These varieties of form and position are well exemplified in a single group of islands-the Feejees; a chart of which Archipelago by the Expedition is inserted at the close of this volume. Near the middle of the chart is the island Goro; its shores, excepting the western, are bordered by a fringing reef. The island Anzgan, south of Goro, is encircled by a coral breakwater, which on the southern and western sides runs far from the shores, and is a proper barrier reef, while on the eastern side, the same reef is attached to the coast and is a fringing reef. From these examples we perceive the close relation of barrier and fringing reefs. While a reef is sometimes quite encircling, in other instances it is interrupted or wholly wanting along certain shores; and occasionally it may be confined to a single point of an island. 8 ON CORAL REEFS AND ISLANDS. Above Angan lies Nairai; though a smaller island than Angau the barrier reef is of greater extent, and stretches off far from the shores. To the eastward of Nairai are Vatu Rera, Chichia, and Naiau, other examples of islands fringed around with narrow reefs. Lakemba, a little more to the southward, is also encircled with coral: but on the east side the reef is a distant barrier. In Aiva, immediately south of Lakemba, the same structure is exemplified; but the coral ring is singularly large for the little spots of land it encloses. The Argo Reef, east of Lakemba, is a still larger barrier, encircling two points of rock called Bacon's isles. It is actually a large lagoon island, twenty miles long, with some coral islets in the lagoon, and two of basaltic constitution, of which the largest is only a mile in diameter, Aiva and Lakernba are in fact other lagoon islands, in which the rocky islands of the interior bear a larger proportion to the whole area. The same view is further illustrated by comparing the Argo reef with Nairai, Angau, or Moala: these cases differ only in the greater or less distance of the reef from the shores and the extent of the enclosed land. Passing to the large islands Vanua Levu and Viti Levu, we observe the same peculiarities illustrated on a much grander scale. Along the southern shores of Viti Levu, the coral reef lies close against the coast; and the same is seen on the east side and north extremity of Vanua Levu. But on the west side of these islands, this reef stretches far off from the land, and in some parts is even twenty-five miles distant, with a broad sea within. This sea) however, is obstructed by reefs, and along the shores there are proper fringing reefs. The forms of encircling reefs depend evidently to a great extent on that of the land they enclose. That this is the case even in the Argo reef and such other examples as offer now but a single rock above the surface of the enclosed lagoon, we shall endeavor to make apparent, if not already so, when the cause of the forms of coral islands is under discussion. Yet it is also evident that this correspondence is not exact, for many parts of the shores, and sometimes more than half the coast, may be exposed to the sea, while other portions are protected by a wide barrier. In recapitulation, we remark, that reefs around islands may be (1) entirely encircling; or they may be (2) confined to a larger or a smaller portion of the coast, either contintuous or interrupted: they miyay (3) constitute throughout a distant barrier; or (4) the reef may be fringing in one part and a barrier in another; or (5) it may be fringing alone: the barrier may be (6) at great distances from the shores, with a wide sea within, or (7) it may so unite to the fringing reef that the channel between will hardly float a canoe. These points are sustained by all reef regions. STRUCTtURE OF CORAL REEFS. 9 A wide difference in the extent of reefs would be inferred frotm these facts. There is the mere point of coral rock; and again, as for example, west of the two large Feejee islands, there may be three thousand square miles of continuous reef-ground, occupied with coral patches and intermediate channels or seas. The enclosing barrier off Vanua Levu alone is more than one hundred miles long. The Exploring Isles, in the eastern part of the Feejee group, have a barrier eighty miles in circuit. New Caledonia, as often cited, has a reef along its whole western shores, a distance of two hundred and fifty miles. and it extends one hundred and fifty miles farther north, adding this mruch to the length of the island. The great Australian barrier forms a broken line, a thousand miles in length, lying off the coast from the Northern Cape to the tropical circle; and the channel within is in somne parts sixty miles from the coast, with a depth of thirty to sixty fathoms. The seas outside of the lines of coral reef are often unfatthomable within a short distance of the line of breakers. b. Struciure of Reef Fornmationts. In the description of reef grounds or reef-formations there are several distinct subjects for consideration, as is obvious firom the preceding remnarks. These are1. Outer reefs, or reefs formed from the growth of corals exposed to the open seas. Of this character, are all proper barrier reefs, arid such fringing reefs as are unprotected by a barrier. 2. Ilnner reefs, or reefs formed in quiet water between a barrier and the shores of an island. 3. Channels or seas within barriers, which may receive detritus either f'orn the reefs, or the shores, or from both of these sources combined. 4. Beaches and beachjormationls, produced by coral acecumulations on the shores through the action of the sea and winds. The outer and inner reefs, channels, and beaches, act each their part in producing the coral formations in progress about islands. Outer reefs.'I-he outer reefs or flats of coral rock receive the waves along their mla;rgin; and the outline exposed to this action is very much cut up withi deep channels which give passage to the advancing waters, and to the clurrents that flow back in preparation for the next breaker. This margin, which we have said rises but little above low-tide level, usuaily slopes beneath the water at an angle of forty to seventy degrees to a depth of three to eight fathoms; thence the waters deepen very gradually for one to five hundred yards out, and from this there is finally an abrupt descent, generally by an angle of at least forty degrees to depths beyond the reach of a sounding lead. There is a great difiference in the rapidity with which the water deepens, as might 2 10 ON CORAL REEFS AND ISLANDS. be inferred from the varied character of submarine slopes; in some cases. the shallow waters may extend for two or three miles beyond the reef;, but it is far more common to meet with the opposite extreme-unfathomable depths within a few hundred feet. The growing corals are mostly confined to the shallow waters of the reef, and to its sloping margin up which they extend to.withirl a foot or less of the surface. In these shallow waters the various zoophytes at times are crowded over extensive areas; yet very often they occur ontly in patches scattered throughout large fields of coral debris. The top of the reef is mostly destitute of life, and consists of the naked coral rock, more or less covered with coral sand. Yet there are some shallow pools, especially towards the outer limits, which abound in corals. The exposed edge of the reef is comlmonly raised a few inches above the general surface, anld is, therefore, the first part laid bare by the retreating tide, although a dangerous place for a ramble, oil account of the heavy.breakers. Though very uneven. the surfatce has generally a smooth, water-worn appearance, and is spotted with various shades of pink and purple. These colors, as observed by Chamlisso, are due to incrusting Nullipores, that grow like lichens over the rock: they are vegetable in nature, though cormposed mostly of lime. Other nodular and branching Nullipores, some sprigs of Madrepores, and a few of Astraas grow in the more sheltered cavities, where they are not easily dislodged by the waves; and among them, despite the breakers, cling numerous echini, asterias, and actinim. The gradual wear of the reefs by the wash of the sea is prevenlted, to a great extent, by these Nullipore incrustationls, as was pointed out by Darwvin.* He states that on Keeling's Island they constitute a layer two or three feet in thickness, with a breadth of twenty feet. They are abundant on the Paumilotrl reefs. The outer reefs are distinguished in many parts fiom the inner by becoming covered with accumulations of coral fiagments and sand, which are thrown tip by the waves: finding a lodgment some distance back from the margin of the reef, the aecnmnulations gradually increase, till in many instances they form dry land, and prepare the way for vegetation. Such effects are mostly confined, however, to the sides open to the prevailing wind, and are generally of limited extent. Occasionally, as at Bolabola, the reef for miles in length is changed from the submerged coral bank into a habitable islet-a green belt to the island of rocks and forests within. The causes and the result are much the same as in the case of the lagoon island, and the steps in the process will be more particularly described when treating of the coral atoll.' Darwin on Coral Reefs, London, 1842, page 9, and elsewhere. STRUCTURE OF CORAL REEFS. 11 The rock of the reef, wherever broken. exhibits a compact texture. In some parts it consists of coral fragments of quite large size firmly celmellted: other portions are a finer coral'breccia, or conglomerate: and still others, more common, are solid white limestones, as impalpable and homogeneous in texture as the secondary lilmestonle of' our continents, and usually much harder. It is rare to meet with any corals in this reef-rock retaining the original position of growth. It is at once apparent that the rock consists of the debris of the coral fields, consolidated by a calcareous cement; and the great abundance of the finer variety of rock indicates that much of it has originated from coral sand or mud. Wherever broken, it is found to present the same character as here described, a texture indicating a detritus origin. This reef-rock is formed in the midst of the waves; and we shall hereafter show that to this fact it owes many of its peculiarities. Besides corals, the shells of the seas contribute to it, and it sometimes contains them as fossils, along with bones of fish, exuvim of crabs, spines and fragments of echini, and other remains of organic life inhabiting reef-grounds. Inner reefs. —In the still waters of the inner channels or lagoons, when of large extent, we find corals growing in their greatest perfection, and the richest views are presented to the explorer of coral scenery. There are many regions —in the Feejes, examples are colmmon-where a remote barrier encloses as pure a sea as the ocean beyond; and the greatest agitation is only such as the wind may excite on a narrow lake or channel. This condition gives rise to some important peculiarities of structure in the inner reefs. Iun the general appearance of the surface, however, they munch resemble the outer reefs. They are nearly flat, and though mostly bare of life, and much covered with coral sand, there are seldom any large accumulations of coral debris. The margin is generally less abrulpt; yet there is every variety, from the gradually sloping bed of corals to the bltff declivity with its clinging clumps. In different parts, there are many portions still under water at the lowest tides; and here, (as well as upon the outer banks,) fine fishing sport is afforded the natives, who wade out at ebb tide with spears, pronged sticks, and nets, to supply themselves with food. The lover of the marvellous may find abundant gratification by joining in such a ramble; among coral plants and flowers, witi fishes of fantastic colors, starfish, echitni, anld myriads of other beings which science alone has named, fit inhabitants of a coral world, there is on every side occasion for surprise and admiration. Between the large reefi, which spread a broad surface at the water's edge of lifeless coral rock, sometimes of great extent, there are other patches, still submerged, which are covered with growing corals throughout. They are of differen!t elevations; and thotgh at times but a few yards in breadth, there is often along 12 ON CORAL REEFS AND ISLANDS. side of them a depth of many fathoms. They sometinles seem to grow up from a narrow base, like a mushroom; and a ship striking one with her keel may crush it and glide on. More frequently, they are below like the solid reef above described, and the contest is more likely to be fatal to the vessel than to the coral patch. Corals grow over them, as in tile shallow waters about other reefs; and, as elsewhere, there are deep cavities among the congregated corals, in which a lead will sometimes sink to a depth of several feet, or even fathoms. These holes about growing reefs often give much annoyance to the boat which may venture to anchor upon them; and in many an instance in the course of the surveys, diving was the only resource left for freeing the foul anchor. iThe rock of the inner reefs is peculiar in being but sparingly flagnmenta'ry. The corals composing it stand to a great extent as they grew; yet it is not less compact or firm in its texture than the rock of the outer reefs. The cavities among the branches and growing masses gradually becolme filled with coral sand, and the whole is finally cemented and thoroughly compacted. At Tongatabu and among the Feejee Islands, reefs thus made of corals stantding in their growing positions are coimmnon. Though now omere dead rock, the limits of the several constituent coral masses may be distinctly made out. Some individual specimens of Porites in the rock of' the inner reef of ToIngatabu were twenty-five feet in diameter; arid Astraas and Meandrinas, both there and in the Feejees, measured twelve to fifteen feet. These corals, when growing beneath the water, form solid hemispheres, or rounded hillocks-; but on reaching the surface, the top dies, and enlargement takes place only on the sides. In this manner the hemisphere is finally changed to a broad cylinder with a flat top. This was the condition of the Astreas and Porites in the reef-rock referred to. The platform looks like a Cyclorean paverment, except that the cementing material, filling in between the huge masses, is more solid than any work of' art: it even exceeds in compactness the corals themselves. Other portions of these reefs consist of' branchinl corals, with the intervals filled in by sand and small fragnments; for even in the more still waters fragments are to some extetnt produced. There is also to be found here, and frequently over large areas, the solid white limestone already described, showing internally no evidence of' its coral origin, and containing rarely a few shells or imbedded fossils. The formation of the inner reefs goes on at a less rapid rate than that of the outer, as the process depends on growth unaided, except in a comparatively small degree, by the action of the waves. Moreover, as we shall explain more particularly in another place, " A rock of this kind is often used for buildings and for walls on the island of Oahu. It consists mainly of Porites, and in many parts is still cavernous, or but imperfectly cemented. It is the material of the large church at Honolulu. STRUCTURE OF CORAL REEFS. 13 implre or fresh waters and currents often operate to retard their growth. Owing to the last mentioned cause, the inner reefs are not usually joined close to the beach. They stand off a little, separated by an interval of shallow water. At Mathuata, in the Feejees, however, the reef extends quite up; and it is the more remarkable as the country is a plain, the site of a Feejee village, and a mile or two back stands a high bluff. On a-n island off this part of Vauna Lebu is another example of this fact, and many more might be cited. In such cases, however, there is evidence that the shores upon which the corals grew were bare rocks, instead of moving beach-salds. From these descriptions it appears that the main distinction between the inner and outer reefs consists in the less fragmentary character of the rock in the former case, the less fiequent accumulations of debris on their upper surface, and the more varied features and slopes of the margin. Moreover, the Nulllipores, which seem to flourish best in the breakers, are of less extent, or but sparingly met with elsewhere.''The inner margin of a barrier reef, it should be observed, is entitled to rank with inner reefs, as its corals grow in the samne quiet waters, and under like circumstances. rThe variety of coral zoophytes is also greater in the stiller waters, and there are species peculiar to the dilfferent regions, as explained in anothler plaee. Charlnnels among reefs.-To complete this review of the general appearance and constitution of reef formations, it remains to add some particulars respecting the channels which intervene between coral patches, or separate them from the shores of an island, and also to describe the coral accumulations forming beaches. The reef of New Holland has been instanced as affording an example of one of the larger reef-channels, varying from thirty to sixty miles in width. and as many fathoms in depth. TIhe reefs west of the large Feejee Islands offer another remarkable example, the reef-grounds being in some parts twenty-five miles wide, and the waters within the barrier, where sounded, twelve to forty fathoms in depth. The barrier in this instance may be from a few hundred yards to a half a mile in width; and some of the inner patches are of the same extent; but by far the larger part of the reef'-ground is covered with deep waters, mostly blue like the ocean, and as clear and pure. The sloop of war Peacock sailed along the west coast of both Viti Lebu and Vanua Lebu, within the inner reefs, a distance exceeding two hundred miles. The island of Tahiti on its northern side presents us with a good illustration of a narrow channel, and at the samle time exhibits the usual broken or interrupted character of reefs. This is seen in the following cut in which the reefs, both fringing and barrier, are the parts enclosed by dotted lines. The outer reef 14 ON CORAL REEFS AND ISLANDS. extends half to two-thirds of a mile from the shore. Within it, between Papieti and Matavai, there is an irregular ship channel, varying from three to twenty fathoms in depth. Occasionally it enlarges into harbors; and in other parts it is very intricate, though throughout navigable by large vessels. rThe island of UpolL, of the Samoan Group, is bordered by a reef nearly a mile wide on part of its northern shore; but the waters within are too shallow for a canoe at low tide; and therefore, notwithstanding its extent, the reef is rather a fringing than a barrier reef. /............ "' ~ ~ o~ x a............,~-'XTOX... G / ~./....... v:,,:. ~mile, PART OF' THE NORH SHORE OF TAHITI. The bottom of the channels or lagoons takes its character, as regards the material constituting it, either from the reefs, a source of calcareous sand arid fragments, or from the earthy detritus of the island streams. At Upolu, the white coral sand of the reefs, (or in more general terms the reef debris,) forms the bottom; in some places it had the consistence of mud, and it was seldom observed to be covered with coarse material. There were some small patches of coral over it, and here and there a growing mass of Porites. The fresh waters of the shores do not flow over these wide reefs as there is no proper inner channel, and there is consequently no shore detritus mingled with the reef debris. At Tahiti, the sounding lead usually brought up sand, shells, and fragments of coral. At Tongatabu, the bottom, where the Peacock anchored,.was a grayish blue mud, appearing as plastic as common clay; it consisted solely of comminuted coral and shells,'with coloring matter probably from vegetable decomposition. STRUCTURE OF CORAL REEFS. 15 To the west of the larger Feejee islands, soundings commonly indicated a bottom of basaltic mud, and this material was fiequently brought up with our dredges. On the north side of Vanua Lebu, a stream has so filled with its detritus the wide channel into which it empties, that for a mile our ship dragged its keel in the mud, although elsewhere the water had been from twelve to twenty fathoms deep; and at least half a dozen square miles of land had been added to the shores from this source. Though due principally to shore material, the reefs have probably added somewhat'to these accumulations; yet little coral sand carl be detected in the mud by the eye, -and the proportion is certainly very small. In many places where we anchored, having the reef not more than five hundred yards from the ship, we might have judged, from the character of the bottom, that there were no corals nor shells within many miles. When the materials from both sources, the shore and the reef, are mingled, the proportion will necessarily depend on the proximity to the mouths of streams, the breadth of the inner waters or channels, and the direction and force of the currents. These tidal currents often ]have great strength, and are much modified and increased in force at certailn places, or diminished in others, by the position of the reef with reference to the land. Sweeping on, they carry off the coral debris from some regions to others distant; and again they bear along only the shore detritus, and distribute it. It is thus seen that the same region may differ widely in its adjacent parts, and seemingly afford evidence in one place that there is no coral near, and in another no basaltic land, although either is within a few rods, or even close along side. The extent of the land in proportion to the reef will have an obvious effect upon the character of the channel or lagoon depositions. When the island stands liike Bacon's isles, (Peejees,) as a mere point of rock in a wide sea enclosed by a distant barrier, the streams of the land are small and their detritus quite limited in amount. In such a case, the reef and the growing patches scattered over the lagoon, are the sources of nearly all the material that is accumulated upon the bottom. Shoore accumulations. —The wide coral banks and the enclosed channels greatly enlarge the limits tributary to the islands they encircle. They afford extensive fishing grounds for the natives and internal waters which enable them to practice and improve their skill in navigation, and communicate wxithout danger between distant settlements; and the effect is evident in the spirit of maritime enterprise which characterizes the islanders: for these circumstances have favored the construction of large sailcanoes, in which they venture beyond their own land, and often undertake voyages hundreds of miles in length. Instead of a rock-bound coast, harborless and thinly habitable, like most extratropical islands, the shores are blooming to the very edge, and 16 ON CORAL REEFS AND ISLANDS. wide plains are spread out with breadfruit and other tropical productions. Ports, safe for scores of vessels, are also opened by tile same means, and some islands number a dozen, when the unprotected shores would have hardly offered a single good anchorage. Coral reefs are sometimes viewed as only traps to surprise and wreck the unwary mariner. But one-who has visited the dreary prison-house, St. Helena, can have some appreciation of the benefits derived firom the growth of the zoophyte. The area of level shores, alluded to as added to many of the high islands by this means, is one of the most striking of these benefits. These plains are sometimes of large extent. T''he reefs stop the detritus from the hills, and are thus the means of its being added again to the land: they prevent, therefore, the waste which is constantly going on about islands without such barriers for the ocean not only encroaches upon the unguarded shores of the smaller islands, but carries off whatever the streams may empty into it. The delta of Rewa, on Viti Leblu, resulting from the detritus accumulations of a large river, covers nearly sixty square miles. This is an extreme case in the Pacific, as few islands are so large, and consequently rivers of' such magnitride are not common. But there is rarely an island which has riot at least some narrow plains from this source; and upon them the villages of the natives are usually situated. Around Tahiti these plains are frorm half a mile to two or three miles iin w\idth, and the cocoanut and breadfruit groves are mostly confirmed to them. Beach sandroc. —Besides the acectumllations firom a shore source, there are also beach formations derived fiom the reefs. The tides and the attending currents carry to the shores m-ore or less coral sand with shells and other reef-relics, and these somettimes form large deposits. The material is mostly like common sand in fineness, but often somewhat coarser. or even like a bank of pebbles. When the barrier is distant, only the sand and smaller pebbles are met with; but if the reef is quite narrow, there may be larger fiagnients and masses of coral rock. These deposits become cemented by being alternately moistened and dried, through the action of the recurring tides, and the wash of the sea on the shores. TBhe waters take up some carbonate of lime whichi is deposited and hardens among the particles on the evaporation of the moistlre at the retreat of' the tides. In some pla6es the grains are loosely coherent, and seem to be united only by the fewv points in contact; and with a little care, the calcareous coating which caused the union may be distinctly traced out. Ill other cases, the sand has been changed to an oolite, or to a solid rock, the interstices having been filled till a compact mass was formed. Generally, even the most solid varieties show evidence of a sand origin, and in this they differ from the reefrock. The pebbly beds produce a pudding-stone of coral. STRUCTURE OF CORAL REEFS. 17 In all instances observed, these calcareous sand-rocks or conglomerates form a number of parallel layers along the coast, which dip regularly at an angle of five to eight degrees towards the water. The layers are from a few inches to a foot in thickness. They appear as if they had been tilted by some force from below, and are seen to outcrop successively, onl receding from the water. Tutuila and Upolu in the Navigator Group, and Oahu in the Hawaiian, afforded us many examples of these beach formations. They seldom rise more than a few inches above high tide. At certain localities they appear to have been washed away after they were formed; and occasionally large masses or slabs have been uplifted by the sea, and thrown back on the beach. Deposits of the same kind sometimes included detritus from the hills. Black basaltic pebbles are thus cemented by the white calcareous material, producing a rock of very singular appearance. Near Diamond Hill on Oahu. is a good locality for observing the steps in its formation. Many of the pebbles of the beach are covered with a thin incrustation of carbonate of lime, appearing as if they had been dipped in milk, and others are actually cemented, yet so weakly that the fingers easily break them apart. The lime in solution in waters washing over these coral shores, is also at times deposited in the cavities or seams of the basaltic rocks; the cavities of the lava or basalt become filled with white calcareous kernels, and the cellular lava is changed into all amygdaloid. In large cavities or caverns, it often forms stalactites or stalagmitic in crustations.~ Drift sand-rock.-Still another kind of beach formation is going on in some regions through the agency of the winds in connection wvith the sea. It occurs only on the windward side of islands when tile reefs are narrow, and proceeds from the drift sands. The drifts resemble ordinary sand-drifts, and are often quite extensive. On Oahu, they occur at intervals around the eastern shores, from the northern cape, to Diamond Point which forms the south cape of the island,-the part exposed to the trades; and they are in some places twenty to forty feet in height. They are most remarkable on the north cape, a prominent point exposed to the winds that blow occasionally from the westward, as well as to the regular trades. They also occur on Kauai, another of the Hawaiian Islands. But at Upolu, (Samoa,) where the pro tecting reefs are broad, I met with no instance worthy of mention. " These sand-banks, through the agency of infiltrating waters, fresh or salt, become cemented into a sand-rock, more or less fri* Sinmilar facts are stated by Mr. Darwin as observed on the shores of Ascension, and many interesting particulars are given respecting calcareous incrustations on coasts.-See Volc. Islands, p. 49. They were observed by the writer upon Madeira in St. Jago, one of the Cape Verds, as well as among the basaltic islands of the Pacitic. 18 ON CORAL REEFS AND ISLANDS. able. The rock consists of thin layers or laminwe, which are very distinct, and indicate, generally, every successive drift of sand which puffs of wind had added in the course of its formation: and where a heavier gale had blown off the top of a drift, and new accumulations again completed it, the whole history is distinctly displayed in the rock. Several catastrophes of this kind may be made out from the character of the lamination in the sand-bluffs on the north side of Oahu. Since their formation, this island has undergone an elevation of twenty-five or thirty feet; these hills, once on the shores, are now seventy feet above the level of the sea, and they face the water with a bluff front (due to degradation), in which the lamination is finely exposed to view. The structure is best seen in a transverse section, presented on the west side. The layers are but a fracetIon of an inch thick: at one of the hills large slate-like slabs may be obtained; they have a sanded surftice, but are so hard within as to clink under the hammer. A particular description of these bluffs is given in the author's remarks on the geology of the Hawaiian islands. inl I III I fie s f BLUFFS OF CORAL SAND-{OCK, NORTH SHORE OF OAIU. these sources. The material is a fine sand, without organic r-vS~~~~~"~C~ii ~-N — BLUFFS OF CORAL SAUD ROCK, NORTH SHORE OF OAHC. these drift lulls, is the absence of shells, and even of fragments mains, although situated on shores off which, within a hundred yards, there are shells and corals innumerable. STRUCTURE OF CORAL REEFS. 19 e. Thickness -of reefs. We have considered in the preceding pages the peculiarities of form and structure characterizing the reef formations bordering islands and continents, and their influence upon the enclosed land. Could we raise one of these coral-bound islands fromt the waves, we should find that the reefs stand upon the submarine slopes, like massy structures of artificial masonry; some forming a broad flat platform or shelf ranging around the land, and others encircling it like vast ramparts, perhaps a hundred miles or more in circuit. The reefs that were near the water-line of the coast would be seen to have stood in the shallowest water, while the outer ramparts rested on the more deeply submerged slopes. Indeed, it is obvious that with a given slope to the declivity of the land, the thickness of the reef resting upon it may be directly determined, as it would be twice as great two hundred feet from the shore as at one hundred feet. The only difficulty, therefore, in correctly determining the depth or thickness of any given reef, arises from the uncertainty with regard to the submarine slope of the land. It is, however, admitted as the result of extensive observation, that in general these slopes correspond nearly with those of the land above water. Mr. Darwin has thus estimated the thickness of the reefs of the Gambier Group and some other Pacific islands, and he arrives at the conclusion, as his figures indicate, that some coral reefs, at their outer-lirmits, are at least two thoiusand feet int thickness. It will be shown in another part of this volume, that the mountain slopes of the islands of the Pacific, except when increased by degrading agents, cannot be assumerd to exceed twelve or fourteen degrees, and they are often but half this amount. The slopes of Maina Kea and Manna Loa, island of Hawaii, do not average over eight degrees. On the north side of Upolu, where the reefs are wide, the inclination is from three to six degrees. Throughout -the Pacific, the steeper slopes of the mountains are due to agencies which cannot be shown to have affected the submarine slopes, excepting in cases of disruption of islands.by forces below. Assuming eight degrees as the mean inclination, we should have for the depth of ree.f (or water,) one mile friom the shore., 740 feet; or assuming five degrees, 460 feet. Adopting the first estimate, the Gamzbier Group would give for the outer reef a thickness of at least 1 750 feet; or with the second, 1150 feet. FThe island of Tahiti, (taking the north side for data,) would give in the same manner 250 feet by the last estimate, which we judge to be most correct; Upolu, by the same estimate, 440 feet. rjThe deduction for Upolu may be too large: taking three degrees as the inclination, it gives 260 for the thickness at the outer margin. The results are sufficiently accurate to satisfy us of the great thickness of many barrier reefs. 20 ON CORAL REEPS AND ISLANDS. These calculations, however, are liable to error from many sources. Very different results might generally be obtained from different sides of the same island; and the same group often contains islands without reefs, and others with reefs one or even several miles from the shores. But since we may show that the absence of a reef or its limited extent may be traced to some causes restricting or modifying its formation, it is obvious that the error would probably be on the side of too low atn estimate. Adjacent to the larger islands, such as those of Vanua Levu and New Holland, the error might be of the opposite kind; for the slopes of the land are of a more complex or irregular character than on the smaller islands. In the latter, they may be shown to belong generally to a single elevation of igneous origin, or at the most to two or three combined; while in the former, they may pertain to different ranges of hills or mountains. For correct results in any instance, the land and its declivities should be carefully studied beforehand, and the system in its inclinations determined by observation. With regard to Tahiti and Upolu, information bearing upon this point was obtained, and the above conclusions may be received with much confidence. Many' of the Feejee reefs, on the same principle, cannot be less than 2000 feet in thickness. Such accumulations of calcareous rock may appear to be an incredible work for the coral polyp, but only so, because we are not accustomed to contemplate the results which may proceed from the smallest agencies long continued. The operatives in the inorganic world are invisible molecules; and among living organisms, it is the lowest grade, the minims of existence, that have accomplished the grandest results in the earth's history. 3. CORAL ISLANDS. A. Formes and general fecttures of Coral Islands. A barrier reef, and a lagoon enclosed by it, are the prominent features of a coral island; yet there are a few of small size in which the lagoon is wanting. In the larger islands, the waters within look like the ocean, and are similarly roughened by the wind,,though not to the same extent. Standing on the north shore of the Raraka lagoon, (in the Paumotus,) and looking southwest, nothing is descried but blue waters;-far in the distance, to the right or left, a few faint dots are distinguished; and as the eye sweeps around, these gradually enlarge into lines of palms and other verdure, which finally become distinct groves on nearing the observer. At Dean's Island, another of the Paurnotus, and at many of the Carolines, the resemblance to the ocean is still more striking. The lagoon is in fact but a fragment of the ocean cut CENERAL FEATURES OF CORAL ISLANDS. 21 off by nore or less perfect walls of coral reef-rock; and the reef is here and there surmounted by verdure, forming a series of islets. In many of the smaller coral islands, the lagoon has lost its ocean character, and become a shallow lake, and the green islets of the margin have coalesced in some instances into a continuous line of foliage. Traces may perhaps be still detected of the passage or passages over which the sea once communicated with the internal waters, thollgh mostly concealed by the trees and shrubbery which have spread around and completed the belt of verdure. The coral island is now ill its most finished state: the lake rests quietly in its bed of palms, hardly ruffled by the storms that madden the surrounding ocean. From the islands with small lagoons, there is every variety in gradation down to those in which there is no trace of a lagoon. These simple banks of coral are the smallest of coral islands. These remarks, in connection with the general view given on a preceding page, will prepare the reader to appreciate the following descriptions of various coral islands, illustrating their forms, actual size, and condition. A single group of islands, the Tarawan or Kingsmills, (see Plate,) affords good examples of the principal varieties. TL'he irregularity of shape and size is at once apparent to the eye. In the southernmost, Taputeoueca, the form is very narrow, the length being thirty-three miles, with the width of the southern portion scarcely exceeding six miles, and that of the northern more than one-half less. The emerged land is confined to one side, and consists of a series of islets upon the eastern line of coral reef. The western side is for the most part some feet under water, and there is hardly a proper lagoon. Sailing by the island, to windward, the patches of verdure thus strung together seem to rise out of a long white line of breakers, the sea surging violently against the unseen coral reef upon which they rest. Namnouti, the next island north, it about twenty miles long by eight broad. The rim of land, though in fewer islets, is similar to that of Taputeouea in being confind to the reef fronting northeast. The reef of the opposite side, though bare of vegetation, stands near low tide level, and the whole encloses a large lagoon. Nanouki and Apamnama, though smaller than Namonti, have the same general character. Nanouki is triangular in shape, and has an islet on the western point or cape, which is quite prorninent. Apamama differs from either of the preceding in having two narrow ship entrances to the lagoon, one through the northwestern reef, and another through the southwestern. - The plate is a reduced copy of the chart of these islands, as surveyed by the Exploring Expedition. ON CORAL REEFS AND ISLANDS. Kalria is a remarkable double island, without a proper lagoon It consists of two neighboring groves, each about a square mile in extent, on adjacent patches of reef. ll/aiana is quite regularly quadrangular, with an uninterrupted range of land on two of the four sides, and an exposed reef constituting the other two. 7Trawa consists of two sides of a triangle. The western reef is wanting, and the sea and lagoon have unbroken communication. In place of it, there are two to ten fathoms water, and a bottom of coral sand. Small vessels may sail in almost anywhere on this side to a good anchorage, and there is a passage for ships of the largest size. The depth within is greater than on the bar, and these inner waters obviously correspond to the lagoon of other islands. Apia has much resemblance to Apamama in its forest border and lagoon. Moreover, there is a ship-entrance through the southwestern reef. Maraki is one of the prettiest coral islands of the Pacific. The line of vegetation is unbroken; and from the mast-head it lies like a garland thrown upon the waters. The unpracticed eye scarcely perceives, in such a view, the variation from a circular form, however great it may be. The grove is partially interrupted at one point, where there are indications of a former passage through the reef. Tacri-tari is a large triangular atoll. It is wooded almost continuously on the reef facing southeast, and has a few spots of verdure on the southwest, with three entrances to the extensive lagoon. The northern side is a naked reef throughout, scarcely apparent from a ship's deck, except by the long line of breakers. llak:in, just north of Tari-tari, is a mere patch of coral reef without a lagoon. We add a few more descriptions of Pacific islands, with figures reduced from the maps of the Expedition to a scale of four tenths of an inch to a mite. 1. 2. W it IIENUAKIE Or1 IONDEN. TATARA. Taiara and Henuake, (figs. I and 2,) are two smlall belts of foliage, somewhat similar to Maraki. Henuake possessed an additional charm in. being tenanted only by birds; and they were so tame that we took them from the trees as if they had been their flowers. GENERAL FEATURES OF CORAL ISLANDS. 23 Swain's and Jarvis Islands, (figs. 4. 3 and 4,) are of still smaller size,: and have no lagoon. The former,PEP... - is densely covered with foliage,,:,t:e >. while the surface of the latter is SWnIN'S ISLA.ND JARVIS ISLAND.S sandy. Swain's Island is a little depressed about the centre, a fact indicating that there was formerly a lagoon. FaAkaafo, or Bowditch, (fig. 5. 5,) 200 miles north of Samoa, is the type of a large part of coral islands. The bank of reef has only here and there emerged,' from the waves and become verdant; in other portions the reef' Ais of the usual height,-that is, Ct'\ near low tide level,-excepting f a few spots elevated a little by'-; the accumulation of sand. The Paumlotu Archipelago, the.... crowded cluster of coral islands just northeast of Tahiti, is a most instructive study for the ~"' reader; and a map of these islands by the Expedition, in- serted in the Narrative of the e,, Expedition, and also in the Hydrographical Atlas, will well repay close examination. Sailing FAIAAFO among these islands-over eighty in number, only four of which are over twelve feet high exclusive of the vegetation, —two or three are almost constantly in sight from the mast-head. The small amount of habitable land on these reef-islands is one of their most peculiar features. Nearly the whole surface is water; and the land around the lagoon is but a narrow rim, the greater part of which is usually under water at high tide. This fact will be rendered more apparent from the following table, containing a statement of the sizes and areas of several islands, with the amount of habitable land. The measures are given in geographical miles. Length. Greatest breadth. Area in square Iabitable parts in Length. Greatest breadth. miles. square miles. Carlshoff; Paumotus, 27 13 200 10 Wolchonsky, " 15 3 40 3 Raraka, 15 10 90 8 Manhii, " 14 6. 50 9 Nairsa or Deans," 50 19 1000 16 Fakaafo, Union Group, 7. 44 20 24 Clarence, " 8 5t 27 2 Taputeouea, Kingsmills, 33 6 60 6 Tarawa,' 20 10 130 8 Namouti, " 22 9 125 7 Tari-tari, " 18 11 110 4 24 ON CORAL REEFS AND ISLANDS. The ten islands here enumerated have an aggregate area of 1952o square miles, while the amount of actual dry habitable land is but seventy-six miles, or less than one twenty-fourth. In the Caroline Archipelago the proportion of land is still smaller. Menchikoff atoll covers an area of 500 square miles, and includes hardly six square miles of wooded land. In the Marshall Islands the dry land is not over one-hundredth of the whole surface; while in the Pescadores the proportion of land to the whole area is about as 1 to 200. The distribution of the land upon the reef is obvious from the sketches already given. It was long since remarked that the windward side was in general the highest. It is also apparent that there are not only great irregularities of form, but the reef may at times be wholly wanting or deeply submerged on one side. In many islands there is a ship entrance, sometimes six or eight fathoms deep, through the reef to the lagoons, where good anchorage may be had; but the larger part have only shallow passages, or none at all. In the Paumotus, out of the twenty-eight visited by the Expedition, not one half were found to have navigable entrances. In the Carollnes, where the islands are large and not so much wooded, entrances are of more common occurrence. About half of the Kingsmill Islands afford a good entrance and safe anchorage. Through these openings in the reefs, there is usually a rapid outward current, especially during the ebbing tide. At Depeyster Island, it was found to run at the rate of two and a half miles an hour. It was as rapid at Raraka, in the Paumotus, and as Capt. Wilkes remarks, it was difficult to pull a boat against it, into the lagoon. Sourzdinhgs about Coral Islands.-The water around coral islands deepens as rapidly and in much the same way as off the reefs about high islands. The atoll usually seems to stand as if stilted up in a fathomless sea. The soundings of the Expedition afford some interesting results. Seven miles east of Clermont Tonnerre, the lead ran out to 1145 fathoms (6870 feet), without reaching bottom. Within three quarters of a mile of the southern point of this island, the lead, at another throw, after running out for a while, brought up an instant at 350 fathoms, and then dropped off again and descended to 600 fathoms without reaching bottoim. On the lead, which appeared bruised, a small piece of white coral was found, and another of red; but no evidence of living zoophytes. On the east side of the island, three hundred feet from the reef, a bottom of coral sand was found in 90 fathoms; at one hundred and eighty feet, the same kind of bottom in 85 fathoms; at one hundred and thirty feet, a coral bottom in 7 fathoms;-and from this it decreased irregularly to the edge of the shore reef. GENERAL FEATURES OF CORAL ISLANDS, 25 Off the southeast side of Ahii (another of the Paumotus), about a cable's length from the shore, the lead after descending 150 fathoms, strLuck a ledge of rock, and then fell off and finally brought up at a depth of 300 fathoms. Two miles east of Serle's Island, no bottom was found at 600 fathoms. A mile and a half south of the larger Disappointment Island, there was no bottom at 550 fathoms. Near the eastern end of M[etia, no bottom was found with a line of 150 fathoms; and a mile distant, no bottom was reached at 600 fathoms.* In general, for one to five hu:lndred yards from the margin of the shore reef, the water slowly deepens, and then there is an abrupt descent, at an angle of 40 or 50 degrees. The results of earlier voyagers, among whom Beechey stands pre-eminent, correspond with this statement. At considerable depths, as would appear from the above facts, the sides of the coral structure may be vertical or even may overhang the bottom below. There are examples also of less abrupt slopes. Northwest of the Hawaiian Group, Lisiansky, at the island bearing his name, found shallow water for a distance of six or seven miles; the water deepened to ten or eleven fathoms the first mile, fifteen the second, and at the last throw of the lead there were still but twentyfive fathoms.t Christmas Island affords on its western side another example of gradually deepening waters. Yet these shallow waters terminate finally in a rapid declivity of forty or fifty degrees. Off the prominent angles of an atoll, soundings generally continue much beyond the distance elsewhere, as was first observed by Beechey. At Washington Island, mostly abrupt in its shores, there is a bank, according to the surveys of the Expedition, extending from the east point to a distance of half a mile, and another on the west extending to a distance of nearly two C Beechlley, whose observations on soundings are the fullest hitherto published, states Inany facts of great interest. At Carysfort Island, he found the depth 60 yards from the surf line, 5 fathoms;-80 yards, 13 fathoms; —120 yards, 18 fathoms; —200 yards, 24 fathoms; —and immediately beyond, no bottom with 35 fathoms. At Henderson's Island, soundings continued out 250 yards, where the depth was 25 fathoms, and then terminated abruptly. Off Whitsunday, 500 feet out there was no bottom at 1500 feet. Darwin states many facts bearing upon this subject, of which we may cite the following. —At Heawandoo Pholo (one of the Maldives) Lieutenant Powell found 50 or 60 fathoms close to the edge of the reef: One hundred fathoms from the mouth of the lagoon of Diego Garcia, Captain Moresby found no bottom with 150 fathoms. At Egmont Island, 50 fathoms from the reef, soundings were struck in 150 fathoms. At Cardoo Atoll, only 60 yards friom the reef, no bottom was obtained with a line of 200 fathoms. Off Keeling Island, 2200 Vards firom the breakers, Captain Fitzroy found no bottom at 1200 fathoms. Mr. Darwin also states that at a depth betwveen five and six hundred fathoms, the line was partly cut as if it had rubbed against a projecting ledge of rock; and deduces from the fact " the probable existence of submrarine cliffs." t Voyage round the world, iin the years 1803-6, in the ship Neva,by N. Lisiansky, Captain in the Russian Navy, 4to, London: pp. 254-257. 4 26 ON CORAL REEFS AND ISLANDS. miles. At Kuria, one of the Kingsmills, soundings continue for three miles from the north extremity, along a bank stretching off from this point to the north-northwest. Many other instances might be cited, but they are seldom as remarkable; yet nearly all islands, especially if the points are much prominent, afford similar facts. It has been said that the reef to leeward is generally less abrupt than that to windward, but no facts were obtained by the Expedition sufficiently definite or extensive to settle this question. It is probably true, yet the difference if any must be slight. B. Structure of Coral Islands. The descriptions of reefs and their islets apply with equal force to coral islands. By transferring here the statements respecting the former, we should have a nearly complete account of the latter. The same causes, with scarcely an exception, are at work:-the growing of coral-zoophytes, the action of the waves, oceanic currents, and the winds. This resemblance will be rendered more apparent by a review of their characters; the description will be found to be a simple recapitulation of a formner paragraph. The reef of the coral atoll, as it lies at the surface still uncovered with vegetation, is a platform of coral rock, usually two to four hundred yards wide, and situated so low as to be swept by the waves at high tide. The outer edge, directly exposed to the surf, is generally broken into channels and jagged indentations, along which the waters of the resurging wave drive with great force. Though in the midst of the breakers, the edge stands a fevw inches, and sometimes a foot, above other parts of the platform; the incrusting Nullipores cover it with varied tints, and afford protection from the abrading action of the waves. There are usually three to five fathoms water near the margin; and below, over the bottom which gradually deepens outward, beds of corals are growing profusely among lifeless patches of coral sand and fragments. Often the dead areas much exceed those flourishing with zoophytes, and not unfrequently the clusters are scattered like tufts of vegetation in a sandy plain. The growing corals extend up the sloping edge of the reef, nearly to low tide level. For ten to twenty yards from the margin, the reef is usually very cavernous or pierced with holes or sinuous recesses, a hiding-place for various crabs, or a'retreat for the echini, asterias, the sea-anemones, and many a pretty mollusc; and over this portion, the gigantic Chama or Tridacna is generally found lying more than half buried in the solid rock, with barely room to gape a little its ponderous shell, and expose to the waters a gorgeously colored mantle. Farther in are occasional pools and basins, alive with all that lives in these strange coral seas. STRUCTURE OF CORAL ISLANDS. 27 The reef-rock, wherever broken, shows a detritus origin. Parts are of compact homogeneous texture, a solid white limestone, without a, piece of coral distinguishable, and rarely an imbedded shell. But generally the rock is a breccia or conglomerate, made up of corals cemented into a compact mass, and the fiagments of which it consists are sometimes many cubic feet in size. It is apparent that we are describing a second time an outer reef. Without dwelling firther,' upon its characters, wve may pass to the features of the reef when raised above the waters and covered with vegetation. Sections of coral islands and their lagoons have i been given by Captain Beechev and Mr. Darwil. We add another, by way of illustrateion, although a x. little may be presented that is novel after h.e excellent descriptions of these auLthors. Sketches of several of these islands, showing the general relation of the rim of land to the reef and the lagoonl i within, are given in the Plate of the Kingsmill, Group. The following sketch represents a sec- tion of the rim of land from the sea on one side, o 0 (the left,) to the lagoon on the other. In the > view, the part m a, represents the shallow sea'' y bordering an island, and abruptly deepening one g. to six hundred feet from the line of breakers. X In these shallow waters are the growing corals; yet, as before stated, a large part is barren sand t or coral rock..i From a to b is the shore platform of reef-rock,: nearly at low tide level, with the margin (a) slight.y elevated, and much incrusted at the top with Nullipores. From the platform there is a rise by a steep beach (b c,) of six or eight feet, to the wooded part of the coral belt represented between c and d. From d to e there is a gently sloping beach bordering the lagoon. Beyond e, the waters of the lagoon at first deepen gradually, and then fall off mnore or less abruptly. In the Paumotus, the shore platform, the steep beach, and the more gently sloping shore of the lagoon are almost constant characteristics. x i The width of the whole rim of,land, when the island gives no evidence of late elevation, varies from three hundred yards to one-third of a mile, excepting~certain prominent points, rmore exposed to the united action of -winds and waves and from opposite directions,' which occasionally exceed half a mile.* * Beechey states that the rim is generally three to four hundred yards in width, and never exceeds half a mile.- Voyage, Amer. ed., p. 160. 28 ON CORAL REEFS AND ISLANDS. Shore platform and emerged land.-The shore platform is from one to three hundred feet in width, and has the general features of a half-submerged outer reef. Its peculiarities arise solely from the accumulations which have changed the reef into an island. Much of it is colmonly bare at low tide, though there are places where it is always covered with a few inches or a foot of water; and the elevated edge, the only part exposed, often seems like an embankment preventing the water from running off. The tides, as they rise, cover it with water throughout, and bear over it coral fragments and sand, comminuted shells and other animal remains, to add them to the beach. The heavier seas transport larger fragments; and at the foot of the beach there is often a deposit of blocks of coral or coral rock, a cubic foot or so in size, which low tide commonly leaves standing in a few inches of water.t Besides the deep channels cutting into the margin of the reef and giving it a broken outline, there are in some instances long fissures intersecting its surface. On Aratica, (Carlshoff,) and Ahii, (Peacock Island,) they extended along for a fourth to half a mile, generally running nearly parallel with the shore, and at top were from a fourth to half an inch wide. These fissures are not essential features of the reef, and will come up for consideration on a fliture page of this work. The beach usually slopes at an angle of 35 to 45 degrees, and consists of coral pebbles or sand, with some wvorn shells, and occasionally the exuviam of crabs and bones of fishes. Owing to its whiteness, and the contrast it affords to the massy verdure above, it is a remarkable feature in the distant view of these islands, and often seemed like an artificial wall or embankment running parallel with the shores. On Clermont Tonnerre, the first of these islands visited by us, the natives seen from shipboard, standing spear in hand along the top of the beach, were believed by some to be keepillng patrol on the ramparts of a kind of fortification. This deception Arose fiom the dazzling whiteness of the coral sand, in consequence of which, the slope of the beach was not distinguished in so distant a view. The emerged land beyond the beach, in its earliest stage when barely raised above the tides, appears like a vast field of ruins. Angular masses of coral rock, varying in dimensions from one to a hundred cubic feet, lie piled together in the utmost confusion; and they are so blackened by exposure, or frorn incrusting lich{ On moving these masses, which generally rest on their projecting angles and have an open space beneath, the waters at once become alive with fish, shrimps, and crabs, escaping from their disturbed shelter; and beneath, appear various Actinret or living flowers, the spiny echini and sluggish biche-la-mar, while swarms of shells having a soldier crab for their tenant walk off with unusual life and stateliness. Moreover, delicate corallines, Ascidia and sponges tint with lively shades of red, green, and pink, the under surface of the block of coral which had formed the roof of the little grotto. STRUCTURE OF CORAL ISLANDS. 29 ens, as to resemble the clinkers of Mauna Loa; moreover, they ring like metal under the hammer. Such regions may be travelled over by leaping along from block to block, with the risk of falling into the. many recesses among the huge masses. On breaking an edge from the black masses, the usual white color of coral is at once apparent. Some of the blocks, measuring five or six feet in each of their dimensions, were found to be portions of individual corals, while others have the usual conglomerate character of the reef-rock. In the next stage, coral sandl has fouLnd lodgment among the blocks; and though so scantily supplied as hardly to be detected without close attention, some seeds have taken root, and vines, purslane, and a few shrubs begin to grow, relieving the scene, by their green leaves, of much of its desolate aspect. Both of these stages are illustrated on the greater part of coral islands. In the last stage, the island stands six to ten feet out of water. The surface consists of coral sand, more or less discolored by vegetable or animal decomposition. There is but little depth of coral soil, although the land may appear buried in the richest foliage: and scattered among the trees, stand. still uncovered, many of the larger blocks of coral, with their usual rough angular features and blackened surface. The soil is seldom discolored beyond four or five inches, and but little of it to this depth; there is no proper vegetable mould, but a simple mixture of darker particles with the white grains of coral sand. It is often rather a coral gravel, and below a foot or two, it is usually cemented together into a more or less compact coral rock. One singular feature of the shore platform, occasionally observed, remains to be mentioned. Hiuge masses of reef-rock are sometimes found upon it, some of which lie loose upon the reef, while others are firmly imbedded in it below, and so cemented to it as to appear to be actually a part of the platform rock. Sketches of some of these masses are here given. 1. 2. Figure 1 represents a mass on the island of Waterland, (one of the Paumotus,) six feet high, and about five in diameter; it was solid with the reef-rock below, as though a part of it, and about two feet above its base, it had been so nearly worn off by the waters as to have become irregularly top-shape. Figure 2 is ON CORAL REEFS AND ISLAND:,. another mass, similarly attached to the reef at base, observed on,awehe, (Vincennes Island.) It was six feet high above low water level, and seven feet in its longest diameter. Belov, it had been worn like the one just described, though to a less extent. Another similar mass was eight feet high. Figure 3 represents a block six feet high. and ten feet in its longest dinameter, seen on Waterland; it,l was unattached below, and lay with one end raised on a sn-al-,, ". Ser block. On Aratica, (Carlshoff,) others were observed. One loose mass like the last was.eight feet high and fifteen feet in diameter, and contained at least a thousand ctbic feet. Raraka also afforded examples of these attached and unattached blocks, -some standing with their tops six feet above high-water marlk. These masses are similar in character to many met with among tfhe fields of blocks just described, and differ only in having been left on the platform instead of being transported over it. Some,of them are near the margin of the reef, while others are quite:at its inner limit. The third mass figured above was a solid congilomerate, consisting of large fragments of Astrwas and Madrepores, and contained some imbedded shells, among which an OsItrrea and a Cypraea were noticed. This is their usual character.'The other two were parts of large individual corals, (Porites;)'but there was evidence in the direction of the cells that they did not stand as they grew; on the contrary, they had been upahrown, and were afterwards cemented with the material of the rock beneath -them, probably at the time this rock itself was con-:solidated. Below some of the loose masses like figure 3, (as on Aratica,) the platform was at times six inches higher than on,either side of the mass, owing to the protection from wear given to the surface beneath it. These blocks are always extremely rough and uneven, like those of the emerging land beyond; and'Ithe angular features are partly owing in both cases to solution from rains or from the sea-water that may be dashed over them. It should be distinctly understood that these masses here de-'scribed were found isolated, and only at considerable intervals. In no instance were they observed clustered. The loose blocks and those cemenlted below had the same general character, and must have been placed where they were by the sarnme cause, though it may have been at different periods. T'he shore of the lag.oon is generally low and gently inclined, yet in the larger islands, in which the waters of the lagoon are much disturbed by the winds, there is usually a beach resem-.bling that on the seaw,.ard side; though of less extent. A plat STRGCTURE OF CORAL ISLANDS. 31 form of reef-rccl at the same elevation as the shore platform sometimes extends out into the lagoon; but it is more common to find it a little submerged and covered for the most part with growing corals: and in either case, the balik terminates outward ill an abrupt descent of a few yards or fathoms, to a lower area of growing corals, or a bottom of sand. Still more commonly, we mneet with a sandy bottom,graduallv deepening from the shores without growing coral. These three varieties of condition are generally found in the sam-e lagoon, characterizing its different parts. The lower area of growing corals slopes outward, and usually ceases where the depth is 10 to 12 fathoms; from this there is another descent to the depth wh:ieh prevails. over the lagoon. On some small lagoons the shore is a thiclk plastic mud, either white or like clay, and forms a low flat which is very gently sloping. On HIenuake, these mnlud deposits are quite extensive, and of a white color. At Enderby's Island, another having a shallow lagoon, the mud was so deep and thick that there was, some difficulty in reaching the waters of the lagoon; the foot sunk in 8 or 10 inches and was not extricated without some difficulty. The color at this island was a dirty brownish clay. This mud is nothing but comminuted coral, so fine as to be almost impalpable. The lagoons of the smaller islands are usually very shallow; and in some, merely a dry bed remains, indicating the former existence of water. Instances of the latter kind are met with only in islands less than three miles in diameter; and those with shallow lagoons are seldom' much larger. These shallow waters, when direct communication with the sea is cut off, become, ii' some instances, very salt by evaporation, and contain no growing: coral, with few signs of life of any kind: and in other cases, they are made too fresh for marine life, through the rains. At, Enderby's Island the water was not only extremely saline, but the shores of the lagoon were in some places incrusted with salt.. But when there is an open channel, or the tides gain access over a bare reef, corals continue to grow, and a considerable portion of the lagoon may be obstructed by them. At Henuake, the. sea is shut out except at high water, and there were consequently but few species of corals, and those of small size. At Ahii (Peacock's Island) there was a small entrance to the lagoon, and though comparatively shallow, corals were growing over a large part of it. In the larger islands, the lagoons contain but small reefs compared with their whole extent; the greater part is an open sea, with deep waters and a sandy bottom. There are instances, as at the southern Maldives, of a depth of 50 and 60 fathoms. Twenty to thirty-five fathoms is the usual depth in the Paunmo 32 ON CORAL REEFS AND ISLANDS, tus. This was the result of Captain Beechey's investigations; anrid those of the Expedition, though few, correspond. It is however probable that deeper soundings would be found in the large island of Nairsa (Dean's). In the Tarawan Group, southeast of the Carolines, the depth, where examined by the Expedition, varied from 2 to 35 fathoms. Mr. Darwin found the latter depth at Keeling's Island. Charnisso found 25 to 35 fathoms at the Marshall Islands. The bottom of these large lagoons is very nearly uniform, varying but little except from the occasional abrupt shallowings produced by growing patches of reef. Soundings bring up sand, pebbles, shells, and coral mud; and the last mentioned material appears to be quite common, even in lagoons of considerable size. It has the same character as above described. The bluish clay-like mud of the harbor of Tongatabu may be classed with these deposits.* It appears, therefore, that the finer coral material of the shores prevails throughout the depths of the lagoon. The growing reefs within the lagoons, are in the condition of the irnner reefs about high islands. The corals grow but little disturbed by the waves, and the reef-rock often contains them in the position of growth. At Taputeouea (Kingsmills or Tarawan1 Group), reefs very similar to those of the Feejees occur; they present the same large Astrreas 10 to 12 feet in diameter, which once were growing where they stand but are now a part of the solid lifeless rock. Beachformrations of coral sandrock are common on the coral islands, and they present the same features in every respect, as those described. They were observed among the Paumotus, on Raraka, lionden, Kawehe, and other islands. The stratified character is always distinct and the layers slope toward the water at the usual small angle, amounting to 5-7 degrees bordering the lagoon, and 7-8 degrees on the seashore side of the land. They often occupy a breadth of 30 to 50 yards, appearing like a series of outcrops; yet not unfrequently they are mostly concealed by the sands of the beach,ed and it is probable that they generally underlie the loose surface material of the land. The rock is a fine or coarse sandrock, or a coral pudding-stone, and consists of beach materials. Occasionally it is quite compact, and resembles common limestone, excepting in its whiter color; but generally its sand origin is very apparent. The drift sandrock was not met with by the writer on any coral islands visited, and probably for the reason that opportuni-: Darwin describes this mud as occurring at the Min.ldives, and at Keeling Island, (op. cit. p. 26;) Kotzebue mentions it as common at the Marshall Atolls, and Lieutenant Nelson observed it at the Bermudas. ] On the northern atolls of the Maldives, the beach sandrock is said to be quarried out in square blocks and used for building. —-Jotrn. Gcog. Soc., v, 400. STRUCTURE OF CORAL ISLANDS. 33 ties were not favorable for a thorough exploration. It has been stated that the more exposed points towards the trades, especially the northeast and southwest, are commonly a little higher than other parts; and it is altogether probable that some of the sand heaps, there formed, will prove on examination to afford examples of this variety of coral-rock. Such situations are exactly identical with those on Oahu, where they occur on so remarkable a scale. Mr. R. H. Schonmburgh states that on the island of Anegada in the West Indies, the drift banks on the windward shores are forty feet in height.* Although in these descriptions of atolls, we have dwelt on some points more at length than when describing barrier reefs, still it will be observed that the former have no essential peculiarities of structure apart from such as necessarily arise from the absence of high rocky lands. The encircling atoll-reef, corresponds with the outer reefs that enclose high islands; and the green islets with the beach formations, in the two cases, originate in the samne manner. The lagoons, moreover, are similar in character and position to the inner channels within barrier reefs; they receive only coral material from the action of' degrading agents, because no other source of detritus but the reefs is at hand. The accumulations going on within them are, therefore, wholly of coral. The reefs within the lagoons, correspond very exactly in mode of growth and other characters to the inner reefs under the lee of a barrier. The corals grow but little disturbed by the waves, and the reefrock thus fortned, often contains them in their natural positions. The preceding descriptions represent the general character of atolls, but are more especially drawn front the Paumlotus. There are some peculiarities in other seas, to which we may briefly allude. Among the scattered coral islands north of the Samoan Group, the shore platform is seldom as extensive as at the Paumotus. It rarely exceeds fifty yards in width, and is cut up by passages often reaching almost to the beach. It was not unusual for our boats to obtain a landing by watching for a favorable opportunity at the entrance of one of these channels to mount a wave and ride in on its top. In some places the platform is broken into islets. Enderby's Island is one of the number to which this description applies: the beach is eleven or twelve feet high. For the first eight feet, it slopes very regularly at an angle of 30 to 35 degrees, and consists of sand, coarse pebbles, or rounded stones of coral, with some shells; and there is the usual beach conglomerate near the water's edge. After this first slope, it is hori* Journal of the Royal Geographical Society, ii, 152. Mr. Schomburgh describes the sandhills as 40 feet in height, and behind the first range, a second, and even a third. 5 34 ON CORAL REEFS AND ISLANDS. zontal for eighty to two hundred feet, and then there is a gradual rise of three to four feet. Over this portion there are large slabs of the beach conglomerate, along with masses from the reef-rock, and some thick plates of a huge foliaceous Madrepora; anrd these slabs, many of which are six feet square, lie inclining quite regularly against one another, as if they had been takeni up and laid there by hand. They incline in the same direction with the slope of the beach. The large Madrepora alluded to has the mode of growth of the Madrepora palmata; aind probably the entire zoophyte extended over an area twelve or fifteen feet in diameter. The fragments are three to four inches thick, arcd thirty square feet in surface. As a key to the explanation of the peculiarities here observed, it may be remarked that the tides in the Paiumotus are two to three feet, and about Enderby's Islalndl five to six feet in height. Itlaldives.-C(hargos Bank.-The Maldives have been often appealed to in illustration of coral structures. They are particularly described by Mr. Darwin, from information communicated to hkiJm by Captain Moresby, and from the charts of this officer and Lienltenant Powell.* The point of special interest in their structure is the occurrence of atolls or rings within the larger atolls. The islets of the lagoon, and those of the encircling reef, are in many instances annular reefs, each with its own little lake. Gems witlhin gems are here clustered together. The annular islets of the main encircling reef are oblong, and lie with the longest diameter, which is sometimes three miles long, in the line of the reef. Those of the lagoon are generally less than two miles across. Th e lagoons they contain vary fron five fathoms or less to twelve fathoms in depth. ~The Maldives are among the largest atoll-reefs known; and they are intersected by many large open channels; and Mr. Darwin observes, that the interior atolls occur only near these charinels, where the sea has free access. We may view each large island in the archipelago as a sub-archipelago of itself. Although thus singular in their features, they illustrate no new principles with regard to reef-formations.t * Darwin on Coral Reefs, p. 32. See also Journal of the Royal Geographical Society, on the Geography of the 3Ialdives, by J. J. Horsburgh, ii, p.'2; and by Captain W. F. W. Owen, ibid, p. 81; also vol. v, p. 398, on the Northern Atolls of the Maldives, by Captain Moresby. - Mr. Darwin thus remarks, (Op. cit. pp. 33, 34,)-" 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 characterized atolls, excepting that the ring-formed reefs are based on a shallow foundation instead of the floor of the open sea, and that instead of being scattered irregularly, they are grouped closely together."-" It appears from the charts on a large scale, that the ring-like structure is contingent on the marginal channels or branches being wide, and consequently on the whole interior of the atoll being freely exposed to the waters of the open sea. When the STRUCTURE OF CORAL ISLANDS. 35 The Chagos Banzk lies about ten degrees south of the Maldives, and is ninety miles long and seventy broad. The rim is mostly submerged from five to ten fathoms. Mr. Darwin confirms the opinion of Captain Moresby, that this bank has the character of a lagoon reef, resembling one of the fMaldives; and he states on the evidence of extensive soundings, that, if raised to the surface, it would actually become a coral island, with a lagoon forty fathoms deep. In the words of Capt. Moresby, it is ill truth nothing more than a half-drowned atoll.* Metia and otler elevated Coral Islands. —In the Chagos Group we have an example of a sunken coral atoll. Metia affords an instance of one that has been elevated by some force; and several such are met with in the Pacific. Metia, or Aurora Island, is one of the western Panmotus. It is a small island about foulr miles by two and a half in width, and two hundred and fifty feet in height; and it consists throughout of coral limestone. As we approached it from the northeast, its high vertical cliffs were supposed to be basaltic, and had mluch resemblance to the Palisades of the Hudson.t- This appearance of a vertical structure was afterwards traced to vertical furrowings by the waters dripping down its front, and the consequent formation of stalagmnitic incrustations. Deep caverns were also seen. The cliff, though vertical in some parts, is roughly sloping in others, and on the west side, the surface of the island gradually declines to the sea. The rock was found to be a white and solid limestone, seldom presenting any traces of its coral origin. In some few layers there were disseminated corals, looking like imbedded fossils, along with beautifutl casts of shells; but for the most part it was as compact as any secondary marble, and as uniform in texture. Occasionally there were disseminated spots of crystallized calcspar. The caverns presented us with coarse stalactites, some of which were six feet in diameter; and interesting specimens were obtained containing recent land shells, which 1had been enclosed while hibernating.T channels are narrow, or few in number, although the lagoona be of great size and depth, (as in Suadiva,) there are no ring-formed reefs; where the cllhannels 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, ahlmost every reef throughout the atoll is more or less perfectly ring-formed. Although their presence is thus contingent on the openness of the nlarginal 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." * Darwin, op. cit., p. 39. ~ For a sketch of this islancl, see Narrative Exp. Exp., vol. i, p. 338.:~. It is probable t.hat more extensive caverns would have been found, had there been more than a few hours for the examination of the island. The Rev. Mr. Williams, in his work on Missionary Enterprises in the Pacific, gives very interesting 36 ON CORAL REEFS AND ISLANDS. The surface of the island is singularly rough, owing to erosion by rains. The paths that cross it wind through narrow passages among ragged needles and ridges of rock as high as the head, the peaks and narrow defiles forming a miniature model of the grandest Alpine scenery. There is but little soil, yet the island is covered with trees and shrubbery. The shores, at the first elevation of the island, must have been worn away to a large extent by the sea; and the cliff and some isolated pinnacles of coral rock still standing on the coast are evidence of the degradation. But at present there is a wide shore platform of coral reef, two hundred or two hundred and fifty feet wide, resembling that of the low coral islands, and having growing coral as usual about its margin and in the shallow depths beyond. In the face of the cliff there are two horizontal lines, along which cavities or caverns are most frequent, which consequently give an appearance of stratification to the rock, dividing it into three nearly equal layers. We might continue this account of coral reefs and islands, by particular descriptions of those visited by the Expedition. But the similarity among them is so great, and their peculiarities are already so fully detailed, that this would amount only to a succession of repetitions. And moreover the facts will be found in the geographical report by Captain Wilkes, and are to a great extent well exhibited on the map of the Paumotus and on the other valuable charts of the Expedition. The characters of a few briefly stated will suffice in this place. We commence with the smallest. Jarvis's Island. —(Fig. 4, page 23.) Lat. 0~ 22' S. Long. 159~ 31' W. Length 13 miles trending east and west. No lagoon. Shape triangular. A low sandy flat, eighteen or twenty feet high, without trees, and partly covered with small shrubs. A high sloping beach continuous around. Trends east and west. We did not land on this island. Birnie's. —Lat. 30 35' S. Long. 1710 39' W. Four-fifths of a mile by one-third, trending northwest. No lagoon. A sandy flat about ten feet high, except near the north-northeast extremity, descriptions of caverns in the elevated coral rock of Atiu, one of the Hervey Group.* In one, he wandered two hours without findingo a termination to its windings, passing' through chambers with "fretwork ceilings of stalagmite and stalactite columns, which,'nicd the darlmess, sparkled brilliantly with the reflected torch-light." This author remarks, "that while the madrepores, the brain, and every other species of coral are full of little cells, these islands, (including those resembling Atiu,) appear to be solid masses of compact limestone, in zolwich nothing like a cell can be detected." Beechey, in his description of Henderson Island, another of this character, speaks of the rock as compact, and having the fracture of a secondary limestone. * Wateoo of Cook. STRUCTURE OF CORAL ISLANDS. 37 where it is about twelve feet. To the south-southwest the submerged reef extends out nearly a mile, over which the sea breaks. Distinguished no vegetation except the low purslane and some trailing plants. Did not land. Swain's.-(Fig. 3, page 23.) Lat. 110 10' S. Long. 1700 52' W. 1l miles by 2-; shape nearly rectangular; trends east and west. No lagoon, but the centre a little lower than the sides. Surface covered with shrubbery and large trees, among the latter many cocoanuts; the centre more sparsely wooded. Height fifteen to eighteen feet, excepting on the middle of western side, where the surface is covered with loose fragments of coral of small size; there appears to have been a former entrance to the lagoon at this place. Shore reef or platform, one hundred yards in average width, and one hundred and fifty yards at the place where we landed. Beach high, ten to twelve feet. At lower part of beach for a height of two to three feet, the coral reef rock was exposed, indicating an elevation of the island. For three or four feet above this, layers of the beach sandrock were often in view, consisting of coral pebbles firmly cemented, and having the usual dip of seven or eight degrees seaward; in many places it was concealed by the beach sands and pebbles. There was no growing coral on the platform excepting Nullipores. The outer margin of this platform was very uneven, and much intersected by channels, though less so than at Enderby's Island.* Great numbers of Birgi, (large Crustacea,) were burrowing over the island, some of which were six inches in breadth. Otultu, Paumotu Archipelago.-140 5' S. 1410 30' W. 12 miles by-2, trending north and south. No lagoon. Wooded. Margaret, Paumotu Archipelago.-200 42' S. 1430 4' W. Diameter one mile, nearly circular. A small shallow lagoon with no entrance. Northeast side alone wooded, and in two patches. Teku or Four Crowns, Paumotu Archipelago.-200 28' S. 1430 18' W. Diameter 1i4 miles, nearly circular. A small lagoon with no entrance. Southwestern reef bare; five patches of forest on the other part. * The sea was quite heavy when we attempted to land at low tide upon the edge of the shore platform. As we pulled towards the reef, an anchor was dropped, as usual, some distance out, to hold on and save the boat, from being carried by the surges against the rocks. After some heavy seas had passed, a partial lull seemed to favor, and the boat was pulled in. Taking advantage of the favorable moment, I jumped out, and made rapid speed over the, reef to escape the breakers which followed. Soon turning about, I was surprised to find the boat just behind me, and the crew in the water alongside trying to steady her and save her from destruction. The man who held to the anchor behind had let go his hold, and the next sea, as it ocame careering on, had borne the boat over the edge of the reef, and far on its surface. With even greater risk, after our ramble was completed, we succeeded in launching again and reached the open sea. This was one of many similar dangers experienced in these seas. 38 ON CORAL REEFS AND ISLANDS. Washington Jsland.-Lat. 40 41' N. Long. 160O 15' W. 3 miles by 1-, trending east and west. It is a dense cocoanut grove with luxuriant shrubbery. No lagoon. The shore platform is rather narrow. A point of submerged reef one and a half miles long stretches out from southwest end. Could not land on account of bad weather. Enderby's.-30~' S. 171~ 16' W. 2" miles by 1 mile nearly, trending N. N. W. and S. S. E.; form trapezoidal or nearly rectangular. Little vegetation on any part, and but few trees. The lagoon very shallow and containing no growing coral; its shores a coral mud, allowing the foot to sink in eight or ten inches, and covered in places with saline incrustations. Shore platform one hundred feet or less in width, and surface inclined outward at a very small angle; covered with three or four feet of water at high tide, and with few corals or shells; beyond this, falls off four to six feet, and then the bottom gradually inclines for one hundred yards or more. The beach very high and regular; rises eight feet, at an inclination of thirty to thirty-five degrees; then horizontal for eighty to two hundred, after which another rise of three or four feet. It consists below of pebbles and fine sand, but above of slabs and blocks of coral rock and the beach sand-.rock, those of the latter nearly rectangular and flat. This beach sandrock occurs in layers from ten to twenty inches thick along the shore, and is inclined from five to seven degrees seaward. Some portions are very compact, and ring under the hammer, while others enclose fragments of different sizes to a foot or more in diameter. The most common coral of the beach was an Astr ma with small cells, (near A. cerium, D.-the specimens were afterwards lost.) There were also other Astreas, a large lamellar Madrepore, (M. cyclopea,) some fragments of which were six feet square and three inches thick; also Meandrine, Porites, &c. Large trunks of transported trees lay upon the island, one of which was forty feet long and four ill diameter. T'he shore platform was much intersected by channels. Captain Hudson obtained:soundings half a mile off in two hundred fathoms; the lead struck upon a sandy bottom but was indented by coral. Honden, or Henuake, Paumotu Archipelago.-Size 3- miles by 2 miles. Oblong, five-sided; trending west-northwest. A small shallow lagoon, communicating wvith the sea only at high tide, on the west side. There are two other entrances, which are seldom if ever covered with water, and appeared merely as dry beds of coral rock. Height of the island, twelve feet: lowest on the south side. Belt of verdure complete, and consisting of large forest trees, with the Pandanus and other species, but no cocoanuts; its breadth ~ mile, and in some parts ~-. Among the trees, large masses of coral rock often exposed to view, and the STRUCTURE OF COIAL ISLANDS. 39 surface in many parts very rough. It seenled surprising at all these islands to find so luxuriant a growth of trees and shrubbery over so rocky a surface. Shores of the lagoon nearly flat. On one side there was a large area of extremely fine coral sand and mud, which extended a long distance into the lagoon. Elsewhere about the centre of the island, the reef-rock was bare, and contained numerous shells of Tridacnue. A few small Madrepores still growing in the lagoon. Beach on the sea-shore side eight feet high. In lowver part of beach, several layers of white limestone, (the beach sandrock,) formed of coral fragments or sand, shells, &c., rmuch of which was very compact. Tlphe layers inclined towards the sea at an angle of about five degrees. Shore platform as elsewhere in this archipelago. The facts above stated are evidence of a slight elevation, not exceeding two or three feet. TCaiarta, or King's, Paumotu Archipelago.-150 42' S.; 1440 46' WV. 2-.miles by 1-a, trending northwest. A small lagoon with no entrallce. Reef almost continuously wooded around, somea what broken into patches. lIaraki, Tarawan or Kingsmills Group.-5 miles by 2, and having a lagoon. Trending north. Shape oblong triangular. Belt of forest complete. Appearance of a former entrance toy the lagoon on the east side. Whytathu, one of the two Disappointment Islands, Paumotu Archllipelago.- 14~ 10' S., 1410 24' YTW. 5~ by 2 miles, trending northwest. The reef fronting northeast almost continuously wooded. On the opposite side, three islets, one of rather large' size. 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