[Geology.] On Coral Reefs and Islands ... From the author's Exploring Expedition Report on Geology, with additions

Front matter

ON CORAL REEES AND ISLANDS BY JAMES D. DANA, GEOLOGIST OF THE EXPLORING EXPEDITION ABOUND THE WORLD DURING THE TEARS 1838 1842, C. WILKES, U. S. N., COMMANDER. " 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." TUUL1A, J. C. PALMER, U. S. N., EXPL. EXP. FROM 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_tl_ep__istrict 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 1 Structure of Reef Formations, 9 Outer Reefs, 9 Inner Reefs, 11 Channels among Reefs, IS 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, 65 Causes influencing the Growth of Coral Zoophytes, 59 Rate of Growth of Zoophytes, CONTENTS.

Chapter III. Formation Of Reefs And Causes Of Their Features And Geo

GRAPHICAL 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 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.

Chapter I. Structure Of Coral Reefs And 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 intersect ed by irregular boat channels, or occasionally encloses large bays, affording harbor protection to scores of ships.

In very many in stances the reef stands at a distance from the shores like an artificial mole, 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 ofthe barrier; but the still waters of a lake pre vail within, affording safe navigation for the tottling canoe some times 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 along the barrier for miles, where the action of the sea has raised the coral structure above the waves. 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 chan nel 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. Coral 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 sur face.

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 be yond, — the white coral beach, the massy foliage of the grove, and the embosomed 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 in termingled, 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 BEEFS. of bare reef; and through one or more of these intervals, a ship channel 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 par ticular 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, render ing 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 Archi pelago 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 Angau, 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 exam ples 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 occa sionally it may be confined to a single point of an island.

ON CORsVL HEEFS AND ISLANDS. Above Angau 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 Lakemba 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 dis tance 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 ex tent 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 sin gle rock above the surface of the enclosed lagoon, we shall en deavor 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 continuous or interrupted : they may (3) constitute throughout a distant barrier ; or (4) the reef may be fringing in one part and a barrier in another ; or (5) it maybe fringing alone: the barrier maybe (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. A wide difference in the extent of reefs would be inferred from 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 en closing 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 Cale donia, 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 much 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 lo the tropical circle; and the channel within is in some 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 unfathomable within a short distance of the line of breakers. b. Structure of Reef Formations. In the description of reef grounds or reef-formations there are several distinct subjects for consideration, as is obvious from the preceding remarks. These are — 1. Outer reefs, or reefs formed from the growth of corals ex posed to the open seas. Of this character, are all proper barrier reefs, and such fringing reefs as are unprotected by a barrier. 2. Inner 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 de tritus either from the reefs, or the shores, or from both of these sources combined. 4. Beaches and beach formations, produced by coral accumula tions 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. — The outer reefs or flats of coral rock receive the waves along their margin ; and the outline exposed to this action is very much cut up with deep channels which give passage to the advancing waters, and to the currents that flow back in prep aration for the next breaker. This margin, which we have said rises but little above low-tide level, usually 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 difference in the rapidity with which the water deepens, as might 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 op posite 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 within 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 only 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 commonly raised a few inches above the general surface, and is, therefore, the first part laid bare by the retreating tide, although a dangerous place for a ramble, on account ofthe heavy breakers. Though very uneven, the surface has generally a smooth, water-worn appearance, and is spotted with various shades of pink aud purple. These colors, as observ ed by Chamisso, are due to incrusting Nullipores, that grow like lichens over the rock : they are vegettible in nature, though com posed mostly of lime. Other nodular and branching Nullipores, some sprigs of Madrepores, and a few of Astræas 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 actiniæ.

The gradual wear of the reefs by the wash of the sea is prevented, to a great extent, by these Nul lipore incrustations, as was pointed out by Darwin.* He states that on Reeling's Island they constitute a layer two or three feet in thickness, with a breadth of twenty feet. They are abundant on the Paumotu reefs. The outer reefs are distinguished in many parts from the inner by becoming covered with accumulations of coral fragments and sand, which are thrown up by the waves: finding a lodgment some distance back from the margin of the reef, the accumula tions gradually increase, till in many instances they form dry land, and prepare the way for vegetation. Such effects are mostly con fined, 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 for ests 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. The rock of the reef, wherever broken, exhibits a compact tex ture. In some parts it consists of coral fragments of quite large size firmly cemented : 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 limestone 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 charac ter as here described, a texture indicating a detritus origin. This reef-rock is formed in the midst of the waves ; and we shall here after show that to this fact it owes many of its peculiarities. Be sides corals, the shells of the seas contribute to it, and it some times contains them as fossils, along with bones of fish, exuviæ of crabs, spines and fragments of echini, and other remains of or ganic life inhabiting reef-grounds. Inner reefs. — In the still waters of the inner channels or la goons, when of large extent, we find corals growing in their great est perfection, and the richest views are presented to the explorer of coral scenery. There are many regions — in the Feejes, examples are common — 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. In the general appearance of the surface, however, they much resemble the outer reefs. They are neatly 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 abrupt ; yet there is every variety, from the gradually sloping bed of corals to the bluff declivity with its clinging clumps. In different parts, there are many portions still under water at the low est tides; and here, (as well as upon the outer banks,) fine fish ing 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, with fishes of fantastic colors, starfish, echini, and 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 reefs, 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 different elevations; and though at times but a few yards in breadth, there is often alongside of them a depth of many fathoms. They sometimes 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 the 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. The rock of the inner reefs is peculiar in being but sparingly fragmentary. 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 become 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 standing in their growing positions are common. Though now mere 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 Tongatabu were twen ty-five feet in diameter; and Astræas and Meandrinas, both there and in the Feejees, measured twelve to fifteen feet. These corals, when growing beneath the water, form solid hemispheres, or i-fliinded 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 Astræas and Porites in the reef-rock referred to. The platform looks like a Cyclopean pavement, ex cept 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 branching corals, with the intervals filled in by sand and small fragments; for even in the more still waters fragments are to some extent 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, ex cept 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 im perfectly cemented. It is the material of the large church at Honolulu. impure or fresh waters and currents often operate to retard their growth. Owing to the last mentioned cause, the inner reefs are not usu ally 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 remarka ble as the country is a plain, the site of a Feejee village, and a mile or two back stands a high bluff.

On an island off this part of Vanua 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-sands. From these descriptions it appears that the main distinction between the inner and outer reel's consists in the less fragmentary character of the rock in the former case, the less frequent accumu lations of debris on their upper surface, and the more varied features and slopes of the margin. Moreover, the Nullipores, 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 same quiet waters, and under like circumstances.

The variety of coral zoophytes is also greater in the stiller waters, and there are species peculiar to the different regions, as explained in another plaee. Channels among reefs. — To complete this review of the gen eral appearance and constitution of reef formations, it remains to add some particulars respecting the channels which intervene be tween 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. The 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 ofthe 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 same time exhib its 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 OX 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 chan nel, 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. The island of Upolu, 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. PART OK THE NORTH SHORE OF TAHITI. The bottom of the channels or lagoons takes its charac ter, as regards the material constituting it, either from the reefs, a source of calcareous sand and 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 conse quently no shore detritus mingled with the reef debris. At Ta hiti, the sounding lead usually brought up sand, shells, and frag ments of coral. At Tongatabu, the bottom, where the Peacock anchored, was a grayish blue mud, appearing as plastic as com mon clay ; it consisted solely of comminuted coral and shells, with coloring matter probably from vegetable decomposition. To the west of the larger Feejee islands, soundings commonly indicated a bottom of basaltic mud, and this material was frequently brought up with our dredges. On the north side of Va nua Lebu, a stream has so filled with its detritus the wide chan nel 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.