Chapter XXII
Keeling Island--Singular appearance--Scanty Flora--Transport of seeds--Birds and insects--Ebbing and flowing springs--Coral formations resisting power of ocean--Fields of dead coral--Stones transported by roots of trees--Great crab--Stinging corals --Structure of lagoon islands--Encircling and Barrier reefs--General proofs of subsidence in the Pacific--Theory of lagoon islands caused by subsidence of the land--Pacific and Indian oceans divided into alternate areas of elevation and subsidence--Points of eruption lie within the areas of elevation.
CORAL FORMATIONS.
APRIL 1ST.--We arrived in view of the Keeling or Cocos Islands, situated in the Indian ocean, and about six hundred miles distant from the coast of Sumatra. This is one of the lagoon islands of coral formation, similar to those we passed in the Dangerous Archipelago. An excellent idea of the general appearance of these extraordinary rings of land, which rise out of the depths of the ocean, may be obtained from the characteristic sketch of Whitsunday Island, in Beechey’s Voyage.
When the ship was in the channel at the entrance, Mr. Liesk, an English resident, came off in his boat. The history of the inhabitants of this place, in as few words as possible, is as follows. About nine years ago, a Mr. Hare, a very worthless character, brought from the East Indian archipelago a number of Malay slaves, which now, including children, amount to more than a hundred. Shortly afterwards, Captain Ross, who had before visited these islands in his merchant-ship, arrived from England, bringing with him his family and goods for settlement. Along with him came Mr. Liesk, who had been a mate in his vessel. The Malay slaves soon ran away from the island on which Mr. Hare was settled, and joined Captain Ross’s party. Mr. Hare upon this was ultimately obliged to leave these islands.
The Malays are now nominally in a state of freedom; and certainly are so, as far as regards their personal treatment; but in most other points they are considered as slaves. From the discontented state of the people, the repeated removals, and; perhaps, also from a little mismanagement, things are not very prosperous. The island has no quadruped, excepting the pig, and no vegetable in any quantity excepting the cocoa-nut. On the latter the whole prosperity of the place depends. The only exports are oil from the nut, and the cocoa-nut[1] itself. On it the pigs, which are loaded with fat, almost entirely subsist, as likewise do the poultry and ducks. Even a huge land-crab is furnished by nature with a curious instinct and form of legs to open and feed on this same fruit.
The annular reef of this lagoon island is surmounted in the greater part of its length by linear islets. On the northern or leeward side there is an opening, through which vessels reach the anchorage. On entering, the scene was very curious and rather pretty; its beauty, however, being solely dependant on the brilliancy of the surrounding colours. The shallow, clear, and still water of the lagoon, resting in its greater part on white sand, is, when illuminated by a vertical sun, of a most vivid green. This brilliant expanse, several miles in width, is on all sides divided, either from the dark heaving water of the ocean by a line of snow-white breakers, or from the blue vault of heaven by the strips of land, crowned at an equal height by the tops of the cocoa-nut trees. As a white cloud here and there affords a pleasing contrast with the azure sky, so in the lagoon, dark bands of living coral appear through the emerald green water.
The next morning after anchoring, I went on shore on Direction Island. The strip of dry land is only a few hundred yards wide; on the lagoon side we have a white calcareous beach, the radiation from which in such a climate is very oppressive; and on the outer coast, a solid broad flat of coral rock, which serves to break the violence of the open sea. Excepting near the lagoon where there is some sand, the land is entirely composed of rounded fragments of coral. In such a loose, dry, stony soil, the climate of the intertropical regions alone could produce a vigorous vegetation. On some of the smaller islets, nothing could be more elegant, than the manner in which the young and full-grown cocoa-nut trees, without destroying each other’s symmetry, were mingled into one wood. A beach of glittering white sand formed a border to these fairy spots.
I will now give a sketch of the natural history of these islands, which, from its very paucity, possesses a peculiar interest. The cocoa-nut tree, at the first glance, seems to compose the whole wood; there are, however, five or six other kinds. One of these grows to a very large size, but, from the extreme softness of its wood, is useless; another sort affords excellent timber for ship-building. Besides the trees, the number of plants is exceedingly limited, and consists of insignificant weeds. In my collection, which includes, I believe, nearly the perfect Flora, there are twenty species, without reckoning a moss, lichen, and fungus. To this number two trees must be added; one of which was not in flower, and the other I only heard of. The latter is a solitary tree of its kind in the whole group, and grows near the beach, where, without doubt, the one seed was thrown up by the waves. I do not include in the above list, the sugar-cane, banana, some other vegetables, fruit-trees, and imported grasses. As these islands consist entirely of coral, and at one time probably existed as a mere water-washed reef, all the productions now living here, must have been transported by the waves of the sea. In accordance to this, the Flora has quite the character of a refuge for the destitute: Professor Henslow informs me, that of the twenty species, nineteen belong to different genera, and these again to no less than sixteen orders!
In Holman’s[2] Travels, an account is given on the authority of Mr. A. S. Keating, who resided twelve months on these islands, of the various seeds, and other bodies, which have been known to have been washed on shore. “Seeds and plants from Sumatra and Java have been driven up by the surf on the windward side of the islands. Among them have been found the Kimiri, native of Sumatra and the peninsula of Malacca; the cocoa-nut of Balci, known by its shape and size; the Dadass, which is planted by the Malays with the pepper-vine, the latter intwining round its trunk, and supporting itself by the prickles on its stem; the soap-tree; the castor-oil plant; trunks of the sago palm; and various kinds of seeds unknown to the Malays who settled on the islands. These are all supposed to have been driven on shore by the N.W. monsoon to the coast of New Holland, and thence to these islands by the S.E. trade-wind. Large masses of Java teak, and yellow-wood, have also been found, besides immense trees, of red and white cedar, and the blue gum-wood of New Holland, in a perfectly sound condition. All the hardy seeds, such as creepers, retain their germinating power, but the softer kinds, among which is the mangostin, are destroyed in the passage. Fishing-canoes, apparently from Java, have at times been washed on shore.” It is interesting thus to discover how numerous the seeds are, which, coming from several countries, are drifted over the wide ocean. Professor Henslow tells me, he believes that nearly all the plants which I brought from this island, are common littoral species in the East Indian archipelago. From the direction, however, of the winds and currents, it seems scarcely possible that they can have come here in a direct line. If, as suggested with much probability by Mr. Keating, they have first been carried towards the coast of New Holland, and thence drifted back again, together with the productions of that country, the seeds, before germinating, must have travelled between 1800 and 2400 miles.
Chamisso,[3] when describing the Radack Archipelago, situated in the central part of the Western Pacific, states that, “The sea brings to these islands the seeds and fruits of many trees, most of which have yet not grown here. The greater part of these seeds appear to have not yet lost the capability of growing.” It is also said that trunks of northern firs are washed on shore, which must have been floated from an immense distance. These facts are highly interesting. It cannot be doubted, if there were land-birds to pick up the seeds when first cast on shore, and a soil more adapted for their growth than the loose blocks of coral, that such islands, although so isolated, would soon possess a more abundant Flora.
The list of land-animals is even poorer than that of plants. Some of the islets are inhabited by rats; and their origin is known to be due to a ship from the Mauritius, which was wrecked here. These rats have rather a different appearance from the English kind; they are smaller and much more brightly coloured. There are no true land-birds; for a snipe and a rail (Rallus phillippensis), though living entirely among the dry herbage, belong to the order of Waders. Birds of this order are said to occur on several of the low islands in the Pacific. At Ascension a rail (Porphyrio?) was shot near the summit of the mountain; and it was evidently a solitary straggler. From these circumstances, I believe, the waders are the first colonists of any island, after the innumerable web-footed species. I may add, that whenever I have noticed birds, which were not pelagic, very far out at sea, they always belonged to this order; and hence they would naturally become the earliest colonist of any distant point.
Of reptiles, I saw only one small lizard. Of insects, I took pains to collect every kind. Exclusive of spiders, which were numerous, there were thirteen species.[4] Of these, one only was a beetle. A small species of ant, swarmed by thousands under the loose dry blocks of coral, and was the only true insect which was abundant. Although the productions of the land are thus scanty; if we look to the waters of the surrounding sea, the number of organic beings is indeed infinite.
Chamisso[5] has described the natural history of Romanzoff, a lagoon island in the Radack Archipelago. The number and kind of productions there is very nearly the same with those here. One small lizard was seen: wading birds (Numenius and Scolopax) were numerous, and very tame. Of plants, he states there were nineteen species (including one fern); and some of them are the same species with those I collected here, although on an island situated in a different ocean.
These strips of land are raised only to that height, to which the surf can throw fragments, and the wind heap up sand. Their protection is due to the outward and lateral increase of the reef, which thus breaks the sea. The aspect and constitution of these islets at once call up the idea, that the land and the ocean are here struggling for mastery: although terra-firma has obtained a footing, the denizens of the other element think their claim at least equal. In every part one meets hermit-crabs of more than one species,[6] carrying on their backs the houses they have stolen from the neighbouring beach. Overhead, the trees are occupied by numbers of gannets, frigate-birds, and terns. From the many nests and smell of the atmosphere, this might be called a sea-rookery. The gannets, sitting on their rude nests, look at an intruder with a stupid, yet angry air. The noddies, as their name expresses, are silly little creatures. But there is one charming bird; it is a small and snow-white tern, which smoothly hovers at the distance of an arm’s length from your head; its large black eye scanning with quiet curiosity your expression. Little imagination is required to fancy, that so light and delicate a body must be tenanted by some wandering fairy spirit.
SUNDAY, APRIL 3D.--After service I accompanied Captain FitzRoy to the settlement, situated at the distance of some miles, on a point thickly scattered over with tall cocoa-nut trees. Captain Ross and Mr. Liesk live in a large barn-like house open at both ends, and lined with mats made of woven bark. The houses of the Malays are arranged along the shore of the lagoon. The whole place had rather a desolate aspect, because there were no gardens to show the signs of care and cultivation. The natives belong to different islands in the East Indian Archipelago, but all speak the same language: we saw inhabitants of Borneo, Celebes, Java, and Sumatra. In the colour of their skin they resemble the Tahitians, nor do they widely differ from them in form of features. Some of the women, however, showed a good deal of the Chinese character. I liked both their general expression and the sound of their voices. They appeared poor, and their houses were destitute of furniture; but it was evident, from the plumpness of the little children, that cocoa-nuts and turtle afford no bad sustenance.
On this island the wells are situated from which ships obtain water. At first sight it appears not a little remarkable that the fresh water should regularly ebb and flow with the usual tide.[7] We must believe that the compressed sand or porous coral rock acts like a sponge; and that the rain-water which falls on the ground being specifically lighter than the salt merely floats on its surface, and is subject to the same movements. There can be no actual attraction between salt and fresh water, and the spongy texture must tend to prevent all admixture from slight disturbances. On the other hand, where the foundation consists only of loose fragments, upon a well being dug, salt or brackish water enters; of which fact we saw an instance on this same island.
After dinner we stayed to see a half superstitious scene acted by the Malay women. They dress a large wooden spoon in garments, carry it to the grave of a dead man, and then at the full of the moon they pretend it becomes inspired, and will dance and jump about. After the proper preparations, the spoon held by two women became convulsed, and danced in good time to the song of the surrounding children and women. It was a most foolish spectacle, but Mr. Liesk maintained that many of the Malays believed in its spiritual movement. The dance did not commence till the moon had risen, and it was well worth remaining to behold her bright globe so quietly shining through the long arms of the cocoa-nuts, as they waved in the evening breeze. These scenes of the tropics are in themselves so delicious, that they almost equal those dearer ones to which we are bound by each best feeling of the mind.
The next day I employed myself in examining the very interesting yet simple structure and origin of these islands. The water being unusually smooth I waded in as far as the living mounds of coral, on which the swell of the open sea breaks. In some of the gullies and hollows there were beautiful green and other coloured fishes, and the forms and tints of many of the zoophytes were admirable. It is excusable to grow enthusiastic over the infinite numbers of organic beings with which the sea of the tropics, so prodigal of life, teems; yet I must confess I think those naturalists who have described in well-known words the submarine grottoes decked with a thousand beauties, have indulged in rather exuberant language.
APRIL 6TH.--I accompanied Captain FitzRoy to an island at the head of the lagoon: the channel was exceedingly intricate, winding through fields of delicately branched corals. We saw several turtle, and two boats were then employed in catching them. The method is rather curious: the water is so clear and shallow, that although at first a turtle quickly dives out of sight, yet in a canoe, or boat under sail, the pursuers after no very long chase come up to it. A man standing ready in the bows, at this moment dashes through the water upon the turtle’s back; then clinging with both hands by the shell of its neck, he is carried away till the animal becomes exhausted and is secured. It was quite an interesting chase to see the two boats thus doubling about, and the men dashing into the water trying to seize their prey.
When we arrived at the head of the lagoon, we crossed the narrow islet and found a great surf breaking on the windward coast. I can hardly explain the cause, but there is to my mind a considerable degree of grandeur in the view of the outer shores of these lagoon islands. There is a simplicity in the barrier-like beach, the margin of green bushes and tall cocoa-nuts, the solid flat of coral rock, strewed here and there with great fragments, and the line of furious breakers, all rounding away towards either hand. The ocean throwing its waters over the broad reef appears an invincible, all-powerful enemy, yet we see it resisted and even conquered by means which at first seem most weak and inefficient.
It is not that the ocean spares the rock of coral; the great fragments scattered over the reef, and accumulated on the beach, whence the tall cocoa-nut springs, plainly bespeak the unrelenting power of its waves. Nor are there any periods of repose granted. The long swell, caused by the gentle but steady action of the trade-wind always blowing in one direction over a wide area, causes breakers, which even exceed in violence those of our temperate regions, and which never cease to rage. It is impossible to behold these waves without feeling a conviction that an island, though built of the hardest rock, let it be porphyry, granite, or quartz, would ultimately yield and be demolished by such irresistible forces. Yet these low, insignificant coral islets stand and are victorious: for here another power, as antagonist to the former, takes part in the contest. The organic forces separate the atoms of carbonate of lime one by one from the foaming breakers, and unite them into a symmetrical structure. Let the hurricane tear up its thousand huge fragments; yet what will this tell against the accumulated labour of myriads of architects at work night and day, month after month. Thus do we see the soft and gelatinous body of a polypus, through the agency of the vital laws, conquering the great mechanical power of the waves of an ocean, which neither the art of man, nor the inanimate works of nature could successfully resist.
We did not return on board till late in the evening, as we staid some time in the lagoon collecting specimens of the giant Chama, and looking at the coral fields. Near the head of the lagoon I was much surprised to find a wide area, considerably more than a mile square, covered with a forest of branching coral, which though standing upright was all dead and rotten. At first I was quite at a loss to understand the cause; afterwards it occurred to me that it was owing to the following rather curious combination of circumstances. It should, however, first be stated, that corals are never able to survive even a short exposure in the air to the sun’s rays, so that their upward limit of growth is determined by that of lowest water at spring tides. It appears from some old charts, that the long island to windward was formerly separated by wide channels into several islets; this fact is likewise indicated by the less age of the trees in certain portions. Under this former condition of the reef, a strong breeze, by throwing more water over the barrier, would tend to raise the level of the lagoon. Now it acts in a directly contrary manner; for the water, not only is not increased by currents from the outside, but is blown outwards by the force of the wind. Hence, it is observed, that the tides near the head of the lagoon do not rise so high during strong breezes as on ordinary occasions. This difference of level, although no doubt very small, has I believe caused the death of those coral groves, which under the former condition of things had attained the utmost possible limit of upward growth.
A few miles north of Keeling there is another small lagoon island, the centre of which is nearly filled up. Captain Ross found in the conglomerate of the outer coast a well rounded fragment of greenstone, rather larger than a man’s head; he and the men with him were so much surprised at this, that they brought it away and preserved it as a curiosity. The occurrence of this one stone, where every other-particle of matter is calcareous, certainly is very puzzling. The island has scarcely ever been visited, nor is it probable that a ship had been wrecked there. From the absence of any better explanation, I came to the conclusion that it must have come there entangled in the roots of some large tree: when, however, I considered the great distance from the nearest land, the combination of chances against a stone thus being entangled, the tree washed into the sea, floated so far, then landed safely, and the stone finally so embedded as to allow of its discovery, I was almost ashamed of imagining a means of transport so improbable. It was therefore with great interest that I found Chamisso,[8] the justly distinguished naturalist who accompanied Kotzebue, stating that the inhabitants of the Radack Archipelago, a group of lagoon islands in the midst of the Pacific, obtained stones for sharpening their instruments by searching the roots of trees which are cast up on the beach. It will be evident that this must have happened several times, since laws have been established that such stones belong to the chief, and a punishment is inflicted on any one who attempts to defraud him of this right. When the isolated position of these small islands in the midst of a vast ocean--their great distance from any land excepting that of coral formation, a fact well-attested by the value which the inhabitants, who are such bold navigators, attach to a stone of any kind,[9]--and the slowness of the currents of the open sea are all considered, the occurrence of pebbles thus transported does appear wonderful. Stones may often be thus transported; and if the island on which they are stranded is constructed of any other substance besides coral, they would scarcely attract attention, and their origin at least would never have been guessed. Moreover this agency may long escape discovery from the probability of trees, especially those loaded with stones, floating beneath the surface. In the channels of Tierra del Fuego large quantities of drift timber are cast upon the beach, yet it is extremely rare to meet a tree swimming on the water. It is easy to conceive that waterlogged wood might be transported, when floating close to the bottom, and occasionally even just touching it. The knowledge of any result which (with sufficient time allowed) can be produced by causes, though appearing infinitely improbable, is valuable to the geologist, for he by his creed deals with centuries and thousands of years as others do with minutes. If a few isolated stones are discovered in a mass of fine sedimentary strata, it cannot, after the above facts, be considered as very improbable that they may have been drifted there by the floating timber of a former epoch.
During another day I visited Horsburg and West Island. In the latter, the vegetation was perhaps more luxuriant than in any other part. The cocoa-nut trees generally grow separate, but here the young ones flourished beneath their tall parents, and formed with their long and curved fronds the most shady arbours. Those alone who have tried it know how delicious it is to be seated in such shade, and drink the cool pleasant fluid of the cocoa-nut, which hangs in great bunches close by. In this island there is a large bay or little lagoon, composed of the finest white sand: it is quite level, and is only covered by the tide at high water; from this large bay smaller creeks penetrated the surrounding woods. To see a field of glittering sand, representing water, and around the border of which the cocoa-nut trees extended their tall and waving trunks, formed a singular and very pretty view.
I will now briefly mention a few zoological observations which I made during our stay at these islands. I have before alluded to a crab which lives on the cocoa-nuts; it is very common on all parts of the dry land, and grows to a monstrous size. It is closely allied or identical with Birgos latro. This crab has its front pair of legs terminated by very strong and heavy pincers, and the last pair by others which are narrow and weak. It would at first be thought quite impossible for a crab to open a strong cocoa-nut covered with the husk; but Mr. Liesk assures me he has repeatedly seen the operation effected. The crab begins by tearing the husk, fibre by fibre, and always from that end under which the three eye holes are situated; when this is completed, the crab commences hammering with its heavy claws on one of these eye holes, till an opening is made. Then turning round its body, by the aid of its posterior and narrow pair of pincers, it extracts the white albuminous substance. I think this is as curious a case of instinct as ever I heard of, and likewise of adaptation in structure between two objects apparently so remote from each other in the scheme of nature as a crab and a cocoa-nut tree. The Birgos is diurnal in its habits, but every night it is said to pay a visit to the sea, no doubt for the purpose of moistening its branchiæ. The young are likewise hatched, and live for some time, on the coast. These crabs inhabit deep burrows, which they excavate beneath the roots of trees; and here they accumulate surprising quantities of the picked fibres of the cocoa-nut husk, on which they rest as on a bed. The Malays sometimes take advantage of their labour by collecting the course fibrous substance and using it as junk. These crabs are very good to eat; moreover under the tail of the larger ones there is a great mass of fat, which when melted sometimes yields as much as a quart bottle full of limpid oil. It has been stated by some authors that the Birgos latro crawls up the cocoa-nut trees for the purpose of stealing the nuts: I very much doubt the possibility of this; but with the Pandanus[10] the task would be very much easier. I understood from Mr. Liesk that on these islands the Birgos lives only on the nuts which fall to the ground.
I was a good deal surprised by finding two species of coral of the genus Millepora, possessed of the property of stinging. The stony branches or plates when taken fresh from the water have a harsh feel and are not slimy, although possessing a strong and disagreeable odour. The stinging property seems to vary within certain limits in different specimens: when a piece was pressed or rubbed on the tender skin of the face or arm, a pricking sensation was generally caused, which came on after the interval of a second, and lasted only for a short time. One day, however, by merely touching my face with one of the branches the pain was instantaneous; it increased as usual after a few seconds, and remaining sharp for some minutes, was perceptible for half an hour afterwards. The sensation was as bad as that from a nettle, but more like that caused by the Portuguese man-of-war (Physalia). Little red spots were produced on the tender skin of the arm, which appeared as if they would have formed watery pustules, but did not. The circumstance of this stinging property is not new, though it has scarcely been sufficiently remarked on. M. Quoy[11] mentions it, and I have heard of stinging corals in the West Indies. In the East Indian sea a stinging sea-weed also is found.
There was another and quite distinct kind of coral, which was remarkable from the change of colour, which it underwent shortly after death; when alive it was of a honey-yellow, but some hours after being taken out of water, it became as black as ink. I may just mention, as partly connected with the above subjects, that there are here two species of fish, of the genus Sparus, which exclusively feed on coral. Both are coloured of a splendid bluish-green, one living invariably in the lagoon, and the other amongst the outer breakers. Mr. Liesk assured us that he had repeatedly seen whole shoals grazing with their strong bony jaws on the tops of the coral branches.[12] I opened the intestines of several, and found them distended with a yellowish calcareous matter. These fish, together with the lithophagous shells and nereidous animals, which perforate every block of dead coral, must be very efficient agents in producing the finest kind of mud, and this, when derived from such materials, appears to be the same with chalk.
APRIL 12TH.--In the morning, we stood out of the Lagoon. I am glad we have visited these islands: such formations surely rank high amongst the wonderful objects of this world. It is not a wonder, which at first strikes the eye of the body, but rather, after reflection, the eye of reason. We feel surprised, when travellers relate accounts of the vast extent of certain ancient ruins; but how utterly insignificant are the greatest of these, when compared to the pile of stone here accumulated by the work of various minute animals. Throughout the whole group of islands, every single atom,[13] even from the smallest particle to large fragments of rock, bears the stamp of having been subjected to the power of organic arrangement. Captain FitzRoy, at the distance of but little more than a mile from the shore, sounded with a line, 7200 feet long, and found no bottom. This island is, therefore, a lofty submarine mountain, which has a greater inclination than even those of volcanic origin on the land. I will now give a sketch[14] of the general results at which I have arrived, respecting the origin of the various classes of reefs, which occur scattered over such large spaces of the intertropical seas.
The first consideration to attend to, is, that every observation leads to the conclusion that those lamelliform corals, which are the efficient agents in forming a reef, cannot live at any considerable depth. As far as I have personally seen, I judge of this from carefully examining the impressions on the soundings, which were taken by Captain FitzRoy at Keeling Island, close outside the breakers, and from some others which I obtained at the Mauritius. At a depth under ten fathoms, the arming came up as clean as if it had been dropped on a carpet of thick turf; but as the depth increased, the particles of sand brought up became more and more numerous, until, at last, it was evident the bottom consisted of a smooth layer of calcareous sand, interrupted only at intervals by shelves, composed probably of dead coral rock. To carry on the analogy, the blades of grass grew thinner and thinner, till, at last, the soil was so sterile, that nothing sprung from it.
As long as no facts, beyond those relating to the structure of lagoon islands were known, so as to establish some more comprehensive theory, the belief that corals constructed their habitations, or, speaking more correctly, their skeletons, on the circular crests of submarine craters, was both ingenious and very plausible. Yet the sinuous margin of some, as in the Radack Islands of Kotzebue, one of which is fifty-two miles long, by twenty broad, and the narrowness of others, as in Bow Island (of which there is a chart on a large scale, forming part of the admirable labours of Captain Beechey), must have startled every one who considered this subject.
The very general surprise of all those who have beheld lagoon islands, has perhaps been one chief cause why other reefs, of an equally curious structure have been almost overlooked:[15] I allude to the encircling reefs. We will take, as an instance, Vanikoro, celebrated on account of the shipwreck of La Peyrouse. The reef there runs at the distance of nearly two, and in some parts three miles from the shore, and is separated from it by a channel having a general depth between thirty and forty fathoms, and, in one part, no less than fifty, or three hundred feet. Externally, the reef rises from an ocean profoundly deep. Can any thing be more singular than this structure? It is analogous to that of a lagoon, but with an island standing, like a picture in its frame, in the middle. A fringe of low alluvial land in these cases generally surrounds the base of the mountains; this, covered by the most beautiful productions of a tropical land, backed by the abrupt mountains and fronted by a lake of smooth water, only separated from the dark waves of the ocean by a line of breakers, form the elements of the beautiful scenery of Tahiti--so well called the Queen of Islands. We cannot suppose these encircling reefs are based on an external crater, for the central mass sometimes consists of primary rock, or on any accumulation of sedimentary deposits, for the reefs follow indifferently the island itself, or its submarine prolongation. Of this latter case there is a grand instance in New Caledonia, where the reefs extend no less than 140 miles beyond the island.
The great Barrier which fronts the N.E. coast of Australia, forms a third class of reef. It is described by Flinders as having a length of nearly one thousand miles, and as running parallel to the shore, at a distance of between twenty and thirty miles from it, and, in some parts, even of fifty and seventy. The great arm of the sea thus included, has a usual depth of between ten and twenty fathoms, but this increases towards one end to forty and even sixty. This probably is both the grandest and most extraordinary reef now existing in any part of the world.
It must be observed, that the reef itself in the three classes, namely, lagoon, encircling, and barrier, agrees in structure, even in the most minute details: but these I have not space here even to allude to. The difference entirely lies in the absence or presence of neighbouring land, and the relative position which the reefs bear to it. In the two last-mentioned classes, there is one difficulty in undertaking their origin, which must be pointed out. Since the time of Dampier it has been remarked, that high land and deep seas go together. Now when we see a number of mountainous islands coming abruptly down to the sea-shore, we must suppose the strata of which they are composed, are continued with nearly the same inclination beneath the water. But, in such cases, where the reef is distant several miles from the coast, it will be evident upon a little consideration, that a line drawn perpendicularly from its outer edge down to the solid rock on which the reef must be based, very far exceeds that small limit at which the efficient lamelliform corals exist.
In some parts of the sea, as we shall hereafter mention, reefs do occur which fringe rather than encircle islands--the distance from the shore being so small, where the inclination of the land is great, that there is no difficulty in understanding the growth of the coral. Even in these “fringing” reefs, as I shall call them in contradistinction to the “encircling,” the reef is not attached quite close to the shore. This appears to be the result of two causes: namely, first, that the water immediately adjoining the beach is rendered turbid by the surf, and therefore injurious to all zoophytes; and, secondly, that the larger and efficient kinds only flourish on the outer edge amidst the breakers of the open sea. The shallow space between the skirting reef and the shore has, however, a very different character from the deep channel, similarly situated with respect to those of the encircling order.
Having thus specified the several kinds of reefs, which differ in their forms and relative position with regard to the neighbouring land, but which are most closely similar in all other respects (as I could show if I had space), it will, I think, be allowed that no explanation can be satisfactory which does not include the whole series. The theory which I would offer, is simply, that as the land with the attached reefs subsides very gradually from the action of subterranean causes, the coral-building polypi soon raise again their solid masses to the level of the water: but not so with the land; each inch lost is irreclaimably gone;--as the whole gradually sinks, the water gains foot by foot on the shore, till the last and highest peak is finally submerged.
Before I explain this view more in detail, I must enter on a few considerations, which render such changes of level not improbable. Indeed, the simple fact of a large portion of the continent of South America, still rising under our eyes, and abounding with proofs of similar elevations on a grander scale during the recent period, takes away any excessive improbability of a movement similar in kind, but in an opposite direction. Mr. Lyell, who first suggested the idea of a general subsidence with reference to coral reefs, has remarked that the existence of so small a portion of land in the Pacific, where so many causes both aqueous and igneous tend to its production, renders such sinking of the foundation probable. There is, however, another argument of much greater weight, which may be inferred from the inconsiderable depth at which corals grow. We see large extents of ocean, of more than a thousand miles in one direction and several hundreds in another, scattered over with islands, none of which rise to a greater height than that to which waves can throw fragments, or the wind heap up sand. Now if we leave out of the question subsidence, the foundation on which these reefs are built, must in every case come to the surface within that small limit (we may say twenty fathoms) at which corals can live. This conclusion is so extremely improbable that it may at once be rejected: for in what country can there be found a broad and grand range of mountains of the same height within a hundred and twenty feet? But on the idea of subsidence, the case is at once clear: as each point, one after the other according to its altitude, was submerged, the coral grew upwards, and formed the many islets now standing at one level.
Having endeavoured on general grounds not only to remove any extreme degree of improbability in the belief of a general subsidence, but likewise to show that it is almost necessary to account for the existence of a vast number of reefs on one level, we will now see how far the same idea will apply to the peculiar configuration in the several classes. Let us imagine an island merely fringed by reefs extending to a short distance from the shore; in which case, as we have before remarked, there is no difficulty in understanding their structure. Now let this island subside by a series of movements of extreme slowness, the coral at each interval growing up to the surface. Without the aid of sections it is not very easy to follow out the result, but a little reflection will show that a reef encircling the shore at a greater or less distance, according to the amount of subsidence, would be produced. If we suppose the sinking to continue, the encircled island must, by the submergence of the central land but upward growth of the ring of coral, be converted into a lagoon island. If we take a section of some encircled island on a true scale, as for instance Gambier, which has been so well described by Captain Beechey, we shall not find the amount of movement very great, which would be necessary to change a well-characterized encircling reef, into as characteristic a lagoon island.
It will at once be evident that a coral reef, closely skirting the shore of a continent, would, in like manner after each subsidence, rise to the surface; the water, however, always encroaching on the land. Would not a barrier reef necessarily be produced, similar to the one extending parallel to the coast of Australia? It is indeed but uncoiling one of those reefs which encircle at a distance so many islands.
Thus the three great classes of reef, lagoon, encircling, and barrier, are connected by one theory. It will perhaps be remarked, if this be true, there ought to exist every intermediate form between a closely-encircled and a lagoon island. Such forms actually occur in various parts of the ocean: we have one, two, or more islands encircled in one reef; and of these some are of small proportional size to the area enclosed by the coral formation; so that a series of charts might be given, showing a gradation of character between the two classes. In New Caledonia, where the double line of reef projects 140 miles beyond the island, we may imagine we see this change in progress. At the northern extremity, reefs occur, some of which are of the encircling kind, and others almost with the character of true lagoon islands. The line of reef which fronts the whole west coast of this great island, has by some been called a barrier. It is four hundred miles long; and may be said thus to form a link between an ordinary encircling reef and the great Australian barrier.
I should perhaps have entered before into the consideration of one apparent difficulty in the origin of lagoon islands. It may be said, granting the theory of subsidence, a mere circular disc of coral would be formed, and not a cup-shaped mass. In the first place, even in reefs closely fringing the land (as before remarked), the corals do not grow on the shore itself, but leave a shallow channel. Secondly, the strong and vigorous species which alone build a solid reef, are never found within the lagoon; they only flourish amidst the foam of the never-tiring breakers. Nevertheless, the more delicate corals, though checked by several causes, such as strong tides and deposits of sand, do constantly tend to fill up the lagoon; but the process must become slower and slower, as the water in the shallow expanse is rendered subject to accidental impurities. A curious instance of this happened at Keeling Island, where a heavy tropical storm of rain killed nearly all the fish. When the coral at last has filled up the lagoon to the height of lowest water at spring tides, which is the extreme limit possible,--how, afterwards, is the work to be completed? There is no high land whence sediment can be poured down; and the dark-blue colour of the ocean bespeaks its purity. The wind, carrying calcareous dust from the outer coast, is the only agent which can finally convert the lagoon island into solid land, and how slow must this process be!
Subsidence of the land must always be most difficult to detect, excepting in countries long civilized,--for the movement itself tends to conceal all evidence of it. Nevertheless, at Keeling Island, tolerably conclusive evidence of such movement could be observed. On every side of the lagoon, in which the water is as tranquil as in the most sheltered lake, old cocoa-nut trees were undermined and falling. Captain FitzRoy likewise pointed out to me on the beach the foundation-posts of a storehouse, which the inhabitants said had stood, seven years before, just above high-water mark, but now was daily washed by the tide. Upon asking the people whether they ever experienced earthquakes, they said, that lately the island had been shaken by a very bad one; and that they remembered two others during the last ten years. I no longer doubted concerning the cause which made the trees fall, and the storehouse to be washed by the daily tide.
At Vanikoro, the encircled island already mentioned, I gathered from Captain Dillon’s account, that the alluvial land at the foot of the mountain was very small in quantity, the channel extremely deep, and the islets on the reef itself, which result from the gradual accumulation of fragments, singularly few in number; all of which, together with the wall-like structure of the reef both inside as well as outside, indicated to my mind, that, without doubt, the movements of subsidence had lately been rapid. At the end of the chapter, it is stated that this island is shaken by earthquakes of extreme violence.
I may here mention a circumstance, which to my mind had the same weight as positive evidence, though bearing on another part of the question. M. Quoy, when discussing in general terms the nature of coral reefs, gives a description which is applicable only to those which, skirting the shore, do not require a foundation at any greater depth than that from which the coral-building polypi can spring. I was at first astonished at this, as I knew he had crossed both the Pacific and Indian oceans, and must, as I thought, have seen the class of widely-encircling reefs, which indicate a subsiding land. He subsequently mentions several islands as instances of his description of the general structure; by a singular chance, the whole can be shown, by his own words, in different parts of his account, to have been recently elevated. Therefore, that which appeared so adverse to the theory, became as strong in its confirmation.
Continental elevations, as observed in South America and other parts, seem to act over wide areas with a very uniform force; we may therefore suppose that continental subsidences act in a nearly similar manner. On this assumption, and taking on the one hand, lagoon islands, encircling and barrier reefs, as indications of subsidence; and on the other, raised shells and corals, together with mere skirting reefs, as our proof of elevation, we may test the truth of the theory,--that their configuration has been determined by the kind of subterranean movement,--by observing whether any uniform results can be obtained. I think it can be shown that such is the case in a very remarkable degree; and that certain laws may be inferred from the examination, of far more importance than the mere explanation of the origin of the circular or other kinds of reef.
If there had been space, I should have made a few general remarks, before entering into any detail. I may, however, just notice the remarkable absence of the reef-building polypi over certain wide areas within the tropical sea: for instance, on the whole west coast of America, and, as I believe, of Africa (?), and round the eastern islands in the Atlantic ocean. Although certain species of lamelliform zoophytes are found on the shores of the latter islands, and though calcareous matter is abundant to excess, yet reefs are never formed. It would appear that the effective species do not occur there; of which circumstance I apprehend no explanation can be given, any more than why it has been ordained that certain plants, as heaths, should be absent from the New World, although so common in the Old.
Without entering into any minute geographical details, I must observe, that the usual direction of the island groups in the central parts of the Pacific, is N.W. and S.E. This must be noticed, because subterranean disturbances are known to follow the coast-lines of the land. Commencing on the shores of America, there are abundant proofs that the greater part has been elevated within the recent period, but as coral reefs do not occur there, it is not immediately connected with our present subject. Immediately adjoining the continent there is an extent of ocean remarkably free from islands, and where of course there exists no possible indication of any change of level. We then come to a N.W. by W. line dividing the open sea from one strewed with lagoon islands, and including the two beautiful groups of encircled islands the Society and Georgian Archipelagoes. This great band having a length of more than four thousand miles by six hundred broad must, according to our view, be an area of subsidence. We will at present for convenience sake pass over the space of ocean immediately adjoining it, and proceed to the chain of islands including the New Hebrides, Solomon, and New Ireland. Any one who examines the charts of the separate islands in the Pacific, engraved on a large scale, will be struck with the absence of all distant or encircling reefs round these groups: yet it is known that coral occurs abundantly close in shore. Here, then, according to the theory, there are no proofs of subsidence; and in conformity to this we find in the works of Forster, Lesson, Labillardière, Quoy, and Bennett, constant allusion to the masses of elevated coral. These islands form, therefore, a well-determined band of elevation: between it and the great area of subsidence first mentioned there is a broad space of sea irregularly scattered with islets of all classes; some with proofs of recent elevation and merely fringed by reefs; others encircled; and some lagoon islands. One of the latter is described by Captain Cook as a grand circle of breakers without a single spot of land; in this case we may believe that an ordinary lagoon island has been recently submerged. On the other hand, there are proofs of other lagoon islands having been lifted up several yards above the level of the sea, but which still retain a pool of salt water in their centres. These facts show an irregular action in the subterranean forces; and when we remember that the space lies directly between the well-marked area of elevation and the enormous one of subsidence, an alternate and irregular movement seems almost probable.
To the westward of the New Hebrides line of elevation we have New Caledonia, and the space included between it and the Australian barrier, which Flinders, on account of the number of reefs, proposed to call the Corallian Sea. It is bounded on two sides by the grandest and most extraordinary reefs in the world, and is likewise terminated to the northward by the coast of Louisiade,--most dangerous on account of its distant reefs. This, then, according to our theory, is an area of subsidence. I may here remark, that as the Barrier is supposed to be produced by the subsidence of the coast of the mainland, it may be expected that any outlying islands would have formed lagoon islands. Now Bligh and others distinctly state that some of the islands there are precisely similar to the well-known lagoon islands in the Pacific; there are also encircled islands, so that the three classes supposed to be produced by the same movement are there found in juxtaposition; as likewise happens, but in a less evident manner, at New Caledonia and in the Society Archipelago.
The New Hebrides line of islands, may be observed to bend abruptly at New Britain, thence to run nearly east and west; and, lastly, to resume its former north-west direction in Sumatra and the peninsula of Malacca. The figure may be compared to the letter S laid obliquely, but the line is often double. We have shown that the southern part, as far north as New Ireland, abounds with proofs of elevation, so is it with the rest. Since the time of Bougainville every voyager adduces some fresh instance of such changes throughout a great part of the East Indian archipelago. I may specify New Guinea, Wageeoo, Ceram, Timor, Java, and Sumatra. Coral reefs are abundant in the greater part of these seas, but they merely skirt the shores. In the same manner as we have followed the curved line of elevation, so may we that of subsidence. At Keeling Island, I have already mentioned that there exist proofs of the latter movement: and it is a very interesting circumstance, that during the last earthquake, by which that island was affected, Sumatra, though distant nearly 600 miles, was violently shaken. Bearing in mind that there is evidence of recent elevation on the coast of the latter, one is strongly tempted to believe that as one end of the lever goes up, the other goes down: that as the East Indian archipelago rises, the bottom of the neighbouring sea sinks and carries with it Keeling Island, which would have been submerged long ago in the depths of the ocean, had it not been for the wonderful labours of the reef-building polypi.
As I have remarked, the islands in this great archipelago are only skirted with reefs; and it appears from the statements of those who have visited them, as well as from an examination of the charts, that lagoon islands are not found there. This in itself is remarkable, but it becomes far more so when it is known, that according to all accounts (and distinctly stated by Mr. De la Beche[16]) they are likewise absent in the West Indian sea, where coral is most abundant: now every one is aware of the numerous proofs of recent elevation in most parts of that archipelago. Again, Ehrenberg has observed that lagoon islands do not occur in the Red Sea: in Lyell’s Geology, and in the Geographical Journal, proofs are given of recent elevation on the shores of a large part of that sea. Excepting on the theory of the form of reefs being determined by the kind of movement to which they have been subjected; it is a most anomalous circumstance, and which has never been attempted to be solved, that the lagoon structure being universal and considered as characteristic in certain parts of the ocean, should be entirely absent in others of equal extent.
I may here also just recall to mind the cases of skirting reefs mentioned by M. Quoy (to which number several others might be added), where proofs of elevation occurred. Some general law must determine the marked difference between reefs merely skirting the shore, and others rising from a deep ocean in the form of distant rings. We have endeavoured to show that with a subsiding movement, the first and simple class must necessarily pass into the second, and more remarkable structure.
To proceed with our examination: to the westward of the prolongation of the line of subsidence, of which Keeling Island is the index, we have an area of elevation. For on the northern end of Ceylon and on the eastern shores of India, elevated shells and corals, such as now exist in the neighbouring sea, have been observed. Again in the middle of the Indian ocean, the Laccadive, Maldive, and Chagos line of atolls or lagoons show a line of subsidence. The best-characterized of these, namely, the Maldive islands, extend in length for 480 miles, with an average breadth of sixty. These atolls agree in most respects with the lagoons of the Pacific; they differ, however, in several of them being crowded together--such little groups being separated from other groups by profoundly deep channels. Now if we look in a chart, at the prolongation of the reef towards the northern end of New Caledonia, and then complete the work of subsidence, so as to continue producing the same results; we should have the original reef broken up into many patches; each of which, from the vigorous growth of coral on the outside, would have a constant tendency to assume a rounded form. Every accidental break in the continuity of the first line would determine a fresh circle. In the case, therefore, of the Low or Dangerous Archipelago in the Pacific, I believe that the lagoon islands were moulded round the flanks of so many distinct islands; but in the Maldives, that one single mountainous island, bordered by reefs, and very nearly of the same actual figure and dimensions with New Caledonia, formerly occupied that part of the ocean.
Lastly, to the extreme westward, the coast of Africa is closely skirted by coral reefs, and according to facts stated in Captain Owen’s voyage, has probably been uplifted within a recent period. The same remark applies to the northern part of Madagascar, and, judging from the reefs likewise at the Seychelles, situated on the submarine prolongation of that great island. Between these two, N.N.E. and S.S.W. lines of elevation, some lagoon and widely-encircled islands indicate a band of subsidence.
When we consider the absence both of widely-encircling reefs and lagoon islands in the several archipelagoes and wide areas, where there are proofs of elevations; and on the other hand the converse case of the absence of such proof where reefs of those classes do occur; together with the juxtaposition of the different kinds produced by movements of the same order, and the symmetry of the whole, I think it will be difficult (even independently of the explanation it offers of the peculiar configuration of each class) to deny a great probability to this theory. Its importance, if true, is evident; because we get at one glance an insight into the system by which the surface of the land has been broken up, in a manner somewhat similar, but certainly far less perfect, to what a geologist would have done who had lived his ten thousand years, and kept a record of the passing changes. We see the law almost established, that linear areas of great extent undergo movements of an astonishing uniformity, and that the bands of elevation and subsidence alternate. Such phenomena at once impress the mind with the idea of a fluid most gradually propelled onwards, from beneath one part of the solid crust to another.
I cannot at present do more than allude to some of the results which may be deduced from these views. If we examine the points of eruption over the Pacific and Indian oceans, we shall find that all the active volcanoes occur within the areas of elevation. (The Asiatic band must be excepted; inasmuch as we are entirely in want of information of all kinds respecting it.) On the other hand, in the great spaces supposed to be now subsiding, between the Radack and Dangerous Archipelagoes, in the Corallian sea, and among the atolls which front the west coast of India, not one occurs. If we look at the changes of level as a consequence of the propulsion of fluid matter beneath the crust, as before suggested, then the area to which the force is directed might be expected to yield more readily than that whence it was gradually retiring. I am the more convinced that the above law is true, because, if we look to other parts of the world, proofs of recent elevation almost invariably occur, where there are active vents: I may instance the West Indies, the Cape de Verds, Canary Islands, southern Italy, Sicily, and other places. But in answer to this, those geologists, who, judging from the history of the isolated volcanic mounds of Europe, were inclined to believe that the level of the ground was constantly oscillating up and down, might maintain that on these same areas, the amount of subsidence had been equal to that of elevation, but that we possessed no means of knowing it. I conceive it is by eliminating this source of doubt, that the alternate bands of opposite movement, deduced from the configuration of the reefs, directly bear on this law. I need not do more than simply state, that we thus obtain (if the view is correct) a means of forming some judgment of the prevailing movements, during the formation of even the oldest series, where volcanic rocks occur interstratified with sedimentary deposits.
Any thing which throws light on the movements of the ground is well worthy of consideration; and the history of coral reefs may, in another manner, elucidate such changes in the older formations. As there is every reason to believe that the lamelliform corals grow only abundantly at a small depth, we may feel sure, where a great thickness of coral limestone occurs, that the reefs on which the zoophytes flourished, must have been sinking. Until we are enabled to judge by some means what were the prevailing movements at different epochs, it will scarcely ever be possible to speculate with any safety on the circumstances under which the complicated European formations, composed of such different materials and in such different states, were accumulated.
Nor can I quite pass over the probability of the above views illustrating those admirable laws first brought forward by Mr. Lyell,--of the geographical distribution of plants and animals, as consequent on geological changes. M. Lesson has remarked on the singular uniformity[17] of the Indio-Polynesian Flora throughout the immense area of the Pacific;--the dispersion of forms having been directed against the course of the trade-wind. If we believe that lagoon islands, those monuments raised by infinite numbers of minute architects, record the former existence of an archipelago or continent in the central part of Polynesia, whence the germs could be disseminated, the problem is rendered far more intelligible. Again, if the theory should hereafter be so far established, as to allow us to pronounce that certain districts fall within areas either of elevation or subsidence, it will directly bear upon that most mysterious question,--whether the series of organized beings peculiar to some isolated points, are the last remnants of a former population, or the first creatures of a new one springing into existence.
Briefly to recapitulate. In the first place, reefs are formed around islands, or on the coast of the mainland, at that limited depth at which the efficient classes of zoophytes can live; and where the sea is shallow, irregular patches may likewise be produced. Afterwards from the effects of a series of small subsidences, encircling reefs, grand barriers, or lagoon islands, are mere modifications of one necessary result. Secondly, it can be shown on the above views, that the intertropical ocean, throughout more than a hemisphere, may be divided into linear and parallel bands, of which the alternate ones have undergone, within a recent period, the opposite movements of elevation and subsidence. Thirdly, that the points of eruption seem invariably to fall within areas subject to a propulsion from below. The traveller who is an eyewitness of some great and overwhelming earthquake, at one moment of time loses all former associations of the land being the type of solidity, so will the geologist, if he believe in these oscillations of level (the deeply-seated origin of which is betrayed by their forms and vast dimensions), perhaps be more deeply impressed with the never-ceasing mutability of the crust of this our World.
FOOTNOTES:
[1] The nuts are carried to Singapore and to Mauritius; the white part being grated into a pulp, is used in making curries, and is said greatly to improve that dish.
[2] Holman’s Travels, vol. iv., p. 378.
[3] Kotzebue’s First Voyage, vol. iii., p. 155.
[4] The thirteen species belong to the following orders. Coleoptera, a species of minute Elater; Orthoptera, a Gryllus and Blatta; Hemiptera, one; Homoptera, two; Neuroptera, a Chrysopa; Hymenoptera, two ants; Lepidoptera Nocturna, a Diopæa, and a Pterophorus(?). Diptera, two.
[5] Kotzebue’s First Voyage, vol. iii., p. 222.
[6] The large claws or pincers of some of these crabs, are most beautifully adapted, when drawn back, to form an operculum to the shell, which is nearly as perfect as the proper one that belonged to the original molluscous animal. I was assured, and as far as my observation went it was confirmed, that there are certain kinds of these hermits, which always use certain kinds only of old shells.
[7] These ebbing and flowing wells are common in parts of the West Indies. The simple fact, that in low islands of small extent, and composed of porous materials, the rain-water can have no tendency to sink lower than the level of the surrounding sea, and must therefore accumulate near the surface, having been overlooked; it has been supposed that sand possesses the power of filtering the salt which is dissolved in the sea-water.
[8] Kotzebue’s first voyage, vol. iii., p. 155. It is said, “The sea throws up on the reefs of Radack the trunks of northern firs (!) and trees of the torrid zone (palms, bamboos). It provides the inhabitants not only with timber for boats, but it also brings them in wrecks of European ships, the iron which they want.”--“They receive, in a similar manner, another treasure, hard stones fit for whetting. They are sought for in the roots and hollows of the trees which the sea throws up.”
[9] Some natives carried by Kotzebue to Kamtschatka collected stones among other valuable articles to take back to their country.
[10] See Proceedings of Zoological Society, 1832, p. 17.
[11] Freycinet’s Voyage, vol. i., p. 597.
[12] It has sometimes been thought (vide Quoy in Freycinet’s Voyage), that coral-eating fish were poisonous; such certainly was not the case with these Spari.
[13] I exclude, of course, the soil which has been brought here in vessels from Malacca and Java, and the small fragments of pumice, drifted here, together with the seeds of East Indian plants. The one block of greenstone, moreover, on the Northern Lagoon must be excepted.
[14] This sketch was read before the Geological Society, May, 1837.
[15] Mr. De la Beche, however, seems to have been fully aware of the difficulty. He says, “there are certain situations, where coral reefs run, as it were, in a line with the coast, but separated from it by deep water, which would seem to require a different explanation.”--Geological Manual, p. 142.
[16] Geological Manual, p. 141.
[17] Perhaps this is stated rather too strongly; but M. Lesson, of course, had grounds for his assertion.