ON THE SAND-FLOOD.
In different parts of Scotland, as in Aberdeenshire, Hebrides, and Shetland Islands, there are examples of the natural chronometer mentioned in the text. In Morayshire there is a striking example of the sand-flood, concerning which the following details have been furnished by my young friend the Rev. Mr Ritchie.
Sand-Flood in Morayshire.
“Westward from the mouth of the river Findhorn in Morayshire, a district, consisting of upwards of ten square miles of land, which, owing to its extreme fertility, was once termed the Granary of Moray, has been depopulated and rendered utterly unproductive by the sand-flood. This barren waste may be characterised as hilly; the accumulations of sand composing these hills frequently varying in their height, and changing their situation.
There is historical evidence, that, in the year 1097, the Moray Firth overflowed the low country on its southern shore, and threw out sand. But the destruction of the barony of Coubine (which includes the greater part of the desert mentioned above) was long subsequent to this, as might be proved from the inscription on a tombstone in the church yard of Dyke. From historical notices, also, in regard to the Kinnairds of Coubine, preparing for publication, it appears that the eruption of sand commenced about the year 1677; that its progress was gradual; that, in 1697, not a vestige was to be seen of the manor-place, orchards, and offices of Coubine; that two-thirds of the barony were already ruined, and that the sand was daily gaining ground.
This sand, which overwhelmed Coubine, came from Mavieston, situated on the shore, about seven miles west from the mouth of the Findhorn, where, from time immemorial, there have been large accumulations of sand. The sands at Mavieston had formerly been covered with vegetation. In an act of the Scottish Parliament, dated 16th July 1695, for the preservation of lands adjacent to sand-hills, it is stated, that the destruction of Coubine “was occasioned by the bad practice of pulling bent and juniper.” Having been thus set at liberty, the sand moved towards the north-east, as appears from the desolation which marks its progress. The moving cause is the wind. I have had opportunities of witnessing the effect of the wind on the loose sand. When the breeze is moderate it carries along with it successive waves of sand, each wave (if I may be allowed the expression) being of a small size, and moving with greater or less velocity, in proportion to the strength of the breeze, and presenting a very beautiful appearance. When the wind is high the heavier particles are drifted forwards, the more minute are raised to a considerable height in the atmosphere, occasioning no small inconvenience to the spectator, who finds his ears and nostrils filled with sand. The movements of the sand are still towards the north-east. In the winter of 1816 a large portion of Binsness, the only remaining farm on the west side of the Findhorn, situated in the line of the sand’s progress, was overwhelmed. Since that period large accumulations of sand have disappeared altogether, and rich soil, marked with the plough, has been left bare, after having been buried for upwards of a century.
The very minute particles, which, as has been stated, the wind raises to a considerable height, are occasionally carried across the Bay of Findhorn. In the statistical account of Dyke, the parish in which Coubine is situated, it is said, “that, at the town of Findern, in a blowing day, one may feel the sand sharply striking on his face, from the west side.” This sand, of extreme fineness, is to be seen in and around the town of Findhorn, and along the coast much rich land is said to have been covered by sand brought from the west.
The greater quantity of the sand is drifted into the river, and its effects have been very remarkable. Many years ago the mouth of the river having become blocked up with sand, it cut out for itself its present channel, which conducts it, by a more direct course, to the sea. In consequence of this, the old town of Findhorn had changed its situation, from the east to the west side of the river, and its site has since been covered by the sea. Previous to this, however, the inhabitants, carrying with them the stones of their former houses, had removed across the river, and erected the present village. On the retiring of the tide from the bay, the river almost disappears, being swallowed up by the sand, and quick-sands are formed. The effect resulting from the same cause, the drifting in of the sand is very different at high water. In consequence of the channel of the river having been filled up, the bay has increased in breadth. The sand constantly carried down by the river has formed a bar, which prevents the entrance of large vessels; and the river, probably owing to its increased breadth, and this bar depriving it of the impetus acquired in the course of its descent, is, at spring-tides, unable to force its way into the sea, when it is made to flow back, and inundate a considerable extent of carse-land situated at the head of the bay. It was at one time proposed to render the river navigable by dredging. And it is proposed to endeavour to save the adjoining carse-land, which is of the richest quality, from the monthly inundation to which it is at present subject, by building a wall along the river side.
I venture to suggest, that the plan Nature employs for fettering down sand should first be imitated, and that seeds of the Arundo arenaria, Elymus arenarius, and other plants, which grow readily in sand, should be, from time to time, strewed over the Mavieston Hills. The seeds of the Arundo arenaria are not always to be had; but plants might easily be procured in abundance, and be dibbled into the sand-hills. The circumstance of great accumulations of sand having of late disappeared from Coubine, has given rise to the expectation, that the barony is at no distant period to become again serviceable to man. By cutting off fresh supplies from Mavieston, this period would be accelerated, and the proposed improvements rendered comparatively easy.
There is at present little bent on Coubine. It is chiefly confined to a range of knolls, which forms the southern boundary of the sand, and protects the adjoining cultivated fields from its encroachments; and yet, notwithstanding the terrible calamity the inhabitants of Moray brought upon themselves, by the pulling of bent, this “bad practice” still prevails; this plant being in no other district of country which I have visited so generally employed for thatching cottars’ houses, and other economical purposes.”
In the Outer Hebrides the effects of the sand-flood are also considerable, as shewn in the following notice communicated by my intelligent assistant Mr Macgillivray.
Sand-Flood in the Hebrides, and other parts of Scotland.
“The bottom of the sea, along the whole west coast of the Outer Hebrides, from Barray Head to the Butt of the Lewis, appears to consist of sand. Along the shores of these islands this sand appears here and there, in patches of several miles, separated by intervals of rock, of equal or greater extent. In some places the sandy shores are flat, or very gently sloping, forming what are here called Fords; in others, behind the beach, there is an accumulation of sand to the height of from twenty to sixty feet, formed into hillocks. This sand is constantly drifting; and in several places islands have been formed by the removal of isthmi. The parts immediately behind the beach are also liable to be inundated by the sand; and in this manner most of the islands have suffered very considerable damage. Those of Pabbay and Berneray in Harris may be particularised; in the former of which, a tract of about a mile and a-half long, by half a mile in breadth, has been converted into a desert of drifting sand; and in the latter a large plain, that was formerly noted for its fertility, has been entirely swept away. The sand consists almost entirely of comminuted shells, apparently of the species which are found in the neighbouring seas. It is rather coarse in the grain; but, during high winds, by the rubbing of its particles upon each other, a sort of dust is formed, which, at a distance, resembles smoke, and which, in the Island of Berneray, I have seen driven into the sea, to the distance of upwards of two miles, appearing like a thin white fog. The cure of sand drift has been attempted in these islands in two different ways. Mr Alexander Macleod, surgeon of North Uist, is the inventor of the most efficacious method, which is that of cutting thin square turfs from the neighbouring pasture grounds, and laying them down at intervals of some inches. In the course of a very few years the turfs coalesce, and the stript ground is little the worse; for the roots remaining in it, a new vegetation rapidly springs up. The other method was introduced by Mr Macleod of Harris, and tried extensively upon his estate. It consists of planting small bundles of Arundo arenaria, at distances of about a foot and a-half. These take root, and prevent the drifting to a certain degree. But often vegetation is tardy in establishing itself, and if the turf plan be not considerably more expensive, it seems preferable, because it very effectually prevents the drift, and moreover, produces excellent pasture ground; the former of which indications, the planting system, does not completely effect, and the latter in a very imperfect degree.”
We may add, as this subject is a very interesting one, that further details, in regard to the moving sands of Scotland, will be found, on consulting the Statistical Account of Scotland, vol. xx. p. 220. In the Appendix to the Account of the parish of Dyke, vol. xx. p. 228. et seq. there is an account of the Sand-Hills of Mavieston, which overwhelmed the barony of Coubine, as mentioned in Mr Ritchie’s communication. In vol. xix. p. 622. is a notice of the shifting of two hills of the Mavieston Range 500 yards in twenty years. In vol. xxi. p. 207. is a notice of some hundred acres in Duffus’ parish covered three feet deep by drift sand; fourteen inches accumulating in one night. In Neill’s Tour in Orkney and Shetland 1804, it is observed, that, in the neighbourhood of the Castle of Noltland, in Westra, much havoc has been done by the blowing of the sand. No measures are there employed for putting a stop to this kind of devastation. In the 6th volume of the Highland Society’s Transactions will be found a report of the operations carried on in Harris, and alluded to in Mr Macgillivray’s communication. And in Dr Walker’s Account of the Hebrides, and Mr Macdonald’s Work on the Hebrides, farther details may be seen. In Jameson’s Account of the Shetland Islands, and in Shirreff and Fleming’s Reports on these islands, are also facts connected with this devastating agent. We may add, that Dr Oudney, Major Denham, and Captain Clapperton, have added to our knowledge of the blowing sands of the African deserts. The coloured engraving of the sand-hills of the African Desert in Denham, Oudney and Clapperton’s Narrative, is a striking and interesting representation of the form of the moving sand-hills of Africa.
The moving Sands of Africa and their effects are thus described in the Mercure de France for September 1809, by De Luc.
The sands of the Lybian desert, he says, driven by the west winds, have left no lands capable of tillage on any parts of the western banks of the Nile not sheltered by mountains. The encroachment of these sands on soils which were formerly inhabited and cultivated is evidently seen. M. Denon informs us, in the account of his Travels in Lower and Upper Egypt, that summits of the ruins of ancient cities buried under these sands still appear externally; and that, but for a ridge of mountains called the Lybian chain, which borders the left bank of the Nile, and forms, in the parts where it rises, a barrier against the invasion of these sands, the shores of the river, on that side, would long since have ceased to be habitable. Nothing can be more melancholy, says this traveller, than to walk over villages swallowed up by the sand of the desert, to trample under foot their roofs, to strike against the summits of their minarets, to reflect that yonder were cultivated fields, that there grew trees, that here were even the dwellings of men, and that all has vanished.
If, then, our continents were as ancient as has been pretended, no traces of the habitation of men would appear on any part of the western bank of the Nile, which is exposed to this scourge of the sands of the desert. The existence, therefore, of such monuments attests the successive progress of the encroachments of the sand; and those parts of the bank, formerly inhabited, will for ever remain arid and waste. Thus the great population of Egypt, announced by the vast and numerous ruins of its cities, was in great part due to a cause of fertility which no longer exists, and to which sufficient attention has not been given. The sands of the desert were formerly remote from Egypt; the Oases, or habitable spots, still appearing in the midst of the sands, being the remains of the soils formerly extending the whole way to the Nile; but these sands, transported hither by the western winds, have overwhelmed and buried this extensive tract, and doomed to sterility a land which was once remarkable for its fruitfulness.
It is therefore not solely to her revolutions and changes of sovereigns that Egypt owes the loss of her ancient splendour; it is also to her having been thus irrecoverably deprived of a tract of land, by which, before the sands of the desert had covered it, and caused it to disappear, her wants had been abundantly supplied. Now, if we fix our attention on this fact, and reflect on the consequences which would have attended it if thousands, or only some hundreds, of centuries had elapsed since our continents first existed above the level of the sea, does it not evidently appear that all the country on the west of the Nile would have been buried under this sand before the erection of the cities of ancient Egypt, how remote soever that period may be supposed; and that in a country so long afflicted with sterility, no idea would even have been formed of constructing such vast and numerous edifices? When these cities indeed were built, another cause concurred in favouring their prosperity. The navigation of the Red Sea was not then attended with any danger on the coasts; all its ports, now nearly blocked up with reefs of coral, had a safe and easy access; the vessels laden with merchandize and provisions could enter them and depart without risk of being wrecked on these shoals, which have risen since that time, and are still increasing in extent.
The defects of the present government of Egypt, and the discovery of the passage from Europe to India round the Cape of Good Hope, are therefore not the only causes of the present state of decline of this country. If the sands of the desert had not invaded the bordering lands on the west, if the work of the sea polypi in the Red Sea had not rendered dangerous the access to its coasts and to its ports, and even filled up some of the latter, the population of Egypt and the adjacent countries, together with their product, would alone have sufficed to maintain them in a state of prosperity and abundance. But now, though the passage to India by the Cape of Good Hope should cease to exist, though the political advantages which Egypt enjoyed during the brilliant period of Thebes and Memphis should be re-established, she could never again attain the same degree of splendour.
Thus the reefs of coral which had been raised in the Red Sea on the east of Egypt, and the sands of the desert which invade it on the west, concur in attesting this truth: That our continents are not of a more remote antiquity than has been assigned to them by the sacred historian in the book of Genesis, from the great era of the deluge.
Note H, [p. 30.]
Action of the Sea upon Coasts.
The ocean, in its action upon the cliffs and banks situated on the coast, breaks them down to a greater or less extent, and either accumulates the debris at their basis in the form of sea beaches of greater or less magnitude, or by currents carries it away to be deposited upon other shores, or to give rise to sand-banks near the coast, which, in the course of time, become united to the land, and thus secure it from the further action of the sea. These destroying and forming effects of the waters of the ocean are to be observed all around the coasts of this island; and beautiful examples of such actions are to be seen on the coasts of Ireland, and in many of the islands that lie to the west and north of Great Britain. In a paper read before the Wernerian Natural History Society, Mr Stevenson, engineer, mentions many facts illustrative of the destroying effects of the ocean on our coasts.—Thus he informs us that the waters of the sea are wearing away the land upon both sides of the Frith of Forth, not only in exposed, but also in sheltered situations, and the solid strata, as well as the looser alluvial formations, which owe their origin to the destroying agency of the ocean at a former period, are again yielding to its action. At Saint Andrew’s, the famous castle of Cardinal Beaton, which is said originally to have been some distance from the sea, now almost overhangs it: From St Andrew’s northward to Eden water and the River Tay, the coast presents a sandy beach, and is so liable to shift, that it is difficult to trace the change it may have undergone. It is certain, however, that, within this last century, the sea has made such an impression upon the sands of Barray, on the northern side of the Tay, that the light-houses at the entrance of the river, which were formerly erected at the southern extremity of Button-ness, have been from time to time removed about a mile and a quarter further northward, on account of the wasting and shifting of these sandy shores, and that the spot on which the outer light-house stood in the 17th century, is now two or three fathoms under water, and is at least three quarters of a mile within flood-mark.
NOTE, [p. 32.]
On the growth of Coral Islands.
Of all the genera of lithophytes, the madrepore is the most abundant. It occurs most frequently in tropical countries, and decreases in number and variety as we approach the poles. It encircles in prodigious rocks and vast reefs many of the basaltic and other rocky islands in the South Sea and Indian Ocean, and, by its daily growth, adds to their magnitude. The coasts of the islands in the West Indies, also those of the islands on the east coast of Africa, and the shores and shoals of the Red Sea, are encircled and incrusted with rocks of coral. Several different tribes of madrepore contribute to form these coral reefs; but by far the most abundant are those of the genera carophylla, astrea and meandrina. These lithophytic animals not only add to the magnitude of land already existing, but, according to some naturalists, they form whole islands. Dr Forster, in his Observations made during a Voyage round the World, gives an account of the formation of these coral islands in the South Sea.
All the low isles, he says, seem to me to be a production of the sea, or rather its inhabitants, the polype-like animals forming the lithophytes. These animalcules raise their habitation gradually from a small base, always spreading more and more, in proportion as the structure grows higher. The materials are a kind of lime mixed with some animal substances. I have seen these large structures in all stages, and of various extent. Near Turtle Island, we found, at a few miles distance, and to leeward of it, a considerable large circular reef, over which the sea broke every where, and no part of it was above water; it included a large deep lagoon. To the east and north-east of the Society Isles, are a great many isles, which in some parts are above water; in others, the elevated parts are connected by reefs, some of which are dry at low water, and others are constantly under water. The elevated parts consist of a soil formed by a sand of shells and coral rocks, mixed with a light black mould, produced from putrified vegetables, and the dung of sea-fowls; and are commonly covered by cocoa-nut trees and other shrubs, and a few antiscorbutic plants. The lower parts have only a few shrubs and the above plants; others still lower, are washed by the sea at high-water. All these isles are connected, and include a lagoon in the middle, which is full of the finest fish; and sometimes there is an opening, admitting a boat or canoe, in the reef, but I never saw or heard of an opening that would admit a ship.
The reef, or the first origin of these isles, is formed by the animalcules inhabiting the lithophytes. They raise their habitation within a little of the surface of the sea, which gradually throws shells, weeds, sand, small bits of corals, and other things, on the tops of these coral rocks, and at last fairly raises them above water; where the above things continue to be accumulated by the sea, till by a bird, or by the sea, a few seeds of plants that commonly grow on the sea-shore, are thrown up, and begin to vegetate; and by their annual decay and reproduction from seeds, create a little mould, yearly accumulated by the mixture with sand, increasing the dry spot on every side; till another sea happens to carry a cocoa-nut hither, which preserves its vegetative power a long time in the sea, and therefore will soon begin to grow on this soil; especially as it thrives equally in all kinds of soil; and thus may all these low isles have become covered with the finest cocoa-nut trees.
The animalcules forming these reefs want to shelter their habitation from the impetuosity of the winds, and the power and rage of the ocean; but as, within the tropics, the winds blow commonly from one quarter, they, by instinct, endeavour to stretch only a ledge, within which is a lagoon, which is certainly entirely screened against the power of both. This, therefore, might account for the method employed by the animalcules in building only narrow ledges of coral rocks, to secure in their middle a calm and sheltered place; and this seems to me to be the most probable cause of the origin of all the Tropical Low Isles, over the whole South Sea.
That excellent navigator, the late Captain Flinders, gives the following interesting account of the formation of Coral Islands, particularly of Half-way Island on the north coast of Terra Australis[381].
“This little island, or rather the surrounding reef, which is three or four miles long, affords shelter from the south-east winds; and being at a moderate day’s run from Murray’s Isles, it forms a convenient anchorage for the night to a ship passing through Torres’ Strait: I named it Half-way Island. It is scarcely more than a mile in circumference, but appears to be increasing both in elevation and extent. At no very distant period of time, it was one of those banks produced by the washing up of sand and broken coral, of which most reefs afford instances, and those of Torres’ Strait a great many. These banks are in different stages of progress: some, like this, are become islands, but not yet habitable; some are above high-water mark, but destitute of vegetation; whilst others are overflowed with every returning tide.
“It seems to me, that, when the animalcules which form the corals at the bottom of the ocean cease to live, their structures adhere to each other, by virtue either of the glutinous remains within, or of some property in salt water; and the interstices being gradually filled up with sand and broken pieces of coral washed by the sea, which also adhere, a mass of rock is at length formed. Future races of these animalcules erect their habitations upon the rising bank, and die in their turn, to increase, but principally to elevate, this monument of their wonderful labours. The care taken to work perpendicularly in the early stages, would mark a surprising instinct in these diminutive creatures. Their wall of coral, for the most part, in situations where the winds are constant, being arrived at the surface, affords a shelter, to leeward of which their infant colonies may be safely sent forth; and to this, their instinctive foresight, it seems to be owing, that the windward side of a reef exposed to the open sea, is generally, if not always, the highest part, and rises almost perpendicular, sometimes from the depth of 200, and perhaps many more fathoms. To be constantly covered with water, seems necessary to the existence of the animalcules, for they do not work, except in holes upon the reef, beyond low-water mark; but the coral, sand, and other broken remnants thrown up by the sea, adhere to the rock, and form a solid mass with it, as high as the common tides reach. That elevation surpassed, the future remnants, being rarely covered, lose their adhesive property; and remaining in a loose state, form what is usually called a key, upon the top of the reef. The new bank is not long in being visited by sea-birds: salt plants take root upon it, and a soil begins to be formed; a cocoa-nut, or the drupe of a pandanus, is thrown on shore; land birds visit it, and deposit the seeds of shrubs and trees; every high tide, and still more every gale, adds something to the bank; the form of an island is gradually assumed; and last of all comes man to take possession.
“Half-way Island is well advanced in the above progressive state; having been many years, probably some ages, above the reach of the highest spring tides, or the wash of the surf in the heaviest gales. I distinguished, however, in the rock which forms its basis, the sand, coral, and shells, formerly thrown up, in a more or less perfect state of cohesion. Small pieces of wood, pumice stone, and other extraneous bodies which chance had mixed with the calcareous substances when the cohesion began, were inclosed in the rock; and in some cases were still separable from it without much force. The upper part of the island is a mixture of the same substances in a loose state, with a little vegetable soil; and is covered with the casuarina and a variety of other trees and shrubs, which give food to parroquets, pigeons, and some other birds; to whose ancestors, it is probable, the island was originally indebted for this vegetation.”
Mr Chamisso, who accompanied Kotzebue in his voyage, has published interesting observations on this subject. He informs us that the low islands of the South Sea and Indian Ocean owe their origin principally to the operations of several species of coral. Their situation with respect to each other, as they often form rows, their union in several places in large groups, and their total absence in other parts of the same seas, induce us to conclude, that the corals have founded their building on shoals of the sea; or, to speak more correctly, on the tops of mountains lying under water. On the one side, as they increase, they continue to approach the surface of the sea, on the other side they enlarge the extent of their earth. The larger species of corals, which form blocks, measuring several fathoms in thickness, seem to prefer the more violent surf on the external edge of the reef; this, and the obstacles opposed to the continuation of their life, in the middle of a broad reef, by the amassing of the shells abandoned by the animals, and fragments of corals, are probably the reason that the outer edge of the reef first approaches the surface. As soon as it has reached such a height, that it remains almost dry at low water, the corals leave off building higher; sea-shells, fragments of coral, shells of echini, and their broken-off prickles, are united by the burning sun, through the medium of the cementing calcareous sand, which has arisen from the pulverization of the above mentioned shells into one whole or solid stone, which, strengthened by the continual throwing up of new materials, gradually increases in thickness till it at last becomes so high, that it is covered only during some seasons of the year by the high tides. The heat of the sun so penetrates the mass of stone when it is dry, that it splits in many places, and breaks off in flakes. These flakes, so separated, are raised one upon another by the waves at the time of high water. The always active surf throws blocks of coral, (frequently of a fathom in length, and three or four feet thick,) and shells of marine animals, between and upon the foundation stones; after this the calcareous sand lies undisturbed, and offers to the seeds of trees and plants, cast upon it by the waves, a soil upon which they rapidly grow, to overshadow its dazzling white surface. Entire trunks of trees, which are carried by the rivers from other countries and islands, find here, at length, a resting place after their long wanderings; with them come some small animals, such as lizards and insects, as the first inhabitants. Even before the trees form a wood, the real sea-birds nestle here; strayed land-birds take refuge in the bushes; and at a much later period, when the work has been long since completed, man also appears, builds his hut on the fruitful soil formed by the corruption of the leaves of the trees, and calls himself lord and proprietor of this new creation.
In the preceding account, we have seen how the exterior edge of a submarine coral edifice first approaches the surface of the water, and how this reef gradually assumes the properties of land; the island, therefore, necessarily has a circular form, and in the middle of it an inclosed lake. This lake, however, is not entirely inclosed; (and it could not be, for without supply from the sea it would soon be dried up by the rays of the sun,) but the exterior wall consists of a great number of smaller islands, which are separated from each other by sometimes larger, sometimes smaller spaces. The number of these islets amounts, in the larger coral islands, to sixty; and between them it is not so deep but that it becomes dry at the time of ebb. The interior sea has in the middle generally a depth of from thirty to five-and-thirty fathoms; but on all sides towards the land the depth gradually increases. In those seas where the constant monsoons prevail, where, consequently, the waves beat only on one side of the reef or island, it is natural that this side of the reef, exposed to the unremitting fury of the ocean, should be formed chiefly by broken-off blocks of coral, and fragments of shells, and first rise above the elements that created it. It is only these islands respecting the formation and nature of which we hitherto know any thing with certainty; we are almost entirely without any observations on those in the Indian and Chinese Sea, which lie in the regions of the six months’ monsoons. From the charts given of them, it is to be inferred that every side is equally advanced in formation. The lee side of such a coral reef in the Pacific Ocean, which is governed by the constant monsoons, frequently does not shew itself above the water, when the opposite side, from time immemorial, has attained perfection in the atmospheric region; the former reef is even interrupted in many places by intervals tolerably broad, and of the same depth as the inner sea, which have been left by nature, like open gates, for the exploring mariner to enter the internal calm and secure harbour. In their external form the coral islands do not resemble each other; but this, and the extent of each, probably depends on the size of the submarine mountain tops, on which their basis is founded. Those islands which have more length than breadth, and are opposed in their greatest extent to the winds and waves, are richer in fruitful islets than those whose situation is not so adapted to a quick formation. In the large island-chains, there are always some single islets which have the appearance of high land; these lie upon an angle projecting into the sea, are exposed to the surf upon two sides, consist therefore almost entirely of large blocks of coral, and are destitute of smaller fragments of shells and coral sand to fill up the intervals. They are, therefore, not adapted to support plants requiring a depth of soil, and only afford a basis to high trees, provided with fibrous roots, (as the Pisonia, Cordia Sebastiana, L.; Morinda citrifolia, L.; and Pandanus odoratissimus, L.), which, at a distance, give to these, always very small islands, the form of a hill. The inner shores of the island, exposed to the surf, consist of fine sand, which is washed up by the tide. Between the small islands under their protection, and even in the middle of the inner sea, are found smaller pieces of coral, which seek a quiet abode, form in time, though very slowly, banks, till they at last reach the surface of the water; gradually increase in extent; unite with the islands that surround them; and at length fill up the minor seas, so that what was at first a ring of islands, becomes one connected land. The islands which are so far formed, retain in the middle a flat plain, which is always lower than the wall that surrounds them on the banks; for which reason pools of water are formed in them, after a continued rain,—the only springs and wells they possess. One of the peculiarities of these islands is, that no dew falls in the evening, that they cause no tempests, and do not check the course of the wind. The very low situation of the country sometimes exposes the inhabitants to great danger, and threatens their lives when the waves roll over their islands, if it happens that the equinox and full moon fall on the same day (consequently when the water has reached its greatest height), and a storm agitates the sea at the same time. These islands are said to be also shaken by earthquakes.
MM. Quoy and Gaimard, in a lately published memoir, propose, 1st, To examine how corals raise their habitations upon rocks, and what circumstances are favourable or unfavourable to their growth. 2d, To shew that there are no islands of any extent, constantly inhabited by man, which are entirely formed of corals; and that far from raising from the depths of the ocean perpendicular walls, as has been alleged, these animals form only layers or crusts of a few fathoms thickness.
The following, according to the French naturalists, is the manner in which this addition or superposition of madrepores is effected. In the places where the heat is constantly intense, where the land is indented by bays containing shallow and quiet water, which is not liable to be agitated by great surges, or by the regular breezes of the tropics, there also the saxigenous polypi multiply. They construct their habitations on the submarine rocks, envelope these rocks in whole or in part, but do not form them properly speaking. Thus, all those reefs, those girdles of madrepore, which are so frequently met with in the South Sea, to the leeward of islands, are shoals depending upon the conformation of the original ground, which will be perceived to belong to it when the direction of the mountains and hills has been attentively observed. It is always where the slopes are gentle, and the sea shallowest, that the greatest masses of madrepores are found. They sprout up if it is calm; in the contrary case, they form only scattered tufts, belonging to species which seem to be least affected by the agitation of the waters.
It has been said, and it is even a matter of general belief among mariners, say MM. Quoy and Gaimard, that there occur in the equatorial seas shoals composed of corals, which rise from the greatest depths, like walls at the bottom of which the sounding line finds no ground. The fact certainly does exist in so far as regards the depth spoken of; and it is this very circumstance which is productive of so much danger to vessels, which, when taken in a calm and carried away by currents, cannot cast anchor in such places. But it is not correct to say that these reefs are entirety formed of madrepores. First, because the species which always form the most considerable banks, such as some meandrinæ, certain caryophylleæ, but especially the astreæ, adorned with the most beautiful and velvety colours, require the influence of light to perfect them; because they are not seen to grow beyond a few yards of depth; and because they cannot consequently be developed at a depth of ten or twelve hundred feet, as they would necessarily be, did they raise the cliffs in question. Besides, these different species of animals would then almost exclusively enjoy the privilege of living at all depths, under all degrees of pressure, and, so to speak, in all temperatures.
Another circumstance to which navigators have not adverted, which corroborates the opinion here stated, is, that, in depths so great as those to which we allude, the sea, always agitated at the surface, breaks with force upon these reefs, without requiring for that purpose any additional impulse from the winds. And by merely attending to the necessary consequences of the observations of these same navigators, who say (what is very true) that, wherever the waves are agitated, the lithophytes are unable to go on with their work, because they destroy their frail edifices, we shall acquire the moral certainty that these submarine steeps are not produced by these animalcules. But, in these same places, let there occur a hollow, a sheltered spot of some kind, and then they will immediately raise their habitations, and will contribute to diminish the little depth that already exists there. And this is what may be seen in almost all the places where an elevated temperature permits these animals to grow in abundance.
In the localities where the tides are sensible, their currents alone may sometimes form irregular canals between the madrepores, without their ever being encumbered with their species, from the twofold cause united, of the motion and the coldness of the water; while, on the other hand, the flexible alcyonia are seen to multiply there.
When these geological dispositions are carefully observed, we see that the zoophytes rise to the surface of the waves, never beyond it; after which the generation which has attained thus far appears to die. It is destroyed much sooner, if, from the effect of the tides, these frail animalcules are exposed naked to the action of a burning sun. When there occur small hollows in these heaps of inert spoils, deprived of their inhabitants, which are always covered by the water, several tufts of those lithophytes are still remarked, which, having escaped from the almost general destruction, glow with the most lively colours. Then, the families which are developed anew, not being able to build on the outside of those reefs on which the sea is constantly breaking, draw nearer and nearer the shore, where the waves now deadened have scarcely any more action upon them, as in the Isle of France, at Timor, the Papua, the Marian, and the Sandwich Islands; provided always the waters had not a great depth, as is the case at Turtle Island, of which Cook speaks, where no bottom is found between the madrepore reefs and the island, notwithstanding the shortness of the space which exists between these two points.
If we examine these animals in the places best adapted to their growth, we shall see their different species, the forms of which, as varied as they are elegant, become rounded into balls, spread out into fans, or ramify into trees, mingling together, blending with each other, and reflecting the varied hues of red, yellow, blue and violet.
It is well known that all these alleged walls, exclusively formed of corals, are intersected with openings through which the sea enters and retires with violence; and every body knows the danger which Captain Cook ran on one occasion, on the coast of New Holland, when he had no other resource, in order to save himself from destruction, than to take the sudden resolution of attempting one of these narrow passes, where one is always sure of finding plenty of water. And this circumstance also comes in support of what we have advanced; for, if these perpendicular walls were entirely composed of madrepores, they would present no deep openings in their continuity, because it is the property of zoophytes to build in masses that have no interruption; and because, again, could they raise themselves from very great depths, they would end with encumbering and shutting up these passages; a circumstance which does not take place, and probably never will, from the causes which we have related.
If these facts prove, that madrepores cannot exist at very great depths, the submarine rocks, which they only increase in height, are not, therefore, exclusively formed by them.
We now come to the second part of the argument; and we assert, that there are no islands of any magnitude and constantly inhabited by man, that are formed by corals; and that the layers which they construct under the water, are not more than a few fathoms in thickness.
We shall commence with the second part of this question. The impossibility of penetrating to the bottom of the sea to examine at what precise depth the solid zoophytes establish themselves, constrains us to confine ourselves to what has taken place in former times; and the monuments which the ancient revolutions of the globe have disclosed to our view, will serve to prove what is going on in our own days. We shall mention what has been seen in several places, and we shall first speak of the island which Peron took for the theatre of the great works of these polypi, namely the island of Timor.
The banks of coral which the sea has left exposed in the land, as it retired, are remarkable for their uncommon magnitude. The whole shores of Coupang are formed of them, and the low hills in its vicinity are enveloped in them; but a few hundred yards from the town, they disappear, when distinct strata of slate make their appearance. The corals form a bed over the subjacent rocks from 25 to 80 feet thick.
Every thing announces that, in the Island of Timor, there exist no mountains exclusively formed of corals. As in all extensive countries, they are composed of various substances. Quoy and Gaimard having coasted it for about fifty leagues, sufficiently near to enable them to form an idea of its geography, were able to see that it exhibited volcanic appearances in several parts. Besides it abounds in mines of gold and copper, which, in conjunction with what we have already mentioned, shews in a general way the nature of the rocks of which it is composed.
Perhaps, remarks Quoy and Gaimard, the Bald-Head, a mountain of King George’s harbour in New Holland, which Vancouver has described in passing, and on the summit of which he saw perfectly preserved branches of coral, might be adduced as a fact in opposition to the opinion here advanced. Yet the phenomenon exhibited there, is still precisely the same as at Timor, and in a thousand other places[382]. The zoophytes have built upon a basis previously existing, and they occupy only the surface of it. For why should this Bald-Head differ from Mount Gardner, which, although close by it, is formed of primitive rocks? Besides, Peron says, that it has the same geological constitution. (T. ii. p. 133.)
At Rota, one of the Marian Isles, M. Gaudichaud, detached from the limestone rock, at about a hundred toises above the level of the sea, branches of true madrepores, in perfect preservation. Here are, then, three localities in which they are found at great heights. We have observed them, say the French naturalists, at infinitely lower elevations in several other places, as at the Isle of France, where they form a bed more than six feet thick, between two streams of lava; at Wahou, one of the Sandwich Islands, where they have not a greater elevation, but extend for several hundred toises over the surface of the island. In all these cases, it is necessary to distinguish between the lithophytes, which have, by their living powers, formed continuous masses, from those which, after having been rolled about, broken down by the water, and mixed with sea shells, contribute to form those deposits known by the name of madrepore limestone. The latter sort is nothing but the debris of the former. Deposits of this description occur in the Marian Isles, and in those of the Papous; they occur also on the coasts of France, and in several other places.
It would appear from observations made in Timor and other places, that the species of the genus Astræa which are the only ones capable of covering immense extents of surface, do not commence their operations at a greater depth than twenty-five or thirty feet, in order to raise their habitations to near the surface of the sea. Fragments of these species are never obtained, either with the sounding line, or upon the anchors; nor do we ever see them, unless in places where the water is shallow; while the branched madrepores, which do not form thick and continuous beds, either on the elevated places which the ocean has left, or on the shores where they still exist, live at considerable depths.
It is evident, then, that these corals have erected their fabrics on the summits of submarine hills and mountains; and that all those reefs of Taiti, the Dangerous Archipelago, Navigators’ Islands, the Friendly Islands, &c. are composed of madrepores only at the surface.
We thus consider it demonstrated, that the rocks of the solid zoophytes or coral, are not capable of forming the immense bases on which the greater number of the islands that occur in the Pacific Ocean rest.
There now remains for us to state how these animals, by their union, are capable of raising small islets. Forster, as already stated, has given a very good description of the manner in which this is effected. In fact, when these animalcules have raised their habitations to the surface of the water, under the shelter of the land, and they remain uncovered during the reflux of the tide, the hurricanes which sometimes supervene, by the agitation which they produce in those shallow waters, throw up from the bottom sand and mud. These substances are detained in the sinuosities and cavities formed between the corals, and thus serve to fix them together, and connect them into masses. Whenever the summit of this new island can remain constantly uncovered by the sea, and the waves can no longer destroy what they themselves have contributed to form, then its extent is enlarged, and its edges are gradually raised by the successive addition of sand. According to the direction of the winds and currents they may long remain sterile; but if the seeds of vegetables be transported to them from the neighbouring shores, by the action of these two causes, then, in latitudes favourable to their development, we presently see these islands becoming covered with verdure, the successively accumulated remains of which form layers of soil, which contribute to the elevation of the surface.
But, in order that this phenomenon of growth be accomplished, the distance from land must not be too great, because then the vegetables cannot get so easily to the islets in question, which then remain almost always bare and sterile. And for this reason what navigators report of those madrepore Islands of the Great Ocean, which are covered with verdure, and are yet at a great distance from any known land, has always appeared to us extraordinary; and that so much the more, that, in those vast spaces, the violence of the waves, which nothing can break there, must disturb the operations of the zoophytes. We do not, however, deny the existence of these islands, which it would be interesting carefully to examine anew; for, whenever navigators meet with low islands between the Tropics, they do not hesitate, in compliance with the generally received opinion, to say that they consist of madrepores. Yet how many islands, which scarcely rise above the surface of the water, recognise no such origin? We may mention, as an example, the Island of Boni, situated under the equator, the beautiful vegetation of which rises upon limestone. Cocoa Island, near Guam, is in the same condition, being also composed of limestone. In general, if they are inhabited, consequently they have springs or lakes of fresh water, we may almost be certain that they are not composed of lithophytes, or are only so in part, because springs could not be formed in their porous substances. Some of the Caroline Isles are excessively low; we supposed them encrusted with madrepores; and as they have inhabitants there must be somewhere in them a soil favourable to the accumulation of fresh water[383].
In restraining the power of these animalcules, concludes Quoy and Gaimard, and in pointing out the limits which nature has prescribed them, we have no other object than to furnish more correct data to the naturalists who aspire to great hypothetical considerations, regarding the conformation of the globe. On reconsidering these zoophytes with greater attention, they will no longer be seen filling up the basins of the seas, raising islands, increasing the size of the continents, threatening future generations with a solid equatorial circle formed of their spoils. Their influence, with regard to the road-steads or harbours, in which they multiply, is already great enough, without adding more to it. But, compared with the masses on which they rest, what are their layers, often interrupted, and which must be searched for with care, before they can be recognised, to the enormous volcanic peaks of the Sandwich Islands, the Island of Bourbon, the Moluccas, the Marian Islands, the mountains of Timor, New Guinea, &c. &c.? Nothing, certainly; and the solid zoophytes are in no degree capable of being compared with the testaceous mollusca, with reference to the materials which they have furnished, and still continue to furnish to the crust of the Globe.
Note I, [p. 33.]