The Island of St. Helena being extremely remote from all continental land and other islands, and so very singular in its appearance and structure, has been justly considered a proper subject for philosophic investigation.
Of its primary formation different opinions are entertained. Some ascribe its elevation above the surface of the ocean, wholly to subterranean operations: whilst others contend, from the striking dissimilarity between the exterior and interior parts, that it is the remnant of a large island, of which the greater part has been sunk under water by the force of earthquakes and volcanoes.
Although conjectures of this sort are sometimes founded upon rather a superficial view of things, and without that knowledge of facts which can alone lead to probable conclusions, yet, from all I have observed, during a residence of five years, it seems to me that the last of these conjectures, which is that adopted by Mr. Forster, is the most plausible.
If this conjecture could by any means be satisfactorily verified from the facts I am about to relate (some of which have hitherto wholly escaped notice) it seems to me that it would be highly important in the progress of geology. It is well known that islands have emerged from the sea; but we have no record (excepting the tradition of the Atlantica Insula) of the immersion of any large portion of land by the effects of volcanoes and earthquakes.
In viewing the hills on the east and west sides of James's Town from the anchorage, particularly the north end of Ladder Hill, we observe numerous strata rising from the base to the summit (600 feet above the level of the sea) which bear evident marks that the whole of this huge mass, extending several hundred yards to the westward, has been formed by a long series of floating lava.
The extreme ends of the strata on the coast, are placed horizontally: but upon examining the sides of the hills, the lava seems to have descended, at a depression of about 20 degrees, and apparently from a crater somewhere about the site of the waterfall.
Of this crater no positive vestige remains: yet there are some appearances which might lead to a supposition that the present waterfall may have been its southern edge, or extremity—that it was bounded on the north-west by High Knoll, and that subsequently to the formation of that mountain and Ladder Hill, some tremendous explosion opened a chasm, extending from the sea to the waterfall, and at the same moment, by tearing away, and precipitating, a large portion of High Knoll into the crater, may have extinguished it.
This conjecture is at least plausible (if not clearly verified) by the appearance of High Knoll. The western side is sloping, and the base of that part (extending more than 1200 yards from north to south) rests upon the surface of Ladder Hill; whilst the eastern side is a stupendous precipitous cliff, the foot of which stands in James's Valley, at least 260 feet lower than the opposite base on Ladder Hill. In short, High Knoll, which rises 1903 feet above the level of the sea, is decidedly but a fragment of what it has originally been. One third at least of this mountain has been precipitated to the eastward, apparently close to the water-fall, and into that very spot whence the eruptions of lava seem to have proceeded, and afterwards descended, even to the northern extremity of Ladder Hill: a distance of 3000 yards. The disruption of High Knoll has exposed to view many layers of lava, from the base almost to the summit, which may be traced throughout the whole distance to the north extreme of Ladder Hill. Hence it seems reasonable to infer that High Knoll was raised by the eruptions of a volcano.
The strata on the east side of Ladder Hill point directly to High Knoll and the waterfall: and it is remarkable that there are several layers near to High Knoll, high up the cliffs, from which salt springs ooze, and more especially during the rainy seasons. These layers have the same declination as the other strata.
There is moreover a very singular vein of red clay on the eastern side, and near the summit of Ladder Hill (two hundred yards above the level of the sea) which corresponds in dimensions, elevation, and colour, with another vein on the opposite hill. Several strata of slimy mud, very strongly impregnated with marine salt, are also to be seen on the same face of Ladder Hill; one at the perpendicular height of 30, and another at 300 feet. Hence it may be inferred, more especially from the correspondency of the veins of red clay, that some violent convulsion has laid open the chasm which is now called James's Valley.
Although, from all these circumstances, it seems most demonstratively certain that the hills on each side of James's Valley have been gradually raised by eruptions from a volcano, succeeded some time after by tremendous shocks, yet upon viewing many other parts of the coast, and the interior of the island, we find no such decisive indications from which a similar deduction can be formed: but there are many indubitable proofs of considerable agitations, or changes, apparently unaccompanied with volcanic eruption.
The most plausible arguments in support of Mr. Forster's opinion, "that St. Helena has undergone a great and total change from a volcano and earthquake, which perhaps sunk the greatest part of it into the sea," may be adduced from the circumstances of the Great Wood; now called the Plain of Long Wood and Dead Wood.
This plain, comprising 1500 acres of fine land, is elevated 2000 feet above the sea, and slopes gently towards the south-east. In former times it was covered with wood, and was therefore called "The Great Wood." So late as the year 1716, there were many trees upon it: but in 1724 the old trees had mostly fallen; and, as goats and hogs were at the time suffered to range, all the young trees were devoured. It appears also by the official records that the trees were, unexpectedly, some years after, succeeded by indigenous wire grass; which now spreads over its whole extent.
How this extensive and beautiful plain could have escaped in a general devastation, is a question not easily to be solved. Its gentle slope, and smooth and even surface, and its fertility, present so striking a contrast to all the surrounding parts, that one might be disposed to believe it a remnant of primitive land, which has remained, untouched and unshaken, amidst all those dreadful convulsions which have agitated and overturned every thing in its vicinity: or it might, with some plausibility, be considered a part of "those countries where the earth appears in a rude state—where every place capable of producing trees is totally covered with wood."
About a mile and a half to the eastward of Long Wood House, there are the remains of many gum-wood trees. Nothing is left but the roots and a few inches of bark. Wherefore, it seems highly probable that the Great Wood may have been of much larger extent; and that it also covered the flat piece of land between Long Wood and the sea: in this case, the whole of the Great Wood must have occupied a space of not less than two thousand acres.
It seems to me that the circumstances of this plain may be of some importance to the science of geology; because it affords a curious and singular contrast, and comparison, with every other part of the island. If such a comparison were undertaken by a skilful geologist, it might lead to some valuable discoveries; or, at least, plausible deductions—not merely concerning the original formation of St. Helena, but of islands in general.
The first notice on record of this plain, is by Governor Roberts; who, in a minute of consultation, dated the 9th of April, 1711, denominates it, "that glorious plain—the finest I ever saw in my life, any where."
It is afterwards more particularly described by the Governor and Council on the 16th October, 1716, in their minutes of a consultation held upon the spot.
"The Governor and Council met here by appointment. We find the place, called the Great Wood, in a flourishing condition, full of young trees, where the hogs (of which there is a great abundance) do not come to root them up: but the Great Wood is miserably lessened and destroyed within our memories; and it is not clear the circuit and length it was. We believe it does not contain less now than fifteen hundred acres of fine wood land and good ground; there are no springs of water but what are salt or brackish; which we take to be the reason that this part was not inhabited when the people first chose out their settlements and made plantations: but if wells could be sunk, which the Governor says he will attempt when we have more hands, we should then think it the most pleasant, and healthiest part of the island. But as to healthiness we don't think 'twill hold so, if the wood that keeps the land warm were destroyed; for then the rains, which are violent here, would carry away the upper soil; and it being a clay marle underneath it would produce but little. As it is, we think in case it were enclosed, it might be greatly improved; but doing that would require many hands, the stone, most of it being to be brought a good distance,—but the ground being near to a level for about five hundred acres of it, carts may be used. The enclosing the whole, we think, would be too great an attempt to begin at once; yet we think nothing more proper than to enclose some of the best part, for when once this wood is gone the island will soon be ruined."
No traces of lava nor shells are to be seen on this plain, nor under its surface: throughout its whole extent there is seldom to be found a stone, of any sort, larger than a walnut: but in digging ditches for the fences at Long Wood, we lately discovered considerable beds of small stones resembling gravel. They are, however, a sort of soft iron stone, easily pulverised: some are close to the surface, others a foot or two underneath; and it has been observed that the crops are always more exuberant where the soil is intermixed with these stones.
The plough might therefore range over at least 1200 acres of this plain, without meeting the smallest impediment. The soil is excellent, and, in many places, of great depth. This was ascertained by the Lieutenant Governor, who resided there some years ago, in making an unsuccessful attempt to find water. In a ravine, many feet below the surface of the plain, he dug a pit 80 feet deep: in which nothing was found but the same fine soil, composed of mould and friable clay.
Indeed, if we may judge from a deep ravine, which partly divides the plains of Long Wood and Dead Wood, the soil and substratum of friable clay in this part of the island may very possibly be several hundred feet deep. This ravine has evidently been formed by torrents of rain, which, in the course of ages have opened a large hollow, more than 500 feet across, and about 150 deep; the sides of this hollow expose to view an infinite number and variety of beautiful layers of coloured earths, which are supposed to be of volcanic origin. I am, however, inclined to believe that the colours are only superficial: this will be noticed hereafter.
But, it is not merely in this eastern part of the island that there is soil of extraordinary depth. At the Plantation House, the country residence of the Governor, which is 1700 feet above the sea, and four miles west from Long Wood, I ascertained to the depth of 25 feet, that the soil is uniformly of the same fine quality as the upper stratum; and probably it may be so to a far greater depth.
These accounts of the depth of soil, and of the fine plain of Long Wood and Dead Wood, will no doubt surprise many who have been taught to believe, that "St. Helena is a barren and unproductive rock:" indeed it will be seen by many facts stated in the first part of this work, that its lands, of which 2 or 3000 acres might be ploughed with the greatest facility (and even much more brought into cultivation) are not inferior in the production of wheat and every other grain, and of potatoes and all sorts of esculents, to the very best lands in Europe. The annual produce is indeed much greater, on account of the certainty of two seasons of rain, and two harvests in the year.
From what has been noticed of the depth of soil, it seems at least probable that neither the plain of Long Wood and Dead Wood, nor the lands in the vicinity of Plantation House, have ever been touched by volcanic eruptions: otherwise some traces would at this day have remained; and notwithstanding there be evident marks of some parts of the exterior boundary of St. Helena having been formed by lava, and afterwards rent open and changed by subterraneous convulsion, yet, upon the whole, there appears to be strong grounds for supposing that the finest parts of the interior are the remnants of primitive land.
Although the plain above-mentioned appears to have escaped in the general devastation, yet in its vicinity are evident traces of the most terrible agitations. The Devil's Punch Bowl, on the west, numerous dreary and barren conical hills on the north, between the plain and the sea, and the deep chasms at Turk's Cap, and Prosperous Bay, are indubitable proofs that all those parts have been violently convulsed: but perhaps the most unaccountable of the whole is, that amidst so much disorder and confusion there should remain a piece of level land which measures 200 acres.
This remarkable spot is situated a little to the south of the Great Wood, and half a mile west from the promontory on which the Prosperous Bay telegraph is erected. It is elevated 5 or 600 feet above the level of the sea; and is encompassed by low hills on the south and east, and rather open towards the other points. The soil is mud, and very strongly impregnated with salt: nothing grows upon it but marine plants. A water-spout which deluged the island in March 1786, left a sheet of water, which lay upon this flat for five or six days afterwards. How so large a portion of land could have been raised to so great an elevation, without deranging its original level seems to be more inexplicable than the formation of all the surrounding parts. These are indeed nothing more than what is common to many islands, and to many other parts of the terrestrial globe. The corresponding strata of the opposite shores of Britain and France, leave no room to doubt but they were once united. The stratum of shells and mud on the hills at Agrigentum, three miles from the harbour, and 1200 feet above the sea, the oyster-shells found on the high mountains in Jamaica, the fossil bones of elephants found by Mr. Humboldt, on the Andes, 3280 yards above the level of the sea, and many other instances that might be adduced, serve only to furnish most incontestable proof that this globe has undergone many surprising changes since it was first created.
There are some other circumstances which seem to have escaped the notice of those who have written upon St. Helena, arising probably from having taken too short a time to explore it; or from not being able to obtain information. Their accounts have certainly led to several erroneous impressions.
For my own part, I perfectly recollect the idea I had formed of this place before I resided upon it. I considered it merely as a rocky island, rising abruptly out of the ocean, and having an unfathomable depth all around it; excepting at James's Bay and Sandy Bay, where the anchorage grounds, as I then imagined, had been formed by the deposition of soil washed down by the rains.
Such were my own ideas from the accounts I had read and heard; and as I have very strong reason to believe that this is the general notion of St. Helena, at the present time, it is proper I should enter a little more into its local and physical circumstances.
That part of St. Helena, which is elevated above water, measures 10½ miles long, 6¾ broad, and is 28 miles in circumference. The coast is on all sides formed by stupendous and almost perpendicular cliffs, rising to the height of from six to more than twelve hundred feet. The principal accessible inlets are at James's Town, Rupert's Bay, Lemon Valley, and Sandy Bay: all these have been strongly fortified. Several reefs of rocks, called ledges, jut out, to the distance of two to four miles; others are detached, some commencing at half a mile or more; and there is one in particular called "New Ledge Fishing Bank," whose outer edge, or extremity, is said to be not less than nine miles distant from the coast. The soundings, at the farthest part, are 45 to 70 fathoms.
Besides these Ledges there are several detached rocks or small islands at a little distance; of which the principal are Egg Island, Speery, and George's Island. All these, excepting the last, which is on the south-east, are situated to the west or south-west; and as the New Ledge, which is the largest of the Ledges, or fishing banks, trends in that direction, it may be presumed that this is also the direction of the higher part of the projecting base of the island under water.
In respect to the depth of water on the south and east, I have no particular information—perhaps being to windward of the island, and not so convenient to the fishing boats, may be the reason that those parts have not been hitherto explored.
According to a minute survey, taken by Captain Austin of the Royal Navy, along the northern face of the island, the bottom of the sea, extending from Flag-staff Bay to Horse-pasture Point, comprising about 16 square miles, shelves very gradually. Three miles north of the coast at Flag-staff there is ground at 82 fathoms; and at three-quarters of a mile from Horse-pasture, there are 36 fathoms. It appears also by this survey that the bottom is in general, smooth and even; consisting of mud, mud and shells, sand with specks, here and there coral, and at one or two places, rock. But, in sounding to the westward the surface was found by Captains Cowan and Beville more irregular; and apparently resembling the surface of the island, consisting of hollows and ridges.
Hence it is evident that this island, resting upon a base, which extends at least 25 miles from east to west, is not "a rock rising abruptly," as had been erroneously supposed; but is rather the pinnacle of a prominence in the bed of the ocean, gradually ascending, from unfathomable depths, to 2700 feet above water: which is the elevation of Diana's Peak, the highest mountain on the island.
This deduction seems consonant to the opinions of some theorists, who have considered "islands as the tops of lofty mountains; the eminences of a great continent, converted into islands by a tremendous concussion of nature:" but whether the circumstances, above stated, may be in any way useful to geologists, or whether they may throw further light upon the origin and formation of islands, or lead to new conjectures upon the probable site of the Atlantica Insula, mentioned by Plato, to have been partially destroyed by an earthquake and deluge, I shall not presume to say.
If, however, any large island ever did exist in the part of the Atlantic under consideration, it might be inferred, according to those theorists, that the islands of St. Helena, Ascension, Saxemberg, Tristan d'Acunha, and Gough's Island, may have been its "lofty mountains and eminences;" and that the whole space within that chain of islands, which is 1800 miles in length, and about 500 in breadth, has been sunk into the sea.
It is very remarkable, and well deserving the attention of naturalists, that a species of gum-wood tree (Conyza gummifera), which is indigenous to the climate of St. Helena, and which has not, I believe, been discovered upon the continent of Africa, has been found upon Gough's Island and Tristan d'Acunha. I have in my possession a sketch of the Island of Saxemberg, upon which some trees are also represented; of what sort I am not informed. But, if it should be ascertained hereafter, that they are of the same species as those on the other three islands, this might be an additional reason for supposing that all those islands, and perhaps Ascension, which has now no trees upon it, may have been, at some remote period, united.
If the possibility of this connection be, for a moment, admitted, the question of immersion, according to M. Buffon's hypothesis, might readily be solved. "History," says this celebrated naturalist, "informs us of inundations and deluges of an extensive nature. Ought not all this to convince us, that the surface of the earth has experienced very great revolutions? Let us suppose, for example, that the old and new worlds were formerly but one continent; and that, by an earthquake, the ancient Atlantis of Plato was sunk; the consequence of this mighty revolution must necessarily be, that the sea would rush in from all quarters, and form what is now called the Atlantic Ocean."
Having now adverted to such circumstances as appear to substantiate the opinion obtained by Mr. Forster, that St. Helena must have existed above water, before it had a volcano, and was afterwards violently changed, and partly subverted by subterraneous fire, it may be proper to shew in what manner this writer supports and illustrates that opinion by the appearances of Ascension and St. Helena.
"The dreariness of Ascension," says Mr. Forster, "surpassed all the horrors of Easter Island, and Terra del Fuego, even without the assistance of snow. It was a ruinous heap of rocks, changed by the fire of a volcano. Nearly in the centre of the island, rises a broad, white mountain of great height, on which we discovered some verdure by the help of our glasses, from whence it has obtained the name of Green Mountain. On landing, we ascended among heaps of black cavernous stone, which perfectly resembles the most common lavas of Vesuvius and Iceland; and of which the broken pieces looked as if they had been accumulated by art. The lava currents, cooling very suddenly, may easily be imagined to produce such an effect. Having ascended about fifteen yards perpendicular, we found ourselves on a great level plain of six or eight miles in circuit; in one corner of which, we observed a large hill of an exact conical shape, and of a reddish colour, standing perfectly insulated. Part of the plain between these hills was covered with great numbers of smaller hillocks, consisting of the same wild and ragged lava as that near the sea, and ringing like glass, when two pieces are knocked together. The ground between the heaps of lava was covered with black earth; but where these heaps did not appear, the whole was red earth. The conic hills consisted of a very different sort of lava, which was red, soft, and crumbling into earth. We concluded, that the plain on which we stood was once the crater, or seat of a volcano, by the accumulation of whose cinders and pumice-stones, the conic hills had been gradually formed; and that the currents of lava, which we now saw, divided into many heaps, had perhaps been gradually buried in fresh cinders and ashes; and the waters coming down from the interior mountains, in the rainy season, had smoothened every thing in their way, and filled up by degrees the cavity of the crater. The rocky black lava was the residence of numberless man-of-war birds and boobies, which sat on their eggs, and suffered us to come close to them. On all this rocky ground, we only met with ten shrivelled plants, which were of two sorts, a species of spurge and a bind weed.
"Having climbed over an extensive and tremendous current of lava, more solid than that near the shore, we came to the foot of the Green Mountain, which even from the ship, we had plainly distinguished to be of a different nature from the rest of the country. The lava which surrounded it, was covered with a prodigious quantity of purslane and a kind of new fern. The great mountain is divided in its extremities, by various cliffs into several bodies; but in the centre they all unite and form one broad mass of great height. The whole appears to consist of a gritty tophaceous lime-stone, which has never been attacked by the volcano, but probably existed prior to its eruption.
"St. Helena has on its outside, especially where the ships lie at anchor, an appearance, if possible, more dreadful and dreary than Ascension: but the further you advance, the less desolate the country appears, and the most interior parts are always covered with plants, trees, and verdure. However, there are every where the most evident marks of its having undergone a great and total change, from a volcano and earthquake, which perhaps sunk the greatest part of it in the sea.
"We visited (says the same author) isles that had still volcanoes burning; others that had only elevation, and marks of being formed in remote ages by a volcano; and lastly, we found isles that had no remains of a volcano, but strong and undoubted vestiges of having been violently changed, and partly overturned by an earthquake, subterraneous fire, and a volcano. I cannot help referring Easter Island, St. Helena, and Ascension, to the last."
The circumstances which have been noticed concerning the strata of Ladder Hill and High Knoll, cannot, I presume, admit of a doubt, that these parts have undergone a great change since the island was formed; and that this change has been effected by eruptions from a volcano, succeeded by an earthquake. But, whether those other changes, which are visible in many other parts have been produced by similar causes, or subterraneous fire, it is perhaps wholly impossible to ascertain: nor does it even seem probable, that the effects from either, or from all these causes, could have left the island in the state it now is.
There is a central ridge, as will be seen by the annexed sketch, which, running east and west, divides the island into, nearly, two equal portions. In no part of this ridge (which is elevated 2000 feet above the level of the sea) is there a single chasm or opening. It seems, therefore, wholly unaccountable that it should have escaped being broken and shattered, if earthquakes, or subterraneous fires, had occasioned the "overturnings." This ridge is narrow on the top; and very abrupt on the south, especially between Diana's Peak and Manatee Bay. It continues eastward to the Devil's Punch Bowl, where it again narrows, and then spreads out and forms the Great Wood Plain. At the Punch Bowl it becomes so narrow that it seems nearly to have been broken: yet the whole extent, including the Great Wood, has stood fast, in spite of all appearances of disruption on either side.
When the island was discovered (three hundred and thirteen years ago), it is said, there were no living animals upon it; and that seals, sea-lions, turtle, and sea-fowl, occasionally frequented its shores. All this is perfectly reconcilable to the idea that St. Helena is a new land, raised from the bed of the ocean. But if we admit this to be the fact, how are we to account for the origin of the present insect tribe; which are pretty numerous, consisting of various sorts of beetles, grubs, and worms? If these did not exist at the period of discovery, there seems to be no other mode of explaining their present existence, than by reviving the exploded doctrine of equivocal or spontaneous generation. Under this difficulty, it may possibly be inferred, that these sorts of insects actually must have existed: and, hence it would follow, as a natural consequence, that subterraneous fire, and volcanic eruption, have not been the sole causes of formation: because, in either case, no creature whatever could have remained alive.
The introduction of quadrupeds, domestic fowls, and birds, remaining at present, may be easily conceived. There is, however, a species of land bird, inhabiting the interior, and found in considerable numbers, of which, I believe, no notice has ever been taken by any writer on St. Helena. It is of that description not likely to be brought there by shipping, and seems for this reason particularly to deserve attention. It is not a bird of passage, for it is seen throughout the year: indeed none of that description have ever found their way to St. Helena. In appearance, and size, and in some of its habits, it resembles the common sand-lark frequently seen on the shores of Europe. It is called the "Wire-Bird;" probably from its very long legs, resembling wires, which enable it to run with uncommon swiftness. The legs are of a greenish colour; the body and wings gray; the breast white; eyes large, and the bill moderately long. In its nature it is rather shy; and as it does not seem to possess those powers of flight which could have brought it from America or Africa, it seems, therefore, not unreasonable to conclude, that it is indigenous to the island. If so, it may be considered as an additional fact, favourable to the conjecture that St. Helena is a fragment of a larger island.
I have already mentioned, upon the authority of Captain Heywood, who commanded his Majesty's ship Nereus, that the indigenous gum-wood trees of St. Helena are the same as on Tristan d'Acunha and Gough's Island. It would be of importance to ascertain with accuracy this fact; and whether the wire-bird be also a native of those islands, and of Ascension and Saxemburg. If all these points should be verified, trifling as they may appear, they would be important to the cause of geology.
The mountainous ridge which divides the island by its greatest length, appears chiefly to consist of indurated clay, in some parts surmounted, and in others mixed, with shattered rock; which, being of brittle nature and easily broken, has never been perfected into solid stone. This clay may possibly appear to be the material which composes the greater part of the stones found on the island. It has no visible intermixture of sand, and is not tenacious, but friable. It is easily excavated in the form of buildings, with interior upright walls; and in this manner it is used for cart sheds, &c.
Towards the western extremity, within a mile of the coast, the ridge assumes a different appearance: it is there very abrupt, and craggy, on the south, and slopes gently to the north: here are solid rocks, from which large fragments have been detached and lie scattered on the surface. The most remarkable of these are at the west extreme of the ridge; where a great number, standing on end, occupy a flat of about three acres. From their striking resemblance to tomb-stones, this place is called "the church-yard." I examined it with attention, and was led to conclude that its very singular appearance has been occasioned by a water-spout, descending with fury; and by dashing out the soil and washing it down the adjacent ravine, has thus entirely uncovered the stones.
An attempt was made to form some of the stones, taken from the vicinity of the church-yard, into mill-stones: but being brittle and laminated, and withal extremely hard, it was found impossible to manufacture them. This sort of stone, when struck, emits an agreeable sound.
The sea-coast is generally bordered with rocks; and in its vicinity as well as in some parts of the interior, are excellent quarries of honeycomb stone and basaltes. The former being of a softer and tougher texture than the latter, is therefore esteemed by far the best on the island for buildings. Both, however, are evidently composed of the same material; that is, of the clay above mentioned.
It seems to me that basaltes is changed into honeycomb by coming in contact with fire; for I have observed in many quarries an exterior coating of honeycomb passing so insensibly into an interior basaltic bed, and so intimately blended with it, that it was impossible to discover at what point the honeycomb ended, or the basaltes began.
The most remarkable quarry which I observed of this sort, is on the high road between High Knoll and Plantation-house. Here, upon viewing the exterior, it was expected to find a valuable quarry; but after penetrating a few feet into the hill, it was found that the honeycomb was insensibly blended with the basaltic stone. This fact seems to correspond with what is stated by Sir William Hamilton, "that basaltes is fusible per se; that it agrees almost entirely with lava in elementary principles, in its grain, and in all the diversities of its texture." And it seems also to afford a demonstration, that after St. Helena was formed, fires must have rushed through chasms and openings, and scorified the clays, and the vertical rocks of basaltes, in their primitive state.
Of the honeycomb stone at Ladder Hill, and near the Plantation-house, the Chinese, who are expert stone-cutters, have manufactured troughs, bowls, urns, pestles and mortars, and hand-mills. These last answer the purpose extremely well, leaving no grit in the flour. Wherefore, it may be presumed that mill-stones in abundance, and of the largest dimensions, might be furnished from this island.
Honeycomb stone is likewise found in various other parts. At Halley's Mount, and in its vicinity, are two quarries of an imperfect kind. The one is on the eastern face of the mount, and the other on the ridge which forms the south-west edge of the Devil's Punch-bowl. The stones, particularly in the latter, have not attained the solidity of the more perfect sorts. At Halley's mount, amongst the cracks and fissures, are sometimes discovered bits of spar, and thin, hard, flaky substances, having small protuberances on their surfaces, exhibiting a variety of colours. It is remarkable, that on the western face of Halley's mount the rocks are of the same raw, shattered, and brittle sort, as those on the central ridge. Some indeed are of a consistence between clay and stone: and as there are no appearances of their having been touched with fire, it may be inferred that the Punch-bowl itself has been a crater, and that the flames issuing from it have scorched the eastern face of Halley's mount and formed the honeycomb stone, whilst the western face was in a manner hid from the flames; and thereby the stone and clay have been preserved in their primitive state. It is also deserving notice, that to the eastward of the Punch-bowl, and in the vicinity of the Great Wood plain, there is not to be found a stone, of any sort whatever, fit for the purpose of building. This seems to furnish a further proof, that all that portion of the island is of a different formation from the other parts.
On the surface of Ladder Hill are some beds of basaltes, producing flaky stones of extreme hardness. These lie horizontally, and when quarried, yield flags of from six to twelve superficial feet, and from four to ten inches in thickness. They are also sonorous, and as they require only to be squared, they are admirably adapted to the floorings of warehouses, granaries, and other buildings. In blasting some of the basaltic beds on Ladder Hill, both sides of the septa, or cracks, were sometimes covered with figures, apparently of metal; of a bright silver colour, beautifully representing trees and other objects of landscape.
On the western side of High Knoll there are quarries of a very different description. They produce a stone very much resembling the Tufa I have seen in Sicily. Indeed this stone, and the small pieces of very light pumice which are scattered at the northern base of High Knoll, and upon many parts of Ladder Hill, are the only sorts that bear the smallest resemblance to the productions of Etna, Strombolo, and Vesuvius. I had a complete collection of these in my possession, which enabled me to make the comparison.
The tufa, or red-stone of St. Helena, is of a brownish red colour. It is rather soft when taken out of the quarry, but hardens by exposure to the air. It has been used successfully in building; and in forming a water-course for supplying the garrison of Ladder Hill, from the springs of Plantation-house; and from the tank at High Knoll.
There is moreover a very extensive quarry of pozzolana on the eastern side of James's Valley, about a mile from the sea. In the year 1807, my predecessor, Governor Patton, to whose talents and indefatigable exertions the island is much indebted, first introduced this valuable ingredient in mortar cements. It was pointed out to his notice by M. Joinville, who had been attached to the suite of the Honourable Frederick North at Ceylon. This pozzolana, mixed with one-third of the Sandy Bay lime, hardens in sea-water, and has withstood the beating of the waves for several years without being in the least degree diminished or affected. Even when used without any admixture of lime, or water, but merely after spreading it dry, and beating it well together, it soon consolidates after being moistened with rain into a mass almost resembling stone. In some parts of James's Town it has been used as a substitute for paving, and has been found to answer that purpose, not being liable to crack; nor is it in the least affected by the heaviest wheel carriages.
Limestone of excellent quality is in the greatest abundance. It is said to be a concretion of shells and sand, and sometimes of clay. In consistence it is not harder than a sugar loaf, and is therefore easily calcined; one bushel of coals producing ten of lime. The mountains close to Sandy Bay, and not far above the level of the sea, are chiefly composed of it. Lot's Wife Beach, adjoining those mountains, is covered with a white sand, which consists almost wholly of small fragments of limestone. On the opposite side of the island, lime has also been discovered, particularly near Banks's Battery, and at Rupert's Bay, and, it is said, in Breakneck Valley. But there are neither shells, nor lime, nor any calcareous matter in the interior of the island. At Rupert's Bay the limestone rocks project into the sea. It would seem that sea-water has a tendency to harden, or perhaps to form, this concretion; for upon tracing this vein, we found that it gradually and insensibly softens as it recedes from the beach: so that at the distance of thirty yards it assumes the appearance, and is in fact, a limestone sand.
Calcareous clays are found in James's valley and Friar's valley; and a calcareous spar, together with small oblong round stones, corresponding with the description of plaster of Paris, are found in considerable quantity in the hills adjoining Turk's Cap Bay, and Prosperous Bay. Here also has been discovered a variety of pebbles extremely hard, which bear a fine polish, and have been made into seals and ornaments.
Calcareous spar is also found in the body of a very hard and ponderous stone at Munden's Cove and Sandy Bay; and even in the interior of the island: and it is said, that on George's Island might be collected a considerable quantity of gypsum.
The following are the analyses, by the late Doctor Adam Baildon, of the lime quarries already opened. The specimens of plaster of Paris have not yet been analysed.
|2||Rupert's hard sort||74||1||25||ü Too close to the|
|3||Ditto. Soft ditto||63||1||36||þ Sea for working.|
|7||Lot's Wife Beach||75||3||22|
|8||Potatoe Bay||71||1||28||In abundance.|
|9||Turk's Cap Spar||98||-||2|
|10||Ditto. Plaster of Paris||-||-||-||Not analyzed.|
A species of natural cement is found on several parts of the coast of St. Helena, which seems to resemble that seen by M. Peron on the coasts of the isles of Kangaroos, St. Peter, and St. Francis. This writer ascribes its formation, at these places, to numerous shells rolled incessantly by the action of the waves on the neighbouring shores; which being broken into very minute fragments, and mixed with quartzose sand, speedily constitute the principle of calcareous cement of a superior quality. The finest specimen I observed at St. Helena, is between Egg Island and Union Cove. The rocks bear evident marks of the action of fire. They resemble the refuse of iron at smelting places. There are also numerous pumice stones, and a sort of shingle scattered over the surface of these rocks. Some very hard stones are so firmly bedded in this cement, that, in attempting to take them out, they broke in pieces. If such a cement could artificially be made, it would undoubtedly be far superior to the Roman, or any other cement hitherto invented.
It seems to me that a likely mode of ascertaining the process of formation would be to imitate that which appears to have been the process of nature; that is, to collect a certain quantity of powdered St. Helena limestone, to mix it with small fragments of basaltic stone, the size of coarse sand, varying the proportions; then to sprinkle the mixtures occasionally with sea water, and leave them to dry in the sun. If these ingredients should not consolidate at first, the sprinkling of sea water should be continued; and possibly in the course of time, the consolidation might take place. This appears to have been the process of nature; for the natural cement bears an exact resemblance to mortar made with white lime, and an admixture of coarse black sand: but, as there is no sand on the coast, nor in the interior of the island, it may be presumed that those black specks in the cement are merely fragments of basaltic, or pumice stones. If, however, the mixture I have suggested should not consolidate, there might then be reason to infer, that the calcareous matter laying on the rocks near Egg island, may have undergone calcination, at the time the rocks were in a state of fusion.
Along the coast of St. Helena are many deep excavations, forming caves, some of which are raised several feet above the high water mark, and afford commodious retreats for fishermen in rainy weather; others are under the level of the lowest tides; consequently, there is a continual flux and reflux of water, occasioned by the swell of the sea, which in the course of time has penetrated very far into the base of the island.
It would be impossible to explore these cavernous holes, because the swell rises, at every ingress, to the top of the entrance; which of course excludes all external air; and the internal air being powerfully compressed by the rushing in of the water, produces, by its re-action, a very singular effect. On the perpendicular cliff, forming the coast opposite to Egg Island, there are two or three very curious jets d'eau. By the compression of air within the caverns, where they are probably more enlarged than at the exterior openings, it appears that the waters must have gradually perforated amongst the cracks and fissures, vertical, or more probably, oblique, and irregular passages within, and behind, the exterior rocks. The orifices of those passages appear on the face of the cliff at the height of 60 or 70 feet above the sea; from which I observed the water to spout, exactly like the blowing of a whale, and, at intervals, corresponding with the times of the rise and fall of the swell.
On the northern coast, at Munden's cove, and also to the eastward of Rupert's Valley, at a considerable height above the sea, is found a sort of saponaceous argil, which is used by the soldiers as a substitute for soap in washing. It is secreted in either small elliptical cavities, or partly occupies longitudinal vacancies in the rock. Formed into a lump, I have used it as a wash-ball, by way of experiment: it lathered in a small degree like soap; but left a prickling sensation on my hands, something resembling that which is occasioned by soap when overcharged with alkali: it dissolves entirely in water, and forms a smooth liquid mass, without the smallest grit, of the consistence of cream, of a reddish colour; and when left to dry, it cracked; but still retained a sort of unctuous quality. Of this production there has been as yet no analysis. I am therefore not informed whether it possess the same properties of steatites, which, according to the analysis of Bergman, contain in 100 parts, 80 of silex, 17 of mild magnesia, 2 of argillaceous earth, and nearly 1 of iron, in a semi-oxydated state.
Various coloured earths or clays abound in many parts of St. Helena. Upon the hills towards the sea they are discovered, only here and there, in thin veins, bedded between layers of rock. In the interior, particularly about a mile to the eastward of Long Wood House, the deep and sloping sides of some of the ravines, which are of great extent, are clothed with a variety of beautiful tints of white, blue, grey, and red. Of the brilliance of these earths, when the sun shines upon them, no words, nor even the finest touches of the pencil, could convey an adequate idea.
Whether those colours penetrate to any great depth within the surface, appears to me doubtful; because, in digging the ditches for the new fences at Long Wood, at a short distance from the ravines, we rarely found any sort of coloured earth; and this was generally of a reddish tint. We know, that by frequently stirring and exposing common earth to the influence of the air and atmosphere, that a pale brown soil is changed to a much darker hue. We observe, that as flowers gradually open, and are exposed to the air, they throw off their old colour, and acquire a new one; but, whether certain component parts of earth or clay, are capable of being changed into any other colours than dark brown, or black, by a long exposure to the sun, air and moisture, I shall leave to the decision of those who are skilled in the sciences of natural philosophy and chemistry.
Some of the white clays of St. Helena appear to be a species of lithomarga, or stone-marrow, being as fat and slippery as soap. Possibly the white are the primitive clays of the eastern part of the island, which have, in the course of ages, been changed into a variety of colours by some unsearchable cause in nature.