In several parts of Asia and Africa travellers have observed these shells in great abundance. In the mountains of Castravan, which lie above the city Barut, they quarry out a white stone, every part of which contains petrified fishes in great numbers, and of surprising diversity. They also seem to continue in such preservation, that their fins, scales, and all the minutest distinctions of their make, can be perfectly discerned.

From all these instances, we may conclude that fossils are very numerous; and, indeed, independent of their situations, they afford no small entertainment to observe them as preserved in the cabinets of the curious. The variety of their kinds is astonishing. Most of the seashells which are known, and many others to which we are entirely strangers, are to be seen either in their natural state, or in various degrees of petrifaction. In the place of some we have mere spar, or stone, exactly expressing all the lineaments of animals, as having been wholly formed from them. For it has happened, that the shells dissolving by very slow degrees, and the matter having nicely and exactly filled all the cavities within, this matter, after the shells have perished, has preserved exactly and regularly the whole print of their internal surface. Of these there are various kinds found in our pits many of them resembling those of our own shores: and many others that are only to be found on the coasts of other countries. There are some shells resembling those that are never stranded upon our coasts, but that always remain in the deep and many more there are which we can assimilate with no shells in our pits, but also fishes and corals in great abundance; together with almost every sort of marine production.

It is extraordinary enough, however, that the common red coral, though so very frequent at sea, is scarce seen in the fossil world, nor is there any account of its having ever been met with. But to compensate for this, there are all the kinds of the white coral now known; and many other kinds of that substance with which we are acquainted. Of animals there are various parts; the vertebræ of whales, and the mouths of lesser fishes; these, with teeth also of various kinds, are found in the cabinets of the curious: where they receive long Greek names, which it is neither the intention nor the province of this work to enumerate. Indeed, few readers would think themselves much improved, should I proceed with enumerating the various classes of the Conicthyodontes, Polyleptoginglimi, or the Orthoceratites. These names, which mean no great matter when they are explained, may serve to guide in the furnishing the page of instructive history.

From all these instances we see in what abundance these petrefactions are to be found; and, indeed, Mr. Buffon (to whose accounts we added some) has not been sparing in the variety of his quotations concerning the places where they are mostly to be found. However, I am surprised that he should have omitted the mention of one which, in some measure more than any of the rest, would have much strengthened his theory. We are informed, by almost every traveller that has described the pyramids of Egypt, that one of them is entirely built of a kind of freestone, in which these petrified shells are found in great abundance. This being the case, it may be conjectured-as we have accounts of these pyramids among the earli st records of mankind, and of their being built so long before the age of Herodotus (who lived but fifteen hundred years after the flood) that even the Egyptian priests could tell neither the time nor the cause of their erection-I say it may be conjectured that they were erected but a short time after the flood. It is not very likely, therefore, that the marine substances found in one of them had time to be formed into a part of the solid stone, either during the deluge or immediately after it; and consequently their petrifaction must have been before that period And this is the opinion Mr. Buffon has all along so strenu

ously endeavoured to maintain-having given specious reasons to prove that such shells were laid in the beds where they are now found, not only before the deluge, but even antecedent to the formation oman, at the time when the whole earth, as he supposes, was buried beneath a covering of waters.

But while there are many reasons to persuade us that these extraneous fossils have been deposited by the sea, there is one fact that will abundantly serve to convince us that the earth was habitable, if not inhabited, before these marine substances came to be thus deposited; for we find fossil-trees, which no doubt once grew upon the earth, as deep, and as much in the body of solid rocks, as these shells are found to be. Some of these fallen trees, also, have lain at least as long, if not longer, in the earth than the shells, as they have been found sunk deep in a marly substance composed of decayed shells and other marine productions. Mr. Buffon has proved that fossil-shells could not have been deposited in such quantities all at once by the flood; and I think, from the above instance, it is pretty plain that, howsoever they were deposited, the earth was covered with trees before their deposition; and, consequently, that the sea could not have made a very permanent stay. How, then, shall we account for these extraordinary appearances in Nature? A suspension of all assent is certainly the first, although the most mortifying conduct. For my own part, were I to offer a conjecture-and all that has been said upon this subject is only conjecture—instead of supposing them to be the remains of animals belong ing to the sea, I would consider them rather as bred in the numerous fresh-water lakes that, in primeval times, covered the face of uncultivated Nature. Some of these shells we know to belong to fresh waters; some can be assimilated to none of the marine shells now known: why, therefore, may we not as well ascribe the production of all to fresh waters where we do not find them, as we do that of the latter to the sea only where we never find them? We know that lakes, and lands also, have produced animals that are now no longer existing; why, therefore, might not these fossil productions be among the number? I grant that this is making a very harsh supposition; but I cannot avoid thinking that it is not attended with so many embarrassments as some of the former, and that it is much easier to believe that these shells were bred in fresh water, than that the sea had for a long time covered the tops of the highest mountains.

CHAP. VI.

OF THE INTERNAL STRUCTURE OF THE EARTH.

Having, in some measure, got free from the regions of conjecture, let us now proceed to a description of the earth as we find it by examination, and observe its interual composition, as far as it has been the subject of experience, or exposed to human inquiry. These inquiries, indeed, have been carried but to a very little depth below its surface; and even in that disquisition men have been conducted more by motives of avarice than of curiosity. The deepest mine (which is that at Cotteburg, in Hungary) reaches not more than three thousand feet deep; but what proportion does that bear to the depth of the terrestrial globe, down to the centre, which is above four thousand miles? All, therefore, that has been said of the earth, to a deeper degree, is merely fabulous or conjectural. We may suppose with one, that it as a globe of glass (Buffon); with another, a sphere of heated iron (Whiston); with a third, a great mass of waters (Burnet); and with a fourth, one dreadful volcano (Kircher). But let us, at the same time, show our consciousness that all these are but supposi tions.

Upon examining the earth where it has been opened to any depth, the first thing that occurs is the different layers or beds of which it is composed-these all lying horizontally one over the other like the leaves of a book, and each of them composed of materials that increase in weight as they lie deeper. This is, in general, the disposition of the different materials where the earth seems to have remained unmolested; but this order is frequently inverted-and we cannot tell whether from its original formation or from accidental causes. Of different substances, thus disposed, the far greatest part of our globe consists, from its surface downwards to the greatest depths we ever dig or mine.

The first coat that is most commonly found at the surface is that light coat of blackish mold, which is by some called "garden-earth." With this the earth is everywhere invested, unless it be washed off by rains, or removed by some other external violence. This seems to have been formed from animal and vegetable bodies decaying, and thus turning into its substance. It also serves again as a store-house, from whence animal and vegetable nature are renewed; and thus are all vital blessings continued with unceasing circulation. This earth, however, is not to be supposed entirely pure, but is mixed up with much stony and gravelly matter from the layers lying immediately beneath it. It generally happens that the soil is fertile in proportion to the quantity that this purified mold bears to the gravelly mixture; and as the former predominates, so far is the vegetation upon it more luxuriant. It is this external covering that supplies man with all the true riches he enjoys. He may bring up gold and jewels from greater depths; but they are merely the toys of a capricious being-things upon which he has placed an imaginary value, and for which fools alone part with the more sub stantial blessings of life. "It is this earth," says Pliny, "that like a kind mother, receives us at our birth, and sustains us when born. It is this alone, of all the elements around us, that is never found an enemy to man. The body of waters deluge him with rains, oppress him with hail, and drown him with inundations. The air rushes in storms, prepares the tempest, or lights up the volcano. But the earth, gentle and indulgent, ever sub servient to the wants of man, spreads his walks with flowers and his table with plenty; returns with interest every good committed to her care; and, though she produces the poison, she still supplies the antidote; though constantly teized more to furnish the luxuries of man than his necessities, yet even to the last she continues her kind indulgence, and, when life is over, she piously

covers his remains in her bosom."

This external and fruitful layer which covers the earth is, as was said, in a state of continual change. Vegetables, which are naturally fixed and rooted to the same place, receive their adventitious nourishment from the surrounding earth and water; animals, which change from place to place, are supported by these, or by each other. Both, however, having for a time enjoyed a life adapted to their nature, give back to the earth those spoils which they had borrowed for a short space, yet still to be quickened again into fresh existence. But the deposits they make are of very dissimilar kinds, and the earth is very differently enriched by their continuThose countries that have for a long time supported men and animals have been observed to become every day more barren; while, on the contrary, those desolate places in which vegetables only are abundantly produced are known to be possessed of amazing fertility. In regions which are uninhabited," says Mr. Buffon, "where the forests are not cut down, and where animals do not feed upon the plants, the bed of vegetable earth 1s constantly increasing. In all woods, and even in those which are often cut, there is a layer of earth, five or six inches thick, which has been formed by the leaves, branches, and bark which fall and rot upon the ground.

ance.

I have frequently observed, on a Roman way that crosses Burgundy for a long extent, that there is a bed of black earth, of more than a foot thick, gathered over the stony pavement, on which several trees of a considerable size: are supported. This I have found to be nothing else than an earth formed by decayed leaves and branches, which have been converted by time into a black soil. Now, as vegetables draw much more of their nourish.. ment from the air and water than they do from the earth, it must follow that, in rotting upon the ground, they must give more to the soil than they have taken: from it. Hence, therefore, in woods kept a long time without cutting, the soil below increases to a considerable depth; and such we actually find the soil in those American wilds where the forests have been undisturbed for ages. But it is otherwise where men and animals have long subsisted; for as they make a considerable consumption of wood and plants, both for firing and other uses, they take more from the earth than they return to it. It follows, therefore, that the bed of vegetable earth in an inhabited country must be always diminishing, and must at length resemble the soil of Arabia Petrea, and other provinces in the East, which, having been long inhabited, are now become plains of salt and sand, the fixed salt always remaining while the other volatile parts have flown away."

If from this external surface we descend deeper, and view the earth cut perpendicularly downwards, either in the banks of great rivers or steepy sea-shores; or, going still deeper, if we observe it in quarries or mines, we shall find its layers regularly disposed in their proper order. We must not expect, however, to find them of the same kind and thickness in every place, as they differ in different soils and situations. Sometimes marl is seen to be over sand, and sometimes under it. The most common disposition is, that under the first earth is found gravel or sand, then clay or marl, then chalk or coal, marbles, ores, sands, gravels-and thus an alternation of these substances, each growing more dense as it sinks. deeper. The clay, for instance, found at the depth of a hundred feet, is usually more heavy than that found not far from the surface. In a well which was dug at Amsterdam, to the depth of two hundred and thirty feet, the following substances were found in succession:-Seven feet of vegetable earth, nine of turf, nine of soft clay, eight of sand, four of earth, ten of clay, four of earth, ten of sand, two of clay, four of white sand, one of soft earth, fourteen of sand, eight of clay mixed with sand, four of sea-sand mixed with shells, then a hundred and two feet of soft clay, and then thirty-one feet of sand.

In a well dug at Marly, to the depth of a hundred feet, Mr. Buffon gives us a still more exact enumeration of its layers of earth. Thirteen of a reddish gravel, two of gravel mingled with a vitrifiable sand, three of slime, two of marl, four of marly stone, five of marl in dust mixed with vitrifiable sand, six of very fine vitrifiable sand, three of earthy marl, three of hard marl, one of gravel, one of eglantine, a stone of the hardness and grain of marble, one of gravelly marl, one of stouy marl, two of a coarser kind still, one of vitrifiable sand mixed with fossil-shells, two of fine gravel, three of stony marl, one of coarse powdered marl, one of stone, calcinable like marble, three of grey sand, two of white sand, one of red sand streaked with white, eight of grey sand with shells, three of very fine sand, three of grit, four of red sand streaked with white, three of white sand, and fifteen of reddish vitrifiable sand."

In this manner the earth is everywhere found in beds over beds, and, what is still remarkable, each of them, as far as it extends, always maintains exactly the same thickness. It is found, also, that, as we proceed to considerable depths, every layer grows thicker. Thus, in the adduced instances we might have observed that the last layer was fifteen feet thick, while most of the others were not above eight; and this might have gone much.

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deeper for aught we can tell, as before they got through it the workmen ceased digging.

These layers are sometimes very extensive, and often are found to spread over a space of some leagues in circumference. But it must not be supposed that they are uniformly continued over the whole globe without any interruption; on the contrary, they are ever, at small intervals, cracked through as it were by perpendicular fissures-the earth resembling, in this respect, the muddy bottom of a pond, from whence the water has been dried up by the sun, and thus gaping in several chinks, which descend in a direction perpendicular to its surface. These fissures are many times found empty, but oftener closed up with adventitious substances which the rain, or some other accidental causes, have conveyed to fill their cavities. Their openings are not less different than their contents-some being not above half an inch wide, some a foot, and some several hundred yards asunder. These last form those dreadful chasms that are to be found in the Alps, at the edge of which the traveller stands dreading to look down at the immeasurable gulf below. These amazing clefts are well known to such as have passed these mountains, where a chasm frequently presents itself several hundred feet deep, and as many over, at the edge of which the way lies. It often happens, also, that the road leads along the bottom, and then the spectator observes on each side frightful precipices several hundred yards above him, the sides of which correspond so exactly with each other, that they evidently seem torn asunder.

But these chasms to be found in the Alps are nothing to what Ovalle tells us are to be seen in the Andes. These amazing mountains, in comparison of which the former are but little hills, have their fissures in proportion to their greatness. In some places they are a mile wide, and deep in proportion; and there are some others that, running under-ground, in extent resemble a forest. Of this kind also is that cavern called "Eldenhole," in Derbyshire; which, Dr. Plot tells us, was founded by a line of eight and twenty hundred feet, without finding the bottom, or meeting with water: and yet the mouth at the top is not above forty yards over. This immeasurable cavern runs perpendicularly downward; and the sides of it seem to tally so plainly as to show that they once were united. Those who come to visit the place generally procure stones to be thrown into its mouth; and these are heard, for several minutes, falling and striking against the sides of the cavern, producing a sonnd that resembles distant thunder, dying away as the stone goes deeper.

Of this kind also is that dreadful cavern described by Ælian, his account of which the reader may not have met with. In the country of the Arrian Indians is to be seen an amazing chasm, which is called The Gulph of Pluto. The depth and the recesses of this horrid place are as extensive as they are unknown. Neither the natives, nor the curious who visit it, are able to tell how it first was made, or to what depths it descends. The Indians continually drive thither great multitudes of animals, more than three thousand at a time, of different kinds, sheep, horses, and goats; and, with an absurd superstition, force them into the cavity, from whence they never return. Their several sounds, however, are heard as they descend; the bleating of sheep, the lowing of oxen, and the neighing of horses, issuing up to the mouth of the cavern. Nor do these sounds cease, as the place is continually furnished with a fresh supply."

There are many more of these dreadful perpendicular fissures in different parts of the earth; with accounts 、of which Kircher, Gaffarellus, and others who have given histories of the wonders of the subterranean world, abundantly supplied us. The generality of readers, however, will consider them with less astonishment, when they are informed of their being common all over the earth; that in every field, in every quarry, these per

pendicular fissures are to be found, either still gaping, or filled with matter that has accidently closed their interstices. The inattentive spectator neglects the inquiry, but their being common is partly the cause that excites the philosophers attention to them; the irregularities of Nature he is often content to let pass unexamined; but when a constant and a common appearance presents itself, every return of the object is a fresh call to his curiosity; and the chink in the next quarry becomes as great a matter of wonder as the chasm in Eldenhole. Philosophers have long, therefore, endea. voured to find out the cause of these perpendicular fissures, which our own countrymen, Woodward and Ray, were the first that found to be so common and universal. Mr. Buffon supposes them to be cracks made by the sun, in drying up the earth immediately after its emersion from the deep. The heat of the sun is very probably a principal cause; but it is not right to ascribe to one only, what we find may be the result of many. Earthquakes, severe frosts, bursting waters, and storms tearing up the roots of trees, have in our own times produced them and to this variety of causes we must, at present, be content to assign those that have happened before we had opportunities for observation

CHAP. VII.

OF CAVES AND SUBTERRANEOUS PASSAGES THAT SINK, BUT NOT PERPENDICULARLY, INTO THE EARTH.

In surveying the subterranean wonders of the globe, besides those fissures that descend perpendicularly, we frequently find others that descend but a little way, and then spread themselves often to a great extent below the surface. Many of these caverns, it must be confessed, may be the production of art and human industryretreats made to protect the oppressed or to shelter the spoiler. The famous labyrinth of Candia, for instance, is supposed to be entirely the work of art. Mr. Tourne fort assures us that it bears the impression of human industry, and that great pains have been bestowed upon its formation. The stone-quarry of Maestricht is evidently made by labour: carts enter at its mouth and load within, then return and discharge their freight into boats that lie on the brink of the river Maese. This quarry is so large, that forty thousand people may take shelter in it; and in general serves for this purpose when armies march that way-becoming then an impregnable retreat to the people that live thereabout. Nothing can be more beautiful than this cavern when lighted up with torches; for there are thousands of square pillars, in large level walks, about twenty feet high, and all wrought with much neatness and regularity. In this vast grotto there is very little rubbishwhich shows both the goodness of the stone and the carefulness of the workmen. To add to its beauty, there are also, in various parts of it, little pools of water for the convenience of the men and cattle. It is also remarkable that no droppings are seen to fall from the roof, nor are the walks any way wet under foot, except in cases of great rains, where the water gets in by the airshafts. The salt mines in Poland are still more spacious than these. Some of the catacombs, both in Egypt and Italy, are said to be very extensive. But no part of the world has a greater number of artificial caverns than Spain, which were made to serve as retreats to the Christians against the fury of the Moors, when the latter conquered that country. However, an account of the works of Art does not properly belong to a Natural History. It will be enough to observe, that though caverns be found in every country, far the greatest part of them have been fashioned only by the hand of Nature. Their size is found to be beyond the power of man to have effected,

and their forms but ill-appointed to the conveniences of a human habitation. In some places, indeed, we find mankind still make use of them as houses, particularly in those countries where the climate is very severe; but in general they are deserted by every race of meaner animals except the bat; these nocturnal, solitary crea tures are the only inhabitants-and these only in such whose descent is sloping, or, at least, not directly perpendicular.

There is scarce a country in the world without its natural caverns; and many new ones are discovered every day. Of those in England, Oakley-hole, the Devil's-hole, and Penpark-hole, have been often described. The former, which lies on the south-side of Mendip-hills, within a mile of the town of Wells. is much resorted to by travellers. To conceive a just idea of this, we must imagine a precipice of more than a hundred yards high, on the side of a mountain which shelves away a mile above it. In this is an opening not very large, into which you enter, going along upon a rocky, uneven pavement, sometimes ascending, and sometimes descending. The roof as you advance grows higher, and in some places is fifty feet from the floor; in other places, however, it is so low, that a man must stoop to pass. It extends itself in length about two hundred yards; and from every part of the roof, and the floor, there are formed sparry concretions of various figures, which, by strong imaginations, have been likened to men, lions, and organs. At the farthest part of this cavern rises a stream of water, well stored with fish, large enough to turn a mill, and which discharges itself near the en

trance.

Penpark-hole, in Gloucestershire, is almost as remarkable as the former. Captain Sturmey descended into this by a rope twenty-five fathoms perpendicular, and at the bottom found a very large vault in the shape of a horse-shoe. The floors consisted of a kind of white stone enamelled with lead ore, and the pendant rocks were glazed with spar. Walking forward on this stony pavement for some time, he came to a great river twenty fathoms broad and eight fathoms deep; and, having been informed that it ebbed and flowed with the sea, he remained in this gloomy abode for five hours, to make an exact observation. He did not find, however, any alteration whatsoever in its appearance. But his curiosity was ill requited, for it cost this unfortunate gentleman his life: immediately after his return he was seized with an unusual and violent head-ache, which threw him into a fever, of which he died soon after.

But of all the subterranean caverns now known, the grotto of Antiparos is the most remarkable, as well for its extent, as for the beauty of its sparry incrustations. This celebrated cavern was first discovered by one Magni, an Italian traveller, about a hundred years ago, at Antiparos, an inconsiderable island of the Archipelago. The account he gives of it is long and inflated, but upon the whole amusing. "Having been informed," says he, "by the natives of Paros, that in the little island of Antiparos, which lies about two miles from the former, of a gigantic statue that was to be seen at the mouth of a cavern in that place, it was resolved that we (the French consul and himself) should pay it a visit. In pursuance of this resolution, after we had landed on the island, and walked about four miles through the midst of beautiful plains and sloping woodlands, we at length came to a little hill, on the side of which yawned a most horrid cavern, that with its gloom at first struck us with terror, and almost repressed curiosity. Recovering the first surprise, however, we entered boldly; and had not proceeded above twenty paces when the supposed statue of the gaint presented itself to our view. We quickly perceived that what the ignorant natives had been terrified at as a giant, was nothing more than a sparry concretion, formed by the water dropping from the roof of the cave, and by degrees hardening into a

In

figure that their fears had formed into a monster. cited by this extraordinary appearance, we were induced to proceed still farther, in quest of new adventures in this subterranean abode. As we proceeded, new wonders offered themselves; the spars, formed into trees and shrubs, presented a kind of petrified grove; some white, some green and all receding in due perspective. They struck us with the more amazement, as we knew them to be mere productions of Nature, who, hitherto in solitude, had, in her playful moments, dressed the scene as if for her own amusement.

In

"But we had as yet seen but a few of the wonders of the place; and were introduced only into the portico of this amazing temple. In one corner of this halfilluminated recess, there appeared an opening of about three feet wide, which seemed to lead to a place totally dark, and that, one of the natives assured us, contained nothing more than a reservoir of water. Upon this we tried by throwing down some stones, which, rumbling along the sides of the descent for some time, the sound seemed at last quashed in a bed of water. order, however, to be more certain, we sent in a Levantine mariner, who, by the promise of a good reward, with a flambeau in his hand, ventured into this narrow aperture. After continuing within it for about a quarter of an hour, he returned, carrying some beautiful pieces of white spar in his hand, which Art could neither imitate nor equal. Upon being informed by him that the place was full of these beautiful incrustations, I ventured in once more with him, for about fifty paces, anxiously and cautiously descending by a steep and dangerous way. Finding, however, that we came to a precipice which led into a spacious amphitheatre, if I may so call it, still deeper than any other part, we returned, and being provided with a ladder, flambeaux, and other things to expedite our descent, our whole company, man by man, ventured into the same opening, and, descending one after another, we at last saw ourselves all together in the most magnificent part of the cavern.

be

"Our candles being now all lighted up, and the whole place completely illuminated, never could the eye presented with a more glittering or a more magnificent scene. The roof all hung with solid icicles, transparent as glass, yet solid as marble. The eye could scarce reach the lofty and noble ceiling; the sides were regularly formed with spars; and the whole presented the idea of a magnificent theatre, illuminated with an immense profusion of lights. The floor consisted of solid marble; and, in several places, magnificent columns, thorns, altars, and other objects appeared, as if Nature had designed to mock the curiosities of Art. voices, upon speaking or singing, were redoubled to an astonishing loudness; and upon the firing of a gun, the noise and reverberations were almost deafening, In the midst of this grand amphitheatre rose a concretion of about fifteen feet high, that in some measure resembled an altar.

Our

"Below even this spacious grotto there seemed ano. ther cavern, down which I ventured with my former mariner, and descended about fifty paces by means of a rope. I at last arrived at a small spot of level ground, where the bottom appeared different from that of the amphitheatre, being composed of soft clay, yielding to the pressure, and in which I thrust a stick to about six feet deep. In this, however, as above, numbers of the most beautful crystals were formed, one of which particularly resembled a table. Upon our egress from this amazing cavern we perceived a Greek inscription upon a rock at the mouth, but so obliterated by time that we could not read it. It seemed to import that one Anti pater, in the time of Alexander, had come thither; but whether he had penetrated into the depths of the cavern he does not think fit to inform us."

Such is the account of this beautiful scene as commu

nicated in a letter to Kircher. We have anotner, and a more copious, description by Tournefort; but I have given the above, both because it was communicated by the first discoverer, and because it is a simple narrative of facts without any reasoning upon them. According to Tournefort's account, indeed, we might conclude, from the rapid growth of the spars in this grotto, that it must every year be growing narrower, and that it must in time be entirely choked up with them; but no such thing has happened hitherto, and the grotto at this day continues as spacious as we ever knew it.

This is not the place for an inquiry into the seeming vegetation of those stony substances with which this and almost every cavern are incrusted; it is enough to observe, in general, that they are formed by an accumulation of that little gritty matter which is carried thither by the waters, and which in time acquires the hardness of marble. What in this place more imports us to know is, how these amazing hollows in the earth came to be formed; and I think, in the three instances above mentioned, it is pretty evident that their excavation has been owing to water. These finding subterranean passages under the earth, and by long degrees hollowing the beds in which they flowed, the ground above them has slipped down closer to their surface, leaving the upper layers of the earth or stone still suspended-the ground which sinks upon the face of the waters forming the floor of the cavern; the ground, or rock that keeps suspended, forming the roof. And, indeed, there are but few of these caverns found without water, either within them, or near enough to point out their formation.

CHAP. VIII.

OF MINES, DAMPS, AND MINERAL VAPOURS. The caverns which we have been describing generally carry us but a very little way below the surface of the earth: two hundred feet, at the utmost, is as much as the lowest of them is found to sink. The perpendicular fissures run much deeper, but few persons have been bold enough to venture down to their deepest recesses; and some few who have tried have been able to bring back no tidings of the place-for unfortunately they left their lives below. The excavations of art have conducted us much further into the bowels of the globe. Some mines in Hungary are known to be a thousand yards perpendicular downwards; and I have been informed, by good authority, of a coal-mine in the north of England a hundred yards deeper still.

It is beside our present purpose to inquire into the peculiar construction and contrivance of these, which more properly belong to the history of fossils. It will be sufficient to observe in this place, that as we descend into the mines the various layers of earth are seen as we have described them, and in some of these are always found the metals or minerals for which the mine has been dag. Thus frequently gold is found dispersed and mixed with clay and gravel; sometimes it is mixed with other metallic bodies, stones, or bitumens: and sometimes united with that most obstinate of all substances, platina, from which scarce any art can separate it Silver is sometimes found quite pure, sometimes mixed with other substances and minerals. Copper is found in beds mixed with various substances-marbles, sulphurs, and pyrites. Tin (the ore of which is heavier than that of any other metal) is generally found mixed with every kind of matter. Lead is also equally common; and iron we well know can be extracted from all the substances upon earth.

The variety of substances which are thus found in he bowels of the earth in their native state have a very different appearance from what they are afterwards

taught to assume by human industry. The richest metals are very often less glittering and splendid than the most useless marcasites, and the basest ores are in general the most beautiful to the eye.

This variety of substances which composes the internal parts of our globe is productive of equal varieties, both above and below its surface. The combination of the different minerals with each other-the heats which arise from their mixture-the vapours they diffuse-the fires which they generate, or the colds which they sometimes produce, are all either noxious or salutary to man; so that in this great elaboratory of Nature a thousand benefits and calamities are forging of which we are wholly unconscious; and it is happy for us that we

are so.

Upon our descent into mines of considerable depth, the cold seems to increase from the mouth as we descend; but after passing very low down, we begin by degrees to come into a warmer air, which sensibly grows hotter as we go deeper, till at last the labourers can scarce bear any covering as they continue working.

This difference in the air was supposed by Boyle to proceed from magazines of fire that lay nearer the centre, and that diffused their heat to the adjacent regions. But we now know that it may be ascribed to more obvious causes. In some mines, the composition of the earth all around is of such a nature, that upon the admission of water or air it frequently becomes hot, and often bursts out into eruptions. Besides this, as the external air cannot readily reach the bottom, or be renewed there, an observable heat is perceived below, without the necessity of recurring to the central heat for an explanation.

Hence, therefore, there are two principal causes of the warmth at the bottom of mines-the heat of the substances of which the sides are composed, and the want of renovation in the air below. Any sulphureous substance mixed with iron produces a very great heat by the admission of water. If, for instance, a quantity of sulphur be mixed with a proportionable share of iron filings, and both kneaded together into a soft paste with water, they will soon grow hot, and at last produce a flame. This experiment, produced by art, is very commonly effected within the bowels of the earth by Nature. Sulphurs and irons are intimately blended together, and want only the mixture of water or air to excite their heat; and this, when once raised, is communicated to all bodies that lie within the sphere of their operation. Those beautiful minerals called "marcasites” and “pyrites," are often of this composition; and wherever they are found, either by imbibing the moisture of the air or having been by any means combined with water, they render the mine considerably hot.

The want of fresh air, also, at these depths is, as we have said, another reason for their being found much hotter. Indeed, without the assistance of art, the bottom of most mines would from this cause be insupportable. To remedy this inconvenience, the miners are often obliged to sink, at some convenient distance from the mouth of the pit where they are at work, another pit, whi h joins the former below, and which in Derbyshire is called an air-shaft. Through this the air circulates; and thus the workmen are enabled to breathe freely at the bottom of the place, which becomes, as Mr. Boyle affirms, very commodious for respiration, and also very temperate as to i eat and cold. Mr. Locke, however, who has left us an account of the Mendip-mines, seems to present a different picture. "The descent into these is exceeding difficult and dangerous; for they are not sunk, like wells, perpendicularly, but as the crannies of the rocks happen to run. The constant method is to swing down by a rope placed under the arms, and clamber along by ap plying both feet and hands to the sides of the narrow passage The air is conveyed into them through a little passage that runs along the sides from the top, where