difcovered the whole affair to his mafter Villa Roel, who lived at Porco. Villa Roel immediately repaired to the spot to ascertain the truth of the report; which being done, he caufed Huanca inroll himself in the regifter for his claim to that portion of land which the laws allow to thofe who discover a mine. In confequence of this, Villa Roel and he became joint proprietors of the district in which the mine is fituated, after having communicated the discovery to government, and engaged to pay the fifth of the produce to the king. This happened the 21st of April 1545. A few days afterwards feveral other veins were difcovered, in all of which, however, the ore, though very rich, was at the fame time very hard in the operation of reducing to metal. In Spanish, this hard mineral is called Mine of Tin. The 31st of Auguft in the fame year, the mine of Mendieta was difcovered, which was alfo registered. These are the four principal mines of Potofi. It is faid, that the mine called the Rich, formed a rock iffuing above the furface of the ground about the height of a vara, and extending to the length of three hundred feet by a breadth of thirteen. The ore was fo rich as to produce the half of its weight of filver. This rich proportion continued till they had funk to fifty or fixty fa

thoms below ground, when the pre duce began to leffen.

It appears, from the accounts of the Caifes Royales, that while Polo was governor of Peru, there was paid in every Sunday evening the fifth of an hundred and fifty, or two hundred thoufand pounds weight of filver, amounting in the year to nearly a million and a half. This calculation comprehended only the filver which paid the fifth, and of which the accounts were checked. But it is well known, that it has long been a cuftom in Peru not to pay the fifth of the filver, which goes by the name of Argent de Cours, and of which the accounts are not checked. Now, thofe who are acquainted with the mines of Potofi, alledge that a very great proportion of the filver which they yield is not subjected to the fifth, particularly that which ferves the purpose of current fpecie among the Indians and Spaniards. It may be prefumed on these grounds, that the third, or perhaps even the half of the whole produce is never exhibited to the Caifjes Royales, and confequently pays no tax to the king. It is a remarkable fingularity in the mines of Potofi, that they have never been subject to inundation, although the pits have been funk to the depth of above two hundred fathoms.

I'

F the name of Volcano were not given exclufively to fuch mountains as vomit forth fire, and if it did not particularly ferve to exprefs the effects produced by this terrible element, but were applicable to every mountain that is formed by the accumulation of its own ejected matter, I would beftow the name upon a fingular phenomenon which I had an op

which

which eject the fubftances it throws out to the height of three hundred feet and upwards. But by whatever name this mountain may be defigned, its phenomena are certainly very fingular and furprifing.

On the 18th of September 1781, as I was travelling from Arragona to Girgenti, I quitted the road leading to this laft town, in order to view a place which, from the variety of accounts I had received of it, excited my curiofity. The foil of the country is chiefly calcarious. It is interperfed with hills and little eminences of clay, which are worn and deeply excavated by the rains, fome of them having a nucleus of gypfum. After an hour's journey, I arrived at the place that had been defcribed to me. I found a mountain with a truncated top, its bafe having nothing remarkable; but on the plain which terminates it, I obferved the moft fingular phenomenon I had ever met with.

This mountain has a circular bafe, it imperfectly reprefents a truncated cone, and is about 150 feet high from the valley below which furrounds it. It is abfolutely fterile, and produces not the fmalleft appearance of vegetation. On its fummit there is a valt number of fmaller ones in the fhape of truncated cones, at different diftances and of different heights. The largest are about two feet and an half high, the fmalleft only a few lines. They are all furnished at the top with little funnel-fhaped craters proportioned to their fize, and thefe are nearly half the depth of the whole elevation. The foil on which they ftand is a greyish clay, hardened and interfected with chinks in every direction, breaking into pieces of four or five inches in thickness. The very fenfible vibrating motion which a perfon feels in walking over this plain, fufficiently fhews that he is fupported by a very thin cruft, incumbent on a foft and femi-fluid mafs; and he is foon convinced that this dried clay really

covers a vast and immense gulf of mud, into which there is the greatest danger of falling.

The infide of each crater is always moift, and in a state of continual motion.

From the bottom of the funnel there conftantly rifes a quantity of diluted clay, of a greyish colour, convex on the furface, which reaches and rests upon the edges of the crater in an hemifpherical form; this hemifphere at laft burfts, and a bubble of air, which was the occafion of the appearance, immediately efcapes. The bubble burfts with a noife like that made by a bottle when fuddenly uncorked; it throws out from the crater part of the clay that enveloped it, and this runs like lava down the fides of the eminence till it reaches the bottom, where it extends itfelf around to a greater or lefs diftance. When the air is difengaged, the refidue of the clay falls back into the crater, where it refumes and preferves its firft form till a new bubble is ready to efcape. Thus there is a continual motion of rifing and failing more or lefs rapid, at intervals of two or three minutes. It is accelerated by fhaking the cruft with one's foot.

When a stick is thruft into one of thefe craters, it is pushed back again by little and little, and by jerks; but it is not thrown to a distance, as I had been told it was. While I was bufy in obferving the phenomena of this mountain, three of my fervants amufed themfelves by putting into one of the large craters bits of the hardened clay from the surface; these were abforbed; and after an hour, during which this operation continued, the orifice was only a little dilated, but not filled. Some of thefe eminences are quite dry, and afford no paffage to the air; the number of both kinds generally amounts to more than a hundred, but varies daily. Befides the fmall cones, there are cavities in the ground itfelf, efpecially towards the

Wefts

Weft, where it inclines a little. Thefe round holes, of one or two inches in diameter, are full of muddy water, which has a faline tafte; from thefe arife, and immediately iffue, bubbles of air which caufe an ebullition like that of boiling water, and they burft without noife or explofion. I found at the furface of fome of thefe cavities a pellicle of bituminous oil of a ftrong fmell, which is often mistaken for that of fulphur.

Such is the ftate of the mountain during the Summer and Autumn while it is dry weather, and it was then that I faw it. But in Winter the circumstances are different: the Fains foften and dilute the dried clay of the fummit, the conical eminences are obliterated, the furface becomes level, and the whole appears a vaft gulf of mire and clay, the depth of which is unknown, as it cannot be approached without the greatest danger. A continual ebullition takes place over all the furface, the air that produces it has no particular vent, but flues from all places indifcriminately. Thefe two different ftates, which I have just described, fubfift only while the mountain is at reft. It has likewife its moments of great agitation, when it prefents phenomena that ftrike terFor into the people of its neighbour hood, and that resemble those which precede eruptions in ordinary volcapoes. At two or three miles diftance are fometimes perceived very violent fhocks of an earthquake; a noise of fubterraneous thunder is heard, and, after a continued agitation for feveral days, and progreffive augmentation of the internal commotion, there fucceed violent eruptions, accompanied with Boife; and maffes of earth, mud, moiftened clay mixed with a few ftones, are ejected perpendicularly to the height of two or three hundred feet. Thefe fubftances fall down again upon the fpot from whence they iffued. The explosions recur three or four times in wenty-four hours: they are attended

with a fetid fmell of liver of fulphur, which is felt all over the neighbour hood, and fometimes, it is faid, smoke is feen. Afterwards thefe preliminary phenomena ceafe, and the mountain re-affumes one of the two ftates in which I have reprefented it.

The eruptions of this fingular vol. cano happen in Autumn, after warm Summers and great droughts, but at different intervals. Sometimes a great number of years intervenes, then they take place two years fucceffively, or twice in three years, as was the cafe in 1777 and 1779. Some authors have afferted that there is a regular intermiflion of five years, but this is not confirmed by obfervation.

1 fhall here give a literal translation of an account drawn up at the time by an eye-witnefs of the eruption in the year 1777.

"About a league from the fea be hind Girgenti, there is a place called Moruca by the antients, now Maca lubi, where, upon an eminence fituated in a falt plain (falma) of sterile ground, are obferved different apertures from whence clay and troubled water are difcharged with flow ebullition. On the 30th of laft September, (1777) about half an hour after fun-rife, a dull noife was heard at this place, which, increafing by degrees, exceed ed that of the loudest thunder. Af terwards the ground in the neighbourhood began to fake, and the large chafms that were then made in it are ftill to be feen; the principal aperture from which the clay and the muddy water generally flow increafed to the fize of ten fpans, (palmi) in diame ter: then there arofe fomething like a cloud of smoke, which gained in a few moments the height of eighty fpans: although this explofion appeared in fome places of a flame colour, it however confifted of mud and bits of clay; fome of which as they fell back again fpread themselves all over the plain, but the greater part fell into the aper tures from which they were ejected.

The

The eruption lafted for half an hour, and was renewed three other times at intervals of a quarter of an hour, and each eruption continued a quarter of an hour. In the mean time, the motion and agitation of the great mafs were heard under ground, and, at the diftance of three miles, a noife was obferved like that of the roaring of the fea. While these terrible convulfions lafted, people thought the end of the world was come, and were afraid of being buried under the fubftances difcharged from the aperture, and which covered the ground to the depth of fix fpans, befides filling up the neighbouring vallies; and altho' the clay was liquid on the day of the eruption, it appeared on the next day to have regained its ufual confiftence, allowing the curious to approach and examine the great aperture fituated in the middle of the plain. This mud Rill preferves the fmell of fulphur, though not fo ftrongly as at the time of the eruption. The other orifices that had been fhut during the explofion again appeared, and a flight fubterranean noife is yet heard that makes us dread another eruption."

We are always apt to attribute effects nearly fimilar to a fimilar caufe. As this mountain is liable to eruptions like Ætna, the inhabitants of the neighbourhood, and the few travellers who have seen it, confider this as fufficient to make them fuppofe that all the phenomena are owing folely to fubterranean fire. I was prepoffeffed with the fame opinion, and thought I was going to fee an ordinary volcano at the commencement of its convulfions, or after they had ceafed. I never fufpected that there was any other agent in nature capable of producing the phenomena that had been defcribed to me; but I was foon undeceived. I faw nothing' around me that indicated the prefence of the igneous element, which impreffes every thing it acts on with fuch diftinctive characters; and I was foon convinced, that nature employs very diffimilar means VOL. VII. No 39.

to operate fimilar effects. I perceived that fire was not here the principal agent, that it produced none of the phenomena of this mountain; and though in fome of the eruptions fmoke and heat were obfervable, yet that theft were only acceffory circumftances, but by no means the true caufe of the explofions. However, before I attempt to investigate the nature of the new agent, I muft relate fome circumftances which I 'omitted in deferibing the more obvious appearances.

Upon my arrival at Macalubi, I was, in the first place, folicitous to afcertain whether there exifted any heat in the ebullitions which I faw around me. I walked with fear and trembling over this unftable furface; Ithought it hazardous to approach the larger cones, near which the ground was lefs hard than elfewhere, and which might fwallow me up; growing bolder, however, after various attempts I ventured to approach the centre of the plain: I put y hand into the moift clay of the craters, and into the hollows full of water which was then bubbling; but, inftead of the fenfation of heat, I felt a degree of cold. I plunged into them my thermometer, which at that time, in the open air, stood at 231°, when it funk three degrees. I thrust my naked arm into the clay of one of the craters as far as I could, and found it colder than at the furface. No smell of fulphur, no finoke were perceptible; and, in a word, by no other means could I difcover, in the state of the mountain at that time, any veftige of fire. This fact being fufficiently confirmed, we must endeavour to afcertain whether, in the great eruptions, the igneous element is the chief agent, or in any degree acceffory. Of this I foon began to doubt: I traversed every part of the plain and of the mountain, efpecially its external furface, and found nothing on which fire had ever acted: on the contrary, I met with fubftances which proved inconteftibly that this destroying agent

had

had never existed here. I faw, in the ejected matter of the laft eruptions, diluted clay, containing calcareous fpar which had fuffered no change, calcareous stones abfolutely untouched, with regular cryftals of fpar, fragments of foliaceous felenite, or lapis fpecularis. Thefe fubftances, to wit, the fpar and and cryftallifed gypfums, are altered by the leaft touch of fire, and the clay hardens and becomes red. Now, as this clay and thofe ftones bore no marks of fire, it follows, that they had never been exposed to its action; it has never therefore exifted here, and this fingular phenomenon cannot be attributed to it. When my obfervations had convinced me that this mountain was not a common volcano, I easily found the caufe of all the appearances. I collected in a bottle a quantity of the air, difengaged from the diluted clay as well as from the water, and introduced into it a lighted taper, which was inftantly extinguished. This air, when mixed with that of the atmosphere, was neither at aded with inflammation nor explosion. I had no convenience for making other experiments, but this was fufficient to fhew, that the air was fixed air, and the only agent in the phenomena I have defcribed and it occurred to me, that the following explanation was fufficient to folve the problem that had at first embarraffed me.

The foil of the whole country is calcareous, as I have already faid; it is overfpread with mountains of a grey and ductile argilla, which contain pretty frequently a nucleus of gypfum. There happens accidentally to be placed in the midst of that called Macalubi, a foring of falt water, fuch as are frequent in the countries where mines of fal-gem abound. This water keeps the argilla continually in a moift ftate, and oozing out of the mountain, runs down one of its fides. The vitriolic acid of the argilla unites with the bafis of the fea falt, and thus difengages the muriatic acid, which the feizes the calcarcous matter of the foil. Its com

bination with this new bafis produces a confiderable extrication of fixed air, which tranfudes through the whole mafs of fuperincumbent clay, and appears at the furface. The vitriolic acid of the clay may likewife combine at once with the calcareous matter, and thus continually form gypfum. The air in its paffage through the clay gives it a fort of kneading, which angments its ductility and tenacity. During the rains of Winter, the clay is more diluted, the air is more eafily difengaged, and the ebullitions are more frequent. In Summer, the clay on the furface dries, and forms a cruft more or lefs thick. The air at this time makes an effort to escape, and ifues at the place which offers the leaft refiftance. It depofits by degrees the portion of earth which it forces along, and forms the fmaller cones, through which it fecures an exit. But when the Summers have been long, warm, and dry, the clay becomes more and more compact and vifcid; it is only imperfectly diluted by the fpring below, which is then lefs copious; it refifts the elafticity of the air, to which it is no longer permeable, and this air being continually difengaged in the lower parts, which are always moist, makes ineffectual efforts to efcape; and when at laft it is accumulated and comprefied to a certain degree, it produces thofe earthquakes, fubterra neous noifes and eruptions I have before defcribed; and its force is proportioned to the refiftance it meets with. This fixed air is therefore the only agent in all the phenomena of this mountain,

The fmoke which accompanies the eruptions is a circumftance that does not contradict the explanation I have here given, Smoke in general is nothing but water in a flate of vapour: clouds and mifts refemble it, and it is not extraordinary that the air, when it is dilated, and produces the explosions I attribute to it, fhould reduce into va pour the water of the fpring that is under the mountain.

The