Earth- the sides of the mountain in a dreadful and destructive quakes and torrent." Volcanoes.

tom. v.

Others have attempted to account for the existence of volcanic fire, on the supposition that it is derived from central fires, and to these it is supposed that volcanoes act the part of chimneys; while others are of opinion that they are owing to the chemical decomposition of different substances, by which inflammable matters are evolved, with a great deal of heat, and by means of the latter the combustible materials are kindled, and exhibit the phenomena which are thus proposed to be accounted for.

M. Patrin is one of the latest naturalists who, with the assistance of modern chemistry, has attempted to account for the phenomena of volcanoes on the principles of this science. For a full view of his theory, or rather of his fanciful conjectures on this subject, we Hist. Nat. must refer the reader to the work itself. But the folde Mineral. lowing is a recapitulation of the principles on which he gives this explanation. All volcanoes, he observes, in a state of activity, are in the vicinity of the sea, and are never found but in those places where sea salt is abundant. The volcanoes of the Mediterranean abstract the salt which the waters of the ocean hold in solution, and are constantly pouring in by the straits of Gibraltar. The strata of primitive schistus are the great laboratories in which volcanic matters are prepared, by a constant circulation of different fluids; but, according to this theory, these strata contribute no part of their own substance. They suffer no waste in the process.

The sphere of the activity of volcanoes may be far extended in these strata, but they have no other outlet beside spiracles, by which the gaseous substances escape, of which one part is dissipated in the atmosphere, and the other becomes concrete by its combination with oxygen. The concretion of these fluids is supposed to be analogous to the concretion of the primitive matters of the globe, according to the theory of La Place; and the elective attractions determine, in the same way, the formation of stony crystals.

Volcanic eruptions are proportioned, in regard to their violence and duration, to the extent of the strata of schistus in which the volcanic fluids are accumulated. These fluids are,

1. Muriatic acid, which carries off the oxygen from the metallic oxides of the schistus.

2. The oxygen of the atmosphere, which constantly replaces in the metals that which was carried off by the muriatic acid.

3. Carbonic acid gas, which the water absorbs from the atmosphere, and conveys to the schistus, which always abounds in carbone.

4. Hydrogen, which proceeds from the decomposition of water. A part of this hydrogen is inflamed by electric explosions; the other united to carbonic acid forms oil, which becomes petroleum by its combination with sulphuric acid; and it is to this petroleum that the bitterness of sea water is owing.

5. The electric fluid, which is attracted from the atmosphere by the metals contained in the schistus. Sulphur seems to be the most homogeneous portion of this fluid, which has become concrete. Phosphorus is a modification of it, and it contributes to the fixation of oxygen. The sulphur formed in the schistus by means

of the electric fluid, combines with the oxygen, and forms sulphuric acid, which decomposes the sea salt.

6. The metalliferous fluid. This forms the iron in lavas. It is the origin of metallic veins, and the colouring principle of organized bodies. This substance in its undecomposed state affords iron, but by decomposition it produces other metals. It is conjectured to be one of the principles of muriatic acid, and it contributes, along with phosphorus, to fix oxygen under an earthy form.

7. The last of the volcanic fluids is azotic gas. Το this gas is owing the formation of the masses of carbonate of lime which are ejected by Vesuvius, and of the calcareous earth contained in lavas.

Such are the materials with which the author proposes to form the different substances which are produced in volcanoes, and by the operation of which he proposes to explain the phenomena of volcanic eruptions. Our readers will probably agree with us in thinking, that the present state of chemical science, even with the assistance of such hypothetical substances as the metalliferous fluids, is yet inadequate to give any degree of support to such opinions, even in the form of conjecture. We shall therefore dismiss it without farther remark.

Earth. quakes and Volcanoes.

245 We shall now conclude this subject with some interObservations on the esting observations by M. de Luc, on the nature of the nature of strata in which volcanic fires exist. the strata.

"Volcanoes, he observes, have been more numerous on the surface of our continents, when they were under the waters of the ancient sea; and as this class of mountains, raised by subterranean fires, manifest themselves still on the shores of the present sea, and in the middle of its waters, it is of importance to geology and the philosophy of the earth to obtain as just ideas of them as possible.

"I have attended a great deal to this subject from my own observations; and I have shown, at different times, the errors into which several geologists and naturalists, in treating of it, have fallen.

"This class of mountains, in particular, requires that we should see them, that we should behold them during their eruptions, that we should have traced the progress of their lava, and have observed closely their explosions; that we should have made a numerous collection of the matters which they throw up under their different circumstances, that we might afterwards be able to study them in the cabinet, and to judge of their composition according to the phenomena which have been observed on the spot.

"This study is highly necessary when we apply it to geology and the philosophy of the earth, in order that we may avoid falling into those mistakes which make us ascribe to subterranean fires what does not belong to them, or which leads us to refuse them what really belongs to them.

We read in the Journal de Physique for January 1804, under the title, On the Cause of Volcanoes, the following assertions :

What is the nature of the matters which maintain these subterranean fires? We have seen that Chimboraço, all these enormous volcanoes of Peru, and the Peak of Teneriffe, are composed of porphyry.

The Puy-de-Dôme is also composed of porphyry, as well as the Mont d'Or and the Cantal. • Ætna,

Earth

Ætna, Solfatara, and Vesuvius, are also of the porquakes and phyry kind.

Volcanoes.

These facts prove that the most considerable volcanoes with which we are acquainted are of porphyry.' "This opinion, that the fires of volcanoes have their centres in such or such a rock, and that their lavas are produced from these rocks, has always appeared to me not to be founded on any certain data. Opinions also on this subject have varied; some having placed the origin of lava in horn rock, others in granite or schist, and at present it is assigned to porphyry.

"I have always been of opinion that nothing certain could be determined in regard to this point. It ever remains uncertain whether the seat of the matters of which lava is formed be in compact rocks, or in strata in the state of softness, pulverulent, and muddy.

"Those who see lava issue from a volcano in its state of fusion and incandescence, and in its cooling, are convinced that the nature of every thing is changed, that it exhibits a paste in which nothing can be known, except the substances which the volcanic fires have not reduced to fusion.

"But these substances contained in the paste of lava, and those which are the most numerous, show us, that the strata from which they proceed cannot be similar to those exposed to the view, nor even to the most profound strata to which we can penetrate.

"Admitting the hypothesis, that the strata from which the lavas proceed are in a pulverulent and muddy state, containing elements of all these small crystals, one may conceive how they are formed there, insulated, grouped, or solitary, and are found then in the lava in that state of insolution.

"The fragments of natural rocks thrown up by Vesuvius are not of the same kind as the matters of which the lava is composed. Most of these fragments are micaceous rocks with lamine of greater or less size, and of a kind of granite called syenite. I have found some composed of white quartzy rock; it is found sometimes of calcareous rock.

which the eruptions have broken to form for themselves a passage.

"No volcano rests on natural strata: they sometimes show themselves on the exterior; but they have been opened by eruptions, and their edges have remained in their place.

"The focus of no volcano exists or has existed in the cone which appears above the surface of the ground. They have been raised by eruptions, which, proceeding from great depths, have thrown them up through the upper strata. When it is said, therefore, that the volcanic mountains of Auvergne rest on granite, this is a mistake, and an incorrect expression has been used by those who have not formed a just idea of the phenomenon. Lava may have flowed upon granite or any other rock, and rested upon it; but this is never the case with the volcano itself: its bases are below all the rocks visible.

"It is from the bosom even of the lava, when in a state of fusion in the interior of the volcano, that all the explosions proceed. In that state of fusion they contain all the matters which produce fermentations, and the disengagement of expansible fluids.

"I have been enabled to ascertain this on Vesuvius as far as was possible. The continual noise which was heard through the two interior mouths of the crater which I had before my eyes, was that of an ebullition, accompanied with inflammable vapours, and the gerbes of burning matters which they threw up at intervals were separated pieces of the lava in its state of fusion. I saw several of them in the air change their form, and sometimes become flat on the bodies which they struck or embraced in falling. And among the most apparent of these fragments there are always a multitude of small ones of the size of peas and nuts, and still smaller ones, which show at their surface, by their asperities, all the characters of laceration.

point that it has been said that it is the compact part only that we must observe, in order to judge of their nature. The pieces which I took from the flowing lava with an iron hook, have at their surface the same lacerations and the same asperities as the fragments thrown up by explosions, and both contain the same substances.

"Some of these fragments of natural rocks have not been attacked by the fire; others have more or less; which depends, no doubt, on the place which they occupied in the volcano, and on the time which they remained in it. The most of the latter have retained at their surface a crust of lava, and this crust contains substances which are not the same as that of the fragment it covers.

"On Vesuvius the strata pierced by eruptions are lower than the surface of the soil; in Auvergne and several places of Germany they are above; for this reason there are seen there in their place schists or granites, VOL. IX. Part II.

"This separation, by tearing off the parcels of the lava, effected by fermentations and explosions which proceed from their bosom, serves to explain those columns, sometimes prodigious, of volcanic sand, which rise from the principal crater. When seen with a magnifying glass, this sand exhibits nothing but lava reduced very small, the particles of which, rough with inequalities, bave the bright black colour and the varnish of recent lava.

"Parcels of substances which exist in our strata, such as fragments of quartz, scales of mica, and crystals of feldspar, are found sometimes in lava. Similar matters must no doubt be disseminated in the composition of our globe, without there being reason to conclude that the strata from which they proceed are the same as the exterior strata. It is neither in the granites, the porphy 4 K

ries,

Earth- ries, nor the horn rock, and still less in the schists and quakes and calcareous rocks, that the schorls of volcanoes, the leuVolcanoes, cites, and perhaps olivins, will be found. These small crystals are brought to view by the lava, otherwise they would be unknown to us.

"These lavas contain a great deal of iron, which they acquire neither from the granite nor porphyries. Might not one see in the ferruginous sand which is found in abundance on the borders of the sea near Naples, and in the environs of Rome, specimens of that kind of pulverulent strata from which lava proceeds?

"I have here offered enough to prove that it cannot be determined that lava proceeds from strata similar to those with which we are acquainted. The operations of volcanoes, those vast laboratories of nature, will always remain unknown to us, and on this subject our conjectures will always be very uncertain.

"What is the nature of that mixture which gives birth to these eruptions, that produce lava and throw up mountains? What we observe as certain is, that the introduction of the water of the sea is necessary to excite these fermentations, as containing marine acid, and other salts, which, united to the sulphuric acid, the bases of which are contained in abundance in the subterranean strata, determine these fermentations, which produce the disengagement of fire and other fluids, and all the grand effects that are the consequence.

Several naturalists have believed, and still believe, that fresh or rain water is sufficient for this purpose; but they are mistaken this opinion is contradicted by every fact known. To be convinced of this, nothing is necessary but to take a short view of them. I have done it several times, as it is necessary to consider them often. I shall here enumerate the principal ones :No burning mountain exists in the interior part of the earth; and all those which still burn are, without exception, in the neighbourhood of the sea, or surrounded by its waters. Among the deliquescent salts deposited by the smoke of volcanoes, we distinguish chiefly the marine salt, united to different bases. Several of the volcanoes of Iceland, and Hecla itself, sometimes throw up eruptions of water, which deposit marine salt in abundance. No extent of fresh water, however vast, gives birth to a volcano. These facts are sufficient to prove that the concurrence of sea-water is absolutely necessary to excite those fermentations which produce volcanoes.

"I shall here repeat the distinction I have already made between burnt-out volcanoes and the ancient volcanoes, that I may range them in two separate classes. "When we simply give the name of burnt-out or extinguished volcanoes to volcanic mountains which are in

the middle of the continents, it is to represent them as Earthhaving burnt while the land was dry, and inhabited as quakes and it is at present; which is not a just idea. These vol-Volcanoes. canoes have burnt when the land on which they are raised was under the waters of the ancient sea, and none of them have burnt since our continents became dry. It is even very apparent that most of them were extinct before the retreat of the sea, as we find by numerous examples in the present sea.

"Those which I denominate extinct volcanoes are such as no longer burn, though surrounded by the sea, or placed on the borders of it. They would still burn, were not the inflammable matters by which they were raised really exhausted and consumed. Of this kind is the volcano of Agde, in Languedoc. Of this kind also are many of the volcanic islands which bave not thrown up fire since time immemorial.

"M. Humboldt, in his letters written from Peru, speaks of the volcanoes which he visited, but what he says is not sufficiently precise to enable us to form a just idea of them. He represents Chimboraço as being composed of porphyry from its bottom to its summit, and adds, that the porphyry is 1900 toises in thickness; afterwards, he remarks, that it is almost improbable that Chimboraço, as well as Pichincha and Antisana, should be of a volcanic nature: The place by which we ascended, (says he), is composed of burnt and scorified rock, mixed with pumicestone, which resembles all the currents of lava in this country.'

"Here are two characters very different. If Chimboraço be porphyry from the top to the bottom, it is not composed of burnt and scorified rocks, mixed with pumicestone; and if it be composed of burnt rocks, it cannot be porphyry. This expression, burnt and scorified rocks, is not even exact, because it excites the idea of natural rocks, altered in their place by fire, and they are certainly lava which has been thrown up by the volcano. But the truth must be, that Chimboraço, and all the other volcanoes of Peru, are composed of volcanic matters, from their base at the level of the sea to the summit.

"I have just read in the Annales du Muséum d'Histoire Naturelle*, a letter of the same traveller, written y from Mexico, on his retura from Peru, where, speaking of the volcanoes of Popayan, Pasto, Quito, and the other parts of the Andes, he says, 'Great masses of this fossil (obsidian) have issued from the craters; and the sides of these gulfs, which we closely examined, consist of porphyry, the base of which holds a mean between obsidian and pitchstone (pechstein).' M. Humboldt therefore considers obsidian, or black compact glass, as a natural fossil or rock, and not a volcanic glasst."

†Joger.de Mines

INDEX.

N° 93