Theory of the Earth Volume I Part 18

This great naturalist is convinced that the spar had not been here introduced by infiltration, although that is the very method which he employs to form concretions, not only of spar but of crystal, zeolite, and pyrites, in the closest cavities of the most solid rocks of basaltes. These four substances in this stone were so mixed together that nothing but the fusion of the whole ma.s.s could explain the state in which they appeared; but, thinking that such a supposition could not be allowed, this naturalist, like a man of science when his data fail, leaves the matter without any interpretation of his own. This however is what he has not done in the case of basaltes, or that which he mistakes for proper lavas, as I shall have occasion to show.]

Here is an infinite operation, but an operation which is easily performed by the natural arrangement of substances acting freely in a fluid state, and concreting together, each substance, whether more simple or more compound, directing itself by its internal principle of attraction, and affecting mechanically those that are concreting around it.

We see the very same thing happen under our eye, and precisely in the same manner. When a fluid ma.s.s of any mineral or metallic substance is made to congeal by sudden cooling on the outside, while the ma.s.s within is fluid, a cavity is thus sometimes formed by the contraction of the contained fluid; and in this cavity are found artificial _druzen_, as they may be called, being crystallizations similar to those which the mineral cavities exhibit in such beauty and perfection.

Petrification and consolidation, in some degree, may doubtless be performed, in certain circ.u.mstances, by means of the solution of calcareous earth; but the examples given by M. de Luc, of those bodies of lime-stone and agate petrified in the middle of strata of loose or sandy materials, are certainly inexplicable upon any other principle except the fusion of those substances with which the bodies are petrified[35].

[Note 35: Vid. Lettre 28 et Lettre 103. Lettres Physiques et Morales.]

This subject deserves the strictest attention; I propose it as a touchstone for every theory of petrification or perfect consolidation.

First, There are found, among argillaceous strata, insulated bodies of iron-stone, perfectly consolidated; secondly, There are found, in strata of chalk and lime-stone, ma.s.ses of insulated flints; thirdly, There are found, in strata of sea sand, ma.s.ses of that sand cemented by a siliceous substance; fourthly, In the midst of blocks of sand-stone, there are found ma.s.ses of loose or pure sand inclosed in crystallised cavities; and in this sand are found insulated ma.s.ses of crystallised spar, including within them the sand, but without having the sparry or calcareous crystallization disturbed by it. There are also other globular ma.s.ses of the same kind, where the sparry crystallization is either not to be observed, or appears only partially[36]: And now, lastly, In strata of sh.e.l.l-sand, there are found ma.s.ses of consolidated lime-stone or marble. In all those cases, the consolidated bodies are perfectly insulated in the middle of strata, in which they must of necessity have been petrified or consolidated; the stratum around the bodies has not been affected by the petrifying substance, as there is not any vestige of it there; and here are examples of different substances, all conspiring to prove one uniform truth. Therefore, a general theory of petrification or consolidation of mineral bodies must explain this distinct fact, and not suffer it any longer to remain a _lusus naturae_.

[Note 36: Mem. de l'Academie Royale des Sciences, an. 1775.]

Let us now consider what it is that we have to explain, upon the supposition of those concretions being formed from a solution. We have, first, To understand what sort of a solution had been employed for the introducing of those various substances; secondly, How those concretions had been formed from such solutions within those bodies of strata; and, lastly, How such concretions could have been formed, without any vestige appearing of the same substance, or of the same operation, in the surrounding part of the stratum. Whatever may be the difficulty of explaining those particular appearances by means of fusion and mechanical force, it is plainly impossible to conceive those bodies formed in those places by infiltration, or any manner of concretion from a state of solution.

Naturalists, in explaining the formation of stones, often use a chemical language which either has no proper meaning, or which will not apply to the subject of mineral operations. We know the chemical process by which one or two stony concretions may be formed among bodies pa.s.sing from one state to another. When, therefore, a change from a former state of things in mineral bodies is judged by naturalists to have happened, the present state is commonly explained, or the change is supposed to have been made by means of a similar process, without inquiring if this had truly been the case or not. Thus their knowledge of chemistry has led naturalists to reason erroneously, in explaining things upon false principles. It would be needless to give an example of any one particular author in this respect; for, so far as I have seen, it appears to be almost general, every one copying the language of another, and no one understanding that language which has been employed.

These naturalists suppose every thing done by means of solution in the mineral kingdom, and yet they are ignorant of those solvents. They conceive or they imagine concretions and crystallizations to be formed of every different substance, and in every place within the solid body of the earth, without considering how far the thing is possible which they suppose. They are constantly talking of operations which could only take place in the cavities of the earth above the level of the sea, and where the influence of the atmosphere were felt; and yet this is the very place which we have it in our power to examine, and where, besides the stalact.i.te, and one or two more of the same kind, or formed on the same principle, they have never been able to discover one of the many which, according to their theory, ought always to be in action or effect. So far from knowing that general consolidating operation, which they suppose to be exerted in filling up the veins and cavities of the earth by means of the infiltrating water of the surface, they do not seem fully to understand the only operation of this kind which they see.

The concretion of calcareous matter upon the surface of the earth is perhaps the only example upon which their theory is founded; and yet nothing can be more against it than the general history of this transaction.

Calcareous matter, the great _vinculum_ of many mineral bodies, is in a perpetual state of dissolution and decay, in every place where the influences of air and water may pervade. The general tendency of this is to dissolve calcareous matter out of the earth, and deliver that solution into the sea. Were it possible to deny that truth, the very formation of stalact.i.te, that operation which has bewildered naturalists, would prove it; for it is upon the general solubility of calcareous matter exposed to water that those cavities are formed, in which may be found such collections of stalactical concretion; and the general tendency of those operations is to waste the calcareous bodies through which water percolates. But how is the general petrifaction or consolidation of strata, below the surface of the sea, to be explained by the general dissolution of that consolidating substance in the earth above that level? Instead of finding a general petrifying or consolidating operation in the part of the earth which we are able to examine, we find the contrary operation, so far at least as relates to calcareous spar, and many other mineral bodies which are decomposed and dissolved upon the surface of the earth.

Thus in the surface of the earth, above the level of the sea, no petrifying operation of a durable nature is found; and, were such an operation there found, it could not be general, as affecting every kind of substance. But, even suppose that such a general operation were found to take place in the earth above the level of the sea, where there might be a circulation of air and percolation of water, How could the strata of the earth below the level of the sea be petrified? This is a question that does not seem to have entered into the heads of our naturalists who attempt to explain petrifaction or mineral concretion from aqueous solutions. But the consolidation of loose and incoherent things, gathered together at the bottom of the sea, and afterwards raised into rocks of various sorts, forms by far the greatest example of petrification or mineral operation of this globe. It is this that must be explained in a mineral theory; and it is this great process of petrifaction to which the doctrine of infiltration, whether for the mechanical purpose of applying cohesive surfaces, or the chemical one of forming crystallizations and concretions, will not by any means apply.

Nothing shows more how little true science has been employed for the explanation of phenomena, than the language of modern naturalists, who attribute, to stalactical and stalagmical operations, every superficial or distant resemblance to those calcareous bodies, the origin of which we know so well. It is not a mere resemblance that should h.o.m.ologate different things; there should be a specific character in every thing that is to be generalised. It will be our business to show that, in the false stalact.i.tes, there is not the distinctive character of those water formed bodies to be found.

In the formation of stalactical concretions, besides the incrustation as well as crystallization of the stony substance from the aqueous vehicle by which it had been carried in the dissolved state, we have the other necessary accompanyments of the operation, or collateral circ.u.mstances of the case. Such, for example, is that tubular construction of the stalact.i.te, first formed by the concretion of the calcareous substance upon the outside of the pendant gut of water exposed to the evaporation of the atmosphere; we then see the gradual filling up of that pervious tube through which the petrifying water had pa.s.sed for a certain time; and, lastly, we see the continual accretion which this conducting body had received from the water running successively over every part of it. But among the infinite number of siliceous concretions and crystallizations, as well as those of an almost indefinite variety of other substances, all of which are attributed to solution, there is not the least vestige of any collateral operation, by which the nature of that concretion might be ascertained in the same manner. In all those cases, we see nothing but the concreted substances or their crystallizations; but, no mark of any solvent or incrusting process is to be perceived. On the contrary, almost all, or the greatest part of them, are so situated, and attended with such circ.u.mstances, as demonstrate the physical impossibility of that being the manner in which they had been concreted; for, they are situated within close cavities, through which nothing can pervade but heat, electricity, magnetism, etc.; and they fill those cavities more or less, from the thinnest incrustation of crystals to the full content of those cavities with various substances, all regularly concreted or crystallised according to an order which cannot apply to the concretion of any manner of solution.

That there is, in the mineral system, an operation of water which may with great propriety be termed _infiltration_, I make no doubt. But this operation of water, that may be employed in consolidating the strata in the mineral regions, is essentially different from that which is inconsiderately employed or supposed by mineralists when they talk of infiltration; these two operations have nothing in common except employing the water of the surface of the earth to percolate a porous body. Now, the percolation of water may increase the porousness of that body which it pervades, but never can thus change it from a porous to a perfect solid body. But even the percolation of water through the strata deposited at the bottom of the sea, necessarily required, according to the supposition of naturalists, must be refused; for, the interstices of those strata are, from the supposition of the case, already filled with water; consequently, without first removing that stagnant water, it is in vain to propose the infiltration of any fluid from the surface.

This is a difficulty which does not occur in our theory, where the strata, deposited at the bottom of the sea, are to be afterwards heated by the internal fires of the earth. The natural consequence of those heating operations may be considered as the converting of the water contained in the strata into steam, and the expulsion of steam or vapour, by raising it up against the power of gravity, to be delivered upon the surface of the earth and again condensed to the state of water.

Let us now conceive the strata, which had been deposited at the bottom of the sea, as exhausted of their water, and as communicating with the surface of the earth impregnated with water. Here again we have the power of gravity to operate in carrying down water to that place which had been before exhausted by the power of heat; and in this manner, by alternately employing those two great physical agents, we cannot doubt that nature may convey soluble substances from above, and deposit them below for the purpose of consolidating porous bodies, or of filling with saline and earthy matter those interstices which had been originally filled with water, when the strata were deposited at the bottom of the sea. How far any marks of this operation may be perceived, by carefully examining our mines and minerals, I know not; I can only say that, on the contrary, whenever those examined objects were clear and distinct, with the concomitant circ.u.mstances, so as to be understood, I have always found the most certain marks of the solid bodies having concreted from the fluid state of fusion. This, however, does not exclude the case of infiltration having been previously employed; and I would intreat mineralists, who have the opportunity of examining the solid parts of the earth, to attend particularly to this distinction. But do not let them suppose that infiltration can be made to fill either the pores or veins of strata without the operation of mineral heat, or some such process by which the aqueous vehicle may be discharged.

Not only are mineral philosophers so inconsiderate, in forming geological theories upon a mere supposition or false a.n.a.logy, they have even proceeded, upon that erroneous theory, to form a geological supposition for explaining the appearances of strata and other stony ma.s.ses in employing a particular physical operation, which is, that of _crystallization_[37]. Now crystallization may be considered as a species of elective concretion, by which every particular substance, in pa.s.sing from a fluid to a solid state, may a.s.sume a certain peculiar external shape and internal arrangement of its parts, by which it is often distinguished. But, to suppose the solid mineral structure of the earth explained, like an enigma, by the word _crystallization_, is to misunderstand the science by which we would explain the subject of research; and, to form a general mineral theory thus upon that term, is an attempt to generalise without a reason. For, when it were even admitted that every solid body is crystallised, we thus know no more of the geology of this earth, or understand as little of the general theory of mineral concretion, as we did before;--we cannot, from that, say whether it be by the operation of solution or of fusion which had produced the perceived effect.

[Note 37: Journal de Physique; Avril 1753.]

M. de Carosi has wrote a treatise upon certain petrifactions[38]. In the doctrine of this treatise there is something new or extraordinary. It will therefore be proper to make some observations on it.

[Note 38: Sur la Generation du Silex et du Quartz en partie.

Observations faites en Pologne 1783, a Cracovie.]

The object of this treatise is to describe the generation of silex and quartz, with their modifications or compositions, formed within mineral bodies of a different substance. The natural history contained in this little treatise is well described and sufficiently interesting. But It is chiefly in order to examine the means which, according to the theory of this treatise, are employed in petrifying bodies, that I consider it in this place.

The first section of this treatise has for t.i.tle, _Generation du Caillou et du Quartz de la terre calcaire pure_. It may be worth while to compare the natural history of this part of the earth with the flint and chert found in our chalk and lime-stone countries. I shall therefore transcribe what is worth observing upon that subject (p. 5.).

"Nous rencontrons chez nous dans les parties le plus montagneuses, et les moins couvertes de terreau, ou tout-au plus de sable, entre de purs rochers calcaires une quant.i.te incroyable de cailloux (silex) tant en boules, que veines, couches, et debris. Au premier coup d'oeil l'on s'imagine que ce font des debris de montagnes eloignees, qui y furent amenes par les eaux, mais, en examinant la chose de plus pres, on est convaincu, que ce sont tout au contraire, des parties detachees des montagnes de la contree. Car il y a sur presque toute l'etendue de nos montagnes calcaires une couche, ou pour mieux dire, un banc compose de plusieurs couches de base calcaire, mais qui ou sont pa.r.s.emees irregulierement de boules, de rognons, de veines, et de pet.i.ts filons de silex, ou qui contiennent cette pierre en filon, veines, et couches paralleles, et regulierement disposees. Les boules et rognons de silex y font depuis moins de la grandeur d'une pet.i.te noisette, jusqu'au diametre de plus de six pouces de notre mesure. La plupart de ces boules tant qu'elles sont dans l'interieur cache de la roche vive, et qu'elles n'ont rien souffert de l'impression de l'air, ont, pour l'ordinaire, une croute de spath calcaire, au moyen de la quelle elles sont accrues a la roche mere; ou pour mieux dire la croute spatheuse fait l'intermede entre le silex, et la roche calcaire, par ou se fait le pa.s.sage de l'une a l'autre. Mais ceci ne vaut que de boules de silex entierement formees.

C'est dont on peut meme se convaincre a la vue, par beaucoup de pierres dont le pave de la ville de Cracovie est compose. Mais la, ou le silex n'est pas encore entierement acheve, la croute spatheuse manque, en revanche on y voit evidemment le pa.s.sage par degres successifs de la roche calcaire au silex qui y est contenu, et les nuances de ce pa.s.sage sont souvent si peu marquees que meme les acides mineraux ne suffisent pas a les determiner, ce n'est que le briquet, qui nous aide a les decouvrir. On voit bien ou la pierre calcaire s'enfonce en couleur, l'on s'appercoit, ou sa durete, ses ca.s.sures changent, mais, comme elle y souffre encore quelque impression des acides, l'on ne sauroit determiner au juste le point, ou elle a deja plus de la nature du silex, que de celle de la chaux, qu'en la frappant du briquet.

"Tels sont les cailloux en boules et rognons avant leur etat de perfection, il y aura meme au milieu une partie de pierre calcaire non changee.

"Ceux au contraire, ou la nature a acheve son ouvrage, ont une croute de chaux endurcie, et sont purement du silex fini, mais de toutes couleurs, d'un grain et d'une texture plus ou moins fine, qui pa.s.se a.s.sez souvent par degres dans les differentes varietes du n.o.ble silex. Ils ont, pour l'ordinaire, dans leur interieur une cavite, mais pas toujours au centre, et qui vient apparemment de la consommation de cette partie calcaire qui y resta la derniere, et n'en fut changee ou dissolute et separee, que lorsque le reste du silex etoit deja entierement fini. Ces cavites sont toujours, ou enduites de calcedoine en couche concentriques recouverte de pet.i.ts cristaux fort brillans et durs de quartz, ou bien seulement de ces derniers-ci. Par-fois il y a aussi du spath calcaire crystallise, mais cela est extremement rare. Quelque-fois enfin ces cavites sont remplies d'une noix de calcedoine. Je n'ai reussi qu'une seule fois en ca.s.sant un pareil silex en boule d'y trouver encore le reste de l'eau de crystallisation."

The only remark that I would here make is this, that, if the crystallization of those close cavities in the _silex_ had at any time required water of solution, it must always have required it. But, if there had been water of solution contained in those close cavities, for the crystallization of the various things which are often found within them, How comes it that this water is almost never found? I have good reason to believe that water contained within a solid flint will not make its escape, as does that contained in the _anhydrites_ of Mount _Berico_, which are composed of a porous calcedony. But the siliceous crystallizations within close cavities is a curious subject, which we shall have occasion to examine more particularly in treating of agates.

We now proceed to the next section, which is the generation of silex and quartz in marl, (p. 19.)

"Il y a des contrees, chez nous, qui out des etendus a.s.sez considerables en long et en large, de montagnes de pierre de marne calcaire, dans lesquelles on rencontre le meme phenomene que dans celles de chaux pure; c. a. d. nous y trouvons du silex de differentes varietes, et dans tous les degres successifs de leur formation, et de leur perfection. Outre cela, nous y voyons encore quelque chose, qui semble nous conduire a la decouverte des moyens, dont se sort la nature pour effecteur cette operation, et qui nous etoit cache dans les montagnes de chaux pure: ces bancs de pierre marnesilicieuse, contiennent une partie considerable de pyrites sulfureuses, qui non seulement y forment une grande quant.i.te de pet.i.ts sillons, mais toute la ma.s.se de la montagne est rempli de parcelles souvent presqu'imperceptibles de ce mineral. Ces pyrites sont evidemment des productions du phlogistique et de l'acide contenu dans la montagne.

"L'eau, qui s'y trouve ordinairement en a.s.sez grande abondance, en detacha, extraha d'un et l'autre, et les combina apres tous les deux ensemble. Cette meme eau les dissout derechef, et en fait de nouvelles combinaisons. C'est ce qu'on voit evidemment la, ou la nature, ayant commence ses operations, il n'y est reste de la pyrite, qu'une portion de la partie inflammable liee a une base terrestre. Dans ces endroits la marne n'est que fort peu sensible aux acides, et de blanche qu'elle etoit, sa couleur est devenue presque noire. C'est la qu'on observe les differens degres du changement de la marne en silex, contenant, meme encore, par fois, de parties pyriteiques non detruites dans son interieur. Et comme la nature forme ici, de meme, que dans la chaux pure les silex, la plupart en boules ou rognons; comme les different degres de metamorphoses de la marne en silex, sont ici beaucoup plus nombreuses que la, de sorte qu'il y a des bandes entieres, qui meriteroient plutot d'etre appelles bandes silicieuses, que marneuses; comme il y a, enfin, une grande quant.i.te de pyrites, qu'ailleurs, il est tres probable qu'elle se serve la du meme moyen qu'ici pour operer la metamorphose en question.

"Ne nous precipitons, cependant, pas a en tirer plus de consequences; poursuivons plutot le fil de notre recit.

"Le silex, qui se trouve ici, est non seulement de differents degres de perfection, il est de plus d'une espece. Il y a de la pierre a feu, 2 de la calcedoine, 3 des agathes, et 4 differentes nuances et pa.s.sages des especes ordinaires aux fines du silex.

"La pierre a feu, est, ordinairement dans son etat de perfection d'un grain a.s.sez fin, d'une couleur grise plus ou moins foncee, et meme donnant, dans le noiratre, plus ou moins diaphane; ses ca.s.sures sont concentriques ou coquillees, et sa ma.s.se est a.s.sez compacte. Outre sa conformation ordinaire en boules et rognons, elle fait presque toujours la noix de ursins marins, qui y font en grand nombre, et dont la coquille est le plus souvent, et presque toujours de spath calcaire, meme au milieu d'une boule de silex parfait.

"Les calcedoines et agathes de ces couches sont toujours (au moins, je ne les ai pas encore vues autrement) de coraux et autres corps marins petrifies. Donc, il faut que les couches de pierres roulees, d'ou j'ai tire ma collection citee plus haut, soyent des debris de montagne detruites de cette espece. Il y en a qui sont tres parfaites comme celles qui composent ma collection, d'autres meritent plutot d'etre rangees parmi les pa.s.sages du silex ordinaire, et ses especes plus fines; d'autres encore sont, en effet, de vraies agathes, mais qui renferment dans leur interieur plus ou moins de parties non parfaites presque calcaires, qui s'annoncent d'abord par leur couleur blanche, par leur gros grains relativement au reste, par leur opacite, par leur mollesse respective, et souvent meme par leur sensibilite pour les acides mineraux. Mais celles, qui sont finies, quoiqu'elles ayent, pour la plupart, une couleur presque noire, ne laissent, cependant, pas d'avoir aussi des teintes plus claires comme brunatres, verdatres, rougeatres, jaunatres, bleuatres, tachetees, veinees, etc. Leur clarte n'est pas moins variable, que leur couleur, il y en a de presqu'opaques, comme aussi de presque transparentes, sur tout la, ou la calcedoine predomine.

"Le quartz s'y trouve comme dans les pierres de la premiere section, c, a, d, crystallise, en groupes dans de pet.i.tes cavites; quelquefois aussi en veines. La calcedoine y est de meme, ou bien en mamelons, ou bien en stalact.i.tes, lorsqu'elle a de la place pour s'y deposer.

"Un phenomene encore plus curieux que cela est cette belle pyrite sulphureuse jaune, comme de l'or, qui est quelquefois pa.r.s.emee par tout la substance de petrifications agathisees, et qui apparemment y fut deposee apres la dite metamorphose a la faveur des pet.i.ts pores, qui y etoient restes ouverts."

I would beg that mineralists, who use such language as this, would consider if it contains a distinct idea of the operation which they would thereby describe, or if it does not contain either a contradiction or an inconceivable proposition. It supposes a calcareous body to be metamorphosed, somehow by means of the mountain acid, into a siliceous body. But, finding many bodies of pyrites contained within that solid flint, it is said, that, when the calcareous body was flintified, there were left in it cavities which were afterwards filled with pyrites. Let us reflect a moment upon this doctrine. These cavities were first open to the outside of the flinty body; but now the pyrites with which they had been filled is insulated in the solid flint. Here three things are required; first, The calcareous body is to be flintified, at the same time leaving the body full of small cavities open to the outside; secondly, These cavities are to be filled with pyrites; lastly, These mineral bodies are to be so inclosed within the flint, as to leave no vestige of the former processes. This marly mountain itself, which had been formed of loose materials gathered together at the bottom of the sea, was first to be filled with pyrites, in various shapes, by means of the phlogistic and the acid of the mountain. Here is proposed to us an operation which is totally unknown, or of which we have no kind of idea. But, let us suppose pyrites formed in this mountain, (of whatever chemical substances), by means of water; Why should water again undo that pyrites, in order to form other concretions? And, Why should the flint be formed first with cavities, and then made solid, after pyrites had been introduced into those cavities of the agate, and, as our author expresses it, _pa.r.s.emee pour toute la substance?_ Here are suppositions which are not only perfectly gratuitous, but are also inconsistent with any thing that we understand. This is not explaining nature; it is only feigning causes[39].

[Note 39: The description of those insulated siliceous bodies, containing in their closed cavities all the usual concretions of calcedony and crystals, as well as full of small pyrites floating in the solid flint, are extremely interesting to a mineral system, or such a geological theory as should explain the present state of things in those strata that had been formed by deposits of known materials at the bottom of the sea; they are indeed such appearances as may be found, more or less, in all consolidated strata. But it is this author's explanation of that petrifaction which is our present object to consider; and, as he is so particular in giving us his theory upon the subject, it is easy to detect the error of his reasoning. Were those naturalists who explain things only in general, by saying that water is the agent, and infiltration the means employed by nature;--were these naturalists, I say, to give us as particular a description of their process, it would appear as inconsistent with the nature of things as that which we have from this author, who examines nature very minutely, and who sees distinctly that the infiltrating theory is inapplicable for the explanation of those petrifactions.]

The third section has for t.i.tle, "_Generation du Silex et Quartz de la Pierre Puante_." Here we find an example worthy of being recorded, as contributing to throw great light upon those mineral operations; however, the opinion of our author and mine, upon this subject, differ widely. He proceeds thus:

"Cette pierre n'est, comme chacun le scait, qu'une pierre calcaire contenant du bitume.

"Nos montagnes n'en contiennent seulement pas de simples couches, mais il y en a meme de grandes bancs fort epais.

"Le caillou, ou silex qui s'y genere, forme, tantot de gros blocs informes, qui occupent des cavites dans l'interieure des montagnes, tantot, enfin, en forme de filons.

"J'ai remarque cette metamorphose sur trois endroits differens, dans chacun des quels la nature a autrement opere.

"Sur l'un, la pierre puante fait un banc horizontal dans une montagne de pierre calcaire crystalline, ou d'une espece de marbre, qui contient des couches et filons de metal. Ce banc de pierre puante y fait le toit d'une couche de galene de plomb et de pierre calaminaire, et dans ses cavites et fentes il y a non seulement des blocs de grandeur differente, mais aussi des veines et pet.i.tes bandes courtes de silex, tant ordinaire, que n.o.ble c, a, d, de la pierre a feu, de calcedoine, d'agathes, et meme d'une espece de cornaline jaune et rouge pale. Je ne m'arreterai pas a en detailler les varietes, parce qu'elles sont trop accidentelles. Je ne les connois pas meme toutes, il s'en faut de beaucoup, parce qu'elles se trouvent dans des anciennes mines negligees, peut etre depuis plus d'un siecle, et par consequent peu accessibles. Je ne doute, cependant pas, que, si l'on pouvoit mieux sonder le terrain, on y trouveroit bien plus encore du peu que j'ai cite. Parmi ce silex, il y a aussi de pet.i.tes groupes et de pet.i.tes veines de quartz solide et crystallise.

"Au second endroit la pierre puante fait un filon, ou si l'on veut, une couche ou bande verticale, qui partage la montagne en deux parties presqu'egales de l'epaisseur de trois aunes a peu pres. La montagne, ou cela se voit est aussi une ancienne mine de cuivre et de plomb, consistant en plusieurs varietes de marbre, different en couleur et en grain, deposees par couches les unes sur les autres. Le filon de silex est forme de feuilles alternatives de pierre puante et de silex, tous les deux de couleur brun de bois a peu pres; mais le silex est plus fonce que sa compagne. Ces feuilles alternatives, consistent d'autres bien plus minces encore, qui souvent n'ont pas l'epaisseur d'une ligne, mais ce qu'il y a de plus curieux, c'est que la meme feuille est d'un but de pierre porque, qui, vers le milieu, pa.s.se successivement en silex, qui, a son tour, vers l'autre but, qui etoit expose a l'air repa.s.se par les memes gradations en une espece de tuffe calcaire. Ce qui nous fait voir evidemment la generation et la destruction du silex, meme avec une partie des moyens par lesquels elle s'opere. Comme l'endroit de cette decouverte n'est accessible qu'a la superficie, je ne saurois dire s'il y a d'autres varietes de silex outre la dite. Il l'est a supposer autant par a.n.a.logie, que par quelques morceaux qui ont de pet.i.tes veines transversales d'une espece de calcedoine, et qui sont, meme, sur leur fentes, garnis de pet.i.ts cristaux de roche. Mais ce qu'il y a de sur c'est que ce filon, parvenu a une certaine profondeur, s'enn.o.blit et contient du metal, c. a. d. de la galene de plomb, et de la pyrite cuivreuse, j'y en ai trouves de morceaux, qui en font de preuves incontestables. Le caillou d'ici est un grain fin d'une texture forte, peu transparent, donne beaucoup d'etincelles au briquet, mais ses ca.s.sures sont ecailleuses.

"La montagne calcaire du troisieme lieu a une couche de pierre puante epaisse de plusieurs aunes, qui, derechef contient de pet.i.tes couches irregulieres et des bandes transversales de silex, qui ont jusques a six pouces pa.s.ses d'epaisseur. La pierre puante est d'une couleur gris-brune, d'un grain a.s.sez fin, et d'un tissu a.s.sez dur; ses ca.s.sures sont irregulieres, mais plus la pierre s'approche du silex, plus elles donnent dans le coquille. Le silex ordinaire est d'un brun de bois, d'un grain a.s.sez fin, et d'un tissu resistant, et ses ca.s.sures sont egales a la pierre porque. Ce n'est pas la la seule variete, il y a, aussi, de la calcedoine et des agathes de couleurs differentes. Meme la pierre a feu est a.s.sez souvent traversee de veines de calcedoine, de quartz crystallise, et de spath calcaire blanc en feuilles et en crystaux. Il arrive que la meme veine est composee de ces trois especes de pierres a la fois, de sorte que l'une semble pa.s.ser dans l'autre, parce que les limites reciproques sont, souvent, a.s.sez indistinctes. Il est evident, que le silex est forme de la pierre puante, parce qu'on remarque ici les memes phenomenes dont j'ai parle plus haut, c. a. d. les pa.s.sages successifs de l'une dans l'autre pierre, tant en montant qu'en descendant."

There is nothing particular in the siliceous mixture in this species of lime-stone, except the vein of that substance. It is evident that this vein, traversing the mountain, had been introduced in the fluid state of fusion. I do not mean to say, that, in this particular case now described, the evidence of that truth peculiarly appears; but that, from the general nature of mineral veins breaking and traversing the solid strata of the globe, no other conclusion can be formed; and that in the particulars of this example there is nothing that could lead us to suppose any other origin to the petrifactions contained in this vein of stinking lime-stone. It is plain, that our author has imagined to himself an unknown manner of executing his mineral metamorphoses. He sees plainly that the common notion of infiltration will not at all explain the evident confusion of those calcareous and siliceous bodies which appear to him to be metamorphosing into each other. Nothing, indeed, can explain those phenomena but a general cause of fluidity; and there is no such general cause besides that of heat or fusion.

But to show how mineralists of great merit, gentlemen who have examined systematically and with some accuracy, may impose upon themselves in reasoning for the explanation of mineral appearances from limited notions of things, and from the supposition of these having been formed where they now are found, that is, upon the surface of the earth, I would beg leave to transcribe what this author has said upon this species of petrifaction. It is not that he is ignorant of what mineralists have already said upon the subject; it is because he sees the incompetency of their explanations in those particular cases; and that he would employ some other more effectual means. (p. 50.)