Theory of the Earth Volume I Part 3

But, besides this argument taken from what does not appear, the actual form in which those flinty ma.s.ses are found, demonstrates, _first_, That they have been introduced among those strata in a fluid state, by injection from some other place. 2_dly_, That they have been dispersed in a variety of ways among those strata, then deeply immersed at the bottom of the sea; and, _lastly_, That they have been there congealed from the state of fusion, and have remained in that situation, while those strata have been removed from the bottom of the ocean to the surface of the present land.

To describe those particular appearances would draw this paper beyond the bounds of an essay. We must, therefore, refer those who would inquire more minutely into the subject, to examine the chalk-countries of France and England, in which the flint is found variously formed; the land-hills interspersed among those chalk-countries, which have been also injected by melted flint; and the pudding-stone of England, which I have not seen in its natural situation. More particularly, I would recommend an examination of the insulated ma.s.ses of stone, found in the sand-hills by the city of Brussels; a stone which is formed by an injection of flint among sand, similar to that which, in a body of gravel, had formed the pudding-stone of England[7].

[Note 7: Accurate descriptions of those appearances, with drawings, would be, to natural history, a valuable acquisition.]

All these examples would require to be examined upon the spot, as a great part of the proof for the fusion of the flinty substance, arises, in my opinion, from the form in which those bodies are found, and the state of the surrounding parts. But there are specimens brought from many different places, which contain, in themselves, the most evident marks of this injection of the flinty substance in a fluid state. These are pieces of fossil wood, penetrated with a siliceous substance, which are brought from England, Germany, and Lochneagh in Ireland.

It appears from these specimens, that there has sometimes been a prior penetration of the body of wood, either with irony matter, or calcareous substance. Sometimes, again, which is the case with that of Lochneagh, there does not seem to have been any penetration of those two substances. The injected flint appears to have penetrated the body of this wood, immersed at the bottom of the sea, under an immense compression of water. This appears from the wood being penetrated partially, some parts not being penetrated at all.

Now, in the limits between those two parts, we have the most convincing proofs, that it had been flint in a simple fluid state which had penetrated the wood, and not in a state of solution.

_First_, Because, however little of the wood is left unpenetrated, the division is always distinct between the injected part and that which is not penetrated by the fluid flint. In this case, the flinty matter has proceeded a certain length, which is marked, and no farther; and, beyond this boundary, there is no partial impregnation, nor a gradation of the flintifying operation, as must have been the case if siliceous matter had been deposited from a solution. 2_dly_, The termination of the flinty impregnation has a.s.sumed such a form, precisely, as would naturally happen from a fluid flint penetrating that body.

In other specimens of this mineralising operation, fossil wood, penetrated, more or less, with ferruginous and calcareous substances, has been afterwards penetrated with a flinty substance. In this case, with whatever different substances the woody body shall be supposed to have been penetrated in a state of solution by water, the regular structure of the plant would still have remained, with its vacuities, variously filled with the petrifying substances, separated from the aqueous menstruum, and deposited in the vascular structure of the wood.

There cannot be a doubt with regard to the truth of this proposition; for, as it is, we frequently find parts of the consolidated wood, with the vascular structure remaining perfectly in its natural shape and situation; but if it had been by aqueous solution that the wood had been penetrated and consolidated, all the parts of that body would be found in the same natural shape and situation.

This, however, is far from being the case; for while, in some parts, the vascular structure is preserved entire, it is also evident, that, in general, the woody structure is variously broken and dissolved by the fusion and crystallization of the flint. There are so many and such various convincing examples of this, that, to attempt to describe them, would be to exceed the bounds prescribed for this dissertation; but such specimens are in my possession, ready for the inspection of any person who may desire to study the subject.

We may now proceed to consider sulphureous substances, with regard to their solubility in water, and to the part which these bodies have acted in consolidating the strata of the globe.

The sulphureous substances here meant to be considered, are substances not soluble in, water, so far as we know, but fusible by heat, and inflammable or combustible by means of heat and vital air. These substances are of two kinds; the one more simple, the other more compound.

The most simple kind is composed of two different substances, viz.

phlogiston, with certain specific substances; from which result, on the one hand, sulphur, and, on the other, proper coal and metals. The more compound sort, again, is oily matter, produced by vegetables, and forming bituminous bodies.

The _first_ of these is found naturally combined with almost all metallic substances, which are then said to be mineralised with sulphur.

Now, it is well known, that this mineralising operation is performed by means of heat or fusion; and there is no person skilled in chemistry that will pretend to say, this may be done by aqueous solution. The combination of iron and sulphur, for example, may easily be performed by fusion; but, by aqueous solution, this particular combination is again resolved, and forms an acido-metallic, that is, a vitriolic substance, after the phlogiston (by means of which it is insoluble in water) has been separated from the composition, by the a.s.sistance of vital air.

The variety of these sulphureo-metallic substances, in point of composition, is almost indefinite; but, unless they were all soluble in water, this could not have happened by the action of that solvent. If we shall allow any one of those bodies to have been formed by the fluidity of heat, they must all have been formed in the same manner; for there is such a chain of connection among those bodies in the mineral regions, that they must all have been composed, either, on the one hand, by aqueous solution, or, on the other, by means of heat and fusion.

Here, for example, are crystallised together in one ma.s.s, 1_st, Pyrites_, containing sulphur, iron, copper; 2_dly, Blend_, a composition of iron, sulphur, and calamine; 3_dly, Galena_, consisting of lead and sulphur; 4_thly, Marmor metallic.u.m_, being the terra ponderosa, saturated with the vitriolic acid; a substance insoluble in water; 5_thly, Fluor_, a saturation of calcareous earth, with a peculiar acid, called the _acid of spar_, also insoluble in water; 6_thly, Calcareous spar_, of different kinds, being calcareous earth saturated with fixed air, and something besides, which forms a variety in this substance; _lastly, Siliceous substance_, or _Quartz crystals_. All these bodies, each possessing its proper shape, are mixed in such a manner as it would be endless to describe, but which may be expressed in general by saying, that they are mutually contained in, and contain each other.

Unless, therefore; every one of these different substances may be dissolved in water, and crystallised from it, it is in vain to look for the explanation of these appearances in the operations of nature, by the means of aqueous solution.

On the other hand, heat being capable of rendering all these substances fluid, they may be, with the greatest simplicity, transported from one place to another; and they may be made to concrete altogether at the same time, and distinctly separate in any place. Hence, for the explanation of those natural appearances, which are so general, no further conditions are required, than the supposition of a sufficient intensity of subterraneous fire or heat, and a sufficient degree of compression upon those bodies, which are to be subjected to that violent heat, without calcination or change. But, so far as this supposition is not gratuitous, the appearances of nature will be thus explained.

I shall only mention one specimen, which must appear most decisive of the question. It is, I believe, from an Hungarian mine. In this specimen, petro-silex, pyrites, and cinnabar, are so mixed together, and crystallised upon each other, that it is impossible to conceive any one of those bodies to have had its fluidity and concretion from a cause which had not affected the other two. Now, let those who would deny the fusion of this siliceous body explain how water could dissolve these three different bodies, and deposit them in their present shape. If, on the contrary, they have not the least shadow of reason for such a gratuitous supposition, the present argument must be admitted in its full force.

Sulphur and metals are commonly found combined in the mineral regions.

But this rule is not universal; for they are also frequently in a separate state. There is not, perhaps, a metal, among the great number which are now discovered, that may not be found native, as they are called, or in their metallic state.

Metallic substances are also thus found in some proportion to the disposition of the particular metals, to resist the mineralising operations, and to their facility of being metallised by fire and fusion. Gold, which refuses to be mineralised with sulphur, is found generally in its native state. Iron, again, which is so easily mineralised and scorified, is seldom found in its malleable state. The other metals are all found more or less mineralised, though some of them but rarely in the native state.

Besides being found with circ.u.mstances thus corresponding to the natural facility, or to the impediments attending the metallization of those different calces, the native metals are also found in such a shape, and with such marks, as can only agree with the fusion of those bodies; that is to say, those appearances are perfectly irreconcilable with any manner of solution and precipitation.

For the truth of this a.s.sertion, among a thousand other examples, I appeal to that famous ma.s.s of native iron discovered by Mr Pallas in Siberia. This ma.s.s being so well known to all the mineralists of Europe, any comment upon its shape and structure will be unnecessary[8].

[Note 8: Since this Dissertation was written, M. de la Peyrouse has discovered a native manganese. The circ.u.mstances of this mineral are so well adapted for ill.u.s.trating the present doctrine, and so well related by M. de la Peyrouse, that I should be wanting to the interest of mineral knowledge, were I not to give here that part of his Memoir.

"Lorsque je fis inserer dans le journal de physique de l'annee 1780, au mois de Janvier, une Dissertation contenant la cla.s.sification des mines de manganese, je ne connoissois point, a cette epoque, la mine de manganese native. Elle a la couleur de son regule: Elle salit les doigts de la meme teinte. Son tissu parait aussi lamelleux, et les lames semblent affecter une sorte de divergence. Elle a ainsi que lui, l'eclat metallique; comme lui elle se laisse aplatir sous le marteau, et s'exfolie si l'on redouble les coups; mais une circonstance qui est trop frappante pour que je l'omette, c'est la figure de la manganese native, si prodigieus.e.m.e.nt conforme a celle du regule, qu'on s'y laisseroit tromper, si la mine n'etoit encore dans sa gangue: Figure tres-essentielle a observer ici, parce qu'elle est due a la nature meme de la manganese. En effet, pour reduire toutes les mines en general, il faut employer divers flux appropries. Pour la reduction de la manganese, bien loin d'user de ce moyen, il faut, au contraire, eloigner tout flux, produire la fusion, par la seule violence et la prompt.i.tude du feu. Et telle est la propension naturelle et prodigieuse de la manganese a la vitrification, qu'on n'a pu parvenir encore a reduire son regule en un seul culot; on trouve dans le creuset plusieurs pet.i.ts boutons, qui forment autant de culots separes. Dans la mine de manganese native, elle n'est point en une seule ma.s.se; elle est disposee egalement en plusieurs culots separes, et un peu aplatis, comme ceux que l'art produit; beaucoup plus gros, a la verite, parce que les agens de la nature doivent avoir une autre energie, que ceux de nos laboratoires; et cette ressemblance si exacte, semble devoir vous faire penser que la mine native a ete produite par le feu, tout comme son regule. La presence de la chaux argentee de la manganese, me permettroit de croire que la nature n'a fait que reduire cette chaux. Du reste, cette mine native est tres-pure, et ne contient aucune partie attirable a l'aimant. Cette mine, unique jusqu'a ce moment, vient, tout comme les autres manganese que j'ai decrites, des mines de fer de _Sem_, dans la vallee de _Viedersos_, en Comte de Foix."--_Journal de Physique, Janvier 1786_.]

We come now to the _second_ species of inflammable bodies called oily or bituminous. These substances are also found variously mixed with mineral bodies, as well as forming strata of themselves; they are, therefore, a proper subject for a particular examination.

In the process of vegetation, there are produced oily and resinous substances; and, from the collection of these substances at the bottom of the ocean, there are formed strata, which have afterwards undergone various degrees of beat, and have been variously changed, in consequence of the effects of that heat, according as the distillation of the more volatile parts of those bodies has been suffered to proceed.

In order to understand this, it must be considered, that, while immersed in water, and under insuperable compression, the vegetable, oily, and resinous substances, would appear to be unalterable by heat; and it is only in proportion as certain chemical separations take place, that these inflammable bodies are changed in their substance by the application of heat. Now, the most general change of this kind is in consequence of evaporation, or the distillation of their more volatile parts, by which oily substances become bituminous, and bituminous substances become coaly.

There is here a gradation which may be best understood, by comparing the extremes.

On the one hand, we know by experiment, that oily and bituminous substances can be melted and partly changed into vapour by heat, and that they become harder and denser, in proportion as the more volatile parts have evaporated from them. On the other hand, coaly substances are dest.i.tute of fusibility and volatility, in proportion as they have been exposed to greater degrees of heat, and to other circ.u.mstances favourable to the dissipation of their more volatile and fluid parts.

If, therefore, in mineral bodies, we find the two extreme states of this combustible substance, and also the intermediate states, we must either conclude, that this particular operation of heat has been thus actually employed in nature, or we must explain those appearances by some other means, in as satisfactory a manner, and so as shall be consistent with other appearances.

In this case, it will avail nothing to have recourse to the false a.n.a.logy of water dissolving and crystallising salts, which has been so much employed for the explanation of other mineral appearances. The operation here in question is of a different nature, and necessarily requires both the powers of heat and proper conditions for evaporation.

Therefore, in order to decide the point, with regard to what is the power in nature by which mineral bodies have become solid, we have but to find bituminous substance in the most complete state of coal, intimately connected with some other substance, which is more generally found consolidating the strata, and a.s.sisting in the concretion of mineral substances. But I have in my possession the most undoubted proof of this kind. It is a mineral vein, or cavity, in which are blended together coal of the most fixed kind, quartz and marmor metallic.u.m. Nor is this all; for the specimen now referred to is contained in a rock of this kind, which every naturalist now-a-days will allow to have congealed from a fluid state of fusion. I have also similar specimens from the same place, in which the coal is not of that fixed and infusible kind which burns without flame or smoke, but is bituminous or inflammable coal.

We have hitherto been resting the argument upon a single point, for the sake of simplicity or clearness, not for want of those circ.u.mstances which shall be found to corroborate the theory. The strata of fossil coal are found in almost every intermediate state, as well as in those of bitumen and charcoal. Of the one kind is that fossil coal which melts or becomes fluid upon receiving heat; of the other, is that species of coal, found both in Wales and Scotland, which is perfectly infusible in the fire, and burns like coals, without flame or smoke. The one species abounds in oily matter, the other has been distilled by heat, until it has become a _caput mortuum_, or perfect coal.

The more volatile parts of these bituminous bodies are found in their separate state on some occasions. There is a stratum of limestone in Fifeshire, near Raith, which, though but slightly tinged with a black colour, contains bituminous matter, like pitch, in many cavities, which are lined with calcareous spar crystallised. I have a specimen of such a cavity, in which the bitumen is in sphericles, or rounded drops, immersed in the calcareous spar.

Now, it is to be observed, that, if the cavity in the solid limestone or marble, which is lined with calcareous crystals containing pyrites, had been thus encrusted by means of the filtration of water, this water must have dissolved calcareous spar, pyrites, and bitumen. But these natural appearances would not even be explained by this dissolution and supposed filtration of those substances. There is also required, _first_, A cause for the separation of those different substances from the aqueous menstruum in which they had been dissolved; _2dly_, An explanation of the way in which a dissolved bitumen should be formed into round hard bodies of the most solid structure; and, _lastly_, Some probable means for this complicated operation being performed, below the bottom of the ocean, in the close cavity of a marble stratum.

Thus, the additional proof, from the facts relating to the bituminous substances, conspiring with that from the phenomena of other bodies, affords the strongest corroboration of this opinion, that the various concretions found in the internal parts of strata have not been occasioned by means of aqueous solution, but by the power of heat and operation of simple fusion, preparing those different substances to concrete and crystallise in cooling.

The arguments which have been now employed for proving that strata have been consolidated by the power of heat, or by the means of fusion, have been drawn chiefly from the insoluble nature of those consolidating substances in relation to water, which is the only general menstruum that can be allowed for the mineral regions. But there are found, in the mineral kingdom, many solid ma.s.ses of saltgem, which is a soluble substance. It may be now inquired, How far these ma.s.ses, which are not infrequent in the earth, tend either to confirm the present theory, or, on the contrary, to give countenance to that which supposes water the chief instrument in consolidating strata.

The formation of salt at the bottom of the sea, without the a.s.sistance of subterranean fire, is not a thing unsupposable, as at first sight it might appear. Let us but suppose a rock placed across the gut of Gibraltar, (a case nowise unnatural), and the bottom of the Mediterranean would be certainly filled with salt, because the evaporation from the surface of that sea exceeds the measure of its supply.

But strata of salt, formed in this manner at the bottom of the sea, are as far from being consolidated by means of aqueous solution, as a bed of sand in the same situation; and we cannot explain the consolidation of such a stratum of salt by means of water, without supposing subterranean heat employed, to evaporate the brine which would successively occupy the interstices of the saline crystals. But this, it may be observed, is equally departing from the natural operation of water, as the means for consolidating the sediment of the ocean, as if we were to suppose the same thing done by heat and fusion. For the question is not, If subterranean heat be of sufficient intensity for the purpose of consolidating strata by the fusion of their substances; the question is, Whether it be by means of this agent, subterranean heat, or by water alone, without the operation of a melting heat, that those materials have been variously consolidated.

The example now under consideration, consolidated mineral salt, will serve to throw some light upon the subject; for, as it is to be shown, that this body of salt had been consolidated by perfect fusion, and not by means of aqueous solution, the consolidation of strata of indissoluble substances, by the operation of a melting heat, will meet with all that confirmation which the consistency of natural appearances can give.

The salt rock in Cheshire lies in strata of red marl. It is horizontal in its direction. I do not know its thickness, but it is dug thirty or forty feet deep. The body of this rock is perfectly solid, and the salt, in many places, pure, colourless, and transparent, breaking with a sparry cubical structure. But the greatest part is tinged by the admixture of the marl, and that in various degrees, from the slightest tinge of red, to the most perfect opacity. Thus, the rock appears as if it had been a ma.s.s of fluid salt, in which had been floating a quant.i.ty of marly substance, not uniformly mixed, but every where separating and subsiding from the pure saline substance.

There is also to be observed a certain regularity in this separation of the tinging from the colourless substance, which, at a proper distance, gives to the perpendicular section of the rock a distinguishable figure in its structure. When looking at this appearance near the bottom of the rock, it, at first, presented me with the figure of regular stratification; but, upon examining the whole ma.s.s of rock, I found, that it was only towards the bottom that this stratified appearance took place; and that, at the top of the rock, the most beautiful and regular figure was to be observed; but a figure the most opposite to that of stratification. It was all composed of concentric circles; and these appeared to be the section of a ma.s.s, composed altogether of concentric spheres, like those beautiful systems of configuration which agates so frequently present us with in miniature. In about eight or ten feet from the top, the circles growing large, were blended together, and gradually lost their regular appearance, until, at a greater depth, they again appeared in resemblance of a stratification.

This regular arrangement of the floating marly substance in the body of salt, which is that of the structure of a coated pebble, or that of concentric spheres, is altogether inexplicable upon any other supposition, than the perfect fluidity or fusion of the salt, and the attractions and repulsions of the contained substances. It is in vain to look, in the operations of solution and evaporation, for that which nothing but perfect fluidity or fusion can explain.

This example of a mineral salt congealed from a melted state, may be confirmed from another which I have from Dr Black, who suggested it to me. It is an alkaline salt, found in a mineral state, and described in the Philosophical Transactions, _anno_ 1771. But to understand this specimen, something must be premised with regard to the nature of fossil alkali.

The fossil alkali crystallises from a dissolved state, in combining itself with a large portion of the water, in the manner of alum; and, in this case, the water is essential to the const.i.tution of that transparent crystalline body; for, upon the evaporation of the water, the transparent salt loses its solidity, and becomes a white powder. If, instead of being gently dried, the crystalline salt is suddenly exposed to a sufficient degree of heat, that is, somewhat more than boiling water, it enters into the state of aqueous fusion, and it boils, in emitting the water by means of which it had been crystallised in the cold, and rendered fluid in that heated state. It is not possible to crystallise this alkaline salt from a dissolved state, without the combination of that quant.i.ty of water, nor to separate that water without destroying its crystalline state.

But in this mineral specimen, we have a solid crystalline salt, with a structure which, upon fracture, appears to be sparry and radiated, something resembling that of zeolite. It contains no water in its crystallization, but melts in a sufficient heat, without any aqueous fusion. Therefore, this salt must have been in a fluid state of fusion, immediately before its congelation and crystallization.

It would be endless to give examples of particular facts, so many are the different natural appearances that occur, attended with a variety of different circ.u.mstances.

There is one, however, which is peculiarly distinct, admits of sufficiently accurate description, and contains circ.u.mstances from which conclusions may be drawn with clearness. This is the ironstone, which is commonly found among the argillaceous strata, attendant upon fossil coal, both in Scotland and in England.