Illustrations of the Huttonian Theory of the Earth

OF THE PHENOMENA PECULIAR TO STRATIFIED BODIES.

1. Materials of the Strata.

1. IT is well known that, on removing the loose earth which forms the immediate surface of the land, we come to the solid rock, of which a great proportion is found to be regularly disposed in strata, or beds of determinate thickness, inclined at different angles to the horizon, but separated from one another by equidistant superficies, that often maintain their parallelism to a great extent. These strata bear such evident marks of being deposited by water, that they are universally acknowledged to have had their origin at the bottom of the sea; and it is also admitted, that the materials which they consist of, were then either soft, or in such a state of comminution and separation, as renders them capable of arrangement by the action of the water in which they were immersed. Thus far most of the theories of the earth agree; but from this point they begin to diverge, and each to assume a character and direction peculiar to itself. Dr Hutton's does so, by laying down this fundamental proposition, That in all the strata we discover proofs of the materials having existed as elements of bodies, which must have been destroyed before the formation of those of which these materials now actually make a part.[1]

2. The calcareous strata are the portion of the mineral kingdom that gives the clearest testimony to the truth of this assertion. They often contain shells, corals, and other exuviæ of marine animals in so great abundance, that they appear to be composed of no other materials. Though these remains of organized bodies are now converted into stone or into spar, their shape and interior structure are often so well preserved, that the species of animal or plant of which they once made a part, can still be distinguished and pointed out among the living inhabitants of the ocean.

Others of the calcareous strata appear to be composed of fragments of some ancient rocks, which, after having been broken, have been again united into a compact stone. In these we find pieces clearly marked as having been once continuous but now placed at a distance from one another, and exhibiting exactly the same appearances as if they floated in a fluid of the same specific gravity with themselves.

From these, therefore, and a variety of similar appearances, Dr Hutton concludes, that the materials of all the calcareous strata have been furnished, either from the dissolution of former strata, or from the remains of organized bodies. But, though this conclusion is meant to be extended to all the calcareous strata, it is not asserted that every cubic inch of marble or of limestone contains in it the characters of its former condition, and of the changes through which it has passed. It may, however, be safely affirmed, that there is scarce any entire stratum where such characters are not to be found. These must be held as decisive with respect to the whole system of strata to which they belong; they prove the existence of calcareous rocks before the formation of the present; and, as the destruction of those is evidently adequate to the supply of the materials of these that we now see, to look for any other supply were superfluous, and could only embarrass our reasonings by the introduction of unnecessary hypotheses.[2]

3. The same conclusions result from an examination of the siliceous strata; under which we may comprehend the common sandstone, and also those pudding-stones or breccias where the gravel consists of quartz. In all these instances, it is plain, that the sand or gravel existed in a state quite loose and unconnected, at the bottom of the sea, previous to its consolidation into stone. But such bodies of gravel or sand could only be formed from the attrition of large masses of quartz, or from the dissolution of such sandstone strata as exist at present; for it will hardly be alleged, that sand is a crystallization of quartz, formed from that substance, when it passes from a fluid to a solid state.

Those pudding-stones in which the gravel is round and polished, carry the conclusion still farther, as such gravel can only be formed in the beds of rivers or on the shores of the sea; for, in the depths of the ocean, though currents are known to exist, yet there can be no motion of the water sufficiently rapid to produce the attrition required to give a round figure and smooth surface to hard and irregular pieces of stone. There must have existed, therefore, not only a sea, but continents, previously to the formation of the present strata.

The same thing is clearly shown by those petrifactions of wood, where, though the vegetable structure is perfectly preserved, the whole mass is siliceous, and has, perhaps, been found in the heart of some mountain, deep imbedded in the solid rock.

4. Characters of the same import are also found among the argillaceous strata, though perhaps more rarely than among the calcareous or siliceous. Such are the impressions of the leaves and stems of vegetables; also the bodies of fish and amphibious animals, found very often in the different kinds of argillaceous schistus, and in most instances having the figure accurately preserved, but the substance of the animal replaced by clay or pyrites. These are all remains of ancient seas or continents; the latter of which have long since disappeared from the surface of the earth, but have still their memory preserved in those archives, where nature has recorded the revolutions of the globe.

5. Among bituminous bodies, pit-coal is the only one which constitutes regular and extensive strata; and no fossil has its origin from the waste of former continents, marked by stronger and more distinct characters. Not to mention that the coal strata are alternated with those that have been already enumerated, and that they often contain shells and corals, perfectly mineralized, it is sufficient to remark, that there are entire beds of this fossil, which appear to consist wholly of wood, and in which the fibrous structure is perfectly preserved. From these instances, the appearances of vegetable structure may be traced through all possible gradations, down to an evanescent state. This last state is undoubtedly the most common; and though coal does not then, on bare inspection, make known its vegetable origin, yet, if we take it in connection with the other terms of the series, as we may call them; if we consider that the two extremes, viz. coal, with the vegetable structure perfect, and coal without any such structure visible, are often found in the same or in contiguous beds; and, if we remark, that through all these gradations coal contains nearly the same chemical elements, and yields, on analysis, bitumen and charcoal, combined with a greater or less proportion of earth: if we take all these circumstances into account, we cannot doubt that this fossil is every where the same, and derives its origin from the trees and plants that grew on the surface of the earth before the formation of the present land.

6. Dr Hutton has further observed, that if those ancient continents were at all similar to the present, we can be at no loss to account for the want of any distinct mark of vegetable organization in the greater part of the coal strata. It is plain, that the daily waste of animal and vegetable substances on the surface of the earth, must disengage a great quantity of oily as well as carbonic matter, which, with whatever element it is at first combined, is ultimately delivered into the ocean. Thus, the oily or fuliginous parts of animal and vegetable substances, let loose by burning, first ascend into the atmosphere, but are at length precipitated, and either fall immediately into the sea, or are, in part at least, washed down into it from the land. From other causes also, much vegetable matter is carried down by the rivers; and the whole quantity of animal and vegetable substances thus delivered into the sea, must be very considerable, amounting annually to the whole residuum of those substances, not employed in the maintenance or reproduction of animal and vegetable bodies. Whether chemically united to the waters of the ocean, or simply suspended in them, this matter is at last precipitated, and, mingling with earthy substances, is formed into strata, the place of which will be determined by the currents, the position of the present continents, and many other circumstances not easily enumerated.

If, then, an order of things similar to what we now see, existed before the formation of the present strata, it would necessarily happen, that the animal and vegetable substances, diffused through the ocean, being separated from the water, would be deposited at the bottom of the sea, and, in the course of ages, would form beds, less or more pure, according to the quantity of earth and other substances deposited at the same time. These beds being consolidated and mineralized by operations that are afterwards to be considered, have been converted into pit-coal, the parts of which are impalpable, and retain nothing of their primitive structure.[3]

If, then, the formation of coal from animal and vegetable bodies be admitted, the general position which derives the origin of the strata from the waste of former land, as it is applicable to all the kinds already enumerated, and of course to all those with which they are alternated, comprehends a very large portion of the earth's surface. It comprehends, indeed, all the strata usually distinguished by the name of Secondary; but there is another great division of the mineral kingdom, viz. the rocks, called Primitive, which, as they are never alternated with the secondary, but are always inferior to them, must be further examined, before we can decide whether the same conclusion extends to them or not.

7. Here it must be carefully observed, that, among the primary rocks, the granite is not meant to be included, except where that stone is stratified, and either coincides with veined granite or with gneiss. The primitive strata, in Dr Hutton's theory, comprehend, besides gneiss, the micaceous, chlorite, hornblende, and siliceous schistus, together with slate, and some other kinds of argillite; to which we must add, serpentine, micaceous limestone, and the greater part of marbles. These are mostly distinguished by their laminated structure, by having their planes much elevated with respect to the horizon, and by belonging more to the mountainous than the level parts of the earth's surface. They rarely contain vestiges of organized bodies; so rarely, indeed, that they were called primitive by the geologists who first distinguished them from other rocks, on the supposition of their being part of the primeval nucleus of the globe, which had never undergone any change whatsoever; but this, I believe, has now almost ceased to be the opinion of any geologist.[4] The Neptunists hold the rocks, here enumerated, and also granite, to be produced by aqueous deposition; but maintain them to be in the strictest sense primeval, and of a formation antecedent to all organized bodies.

8. In opposition to this, Dr Hutton maintained, that the primary schistus, like all the other strata, was formed of materials deposited at the bottom of the sea, and collected from the waste of rocks still more ancient. When, therefore, he conformed to the received language of mineralogists, by calling these strata primitive, he only meant to describe them as more ancient than any other strata now existing, but not as more ancient than any that ever had existed. They are distinguished, in his system, by the name of Primary, rather than of Primitive strata.

That the account now given of their origin is well founded, may be proved by unquestionable facts. For, first, though, agreeably to the observation just made, the ancient strata do but rarely contain any remains of organized bodies, they are not entirely destitute of them. Different places in this island have been pointed out by Dr Hutton, where marine objects have been discovered in primary limestone, either by himself or others, and it would not be difficult to add more instances of the same kind.[5] In Dauphine, coal, which is certainly a derivative substance, has been found among mountains which have a title to the character of primitive, such as no one will dispute. These facts put the composition of such rocks from loose materials, beyond all doubt, and also prove their formation to be posterior to the existence of an animal and vegetable system. They do indeed prove this in the strictest sense, only of the particular beds in which they are found; but as these beds are in all other respects as much to be accounted primary as any part of the mineral kingdom, it is evident that the negative instances are here of no force, and that nothing can be gained to the adversaries of this opinion by denying it in general, if they are obliged to admit it in a single case.

9. Again, it is certain, as Dr Hutton remarks, that there are few considerable bodies of schistus, even the most decidedly primitive, where sand and gravel may not in some parts be observed. Indeed, it is not only true that they are to be found in some parts of them; but, in fact, among many of the primitive mountains, we find large tracts, composed entirely of a schistose and much indurated sandstone, in beds highly inclined, sometimes alone, sometimes alternated with other schisti. In many of them, the sand of which they consist appears to be entirely of granite, from the detritus of which rock it should seem that they were chiefly formed.

10. Thus we conclude, that the strata both primary and secondary, both those of ancient and those of more recent origin, have had their materials furnished from the ruins of former continents, from the dissolution of rocks, or the destruction of animal or vegetable bodies, similar, at least in some respects, to those that now occupy the surface of the earth. This conclusion is not indeed proved of every individual portion of rock, but it is demonstrated of many and large parts, and those scattered indifferently through all the varieties of the strata; and therefore, from the rules of the strictest reasoning, we must infer, that the whole is derived from the same origin.[6]

Thus far concerning the materials of the strata; and, as these were originally loose and unconnected, we must next consider by what means they were consolidated into stone.

2. Consolidation of the Strata.

11. Though Dr Hutton has no where defined the meaning of the term consolidation, he has been scrupulously exact in using it constantly in the same sense. He understands by it, not merely that quality in a hard body, by which its parts cohere together, but also that by which it fills up the space comprehended within its surface, being to sense without porosity, and impervious to air and moisture.

Now, a porous mass of unconnected materials, such as the strata appear originally to have been, can acquire hardness and solidity only in two ways, that is, either when it is first reduced by heat into a state of fusion, or at least of softness, and afterwards permitted to cool; or when matter that is dissolved in some fluid menstruum, is introduced along with that menstruum into the porous mass, and, being deposited, forms a cement by which the whole is rendered firm and compact. Fire and water, therefore, are the only two physical agents to which we can ascribe the consolidation of the strata; and, in order to determine to which of them that effect is to be attributed, we must inquire whether there are any certain characters that distinguish the action of the one from that of the other, and which may be compared with the phenomena actually observed among mineral substances.

12. First, then, it is evident, that the consolidation produced by the action of water, or of another fluid menstruum, in the manner just referred to, must necessarily be imperfect, and can never entirely banish the porosity of the mass. For the bulk of the solvent, and of the matter it contained in solution, being greater than the bulk of either taken singly, when the latter was deposited, the former would have sufficient room left, and would continue to occupy a certain space in the interior of the strata. A liquid solvent, therefore, could never shut up the pores of a body to the entire exclusion of itself; and, had mineral substances been consolidated, as here supposed, the solvent ought either to remain within them in a liquid state, or if evaporated, should have left the pores empty and the body pervious to water. Neither of these however, is the fact; many stratified bodies are perfectly impervious to water, and few mineral substances contain water in a liquid state. That they sometimes contain it, chemically united to them, is no proof of their solidity having been brought about by that fluid; for such chemical union is as consistent with the supposition of igneous as of aqueous consolidation, since the region in which the fire was applied, on every hypothesis must have abounded with humidity.

13. Again, if water was the solvent by which the consolidating matter was introduced into the interstices of the strata, that matter could consist only of such substances as are soluble in water, whereas it consists of a vast variety of substances, altogether insoluble either in it, or in any single menstruum whatsoever. The strata are consolidated, for example, by quartz, by fluor, by feldspar, and by all the metals, in their endless combinations with sulphureous bodies. To affirm that water was ever capable of dissolving these substances, is to ascribe to it powers which it confessedly has not at present; and, therefore, it is to introduce an hypothesis, not merely gratuitous, but one which, physically speaking, is absurd and impossible.

This is not all, however; for, even if this difficulty were to be passed over, it would still be required to explain, how the water, which, together with the matter which it held in solution, had insinuated itself into the pores of the strata, became suddenly disposed to deposit that matter, and to allow it, by crystallization or concretion, to assume a solid form.[7] The Neptunists must either assign a sufficient reason for this great and universal change, or must expect to see their system treated as an inartificial accumulation of hypotheses which assigns opposite virtues to the same subject, and is alike at variance with nature and with itself; in a word, a system that might pass for the invention of an age, when as yet sound philosophy had not alighted on the earth, nor taught man that he is but the minister and interpreter of nature, and can neither extend his power nor his knowledge a hair's-breadth beyond his experience and observation of the present order of things.[8]

14. Such are the more obvious, but I think unanswerable objections, that may be urged against the aqueous consolidation of the strata. It is true, that stony concretions, some of them much indurated, are formed in the humid way under our eyes. Very particular conditions, however, are required for that purpose, and conditions such as can hardly have existed at the bottom of the sea. First. The water must dissolve the substance of which the concretion is to be formed, as it actually does in the case of calcareous, and in certain circumstances, in that of siliceous, earth. Secondly. It must be separated from that substance, as by evaporation, or by a combination of the matter dissolved with some third substance, to which it has a greater affinity than to water, so as to form with it an insoluble compound. Lastly, the water that is deprived of its solution must be carried off, and more of that which contains the solution must be supplied, as sometimes happens where water runs in a stream, or drops from the roof of a cavern. The two last conditions are peculiarly inapplicable to the bottom of the sea, where the state of the surrounding fluid would neither permit the water that was deprived of its solution from being drawn off, nor that which contained the solution from succeeding it.

It is further to be observed, that the consolidation of stalactitical concretions, that is, the filling up of their pores, is always imperfect, and is brought about by the repeated action of the fluid running through the porous mass, and continuing to deposit there some of the matter it holds in solution. This, which is properly infiltration, is incompatible with the nature of a fluid, either nearly, or altogether quiescent.

15. In order to judge whether objections of equal weight can be opposed to the hypothesis of igneous consolidation, we must attend to a very important remark, first made by Dr Hutton, and applied with wonderful success to explain the most mysterious phenomena of the mineral kingdom.

It is certain, that the effects of fire on bodies vary with the circumstances under which it is applied to them, and, therefore, a considerable allowance must be made, if we would compare the operation of that element when it consolidated the strata, with the results of our daily experience. The materials of the strata were disposed, as we have already seen, loose and unconnected, at the bottom of the sea; that is, even on the most moderate estimation, at the depth of several miles under its surface. At this depth, and under the pressure of a column of water of so great a height, the action of heat would differ much from that which we observe here upon the surface; and, though our experience does not enable us to compute with accuracy the amount of this difference, it nevertheless points out the direction in which it must lie, and even marks certain limits to which it would probably extend.

The tendency of an increased pressure on the bodies to which heat is applied, is to restrain the volatility of those parts which otherwise would make their escape, and to force them to endure a more intense action of heat. At a certain depth under the surface of the sea, the power even of a very intense heat might therefore be unable to drive off the oily or bituminous parts from the inflammable matter there deposited, so that, when the heat was withdrawn, these principles might be found still united to the earthy and carbonic parts, forming a substance very unlike the residuum obtained after combustion under a pressure no greater than the weight of the atmosphere. It is in like manner reasonable to believe, that, on the application of heat to calcareous bodies under great compression, the carbonic gas would be forced to remain; the generation of quicklime would be prevented, and the whole might be softened, or even completely melted; which last effect, though not directly deducible from any experiment yet made, is rendered very probable, from the analogy of certain chemical phenomena.

16. An analogy of this kind, derived from a property of the barytic earth, was suggested by that excellent chemist and philosopher, the late Dr Black. The barytic earth, as is well known, has a stronger attraction for fixed air than common calcareous earth has, so that the carbonate of barytes is able to endure a great degree of heat before its fixed air is expelled. Accordingly, when exposed to an increasing heat, at a certain temperature, it is brought into fusion, the fixed air still remaining united to it: if the heat be further increased, the air is driven off, the earth loses its fluidity, and appears in a caustic state. Here, it is plain, that the barytic earth, which is infusible, or very refractory, per se, as well as the calcareous, owes its fusibility to the presence of the fixed air; and it is therefore probable, that the same thing would happen to the calcareous earth, if by any means the fixed air were prevented from escaping when great heat is applied to it. This escape of the fixed air is exactly what the compression in the subterraneous regions is calculated to prevent, and therefore we are not to wonder if, among the calcareous strata, we find marks of actual fusion having taken place.[9]

17. These effects of pressure to resist the decomposition, and augment the fusibility of bodies, being once supposed, we shall find little difficulty in conceiving the consolidation of the strata by heat, since the intervals between the loose materials of which they originally consisted may have been closed, either by the softening of those materials, or by the introduction of foreign matter among them, in the state of a fluid, or of an elastic vapour. No objection to this hypothesis can arise from the considerations stated in the preceding case; the solvent here employed would want no pores to lodge in after its work was completed, nor would it find any difficulty in making its retreat through the densest and most solid substances in the mineral kingdom. Neither can its incapacity to dissolve the bodies submitted to its action be alleged. Heat is the most powerful and most general of all solvents; and, though some bodies, such as the calcareous, are able to resist its force on the surface of the earth, yet, as has just been shown, it is perfectly agreeable to analogy to suppose, that, under great pressure, the carbonic state of the lime being preserved, the purest limestone or marble might be softened, or even melted. With respect to other substances, less doubt of their fusibility is entertained; and though, in our experiments, the refractory nature of siliceous earth has not been completely subdued, a degree of softness and an incipient fusion have nevertheless been induced.

Thus it appears, in general, that the same difficulties do not press against the two theories of aqueous and of igneous consolidation; and, that the latter employs an agent incomparably more powerful than the former, of more general activity, and, what is of infinite importance in a philosophical theory, vastly more definite in the laws of its operation.

18. A more particular examination of the different kinds of fossils will confirm this conclusion, and will show, that, wherever they bear marks of having been fluid, these marks are such as characterize the fluidity of fusion, and distinguish it from that which is produced by solution in a menstruum. Dr Hutton has enumerated many of these discovered in the course of that careful and accurate examination of fossils, in which he probably never was excelled by any mineralogist. It will be sufficient here to point out a few of the most remarkable examples.

19. Fossil wood, penetrated by siliceous matter, is a substance well known to mineralogists; it is found in great abundance in various situations, and frequently in the heart of great bodies of rock. On examination, the siliceous matter is often observed to have penetrated the wood very unequally, so that the vegetable structure remains in some places entire; and in other places is lost in a homogeneous mass of agate or jasper. Where this happens, it may be remarked, that the line which separates these two parts is quite sharp and distinct, altogether different from what must have taken place, had the flinty matter been introduced into the body of the wood, by any fluid in which it was dissolved, as it would then have pervaded the whole, if not uniformly, yet with a regular gradation. In those specimens of fossil wood that are partly penetrated by agate, and partly not penetrated at all, the same sharpness of termination may be remarked, and is an appearance highly characteristic of the fluidity produced by fusion.

20. The round nodules of flint that are found in chalk, quite insulated and separate from one another, afford an argument of the same kind; since the flinty matter, if it had been carried into the chalk by any solvent, must have been deposited with a certain degree of uniformity, and would not now appear collected into separate masses, without any trace of its existence in the intermediate parts. On the other hand, if we conceive the melted flint to have been forcibly injected among the chalk, and to have penetrated it, somewhat as mercury may, by pressure, be made to penetrate through the pores of wood, it might, on cooling, exhibit the same appearances that the chalk-beds of England do actually present us with.

The siliceous pudding-stone is an instance closely connected with the two last; in it we find both the pebbles, and the cement which unites them, consisting of flint equally hard and consolidated; and this circumstance, for which it is impossible to account by infiltration, or the insinuation of an aqueous solvent, is perfectly consistent with the supposition, that a stream of melted flint has been forcibly injected among a mass of loose gravel.

21. The common grit, or sandstone, though it certainly gives no indication of having possessed fluidity, is strongly expressive of the effects of heat. It is so, especially in those instances where the particles of quartzy sand, of which it is composed, are firmly and closely united, without the help of any cementing substance whatsoever. This appearance, which is very common, seems to be quite inconsistent with every idea of consolidation, except an incipient fusion, which, with the assistance of a suitable compression, has enabled the particles of quartz to unite into stone.

It has indeed been asserted, that the mere apposition of stony particles, so as to permit their corpuscular attraction to take place, was sufficient to form them into stone. To this Dr Hutton has very well replied, that, admitting the possibility of a hard and firm body being produced in this way of which, however, we have no proof, the close and compact texture, the perfect consolidation of the stones we are now speaking of, would still remain to be explained, and of this it is evident that the mere apposition of particles, and the force of their mutual attraction, can afford no solution.

22. These proofs that the strata must have endured the action of intense heat, though immediately deduced from those of the siliceous genus only, extend in reality to all the strata, of every kind, with which they are found alternated. It is impossible that heat, of the intensity here supposed, can have acted on a particular stratum, and not on those that are contiguous to it; and, as there are no strata of any kind with which the quartzy and siliceous are not intermixed, so there are none of which the igneous consolidation is not thus rendered probable. We need rest nothing, however, on this argument, as the fossils of every genus may be shown to speak distinctly for themselves.

23. Those of the calcareous genus do so perhaps more sparingly than the rest; yet even among them there are many facts, that, though taken unconnected with all others, are sufficient to establish the action of subterraneous fire. Such, for example, are the calcareous breccias, composed of fragments of marble or limestone, and not only adapted to each other's shape, but indented into one another, in a manner not a little resembling the sutures of the human cranium. From such instances, it is impossible not to infer the softness of the calcareous fragments when they were consolidated into one mass. Now, this softness could be induced only by heat; for it must be acknowledged, that the action of any other solvent is quite inadequate to the softening of large fragments of stone, without dissolving them altogether.

24. In many other instances it appears certain, that the stones of the calcareous genus have been reduced by heat into a state of fluidity much more perfect. Thus, the saline or finer kinds of marble, and many others that have a structure highly crystallized, must have been softened to a degree little short of fusion, before this crystallization could take place. Even the petrifactions which abound so much in limestones tend to establish the same fact; for they possess a sparry structure, and must have acquired that structure in their transition from a fluid to a solid state.[10]

25. In accounting, by the operation of heat, for these appearances of fluidity, Dr Hutton has proceeded on the principle already laid down, as conformable to analogy, that calcareous earth, under great compression, may have its fixed air retained in it, notwithstanding the action of intense heat, and may, by that means, be reduced into fusion, or into a state approaching to it. In all this I do not think that he has departed from the strictest rules of philosophical investigation. The facts just stated prove, that limestone was once soft, its fragments retaining at the same time their peculiar form, an effect to which we know of none similar but those of fire; and, therefore, though we could not conjecture how heat might be applied to limestone so as to melt it, instead of reducing it to a calx, we should, nevertheless, have been forced to suppose, that this had actually taken place in the bowels of the earth; and was a fact which, though we were not able to explain it, we were not entitled to deny. The principle just mentioned relieves us therefore from a difficulty, that would have embarrassed, but could not have overturned, this theory of the earth.

26. From the arguments which the argillaceous strata afford for the igneous consolidation of fossils, I shall select one on which Dr Hutton used to lay considerable stress, and which some of the adversaries of his system have endeavoured to refute. This argument is founded on the structure of certain ironstones called septaria, often met with among the argillaceous schistus, particularly in the vicinity of coal. These stones are usually of a lenticular or spheroidal form, and are divided in their interior into distinct septa, by veins of calcareous spar, of which one set are circular and concentric, the other rectilineal; diverging from the centre of the former, and diminishing in size as they recede from it. Now, what is chiefly to be remarked is, that these veins terminate before they reach the surface of the stone; so that the matter with which they are filled cannot have been introduced from without by infiltration, or in any other way whatsoever. The only other supposition, therefore, that is left for explaining the singular structure of this fossil, is, that the whole mass was originally fluid, and that, in cooling, the calcareous part separated from the rest, and afterwards crystallized.

27. It has been urged against this theory of the septaria, that these stones are sometimes found with the calcareous veins extending all the way to the circumference, and of course communicating with the outside. But it must be observed, that this fact does not affect the argument drawn from specimens in which no such communication takes place. It is at best only an ambiguous instance, that may be explained by two opposite theories, and may be reconciled either to the notion of igneous or of aqueous consolidation: but if there is a single close septarium in nature, it can, of course, be explained only by one of these theories, and the other must, of necessity, be rejected. Besides, it is plain, that a close septarium can never have been open, though an open septarium may very well have been close; and indeed, as this stone is, in certain circumstances, subject to perpetual exfoliation, it would be wonderful if no one was ever found with the calcareous veins reaching to the surface. With regard to the light, therefore, that they give into their own history, these two kinds of septaria are by no means on an equal footing; and this may serve to show, how necessary it is, in all inductive reasoning, and particularly in a subject so complex as geology, to separate with care such phenomena as admit of two solutions, from such as admit only of one.

28. The bituminous strata come next to be considered; and they are of great consequence in the present argument, because their dissimilarity in so many particulars to all other mineral substances, renders them what Lord Bacon calls an instantia singularis, having the first rank among facts subservient to inductive investigation. But though unlike in substance to other fossils, and composed, as has been shown, of materials that belonged not originally to the mineral kingdom, they agree in many material circumstances with the strata already enumerated. Their beds are disposed in the same manner, and are alternated indiscriminately with those of all the secondary rocks, and, being formed in the same region, must have been subject to the same accidents, and have endured the operation of the same causes. They are traversed too like the other strata, by veins of the metals, of spars, of basaltes, and of other substances; and, whatever argument may hereafter be derived from this to prove the action of fire on the strata so traversed, is as much applicable to coal as to any other mineral. The coal strata also contain pyrites in great abundance, a substance that is perhaps, more than any other, the decided progeny of fire. This compound of metal and sulphur, which is found in mineral bodies of every kind, I believe, without any exception, is destroyed by the contact of moisture, and resolved into a vitriolic salt. At the same time it is found in the strata, not traversing them in veins, which may be supposed of more recent formation than the strata themselves; but existing in the heart of the most solid rocks, often nicely crystallized, and completely inclosed, on all sides, without the most minute vacuity. The pyrites must have been present, therefore, when the strata were consolidated, and it is inconceivable, if their consolidation was brought about in the wet way, that a substance should be so generally found in them, the very existence of which is incompatible with humidity. This argument for the igneous origin of the strata is applicable to them all, but especially to those of coal, as abounding with pyrites more than any other.

29. The difficulty that here naturally present itself, viz. how vegetable matter, such as coal is supposed to have been, could be exposed to the action of intense heat, without being deprived of its inflammable part, is obviated by the principle formerly explained concerning the effects of compression. The weight incumbent on the strata of coal, when they were exposed to the intense heat of the mineral regions, may have been such as to retain the oily and bituminous, as well as sulphureous parts, though the whole was reduced almost to fusion; and thus, on cooling, the sulphur uniting with iron might crystallize, and assume the form of pyrites.

30. The compression, however, has not in every instance preserved the bituminous, in union with the carbonic part of coal; and hence a mark of the operation of fire quite peculiar to this fossil, and found in those infusible kinds of it which contain no bitumen, and burn without flame. These resemble, some of them very precisely, and all them in a great degree, the products obtained by the distillation of the common bituminous coal that is, they consist of charcoal, united to an earthy basis in different proportions. It is natural therefore to conclude, that this substance was prepared in the mineral regions by the action of heat, which, in some instances, has driven off the inflammable part of the coal. That the heat should, in some cases, have done so, is not inconsistent with the general effect attributed to compression. The conditions necessary for retaining the more volatile parts, may not have been present every where in the same degree, so that the latter, though they could not escape, may have been forced from one part of a stratum, or body of strata, to another.

31. In confirmation of this it must be observed, that, as the fixed part of coal is thus found in the bowels of the earth, separate from the volatile or bituminous, so, in the neighbourhood of coal strata, the latter is sometimes found without any mixture of the former. The fountains of naphtha and petroleum are well known; and Dr Hutton has described a stratum of limestone, lying in the centre of a coal country, which is pervaded and tinged by bituminous matter, through its whole mass, and has, at the same time, many close cavities in the heart of it, lined with calcareous spar, and containing fossil pitch, sometimes in large pieces, sometimes in hemispherical drops, scattered over the surface of the cavities. This combination could only be effected by a part of the inflammable matter of the beds of coal underneath, being driven off by heat, and made to penetrate the limestone, while it was yet soft and pervious to heated vapours.[11]

32. Hitherto we have enumerated those fossils that are either not at all, or very sparingly soluble in water. There are, however, saline bodies among the mineral strata, such for instance as rock-salt, which are readily dissolved in water; and it yet remains to examine by what cause their consolidation has been effected.

Here the theorists who consider water as the sole agent in the mineralization of fossils, are indeed delivered from one difficulty, but it is only that they may be harder pressed on by another. It cannot now be said, that the menstruum which they employ is incapable of dissolving the substances exposed to its action, as in the case of metallic or stony bodies; but it may very well be asked, how the water came to deposit the salts which it held in solution, and to deposit them so copiously as it has done in many places, without any vestige of similar deposition in the places immediately contiguous. If they refuse to call to their assistance any other than their favourite element, they will not find it easy to answer this question, and must feel the embarrassment of a system, subject to two difficulties, so nicely, but so unhappily adjusted, that one of them is always prepared to act whenever the other is removed. If, on the other hand, they will admit the operation of subterraneous heat, it appears possible, that the local application of such heat may have driven the water, in vapour, from one place to another, and by such action often repeated in the same spot, may have produced those great accumulations of saline matter, that are actually found in the bowels of the earth.

33. But granting that, either in the way just pointed out, or in some other that is unknown, the salt and the water have been separated, some further action of heat seems requisite, before a compact, and highly indurated body, like rock-salt, could be produced. The mere precipitation of the salt, would, as Dr Hutton has observed, form only an assemblage of loose crystals at the bottom of the sea, without solidity or cohesion: and to convert such a mass into a firm and solid rock, would require the application of such heat as was able to reduce it into fusion. The consolidation of rock-salt, therefore, however its separation from the water is accounted for, cannot be explained but on the hypothesis of subterraneous heat.

34. Some other phenomena that have been observed in salt mines, come in support of the same conclusion. The salt rock of Cheshire, which lies in thick beds, interposed between strata of an argillaceous or marly stone, and is itself mixed with a considerable portion of the same earth, exhibits a very great peculiarity in its structure. Though it forms a mass extremely compact, the salt is found to be arranged in round masses of five or six feet in diameter, not truly spherical, but each compressed by those that surround it, so as to have the shape of an irregular polyhedron. These are formed of concentric coats, distinguishable from one another by their colour, that is, probably by the greater or less quantity of earth which they contain, so that the roof of the mine, as it exhibits a horizontal section of them, is divided into polygonal figures, each with a multitude of polygons within it, having altogether no inconsiderable resemblance to a mosaic pavement. In the triangular spaces without the polygons, the salt is in coats parallel to the sides of the polygons.

The circumstances which gave rise to this singular structure we should in vain endeavour to define; yet some general conclusions concerning them seem to be within our reach. It is clear that the whole mass of salt was fluid at once, and that the forces, whatever they were, which gave solidity to it, and produced the new arrangement of its particles, were all in action at the same time. The uniformity of the coated structure is a proof of this, and, above all, the compression of the polyhedra, which is always mutual, the flat side of one being turned to the flat side of another, and never an angle to an angle, nor an angle to a side. The coats formed as it were round so many different centres of attraction, is also an appearance quite inconsistent with the notion of deposition; both these, however, are compatible with the notion of solidity acquired by the refrigeration of a fluid, where the whole mass is acted on at the same time, and where no solvent remains to be disposed of after the induration of the rest.

35. Another species of fossil salt exhibits appearances equally favourable to the theory of igneous consolidation. This is the Trona of Africa, which is no other than soda, or mineral alkali, in a particular state. The specimen of this fossil in Dr Black's, now Dr Hope's, collection, is of a sparry and radiated structure, and is evidently part of the contents of a vein, having a stony crust adhering to it, on one side, with its own sparry structure complete, on the opposite. It contains but about one sixth of the water of crystallization essential to this salt when obtained in the humid way; and, what is particularly to be remarked, it does not lose this water, nor become covered with a powder, like the common alkali, by simple exposure to the air. It is evident, therefore, that this fossil does not originate from mere precipitation; and when we add, that in its sparry structure it contains evident marks of having once been fluid, we have little reason to entertain much doubt concerning the principle of its consolidation.

Thus, then, the testimony given to the operation of fire, or heat, as the consolidating power of the mineral kingdom, is not confined to a few fossils, but is general over all the strata. How far the unstratified fossils agree in supporting the same conclusion, will be afterwards examined.

3. Position of the Strata.[12]