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date, and both of them have been agitated in ancient as well as in modern times with a considerable degree of warmth as well as of plausible argument.
Among the ancients, Heraclitus seems to have headed the advocates for the former theory, and Thales, or rather Epicurus, the supporters of the latter. In what may be regarded as modern times, Hooke may, perhaps, be held the reviver of the Plutonic system, which has since, as I have already observed, been supported by the cosmological doctrines of Buffon and Dr. Herschel. Its principal chanıpions, however, in the present day are Dr. Hutton, Professor Playfair,* and Sir James Hall; names, unquestionably, of high literary rank, and entitled to the utmost deference, but most powerfully opposed by the distinguished authorities of Werner, whose system I have just glanced at, Saussure, Kirwan, Cuvier, and Jameson, not to mention that the general voice of geologists is very considerably in favour of the latter class of philosophers, and consequently of the Neptunian or aqueous hypothesis. Let us, then, take a brief view of each of these theories in their order.
According to the former, or the Plutonic conjecture, heat is the great source, not only of the original production, but of the perpetual reproduction of things. This theory supposes a regular alternation of decay and renovation. Of decay induced by the action of light, air, and other gases, rain, and other waters, upon the hardest rocks, by which they are worn down and their par ticles progressively carried towards the ocean, and ultimately deposited in its bed; and of renovation, by means of an immense subterranean heat, constantly present at different depths of the mineral regions; which operates in the fusion and recombination of the materials thus carried down and contained there, and afterward in their sublimation and re-exposure to view in new strata of a more compact and perfect character. Hence, the existing strata of every period consist, upon this theory, of the wreck of a former world, more or less completely fused and elevated by the agency of violent heat, and reconsolidated by subsequent cooling: of the general nature of which heat, however, we are still left in a considerable degree of ignorance. “Jt is not fire, in the usual sense of the word,” observes Mr. Play fair, “but heat, which is required for this purpose; and there is nothing chimerical in supposing that nature has the means of producing heat, even in a very great degree, without the assistance of fuel or of vital air. Friction is a source of heat unlimited, for what we know, in its extent; and so, perhaps, are other operations, chemical and mechanical ; nor are either combustible substances or vital air concerned in the heat thus produced. So, also, the heat of the sun's rays in the form of a burning-glass, the most intense that is known, is independent of the substance just mentioned; and though the heat would not cal. cine a metal, nor even burn a piece of wood, without oxygenous gas, it would doubtless produce as high a temperature in the absence as in the presence of that gas.”+
This subterranean heat, moreover, is supposed to derive a very considerable accession of power from the vast superincumbent weight that is perpetually pressing upon its materials; in confirmation of which a variety of curious experiments are appealed to, and especially a very ingenious set lately carried into effect and described by Sir James Hall, by which it has been rendered probable, that when the gases of any fusible substance, as the carbonic acid of carbonate of lime, for example, are rendered incapable of flying off, a much less quantity of actual heat is sufficient for the purpose of fusion than when such gases, freed from a heavy compression, can escape with facility. Now, the subterranean heat heing supposed to exist at prodigious depths below the surface, the substances on which it operates must be so enormously compressed, as not only to render them easily fused, but in many instances to prevent their volatilization after the fusion has taken place; and from this circumstance it is possible, we are told, to explain a
variety of appearances and qualities in minerals, and to answer a variety of a objections which would otherwise weigh heavy against the general theory.
* Illustrations of the lIuttonian Thoory of the Earth. Edinb. 1802
To the principle of an alternate decay and renovation, separated from the means by which they are supposed, upon this theory, to be accomplished, there seems to be no very serious objection. It is as readily allowed by the Neptunian as by the Plutonic geologist, that the strata of the earth are liable to waste, and are, indeed, perpetually wasting; and that the waste materials are carried forward to the sea. But the appearance of shells in limestone and marbles, in which the sparry structure is as perfect as in primary limestone, and through which are distributed veins of crystallized carbonate of lime, together with a variety of similar facts, fatally militate against the agency of heat as a universal cause; since, in such case, allowing it to have been sufficient to produce the general effect of crystallization, every vestige of the structure of the shells must have been destroyed, and every atom of the carbonic acid totally evaporated.
It is, secondly, useless to argue, that there are other sources of heat than combustion or deflagration; because, admitting the fact to Mr. Playfair's utmost desire, it can be satisfactorily proved that all these sources are as little capable of acting in the interior parts of the globe, to the extent supposed in the theory before us, as combustion itself, which is relinquished by its defenders as incompetent to their purpose. But even allowing the full operation of all, or of any one of these causes, we have no method pointed out to us by which this subterranean heat is duly preserved and regulatedno controlling power that directs it to the proper place at the proper season, without which it must be as likely to prove a cause of havoc and disorder as of renovation and harmony. It is useless, therefore, to pursue this theory any farther. In spite of the magnificence of its structure, the universality of its application, the plausibility of its appearance, and the talents with which it has been supported, it is built upon assumption alone; it lays down prin. ciples which it cannot support, and deals in fancy and conjecture rather than in solid facts and firm evidence.
Let us next, then, take a glance at the theory by which this is chiefly opposed, and which, as I have already observed, is denoininated the NEPTUNIAN.
Under this hypothesis, the two substances that were first evolved out of the general chaos on the formation of the earth, and chemically united to each other, were hydrogen and oxygen, in such proportion as to produce water, which is a compound of these substances, and in such quantity as to be able to hold every other material in a state of thin paste or solution.
Of the materials thus held in solution granite is supposed to have been produced first, and in by far the greatest abundance. It hence, consolidated first, probably forms the foundation of the superficies of the globe, and perhaps the entire nucleus of the globe itself; and, as has been already seen, while it constitutes the basis of every other kind of rock, rises higher than any of them. It consists, as we have already observed, of felspar, quartz, and mica, all which must therefore have concreted by a crystallization nearly simultaneous; and from its containing no organic remains, it is obvious that it must have been formed prior to the existence of the animal and vegetable kingdoms. All the other rocks, upon this hypothesis, began to crystallize and consolidate after the formation of granite, in the order in which we have already traced them; and some of these before the whole of the granite was rendered perfectly firm, whence we trace beds of several of them in the granite formation itself; and as the same kind of action appears to apply to the whole, we, in like manner, trace beds of the newer rocks successively in formations of those that are older; and, at last, remains of animal and vegetable materials, which are hence proved to have had an existence coetaneous with the newer classes.
The law of gravity appears to have operated through the whole of this process; and hence water, as the least heavy material, must have risen to the surface, and purified itself by a filtration through the other materials, and at length collected in such hollows as were most convenient for its reception: these hollows constitute the bed of the ocean.
Water, thus collected in the cavity of the ocean, is carried by the atmos. phere over the tops of the most elevated mountains, on which it is precipi.
tated in rain, and forms torrents by which it returns with various degrees of rapidity into the common reservoir. This restless motion and progress of the water in the form of rain or torrents gradually attenuate and wear away the hardest rocks, and carry their detached parts to distances more or less considerable; whence we meet with limestone, clay, quartz, or flint, sand, and mineral ores, in places to which they do not naturally belong. The influence of the air, and the varying temperature of the atmosphere, facilitate the attenuation and destruction of these rocks. Heat acts upon their surface, and renders it more accessible, and more penetrable to the moisture, as it enters into their texture; the limestone rocks are reduced by efflorescence, and the air itself affords the acid principle by which the efflorescence is continued. Such are a few of the numerous causes that contribute to the disunion of concrete bodies, and powerfully co-operate with that wonderful fluid which alternately forms and unforms; which creates, decomposes, and regenerates all nature.
The immediate effects of water in the shape of rain is to depress the mountains. But the materials which compose them must resist in proportion to their hardness; and hence we ought not to be surprised at meeting occasionally with peaks which have stood firm amid the wreck of ages, and still remain to attest the original level of the mountain-breadths which have disappeared. These primitive rocks, alike inaccessible to the assault of time and to that of the once animated beings which cover the less elevated heights with their relics, may be considered as the origin of streams and rivers. The water which falls on their summits flows down in torrents by their lateral surfaces. In its course it wears away the soil upon which it is incessantly acting. It hollows out channels of a depth proportioned to its rapidity, its quantity, and the hardness of the rock over which it passes, and at the same time carries along with it fragments of such stones as it loosens in its progress.
These stones, rolled by the water, strike together, and mutually break off their projecting angles; and hence we obtain collections of rounded flints which line the beds of rivers, and of smaller pebbles which the sea is perpetually throwing upon the shores, often incrusted with a gravelly or calcareous edging. The powder which is produced by the rounding of the flints, or is washed down from the mountains, frequently stagnates, forms a paste, and agglutinates irto fresh masses of the rocky matter of which it consists; often imbedding flints and other materials, and constituting compound substances known by the name of pudding-stones and grit stones, which chiefly differ from each other in the coarseness or fineness of their grains, or in the cement which connects them. And if the water be loaded, as it often is, with minutely-divided particles of quartz, it will proceed to crystallize whenever it becomes quiescent; and will form stalactites, agates, cornelians, rock-crystals, plain or coloured, according as it is destitute of, or combined with, any colouring material: and if the material with which the water be impregnated be lime instead of quartz, the crystallization will be calcareous alabaster, or marble.
Many of the earths are now known to be metallic oxides, and all of them are suspected to be so: and hence a degree of heat capable of fusing them, and depriving them of the xygen which gives them their oxide form, will necessarily convert them into their metallic state. That such currents of heat, from electricity and other causes, are occasionally, and perhaps in different places perpetually, existing beneath the surface of the earth, the Neptunian is as ready to admit as the Plutonic geologist; and hence the ori. gin of metallic minerals, of mines, ores, ochres, and pyrites.
The decomposition of animal and vegetable matter contributes largely, moreover, in the view of the system now before us, to the changes which the globe is perpetually sustaining. The exuviæ of shell and coral animals is pepetually adding to the mass of its earths, and laying a foundation for new islands and numerous beds of limestone, in which we very often perceive impressions of the shells from which the soil has originated. On the other hand we observe numerous quantities of vegetables, both submarine and su. perficial, heaped and deposited together by currents or other causes, constituting distinct strata, which progressively become decomposed, lose their organization, and confound their own principles with those of the earths. Hence the origin of pit-coal, and secondary schists or slates ; to which, however, the decomposition of animal substances has also largely contributed. Hence, too, the formation and extrication of a variety of acids and alkalies, which have essentially administered to the actual phenomena of the face of the earth.
The action of volcanoes has contributed much in all ages, and is still contributing in our own, to the present state of the earth's surface. We have daily proofs of the mountains which it has elevated, and have already noticed it as one source of the numerous islands that stud the face of the ocean; and we have just adverted to the subterranean agencies of electricity, heat, water, and other gases and fluids which form its fuel. But the operation of volca. noes is more limited and local than that of the preceding agents. “They accumulate substances," says M. Cuvier," on the surface that were formerly buried deep in the bowels of the earth, after having changed or modified their nature or appearances, and raise them into mountains; but they have never raised up nor overturned the strata through which their apertures pass, and have in no degree contributed to the elevation of the great mountains, which are not volcanic."
Inundations of seas and rivers have also, from time to time, added their tre. mendous force; but there is no ground for concluding that any catastrophe of this kind has been universal for the last four thousand years; nor, in fact, that such an event has ever occurred more than once since the earth has been rendered habitable.
In examining, then, the merits of the antagonist systems of geology before us, the Plutonic is perhaps best entitled to the praise of boldness of conception and unlimited extent of view. It aspires, in many of its modifications, not only to account for the present appearances of the earth, but for that of the universe; and traces out a scheme by which every planet, or system of planets, may be continued indefinitely, and perhaps for ever, by a perpetual series of restoration and balance.
With this system the Neptunian forms a perfect contrast. It is limited to the earth, and to the present appearances of the earth. It resolves the genuine origin of things into the operation of water; and while it admits the existence of subterranean fires to a certain extent, and that several of the phenomena that strike us most forcibly may be the result of such an agency, it peremptorily denies that such an agency is the sole or universal cause of the existing state of things, or that it could possibly be rendered competent to such an effect.
More especially should we feel disposed to adhere to this theory, from its general coincidence with the geology of the Scriptures. The Mosaic narra. tive, indeed, with bold and soaring pinions, takes a comprehensive sweep through the vast range of the solar system, if not through that of the uni. verse; and in its history of the simultaneous origin of this system touches chiefly upon geology, as the part most interesting to ourselves; but so far as it enters upon this doctrine, it is in sufficiently close accordance with the Neptunian scheme,-with the great volume of nature as now cursorily dipped into. The narrative opens, as I had occasion to observe in the lecture on Matter and a Material World, with a statement of three distinct facts, each following the other in a regular series, in the origin of the visible world. First, an absolute creation, as opposed to a mere remodification of the heaven and the earth, which constituted the earliest step in the creative process. Secondly, the condition of the earth when it was thus primarily brought into being, which was that of an amorphous or shapeless waste. And, thirdly, a commencing effort to reduce the unfashioned mass to a condition of order and harmony. "In the beginning," says the sacred historian," God CREATED 'the heaven and the earth. And the earth was WITHOUT FORM AND VOID: and
darkness was upon the face of the deep (or abyss).- And the Spirit of God MOVED upon the FACE OF THE WATERS."
We are hence, therefore, necessarily led to infer that the first change of the formless chaos, after its existence, was into a state of universal aqueous solution ; for it was upon the surface of the waters that the Divine Spirit commenced his operative power. We are next informed, that this chaotic mass acquired shape, not instantaneously, but by a series of six distinct days, or GENERATIONS (that is, epochs), as Moses afterward calls them ;* and apparently through the agency of the established laws of gravity and crystallization, which regulate it at the present moment.
It tells us, that during the first of these days, or generations, was evolved, what, indeed, agreeably to the laws of gravity, must have been evolved first of all, the matter of light and heat; of all material substances the most subtle and attenuate; those by which alone the sun operates, and has ever operated, upon the earth and the other planets, and which may be the identical substances that constitute his essence.t And it tells us also, that the luminous matter thus evolved produced light without the assistance of the sun or moon, which were not set in the sky or firmament, and had no rule till the fourth day or generation: that the light thus produced flowed by tides, and alternately intermitted, constituting a single day and a single night of each of such epochs or generations, whatever their length might be, of which we have no information communicated to us.
It tells us, that during the second day or generation uprose progressively the fine fluids, or waters, as they are poetically and beautifully denominated, of the firmament, and filled the blue ethereal void with a vital atmosphere. That during the third day or generation the waters more properly so called, or the grosser and compacter fluids of the general mass, were strained off and gathered together into the vast bed of the ocean, and the dry land began to make its appearance, by disclosing the peaks or highest points of the primi. tive mountains ; in consequence of which a progress instantly commenced from inorganic matter to vegetable organization, the surface of the earth, as well above as under the waters, being covered with plants and herbs, bearing seeds after their respective kinds; thus laying a basis for those carbonaceous materials, the remains of vegetable matter, which we have already observed are occasionally to be traced in some of the layers or formations of the class of primitive rocks (the lowest of the whole), without a single particle of animal relics intermixed with them.
It tells us, that during the fourth day, or epoch, the sun and moon, now completed, were set in the firmament, the solar system was finished, its laws were established, and the celestial orrery was put into play ; in consequence of which the harmonious revolutions of signs and of seasons, of days and of years, struck up for the first time their mighty symphony. That the fifth period was allotted exclusively to the formation of water-fowl, and the countless tribes of aquatic creatures, and consequently, to that of those lowest ranks of animal life, testaceous worms, corals, and other zoophytes, whose relics, as we have already observed, are alone to be traced in the second class of rocks or transition-formations, and still more freely in the third or horizontal formations; these being the only animals as yet created, since the air and the water, and the utmost peaks of the loftiest mountains, were the only parts as yet inhabitable. It tells us, still continuing the same grand and exquisite climax, that towards the close of this period, the mass of waters having sufficiently retired into the deep bed appointed for them, the sixth and concluding period was devoted to the formation of terrestrial animals; and, last of all, as the masterpiece of the whole, to that of man himself.
Such is the beautiful but literal progression of the creation, according to the Mosaic account, as must be perceived by every one who will carefully peruse it for himself.
Of the extent, however, of the days or GENERATIONS that preceded the forma.
• Gen. ii. 4.
| Herschel, Phil. Trans. vol. lxxxiv.