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 particles 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 re-combination of the materials thus carried down and contained there, and afterwards in their sublimation and reexposure 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." It is not fire, in the usual sense of the word," observes Mr. Playfair," 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 calcine 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 being supposed to exist at prodigious depths below the surface, the substances on which it operates have been regarded as 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 objections which would otherwise weigh heavily against the general theory.

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

* Illustrations of the Huttonian Theory, &c.

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 an 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 regulated—no 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. Notwithstanding 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 principles 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 denominated 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 hydrogene and oxygene, 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 atmosphere over the tops of the most elevated mountains, on which it is precipitated 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