Illustrations of the Huttonian Theory of the Earth

by John Playfair

428. I have often quoted Buffon in, the course of these Illustrations, and most commonly for the purpose of combating his opinions; but I am very sensible, nevertheless, of the obligations under which he has laid all the sciences connected with the natural history of the earth.

The extent and variety of his knowledge, the justness of his reasonings, the greatness of his views, his correct taste, and manly eloquence, qualified him, better, perhaps, than any other individual, to compose the History of Nature. The errors into which he Has fallen, are almost all the unavoidable consequences of the circumstances in which he was placed; and if their amount is estimated by the proportion that they bear to the general excellence of the work, they will be reckoned but of small account. Buffon began to write when many parts of natural history had made but little progress; when the quantity of authentic information was small, and when scientific and correct description was hardly to be found. Many of the greatest and most important facts in geology were quite unknown, and scarcely any part of the mineral kingdom had been accurately surveyed; and, with such materials as this state of things afforded, it is not wonderful if some parts of the edifice he erected have not proved so solid and durable as the rest. Had he appeared somewhat later; had he been farther removed from the time when reasonings a priori usurped the place of induction; and had he been as willing to correct the errors into which he had been betrayed by imperfect information, as he was ingenious in defending them, his work would probably have reached as great perfection, as it is given for any thing without the sphere of the accurate sciences to attain. If he had examined the natural history of the earth more with his own eyes, and been as careful to delineate it with fidelity as force; if he had listened with greater care to the philosophers around him; had he attended to the demonstrations of Newton more, and despised the arrangements of Linnæus less; he would have produced a work, as singular for its truth as for its beauty, and would have gone near to merit the eulogy pronounced by the enthusiasm of his countrymen, Majestati Naturæ par ingenium.

Note xxv. § 130.

Figure of the Earth.

429. That the earth is a spheroidal body, compressed at the poles, or elevated at the equator, is a fact established by many accurate experiments; and though these experiments do not exactly coincide, as to the degree of oblateness which they give to that spheroid, they agree sufficiently to put it beyond all dispute, that the earth, though solid, has nearly the same figure which it would assume if fluid, in consequence of its rotation on its axis.

Now, it is not at all obvious, to what physical cause this phenomenon is to be ascribed. The earth, as it exists at present, has none of the conditions that render the assumption of the figure of equilibrium in any way necessary to it. Constituted as it is, its parts cohere with forces incomparably too great to obey the laws of statical pressure, or to assume any one figure rather than another, on account of the centrifugal tendency which results from its revolution on its axis. There is no necessity that its superficies should be every where level, or perpendicular to the direction of gravity, nor that every two columns, standing on the same base, any where within it, and reaching from thence to any two points of the surface, should be of such weights as precisely to balance one another. Neither of these, indeed, is at all conformable to fact. They are, however, the very suppositions on which the determination of the spheroid of equilibrium is founded; and as they certainly do in no degree belong to the earth, it seems strange that the result deduced from them should be in any way applicable to it. This coincidence remains, therefore, to be explained; and it must greatly enhance the merit of any geological system, if it can connect this great and enigmatical phenomenon with the other facts in the natural history of the earth.

430. To establish such a connection, has, accordingly, been a favourite object with geologists, whether they have embraced the Neptunian or Volcanic theory: both have thought that they were entitled to suppose the primeval fluidity of the globe, the one by water, and the other by fire; and in whatsoever way that fluidity was produced, the result of it could be no other than the spheroidal figure of the whole mass, agreeably to the laws of hydrostatics. If in this fluid state the earth was homogeneous, the spheroid would be accurately elliptical, and the compression at the poles would be 1/230 of the radius of the equator; if the fluid was denser toward the centre, the flattening would be less: and in either case, the body, as it acquired solidity, may be supposed to have retained its spheroidal figure with little variation. But though the fluidity of the earth will account for the phenomenon of its oblate figure, it may reasonably be questioned, whether this fluidity can be admitted, in consistency with other appearances. According to what is established above, none of the appearances in the mineral kingdom indicate more than a partial fluidity in any former condition of the earth. The present strata, made up as they are of the ruins of former strata, though softened by heat, have not been rendered fluid by it, and have even possessed their softness in parts, and in succession, not altogether, nor at the same time.

The unstratified, and more crystallized substances, were cast in the bosom of others, which were solid at the time when they were fluid. In all this, therefore, there is no indication of a fluidity prevailing through the whole mass, or even over the whole surface of the earth, and therefore nothing that can explain the spheroidal figure which it has acquired. The supposition, then, of the entire body of the earth, or even of its external crust, having been fluid, though it might account for the compression at the poles, does not connect that fact with the other facts in the natural history of the globe, and fails, therefore, in the point most essential to a theory. It is liable, also, to other objections, whether it be conceived to have proceeded from fire or from water; whether it has happened on the principles of Buffon or of Werner.

431. First, let us suppose that the fluidity of the earth, or of the external crust of it, at least to a certain depth, proceeded from a solution of the whole in the waters of the ocean; and, waving all the objections that have been stated to this hypothesis, on account of the absolute insolubility of many mineral substances in water, let us suppose them all soluble in a certain degree, and let us compute the quantity of the menstruum, which, on the suppositions most favourable to the system, must have been required to this great geologico-chemical operation.

The siliceous earth, though not soluble in water per se, yet, after being dissolved in that fluid by means of an alkali, was found by Dr Black, in his analysis of the Geyser water, to remain suspended in a quantity of water, between 500 and 1000 times its own weight. This is one of the facts most favourable to the Neptunian theory; and that every advantage may be given to that theory, we shall take the least of the numbers just mentioned, and suppose that siliceous earth may be dissolved or suspended in 500 times its weight of water.