Illustrations of the Huttonian Theory of the Earth

by John Playfair

453. The first example I shall take from the system of Saussure. It is to be regretted, that this excellent geologist has no where given us a complete account of his theory. Some of the leading principles of it are, however, unfolded in the course of his observations, and enable us to form a notion of its general outline. It was evidently far removed from the system of subterraneous heat, and seems, especially in the latter part of the author's life, to have been very much accommodated to the prevailing system of Werner. Nevertheless, with so little affinity between their general views, Saussure and Hutton agree in that most important article which regards the elevation of the strata. Saussure plainly perceived the impossibility of the strata being formed in the vertical situations which so many of them now occupy; and he takes great pains to demonstrate this impossibility, from some facts that have been referred to above. He also believed that this elevation had been given to strata that were originally level, by a force directed upwards, or by the refoulement of the beds, not by their falling in, as is the opinion of De Luc and some other of the Neptunists.

Now, whoever admits this principle, and reasons on it consistently, without being afraid to follow it through all its consequences, must unavoidably come very close to the Huttonian theory. He must see, that a power which, acting from below, produced this great effect can never have belonged to water, unless rarefied into steam by the application of heat. But if it be once admitted that heat resides in the mineral regions, the great objection to Dr Hutton's system is removed; and the theorist, who was furnished with so active and so powerful an agent, would be very unskilful in the management of his own resources, if he did not employ it in the work of consolidating as well as in that of raising up the strata. A little attention will show, that it is qualified for both purposes; though insuperable objections must, no doubt, offer themselves, where the effects of compression are not understood. We may safely conclude, then, that the accurate and ingenious Geologist of Geneva ought to have been a Plutonist, in order to give consistency to the principles which he had adopted, and to make them coalesce as parts of one and the same system. If he embraced an opposite opinion, it probably was from feeling the force of those objections that arise from our discovering nothing in the bowels of the earth like the remains left by combustion, or inflammation, at its surface. The secret by which these seeming contradictions are to be reconciled, was unknown to this mineralogist, and he has accordingly decided strongly against the action of fire, even in the case of those unstratified substances that have the greatest affinity to volcanic lava.

454. The theoretical conclusions of another accurate and skilful observer, Dolomieu, furnish a still more remarkable example of a tendency to union between systems professedly hostile to one another.

This ingenious mineralogist, observing the interposition of the basalt between stratified rocks, so that it had not only regular beds of sandstone for its base, but was also covered with beds of the same kind, saw plainly that these appearances were inconsistent with the supposition of common volcanic explosions at the surface. He therefore conceived, that the volcanic eruption had happened at the bottom of the sea, (the level of which, in former ages, had been much higher than at present,) and that the materials afterwards deposited on the lava, had been in length of time consolidated into beds of stone. It is evident, that this notion of submarine volcanoes, comes very near, in many respects, to Dr Hutton's explanation of the same appearances. If the only thing to be accounted for were the phenomenon in question, it cannot be denied that Dolomieu's hypothesis would be perfectly sufficient; but Dr Hutton, to whom this phenomenon was familiar, and who, like Dolomieu, conceived the basalt to have been in fusion, was convinced that the retreat of the sea was not a fact well attested by geological appearances, and if admitted, was inadequate to account for the facts usually explained by it. He conceived, therefore, that such lava as the preceding had flowed not only at the bottom of the sea, but in the bowels of the earth, and having been forced up through the fissures of rocks already formed, had heaved up some of these rocks, and interposed itself between them. This agrees with the other facts in the natural history both of the basaltes and the strata.

It is plain, that, in this, there is a great approach of the two theories to one another: both maintain the igneous origin of basaltes, and its affinity to lava; both acknowledge that this lava cannot have flowed at the surface, and that the strata which cover it have been formed at the bottom of the sea. They only differ as to the mode in which the submarine or subterraneous volcano produced its effect, and that difference arises merely from the one geologist having generalized more than the other. Dolomieu sought to connect the basalt with the lavas that proceed from volcanic explosions at the surface; Dr Hutton sought not only to connect these two appearances with one another, but also with the other phenomena of mineralogy, particularly with the veins of basaltes, and the elevation of the strata.

455. In another point, the coincidence of Dolomieu's opinions and Dr Hutton's is still more striking. The former has remarked, that many of the extinguished volcanoes are in granite countries, and that, nevertheless, the lavas that they have erupted contain no granitic stones. There must be, therefore, says he, something under the granite, and this last is not, at least in all cases, to be considered as the basis of the mineral kingdom, or as the body on which all others rest. In this system, therefore, granite is not always a primordial rock, any more than in Dr Hutton's.

But Dolomieu makes a still nearer advance to the Huttonian theory; for he supposes, that under the solid and hard crust of the globe, there is a sphere of melted stone, from which this basaltic lava was thrown up. The system of subterraneous heat is here adopted in its utmost extent, and in that form which is considered as the most liable to objection, viz. the existence of it at the present moment, in such a degree as to melt rocks, and keep them in a state of fusion. In this conclusion, the two theories agree perfectly; and if they do so, it is only because the nature of things has forced them into union, notwithstanding the dissimilitude of their fundamental principles.

This ought to be considered as a strong proof, that the phenomena known to mineralogists are sufficient to justify the attempts to form a theory of the earth, and are such as lead to the same conclusions, where there was not only no previous concert, but even a very marked opposition. I have already observed, that there is a greater tendency to agree among geological theories, than among the authors of those theories.

456. Another circumstance worthy of consideration is, that in the search which the Neptunists have made, for facts most favourable to the aqueous formation of minerals, we find hardly any of a kind that was unknown to the author of the system here explained. The appearances on which Werner grounds his opinion with respect to basaltes, and by which he would exclude the action of fire from any share in the formation of it, are all comprehended in the alternation of that rock with beds, or strata obviously of aqueous origin. Now these appearances were well known to Dr Hutton, and are easily explained by his theory, provided the effects of compression are admitted. From this, and the other circumstances just observed, I am disposed to think, that the great facts on which every geological system must depend, are now known, and that it is not too bold an anticipation to say, that a theory of the earth, which explains all the phenomena with which we are at present acquainted, will be found to explain all those that remain to be discovered.

457. The time indeed was, and we are not yet far removed from it, when one of the most important principles involved in Dr Hutton's theory was not only unknown, but could not be discovered. This was before the causticity produced in limestone by exposure to fire was understood, and when it was not known that it arose from the expulsion of a certain aerial fluid, which before was a component part of the stone. It could not then be perceived, that this aerial part might be retained by pressure, even in spite of the action of fire, and that in a region where great compression existed, the absence of causticity was no proof that great heat had not been applied. The discoveries of Dr Black, therefore, mark an era, before which men were not qualified to judge of the nature of the powers that had acted in the consolidation of mineral substances. Those discoveries were, indeed, destined to produce a memorable change in chemistry, and in all the branches of knowledge allied to it; and have been the foundation of that brilliant progress, by which a collection of practical rules, and of insulated facts, has in a few years risen to the rank of a very perfect science. But even before they had explained the nature of carbonic gas, and its affinity to calcareous earth, I am not sure but that Dr Hutton's theory was, at least, partly formed, though it must certainly have remained, even in his own opinion, exposed to great difficulties. His active and penetrating genius soon perceived, in the experiments of his friend, the solution of those difficulties, and formed that happy combination of principles, which has enabled him to explain the most enigmatical appearances in the natural history of the earth.