Illustrations of the Huttonian Theory of the Earth

by John Playfair

84. In this general conclusion we may distinguish two parts, which, in their degree of certainty, differ perhaps somewhat from one another. The first of these, and that which stands highest in point of evidence, consists of two propositions; namely, that the fluidity which preceded the consolidation of mineral substances was simple, that is, it did not arise from the combination of these substances with any solvent; and, next, that after consolidation, these bodies have been raised up by an expansive force acting from below, and have by that means been brought into their present situation. These two propositions seem to me to be supported by all the evidence that is necessary to constitute the most perfect demonstration.

85. The other part of the general conclusion, that fire, or more properly heat, was the cause of the fluidity of these mineral bodies, and also of their subsequent elevation, is not perhaps to be considered as a truth so fully demonstrated as the two preceding propositions; it is, no doubt, a matter of theory; or a portion of one of those invisible chains by which men seek to connect in the mind the state of nature that is present, with the states of it that are past; and participates of that uncertainty from which our reasonings concerning such causes as are not direct objects of perception, are hardly ever exempted. That it participates of this uncertainty in a very slight degree, will, however, be admitted, when it is considered that the cause assigned has been proved sufficient for the effect; that the same is not true of any other known cause; and that this theory accounts, with singular simplicity and precision, for a system of facts so various and complex, as that which is presented by the natural history of the globe.

86. Neither can it be said that the existence of subterraneous heat is a principle assumed without any evidence, but that of the geological facts which it is intended to explain: on the contrary, it is proved by phenomena within the circle of ordinary experience, namely, those of hot springs, volcanoes, and earthquakes. These leave no doubt of the existence of heat, and of a moving and expansive power, in the bowels of the earth; so that the only questions are, at what depth is this power lodged? to what extent, and with what intensity, does it act? That it is lodged at a very considerable depth, is rendered probable by the permanency of some of the preceding phenomena: from the earliest times many fountains have retained their heat to the present day; and volcanoes, though they become extinguished at length, have a very long period allotted for their duration. The cause of earthquakes is certainly a force that resides very deep under the surface, otherwise the extent of the concussion could not be such as has been observed in many instances.

87. The intensity of volcanic fire, is another circumstance that favours the opinion of its being seated deep under the surface. That this intensity is considerable, is certain from the experiments made by Sir James Hall on the fusibility of whinstone and lava; from which it appears, that the lowest temperature in which either of these stones melt, is about 80° of Wedgewood's pyrometer. Some mineralogists have indeed affirmed, that lava is melted, not by the intensity of the heat applied to it, but in consequence of a certain combination formed between it and bituminous substances, in a manner which they do not attempt to explain, and which has indeed no analogy to any thing that is known. That a hypothesis, formed in such direct opposition to the most obvious principles of inductive reasoning, should have been imagined by a philosopher who had examined the phenomena of Etna and Vesuvius with much attention, and described them with great accuracy and truth, is more wonderful than that it should have been adopted by mineralogists, whose views of nature may have been confined within a cabinet or a laboratory. It is, however, a hypothesis, which, having never had any support but from other hypotheses, hardly merited the direct refutation that it has received from the experiments just mentioned.

88. But, if the intensity of volcanic heat be such as is here stated, it will be found very difficult to account for a fire of such activity, and of such long continuance in the same spot, by any decomposition of mineral substances near the surface. In the place where this combustion is supposed to exist, it must be remembered, that there is no fresh supply of materials to replace those that have been consumed, and that, therefore, the original accumulation of these materials in one spot, must have been very unlike any thing that has ever been observed concerning the disposition of minerals in the bowels of the earth.

89. If, on the other hand, we ascribe the phenomena of volcanoes to the central heat, the account that may be given of them is simple, and consistent with itself. According to all the appearances from which the existence of such heat has been inferred above, it is of a nature so far different from ordinary fire, that it may require no circulation of air, and no supply of combustible materials to support it. It is not accompanied with inflammation or combustion, the great pressure preventing any separation of parts in the substances on which it acts, and the absence of that elastic fluid without which heat seems to have no power to decompose bodies, even the most combustible, contributing to the unalterable nature of all the substances in the mineral regions. There, of consequence, the only effects of heat are fusion and expansion; and that which forms the nucleus of the globe may therefore be a fluid mass, melted, but unchanged by the action of heat.

90. If, from the confines of this nucleus, we conceive certain fissures and openings to traverse the solid crust, and to issue at the surface of the earth, the vapours ascending through these may in time heat the sides of the tubes through which they pass to a vast distance from the lower extremities. It is, indeed, difficult to fix the limit to which this distance may extend, on account of the great difference between the rate at which heat moves when it has a fluid for its vehicle, and when it is left to make its way alone through a solid body. In the present case, the supply of heat is rapid, as being made by a vapour ascending through a tube of solid rock; and the dissipation of it slow, as arising from its transmission through the rock. The waste of heat is therefore small, compared with the supply, and grows smaller at every given point, the longer the stream of heated vapour has continued to flow. Such a stream, therefore, though it may at first be condensed within a small distance of its source, will in time reach higher and higher, and may at last be able to carry its heat to an immense distance from the place of its original derivation. Thus, it is easy to conceive, that vapours from the mineral regions may convey their heat to reservoirs of water near the surface of the earth, and may in that manner produce hot springs, and even boiling fountains, like those of Rycum and Geyser.

91. When, instead of a heated vapour, melted matter is thrown up through the shafts or tubes, which thus communicate with the mineral regions, veins of whinstone and basaltes are formed in the interior of the earth. When the melted matter reaches to the surface, it is thrown out in the form of lava, and all the other phenomena of volcanoes are produced.

Lastly, where melted matter of this kind, or vapours without being condensed, have their progress obstructed, those dreadful concussions are produced, which seem to threaten the existence even of the earth itself. Though terrible, therefore, to the present inhabitants of the globe, the earthquake has its place in the great system of geological operations, and is part of a series of events, essential, as will more clearly appear hereafter, to the general order, and to the preservation of the whole.

Such, according to this theory, are the changes which have befallen mineral substances in the bowels of the earth; and though different for the stratified and unstratified parts of those substances, they are connected together by the same principle, or explained by the same cause. It remains to consider that part of the history of both which describes their changes after their elevation to the surface; and here we shall find new causes introduced, which are more directly the subjects of observation, than those hitherto treated of; causes, also, which act on all fossils alike, and alike prepare them for their ultimate destination.