Illustrations of the Huttonian Theory of the Earth

by John Playfair

48. The probability of this hypothesis will appear greatly increased, when it is considered, that, besides those now enumerated, there are other indications of movement among the bodies of the mineral kingdom, where effects of heat more characteristic than simple expansion are clearly to be discovered. Thus, on examining the marks of disorder and movement which are found among the strata, it cannot fail to be observed, that notwithstanding the fracture and dislocation, of which they afford so many examples, there are few empty spaces to be met with among them, as far as our observation extends. The breaches and separations are numerous, and distinct; but they are, for the most part, completely filled up with minerals of a kind quite different from the rock on each side of them, and remarkable for containing no vestiges of stratification. We are thus led to consider the unstratified minerals, the second of the divisions into which the whole mineral kingdom, viewed geologically, ought to be distinguished. These minerals are immediately connected with the disturbance of the strata, and appear, in many instances, to have been the instruments of their elevation.

SECTION II.

OF THE PHENOMENA PECULIAR TO UNSTRATIFIED BODIES.

1. Metallic Veins.

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49. THE unstratified minerals exist either in veins, intersecting the stratified, or in masses surrounded by them. Veins are of various kinds, and may in general be defined, separations in the continuity of a rock, of a determinate width, but extending indefinitely in length and depth, and filled with mineral substances, different from the rock itself. The mineral veins, strictly so called, are those filled with crystallized substances, and containing the metallic ores.

That these veins are of a formation subsequent to the hardening and consolidation of the strata which they traverse, is too obvious to require any proof; and it is no less clear, from the crystallized and sparry structure of the substances contained in them, that these substances must have concreted from a fluid state. Now, that this fluidity was simple, like that of fusion by heat, and not compound, like that of solution in a menstruum, is inferred from many phenomena. It is inferred from the acknowledged insolubility of the substances that fill the veins, in any one menstruum whatsoever; from the total disappearance of the solvent, if there was any; from the complete filling up of the vein by the substances which that solvent had deposited; from the entire absence of all the appearances of horizontal or gradual deposition; and, lastly, from the existence of close cavities, lined with crystals, and admitting no egress to any thing but heat.

50. To the same effect may be mentioned those groups of crystals composed of substances the most different, that are united in the same specimen, all intersecting and mutually impressing one another. These admit of being explained, on the supposition that they were originally in fusion, and became solid by the loss of heat; a cause that acted on them all alike, and alike impelled them to crystallize: But the appearances of simultaneous crystallization seem incompatible with the nature of deposition from a solvent, where, with respect 16 different substances, the effects must take place slowly, and in succession.

51. The metals contained in the veins which we are now treating of, appear very commonly in the form of an ore, mineralized by sulphur. Their union with this latter substance can be produced, as we know, by heat, but hardly by the way of solution in a menstruum, and certainly not at all, if that menstruum is nothing else than water. The metals, therefore, when mineralized by sulphur, give no countenance to the hypothesis of aqueous solution; and still less do they give any when they are found native, as it is called, that is, malleable, pure and uncombined with any other substance. The great masses of native iron found in Siberia and South America are well known; and nothing certainly can less resemble the products of a chemical precipitation. Gold, however, the most perfect of the metals, is found native most frequently; the others more rarely, in proportion nearly to the facility of their combination with sulphur. Of all such specimens it may be safely affirmed, that if they have ever been fluid, or even soft, they must have been so by the action of heat; for, to suppose that a metal has been precipitated, pure and uncombined from any menstruum, is to trespass against all analogy, and to maintain a physical impossibility. But it is certain, that many of the native metals have once been in a state of softness, because they bear on them impressions which they could not have received but when they were soft. Thus, gold is often impressed by quartz and other stones, which still adhere to it, or are involved in it. Specimens of quartz, containing gold and silver shooting through them, with the most beautiful and varied ramifications, are every where to be met with in the cabinets of the curious; and contain, in their structure, the clearest proof, that the metal and the quartz have been both soft, and have crystallized together. By the compactness, also, of the body which they form, they show, that when they acquired solidity, it was by the concretion of the whole mass, and not by such partial concretion as takes place when a solvent is separated from substances which it held in solution.

52. Native copper is very abundant; and some specimens of it have been found crystallized. Here the crystallization of the metal is a proof that it has passed from a fluid to a solid state; and its purity is a proof that it did not make that transition by being precipitated from a menstruum.