Illustrations of the Huttonian Theory of the Earth

by John Playfair

[20] See some farther remarks on this subject at Note xiii.

60. In the view now given of metallic veins, they have been considered as traversing only the stratified parts of the globe. They do, however, occasionally intersect the unstratified parts, particularly the granite, the same vein often continuing its course across rocks of both kinds, without suffering any material change; and, if we have hitherto paid no attention to this circumstance, it is because the order pursued in this essay required, that the relation of the veins to stratified bodies should be first treated of. Besides, the facts in the natural history of veins, whether contained in stratified or unstratified rocks, are so nearly alike, that in a general view of geology, they do not require to be distinguished. It is material to remark, that, though metallic veins are found indiscriminately in all the different kinds of rock, whether stratified or otherwise, they are most abundant in the class of primary schisti. All the countries most remarkable for their mines, and the mountains distinguished by the name of metalliferous, are primary, and the instance of Derbyshire is perhaps the most considerable exception to this rule that is known. This preference, which the metals appear to give to the primary strata, is very consistent with Dr Hutton's theory, which represents the rocks of that order as being most changed from their original position, and those on which the disturbing forces of the subterraneous regions have acted most frequently, and with greatest energy. The primary strata are the lowest, also, and have the most direct communication with those regions from which the mineral veins derive all their riches.

2. Of Whinstone.

61. Beside the veins filled with spar, and containing the metallic ores, the strata are intersected by veins of whinstone, porphyry, and granite, the characters of which are next to be examined.

The term whin, or whinstone, with Dr. Hutton, like the word trap, with the German mineralogists, denotes a class of stones, comprehending several distinct species, or at least varieties. The common basalt, the wacken, mullen, and crag of Kirwan, the grûnstein of Werner, and the amygdaloid, are comprehended under the name of whin. All these stones have a tendency to a spathose structure, and discover at least the rudiments of crystallization. They are, at the same time, without any mark of stratification in their internal texture, as they are also, for the most part, in their outward configuration; and, as the different species here enumerated compose, not unfrequently, parts of the same continuous rock, the change from one to another being made through a series of insensible gradations, they may safely be regarded by the geologist as belonging to the same genus.

62. Whin, though not stratified, exists in two different ways, that is, either in veins, (called in Scotland dykes,) traversing the strata like the veins already described, or in irregular masses, incumbent on the strata, and sometimes interposed between them. In both these forms, whinstone has nearly the same characters, and bears, in all its varieties, a most striking resemblance to the lavas which have actually flowed from volcanoes on the surface of the earth. This resemblance is so great, that the two substances have been often mistaken for one another; and many rocks, which have been pronounced to be the remains of extinguished volcanoes, by mineralogists of no inconsiderable name, have been found, on closer examination, to be nothing else than masses or veins of whinstone. This latter stone is indeed only to be distinguished from the former, by a careful examination of the internal characters of both; and chiefly from this circumstance, that whinstone often contains calcareous spar and zeolite, whereas neither of these substances is found in such lavas, as are certainly known to have been thrown out by volcanic explosions.

Now, from these circumstances of affinity between lava and whinstone, on the one hand, and of diversity on the other, as the formation of the one is known, it should seem that some probable conclusion may be drawn concerning the formation of the other. The affinity in question is constant and essential; the difference variable and accidental; and this naturally leads to suspect, that the two stones have the same origin; and that, as lava is certainly a production of fire, so probably is whinstone.

63. But, in order to see whether this hypothesis will explain the diversity of the two substances, without which it will not be entitled to much attention, we must remark, that the presence of carbonate of lime in a body that has been fused, argues, agreeably to the principles formerly explained, that the fusion was brought about under a great compressing force, that is to say, deep in the bowels of the earth, or in the great laboratory of the mineral regions. We are, therefore, to suppose that the fusion of the whin was performed in those regions, where the compression was sufficient to preserve the carbonic gas in union with the calcareous earth, so that these two substances melted together, and, on cooling, crystallized into spar. In the lavas, again, thrown out by volcanic eruption, the fusion, as we know, wherever it may begin, continues in the open air, where the pressure is only that of the atmosphere: the calcareous earth, which, therefore, may have been, in the form of a carbonate, among the materials of this lava, must be converted into quicklime, and become infusible; hence the want of calcareous spar in lavas that have flowed at the surface.

Thus, whinstone is to be accounted a subterraneous, or unerupted lava; and our theory has the advantage of explaining both the affinity and the difference between these stony bodies, without the introduction of any new hypothesis. In the Neptunian system, the affinity of whinstone and lava is a paradox which admits of no solution.

64. The columnar structure sometimes found in that species of whinstone called basaltes, is a fact which has given rise to much discussion; and it must be confessed, that though one of the most striking and peculiar characters of this fossil, it is not that which gives the clearest and most direct information concerning its origin. One circumstance, however, very much in favour of the opinion that basaltic rocks owe their formation to fire, is, that the columnar form is sometimes assumed by the lava actually erupted from volcanoes. Now, it is certainly of no small importance, to have the synthetic argument on our side, and to know, that basaltic columns can be produced by fire; though, no doubt, to give absolute certainty to our conclusion, it would be necessary to show, that there are in nature no other means but this by which these columns can be formed. This sort of evidence is hardly to be looked for; but since the power of fusion, to produce the phenomena in question, is perfectly established, and since the production of the same phenomena in the humid way is a mere hypothesis, if there be the least reason to suspect the action of subterraneous heat as one of the causes of mineralization, every maxim of sound philosophy requires that the basaltic structure, in all cases, should be ascribed to it.