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II. On the Vertical Position and Convolutions of certain Strata, and their relation with Granite

Published online by Cambridge University Press:  17 January 2013

James Hall
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Extract

The tract of country extending across this island, from the sea-coast of Galloway to that of Berwickshire, consists, with little interruption, of that species of rock, which has of late been most generally known by the German name of Grauwacke. But as this rock does not differ essentially from what in Cornwall is called Killas, I am disposed, in concurrence with several members of this Society, in particular with Mr Allan, who has of late been in Cornwall, and has paid particular attention to that subject, to reject the uncouth term Grauwacke, and adopt that of Killas, as being more congenial to our language.

Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 7 , Issue 1 , 1815 , pp. 79 - 108
Copyright
Copyright © Royal Society of Edinburgh 1815

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References

page 85 note * I am aware, that this expression of parallel curves is irregular; but I can find no other mode of conveying the idea. It is not easy to trace, à priori, what form would be assumed by these beds, supposing the whole to be held down by a force so powerful as to prevent any vacuity. It is enough, however, for our present purpose, that the forms of nature correspond with those obtained in an experiment soon to be mentioned.

page 89 note * M. de Luc, in his Elementary Treatise on Geology, p. 365. art. 311. has undertaken to shew, that my experiments with compression are not applicable to Dr Hutton's hypothesis. “When calcareous substances,” (he says, p 365.) “are calcined in open air, the fixed air which is produced immediately escapes, and it continues to form until the substance is deprived of its ingredients; but when a solid body prevents its escape, the particles first formed acquire a degree of condensation proportional to the resistance which they experience, and they oppose in their turn the same resistance to the formation of other particles. Under such circumstances, therefore, if the heat be increased, it produces other combinations of the fixed air with the calcareous earth, as in the experiments of Sir James Hall. But, under water, which the particles of that gass can easily penetrate, in which, collecting in bubbles, they will rise rapidly, on account of their inferior specific gravity, there can be no impediment to their formation, any more than that of the aqueous vapour in water, under the pressure of the atmosphere, when the heat is sufficiently intense.”

When M. de Luc says, that in my experiments a solid body preventing the escape of the fixed air, “the particles first formed acquire a degree of condensation proportional to the resistance which they experience,” he must conceive, that during the first application of heat, some fixed air has separated from the lime, and has accumulated in the cavity left in the barrels. But if he will look again into my paper, he will find that I had foreseen this inconvenience, and had guarded against it; that being under the necessity of leaving some cavity, in order to allow for the liquid expansion of the fusible metal, I introduced some water into the barrel, which assuming the gaseous form, and reacting with great power, before the heat had risen to the calcining point, effectually prevented the separation of any fixed air. And the same thing would happen at the bottom of a sea that was deep enough. In some of my experiments, made with a compressing force equal to 171 atmospheres, equal to 5693 feet, or about a mile of sea, the carbonate bore the heat of melting gold without calcination, and entered into fusion. Now, it is obvious, that the same result must take place at the bottom of a sea of this depth, and that a shell lying on its bottom, if met by a lava whose heat was equal to that of melting gold, would enter into fusion, and no fixed air would be separated in the form of gas. M. de Luc's objection, therefore, which is founded on the levity of the substance in that gaseous form, must fall to the ground.

In those experiments which I have made, with a compressing force applied by means of a known and regulated weight, the carbonate has been placed exactly in the predicament ascribed to it by Dr Hutton, when exposed to the same heat under a sea whose pressure is equal to that force.

The fusions, then, which have taken place in my experiments, confirm Dr Hutton's theory, in so far as it depends upon the action of heat on limestone.

page 95 note * Gneiss is found to pass by insensible degrees into granite; that is, specimens of every conceivable intermediate step have been found. We may then conceive one stage more advanced towards granite than the rest, in which all character of the original stratification is removed, and the mass may have become wholly crystalline, but in which the peculiarity of each stratum may still have left a trace of its character, in the quality of the granite thus produced from it. This seems to explain the nature of a great part of the internal ridge of the Alps, which is possessed, as Saussure mentions, of a stratified character. This mass may, again, be traversed from within by granite in higher heat, and in a state of complete liquidity, which would be more ready than the mass first described, to penetrate into the neighbouring masses. Accordingly, Saussure observes, that those veins, and other masses which, project from the central ridge, and penetrate farthest into the neighbouring strata, are devoid of that stratified character.

page 101 note * I have also presented one of these models to the Geological Society of London.