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The Geognosy and Mineralogy of Scotland
Published online by Cambridge University Press: 14 March 2018
Extract
While agreeing with those writers who affirm that many of the members of this series are of the nature of schists, —sometimes almost flaggy schists, still, even when examined throughout their multitudinous varieties, it has to be affirmed that they are all gneissic schists; and, however fissile some of. these members may be, this is far from a sumcient reason for attempting to dispense with a time-honoured appellation;—one moreover which is more fittingly applied to them, than to the somewhat more generally hornblendic rock to which alone it has been proposed to confine its use.
- Type
- Research Article
- Information
- Mineralogical magazine and journal of the Mineralogical Society , Volume 5 , Issue 23 , November 1882 , pp. 71 - 106
- Copyright
- Copyright © The Mineralogical Society of Great Britain and Ireland 1882
References
page 74 note * That no very marked petrological distinction can be drawn between the twaelasses of rocks, may be shown by quoting from the writer who most urgently insisted upon the change of name.
Sir R. Murehison in 1858, writes— “weasccrtained that FarridHead consists of the old gneiss.” In 1859, he writes “that promontory was erroneously referred to the old gneiss in my former memoirs, on conclusions drawn from a hasty visit of my friend Mr. Peach, who brought me specimens from one part of the headland, which had a gnelssose aspect.”
One of two conelusio, s must be drawn from this—either, that the haste lay mostin the writer's inferences,—or, that these specimens so strongly resembled the old gneiss, that the very geologist who was desirous of separating them, was himself unable to distinguish between them.
And yet, writing in 1861 we find Murehison again saying—“we are of opinion that no geologist can confound the fundamental gneiss with the so-called gneiss of the superior crystalline schists, which, instead of being a massive hornblendic and granitic rock, is a flag-like, micaceous, and quartzose deposit of very different character.”
No strata of the upper-semes are more flag-like, mieaceous, and quartzose "than those of Farrid Head; yet their “great dissimilarity” in character to those of the lower-rock would not appear to be so marked as to be always capable of ocular apprehension; and it is therefore strange that they should have formed the text for the above quoted specimen of scientific dogmatism.
The climax as regards the rocks of the Farrid is more c/Irious still. We find in a foot note to the last paper by Mnrchison (Feb. 6th, 1861), that specimens of the rocks from this promontory were exhibited to the meeting “to show their dissimilarity to the oM gneiss of Kean na bin, with which Prof. Nicol connects them.” Many years ago Dr, Maculloch cynically tohl us that geologists lind “to be taught even to see;”—does not the above entitle ns to say, that we should be taught to be slow ,to believe? And what do the words "with which Prof. Nicol connects them" teach us,—eeing thag Murchison was himself the first to make the connection
page 87 note * The italics throughout are the present writer's.
page 91 note * Bisehof writes :— “The adherents of the plutonic theory regard granular limestone as sedimentary limestone that has been altered by volcanic action; and they would, therefore, suppose Wollastonite to have originated in the same way, more especially as it occurs in the masses erupted by Vesuvius. It is remarkable that Wollastonite occurs only in granular limestone; not in sedimentary limestone, where the conditions for its formation obtain equally.” i.e., they obtain equally according to Noptunian views.
page 92 note * I shall afterwards adduce facts which almost incontestably show that many so.called "igneous rocks" in Scotland owed that plasticity which enabled them to be “injected” into fissures of moderate size, to an exalted metamorphism in situ; their heat being due to crushing and folding. At present I confine myself to an auymentation of that heat, cozsequent, upon thw presence of lime.
page 92 note † So great and so invwriohle is the occurrence of one or other form of augite in connection with the “primary limestone” throughout the whole of the Highlands of Scotland, that it was with no little wonder that the writer observed the iollowing statement—in a critical review of a work by Messrs. King and Rowney, in which these authors are being taken severely to task for postulating points in dispute. “Augite and hypersthene are placed among mineral species ‘common to ordinary metamorphics.’ The authors are surely aware that at any rate normal angite cannot be called common in metamorphic rocks.” While prepared to admit that the augito present here in the outer-zone of the contact-band, may, if it shews a uralitic structure, possibly result from a transmutation of the adjacent hornblende, yet there are many localities in which such augite is seen, where there is no hornblende in the neighbouring gneiss; while the mode of occurrence and the amount of the sablite and malacolite in the limestone, is such as to entitle us to say that it becomes the mineral typical of this metamorphic rock;—while in some localities it actually forms its mass, and this it does to such an extent that the quantity of augite iu one form or another, cannot, in the upper-gneiss of Scotland, be much smaller than that of the hornblende.
Very possibly the above writer will hold primary limestone to be not an “ordinary metamorphic”; and, as he seems to regard augito as being typical of rocks which have at least been subjected to a high heat, we should expect him to agree more or tess with Our views relating to this limestone; his adhesion to our views would bring satisfaction, if we did not observe that the microscope seems to be by him regarded as an instrument of precision instead of being at present a mere adjunct—however valusble,—in lithology.
page 94 note * I do not here, for want of sufficient data, enter into a consideration of the modifications produced by the presence of certain media,—ealcitc, however, does not generally induce complex moditicatiuus
page 100 note * I have since treated the andesine from Urquhart and that from Dalnein—both of which analysis has proved to be andeaine, and both of whi ch are striated—with acidlof one to four of water, and also with strong acid, without observing any effect.
page 102 note * In figure, 6 the, focus marked o should be d, in the lower half.