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I.—On the Metamorphic Origin of Certain Granitold Rocks and Granites in the Southern Uplands of Scotland
Published online by Cambridge University Press: 01 May 2009
Extract
The special proofs of metamorphism which it is the object of this paper to point out, are rather of a geological than chemical nature. They consist, in short, of such evidence as may be readily gathered in the field, and although, to render them complete, they ought perhaps, to be followed up by analyses of the rocks, yet it is believed that the phenomena to be described tell a plain enough story.
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- Copyright © Cambridge University Press 1866
References
page 529 note 1 Most of the trap dykes of the Scottish Carboniferous strata, however, are not hornblendic but augitic greenstones, or dolerites.
page 530 note 1 In a paper read before the Geological Society, 06 6, 1866: vide Geol. Mag.Google Scholar
page 531 note 1 The best exposures of these metamorphic rocks occur on Tineorn Hill and Blackside; the unaltered strata are well seen in a few quarries in the same neighbourhood, and fine sections are also obtained in the tributaries of the Gower water, a stream that joins the Irvine above the village of Darvel.
page 531 note 2 It ought to be mentioned that here and there this minette contains a few crystals of hornblende.
page 531 note 3 It is impossible not to be impressed with the exact resemblance of these altered fragments to the similar “nests” of metamorphic rock in granite, which are described in the sequel. One is also interested by observing how the minette sometimes takes on a faintly “schistoze ” arrangement of its minerals. In a few places dark mica lies ia Certain horizontal lines which seem to coincide with the bedding, leaving intermediate spaces of a finely crystalline greyish felstone. Flakes of dark shale, somewhat less highly altered than those alluded to above, occur in these incipient schistoze portions of the minette. But from this, it is not inferred that an amorphous crystalline state is necessarily preceded by a streaky or schistoze condition of the minerals. With regard to the peculiar arrangement of the minerals in gneissoze and schistoze rocks we have still much to learn. It is certain, however, that rocks such as diallagite, hypersthenite, diorite, syenite, and even granite itself can be developed directly from aqueous rocks, without passing through an intermediate gneissic or schistoze state. This will probably always be the case when the aqueous rocks acted upon are thick-bedded and of an equally diffused composition. The exact conditions which gave rise to our schistoze-, hornblende-, diallage-, and hypersthene-rocks, and to gneiss itself have yet to be discovered, but it is not unlikely that thin bedding and variable composition of the alternating layers may have been among the causes that determined a schistose.
page 533 note 1 They are well shown in many granites of the north of Scotland as well as in those of the districts more immediately referred to.