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I.—Notes on the Cheviot Andesites and Porphyrites

Published online by Cambridge University Press:  01 May 2009

J. J. Harris Teall
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Extract

Before proceeding to describe the more common porphyrites, I wish to return to the subject of the true andesites, referred to in my last paper, and their relation to Tertiary and recent rocks of a similar chemical and mineralogical composition. On referring to the note appended to that paper, it will be seen that my description of the pyroxenic constituents of these rocks was very imperfect, not to say inaccurate. The number of elongated sections which extinguished parallel with the vibration-planes of the crossed Nicols attracted my attention when first I examined the rock, but seeing that the same feature occurred in the so-called augite-andesites of Hungary and Santorin, and seeing further that as regards cleavage and crystalline form the pyroxenic constituents of the Cheviot rock resembled those of the augite-andesites above mentioned, I came to the conclusion that the prevailing pyroxene in both cases was the same, and also that it was augite. In the former conclusion I still think I was right, but in the latter I was unquestionably wrong.

Type
Original Articles
Information
Geological Magazine , Volume 10 , Issue 4 , April 1883 , pp. 145 - 152
Copyright
Copyright © Cambridge University Press 1883

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References

page 146 note 1 Mr. Whitman Cross expresses doubt as to the monoclinic character of the pyroxene associated with hypersthene in the American rocks. I have not examined a sufficient number of sections of the corresponding mineral in the Cheviot rocks to speak positively on this point, but I have seen nothing to suggest a doubt as to its being a true augite. I therefore speak of it as monoclinic in thi paper.

page 147 note 1 I do not wish to be understood as holding the view that there is no difference between igneous rocks of different geological epochs. I do hold, however, that a petrologist should be able to name a rock without reference to its geological age, just as a palæontologist should be able to name a well-preserved fossil without reference to its age. If geological age, per se, is to be accepted as a point of classificatory value in petrology, then there are, as every field geologist knows, many intrusive rocks that we can never name, for we can never determine their geological age. Such, for instance, are the rocks forming many of the dykes in the North of England.

page 147 note 2 Trachyte eingetheilt nach den natürliehen System. Exposition TJniverselle, Vienne, 1873.

Classification macrographique des trachytes par J. Szabóo.

Sur la classification et la chronologie des roches éruptives tertiaire de la Hongrie, Paris, 1880. Extrait du compte rendu sténographique du Congrés international de Geologie tenu à Paris du 29 au 31 août et dn 2 au 4 Sept. 1878.

page 148 note 1 Trans. Royal Soc. Edin., vol. xxix. part 1.Google Scholar

page 151 note 1 Estimated as Fe2 O3.

page 152 note 1 Levy and Fouqué, Min. Micro, p. 383.