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II.—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

The Cheviot district is largely composed of those quartzless porphyritic rocks to which the term porphyrite has been applied both by English and Continental petrologists. The Cheviot porphyrites are characterized by a compact felsitic ground-mass, throughout which are scattered numerous crystals of felspar, mostly belonging to triclinic species. The ground-mass varies in colour; the two principal shades being dark purple and red. These porphyrites are evidently more or less altered rocks and sometimes the alteration has been carried so far as to make it almost, if not quite impossible to recognize their original nature by macroscopic examination. Amygdaloidal varieties are not uncommon, and in these agates are frequently found. In addition to the ordinary porphyrites, there occur masses of volcanic ash and breccia, and also a remarkable rock which has been called pitchstone-porphyrite. This is dark almost black in colour, with a well-marked resinous lustre and a specific gravity lying between 2·53 and 2·62. It is porphyritic in texture; the large crystals consisting of a fresh glassy-looking triclinic felspar. Red veins usually traverse the rock in different directions. The boulders lying in the streams are covered with a thin light-coloured crust due to weathering, but at a distance of a quarter of an inch from the surface, or even less, the fresh unaltered rock is always found.

Type
Original Articles
Information
Geological Magazine , Volume 10 , Issue 3 , March 1883 , pp. 100 - 108
Copyright
Copyright © Cambridge University Press 1883

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References

page 100 note 1 The term “felsitic” is used with exclusive reference to the macroscopic characters of the ground-mass.

page 101 note 1 These were recognized on the Scotch side by Prof. James Geikie, and on the English by Mr. Clough.

page 101 note 2 Good Words, 1876, p. 265.

page 101 note 3 Geology of Northumberland, p. 43.

page 102 note 1 Prof. A. Geikie states that these volcanic rocks (porphyrites and tuffs) attain a maximum thickness of 6000 feet. Text-Book of Geology, 1882, page 707.

page 102 note 2 Text-Book of Geology, page 739.

page 103 note 1 Rosenbusch, Mikroskopische Physiographie der massigen Gesteine, p. 410.

page 104 note 1 I assume they are glass cavities, because I have not seen any spontaneous movement of the bubbles even in the smallest of them.

page 104 note 2 Zirkel, F., Ueber die mikroskopische Zusammensetzung der diessjährigen Laven von Nea Kammeni, Neues Jahrbuch, 1876, p. 769. Also Mik. Besch. p. 390.

page 104 note 3 The same author describes the augite-andesites of the 40th parallel of North America as having in most cases a resinous lustre. Microscopical Petrography of the 40th Parallel, Washington, 1876, p. 221.

page 104 note 4 Santorin et ses Eruptions, 1879, Paris.

page 105 note 1 Rosenbusch, H., Ueber das Wesen der kömigen und porphyriscke Structur bei Massengesteinen, Neues Jahrbuch, 1882, p. 13.

page 105 note 2 Santorin et ses Eruptions, p. xi.

page 105 note 3 Die Krystalliten, Bonn, 1875, page 139.

page 106 note 1 Abich, A., Ueber die Katur und die Zusammenhang der vulkamschen Bildungen. Brunswick, 1841.

page 106 note 2 Humboldt's Kosmos.

page 106 note 3 Mik. Phy. d. massigen Gesteine, p. 407.

page 106 note 4 Mineralogische Mittheilungen, 1878, vol. i. p. 538.Google Scholar

page 106 note 5 Min. Mitt. 1878, p. 280.