II.—On some Quartz-Felsites and Augite-Granites from the Cheviot District

J. J. Harris Teall

doi:10.1017/S001675680000546X

II.—On some Quartz-Felsites and Augite-Granites from the Cheviot District

Published online by Cambridge University Press: 01 May 2009

J. J. Harris Teall

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In previous communications to the Geological Magazine I have described at some length the petrographical characters of the lavas and tuffs of the Cheviot District. It was proved that they are, at any rate for the most part, of an andesitic character, and that some of them are so little altered as to be thoroughly entitled to the term andesite, unless we are prepared to adopt the unphilosophical system of making geological age, per se, a factor in petrographical nomenclature. If we leave out of account the modifications in structure and composition which have been superinduced on the rocks by the various agents of change, then the Cheviot lavas and tuffs belong to the three fairly well characterized groups of the augite-, hypersthene-, and mica-andesites.

Type: Original Articles
Information: Geological Magazine , Volume 2 , Issue 3 , March 1885 , pp. 106 - 121

DOI: https://doi.org/10.1017/S001675680000546X [Opens in a new window]
Copyright: Copyright © Cambridge University Press 1885

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^{page 106 note 1} Notes on the Cheviot Andesites and Porphyrites, Geol. Mag. 1883, pp. 100, 145, 252Google Scholar

^{page 108 note 1} It must be remembered that the terms quartz-porphyry and quartz-felsite are not synonymous. The former term is the more comprehensive, as it includes such rocks as the porphyritic pitchstones (Vitrophyr of Vogelsang).

^{page 108 note 2} Page 60.

^{page 108 note 3} Die Krystalliten, Bonn, 1875, p. 160.

^{page 108 note 4} Zusammensetzung und Structur granitischer Gesteine, Z.D.G.G. 1876, p. 369.

^{page 109 note 1} Q. J. G. S. vol. xxxii. p. 20.

^{page 109 note 2} Die Steiger Schiefer, p. 345.

^{page 110 note 1} On Devitrified Pitchstones and Perlites of Shropshire, Q.J.G.S. vol. xxxiii. p. 449.

^{page 112 note 1} Z. D. G. G. 1876, p. 369.

^{page 112 note 2} Neues, Jahr. 1883, vol. i. p. 200.Google Scholar

^{page 112 note 3} Studien an gesteinsbildenden Pyroxenen. Neues Jahrbuch III. Beilage Band, p. 262.

^{page 114 note 1} In is interesting to note that the augite-syenites and their porphyritic equivalents of the Christiania district contain felspar rich in soda which appears to be sometimes monoclinic and sometimes triclinic, with a cleavage angle differing but slightly from 90°. Brögger has described these two forms in great detail (Die Silurischen Etagen 2 und 3, p. 203, et seq.). He names them soda-orthoclase and sodamicrocline.

^{page 115 note 1} Massige Gesteine, p. 58.

^{page 115 note 2} Die pyroxen-führenden Gesteine des nord-sächsichen Porphyrgebietes, Min. Mitth. 1881, p. 72.Google Scholar

^{page 115 note 3} Massige Gesteine, p. 58.

^{page 115 note 4} Neues, Jahr. 1871, p. 246.Google Scholar

^{page 115 note 5} Z. D. G. G. 1877, p. 285.

^{page 115 note 6} Die Entstehung der alt-krystallinischen Schiefer-gesteine, p. 228.

^{page 115 note 7} Z. D. G. G. 1875, 343–357.

^{page 116 note 1} Die silurischen Etagen 2 und 3.

^{page 116 note 2} Studien an gesteinsbildenden Pyroxenen, Merian.

^{page 116 note 3} Neues, Jahr. 1883. I. 201.Google Scholar

^{page 116 note 4} Z.D.G.G. 1876, p. 369.

^{page 117 note 1} Geol,. Mag. Dec. II. Vol. X. p. 151.

^{page 117 note 2} Neues, Jahr. 1882, ii. p. 1.Google Scholar

^{page 118 note 1} North of England Dykes, Q.J.G.S. 1884, p. 225.

^{page 118 note 2} Midland Naturalist, 1885, p. 6. Compare the concretionary (?) patches (e.g. certain inclusions in granite described by Mr. Phillips. Q.J.G.S., vol. xxxvi. p. 1, and the nodules of peridotite in basalt) with the so-called contemporaneous veins. The former are as a rule more basic, the latter more acid than the normal rock. The former may represent the earliest, the latter certainly represent in many cases the latest products of consolidation of an originally homogeneous magma.

^{page 119 note 1} Separation may be effected without gravity. Suppose a half-consolidated plutonic mass to become subject to the lateral thrust of which there is such striking evidence in certain districts. The mother-liquor may be squeezed out of the rock as water out of a sponge. In this way contemporaneous veins might be produced in the plutonic rock, dykes in the surrounding rock, and lavas at the surface.

^{page 119 note 2} The subject of the separation of crystals in a molten magma has been discussed by many writers. It is important to notice that it must occur if there be any difference between the specific gravity of the crystals and that of the magma in which they exist. The only question that can arise is as to the geological significance of the operation. Mr. Darwin deals with the subject in his Geological Observations (2nd edition, p. 132), where he describes certain phenomena in the Galapagos Islands which he attributes to this cause. He also quotes a statement from Von Buch that M. Drée found, on melting lava, that the felspars always tended to fall to the bottom of the crucible. The Pattison process for separating lead from silver is mentioned by Mr. Darwin, and it certainly has a very interesting bearing on the subject under consideration.

Again, Mr. Clarence King (Systematic Geology, p. 678) says: “During an eruption in the crater of Kilauea at the time of my visit, a fluid stream of basalt overflowed from the molten lake at the west end of the crater and poured eastward along the level floor of the pit. Numerous little branchlets spurted out from the sides of the flow and ran along the depressions of the basaltic floor, for a few feet and then congealed. I repeatedly broke these small branch streams and examined their section. In every case the bottom of the flow was thickly crowded with triclinic felspars and augites, while the whole upper part of the stream was of nearly pure isotropic and acid glass.’ The sinking of felspars in a basaltic magma strikes one at first sight as an impossibility; but the observations of Darwin and King and the experiments of M. Drée appear to prove the fact in a conclusive manner. We must infer, therefore, that the specific gravity of the molten magma is less than that of the felspars.

^{page 120 note 1} Propylite, the rock which has long been supposed to mark the first period of volcanic action in Tertiary times, has been shown by the recent work of Dr. Becker (Geology of the Comstock Lode) to comprise, in America, altered forms of wellknown rocks, especially andesite, and a similar conclusion had been previously arrived at by Dr. Wadsworth.

^{page 120 note 2} Geol. Mag. 1875.

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II.—On some Quartz-Felsites and Augite-Granites from the Cheviot District

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