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II.—On Hypersthene Andesite

Published online by Cambridge University Press:  01 May 2009

J. J. Harris Teall
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Extract

At the conclusion of my last paper on the Cheviot andesites and porphyrites (Geol. Mag. Vol. X. p. 262), I called attention to the close resemblance between the Cheviot hypersthene andesite (or porphyrite) and a rock from Steinerne Mann, Nahe, which has been called a proterobase. Both rocks are black in colour and consist macroscopically of glassy felspars (Mikrolin) imbedded in a groundmass which has a resinous or semi-resinous lustre. Microscopically they consist of two generations of felspar, pyroxene mostly rhombic, magnetite, and a glassy base with various devitrification products.

Type
Original Articles
Information
Geological Magazine , Volume 10 , Issue 8 , August 1883 , pp. 344 - 348
Copyright
Copyright © Cambridge University Press 1883

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References

page 344 note 1 To this relation of sunken trough and mountain arch, there can, indeed, be no exception. The grand arch constituting a mountain chain is most properly regarded as an elevated anticlinal limited exteriorly by two depressed synclinals. Hence the paragraph ix. (a) 4 (p. 198) is mechanically incorrect as it stands, and should read:— (IX. 4.) At the foot of either bounding synclinal of a mountain range the thrust is wholly horizontal; the twisting or shearing force is zero or very small, and the resulting folds are normal (symmetrical) and regular. But as we proceed towards the centre of the anticlinal of the range, this horizontal thrust is combined with a vertical component due to the weight of the mass above; the total thrust therefore becomes greater, and, moreover, its direction is more or less oblique to the transverse section of the rock, i.e. the twisting or shearing force becomes greater also. (The maximum of these latter effects occurs somewhere near the junction of the syncline a d the anticline.) As a natural consequence the axes of the folds no longer remain vertical, but slope obliquely outwards, in the manner of inclined or reflexed folds.

page 344 note 2 The term proterobase was introduced by Gümbel (Die paläolithischeu Eruptivgesteine des Fichtelgebirges, München, 1874) for certain rocks occurring in the Fichtelgebirge. It is defined by Rosenbusch (Massige Gesteine, p. 346) as a hornblende-bearing diabase. As the Steinerne Mann rock does not appear to contain hornblende, the term seems inappropriate.

page 345 note 1 If I were repeating this isolation, I should use the solution of bi-iodide of mercury and potassium. The following directions for preparing this solution may be of interest to readers of the Geological Magazine. Mix together in a mortar carefully weighed portions of the two salts, bi-iodide of mercury and iodide of potassium in the proportion of 124 grammes of the former to 100 of the latter. Add the mixed salts gradually to a small quantity of water (500 grammes of the mixed salts will dissolve in 80 c.c. of water) in the cold. Then place the vessel containing the solution on a water bath and evaporate till a film begins to form on the surface of the solution. Allow to cool, and filter. The most convenient way of filtering is to place a little asbestos at the bottom of an ordinary glass filter, as the dense liquid passes very slowly through ordinary filter-paper. The clear solution thus obtained will support fluor spar. My own stock solution has a specific gravity of 3·161. The above instructions are taken from a paper by V. Goldschmidt in the Neues Jabrbuch, I. Beilage Band, p. 179. The maximum density obtained by Herr Goldschmidt in the winter was 3·196. Any lower specific gravity may of course be obtained by dilution, and the original density may be_ again reached by evaporation over the water bath.