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Published online by Cambridge University Press: 01 May 2009
In the very valuable work on “British Petrography” by my friend Mr. J. J. II. Teall, nowinprocess of publication, there is a rather full notice of an interesting serpentine which occurs in the Rauenthal (Vosges), and has been investigated by Herr Weigand. According to the latter author, as summarized by Mr. Teall,2 this serpentine “has been formed by the alteration of a hornblende rook, and the change has been accompanied by the removal of lime and the development of a chlorite, inwhich the original alumina of the hornblende is retained.”
page 66 note 1 Tsehermaek, Mm. Mitt. 1875, p. 175Google Scholar
page 66 note 2 British Tetrog. p. 112.
page 66 note 3 I find that the dialhige rock at Lendalfoot, described by me, Q.J.G.S. vol. xxxiv. p. 778, is not quite free from olivine. This too is a comparatively small mass.
page 66 note 4 That is to say, a rock which lias resulted from the alteration of a rock consisting of divine with a certain amount of a magnesian bisilicate. a serpentine formed from a perfectly typical duuite should have MgOinexcess of SiO2.
page 67 note 1 For myself I think it better to apply the term picrite to the rather indefinite group which is intermediate between the true peridotites (or felsparless rocks where olivine dominates) and the olivine-dolerites. Olivine + augite and olivine + hornblende need names.
page 67 note 2 Suppose, for purposes of comparison, we assume that no magnesia has been removed.in(B) Al2O3:MgO = 14:360;in(C) Al203:MgO = 67:263. These proportions expressedindecimals are.038 and.254. Againin(A) SiO2:MgO = 369:360—almost equal;in(C) SiO2:MgO = 464:262—about 17:10.
page 67 note 3 Anthophyllite would also he a very suitable mineral, but it is evident that we are not dealiug here with a rhombic mineral, but one which much resembles tremolite.
page 67 note 4 These happen to be slightly thicker than Mr. Teall's slide, so that the distinction, of the constituents is rather more conspicuous.
page 68 note 1 I observe that very commonly there is a slight green band between the cracks (cleavages) of this mineral, as though a liquid had been injected from without. I nave noted the same thing in the case of the pyroxenic minerals present in other serpentines. The mineral may be derived from the olivine and “injected” during the nydration of the olivine, which process must set up considerable strains and pressures.
page 68 note 1 See, for instance, Wadsworth's Lithological Studies, pi. iv. fig. i. The decided green colour is probably due to imperfect separation of the iron.
page 69 note 1 As mentionedinmy paper, Q.J.G.S. vol. xxxix. p. 23.Google Scholar