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III.—On Some Quartz-schists from the Alps
Published online by Cambridge University Press: 01 May 2009
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A thick mass of bedded schists, as I have described in more than one paper, occupies the highest position among the socalled metamorphic rocks of the Alps. With the exception of some gneisses—so far as I know of a porphyritic character, and almost certainly intrusive granites modified by pressure— and various green schists, which, in part at least, are basic igneous rocks, similarly affected, the group, which may be traced from one end of the chain to the other, consists of altered sediments.
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References
page 204 note 1 Q.J.G.S. vol. xlii (Presid. Address) xlv. p. 67; vol. xlvi. p. 187.Google Scholar
page 204 note 2 Q.J.G.S. vol. xlix. p. 94.Google Scholar
page 204 note 3 Sometimes dolomitic.
page 204 note 4 I have examined them more or less minutely, over various Alpine regions, from the Viso to the Gross-Glockner.
page 204 note 5 Q.J.G.S. vol. xlvi. p. 199, etc.Google Scholar
page 204 note 6 I have not been east of the former among the crystalline schists, nor found them west of the latter in any journey made since I began to pay minute attention to the Alpine schists.
page 205 note 1 It was my intention to give some particulars of these gneisses, but they would make the article too long, so I reserve them for a future communication.
page 205 note 2 Between Saas-Fee and Saas im-Grund and on the peak of the Mittaghorn.
page 205 note 3 Many details are given in the papers to which reference has already been made.
page 205 note 4 I avoid the term quartzite for reasons which will presently appear.
page 205 note 5 Other instances may be found both in this district and elsewhere in the Alps. The effect of pressure in crystalline limestones is discussed in this MAGAZINE, 1889, p. 483.Google Scholar
page 206 note 1 It is indicated as correctly as the scale permits on the Swiss map, sheet xxiii. Here the gneiss above is distinguished from that below. There are some differences, but I think that we may regard Ihem as of minor importance, and the gneiss broadly as one group.
page 206 note 2 To give an idea of the possible complication I may mention that in one place we found a small patch of a cavernous, yellowish, soft, calcareous rock, containing numerous fragments from the calc-schist group. It might have beeu a tufa; but the position made this interpretation improbable, and I strongly suspect it was the last remains of an infold of rauchwacke (Trias). This rock is mapped as here and there in the middle of the crystallines in this part of the Pennines, always in mere strips, and with the once overlying Jurassic rocks wholly gone.
page 206 note 3 I refer only to the apparent position.
page 206 note 4 The path from Saas-Fee ascends and crosses the northern spur of the Mittagliorn and then runs for some little time along the eastern face at a height probably of rather more than 3000 feet above the Saas-thal. The sections described are found soon after the corner is turned, but the calc-mica-schist and quartz-schist can be also seen for some distance running obliquely down the western slope of the mountain.
page 207 note 1 Small zircons, occasional grains of iron oxide, probably also of zoisite, may be detected, but on these it is needless to dwell, as they have no apparent bearing on the history of the rock.
page 207 note 2 Most of these require a one-eighth objective and many can only just be seen with that; some are irregular in form, the majority oval.
page 208 note 1 Thus labelled in the field: “seems to be the base of the quartz-schist, just against wrinkled gneiss.”
page 208 note 2 In these slides a few rather irregular grains of a clear mineral are present, which is probably zoisite, and in one garnet (one or two grains of rather fragmental aspect) is probably present.
page 209 note 1 The structure of these schists sometimes recalls that of certain of the rocks on the Eastern Gneiss near Kinlochewe, but the structure is coarser, the fragmental character (due in the latter to crushing) is less distinct, as if mineral reconstitution. had been more complete in these Alpine rocks.
page 209 note 2 See Q.J.G.S. vol. xliv. p. 11.Google Scholar
page 209 note 3 I have not a few slides in my collection, representing, among others, the ‘ Durness,’ Wrekin, Hartsliill, Lickey, Stiperstones, Cherbourg, and Ardennes quartzsites.
page 209 note 4 I am indebted to the kindness of Messrs. Peach and Goodchild, and the favour of Sir A. Geikie, for the opportunity of examining a fine specimen of the ‘piped’ quartzite (Basal Cambrian) from Glendhu, Sutherlandshire, which has become, through great pressure, slabby and schistose. It exhibits a structuie with which I was already familiar from other rocks affected by the great thrust faults of the N.W. Highlands, and this is so different from that of the quartz-schists described in this paper that I could distinguish the one from the other at the first glance through the microscope.
page 209 note 5 I except, of course, the cases where the rock exhibits flexures. Here it may be added that since the first notes of this paper were written I have had the opportunity of seeing the quartz-rocks near Clifden (Connemara)—a prolongation westward of the Twelve Pins. These in the field reminded me frequently of the Alpine quartzschists, and always appeared to differ from the Durness quartzite; they seem to be associated with crystalline schists. Also the quartz-schists in the district near Braemar; some of which occur near to calc-mica-schists, very like those of the Alps, others pass into mica-schists or a kind of gneiss. In microscopic structure both rocks agree in the main with these Alpine quartz-schists. The gneissoid bands seem likely to confirm the idea expressed above that one kind of gneiss may result from the alteration of a quartz felspar grit, but I must verify some points before I can venture to say more.
page 210 note 1 Q.J.G.S. vol. xliv. p. 32.Google Scholar
page 210 note 2 Geol. Mag. Dec. III. Vol. VI. (1889), pp. 315–318.Google Scholar
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