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VIII.—Notes on the Probable Origin of Some Slates

Published online by Cambridge University Press:  01 May 2009

W. Maynard Hutchings
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Extract

In the deposits previously described what takes place appears to be about as follows. The larger flakes of biotite undergo an alteration which largely produces epidote, and not improbably granular rutile, though this is not at all proved. Crystals of rutile very rarely result during the process. Such of the iron of the biotite as is not combined in the epidote (which is strongly coloured) is removed in solution, some of it being deposited again as limonite diffused throughout the deposit, and some taking part in the formation of the thin bands of ironstone, being precipitated as carbonate.

Type
Original Articles
Information
Geological Magazine , Volume 7 , Issue 7 , July 1890 , pp. 316 - 322
Copyright
Copyright © Cambridge University Press 1890

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References

page 316 note 1 The experiments of Daubrée (Geologie expérimentale, 1879) on the effects of the mutual attrition of fragments of granite in water, are of very great interest and bear strongly on the special point here in question. It is to be particularly noted that he shows how comparatively rapidly the felspar is reduced to the condition of the finest mud (“limon”); and further, that the felspathic mud so produced has not been simply reduced mechanically to powder, but has been chemically acted upon to a noticeable extent in a relatively short time. The felspar has given up some of its alkali to the water and has become hydrated by taking water into combination. The granite used in these experiments was fresh and its felspar not weathered.

This fine mud of hydrated felspar, so fine as to remain suspended in water several days, and depositing as a plastic mass, is in just the condition for rapidly undergoing chemical changes and acting powerfully upon the minute fragments of biotite, etc., with which it is intimately intermingled.

page 317 note 1 Although “sericite” is now stated to be only a form of muscovite, and the use of the term is open to some objections, it will still be well to follow Rénard, who elects to continue its employment when writing of mica haying the special physical characters which it shows in the class of rocks now under discussion.

page 318 note 1 We are indebted to Rénard (“Becherches sur la composition et la structure des phyllades Ardennais,” Bulletia du musée royal d'histoire uaturelle de Belgique, 1882–1883), for a series of very careful analyses of phyllites, many of which very closely resemble those of North Cornwall.

In ten analyses the titanic acid present varies from 2·28 p c. to 0·13 p.c, the average being 1·20 p.c.

In the same analyses magnesia ranges from 2·35 p.c. to 1·13 p.c. Combined water is mostly between 3 and 4 p.c, the maximum (unusual) being 4·94 p.c.

page 320 note 1 At about the same date as Dr. Sorby's address, Pfaff published a paper on some more recent clay-slates (“Petrographische Untersuchungen über die eocänen Thonschiefer der Glarner Alpen,” Sitzungsbericht der K. Bayer. Akad. der Wissenschaften, 1880). The examination of these slates and shales showed a “surprising amount of resemblance to the Palæozoic clay-slates” down to the presence of the “clay-slate needles.” Pfaff arrives at the same conclusion as does Sorby as to the slates being simply compacted original sediments; but while Sorby seems to have formed this opinion from the study of the slates simply, and from some reasoning as to what might be looked for as a result of disintegration of granites and other rocks, Pfaff bases his verdict more on the fact that he has studied clays of various ages (including Carboniferous, Triassic, Jurassic, and Tertiary), in all of which he sees the microlites (rutile) and the micaceous base, as in the slates.

If the result of a full study of clays and shales were to convince the observer that the rutile, etc., and the micaceous base are all original deposit, then the very great similarity in these respects of clays and slates would go far to compel the adoption of the same conclusions as are advocated by Sorby and Pfaff.

As shown above, the key lies in the selection for study of clays whose exact and direct origin can be pretty clearly demonstrated. In such clays I do not consider it is possible, in view of all the characteristics, to look on the rutile, tourmaline, mica, etc., as of clastic origin.

Pfaff's paper is abstracted and criticized by Rosenbusch (Neues Jahrbuch für Min. etc., 1881), who expresses very decidedly contrary opinions to those of Pfaff, stating that he holds the cardinal point as to the question of the formation of slates to lie in the large amount of mica, from which he concludes that chemical processes have played a large part. “Gewiss wurde das Material zu den Thonschietern mechanisch herbeigeführt, der Mineralbestand aber der vorwiegend glimmerhaltigen und felspathfreien Abtheilung derselben ist gewiss durch metamorphe Processe bedingt.”

In dealing with, the same question on a previous occasion (Die Steiger Schiefer, 1877, pp. 116–118), though he was very positive as to the non-admissibility of the clastic origin of the rutiles, tourmalines, etc., of the slates, he was much more reserved in any expression of opinion as to the mica.

Rénard also, who has so deeply studied the slates of the Ardennes, appears to have arrived at views quite opposed to the idea of such rocks being simply compacted original sediments (op. cit.).

page 321 note 1 The first decided proof that these needles are rutile was given by Cathrein (“Ein Beitrag zur Kenntniss der Wildschönaner Schiefer und der Thonschiefernäadelchen,” Neues Jahrhuch für Min. Geologie, etc., 1881).

In dealing with his method of isolation of the rutile, Cathrein mentions the “roundish grains” as well as crystals, and looks on these as “partly imperfectly developed rutile needles.”

Very soon after Cathrein's paper, in the same year, an article appeared in the “Neues Jahrbuch” by Dr. Sauer (“Rutil als mikroskopischer Gemengtheil in der Gneiss- und Glimmerformation, sowie als Thonschiefernädelchen in der Phyllit-formation”), in which the proof of the nature of the slate-needles is again fully carried out; and a note by Prof. Eosenbusch states that Sauer had communicated the contents of this article to him in private correspondence before the appearance of Cathrein's paper