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III.—Notes on the composition of Clays, Slates, etc., And on some points in their Contact-Metamorphism
Published online by Cambridge University Press: 01 May 2009
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After this substance has been well observed in good occurrences, it is always recognized at once, even as isolated small patches in a slide. It has a very characteristic appearance among the other constituents. It is marked off, for instance, by its special refraction from anything else that occurs in these rocks, and has other characteristics not to be missed when once observed, but not very easy to exactly describe. As soon as it ceases to be quite inert in polarized light, as soon as the speckly polarization sets in, it has also a more or less granular appearance in ordinary light with lowered condenser, which is very characteristic.indifferent slides, and often in one and the same slide, various stages of development may be observed in it from the first speckled appearance in polarized light up to a point where flakelets of white mica are recognized as forminginit, lying “criss-cross”inall directions. It passes in this way in some cases almost wholly into mica, with apparently other substances which cannot be identified with certainty, the transition, and connection with the original substance, being all the time distinct and beyond mistake.
In the inner zones, the “hornfels-zones,“ this substance is usually present in smaller amount, as already stated; but examples of its abundant occurrence are not wanting, as for instance in a hornfels from Spitzenberg, in the Harz. There is a mosaic of quartz and felspar, with brown mica and some white mica. The yellow substance lies in amongst, and surrounds, the minerals of this mosaic. It has no definite form of its own, simply occupying spaces and taking the outlines of the other minerals. It is all either in the minutely-speckly stage or variously developed from this upwards to mica; and the impression made is that if it had all developed as far as some portions of it have done, the whole mosaic-grains of the slides would be mixed in with and surrounded by white mica, as may not infrequently be observed in these contact-rocks.
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References
page 69 note 1 Even this list of changes does not exhaust the number of transformations the titanic acid may undergo. At later stages of the metamorphism it appears that part of it can be taken up into the newly-formed contact-biotite, thus ending up where it began,—combined in dark mica. Such contact-biotite has been shown, by Lang and Jannasch, to contain as much as 3.40 per cent, of titanic acid, and Beck records (op. cit.) cases of decayed contact-biotite in which rutile has once more been separated out.
page 70 note 1 It should be noted that the “tesselated” or “mosaic” structure is by no means peculiar to the alteration of sedimentary rocks. At Shap, for instance, it is very beautifully developed in some of the altered rhyolitic and ardesitic ashes in the contact-zone; sometimes it is quartz alone, sometimes quartz and felspar, and sometimes felspar alone which forms these mosaics.
A tesselated structure has been looked upon by some observers as quite specially a result of dynamic metamorphism, but this idea can, of course, no longer be enter tained. It may be well seen in igneous rocks. Thus, General Mcllahon shows it to occur in the marginal portions of Dartmoor granite (Q.J.G.S. vol. 49, p. 388), and has previously shown its occurrence in granite of the Himalayas. It is also, as is well known, seen perfectly developed in the groundmass of some quartz-porphyries, not to be distinguished, as to its structure and general appearance, orinanything except its inclusions, from some of the finer-grained mosaics of altered slates.inconsidering this structure in the contact-metamorphism of slates it is a point of some importance that exactly similar structures can be seen to result during the consolidation of purely igneous magmas.Google Scholar
page 72 note 1 It seems desirable to point out that there is risk of sometimes confusing cordierite and felspar in such rocks, where the cordierite occurs as smaller grains and is not a striking constituent. in the books we are warned against mistaking cordierite and quartz; but a similar warning does not seem to be given as regards felspar. As regards quartz the risk does not appear to be great. Where there are several grains of cordieriteina slide it will nearly always be possible to prove that we have a biaxal mineral before us, and therefore not quartz. Butina thin section it may often happen that we cannot say whether the biaxal grain is cordierite or felspar, simply by its optic examination. Felspar grains, without any cleavages or definite forms, occurinthese rocks. The refraction and bi refraction are not sufficiently different to serve as guides when cut thin, and in many of the best occurrences of cordierite in contact-slates the dichroism of the mineral, and the “halos” round its enclosures, are not discernible at all in most grains. Enclosures, etc., may aid in discrimination, and usually do so; but I have satisfied myself that the determination is often a difficult one, and think that there is considerable chance of cordierite being overlooked in some cases in consequence.