The distribution & significance of deviations from the normal order of crystallization : also the distribution & significance of micropegmatite in granites, as illustrated by the granites of the north of Scotland / by William Mackie.

  • Mackie, William.
Date:
[1908?]
Catalogue details

Licence: In copyright

Credit: The distribution & significance of deviations from the normal order of crystallization : also the distribution & significance of micropegmatite in granites, as illustrated by the granites of the north of Scotland / by William Mackie. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    Actual Observations hearing on a Double Period of Develop- foment of the B Type of Micropegmatite. The actual observation of appearances pointing to the develop- ment of micropegmatite of the B type at two different periods, has been made in one or two instances. It would be natural to infer, in view of what has just been said, that one of these would be found synchronous with the development of ortho- clase and the other synchronous with the development of micro- cline. Nothing to indicate such a relation, however, has been noted in the one or two instances, where the appearances occur, and it does not necessarily follow that their periods of appear- ance have been as indicated. One occurrence of two areas of micropegmatite of very different degrees of fineness in close association was observed in a double-mica granite from Aitnock Hill, Dara, Morayshire, where an area of coarse micropegmatite, probably representing an original plagioclase, was seen to be separated from biotite by an area of very fine micropegmatite. Both orthoclase and microcline, the latter in abundance, are present in the rock, but the latter mineral at least has no im- mediate relation with either of the manifestations. Another instance of two areas of micropegmatite of different degrees of fineness in juxtaposition was observed in Moy granite, Inver- ness-shire, where an area of fairly coarse micropegmatite was seen radiating from muscovite and biotite. A small roughly quad- rangular white spot at its periphery in a different position of the nicols was observed to show an exceedingly fine micropegmatite. Microcline is present in this rock in sparing amoimt, but it does not occur in immediate relation to these particular areas. These observations in themselves suggest different periods of origin for the two forms of concomitant micropegmatite. They mani- festly, when taken in conjunction with the generalizations as to the periods of appearance of the different forms of the B type of micropegmatite already made, also suggest the further question as to whether the mineral microcline might not be associated with changes at some distance from the locus of its development. We shall subsequently see that some reason exists for concluding that such action may possibly occur. The C Type of Micropegmatite. The C type of micropegmatite is of limited occurrence in the true granites, and its presence in these is difficult of determination as it is associated in a number of cases with microgranitic structure, which tends to obscure it and prevent its determination with absolute certainty. In considering this manifestation, regard