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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    by the results of foliation ; but exceptions have been noted in a number of instances. Three out of the 6 show the A type of micropegmatite ; all of them show the B type and one or two of them a particular variety of the B t)rpe, which it is inferred has owed its origin to the movements that produced foliation. A typical instance of this form was observed in the granite of Boggierow, Portsoy. In order to facilitate comparison of the different classes of rocks, I have represented in tables the various exceptions to the order of crystallization as percentages of the rocks examined in each class. The same treatment has been extended to the different forms of micropegmatite in another table. The facts may also be represented in graphic form, which probably gives a better idea—^and at a glance—of the distribution of exceptions to the normal scheme of crystallization, and of the distribution of the different forms of micropegmatite in the various kinds of rocks. From either of these modes of presentation it will be evident as regards exceptions to the order of crystallization that the aberrations of quartz from its normal ]DOsition count for quite half of all exceptions. In the case of the hornblendic rocks, these are seen to occur in greatest numbers in association with hornblende; in the case of the more acid rocks, in association with orthoclase ; that is to say, in highly silicated rocks of both classes, the excess of silica begins to separate in the hornblende rocks with the hornblende—or comparatively early in the scheme of crystallization ; in the non-hornblendic rocks, it separates in greatest abundance with the orthoclase, or comparatively late in the scheme of crystallization, but in both cases in abnormal positions. As regards the inter- mediate minerals, in biotite for instance, the rate of aberration for quartz is practically equal in all the classes of rocks. In regard to the inclusion of quartz in sphene, it is found to occur in more abundance in the hornblendic rocks ; but that is in great part due to the fact that sphene occurs comparatively rarely in the more acid rocks. Orthoclase and plagioclase occur in the hornblende of hornblende granites with about equal frequency and in about one-third of all the rocks examined. Orthoclase occurs in biotite more frequently than plagioclase in this class of rocks, but plagioclase occurs more frequently than orthoclase in biotites of the other classes of rocks. In the double mica granites, indeed, so far as our observations go, orthoclase does not appear to occur in the micas at all. The fact that the two micas taken together present a general composition which is inclusive of the general composition of orthoclase has probably something to do with this result.