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.

    18/84 (page 258)
    Quartz shows Tioo Maxima of Abnormal Crystallization. On the whole, it may be said that as regards its tendency ta crystallize at abnormal periods, quartz shows two maxima— one just after the commencement of crystallization in the horn- blendic rocks, and another towards the end of crystallization in the more acid rocks. In the latter case, it grades into its normal period of crystallization after the orthoclase. The Several Classes of Rocks compared as to the Occurrence of Micropegmatite in them. As regards the occurrence of the different forms of micro- pegmatite in the different classes of rocks, the A form may be considered as typically absent from the hornblendic rocks. The B form is the commonest form in these, though on the whole it is relatively less frequent in them than in the other classes of rocks. In these the C form occasionally occurs but is infrequent. The A form of micropegmatite is typically present in the most acid class of rocks, being, so far as our observations go, absent from the double mica granites. In the foliated granites again, which probably represent granites originally belonging to both the biotite and double mica groups, the A form is present in 50 per cent, of the rocks examined. The B form appears in increasing abundance in the biotite granites, while it has been observed in every specimen of the double mica granites examined. The C form is relatively infrequent in aU the classes, but occurs most often in the most acid rocks, and relatively more frequently in the dyke and efiusive rocks of that class than in the typical granites of the same class. The absence of any form of micro- pegmatite is most marked in the most basic class of rocks, and indeed some inverse relation to basicity appears to be indicated in this negative character in a fairly marked degree, thus bearing out the idea that in the first instance the development of micro- pegmatite is dependent on composition rather than on any temporary physical condition of the magma, in which it has ultimately appeared. Another feature appears to be indicated in the more basic rocks—the granites show the negative character less frequently than the atypical rocks of that class ; while among the more acid rocks the granites show it more frequently than the atypical rocks of that class. But in the former case, the basis of observation is perhaps somewhat too narrow to fomid any very broad generalization upon. One or two points of wide generality emerge in regard to the occurrence of micropegmatite. The presence of the A type is associated with marked acidity of composition. The general