The present paper summarizes the facts at present known regarding a characteristic effect that is often produced in the fresh plagioclase felspars of igneous rocks by thermal metamorphism—namely the development of a special type of cloudiness caused by the appearance of minute inclusions. Reference is also made to certain wel|-known rocks in which the clouding of plagioclase felspars has been described without any suggestion that the peculiarity of the felspars is connected with thermal metamorphism.
Attention is drawn to resemblances between two contrasted styles of mosaic crystallization—tessellate and equigranular as compared with irregularly interlocking (sutured) and inequigranular—that characterize ice in alpine glaciers and quartz in dynamically metamorphosed sandstones of the Moinian area of the northern Scottish Highlands. It is suggested that in both environments the contrasted types of mosaic are due to crystallization under conditions of shearing stress that were respectively minimal and maximal. Similarities are also pointed out between the orientation of the principal crystallographic axes of crystals in ice subjected to shearing stress, and the “girdle” arrangement of the principal crystallographic axes of quartz in Moinian metamorphic sandstones.
In this paper the writer deals with certain mineralogical and textural features found in the Moine and Sub-Moine Series of western Scotland, viz. garnet-zoisite calc-silicate ribs, non-garnetiferous epidotic calcsilicate ribs, sutured (crenulate) quartz-mosaic, foliation oblique to folded bedding, and the presence of abundant yellow epidote and of spots of haematite. The garnet-zoisite calc-silicate ribs have been claimed by Dr. Richey and Professor Kennedy as diagnostic of parts of the Moine Series. For the non-garnetiferous epidotic calc-silicate ribs, which have not previously been described, the name “calc-silicate ribs of Arnipol type” is suggested. These ribs occur in adjacent groups of rocks that have been mapped with the Lower Striped Schists and the Lower Psammitic Group of the Moine and with the outermost psammitic zone of the Sub-Moine.
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