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B the significance of banding the significance of

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b) The significance of banding The significance of banding in the definition of gneiss also varies, for example, Yardley (1989) notes: “English and north American usage emphasises a tendency for different minerals to segregate into layers parallel to the schistosity, known as gneissic layering; typically quartz and feldspar-rich layers segregate out from more micaceous or mafic layers. European usage of gneiss is for coarse, mica-poor, high-grade rocks, irrespective of their fabric”. Banding may be defined in a variety of ways; some workers place the emphasis on the alternation of schistose and granulose layers (Tyrrel, 1929; Bates & Jackson, 1987; Tomkeieff, 1983), while others regard mineral banding as the characteristic feature (Barker, 1990; Mason, 1978) The SCMR debated these differences at length, particularly the requirement for feldspar + quartz in the definition of gneiss. However, the strong association in geological usage of gneiss as a rock possessing a poorer fissility than schist and the usefulness of gneiss as such a term proved decisive. Also, although this could be construed as showing a bias towards English-language usage, it was felt that this was acceptable because of the SCMR’s decision to make its definitions in English. The SCMR also noted that a structural-only definition did not exclude any rocks currently defined as gneiss. On the other hand a major concern for the SCMR was that a structure-only definition might include rocks that in current usage would never be considered as gneisses, for example, metasandstone of low metamorphic grade. In these cases it was felt that a guideline encouraging the use of protolith-based names would provide an adequate safeguard (Schmid et al., this vol.). The SCMR chose the boundary between schist and gneiss based on the definition proposed by Wenk (1963, as given by Winkler 1974), namely: “When hit with a hammer, rocks having a schistose fabric (schists) split perfectly parallel to ‘s’ into plates, 1-10 mm in thickness, or parallel to the lineation into thin pencil-like columns”. www.bgs.ac.uk/scmr/home.html
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