XXII.—Silurian Greywackes in Peeblesshire

Abstract

Sedimentary structures in the greywackes of Peeblesshire, including graded bedding, cross-lamination and load-cast structures, are described and their origin discussed.Petrographically the greywackes are poorly sorted sediments composed of a variety of minerals and rock fragments. The minerals include quartz, felspar, augite and horn-blende, mica and chlorite, while the dominant rock fragments are andesites, spilites, quartz-keratophyres and granites.Three groups of greywackes are recognized according to their mineralogy. The first, the Pyroxenous group, contains augite and hornblende. Rocks of the second group, the Intermediate group, are without these minerals, while the third group consists of rocks rich in garnet—the Garnetiferous group. These mineralogical differences are reflected in differences in the chemical analyses of the three groups. From consideration of the chemical and modal analyses it is suggested that differences in source rocks have influenced the composition of the greywackes more than variations in the intensity of alteration due to weathering and transport.It is concluded that the greywackes have been formed from the rapid erosion of a land mass largely covered by Ordovician volcanic rocks, but also with developments of lowgrade regionally metamorphosed rocks and others resembling Torridonian arkoses. Sedimentation features are due to accumulation controlled largely by the action of turbidity currents.