Abstract

Arthur’s Seat is the well-dissected remains of a volcano which erupted a series of basalts representative of the Scottish Carboniferous province to which it belongs. Some of the microporphyritic basalts of Arthur’s Seat are characterised by ophitic augite which crystallised after flow had ceased. Others have non-ophitic pyroxene, much of which crystallised comparatively early in the course of solidification of the rock. The stage of crystallisation of pyroxene in the cooling history is related to the composition of the basalt: where the proportion of pyroxene to feldspar is low, the crystallisation of augite is late. Chemical analyses of the various rocks have been considered in relation to the quaternary system albite-anorthite-diopside-hypersthene (Barth). All the microporphyritic rocks of Arthur’s Seat lie well within the plagioclase field, but those with ophitic augite lie farthest from the boundary between the plagioclase and pyroxene fields.