Appendix on the Microscopic Structure of some Dimetian and Pebidian Rocks of Pembrokeshire

Abstract

D imetian. 1. The rock from near the Church School is seen macroscopically to consist largely of quartz in individual crystalline grains, or in groups, thickly distributed through a porphyritic-felsite ground-mass. In fissures and nests in the mass of the rock much chlorite and also some epidote are present. Its affinity to the quartz porphyries is only apparent when examined in thin section under the microscope. Here the quartz is seen to be distinctly crystallized, with characteristically rounded edges, and also with inclusions of the ground-mass. The felspar crystals are numerous, and much altered though the indications of banding of a plagioclase felspar are not wanting. The ground-mass is microcrystalline throughout, consisting of quite individualized grains of quartz and felspar, and in many parts presents a peculiar structure. Around the quartz and felspar crystals an incipient spherulitic arrangement has taken place, resembling to some extent that seen in some rhyolites ; but here the radial texture is modified by a tendency to a dendritic or a branched structure, a peripheral outline being in all cases absent. This structure is also developed from numerous individual centres in the ground-mass, having no apparent nucleus ; it bears no resemblance to the rhyolite of the breccia before described, though it is probably but a modified form of spherulitic structure. Much grer chlorite and a black indefinable substance is dispersed through the ground-mass. 2. This fine-grained, chlorite-spotted rock presents in thin section a completely crystalline ground-mass of felspar and quartz grains, the latter being but