Note on the minerals observed in the railway cutting at Barnton, near Edinburgh

Abstract

In the course of one of the Saturday evening excursions of the Heriot-Watt Geology Class in the spring of the present year (1893), the rocks now being exposed in the new railway cuttings to the west of Davidson’s Mains were studied in some detail. The sedimentary rocks outcropping there are of Lower Carboniferous age, and appear to correspond in geological position with the highest beds of the Granton Sandstone, together with the lower part of the overlying Wardie Shales. Their prevailing dip is westward. At this point these rocks have been invaded by a sheet of igneous rock forming part of the intrude of the Corstorphine Hill, and whose bounding surface nearly follows the bedding of the sedimentary rocks. In its outer parts the rock at Barnton consists of a holocrystalline aggregate of plagioclase felspar, and pyroxene, combined in about equal proportions; together with ilmenite, a serpentinous mineral, and some leucoxene. It shows well-marked ophitic structure, and is, therefore a dolerite. The central portion of the mass is different in both structure and composition. It consists of a holocrystalline compound of olivine (and of serpentine representing that mineral), pyroxene, brown mica, and an ore of iron. Felspar occurs in either small proportion or is absent entirely. The structure of the aggregate is granitic; this part of the rock, therefore, is a picrite. Here and there the picrite itself is traversed by segregation-veins, which are, as usual, of a more acid composition than the mass, and which agree closely in character