The Cairnsmore of Carsphairn Igneous Complex

I. Introduction The Cairnsmore of Carsphairn plutonic mass occurs in the north-eastern corner of sheet 8 and adjacent sheets of the Geological Survey 1-inch map of Scotland. It is the smallest of the four large plutonic masses intruded into the Lower Palæozoic rocks of south-western Scotland. The complex has been injected into the Arenig, Glenkiln, and Hartfell beds represented by well-marked conglomerates and pebbly grits, blue greywackes, blue flags, and dark shales. The plutonic rocks occupy a roughly triangular area a little over three miles in width and extending nearly 2½ miles from north to south. The country occupied by the complex is essentially the Cairnsmore mass (2612 feet). The “granite” just fringes Dunool (1758 feet), Beninner (2328 feet), Moorbrock (2136 feet), and Black Shoulder (2257 feet). Hitherto no detailed work has been done on the igneous rocks. The Carsphairn “granite’ was referred to briefly in “The Silurian Rocks of Britain” (Teall, 1899). No microscopical descriptions were published, although the mass was known to include a number of distinguishable types. II. The Plutonic Rocks The intrusion consists of at least four well-defined plutonic rocks. There is a small central mass of granite, surrounded first by a finer grained granite-tonalite hybrid and then by a tonalite, and finally there is an outer heterogeneous area of much varied character. The acid hybrid between the granite and the tonalite forms a continous and comparatively wide outcrop. The tonalite also occurs in a continous ring broadening towards the north, where it is in contact