The Structure of the Bowmore-Portaskaig District of Islay

Abstract

The island of Islay occupies a peculiarly critical position in connexion with existing controversies regarding the Highland metamorphic rocks. The history of the various opinions held on the North-West Highlands is given somewhat fully in the Geological Survey Memoir (2, pp. 11–32). In brief, it was generally believed for many years that the geology of the Highlands was comparatively simple. The apparent succession in the north-west was accepted as the true one, and, as Cambrian rocks there underlay the gneisses and schists which covered the greater part of the Highlands, these were supposed to be Palæozoic rocks, regionally altered by post-Silurian movement. Although Nicol had argued that the succession was deceptive, and in 1860 had in reality indicated the correct solution of the main structural problems, Murchison and his supporters long maintained their position. The chief point at issue was settled by the work of Lapworth, published in 1883, after which the ‘upward succession’ was abandoned. But the unidentified gneisses and schists at the top of the apparent succession: that is, those above the Moine Thrust, covering most of the Highlands, were still regarded as largely altered Palæozoic rocks, although the original evidence for this view had disappeared. Lapworth held the ‘eastern schists’ to be, unlike the Lewisian Gneiss, of post-Silurian dynamical manufacture from Archæan and later rocks. The Geological Survey, mapping down the west coast of Scotland along the great thrusts, accepted, generally speaking, the theories of Lapworth, the tendency being to treat the schists that spread over the