On the torridonian sediments of colonsay and their relationship to the main outcrop in north‐west Scotland

Colonsay is a small island in the Inner Hebrides, about 100 km. south of Skye. The main part of the succession, which is about 2 km. thick, consists of phyllites, fine dark sandstones and flags, occasionally graded, with major intercalations of coarse current‐bedded arkose and, near the top, dolomitic limestone horizons. This series rests with a sharp contact (probably a slight unconformity) on a sequence of graded greywackes and mudstones, not less than 2 km. thick, most of which outcrops to the south in Oronsay, and in Islay, where it rests unconformably on the Lewisian basement. The greywackes and arkoses indicate that currents moved mainly northeastwards. A comparison of the Torridonian rocks to the north with those in Colonsay suggests the equivalence of the beds above the greywackes with at least part of the Diabaig in Skye. Three major fades are distinguished in the Torridonian; the greywacke, Diabaig and arkose fades, and it is tentatively suggested that they mark stages in the infilling of a major sedimentary basin or trough of which the present Torridonian outcrop forms only the western margin.