II. The Geology of a Composite Intrusion at Bennan, South Arran

Abstract

The Composite Dyke at Bennan, South Arran. There occurs in the extreme south of the island of Arran a composite intrusion composed of three sheets. The prominent cliffs produced by the weathering out of the intrusion have been remarked, in their references to Bennan Head, by most writers on Arran for a century. Inland from the sea the position of the dyke is marked by raised barren ground, for the intrusion is more resistant than the sandstones and marls through which it cuts. It is accurately mapped on the colour-printed geological map of Arran published by the Geological Survey of Great Britain in 1910. The intrusion cuts steeply through the Keuper marls and sandstones (Gregory, page 179), which are gently baked, whilst it is cut in turn by numerous basic dykes which throughout the Western Isles are regarded as being of late Tertiary Age. Since the Mesozoic was a period of volcanic quiescence in Scotland, the dyke is a product of early Tertiary volcanic activity. The two outer sheets are quartz “dolerite” and the central one enclosed between them is quartz porphyry. At its northern termination where the dyke approaches the main road the central sheet pinches out and the two other sheets coalesce into one. The “dolerite” thus forms a sheath enclosing the porphyry. The outer margins of the sheath show chilled edges due to rapid cooling, but the junction between the “dolerite” and the porphyry is ragged, with apophyses of the latter intruded into the former. Fragments of This 250-word extract was created in the absence of an abstract