Contact Metamorphism by a Tertiary Dyke at Waterfoot, Co. Antrim

Abstract

Close to Waterfoot and about one and a quarter miles south of Cushendall, in Co. Antrim, a bifurcating dolerite dyke of Tertiary age outcrops on the shore approximately 750 feet south-west of Red Bay pier (see text-fig. 1). The dyke is intruded into practically horizontal beds of Triassic sandstone which contain abundant fragments and pebbles of schist and quartzite. As shown on the map, text-fig. 1, the dyke bifurcates and encloses a lenticular portion of these Triassic beds. Where unaltered by the dyke, that is in the low cliff adjoining the sea-wall to the south-west of the dyke and in that portion of the enclosed mass adjoining the south-western branch of the dyke, the Trias is a friable yellow sandstone, rich in fragments of schist and quartzite ranging in size from a few inches down to microscopic dimensions. On the south-western side of the north-eastern branch of the dyke, however, a marked change takes place. The Trias here changes in colour from yellow to grey and becomes hard and compact; and, as the dyke is approached, the fragments and pebbles become less distinct and eventually merge into the matrix of the rock. This change begins between eight and nine feet from the contact. Within a foot from the contact the alteration is so intense that the main part of the rock has a compact homogeneous appearance, so that at first glance it resembles an igneous rock with sparse xenoliths of white quartzite. The schist fragments are here represented by dafk patches which merge, often imperceptibly, into the matrix of the rock.