Subaerial lavas and intercalated sediments crop out on the islet of Glas Eilean and the Black Rock skerries in the Sound of Islay between Islay and Jura. The visible succession is c. 120 m thick and is traceable c. 2 km along strike (NNW–SSE). The lavas are alkali olivine-basalts containing olivine ± plagioclase and augite phenocrysts. Decreasing Mg, Cr and Ni upwards, coupled with decreasing average flow thickness and increasing amounts of inter-flow sediment, suggest progressively waning volcanic activity marked by decreasing ascent rates and greater degrees of differentiation. K-Ar dating on one sample gave an early Permian age of 285 ± 5 Ma. It is inferred that the lavas erupted from an isolated basalt volcano situated on a NNW–SSE trending fracture, associated with a narrow developing half-graben within the Dalradian metasediments. The tectonism and magmatism is inferred to be related to the ‘Clyde Belt’ of fault-bounded basins extending from Cheshire to the Little Minch.
