Composite sills of Cenozoic age, ranging from basic to acid in composition, intrude the Triassic marls and sandstones of southeastern Arran off the west coast of Scotland. The various rock types are described petrographically and two new rock analyses of quartz basalt and quartz-augite diorite, together with average modal analyses, are given. The origin is ascribed to deep-seated hybridization of contrasting magmas of basic and acid composition, with the ultimate source of the two magmas still unknown, The order of injection is always from basic to acid, and the contaminated magma seems to have been injected in spurts, each becoming more acid.

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