The Palaeogene Eskdalemuir dyke, part of the Mull dyke swarm in the Southern Uplands of Scotland, is ∼60 km long and up to 40 m thick. Its southern tip is 230 km from the inferred source on Mull. The dyke is composite, with tholeiitic basaltic margins and a vitreous central facies ranging from basaltic andesite to andesite in composition. Plagioclase and pyroxene phenocrysts and matrix crystals in the central facies show unusually large compositional ranges and complex textural relationships. Whole-rock major and trace-element abundances show linear variations against MgO content, consistent with the rocks in the central facies having formed by mixing of basalt and rhyolite magmas. The rhyolite can be closely matched by rocks from the Mull centre. The mafic and silicic magmas were intruded from a compositionally zoned chamber beneath Mull, perhaps during collapse of the Centre 1 caldera. The lower-viscosity basaltic magma was emplaced before, but lubricated the lateral propagation of, the silicic magma, which mixed with the partially solidified basalt, the proportion of rhyolite increasing towards the dyke centre. The Eskdalemuir dyke represents an unusual, perhaps unique, example of a rhyolite magma being emplaced >200 km from its inferred source. The supposed correlative of the Eskdalemuir dyke north of the Southern Uplands Fault, the Dalraith-Linburn dyke, is not comagmatic with it.