The Cumbrae islands are unmatched for study of minor intrusions associated with the Early Carboniferous Clyde Lava Plateau. The transitional to subalkaline rock suite is present in a linear swarm of c. 250 ENE to NE dykes – 82 ‘basaltic’ (basalt to hawaiite) and 18 ‘trachytic’ (benmoreite to felsite), with a crustal dilation of 13·3. This swarm provides a unique window on the shallow crustal feeder system of the lava plateau. A ‘trachytic’ secondary swarm, focused on the plateau volcano of Misty Law in the Renfrewshire Hills, is analogous to a similar swarm in the Campsie Fells linked to the Meikle Bin volcano, and a ‘basaltic’ secondary swarm, unusually dense, with crustal dilation of 24·7, may record projection of the Dumbarton–Fintry Volcano–Tectonic lineament from the Campsie and Kilpatrick hills into the Firth of Clyde. Accompanied by bosses, plugs, vents and sheeted structures, the swarms are inferred to overlie fracture systems in the Caledonian basement of the Midland Valley. Internal relationships among extrusive and intrusive rocks of the Early Carboniferous suite and external relationships to dyke suites recording regional tectono-magmatic events of Late Carboniferous and Palaeogene age allow formulation of a new relative chronology for magmatic activity in the western off-shore Midland Valley.