In the western part of the Midland Valley of Scotland, Carboniferous volcanic activity is considered to have been initiated at c. 335 Ma with extrusion of the first lavas of the Clyde Plateau Volcanic Formation (CPF). On the island of Little Cumbrae, in the Firth of Clyde, the basal flow of the CPF lies disconformably on sandstones and conglomerates of the upper part of the Clyde Sandstone Formation. However, at the south end of adjacent Great Cumbrae, the lower part of the Clyde Sandstone Formation (Tournaisian) is host to a series of thin concordant igneous bodies that resemble sills but lack features such as chilled margins. Some display fine bedding and lamination near the base and top. These rocks are interpreted as tuffs with chemical affinities to alkali basalts such as those of the CPF on the neighbouring island of Little Cumbrae and in south Bute. Their major element content has been modified by addition of Ca and volatiles, possibly during ascent of magma through caliche-bearing rocks and water-rich sediments. The tuffs have chondrite-normalized rare earth element patterns similar to those of basaltic rocks forming the lower part of the CPF. They provide new evidence of Tournaisian volcanic activity in the western part of the Midland Valley, thus bringing it into line with that in parts of the Southern Uplands and Scottish Borders region. The Bessy's Port tuffs occur near the base of the Clyde Sandstone Formation whereas, on Little Cumbrae, the first lava of the CPF is near its top. Early magmatic activity is also indicated by the occurrence of peperites associated with mafic dykes intruding mudstones of the Lower Foul Port Member, near the base of the Kinnesswood Formation, in southwestern Great Cumbrae, and by basaltic tuffs in the Ascog Member of the Clyde Sandstone Formation (or basal part of the Birgidale Formation) in east–central Bute.

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