Gneissic clasts occur in Carboniferous volcanic vents in the Scottish Midland Valley. Samples from two widely separated areas are closely similar in petrography and are interpreted as fragments of the same gneiss complex. They exhibit mineral assemblages and textures typical of granulite-facies metamorphism. Mineral chemistry is used to suggest that pressure and temperature of metamorphism exceeded 11 kbar and 850°C in part of the complex. The gneisses are geochemically similar to Lewisian granulites in their depletion in K, Rb, and Y. Seismic evidence indicates derivation from a basement now lying 7 to 8 km down which is unconformably overlain by Ordovician and later rocks. This basement complex may have been an area of positive relief in late Precambrian and early Palaeozoic times, and its existence provides an important constraint on plate tectonic models of the Lower Palaeozoic.