Two Caledonian lamprophyre dykes, intruding Upper Llandovery greywackes in the Central Belt of the Southern Uplands–Down–Longford Terrane on the coast of Down. carry numerous cognate and incognate xenoliths. The incognate xenolith population is dominated by schistose or mylonitic basic and intermediate metavolcanics, some of which are macroscopically banded and rodded. The geochemistry of one large xenolith is that of a calc-alkaline ensialic magmatic-arc basalt, remarkably similar to the basalts of the Borrowdale Volcanics. The xenolith has undergone pumpellyite–actinolite facies metamorphism in a deep-seated shear zone at depths between 7 and 30 km. These incognate xenoliths apparently provide the first direct geological evidence of the lithological nature of the unexposed basement of the Southern Uplands–Down–Longford Terrane. They offer strong support for the hypothesis that the Southern Uplands was underthrust. along the Iapetus Suture. by a basement of Lake District type. The surface interpreted as the Suture on the WINCH 2 seismic reflection profile inclines to the north to a depth of 21 km below the County Down lamprophyre outcrops. The lamprophyres have sampled both basement and Iapetus mylonites. One of the host lamprophyres has been dated by K–Ar hornblende at 415 ± 12 Ma. This implies a minimum of 30 km of underthrusting on the Suture before the end of the Silurian.