Deposition of Dalradian sediments in a stretching ensialic basin peripheral to Iapetus continued into Cambrian times. Late in the history of Dalradian deposition a southern continental landmass was removed by strike-slip motions and a subduction zone then developed beneath the Dalradian sediments. Deformation and moderately high pressure metamorphism of Dalradian rocks occurred and was followed by thermal relaxation and development of a volcanic arc with magmatic centres in Connemara by Tremadoc times and in NE Scotland by the Arenig. The S Mayo Trough was a back-arc basin that opened shortly after the initiation of the arc, the oldest volcanic rocks seen there being the extrusive equivalents of the basic intrusions of Connemara. Dalradian rocks outcropping in Ordovician times over the Highlands and probably extending southwards are a possible source for high pressure metamorphic clasts at Ballantrae, while clasts of arc volcanics may have come from N of the Highland Boundary Fault. Plate models that involve a separate arc in the Midland Valley are rejected because the tectonic elements are impossibly small by comparison with modern convergent plate margins. The arc rooted in Dalradian rocks ceased to be active at the end of the Llanvirn as a result of a further change in plate motions, and there are considerable spatial problems in locating an arc coupled to the Southern Uplands accretionary prism after Llanvirn times.