Abstract The Caledonides of west Mayo and northwest Galway in Ireland can be subdivided into three structural units. First, within a narrow E-W-trending belt of amphibolite-facies schist in the Clew Bay area, there is evidence of pre-Caledonian structural and metamorphic events. This basement schist of uncertain age is overlain tectonically by the Erris schists, which extend 40 mi northward and which, with the Connemara schists of northwest Galway, represent the Early Caledonides. The succession of these schist groups is comparable with that of the Moine and Dalradian of Scotland. Large F 1 -F 4 folds are associated with M 1 -M 4 recrystallization under conditions of the amphibolite and greenschist facies. The E-W- and NE-SW-trending Late Caledonides of south Mayo and northwest Galway consist of a thick succession ranging in age from middle Arenigian to the time of Middle Old Red Sandstone deposition. Ordovician sedimentation was related to a southern landmass in Connemara, and Silurian sediments were deposited south of a landmass in north Mayo. A late Silurian or Early Devonian (Lower Old Red Sandstone) orogeny involved granitic intrusion and polyphase folding and metamorphism showing northward intensification. An orogeny late during Middle Old Red Sandstone deposition, or during Upper Old Red deposition, produced large-scale thrusting, folding, and cleavage; it also resulted in the emplacement of serpentinite and the development of a major sinistral wrench fault of Caledonoid trend, probably related to the Leannan fault of Donegal. Early Carboniferous sedimentation was influenced by this structure. In an area in northeastern Newfoundland which includes the Burlington Peninsula, a comparable