Geologic evidence shows that the Annagh Gneiss Complex on the Atlantic seaboard of Ireland was basement during the Caledonian cycle that began about 700 m.y. ago. Rb-Sr isotopic data show that a deformed granitic injection complex, forming part of the Annagh Gneiss Complex, is younger than 1,300 m.y. This indicates that at least part of the basement of the Caledonides is of late Precambrian age, possibly formed during the Grenville orogeny. A Caledonian imprint is reflected by mineral ages in the range 480 to 445 m.y.