New potassium-argon age determinations are presented for rocks ranging in age from Lewisian to Tertiary. A late-Caledonian metamorphic event at about 422 million years is very important in the Highlands and the relationship of this to previous metamorphisms is discussed. Newer Granite emplacement took place about 400 million years ago and the Moine thrust movement also took place at about this date. Lower Carboniferous basalts yield, by whole rock methods, ages compatible with the time scales of Kulp (1960) and Holmes (1959). Volcanic activity in Mull is confirmed as Palaeocene or Eocene in age.