Contour-maps of (i) percentage crustal-extension (dilation) and (ii) number of dykes per kilometre, for a regional linear WNW-trending Permo-Carboniferous dyke-swarm of northern Argyll, indicate a dextral displacement of the axis of the swarm at the Great Glen Fault by 7–8 km. The dykes are olivine-dolerites, camptonitic dolerites, camptonites and monchiquites. They are distinguished from similarly trending Tertiary dolerite dykes chiefly by their field-characters. Isotopic dates of eight dykes exhibit a range of ages, although five of these agree within the limits of experimental error, giving a mean of 285 ± 5 Ma. K-Ar ages of two Caledonian dykes are also reported. Since the Tertiary dyke-swarm of Skye is not laterally displaced at the Great Glen Fault, the dextral slip dates from between the dyke-swarms. Comparatively lower intensities of both Permo-Carboniferous and Tertiary dykes south of the fault suggest considerable downthrow to the southeast. At least part of the normal faulting is of post- or syn-Tertiary dyke-swarm date, though the Great Glen Fault may, in addition, have acted as a barrier to the southerly continuation of the Skye dyke-swarm.