The British Tertiary Igneous Province: K-Ar Ages of Some Dykes and Lavas, from Mull, Scotland

K-Ar age determinations of some of the oldest lavas, and of dykes from the regional swarm are considered together with geological, petrological and palaeomagnetic evidence. The apparent ages do not correlate with the polarity, and correlate only weakly with the degree of feldspar alteration. For the lavas alone there is a strong correlation with potassium content which is thought to be coincidental, the range of ages being due to argon loss. The reversely magnetized plateau lavas were probably extruded within a period of less than 1 My, in the interval 60±2 My ago. The dykes, of normal, reversed and shallow reversed polarities, probably were intruded during the interval 56-49 My ago.