Previous work left unresolved whether the Tertiary igneous activity had spanned a magnetic polarity succession R-N-R-N-R or, if overprinting had occurred, only R-N-R. New work is reported that demonstrates that thermal overprinting has occurred in a narrow zone, c. 600 m wide, bordering the outermost major intrusions of Centre 2. Criteria for the recognition of overprinting are examined.
The polarity sequence is now believed to be R-N-R. In conjunction with radiometric dates which show that activity occurred from c. 60 Ma to c. 57 Ma, our data show that the sequence corresponds to chrons 26r-26-25r of the polarity time-scale of Harland et al. 1982 or 25r, 25 and 24r of Breggren et al. 1985. Upper and lower limits to the duration of the activity are, respectively, 4.67 Ma and 0.67 Ma, or 4.07 Ma and 0.60 Ma: a duration of about 3.5 Ma is thought probable.
The magnetic evidence supports the view that the Loch Scridain sills correspond to the Early Acid and Intermediate Cone Sheets of Centre 1.