In a previous palaeomagnetic investigation of the Egersund dolerites it was suggested that the total time span recorded was short compared with the time scale of geomagnetic secular variation. The within-dike scatter was ascribed to other causes than field changes. Because of the presence of glass and since the overall mean remanence direction was fairly close to that of the present di pole field (within the range of secular variation), a Tertiary age of this dike system was suggested. However, this conclusion is probably no longer valid as further palaeomagnetic measurements have shown that: a) the genuine mean direction is too far from any expected Tertiary dipole field to be reasonably attributed to secular variation at that time, and b) there is every reason to suggest that the within-dike dispersion is largely affected by palaeosecular variation. Following the last assumption, the attitude of the ancient axis of rotation can be estimated. The calculation of a north pole (in present co-ordinates) at 28°N, 52°E or altematively a south pole at 28°S, 128°W suggests that the considered dolerites are of late Precambrian origin.
Palaeomagnetism and the origin of the Egersund dolorites, S. Norway