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Journal of the Geological Society; 1976; v. 132; issue.1; p. 59;
DOI: 10.1144/gsjgs.132.1.0059
© 1976 Geological Society of London

Discussion

DR A. E. MUSSETT observed that little was known about secular variation in Precambrian times but, to judge by its behaviour of the last few thousand years it should be averaged out if the samples acquired their magnetism in a period of more than a few thousand years. Thus a previous comment that the data may represent a ‘spot’ value of the field is to re-invoke the old idea of catastrophism with a vengeance. Surely the speaker does not believe magnetization could have taken place in so short a time?

The AUTHOR replied: I believe that there were limited differences in the times of magnetization of the stable rock units. I speculate that these time differences were of the order of tens of millions of years. I have suggested the possibility that four of the dykes having high precision parameters may have each been magnetized by specific shocks; if this is correct, ‘spot’ values of the field would have been recorded in these cases. This may be true for the remaining stable units, or secular variation may have been averaged out by shocks occurring over a period of time.

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This 250-word extract was created in the absence of an abstract.