How old is the Cambrian System?

Abstract

PROF. C. F. DAVIDSON has presented no new evidence free from unambiguous interpretation which would refute our proposed extension of the Holmes B time-scale; or indeed on which an alternative scale could be based. The examples which he cites form only a part of a large number of such data which we have considered. We have rejected them because either the stratigraphy of the samples or their measured age is not free from unwarranted assumptions. We have used only samples which are above criticism on these points. We would, however, like to correct Prof. Davidson's remark that we “claim that the base of the Cambrian System should be … at about 750 m.y.”. In fact, we have avoided using the rather meaningless term ‘base of the Cambrian’, and we have but estimated, by extrapolation from our oldest measured date at around 500 m.y., that the upper Cambrian “should be closer to 650 m.y.”. Prof. Davidson's recapitulation of a possible geochemical history of the kolm emphasizes our conclusion that this material provides a poor calibration point. The pattern of discordancy¹ found for lead ages of the kolm material would not justify his argument that the kolm is younger than 650 m.y. Nevertheless, such an age is not incompatible with an extrapolation of our scale. Considering that the precision of measurement is not better than 5 per cent, this extrapolation could give an age for the top of the Cambrian between 600 and 650 m.y., and clearly this age also depends greatly on the (unknown) length of the Ordovician.