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Since the first edition of the Cambrian correlation report was published (Cowie et al. 1972), there have been great improvements in the techniques of radiometric dating. Cowie et al. (1972, p. 12) recognized no reliable radiometric ages from British Cambrian rocks, but provisionally accepted ‘with questionable accuracy’ that the age of the base of the Lower Cambrian was 570 Ma and the top of the Upper Cambrian (including Tremadoc) was about 500 Ma. The subsequent development of the U-Pb dating methods has given much more consistent and reliable ages with small errors, showing that the Cambrian Period was of shorter duration and not as old as Cowie et al. had supposed (Tucker & McKerrow 1995). Thanks to the initiative of Landing and his co-workers (Landing et al. 1998, 2000; Davidek et al. 1998), several radiometric ages have been obtained using zircon crystals extracted from volcanic and volcaniclastic detrital rocks in Avalonia, and from Morocco on the margin of Gondwana, which is depicted as adjacent to Avalonia in Chapter 6. Examples of these radiometric dates that can be correlated to the stratigraphical standard adopted here are shown in Figure 1, whilst interpolated dates, drawn from the syntheses of Ogg et al. (2008), Gradstein et al. (2008) and Peng & Babcock (2008) are given in brackets.

Development of a precise chronology for the four series and ten stages of the Cambrian is a work in progress (Shergold & Cooper 2004; Zhu et al. 2006).

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