Summary

Thirty-five reliable dates for rock units of well-defined biostratigraphy within the Ordovician, Silurian and Devonian periods are listed. Two alternative graphical presentations of the data show that all the data, including Rb-Sr whole rock ages of acid rocks, are internally consistent. Less precise fission track dates are consistent with the other data, but too imprecise sufficiently to constrain the time-scale. Estimates are given for the numerical calibration of the base of the Carboniferous, Frasnian, Eifelian, Gedinnian, Llandovery, Caradoc, Llandeilo and Arenig, by considering the sets of reliable ages for rock units near each boundary.