The extrusion of the Stockdale Rhyolite, an upper Ordovician lava lying within Zone 2 of the Cautleyan Stage of the Ashgill, is dated by the Rb-Sr whole rock isochron method at 421 ± 3 Ma. This datum is more precise than any published Lower Palaeozoic age. Samples of the rhyolite from within the thermal aureole of the Shap Granite show Rb-Sr systematics apparently unaffected by baking. A revised mid-Palaeozoic time-scale is presented.

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