N. H. Gale, R. D. Beckinsale, A. J. Wadge; A Rb-Sr whole rock isochron for the Stockdale Rhyolite of the English Lake District and a revised mid-Palaeozoic time-scale. Journal of the Geological Society ; 136 (2): 235–242. doi: https://doi.org/10.1144/gsjgs.136.2.0235
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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.