Drs J. G. Mitchell & P. R. Ineson write: We wish to bring to the authors’ (O’Nions et al. 1973) notice K-Ar ages obtained from thirteen clay mineral separates from samples from five localities within the Ingletonian at Ingleton. These ages, summarized in the table below, show a range from 395 m.y. to 414 m.y. with a mean of 405 m.y. The analytical precision of all the data is better than ±2%.

Unlike the authors, we observed no correlation between isotopic age and potassium content, nor was the presence of two components of minerals indicated by X-ray diffraction analyses of our samples. The range of potassium contents reflects only the proportion of illite over quartz in the samples. Our interpretation of the mean age of 405 m.y. with such small within site scatter is that it is a consequence of a well-defined thermal event experienced by the rock at this time (close to the Silurian–Devonian boundary). We contend that the spread of K-Ar ages from 419 m.y. to 451 m.y. reported by the authors is due to partial ‘overprinting’ of the age of isoclinal folding and major cleavage in the preAshgillian/Caradocian, by a period of refolding in Siluro-Devonian times, determined more precisely by our data at 405 m.y. It would appear that the response of the illite content of the rocks analysed admits the resolution of this later event better than do whole rock analyses.

The authors have also obtained an excellent Rb–Sr isochron from whole rock Ingletonian

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