We welcome this publication (Bullock et al. 2019) on vitrinite reflectance (VR) in British Carboniferous coals. There has been no unifying account for many years; the article demonstrates the value of inter-disciplinary and international collaboration. However, its brevity precludes a comprehensive review of available data and publications, and massed National Coal Board (NCB) data are excluded, which would greatly enhance understanding of maturity variations through the rock volume, at different rates in very varied coalfields. Nor does the article include VR data from many modern boreholes drilled in unmined areas, records for which are available at the...

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