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 British Geological Survey (BGS).

The authors discuss only 103 seam analysis data points for the entire British Carboniferous, and most cannot be referenced to a specified seam or seam component. Their figure 1 shows the restricted geographical range of their data points, compared with the known extent of exposed and concealed Coal Measures (see fig. 2, British Geological Survey UK Coal Resource for New Exploitation Technologies Final Report CR/04/015N, Jones et al. 2004). Of their 103 samples, 27 are from The BCURA Sample Bank, for which the User's Handbook (BCURA 2002) indicates an even more restricted geographical range; only 17 have a specified seam. The handbook's appendices usefully indicate VR ranges for each data point, in histograms, with a VR statistical mean. These samples were also assessed for Volatile Matter % (VM): Appendix 38 is the NCB Coal Classification System (Williamson 1967), which, like other international classifications, is based on thermal maturity (referred to as coal rank), using VM as a key maturity indicator, coal being a very complex substance. VR is also used, …