R. W. Brown writes: Green (1989) proposed that elevated palaeotemperatures during the early Tertiary in the East Midlands area, were the result of increased burial at this time with a geothermal gradient similar to that observed at present (c. 30°C km-'). The variation of maximum palaeotemperatures estimated for rocks now at the surface was explained by proposing varying amounts of erosion; deeper levels of erosion being correlated with higher palaeotemperatures. There are several implications as- sociated with this suggestion which indicate that the author was overly optimistic in presenting his conclusions and that an alternative explanation might be more appropriate.

An examination of the distribution of present surface heat flow for the UK (Lee et al. 1987) and the distribution of maximum palaeotemperatures presented by Green (1989) reveals a remarkable correlation between areas of high heat flow and high palaeotemperature. These zones of anomalous heat flow are thought to be primarily associated with crustal and/or mantle heat production and are therefore likely to have been present during the early Tertiary. If this is true, then Green's (1989) proposal implies that the distribution of the thermal conductivity of the overlying sediments was such that it exactly cancelled the variation in basal heat flow, i.e. that the pattern of variation of thermal conductivity of the sediments exactly matched and was inversely proportional to the variation of heat flow. This is unlikely to have been the case.

The crux of Green's (1989) argument is that the apatite fission track analysis (AFTA) results indicate

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