Seventy coal samples from the Maudlin (H) horizon (the main seam and two thin seams, one above and one below the main seam), from thirty boreholes on offshore easterly extensions to the contiguous Northumberland and Durham coalfields were analysed for vitrinite reflectance and volatile-matter yield of the whole coals. Geologically, the two extensions are parts of the "Northumberland Basin" and the "Alston Block", lying on the Mid North Sea High, which are separated by important east-west trending structures, the Stublick and Ninety Fathom faults. Data for samples from the Maudlin seam in both Basin and Block, when plotted, show similar trends but lie in different parts of a general envelope formed by published data for the relationship between vitrinite reflectance and volatile-matter yield. Equivalent plots for the same properties for the two thin seams show similar separation and trends. In both cases, the separation between the two groups of measurements can be attributed to differing thermal gradients which were relatively low in the Basin and, together with thermal conductivities, high on the Block. Strong support for this explanation is given by the shift of plots for the relationship between reflectance and volatile-matter yield for coals in the Basin, which have been rapidly heated by igneous intrusion. These plots fall in the part of the reflectance/volatile-matter yield envelope occupied by the more rapidly heated coals of the Block. Stress is again laid on the inadvisability of placing too much reliance, for low-rank coals particularly, on the use of specific correlations between coal parameters for predictive purposes without taking due account of the range of geological circumstances existing within any particular coalfield region.