Arbitrary relations between volatile percentage and depth of the seams can be established readily for both the South Wales and Kent coalfields by means of the Hilt rates: this is perhaps most clearly indicated by the South Wales sequences, which are considered first. If we take as a horizontal datum plane a seam which contains 40 per cent of volatiles we can calculate from the Hilt rate the depth for any given sequence at which the Nine-Foot seam would lie below that datum. Thus, at New Bridge the recorded volatile content of the Nine-Foot seam is 28; a least squares solution gives 26·7; adopting a value of 27 and a Hilt rate of 5·32, that seam would be 2,440 feet below the datum. At Treherbert the recorded volatile is 11, while a least squares solution gives 11·5; the depth of the Nine-Foot seam below the datum would thus be 5,400 feet. It is clear, therefore, that if the 40 per cent seam were regarded as a horizontal plane, depression was at one time more rapid in the west than in the east. It is known that a rapid increase in the thickness of the measures below the Pennant Sandstone takes place westward, but the difference of 3,000 feet indicated above is much more than occurred by differential warping during the formation of the lower measures among which the Nine-Foot seam lies. The tendency which is evident, however, in these lower measures must have continued into much later Coal Measure times, in which case a 40 per cent datum was itself warped down.