If we compared the two general sections now given, on the supposition that the Thick coal of the first was but one coal, we should have only 7 or 8 workable coals in that section to contrast with 21 in the other. By taking into account that the Thick coal is made up in reality of 12 to 14 separate coals, we at once get the fact that the number of coals are in reality nearly the same in both sections. The difference in the two series is then seen to be the result of variations in the other materials, namely, the shales and sandstones with which the coals are interstratified.
It may very well be doubted whether any single bed of coal is ever more than two or three feet in thickness, and we may therefore take it for granted, that every bed which exceeds that thickness over any considerable space is in reality a compound seam made up of two or more beds resting on each other, with or without "partings" of shale, &c, between them. Not only then are many of the beds which are spoken of under one name in the southern part of the district compound seams, but several of those in the northern district also. We may look upon the seams numbered III, VI, VII, VIII, XIII, and XXI, in the second general section as undoubtedly compound seams, liable at any place to be separated into two by interposed shale and sandstone. Some of the others may also be compound seams.
In comparing the two general sections we must recollect that the second does not include any of the 600 or 800 feet of upper measures which form the top of the first, and we shall then see that the bulk of the coal-bearing beds in the second is much greater than the bulk of the coal-bearing beds in the first. Since, however, the quantity of coal disseminated through that bulk is nearly the same in both, or at least is but slightly preponderate in the second, it follows that the increase is in the shales and sandstones interstratified with the coals. Since, moreover, the lower beds are the same in both sections, it follows that the beds above them, those about the Thick coal, are on the same geological horizon, and that the Thick coal of the first is split up somehow into the separated beds of coal to be found in the second.
In Plate I. Figure 1 will be found a representation of this splitting up of the Thick coal and other coals towards the north in a diagrammatic form. The constitution of the beds south of Bilston (which we may call the south central portion of the coal-field) is shown on the one hand in a vertical section on a scale of 120 feet to an inch, and that of the beds at Bentley and north of it, on the other hand, in a similar vertical section on the same scale, each section being a mean or average section deduced from a considerable number of actual pit sections.
The coals known as the Heathen and Sulphur are common to the two sections, the Heathen being assumed as the horizontal datum line.
The Fire-clay and Bottom coals are also common to the two sections being worked continuously from Bilston to Bentley, and thence to Pelsall.
