From the preceding considerations it is scarcely possible that the place of the Thick coal can be at a less depth than 800 or 900 feet (say 300 yards) beneath Wassel Grove, and all the line of country mentioned before where these little upper coals are observed to crop out.
1b. Red Coal-measure Clays.—These facts are represented in Horizontal Section No. 10, sheet 25, which was taken over Hasbury Hill, where there is a capping of red rock believed to be Permian, and thence across Hawn to the Old Lion colliery. In the latter shaft (of which a vertical section is given on Sheet 18 of the Vertical Sections. No. 25) there are about 50 or 60 feet of dark and grey rocks at the top containing a little nine-inch coal, underneath which come about 280 feet (or 93 yards) of red and blue marls, interstratified with ' blue ground," "blue rock," and "mottled ground," many of the so-called blues being rather green than blue, but the predominating colour of the whole a deep purplish red. The bottom of this red group is about 233 feet (or 77 yards) above the Thick coal. A similar group of red and blue beds, mostly clays, but sometimes sandy, is noted in the sections at Congreaves and Baremoor. Red and blue mingled ground is mentioned in the Barrow Hill pit (No. 23. Sheet 18, of Vertical Sections,) in the shafts about Corbyns Hall, at Wordesley Bank, and almost wherever a detailed section is given of the beds lying 100 yards or so above the Thick coal in the southern part of the district.
North of Dudley, again, in the Trough pits at Burnt Tree, where the Thick coal is 617 feet deep, the upper 360 feet are said to consist largely of "mottled ground," "mingled ground," "blue and mingled ground," the " mottling" and the " mingling" almost always alluding to the occurrence of red with the other colours, and the term "ground " signifying an argillaceous material, and not a sandstone (see Vertical Section No. 14, Sheet 17). These reddish clays are worked in large excavations at the surface near Tipton and south of Oldbury, as well as between the Lye Waste and Stourbridge, and used for making a very superior kind of brick, generally of a deep blue colour. In the deep sinking at Great Bridge, near West Bromwich, where the Thick coal is 567 feet deep, there is a bed of "red marl" 6 feet thick, at a height of 205 feet above the Thick coal; and 38 feet above that again, or about 243 feet (or 80 yards) above the Thick coal, there is a group of red marls 64 feet thick, with a few beds of "blue" or "mottled rock." The Upper Sulphur coal then comes in with a "white fire clay" below and a "blue clunch" above, covered by about 63 feet of beds containing no red, over which are 192 feet of other measures, in which beds of "red marl" 8 to 16 feet in thickness, are very frequent.
We may, therefore, assume it as a fact, supported by all the available evidence we possess, that there was deposited a group of beds in the South Staffordshire coal-field lying about 100 yards above the Thick coal, being themselves about 100 yards in thickness, and principally composed of red or red and green (or blue) clays, variously interstratified with beds of other colours, and occasionally containing small coals, fireclays, &c.
This fact is very important, for it enables us to classify some beds in the northern part of the district in their right place as Coal-measures, notwithstanding the red colours which formerly led us to look upon them as belonging to the Permian (or even, still earlier, to the New red) formation. These are the red clays about Walsall Wood,[1] those of
- ↑ Professor Ramsay and Mr. Hull have lately (February 1859) gone over this ground again for the express purpose of comparing these several localities with each other, and with the red beds in the southern part of the coal-field, and have satisfied themselves that they all belong properly to an upper part of the Coal-measure series.
