[117] Jukes, "South Staffordshire Coal-field," Mem. Geol. Surv. 2nd edit. (1859). The area is embraced in Sheet 62 N.W. and S.W. of the Geological Survey, and is illustrated in Sheets 23, 24 and 25 of the Horizontal Sections.
This coal-field is above 20 miles long and 5 miles broad. Its strata rest unconformably on Upper Silurian strata, which, as part of the ancient ridge or island already referred to, project here and there from amidst the Coal-measures. The boundaries of the field on the east and west sides are chiefly made by faults which bring down Permian and Triassic formations against the Carboniferous strata.
Throughout this coal-field sheets of igneous rock are abundant. In the detailed account of them given by Jukes in his admirable essay on the South Staffordshire Coal-field,[118] he distinguished two kinds of igneous material—"basalt," which comes out at the surface, and sometimes overlies the Coal-measures in large cakes like that of the Rowley Hills, which extends for two miles in one direction and more than a mile in another; and "greenstone," which burrows among the coal-bearing strata, and gives off dykes and veins of "white rock-trap." There does not appear, however, to be any essential difference in composition, age or origin between these contrasted kinds of igneous material. They not improbably all belong to one series of extrusions, their distinctions being due rather to the conditions under which they were erupted, and in particular to their comparative thickness, and the influence of adjacent coals and carbonaceous shales upon them.
The igneous rocks seen at the surface in this district form a series of well-marked eminences. Of these the largest extends as a ridge from Dudley to beyond Rowley Regis, a distance of more than two miles. To the west of this tract, a number of small patches of the same material crop out at the surface, the most important forming Barrow Hill. Six miles farther north another group of similar patches may be seen. Of these the largest occurs at Wednesfield, but the most noted forms the Pouk Hill, which has long been noted for the beauty of its columnar structure.
The sheets of "greenstone" met with in the coal-field are more numerous and extensive than the detached areas of more compact rock visible above ground, a single sheet being sometimes traceable in the coal-workings for two miles in one direction.
The eruptive rocks of this district, when examined in their freshest form, consist of well-preserved olivine-dolerite. An examination of the "greenstone" and the "white rock-trap," which runs in fingers and threads through the coal, shows that these are really the same dolerite which has undergone alteration, the ferruginous silicates having especially been decomposed.[119]
[119] Allport, Quart. Journ. Geol. Soc. xxx. (1874), p. 547. Chemical analysis also shows the identity of the rocks and the nature of the alteration of the "white rock." See Jukes, "South Staffordshire Coal-field," pp. 117, 118.
The sills of greenish decomposed material that have been injected amongst and alter the coals, vary from 15 feet to 80 or 90 feet in thickness. The largest of the dolerite cakes on the surface, that of the Rowley Hills, is somewhat irregular in its thickness, but may reach as much as 100 feet.
That nearly the whole of the igneous material is intrusive is admitted by all observers who have studied the ground. The manner in which the "basalts" and "greenstones" send out veins into the Coal-measures shows conclusively that they have been injected into the strata. The only rock about which some doubt has been expressed is that of the Rowley Hills, which Jukes was disposed, though not without some hesitation, to consider as part of an actual lava-stream. He based this inference chiefly on the occurrence, immediately under the dolerite, of what he looked upon as a "trappean breccia or brecciated ash, containing rounded and angular fragments of igneous rock lying in a brown rather ferruginous paste, that looks like the debris of a basaltic rock."[120] This breccia he regarded as belonging to and passing into the Coal-measures, and he was thus inclined to regard the dolerite as a lava of Coal-measure age.