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. [Footnote 118: _Op. cit._ p. 117.] 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] [Footnote 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.]