Page:VCH Staffordshire 1.djvu/45

 GEOLOGY got coal, which has made Birmingham and Wolverhampton the great hardware manufacturing centres of the world. The Dudley Coalfield has been regarded as the typical area for the Middle Coal-measure flora of Great Britain. The genus Sphenopteris in this sub-division attains its maximum development. Stumps of the gigantic lycopod, Lepidodendron, have been met with in such profusion in the workings of the Parkfield Colliery as to form a veritable fossil forest. As in North Staffordshire the commonest mollusc is Carbonicola (Anthra- cosia). In addition to remains of fishes the coalfield has also yielded specimens of Arachnida and insects, types rare or unknown in North Staffordshire. All these fossils, excepting the Fishes, indicate the close proximity, if not the absolute presence, of land ; but below the Thick Coal, fossils such as Lingula, Productus, Discina and Pterinopecten show a temporary incursion of the sea ; though these marine episodes do not appear to have been of such frequent recurrence as in the north. Upper Coal Measures. The gradual infilling of the basin and final change in the character of the sediments, accompanied by the gradual passing away of the fauna, is as clearly illustrated in the southern part of the county as it is in the Potteries. In the districts of Corngreaves and Oldhill the ordinary grey Coal-measures graduate upwards into a con- siderable thickness (over 300 feet) of red clays (Red Coal-measure Clays of Jukes) indistinguishable from the Etruria Marls of the northern coalfield. Moreover they contain similar thin bands of ashy green grits known as ' Espley Rocks,' As the area is not far distant from the Cambrian and Pre-Cambrian ridges of the Lickey Hills, these green grits, as might be expected, contain angular fragments of the Lickey rocks. Occasionally the grits are so coarse as to form a true breccia, interesting as fore- shadowing the breccia conditions so prevalent in the succeeding ' Permian ' rocks of South Staffordshire. The red clays afford some of the material for the famous South Staffordshire blue bricks, and large quarries have been opened round Oldhill. The brick clays pass up near Halesowen (just beyond the county limits) into grey sandstones and marls (Halesowen Sandstone Group], about 400 feet thick, containing an occasional thin seam of coal and a well marked band of Spirorbis limestone near the summit. These in turn are surmounted, quite conformably, by red sandstones and marls, generally included in the ' Permian ' formation, but identical with the Keele type of North Staffordshire. The sequence of the Upper Coal-measures of North Staffordshire is thus at once seen to be repeated around the southern margins of the South Staffordshire Coalfield, and the connection of the two fields either absolutely, or at least as regards the similarity in the sequence of events proved beyond dispute. The same sequence too has been detected in the deep sinkings and borings outside the exposed coalfields, where the green ' Espley Rocks ' at once afford the miner a clue to his position in the Coal-measure sequence. Origin of Coal. As the county abounds in this mineral a few words i 17 3