GEOLOGY destroyed and we pass abruptly from the deserts of the Trias to the arctic conditions of the Pleistocene period. Before describing this wonderful contrast of events we must however retrace our steps and briefly consider the igneous rocks breaking through the formations previously described. IGNEOUS AND VOLCANIC ROCKS The stratified deposits are in many places but a thin skin overlying a reservoir of molten material ever ready to burst forth and intrude itself along lines of weakness. Evidences of such weak spots are to be met with again and again among the formations whose history we have been tracing, yet it was only rarely that the underlying molten matter found egress from its subterranean reservoir. The earliest record is afforded by the limestone quarry on Congleton Edge (p. 8), where it becomes evident that during the closing scenes of the Carboniferous Limestone epoch a volcano was close at hand vomiting forth ashes and dust which fell into the surrounding seas and possibly sending forth a submarine lava stream. The famous basalts or trap rocks intruded into the Coal-measures of South Staffordshire present the next example. These cover no inconsider- able area at Rowley Regis, Barrow Hill, Pouk Hill, and again round Wednesfield. Each occurs as a ' sill ' whose intrusive character is shown by the coal-seams being charred where they came in contact with the molten mass or by the baking of the black Coal-measure shales at their junction with the basalt above and below. The largest sill forms the Rowley Regis mass, through which the tunnel between Rowley Regis Station and Old Hill passes. The lava was here injected into the space of an arched up mass of Coal-measure strata forming what is known as a ' laccolite,' of which the cover has been removed by denudation. During the process of cooling, a beautiful columnar structure, excellently preserved in Turner's Pit, was set up. 1 Huge spheroids of basalt are frequently enclosed between the joints which transversely divide the columns at fairly regular intervals. The Rowley Rag is largely used for road metal. Some uncertainty exists as to the age of the intrusions owing to the want of conclusive field evidence. Professor Watts 3 comments on the fresh appearance of the constituent minerals and the many features they possess in common with the well known Tertiary dykes of the north of Ireland and Scotland, and also on the fact that the Rowley mass partakes in the fractures affecting the coalfield, some of which, such as the Great Boundary Faults, traverse Jurassic rocks. None of the South Staffordshire intrusions pierce rocks later than high Coal-measures, but an interesting dyke met with in North Staffordshire traverses the marls of the Keuper period. This is a very narrow basaltic dyke, never more 1 T. G. Bonney, S>uart. Joum. Geol. Soc. xxxii. 151 (1876). a W. W. Watts, Geologists' Association, p. 399 (1898), op. cit., in which references to the literature on the igneous rocks are also given. I 25 4
