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. 



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