EFFECTS OF TRAP INTRUSION— POUK HILL, BIRCH HILLS, ETC. 501 



Trap of Pouk Hill, Birch Hill. 



Though trap is much diffused underground through the northern or Wolverhampton coal-field, 

 being met with in many of the mining works, it shows itself much less frequently, and never in 

 such large masses as in the Dudley district. The only large mass of the rock which appears at 

 the surface is at Pouk Hill, between Wall sail and Wolverhampton, two miles west of the former 

 place. It is there a basaltic rock, very like that of Rowley (composed of an intimate admixture of 

 felspar and hornblende). It rises to the crest of the hill in clusters of four-sided columns, spreading 

 out in a fan-shape and for the most part slightly inclined to the horizon. The lithological characters 

 of this trap have been described by Mr. Finch, in a short notice in the Philosophical Magazine. 

 (See vol. xii. p. 167-) There is little doubt that Pouk Hill is the centre of eruption of much of the 

 trappean matter which to the right and left of it overflows some of the adjacent coal measures, such 

 as the "blue flats ironstone," and penetrates between those beds and the "gubbing stone." At 

 Bentley Forge, a mass of this rock has been laid open, associated with twisted coal, smut, and shale ; 

 and in the old clay works opposite this forge, at the foot of Birch Hill Collieries, I found the trap 

 in an overlying position, containing fragments of coal and shale. 



At Bentley Forge, the columnar greenstone exfoliates at the angles of the prisms, and shows a 

 tendency to run into spherical concretions, so that it is easy to reduce any of these masses by frac- 

 ture to a small nucleus. In other places, near Wolverhampton, the trap has the appearance of having 

 been forced in laterally for some distance, between the coal and iron strata, with which it seems for 

 short distances to be regularly interstratified, as in the case before cited. 



In the tract now under consideration, the true explanation of such phenomena were long ago 

 pointed out by Mr. Arthur Aikin. As early as the year 1812, that author, being then Secretary of 

 the Geological Society, gave an excellent account (Geol. Trans., vol. iii. Old Series) of a certain 

 mass of the "green rock," exposed in the works of the Birch Hill Collieries; showing from its 

 bulging or irregular form, and its finally wedging out, that it was not a true bed, but simply a dyke, 

 which by its intrusion had altered the sandstone, shale and coal in contact, particularly on the lower 

 surface, the bitumen being driven off and the coal reduced to a cinder. After describing the com- 

 position of the trap with his usual mineralogical acumen, Mr. Aikin remarks, that it is penetrated 

 by contemporaneous small veins of calcareous spar, nearly vertical, and that the rock, though highly 

 compact when fresh quarried, is discoloured and falls rapidly to pieces upon exposure. Notwith- 

 standing these clear evidences, most of the iron masters still consider the green rock a regular bed, 

 and believe that it always lies above {he blue flats. Mr. J. Barker, however, assures me, that its 

 course, position, and thickness are all very uncertain, though generally found just below "the 

 gubbing stone," as mentioned in the section 1, p. 479. At the Chillington Collieries this rock was 

 met with at a depth of 60 yards, overlying the "blue flats" ironstone, the coal in contact being 

 much charred and altered. In parts, however, this apparent bed is only 8 yards thick, whilst in 

 others it expands to 25 and it is therefore proved to be a dyke of lateral injection. To the north 

 of Pool Hays, certain trial shafts having passed through 40 yards of poor coal measures, a mass of 

 this greenstone was met with, which was sunk into to a depth of 53 yards without a change of 



effects produced by this lateral wedge of trap, and also for the account of the faults by which it has been 

 affected. The works in question lie to the north of Corbyns Hall, immediately north of the great fault which 

 ranges from Barrow Hill to the Stand Hills, and are close to the western edge of Barrow Hill, 



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