292 



STRATIFIED AND INTRUSIVE TRAP OF THE BREIDDENS. 



Similar sandstones are found on the other side of this hill, dipping to the south-south- east at angles 

 of eighty and seventy, and passing under the ordinary shale of the district. This transverse section 

 of Bauseley Hill is so clear, that no one can examine it without being convinced that the felspar rocks 

 were formed contemporaneously with the shale and sandstone with which they alternate ; for the 

 planes of deposit of the beds of micaceous sandstone and black shale, are exposed like the sides of 

 party-coloured walls, interstratified with the beds of trap and felspar aggregates. The evidence is 

 further most valuable in showing, that as the Silurian deposits and the alternating traps are not in 

 positions in which they could have been deposited, they must have been thrown into their present 

 vertical positions by convulsions. Continuing this transverse section to the north-west, across 

 a little dell, we come to a conical knoll of intrusive trap rock, which piercing through the strata, 

 dislocates and throws them off abruptly. This rock is made of compact felspar, sometimes por- 

 phyritic, containing crystals of common felspar, associated with concretions of green earth and a 

 little disseminated lime. The diagram, PI. 32. f. 7., will explain the relations of the stratified and 

 intrusive trap rocks to the sedimentary deposits. 



The summit of the Moel-y-Golfa is a slaty porphyry with a base of compact felspar, and has a 

 tendency to columnar structure. It passes on one side into a coarse-grained greenstone, and on 

 the other into a large concretionary trap. The last variety is deeply cut into at the foot of the hill 

 nearest to Middleton, where the large size of the concretions well entitle the quarries to a visit. 

 They consist for the greater part of compact felspar, sometimes porphyritic, and their colours 

 usually differ from those of the matrix. This rock, indeed, is exactly the same as that described on 

 the sides of the Welch Pool Dyke. It reappears at various points along this ridge, particularly in 

 the little bosses between Builthy and Bauseley. Wherever this rock has been long exposed above 

 the turf it has the aspect of a bronze-coloured quartzose conglomerate 3 . 



The hill of Cefn-y-Castel offers little clear evidence of structure. At the south-west end of Moel- 

 y-Golfa the trap of the detached hillock of Little Garreg is rather remarkable, consisting of spotted, 

 fine concretionary greenstone, with small nests of white and black calcareous spar. This spotted 

 rock, passing also into amygdaloid, reappears at a place called the Cefn, on the east side of the 

 road from Shrewsbury to Welchpool, and about four miles from the latter. Iron pyrites is dis- 

 seminated in minute crystals, and white calcareous spar, coats the rifts of the rock and also pene- 

 trates it in veins. Some varieties contain white calc spar enveloping small kernels of green earth, 

 and the dark spots and the discoloration of the flakes and nests of carbonate of lime are probably 

 due to this mineral. This rock is quarried for the roads, and the excavations offer satisfactory 

 evidence of the violent intrusion of the trap into the associated strata. It has risen in round 

 masses, throwing off the schist and sandy limestone, and where the masses have been quarried out, 

 large cavities have been left, the roofs of which exhibit the stratified deposits highly curved and 

 broken. The junction bands are chiefly black, laminated, flinty schists, with some minute stripes 

 or veins of lead ore and steatitic matter : in parts the trap rock is intruded, with jagged edges, into 

 contiguous beds of sandy impure limestone, which on contact are more or less crystalline and in- 

 durated. Another variety of these contact rocks is a breccia of dark-coloured, indurated sandstone, 

 almost quartz rock, with veins of calcareous spar and iron pyrites. At the south end of this low 

 rising ground, consisting of upper Silurian rocks, as proved by the fossils, and where there is no 

 trap, the stratified deposits are unaltered and preserve a strike from north-east to south-west; but 

 on the north side of the boss of trap they belong to the upper calcareous beds of the Caradoc sand- 



1 See account of a similar large concretionary trap at Leigh Hall, p. 276. 



