WELCH POOL. — TRAP DYKE. 



289 



The trap rock of the Standard quarries in the suburbs of this town, is a portion 

 of a large dyke, which cuts through beds of the Caradoc sandstone and associated 

 limestone and shale in a direction from N.N.E. to S.S.W. It has been exposed to a 

 width of about 120 paces, and a maximum height of upwards of ]00 feet. A great 

 portion of the mass is arranged in columns (as expressed in the above vignette) , inclined 

 20° to the S.W. ; the ends of the prisms, as seen at the north-eastern face of the dyke, 

 (being at right angles to the highly inclined strata through which the dyke passes,) dip 

 70° to E.S.E. The columns are divided by joints at right angles to the axis of the 

 columns, or parallel to the strata, on the sides of the dyke, and the workmen are thus 

 enabled to extract large blocks with great facility. This rock, though of unquestionable 

 volcanic origin, is remarkable from being most easily worked and well adapted for ar- 

 chitectural purposes. The best stone is of a light green colour, and is extracted from 

 the centre of the dyke, where the prismatic structure is most distinct, the distance 

 between the transverse joints being frequently seven to ten feet. 



This light green variety is essentially made up of granular felspar, speckled with many minute 

 nests and sometimes kernels of calcareous spar. It contains also crystals of common and arsenical 

 pyrites ; the joints of these prisms are partially coated with serpentine, and where the rock is con- 

 cretionary, the same mineral occasionally envelopes the concretions. Sometimes the joints are lined 

 with thin films of green earth and anthracite, in minute quantities. The soft and sectile nature 

 of this columnar trap, is probably due in some degree to the state of aggregation of the grains of 

 felspar, but doubtless in a greater, to the equable diffusion of small particles of lime. Another 

 variety, better seen in the flanks of the quarry, is blue, harder, more compact, very slightly cal- 

 careous, and fit only for the roads ; it passes into common greenstone without lime. A third variety 

 is a grey granular felspar : a fourth, in the north-eastern prolongation of the dyke, at the spot called 

 the " Old Dog Kennels," is entirely composed of very large concretions, some of which consist of 

 granular felspar, hornblende, and lime. When their surfaces are slightly decomposed, the oxydized 

 iron of the hornblende defines the outline, as if the concretions were encircled by a number of rusty 

 hoops. 



The sudden passage from the fine-grained columnar, into 

 the coarse and large concretionary trap is beautifully displayed 

 at the south-west end of the quarry, beneath a small garden. 

 The columnar rock has here a hard, brittle and conchoidal 

 fracture, and the ends of the prisms pass at once, as expressed 

 in this wood-cut, into the concretions, which vary in size 

 from large gourds to a man's fist : the same rock also occurs 

 in the Moel-y-Golfa Hill, p. 292. 

 The south-eastern and south-western walls of the Welch Pool dyke, consist of highly altered 

 rocks. Shale and sandy limestone are seen near the Old Dog Kennels ; at one point of contact 

 with, the trap, the granular structure of the calcareous sandstone is gone, and replaced by a thin 

 zone of grey, hard, and slightly calcareous quartzose matter ; at another, a few feet of dull grey en- 

 crinital limestone, is penetrated by large veins of white calcareous spar. The beds dip 80° south- 

 east; a thin band of bluish clay, apparently little altered, is, however, partially interposed between 



