EXPLAINED BY DIVERGENT LINEAR ERUPTIONS OF TRAP. 



407 



east and south-west direction of the latter is abundantly explained by the linear and 

 parallel masses of trap ; so is the carboniferous axis of South Pembroke satisfactorily 

 accounted for, by the trappean ridges extending from Benton Castle on the east and 

 by south to Talbenny Cliffs on the west and by north. Thus, in both these tracts 

 the divergences of direction of the strata are distinctly referable to separate linear 

 eruptions. 



Volcanic or eruptive forces, sufficient to determine the east and by south, and west 

 and by north allinement of South Pembroke, could not have taken place in a large 

 mass of matter, most if not all of which had previously a north-east and south-west 

 direction, without subjecting it to prodigious rents and dislocations. Many of these 

 fractures have been pointed out in describing Caermarthenshire, where it has been 

 shown, that Silurian and slaty rocks, previously consolidated, have been snapped across 

 by subsequent movements, which, wrenching them from their original direction, have 

 thrown them into the axis of the Glamorganshire coal basin : but none of the fractures 

 coincident with that axis, either in Glamorganshire or in Pembrokeshire, are accompa- 

 nied by outbursts of volcanic matter. It is therefore highly satisfactory, in following 

 the strata so affected into Pembrokeshire, to find them pierced by trap ridges, having 

 precisely the same course as the major axis of the South Welsh coal-field. 



The interference of two such lines of elevation also explains the contortions and breaks 

 of the carboniferous strata ; since it is evident, that shale, sandstone, and limestone, 

 when compressed between these lines of powerful movement, must have been subjected 

 to extraordinary contortion and dislocation ; while the seams of frangible anthracite 

 would be so shivered into numberless small fragments, as to form the slashes of culm 

 which are characteristic of Pembrokeshire. 



A thorough examination of this region may well excite deep interest, not only in 

 bringing to light the numberless proofs of the violent disturbance resulting from the 

 interference of this great east and west movement with the ancient direction of the 

 strata, but also in showing over how great a breadth of surface its effects extended. 

 Thus, besides the zone specially affected in the southern part of Caermarthen and Pem- 

 broke, the width of which is from eleven to fifteen miles, the east and west strike has 

 been also impressed upon the strata, at intervals, very far to the north, extending along 

 the boundary between Pembrokeshire and Cardiganshire. Examples of it have been 

 cited in the slaty rocks of St. David's, Scillyham, and Abereiddy. Another striking 

 case is south of a spot called Felindre, where the Nevern stream descends from the 

 Precelly ridge to the valley of Newport. The strata on its west bank, have the prevailing 

 strike of the region from north-east to south-west, and parallel to the great trap ridge 

 of Caerningly, while on the east bank, the same strata are broken off, ranging east and 

 west 1 . Similar oscillations between these conflicting directions, are seen as far north 



The dislocation at Felindre was observed by Professor Sedgwick. 



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