Mr. De la Beche on the Geology of Southern Pembrokeshire. 7 



characterized near Millhaven ; and it is indeed the prevailing variety of trap 

 in the tract under consideration. 



By reference to the plate it will be seen that the body of trap rises at 

 Millhaven from beneath old red sandstone^ which is disturbed and bent near 

 the place of contact ; that at the base of the cliff near Gouldtrop Head it rests 

 on a curved mass of carboniferous limestone ; and that at Gouldtrop Road it 

 abuts against, and rests upon, not only the limestone, but also coal-measures, 

 of which the strata are bent back. Here then is a mass of trap, not differing 

 in mineralogical character from those masses of the same rock which have 

 previously been described as associated with and subjacent to greywacke, and 

 which by many would be termed, from those circumstances of position, trans- 

 ition trap. Yet here we find it, at the same time supporting old red sand- 

 stone, and incumbent on carboniferous limestone and on coal-measures. It 

 would be difficult to explain these phaBnomena by any other hypothesis than 

 that of the forcible intrusion of trap among the other rocks, at a period subse- 

 quent to that of their being formed and consolidated. 



Range of Trap from Bolton Beacon to Benton Castle. 



This mass appears to be connected with the former by a dyke which passes 

 through Rosepool and Walwin's Castle to Tiers Cross ; since blocks of trap 

 can be traced along this line, and since a perpendicular dyke of felspathic 

 cornean, a few feet thick, occurs in the face of the cliff at Walwin's Castle. 

 The blocks are far from numerous, and are rather difficult to trace. They 

 mostly consist of greenstone, like that near the parsonage at Walwin's Castle: 

 some however approach in character to cornean. 



From Bolton Beacon eastward, this range is considerably elevated above 

 the neighbouring country, but mostly so towards its two extremities. It varies 

 in substance in different parts of its course. At Bolton Beacon it is a green- 

 stone, approaching in some places to greenstone-porphyry : from Johnston to 

 Trooper's End syenite predominates ; which sometimes, owing to the absence 

 of quartz, becomes a large-grained greenstone. Felspathic cornean occurs 

 near Thurston; and the same rock prevails at the eastern extremity near Ben- 

 ton Castle, where it forms several small tors on the surface. 



This range is bounded on the north, from Tiers Cross to Nash, by coal- 

 measures, except near Johnston, where a small band of mountain limestone 

 intervenes. Both the coal-measures and the limestone between Tiers Cross 

 and Nash, are highly contorted. From Nash to Milford Haven the range is 

 flanked on the north by a rock resembling greywacke (to be described in the 

 sequel), which towards Guilford has its strata nearly vertical. The same rock 



