TRAP ROCKS OF SOUTH PEMBROKE. 



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Ridges of Johnston and Benton Castle. 



These masses of trap are both of intrusive character. They were previously thought to be con- 

 tinuous from the hills north of Johnston to Benton Castle, but they are in fact separated by a band 

 of Silurian rocks. As a continuous mass, the trap of the Johnston ridge has a length of about six 

 miles, from Nash to Tier's Cross, cutting from east and by south to west and by north, through the 

 carboniferous strata on its northern flank, and throwing off the Lower Silurian Rocks to the south. 

 Though rising to the highest land in this division of the county (Bolton Beacon), this trap ridge is 

 nowhere diversified with rocky tors like those of Trafgarn and the northern parts of Pembrokeshire, 

 but forms round-backed sloping hills, a feature chiefly due to the disintegrating nature of the rock. 

 Thus at Nash and Clarisson the greenstone weathers to yellow incoherent sand and gravel, which 

 in some cases are cut through to a considerable depth before the body of hard rock is reached. At 

 Johnston, the greenstone is so quartzose, that by some geologists it has been called syenite, and at 

 Bolton Beacon it passes into true greenstone and porphyritic greenstone. The line of eruption is 

 marked at intervals to the west of Bolton Beacon, as at Walwin's Castle (where there is a dyke of 

 compact felspar), till it reappears in great force in the sea cliffs of Gouldtrop road, where Mr. De 

 la Beche has described it as a large and fine-grained greenstone passing into syenite. He further 

 points out that although this rock does not differ in mineral composition from those associated 

 with much older strata, yet that it traverses equally Old Red Sandstone and coal measures, the 

 latter being bent back at the points of contact, and one large fragment of the carboniferous limestone 

 being actually twisted into the mass of trap. Hence he justly inferred, that it was impracticable to 

 draw geological distinctions between the then so-called greywacke or transition trap and the rocks 

 of similar composition which burst through the younger strata. I have previously alluded to a spur 

 or dyke of this rock which, to the north of Newgale sands, disturbs the coal measures and the Cam- 

 brian System ; and in Marloes Bay the Silurian rocks have been subject to a similar intrusion. 

 South-west of the Johnston jidge, at the farm-house of Roman's Castle, a small insulated point of 

 trap bursts up through the Lower Silurian Rocks, throwing off altered and contorted strata; and 

 directly to the west of this dyke, a similar rock protrudes through the Old Red Sandstone near Little 

 Hasguard. 



Benton Castle Trap. 



This mass has a length of nearly four miles from Benton Castle to a point north-west of Rose- 

 market. It ranges from W.N.W. to E.S.E. In Rosemarket Common it is a sort of trap tuf, 

 enveloping fragments of quartz and hornstone ; and this rock, passing into compact felspar, pro- 

 trudes through altered Silurian sandstone between Rosemarket and Waterless, These feJspar rocks, 

 graduating into porphyry, occupy the high grounds of Hearston Common and run out to a narrow 

 point on which Benton Castle stands, on the right bank of the Haverford river, where they throw off 

 unequivocal Old Red Sandstone on the north, and are flanked on the south by a very narrow stripe 

 of decomposing yellow shale and sand, which in the absence of fossils it is difficult to refer. This 

 zone of trap does not strictly terminate at Benton Castle, but crosses the river, protruding into and 

 dislocating the Old Red Sandstone, and enveloping fragments of that rock. In its final appearance 

 in the cliffs it is a conglomerate tuf, like that at the other extremity near Rosemarket, but in this 

 instance the included quartz pebbles predominate so very much over the felspathic base, that in 



