8 Mr. De la Beche on the Geology of Southern Pembrokeshire » 



forms the entire southern boundary of the trap, and is every where found to 

 dip from it. 



Trap ofMarloes and Skomer Island. 



This mass of trap, which forms the southern point of St. Bride's Bay, is 

 separated from that of Millhaven by well characterized old red sandstone, 

 and is bounded on the south by the rock resembling greywacke. It forms a 

 wedge-shaped mass, which gradually becomes narrower towards the east, and 

 does not extend inland above half a mile eastward of Marloes. It appears to 

 rest upon the sandstone, and to rise from beneath the rock resembling grey- 

 wacke. Both these rocks dip at a considerable angle to the south ; and as the 

 breadth of the trap diminishes, they gradually approach one another, until 

 they unite to the east of Marloes, 



This mass consists principally of greenstone, containing rocks, however, 

 intermediate between cornean and greenstone. Between St. Martin's Haven 

 and Marloes there is a small tor of trap. At a small cove near Mosely Wick 

 Farm, where the trap comes in contact with the old red sandstone, a coarse- 

 grained siliceous rock appears to proceed from the trap in veins, which dimi- 

 nish as they recede from it, and penetrate to a short distance between the 

 strata of sandstone. 



All the north of Skomer Island consists of massive trap, having the character 

 principally of fine-grained compact greenstone, and sometimes approaching to 

 cornean. The small peninsula, however, to the east of the landing-place, 

 must be excepted, where some stratified rocks of ambiguous appearance occur. 

 The southern part of the island consists of stratified greenstone, dipping at 

 about 48° to the south-east. Between this and the greenstone belonging to the 

 north of the island a quartzose cornean, mostly striped, occurs. In some parts 

 of the island hornblende is the prevailing ingredient of the rock. 



Prom the information of intelligent seamen it should appear that Grasholm 

 Island, and the rock on which the Smalls Lighthouse stands, consist of a rock 

 similar to that of the northern side of Skomer Island. If this account be cor- 

 rect, a great dyke of trap must extend westward from Marloes to the Smalls 

 Lighthouse, parts of which only appear above the water. The bottom of the 

 sea along that line is known to be very rocky. 



§ 2. — Greiwack^. 



(A) Great Greywacke District, North of the Coal-basin. 



The great tract of greywacke, which extends through North Pembroke- 

 shire, has its southern boundary-line within the limits of our district. This 



