4 Mr. De la Beche on the Geology of Southern Pembrokeshire, 



I shall now proceed to describe the various insulated masses^ belonging to 

 varieties of trap, that occur within the district. 



Vicinity of St. David's. 



Penberry Rock_, to the north of St. David's, is of an irregular conical shape. 

 Its summit is a greenstone, composed of well-defined grains of hornblende 

 and felspar, of middling size. Felspathic cornean is in some parts associated 

 with the greenstone. Greywacke forms the base of the hill, and appears to 

 rest upon the trap. On the northern side however, in several places, trap is 

 intermixed with greywacke, and in some is interstratified with it. In one 

 instance a continuous mass of trap descends from the top of the hill to the sea. 

 It seems rather to be interposed between the strata of greywacke, than to cut 

 through them. 



At a little distance to the south-west of Penberry a ridge of trap emerges, 

 which forms a remarkable feature in the country ; and after reaching its highest 

 point at a rock called Llacithty, extends to the sea in Whitesand Bay. [t is 

 composed of the same kind of massive unstratified greenstone as Penberry 

 Rock, of which it may be deemed the continuation. 



Greywacke is observable in small quantity to the north of the ridge of 

 Llacithty, between Porthyrow and a bay near Cesselvawr Head. It is much 

 intermixed with trap ; so that it is difficult to determine whether the latter be 

 transverse or conformable to the strata of greywacke. Greywacke forms the 

 north-western base of the ridge of Llacithty ; but further to the north-west 

 another ridge of greenstone appears, extending from Cesselvawr Head to St. 

 David's Head. In substance it resembles the rock of Llacithty, excepting that 

 it is stratified, with a dip to the north-west of about 45°. 



A series of rocky tors, to the west and south-west of St. David's, marks the 

 course of a large body of trap, extending from Crosswoodig on the north, to 

 Pen-y-Main-Melin Point on the south. This mass has not the slightest appear- 

 ance of being stratified. It is very variable in its composition. Near St. David's 

 felspathic cornean forms its base, in which are disseminated numerous crystals 

 of quartz and felspar. At Trevaythan the base is more earthy, and contains 



hood of Builth. Greenstone and porphyritic cornean occur extensively in Cumberland. To the 

 trap-formation belong the insulated ridges of Malvern and Charnwood forest. Of the same aera 

 is the greenstone surrounding Dartmoor, and occurring in other parts of Devonshire, as also in 

 Cornwall, Thcporphyritic elvans of those counties, though agreeing with some varieties of cornean 

 in having for their basis a paste of compact felspar, or perhaps of claystone, appear rather to belong 

 to a more ancient formation than the trap. 



