SLATY CLEAVAGE AND BEDS BOTH DIVERGENT AND COINCIDENT. 399 



and other rocks, with much felspar in the base. These beds dip 40° north, vdiile at Trifleton quar- 

 ries, courses of dark grey, hard, flaglike grits dip 25° south-east, being thrown off the southern 

 flanks of the Trafgarn trap ridge. Similar flagstones are worked at Bwdloi and other places along 

 this line. 



c. The next practicable division in the Cambrian rocks of Pembrokeshire, consists of hard, dark, 

 purple, grey, and green, close-grained sandstones, with a partially slaty cleavage, and perfectly re- 

 sembling the rocks of the Longmynd and Linley Hills, Salop 1 , and of the Lammermuir hills in 

 Scotland. They are displayed in a long range of sea-coast between Brawdy and the north end of 

 Newgale Sands, where they have been mentioned as environing and abutting against the coal mea- 

 sures and extending thence to St. David's, where they pass into still older slaty rocks. Excellent 

 examples of these party-coloured, finely laminated, hard sandstones are also exposed in the cliffs of 

 Skomer, and will be mentioned in describing the trap rocks of that island. 



d. The oldest rock in Pembrokeshire is the schist, which rises from beneath the purple and grey 

 sandstones of St. David's, and constitutes those great masses which, having a more decided slaty 

 structure than the strata just described, are worked at intervals for roofing slates. With this 

 schist are associated many courses of hard sandstone passing almost into quartz rock, and tra- 

 versed by numberless veins of pure quartz, giving to this tract a much more siliceous character, 

 than is possessed by any previously described. 



Slaty Rocks. 



In the southern end of Whitesand Bay near St. David's, the cliffs expose a succession 

 of hard, dark grey and light green, slaty sandstones, which strike from 10° to 15° north 

 of west. These beds are cut through obliquely, from north-east to south-west, by planes 

 of slaty cleavage (in perfect accordance with the prevailing strike of the slates through 

 Wales), and dip 65° to 70° to the north-west. 



At the northern side of the bay, and not half a mile distant, are vertical beds, dif- 

 fering little from the preceding in composition, but without slaty cleavage transverse 

 to the laminse of deposit. In some of these are large veins of white quartz, the strata 

 undulating with a prevailing strike of E.N.E. To these succeed (in vertical cliffs 

 150 feet high) masses of slate, which were formerly extracted for roofing purposes 2 . 



In this instance, the planes of cleavage and stratification are apparently one and the 

 same, the latter being marked by the succession of mineral layers of different colours, 

 frequently separated by very thin laminae of iron pyrites, and the slates are obtained by 

 splitting the stone along these lines of bedding. 



It is, however, well worthy of remark, that these highly inclined beds rise in the 

 form of flagstones, presenting, when very closely examined, slightly uneven surfaces, 



1 See p. 255 et seq. 



2 Since the memoir upon Pembrokeshire was read before the Geological Society, Professor Sedgwick and 

 myself visited the county together. On this last occasion I was called away by a severe domestic misfortune, 

 before I could re-examine Northern Pembroke, so that the portion of the map relating to that tract, has been 

 much improved by my friend's labours alone. 



3 d 2 



