TRAP ROCKS OF NORTH PEMBROKE. 



401 



Again, in the principal range of the slate rocks constituting the Precelly Mountain, 

 where the greatest quantity of the best slate is obtained, there are occasional coincidences 

 of cleavage lines and laminae of deposit. The same phenomenon has since, indeed, been 

 observed by Professor Sedgwick, both in this range, and also along the line of coast cliff 

 on the sides of Abereiddy Bay, where, if there had been any doubts upon the question, 

 they would have been entirely dispelled, as fossil bodies (Graptolites) , occur upon the 

 lines of slaty cleavage, which are thereby proved to be coincident with true layers of 

 deposit. 



Thus, this important modification of the principle of slaty cleavage, which I pointed 

 out to the Geological Society in 1835, has received the full and entire sanction of Pro- 

 fessor Sedgwick. The above-mentioned cases are, however, to be distinctly under- 

 stood as marked exceptions to the prevailing phenomena in Wales. 



The chief objects which I had in view in examining North Pembroke were, 1st. The 

 succession of the strata between the Silurian System and roofing slates of Precelly. 

 2ndly. The direction in which the principal masses were prolonged to the sea-coast. 

 The former has been explained ; to the latter point, after a short description of the trap 

 rocks, I shall specially invite attention. 



On the Trap Rocks of Pembrokeshire. 



Pembrokeshire offers many examples of the two classes of trap rock which have been 

 described in other tracts, viz. stratified masses alternating with sedimentary deposits, 

 and amorphous masses which have burst through the strata. The former or stratified 

 traps are, however, nowhere seen in association with the Silurian rocks, as in Salop and 

 Radnor, but are confined to the Cambrian or older system. In the rugged and arid 

 headlands north-west of Fishguard, noted in modern times for the landing of a small 

 French force, linear parallel ridges of trap occupy a very large portion of the tract, 

 rising in gnarled bosses through altered sandstone and slate. They strike from north- 

 east to south-west along the craggy summits of Gaer-fawr and terminate in St. David's 

 Head. 



Among these older trap rocks are thick-bedded coarse felspathic conglomerates, containing frag- 

 ments of schist and slate, which range from north-east to south-west in allinement with the other 

 ridges of amorphous trap. Some, however, of the more prominent of these parallel ridges, between 

 St. David's and the headlands north-west of Fishguard, appear rather to belong to the class of in- 

 trusive trap, and they generally inclose between them, highly altered greywacke and slaty rocks. 

 They consist of greenstones of several varieties, passing into porphyritic greenstone and porphyry, 

 together with much compact felspar rock ("corneen"). It is, however, difficult to define all these 

 masses in such a manner as to separate those which are of formation contemporaneous with the 

 slates from those which have been subsequently intruded. 



The mineral composition of the rocks near St. David's has been somewhat minutely described 



