394 



COAST SECTION OF LLANDEILO FLAGS. 



The Caradoc sandstones range from this point all along the wild coast cliffs of Marloes Mire, rising in unbroken and 

 regular succession for upwards of a mile, the hardest beds sometimes forming headlands, as at the Rain Rock, and where 

 the sandy shale prevails being scooped into caves. The angle of inclination varies from 50° to 65°, the strike being W.S.W 

 and the dip S.S.E. 



Slaty cleavage prevails more or less throughout this vast succession of strata, but is not per- 

 ceptible where the sandstone is thick-bedded. Innumerable joints occur in many of the masses, 

 and they seem to increase in number near points of violent dislocation, as near the great fault of 

 Gateholm Isle, where the dominant joints trend with the dip, ranging from 35° east of south to 35° 

 w r est of north, or transverse to the strike. 



Llandeilo Flags. (No. 11 of Coast Section.) 



The Caradoc sandstone seem to have its natural termination about a quarter of a mile to the east 

 of Wooltack Park, the most western promontory of the mainland, where it graduates down into 

 thinly laminated, blue and greenish schist and flagstone, containing some of the fossils found in 

 the lowest Silurian rocks. At this point the cliffs are so difficult to examine, that I could not de- 

 tect any well recognised tvilobites ; but in the creek of St. Bride's Bay, called Muscle-wick Mouth, 

 on the opposite side of the promontory, and dipping beneath the great mass of Silurian strata above 

 described, I had great satisfaction in discovering a considerable mass of true Llandeilo flags. They 

 are exposed for upwards of half a mile, in cliffs easy of access at only one or two spots. They can, 

 however, be descended by rugged paths, in one of which, called the Welchman's road, I found spe- 

 cimens of the Asaphus Buchii, A. Tyrannus, and a species of Trinucleus. The flags are of their 

 usual dark colour, but they are more sandy and less calcareous than in parts of the county through 

 which we shall presently trace them. They are indeed chiefly made up of black, finely laminated, 

 shivery schist, with occasional strings, veins, and concretions of black and white calcareous spar. 

 They dip about 35° to S.S.E., and thus plunge under the greater mass of the Silurian System of the 

 Marloes promontory. At this spot, the Llandeilo flags are penetrated and partially overflowed on 

 their western flank by porphyritic greenstone. On the north, where they are in contact with the 

 Old Red Sandstone, the thinly laminated beds become thick masses in which traces of bedding are 

 with difficulty observed, and altogether they are muck altered, indurated, and traversed by many 

 veins, so I had little doubt that trap rock, though not visible, lies at very little depth beneath. On 

 the south, indeed, the beds are abruptly cut off by the trap extending from Marloes by West Hook 

 to the Hays Point, which rock appeared to break through and overlap these beds, but I could not 

 descend the cliff at the point of intersection. As the relations of these Llandeilo flags of Muscle - 

 wick Mouth, to the Old Red Sandstone and the trap, are highly interesting, they are explained in a 

 separate diagram. (PI. 35. f. 11.) The whole of this mass of Llandeilo flag has a slaty cleavage, 

 inclined at angles of 70° to 80°, dipping in the same direction as the beds, and as the latter fold 

 over from verticality to angles of 35°, the planes of cleavage and the laminae have an apparent coin- 

 cidence at one point. (See further observations on this head towards the close of this chapter.) 



Enough has now been said to show the existence in the coast cliffs of Marloes of a great suc- 

 cession of fossiliferous strata, among the lowest of which are flags containing the fossils of the 

 Llandeilo formation. Beds of sandstone, however, unquestionably older than these Llandeilo 

 flags, rise out in the furthest western rocky peninsula of the mainland called Wooltack Park. They 

 consist of hard quartzose grits and sandstones with a few impressions of encrinites, and they may 

 be considered, like those described near Llandeilo, as the true beds of passage into the Cambrian 



