LOWER PALEOZOIC ROCKS OF HATERFORDWEST. 477 



running eastward from Eoch Castle, north also of the Pre-Cambrian 

 ridges, parallel with this, are black iron-stained slates, weathering 

 oHve-grey or yellowish, generally dipping north. They are well 

 seen near Leweston, Trefgarn Bridge, and Spittal Cross. At Lewes- 

 ton Old Mill they have yielded : — 



Agnostus pisiformis, Linn. I Olenus spinulosus, WahZ. 

 , yar. socialis, Tidlh. \ 



At Trefgarn Bridge the following very important section occurs in 

 a quarry by the roadside, close to the fourth milestone from Haver- 

 fordwest (fig. 2). 



The shales are considerably disturbed, and contain a fair number 

 of fossils of the same species as those found at Leweston Old Mill. 



The conglomerate adheres to an ashy-looking rock of Pre-Cambrian 

 (?) age, with nearly vertical divisional planes, the origin of which we 

 were unable to determine. 



The fossils found in these two localities prove that these beds are 

 Lingula Plags. Dr. Hicks has recorded the presence of Lingula 

 ^ Flags about this spot (Q. J. G. S. vol. xxxv. p. 287), but gives no 

 fossil list ; and it is interesting to find that his determination of the 

 age of the beds, based presumably upon lithological characters, is 

 fully borne out by the fossil evidence. 



To the south of the great fault, much newer beds occur, so that 

 we are unable to record the occurrence of Tremadoc and Arenig 

 fossils in the area under consideration. 



2. Didymogy^aptus Shales. — These beds occur in the complicated 

 anticlinal to the east of Narberth, and, next to the Lingula Plags, 

 are the oldest beds we have met with in the tract of country we 

 have examined. They consist of black graptolite-shales of the 

 ordinary type, crowded with "tuning-fork" Didymogmpti, and con- 

 taining also small homy brachiopods and fragments of trilobites. 

 Didymograptus Murcliisoni occurs in abundance. That these beds 

 are underneath the Llandeilo limestone is shown by their occurrence 

 in an anticlinal arch between the limestone of Llan Mill and that 

 of Lampeter Yelfry. The southern arch of this anticlinal is vertical, 

 and even reversed in places, but it is indicated as an anticlinal in 

 the horizontal section No. 2 of the Oeological Survey. The lime- 

 stone of Lampeter Yelfry is a faulted synclinal, and to the north 

 occurs another anticlinal, between Lampeter Yelfry and Llandewi 

 Yelfry, and here again the DidymograiJtKs-shales are found, and have 

 yielded Didymograpti by the roadside west of '•' LI " in " Llandewi 

 Yelfry." 



The same fossils are found in similar shales near Whitland, below 

 the limestone, but at some distance from its outcrop. 



Prof. Lapworth (Ann. & Mag. Nat. Hist. Ser. 5. vol. iii. p. 59) also 

 places the Didymograpiiis-shales of this area below the Llandeilo 

 limestone. 



3. Llandeilo Limestone. — The well-knovm black limestone of Llan 

 Mill, Lampeter Yelfry, Llandewi Yelfry, &c., interstratified with 

 black shales. It is frequently crowded with fossils, and the following 



