LLANDEILO FLAGS AND BLACK LIMESTONE. 



397 



north-west to south-east, the same rock when it reappears at the Ffron and Llandewi 

 ridge, strikes at a right angle to its former direction, viz. from north-east to south-west, 

 whilst the ridges of Llampeter Felfrey run from east to west. These converging direc- 

 tions soon render the two ridges confluent, so that when viewed on the map they appear 

 like the prongs of a fork. Their union takes place to the north of Croynwydd and 

 Narbeth, whence the calcareous strata in their further prolongation, conform to the 

 westerly direction of the rocks in the central part of Pembrokeshire. 



The upper portion of these Llandeilo flags at the Land Mill near Cilrhiw, consists of vertical beds 

 of sandy flags with calcareous courses, the mineral characters agreeing precisely with those in the 

 upper beds at Golden Grove near Llandeilo, and the fossils are also identical, namely, Or this canalis, 

 with corals, and abundance of fragments of trilobites, including the Trinucleus Bigshii and the 

 large Asaphus Buchii. These beds alternate, first with thin bands of black limestone, and are finally 

 succeeded near Llampeter, by large, undulating, calcareous masses, dipping both to the north and 

 south. It is remarkable, however, that the chief inclination is to the north, as seen in the great lime- 

 stone quarries near Llampeter village. The limestone, dipping 35° to 40° north, is quarried through 

 a thickness of 60 to *]0 feet in beds both thick and thin. It is of the usual dark colour, traversed 

 by white veins, has few or no way-boards, and is largely extracted for burning to lime. In the pro- 

 longation of these strata through the fertile farm of Langwathan and the valley of the White Mill 

 stream, and also between Llampeter and the southern frontier, the characteristic fossils, including a 

 profusion of the Asaphus Buchii and A. tyra?inus, may be collected. In the loftier subsidiary ridges 

 of Pen-gaer and the Ffron, north of Llandewi Felfrey, these flagstones also contain, at intervals, 

 thick expansions of limestone sufficiently good to be burnt. Here also the dip is generally reversed 

 and unconformable, for as these hills trend from north-east to south-west, the most prominent 

 escarpments face to the south-east, as under Pengaer. On the whole, however, they may be de- 

 scribed as undulating or dome-shaped masses, since upon their north-western confines, the older 

 rocks plunge under them, dipping to the south and south-east. They sometimes expose the beds 

 of passage upwards into the Caradoc sandstone, as in nearly horizontal strata at Pen-biowin, a mile 

 and a half north of Narbeth, and at the Ffron, where the calcareous beds are again almost horizontal. 

 Here, though occasionally burnt for lime, they are usually extracted for road and coping-stones. 

 In the principal quarries, the backs of the beds expose a succession of small swallow holes or arched 

 cavities, a feature not often observed in limestone of this age. Fossils are abundant, particularly 

 the Orthis canalis, and corals. At Whitley, north-east of Narbeth, where the calcareous ridges of 

 Llandewi and Llampeter unite ; the old quarries expose violently contorted strata, dipping both to 

 the north and south, and appearing to rise from beneath the younger formations. From this point 

 in their course to the westward, the Llandeilo flags become gradually less important, for the lime- 

 stones thin out and the fossils disappear. Their southern frontier or upper member is, however, 

 clearly defined, about half a mile north of Robeston Wathen, where beds of black limestone, two, 

 three, and five feet thick, alternate with dark grey shale, and passing upwards into sandy flags, dip 

 at an angle of 40° under the Caradoc sandstone before mentioned. Fossils at this spot are plen- 

 tiful, particularly a singular serpuline body, and a new species of coral. 



We here perceive that the calcareous matter is thinning out, showing a tendency to run into 

 concretions ; and in following the direction of the strata we lose, for a time, all traces of it. The 

 limestone, however, again emerges directly in the western prolongation of these beds at Sholeshook, 



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