PASSAGE FROM LLANDEILO FLAGS INTO CAMBRIAN ROCKS. 357 



the house of Golden Grove, the principal strike is west-north-west, the strata being nearly vertical, or 

 dipping 80° under the younger group, but in the great quarry the strike is 29° north of west. These 

 strata belong to the younger portion of the formation, passing upwards into shelly sandstone, con- 

 taining, besides the Asaphus Buchii, the casts of shells generally characteristic of the Lower Silu- 

 rian Rocks, some of which are figured PI. 22 1 . 



Between Golden Grove and the point occupied by these flags on the opposite banks of the Towy, 

 is a distance of two miles, occupied by a flat alluvial plain, which has evidently been the scene of 

 great dislocations and subsequent denudation ; for on examining the rocks extending from Llan- 

 gathen through Grongar Hill, to the old castle of Dryslwyn, (see Map), the strike of the same beds 

 is almost completely reversed from that of Golden Grove. In this ridge, the prevailing south- 

 westerly direction is resumed, subject however to striking aberrations, as near the house of Berlland 

 y wull, where vertical beds of impure calcareous nodules and flag, strike from north to south. 



The best examples in Caermarthenshire, where this formation passes into strata of the 

 inferior system, are seen near the hamlet of Rhiw-yr-adar and along the western sides 

 of Llangathen and Grongar Hills. (PL 34. f. 8.) Here the flags, highly inclined and 

 contorted, graduate on their north-western face into irregular concretions of impure 

 limestone, occupying the same place as the limestone of Griig. They first alternate 

 with dark and light grey grits, a coral being sometimes discernible, and are succeeded 

 by beds of schist with soft sandstone and grit, containing casts of encrinites. There is 

 also an imperceptible passage from these sandy beds into still more ancient strata void 

 of fossils ; viz. the black schists, which, in common with Professor Sedgwick, I consider 

 the link connecting the Cambrian and Silurian Systems. Similar successions are ob- 

 servable near Llan-rhaidr, in Denbighshire, p. 307, and in Pembrokeshire. 



The Llandeilo flagstones appear the last on the right bank of the Towy, in little parallel ridges on 

 both banks of, and near the mouth of the river Cothi ; they are also obscurely seen near Llaneg-wad, 

 and in a marked manner at a hamlet called Nant-y-redig. Here the strata striking true north-east 

 and south-west, and dipping 70° to the south-east, are composed of dark impure limestone, with 

 some fossils, and are underlaid by less calcareous beds, traversed by numberless veins of pure white 

 calcareous spar, and these again by black flagstones, of two to four inches thick, having a ferruginous 

 weathered exterior. A little to the north-west of these quarries, or at the foot of the hill of Gait- 

 fawr, one of the lowest masses of limestone of this age, containing casts of large trilobites, protrudes 

 above the high road, exhibiting dark sub-crystalline concretions, overlaid on one side by an arched 

 stratum, a few inches thick, of hard, earthy limestone, and on the other by thin beds of shale, and 

 thick beds inclosing small calcareous nodules with corals and other fossils. The calcareous flags 

 of Nant-y-redig may be traced in their strike to the south-west, reappearing at Capel dewi, on 



1 In all the quarries around Llandeilo, and in many other situations both in Caermarthenshire and Pembroke- 

 shire, the Asaphus Buchii is not the only large trilobite peculiar to these flags and limestones. That species, 

 so easily distinguished by its round form (see PL 23 and 24.), is frequently associated with another, having a 

 caudal extremity more or less pointed, and differing from the A. Buchii in the termination of the post-abdominal 

 segments. To this species I have assigned the name of Asaphus Tyrannus. See Plate 24, figured from a splen- 

 did specimen presented to me by the Earl of Cawdor. 



2 y 



