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LLANDEILO FLAGS AND BLACK LIMESTONE. 



These sandstones with much incoherent shale are traceable to the westward of Haverford, through 

 Sutton and Stember Commons, to the flanks of the Norton coal-field, near which they contain Tri- 

 nuclei, trilobites peculiar to the Lower Silurian Rocks. To the eastward they pass to the north of 

 Narbeth by Arnol's Hill and Robeston Wathen, from which point they become much attenuated and 

 give way to a great expansion of Llandeilo flags. 



The great dislocations which have taken place along the southern frontier of the Silurian System, 

 in its course from Narbeth to Haverfordwest, have been already alluded to ; and it has been shown, 

 that although the uppermost Silurian rocks are exposed at Narbeth, yet at most of the intermediate 

 places upon this line, there are such breaks in the succession, that the strata of the age of the 

 Caradoc sandstone, carboniferous limestone, and Old Red Sandstone are brought into contact. 

 These dislocations seem to have increased in intensity in that great elbow into which they have 

 been thrown to the north-east of Narbeth. (See Map.) Instead of a well- developed zone to mark 

 the Upper Silurian Rocks, like that which has been pointed out in other places, we there find the 

 break has been so powerful, that not only can we observe no traces of the rocks which are so largely 

 exposed within three or four miles, forming a passage into the Old Red Sandstone, but the latter 

 is thrown off in a great dome-shaped mass, flanked on the north-west by a very slender band of 

 grey sandstone and shale, which alone separates it from the Llandeilo flags. (PI. 35. f. 6.) This is 

 most clearly seen in crossing from Cirihiw by the Land Mill, or in any parallel traverse from the 

 old red of Cynic and the hills north of Tavern Spite to the parish of Llampeter Felfrey ; and, al- 

 though, for the most part, the line of junction between the Old Red and Silurian Systems is obscured 

 by vast accumulations of detritus, in several spots are indications of faults, producing unconform- 

 able positions of the strata. One of the faults, passing to the north of Cirihiw by Land Farm, has 

 a direction from 10° north of west to 10° south of east, whilst another, traceable from Pen-a-bac by 

 Cwm-coaly, trends from north-east to south-west. 



We may presume that the hard quartzose and flaglike grits, quarried at Middleway, and ranging 

 thence by Pen-y-graig to the woods above the Taaf, form a portion of the Caradoc sandstone, 

 since they greatly resemble those of Llandwror in the adjacent part of Caermarthenshire, and 

 occupy the same place in the series. These, being upon a line of great dislocation, are arched, 

 snapped off, and in one part being thrown over, they dip north-west at an angle of 60°. 



Llandeilo Flags and Limestone-— inland course. 



The fullest development of the Llandeilo flags in Pembrokeshire, and the largest 

 masses of limestone associated with them, are displayed in the parishes of Llampeter 

 Felfrey and Llandewi Felfrey. The formation is there nearly three miles wide, occupy- 

 ing two tracts, of undulating fertile hills, separated by a brook, which flowing from 

 south-west to north. east, falls into the Taaf near Pont-loyria. The subordinate lime- 

 stone is similar to that of Clog-y-frain in Caermarthenshire (p. 358.), from which it is 

 separated by a denuded tract from three to four miles in breadth, through which the 

 Taaf winds, and in the centre of which is the village of Eglwysfair. This valley of 

 elevation marks powerful dislocations corresponding to the great fault by which the 

 Cynic promontory of Old Red Sandstone has been projected to the north-east, for by 

 inspecting the map it will be seen, that as the limestone of Clog-y-frain trends from 



