UPPER SILURIAN ROCKS — CAERMARTHENSHIRE. 



349 



The stone is of a dark grey colour and of sub-crystalline structure, penetrated by many veins of 

 white calcareous spar. The fossils found on the weathered surfaces at this spot and in the shale of 

 Tynewidd, not far distant, are principally those of the Wenlock formation. Among them are Spirifer 

 radiatus, Productus depressus, Orthis spinosa, Terebratula leviuscula, T. crebricosta, Lingula 

 lata, Orthoceras annulatum, and several corals of the formation. 



There can consequently be little doubt that this calcareous band, slender as it is, re- 

 presents truly the Wenlock limestone, a fact of high interest, as we have been unable 

 to mark the exact place of this rock in the strike of the Silurian System since we took 

 leave of it at Old Radnor, distant from this spot about thirty-six miles. Again, slight 

 traces of this limestone appear on the banks of the Sowdde between Pont-ar-llechau 

 and Rhyd-sant, but they are the last signs of any calcareous beds of this age in the 

 south-western prolongation of the Silurian rocks of Caermarthenshire. At Pwll-calch 

 the calcareous mass is flanked by walls of a hardish, flaglike, slightly micaceous sand- 

 stone of a dingy grey or green colour, traversed by veins of white crystallized quartz ; 

 proving how very materially the strata have changed their lithological characters since 

 we took leave of them in Salop and Hereford. 



From the right bank of the river Sowdde, and thence to the south-west and west below Caer- 

 marthen, the Upper Silurian Rocks thin out considerably, and are quite incapable of subdivision ; 

 for although a partially fossiliferous grey sandstone, which may be termed Ludlow rock, rises out 

 from beneath the tilestones of the Old Red Sandstone, there is little subjacent schist and shale to 

 represent the Lower Ludlow Rock, and no trace of subordinate limestone. This member of the 

 Silurian System in Caermarthenshire is so unlike the beds of the same age in Salop and Hereford 

 and Radnor, that what in these three latter counties is called a mudstone, is here represented by a 

 hard compact sandstone. 



Pursuing these Upper Silurian Rocks along the ridge of the Tri-chrug, we come to the transverse 

 dell by which the little river Cennen flows into the vale of the Towy, and where the strata have 

 been affected by a powerful fault. On the left bank of the Cennen the Upper Silurian Rocks are 

 thrown into a sharp ridge above Golden Grove, the strata containing fossils and dipping 80° E.S.E. 

 (PI. 34. f. 7-) Thence these beds, still maintaining the top of the ridge and in contact with the Old 

 Red Sandstone, resume their south-westerly strike for a short space, when they are suddenly thrown 

 out to the W.N.W. for a mile (see Map), and then regaining a true south-westerly strike, form the 

 promontory of Nelson's Monument. Between this promontory and Middleton Hall is another 

 transverse fault, in which the junction beds with the Old Red Sandstone have been snapped off and 

 moved considerably to the south-east. This fault has produced only the rudiments, if I may so 

 speak, of a transverse valley 1 , for it has caused a slight depression only, in which water runs, 

 although at a height of several hundred feet above the level of the Towy. It therefore differs only 

 from those deeper rents in which rivers flow through ridges, in not having been fissured and chan- 

 nelled to a sufficient depth. It was very interesting to detect in this spot many well characterized 

 fossils of the Lower Ludlow Rock, such as Phragmoceras nautileum, Productus depressus, P. 

 euglyphus, Avicula reticulata, &c. (PI. 10, et seq.) 



Owing to the small development of the Upper Silurian Rocks and their vertical position, the 



1 See similar observations on rudimentary transverse valleys in a subsequent account of the Valley of Woolhope. 



2 x 



