350 



DISLOCATIONS OF THE UPPER SILURIAN ROCKS. 



strata containing these Lower Ludlow fossils are within a field's breadth of the Old Red Sandstone, 

 the intervening distance being apparently occupied by a hardish green or grey sandstone, with few 

 or no fossils, the equivalent of the Upper Ludlow. From Middleton Hall to the Black Pool on the 

 Towy, where the Upper Silurian Rocks pass that river in contact with the Old Red Sandstone, it 

 is still more impracticable to separate this Upper Silurian group into distinct members. Indeed 

 it is even difficult to separate the Upper Silurian Rocks from the Lower ; for the hills dwindle away, 

 calcareous matter disappears, and in great measure the fossils ; thus in short we appear to lose for 

 a time, the features which characterize those rocks through a range of nearly 100 miles. (PI. 34. 

 figs. 8 and 9.) 



Along this portion of their frontier, the Upper Silurian Rocks in the form of hard quartzose sand- 

 stones lie in disjointed masses, separated by a series of transverse faults, each portion usually 

 ranging from east to west, and dipping to the south, till they approach the Towy, when another 

 snap takes place and the strata have the appearance of being shifted back nearly half a mile, and 

 then projected against the left bank of the river, striking from north-east to south-ivest. (See 

 Map.) Here, however, and at Croes ceiliog in the road from Kidwelly to Caermarthen, is a per- 

 fectly conformable junction of the base of the Old Red Sandstone and the inferior grey-coloured 

 sandstone, dipping 40° south-east ; and again on the right bank of the Towy at Castel-moel is a 

 still clearer and more complete passage from the representative of the Ludlow rock into the over- 

 lying Old Red Sandstone. This Ludlow rock is a thick-bedded grey, quartzose, micaceous sand- 

 stone, dipping 45° to the south beneath the Old Red, and underlaid by black schists, the mass being 

 apparently void of fossils. 



The Towy here escapes through a rent in strata once continuous, but which have been thrown 

 into divergent directions by a fault ; so that the case is precisely similar to those of the Severn at 

 Coal Brook Dale, of the Wye near Builth, and of all the intervening streams which escape from 

 north-west to south- east. 



Between the rivers Towy and Taaf the Upper Silurian Rocks are still less exhibited, but being 

 connected with a line of trappean rocks they will be again alluded to. At two miles south of St. 

 Clare, on the right bank of the Taaf, we meet with slightly inclined beds of unaltered Upper Silurian 

 Rocks, which, passing into Pembrokeshire, will be best understood by reference to their description 

 in the next chapter, only here remarking that the river Taaf, as well as the Towy, flows in a great 

 transverse dislocation, to which more pointed allusion will be made in describing the Llandeilo 

 formation as it appears at Clog-y-frain, where it enters into Pembrokeshire. 



"Lower Silurian Rocks" 



We now return to the north-east of Caermarthenshire to observe the relations of 

 the underlying deposits, and to follow their course to the south-west. The Lower 

 Silurian Rocks of this county are in some places not easily separable from the Upper 

 by mineral characters, particularly in the environs of Llandovery, where undulating 

 hills of sandy and argillaceous schist can be proved to belong to the Caradoc sandstone 

 only by containing at wide intervals casts of the well characterized fossils of that 

 formation 1 . 



1 That this range of hills, including the Llandovery building- stone, is in the equivalent of the Caradoc sand- 



