390 



UPPER SILURIAN ROCKS. — PEMBROKESHIRE. 



the principal coast section in which this system appears, affords us one example of 

 shale, in parts calcareous, containing the usual corals and some of the shells of this 

 formation. With such variations in the Upper Silurian Rocks, the Lower Silurian 

 possess, on the contrary, their usual mineral impress, and many characteristic fossils ; 

 the limestone of the age of that of Llandeilo, swelling into masses of even greater thick- 

 ness than in any previous part of its course. Under these circumstances, as it is not 

 always possible to define the course of each formation, I shall first describe the system, 

 as in Caermarthenshire, under the heads of Upper and Lower Silurian Rocks, in doing 

 which I shall indicate those points where the strata can be identified in greater detail. 



Upper Silurian Rocks. 



These rocks, though forming only slender bands, rise, as already stated, with distinctness on the 

 eastern borders of Pembrokeshire from beneath the Old Red Sandstone of Cyme and Tavern Spite. 

 The peculiar junction beds of the Old Red Sandstone, before described, are succeeded by thin-bedded, 

 finely laminated, argillaceous sandstone, sometimes ferruginous, and the laminse of separation occa- 

 sionally coated with black oxide of iron. They graduate downwards into other thin, grey, sandy 

 strata, some of which are flaglike and calcareous as at Parkau, between Tavern Spite and Llan- 

 dwror. These beds are not very fossiliferous, but I have detected the Calymene Blumenhachii and 

 Terebratula Navicula of the Ludlow formation. Though such relations of the Silurian rocks to the 

 Old Red Sandstone are observable to the east and south-east of Cyme, on the sides of both the old 

 and new road to St. Clears, the section in descending order is suddenly disconnected by a powerful 

 dislocation, ranging from south-west to north-east, along the north-western face of these elevated 

 tracts, whereby the relations of the Upper to the Lower Silurian Rocks are much obscured and 

 overlaid by detritus. This line of elevation, it will be observed, is at right angles to another which 

 passes along the valley of the Taaf. 



The passage of the Old Red Sandstone into the Upper Silurian Rocks is again well seen at Nar- 

 beth. A transverse section from the edges of the Old Red Sandstone to the highly inclined Silurian 

 strata on which the castle is built, presents in descending order sandy, yellowish red grits with 

 a base of felspar, constituting the junction beds of the Old Red Sandstone, succeeded by dull 

 greenish and ferruginous, micaceous, soft, thin-bedded sandstone, passing into fragile, sandy, grey 

 flags with shale, and containing under the castle a few fossils (such as Calymene Blumenhachii, 

 Asaphus caudatus, Terebratula Navicula) . As the dip varies from 70° to 90°, a considerable 

 thickness of strata is represented in the short transverse section exposed on the sides of the road. 

 (See PI. 35. f. 4.) 



To the westward of Narbeth the junction of the Silurian rocks and the Old Red Sandstone is so 

 obscured by dislocation and denudation, that it is impossible to determine precisely the age of each 

 underlying stratum which is successively brought into contact with the Old Red Sandstone. Thus 

 at Canaston Wood 1 the fossils contained in the Silurian Rock compel us to place it in the Caradoc 



1 Among the fossils at Canaston wood are Productus sericeus, Orthis radians, Orthis aperturata, Orthis ele- 

 gantula, Dalm., Spirifer, n.s. All these species are found in Shropshire, Radnorshire, and Caermarthenshire, in 

 beds of true Caradoc sandstone. 



