354 



CARADOC SANDSTONE, CAERMARTHENSHIRE. 



singular troughs above described. So various indeed are the positions into which the strata are thrown 

 by these curvatures, that it is remarkable the planes of deposit and those of slaty cleavage should not 

 occasionally coincide ; for instances of this coincidence, though never observable in North Wales, 

 will hereafter be shown to occur in Pembrokeshire. 



There is no spot, indeed, within the range of the Silurian rocks, in which the di- 

 stinctions between cleavage joints and bedding (as laid down by Professor Sedgwick,) 

 are better defined than in the rugged tract of Noeth-grug ; and the preceding wood-cut, 

 made from a rough sketch of my own, will enable the reader to form some estimate 

 of the geological interest attached to this scene 1 . 



The right bank of the Sowdde from Pont-rhydd-sant to Llangadock, offers only low and obscure 

 undulations of schistose beds ; but on the left bank a transverse section, parallel to the river pro- 

 ceeding from Blaen-dyfFrin-garn to Tan-yr-alt, exposes a full succession of Caradoc sandstone, con- 

 taining in their upper beds Productus sericeus, var., and other Caradoc fossils ; and towards their 

 base flagstones with Asaphus Buchii, &c. A zone of sandstone, therefore, is here interpolated 

 between the Upper Silurian Rocks and the true Llandeilo flags, commencing on the sides of the low 

 hillock of trap at Blaen-dyfFrin-garn, and expanding to the south-west into the arid hill of Carn- 

 goch. The operation by which a great portion of the ridge has been changed from sandstone into 

 granular quartz rock, will presently be spoken of in describing the trap of this spot. The sand- 

 stone contains some of the same organic remains as its equivalents in Shropshire, and other parts ; 

 and though much disturbed upon the sides of the trap, and in Carn-goch, yet in the hill of Carreg- 

 gwyn-hir, removed from its influence, the strata are regular, and plunge at 60° beneath the Upper 

 Silurian Rocks of the Tri-chrug. (PL 34, fig. 6.) From this point the sandstone begins to thin out, 

 and the strike to undergo changes. At Tre-gib the direction is from east and by north to 10° south 

 of west, overlying Llandeilo flags at a high angle. These sandstones traverse the high road from 

 Llandeilo to Swansea, near the bridge Pont-ladies, with a strike from east to west, and a dip of 80° 

 south. They consist of small fragments of grey quartz and decomposing felspar, cemented by a 

 ferrugino-silicious paste, and contain a thin layer of white pipe-clay. Similar sandstones are again 

 found in the lower and western sides of the hill of Golden Grove, highly inclined, dipping under a 

 thin zone of the Upper System, and passing downwards into black trilobite flags. (PL 34, fig. 8.) 



Though the Silurian System is here so attenuated that it is scarcely practicable to follow with 

 precision any one of the formations individually, yet it is quite clear that these light-coloured sand- 

 stones represent and occupy the same position as the Caradoc sandstone of Shropshire. To the 

 south-west of Golden Grove they are obscurely developed ; but the hard quartzose sandstone and 

 fine conglomerates of Glas-coed near Llanarthney, and of Capel dewi, may belong to this division, 

 since they lie between the Upper Silurian Rocks and a zone of the Llandeilo flags. The same 

 quartzose beds are again traversed on the sides of a small brook at Nant-y-caws near Pont-pibwr, 



1 The laminae of deposit are the undulating lines, the joints are indicated by those lines which are at right 

 angles to the beds, and the cleavage planes are inclined at 70°. (See wood-cut, p. 352.) In many parts of the 

 environs of Llandovery, particularly in those occupied by schists void of fossils, it is almost impracticable to 

 separate the lines of cleavage, joints and bedding, without examining a large area, and by ascending to the 

 tops of the hills, where the most marked character of the rock is generally to be found. Where the slaty 

 cleavage is very pronounced, as at Noeth-grug, it is of course much easier to determine what is not the laminaa 

 of deposit than in these half amorphous, half slatified masses. 



