aUARTZOSE AND CALCAREOUS SLATES OF CAERMARTHENSHIRE. 361 



it abounds) l , is the only simple mineral to be found throughout large tracts of these slates, the 

 occurrence of ores of real value, such as at Nant-y-moen, being a very rare phenomenon in Caermar- 

 thenshire. In the higher parts of the range called Llansadarn mountain and Carreg-fawr, there are 

 strong sandstone and grits passing into quartzose conglomerates, similar to those in the gorges of 

 the Elan and Wye near Rhayder. If we attempt to follow any one of these bands of grit or con- 

 glomerate to the south-east or north-west, all traces of it are soon lost, the prevailing rotten slate 

 rising to the surface ; but pursuing the direction of the strata, we again meet with similar quartzose 

 masses, such for example are the rocks of Taliaris warren, distant five or six miles from those of 

 Llanwrda and Llansadarn. These quartzose grits and conglomerates are therefore only irregular 

 aggregates or large concretions, where the amount of coarse materials was not sufficient to form 

 continuous strata. As the schistose beds are generally fragile, rarely affording slates worth extract- 

 ing, and as there is not a trace of limestone in the region, these coarse sandstones are the only 

 useful rocks of this part of Caermarthenshire ; and hence they are extracted both for the roads and 

 rough masonry. 



In the neighbourhood of Caermarthen, hard grits of this class are quarried at Galli-cistaniog on 

 the left baiik, and near Abergwylli on the right bank of the Towy. I have before remarked that in 

 the neighbourhood of Caermarthen the Silurian System dwindles to a narrow obscure zone, which 

 is strikingly proved by the proximity of the edge of these slaty greywacke rocks to the Old Red 

 Sandstone. (See Map.) I examined these Cambrian rocks towards the interior by traverses from 

 Llandovery to Llampeter, and from St. Clears to Newcastle Emlyn j but in no portion of the 

 wide space between these places have I detected any striking variety of mineral structure ; the 

 whole tract being occupied by schists, grits and sandstones, more or less impressed with a slaty 

 cleavage, ranging generally from north-east to south-west, and dipping to the north-ivest. This 

 cleavage is rarely perfect, though slates are sometimes worked. Between Llampeter and Newcastle 

 Emlyn the strata occasionally change their prevalent direction from E.N.E. to W.S.W. to east 

 and west. Between Caermarthen and Mydrim are similar oscillations from a south-westerly 

 to a true westerly strike ; but in the transverse section from St. Clears to Newcastle, on the 

 right bank of the Afon-gynin, there is no deviation from the westerly strike with which the rocks 

 of this age, in common with those of the Silurian System, are thence prolonged into Pembroke- 

 shire. (See Map.) This slaty system of Caermarthenshire (we may say the same of the adjoining 

 county of Cardigan) contains very little calcareous matter. The only exceptions occur at intervals 

 in a slaty zone, not far removed from the frontier of Silurian rocks. At Mydrim, four miles north 

 of St. Clears, hard dark-grey, thick flags, slightly calcareous, pass into imperfect concretions of a 

 very impure limestone, underlaid by schists and quartzose grits. In this calcareous mass I could 

 detect no fossils, but the geodes contain crystals of quartz, and the beds are traversed by* veins of 

 the same. It will be observed, by consulting the map, that these strata strike a little to the north 

 of west and dip northwards 45°. Similar calcareous flagstones, quarried for roofing slates, and of 

 about the same age as those at Mydrim, occur at Pont-y-glaen and north-west of Abergwylli near 

 Caermarthen, but they are apparently equally void of organic remains. These beds dip 35° north, 



1 I would partially except from this character those tracts where iron pyrites, by its decomposition, produces 

 medicinal springs, (see pp. 34, 155, &c), as well as those districts where the pyritous clays and marls have 

 been used for the manufacture of alum j but pyritous clays, as far as I have observed, are invariably of slight 

 agricultural value. 



