224 MESSES. T. C. CANTRILL AND H. H. THOMAS ON THE [May I906, 



The south-eastern part of the plateau is occupied by the red 

 marls of the Lower Old Red Sandstone, the base of which crosses 

 the district from north-east to south-west, and rests with great 

 unconformity on the various Ordovician and Cambrian rocks which 

 make up the north-western part of the plateau. 



II. Historical Summary. 



The earliest references to the igneous rocks of the district appear 

 to be the observations published by Murchison in 1839, in his 

 4 Silurian System.' In chap, xxviii, 1 on Caermarthenshire, under 

 the heading ' Trap of Castel cogan, &c/, he calls attention to the 

 outburst of trap which occurs between the rivers Towy and Taf, 

 pointing out that the trap has been erupted along a north-east 

 and south-west fissure, and is traceable at intervals for 3 or 4 

 miles, from Glog, past Capel [Bethesda] and Llangynog, to Castell 

 Cogan and Gallt-y-minde on the left bank of the Taf. He notices 

 the felspathic character of the rock [rhyolite] at Gallt-y-minde 2 

 and Castell Cogan, and observes that it contains minute crystals of 

 iron-pyrites. Having thus briefly noted the course of the supposed 

 outburst, as followed from north-east to south-west, he retraces 

 his steps and points out that, after subsiding for a while, the trap 

 reappears at Capel [Bethesda] in the form of a concretionary 

 felspar-rock [rhyolite]. He observes that the line of outburst is 

 flanked by masses of volcanic grit, passing into a felspar-conglo- 

 merate [Tetragrajptus-Grnta]. At Glog this conglomerate had been 

 deeply quarried, and the base of the quarries exposed 



'a very hard rock, full of oblique rents and cracks, made up essentially of 

 compact felspar, for the most part concretionary, whilst in the upper part, 

 small pebbles of quartz become apparent, and are frequent near the summit. 

 This felspar conglomerate, with quartz pebbles of the size of eggs, varies from 

 that state to a grit, and when deeply laid open, consists of a concretionary and 

 solid mass of felspar.' 3 



He points out that the strata along the line of eruption near 

 Castell Cogan are extensively fractured ; that the associated 

 4 Silurian ' grits are in places vertical and even inverted, and are 

 unconformably covered by the Old Red Sandstone. 



On his map Murchison shows four patches of ' trap ' in the 

 4 Silurian ' rocks of the Llangynog district, and three patches of 

 4 volcanic grit/ and notes the occurrence of ' trap-conglomerate/ 

 Of his four patches of ' trap,' one is evidently the rhyolite of 

 Castell Cogan ; another, the rhyolite of Capel Bethesda ; a third is 

 presumably the mass of rhyolite which he appears to have seen 

 in the deep quarry at Glog ; while the fourth coincides in position 

 with an elliptical outcrop of Tetragraptus-Qxrlt at the old windmill 



1 Pp. 365-66. 



2 This name does not appear on the Ordnance-maps ; Gallt-y-minde is the 

 steep slope west of Castell Cogan. 



3 ' Silurian System » 1839, p. 365. 



