part 1] IGNEOUS AND ASSOCIATED HOCKS OF LLANWRTl'D. 17 



(2) Previous Work. 



The literature relating to this district is scanty. The only 

 description is that given by Sir Roderick Murchison. 1 He states 

 that the presence of mineral springs at Llanwrtyd Wells led him 

 to suspect the presence of igneous rocks in the neighbourhood, and 

 his investigations proved that such rocks did occur. His account 

 occupies two pages in the ' Silurian System ', and is accompanied 

 by two woodcuts by Mrs. Traherne. Curiously enough, the 

 description of the Llanwrtyd rocks is not repeated in ' Siluria ', 

 and the references in that work are very brief. 3 



The district was mapped by the Geological Survey on the 1-inch 

 scale, and the map (Sheet 56 S.W. ) was published in 1850. The 

 structure is fairly accurately shown, but no attempt is made to 

 distinguish between the breccias, ashes, lavas, and intrusions, all 

 being coloured as ' Felspathic Trap.' The structure is further 

 shown in Horizontal Sections 3 & 6. 



Prof. W. Gr. Fearnsides 3 has stated that 



' beds of Arenig age .... form the core of the Carneddau (Llandrindod-Builth) 

 mass, and appear also at Llanwrtyd . . . .' [and] ' at Llanwrtyd the succession 

 seems to begin with the tuning-fork graptolite-beds. In each area, as at 

 Arenig, the volcanic series begins with basic andesites and through rhyolites 

 passes to rhyolitic ashes before the oncoming of the Llandeilo Flag Series.' 



These remarks may apply to the Llandrindod-Builth area, but 

 they are certainly not true of the Llanwrtyd district. Other 

 references to the district are of a general nature, and will be 

 quoted, where necessary, in the sequel. 



Murchison's general description is so clear that we may, with 

 advantage, quote portions of it here. He writes : 



' I found a line of intrusive rock, about three miles in length and half-a- 

 mile in its greatest width, running, like the trap ridges of Radnorshire, 

 from north-east to south-west. A narrow and deep dell, through which 

 flows the rivulet Cerdin, divides this elliptical- shaped ridge into two moun- 

 tains, Caer-cwm and Garn-dwad, each about 1600 feet in height. At the 

 north-eastern extremity of Caer-cwm, trap is seen, for the last time, on 

 the banks of the little stream Nant-einon, alternating in thin courses with 

 slaty schists ; whilst at the south-western end of Garn-dwad the trap crosses 

 the Ithon between Llanwrtyd and the mineral spring, near a boss of 

 rock called Gwern-goch, upon the right bank of that river, and near the 

 farm-house of Dol-y-dymmor. In this ridge of Garn-dwad and Caer-cwm 

 the predominant character of the trap is porphyritic, and the following 

 varieties occur :....' (' The Silurian System ' 1839, loc. supra cit.) 



[Murchison distinguishes seven varieties of ' trap ' ; of these No. 7 is the 

 spilite (' amygdaloidal trap, cellular on the weathered surface'), 5 is the 

 intrusive rock (' greenstone '), 4 is probably the massive form of spilite, 

 the others are varieties of the ashes.] 



' Whilst the porphyritic trap occasionally peeps out in rugged bosses along 

 the summits and sides of the hill of Caer-cwm and Garn-dwad, the little 

 transverse dell of the Cerdin lays bare the true nature of this nucleus in a rock 

 called Craig-castell, which towers above the left bank of the stream. . . .' 



1 ' The Silurian System ' vol. i (1839) pp. 343-46. 



2 ' Siluria' 4th ed. (1867) pp. 57-58. 



3 ' North & Central "Wales ' (Geology in the Field) Jubilee Vol. Geol. 

 Assoc. (1910) pp. 796-97. 



Q. J. G. S. No. 313. o 



