About nine miles in a west-south-westerly direction from the southern extremity of the Builth volcanic area, another much smaller exposure of igneous rocks has been mapped by the Geological Survey at the village of Llanwrtyd. This tract is only about three miles long and half a mile broad. The volcanic rocks are represented as consisting of three or more bands of "felspathic trap" interstratified in the Lower Silurian strata, and folded into an anticline along the ridge of Caer Cwm. No published line of section runs across this ground, and the band of rock does not appear to have been described.[181] [Footnote 181: The locality is referred to by De la Beche, _Mem. Geol. Surv._ vol. i. p. 31, and by Ramsay in the _Descriptive Catalogue of Rock-specimens in the Museum of Practical Geology_, 3rd edit. p. 38, but no specimens from it are in the collection.] Seventeen miles to the south-west a still feebler display of intercalated volcanic material occurs in the Llandeilo formation near the village of Llangadock. The Geological Survey map represents one or more bands of ash associated with limestone, and thrown into a succession of folds. In the _Horizontal Section_ (Sheet III. Section 3) a band, 100 to 200 feet thick, of "trappean ash" with fossils is shown among the shales, limestones and grits, and in the _Catalogue of Rock-specimens_ the same rock is referred to as brecciated ash in connection with specimens of it in the Museum, which are described as not purely ashy, but containing many slate-fragments and broken felspar-crystals together with organic remains.[182] [Footnote 182: _Op. cit._ p. 38.] About twenty-four miles still further in the same south-westerly direction, two patches of "ash" are shown upon the Survey map, near the mouth of the river Taf. No description of these rocks is given.[183] [Footnote 183: One of the patches was shown by J. Phillips in _Horizontal Section_, Sheet III. Section 6, as a "felspathic trap," near which the shales are bleached. The map, however, was subsequently altered, so as to make the igneous rocks pyroclastic.] ii. THE VOLCANOES OF PEMBROKESHIRE In north-western Pembrokeshire, the observations of Murchison, De la Beche and Ramsay showed the existence of an important volcanic district, where numerous igneous bands are interstratified among the Lower Silurian rocks, over an area extending from St. David's Head for thirty miles to the eastward.[184] On the maps of the Geological Survey, lavas, tuffs, sills and bosses were discriminated, but no description of these rocks was published. Since the publication of the Survey map very little has yet been added to our information on the subject. [Footnote 184: See _Silurian System_, p. 401; Sheet 40 of the Geological Survey Map; _Memoir of A. C. Ramsay_, p. 232 _et seq._; De la Beche, _Trans. Geol. Soc._ 2nd series, vol. ii. part i. (1826), p. 3.]