i. THE VOLCANO OF BUILTH AND ITS NEIGHBOURS

So far as the available evidence goes, the most important volcanic centre down the eastern side of Wales during the Llandeilo period was one which lay not far from the centre of the long line of vents just referred to. Its visible remains form an isolated tract of hilly ground, some seven miles long, and four or five miles broad, immediately north from the town of Builth. This area is almost entirely surrounded by unconformable Upper Silurian strata, so that its total extent is not seen, and may be much more considerable than the area now laid bare by denudation.

The volcanic rocks of Builth were first described in the "Silurian System." Murchison clearly recognized that they included some which were "evolved from volcanic apertures during the submarine accumulation of the Lower Silurian rocks," and also "unbedded volcanic masses which had been intruded subsequently, dismembering and altering all the strata with which they came in contact."[177] These igneous rocks were mapped in some detail by the Geological Survey, and their general relations were expressed in lines of horizontal section.[178] They were likewise described by Ramsay in the _Catalogue of the Rock-specimens in the Jermyn Street Museum_, specimens of them being displayed in that collection.[179] The tuffs and lavas were distinguished, and likewise the intrusive "greenstones." But no attempt was made towards petrographical detail.

[Footnote 177: _Silurian System_, 1839, p. 330. The occurrence of "trappean ash" with fossils in the Builth district was noticed by De la Beche, _Mem. Geol. Surv._ vol. i. (1846), p. 31.]

[Footnote 178: See Sheet 56 of the one-inch map and Sheets 5 and 6 of the Horizontal Sections.]

[Footnote 179: _Catalogue of Rock Specimens_, 3rd edit. 1862, p. 36 _et seq._]

This interesting district has recently been studied by Mr. Henry Woods,[180] who has grouped the igneous rocks in probable order of appearance, as follows:--1st, Andesites; 2nd, Andesitic ash; 3rd, Rhyolites; 4th, Diabase-porphyrite; and 5th, Diabase.

[Footnote 180: _Quart. Journ. Geol. Soc._ vol. l. (1894), p. 566.]

Some of the andesites are described as intrusive in the Llandeilo strata. The ash in its lower part contains numerous well-rounded pebbles of andesite, usually five or six inches in diameter, but sometimes having a length of two feet. It contains fossils (_Orthis calligramma_, _Leptæna sericea_, _Serpulites dispar_, etc.), and as it is overlain with shales containing _Ogygia Buchii_, it may be regarded as probably of Lower Llandeilo age. The rhyolites are feebly represented, and some of them may possibly be intrusive. Among them a nodular variety has been noticed, the nodules being solid throughout, varying up to two inches in diameter, and formed of microcrystalline quartz and felspar, with no trace of any radial or concentric internal arrangement. The diabase-porphyrite, the most conspicuous rock of the district, is intrusive in the andesites and ashes, and occurs in four separate masses or sills. The diabases are all intrusive and of later date than any of the other igneous rocks, and as they traverse also the Llandeilo shales, they are probably considerably later than the previous eruptions. But as they do not enter the surrounding Llandovery and Wenlock strata, they are regarded by Mr. Woods as of intermediate age between the time of the Llandeilo and that of the Upper Silurian formations.

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.]