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.