Above the Arenig group with its voluminous volcanic records comes the great group of sediments known as the Llandeilo formation, in which also there are proofs of contemporaneous volcanic activity over various parts of the sea-floor within the site of Britain. We have seen that in the south of Scotland the eruptions of Arenig time were probably continued into the period of the Llandeilo rocks, or even still later into that of the Bala group. But it is in Wales that the history of the Llandeilo volcanoes is most fully preserved. A series of detached areas of volcanic rocks, intercalated among the Llandeilo sediments, may be followed for nearly 100 miles, from the northern end of the Breidden Hills in Montgomeryshire, by Shelve, Builth, Llanwrtyd and Llangadock, to the mouth of the Taf river. But some 35 miles further west another group of lavas and tuffs appears on the coast of Pembrokeshire, from Abereiddy Bay to beyond Fishguard. The want of continuity in these scattered outcrops is no doubt partly due to concealment by geological structure. But from the comparative thinness of the volcanic accumulations and their apparent thinning out along the strike it may be inferred that no large Llandeilo volcano existed in Wales. There would rather seem to have been a long line of minor vents which in the south-east part of the area appear to have only discharged ashes. Certainly, if we may judge from their visible relics, these eruptions never rivalled the magnitude of the discharges from the Arenig volcanoes that preceded, or the Bala volcanoes that followed them. 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.