Among the thick group of sedimentary formations which overlies the great volcanic ridge of the Arans and Arenig, and undulates eastwards across the Bala Valley, occasional thin intercalations of tuff point to the existence of another centre of volcanic activity which lay somewhere in the region of the Berwyn Hills. The structure of this ground, first indicated by Sedgwick, was investigated in detail by J. B. Jukes and his colleagues, whose work was embodied in the Maps, Sections and Memoirs of the Geological Survey.[221] The distinguishing characteristics of the volcanic rocks of this district are the occurrence of both lavas and tuffs as comparatively thin solitary bands in the midst of the ordinary sediments, and the persistence of these bands for a distance of sometimes more than 24 miles. The position of the vent or vents from which this extensive outpouring of volcanic material took place has not been revealed. As the bands tend to thin away eastwards, it may be surmised that the chief focus of eruption lay rather towards the west, perhaps under the trough of Upper Silurian strata somewhere in the neighbourhood of Llandderfel. There was probably another in the Hirnant district. [Footnote 221: See Sheet 74 of the one-inch map; Sheets 32, 35, 37 and 38 of the Horizontal Sections; and chapter xxxi. of the _Memoir_ on the Geology of North Wales.] The mapping of the officers of the Survey showed that in the Berwyn Hills there are representatives of both the great volcanic periods of North Wales. A lower series of "felstones and greenstones" probably belongs to the older period, which began towards the end of Cambrian time and lasted in some districts even into the time of the Llandeilo formation. An upper group of tuffs, lying among the Bala rocks, is evidently equivalent, on the whole, to the much thicker volcanic series of the Snowdon region. The lowest visible volcanic rocks occur among the hills to the north-west of Llanrhaiadr yn Mochnant. They are described as consisting of felstone of a pale greenish-grey colour and compact texture, like those of Arenig, and ashes distinctly interstratified with the slates. No exact petrographical examination of these rocks has yet been made. From the account given in the Survey _Memoir_ there appears to be here a group of lavas and tuffs intercalated in Llandeilo perhaps partly in Upper Arenig, strata which form the broken dome of the Berwyn anticline. The lavas are represented as lying on four or five platforms, a single band reaching a thickness of 300 feet and separated from the next band by sometimes 1000 or 1500 feet of non-volcanic sediment.