Owing to the effects partly of plication and partly of denudation, the rocks of the next volcanic episode in Wales, that of the Bala period, occupy a less compact and defined area than those of the Arenig group in Merionethshire. From the latter they are separated, as we have seen, by a considerable depth of strata,[197] whence we may infer, with the Geological Survey, that the eruptions of Arenig, the Arans and Cader Idris were succeeded by a long period of repose, the Llandeilo outbreaks described in the foregoing pages not having extended apparently into North Wales. When the next outbreaks took place, the vents are found to have shifted northwards into Caernarvonshire, where they fixed themselves along a line not much to the east of where the Cambrian porphyries and tuffs now appear at the surface. The lavas and ashes that were thrown out from these vents form the highest and most picturesque mountains of North Wales, culminating in the noble cone of Snowdon. They stretch northwards to Diganwy, beyond Conway, and southwards, at least as far as the neighbourhood of Criccieth. They die out north-eastwards beyond Bala Lake, and there can be but little doubt that they thin out also eastwards under the Upper Bala rocks. The lavas and tuffs that rise up on a similar horizon among the Bala rocks of the Berwyn Hills evidently came not from the Snowdonian vents, but from another minor volcanic centre some miles to the east, while still more remote lay the vents of the Breidden Hills and the sheets of andesitic tuff that probably spread from them over the ground east of Chirbury (Map II.). [Footnote 197: Estimated at from 6000 to 7000 feet, _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 131.] The Caernarvonshire volcanic group extends from north to south for fully thirty miles, with an extreme breadth of about fifteen miles; while, if we include the rocks of the Lleyn peninsula, the area will be prolonged some twenty miles farther to the south-west.