[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.
These lower lavas, according to the measurements of Jukes, are overlain by more than 4000 feet of sedimentary strata before the upper or Bala volcanic series is reached. Three successive "ash-beds" constitute this upper series. Of these the lowest band, about 50 or 60 feet thick, was named a "greenstone ash" in contradistinction to a felstone ash, and was not traceable for more than a short distance. Above it, after an intervening thickness of several hundred feet of sedimentary strata, comes a second and much more continuous band of tuff, known as the "Lower ash-bed," about 100 feet thick on the west front of the Berwyn range. Still higher, after an interval of about 1500 feet of slates, lies the "Upper ash-bed," which on the same line of section has a thickness of about 200 feet. This is the most persistent of all the volcanic horizons, for it can be followed continuously round the whole range of the Berwyns until it is overlain by the Carboniferous Limestone near Selattyn, a distance of not less than twenty-four miles. The same band, but much more feebly developed, has been traced through the faulted country on both sides of Bala Lake, where it formed a useful platform in the investigation of the complicated geological structure of that area. Along the north side of the Berwyn Hills another thin band of tuff lies from 150 to 200 feet still higher up in the series, and has been traced for a distance of about twelve miles. The Bala limestone comes in about 800 or 1000 feet above the "Upper ash-bed."
Fig. 57.—Section across the Berwyn Hills. (Reduced from Horizontal Section, Geol. Surv., Sheet 35).
L, Llandeilo Flags; B, Bala group; B L, Bala Limestone; t t, volcanic tuffs; D, intrusive "greenstones."
Besides the rocks now enumerated, the Survey maps show the intercalation of four or five sheets of "greenstone," which are represented as following with marked regularity the strike of the strata. Until these sheets have been more precisely examined it is impossible to decide regarding their true petrographical character, or to determine whether they are sills, or interstratified lavas, or include rocks of both these types.
V. THE VOLCANOES OF ANGLESEY
We now turn to another part of the country, about which much has been written and keen controversy has arisen. In the centre of Anglesey, among the rocks grouped together by the Geological Survey as "altered Cambrian," there occur masses of breccia, the probable volcanic origin of which was, so far as I know, first suggested by Professor Hughes.[222] Dr. Callaway regards them as pre-Cambrian,[223] while Professor Blake places them in his "Monian system."[224] When I went over them some years ago, I accepted the view that they are volcanic agglomerates.[225] Subsequent examination, however, has convinced me that notwithstanding their remarkable resemblance to true agglomerates they are not really of volcanic origin, but are essentially "crush-conglomerates," like those in the Isle of Man, so well described by Mr. Lamplugh.[226]
[222] Proc. Camb. Phil. Soc. vol. iii. (1880), p. 347.