The Ordovician Rocks of the Trefriw District (North Wales)

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Abstract
The area described is a rugged and mountainous tract on the eastern flanks of Snowdonia, lying largely between the villages of Dolgarrog, Trefriw, and Capel Curig. The eastern boundary is defined by the alluvial flats of the Conway valley, and the northern by the Afon Porthllwyd and Afon Eigiau. In the south and west, the mapping has been carried as far as was considered necessary for the elucidation of the geology. The area is shown on the six-inch Ordnance Survey Map, Caernarvonshire, Sheets XVIII and XIII, with portions of the quarter-sheets XIV SW. and XIX NW., and on the one-inch Geological Survey Map, Sheet 78 SE. (Old Series). The results of the present work are embodied in the map (PI. VII) accompanying this paper. The area was first described by Sir Andrew Ramsay (1866), who himself mapped the greater portion of the district. His colleague Aveline was responsible for the mapping of the country “east of Llyn Eigiau”. Ramsay regarded the whole succession of “slates and grits with interbedded felspathic traps, felspathic and sandy ashes” as “Caradoc or Bala” age, and recognized at the top of the volcanic series in the neighbourhood of Llyn Crafnant and Llyn Geirionydd a bed of muddy crystal tuff—“a dark blue or black slaty base containing numerous scattered phenocrysts of yellow felspar.” In this “topmost igneous bed” he found at Cwm Llanerch, 2½ miles south of Llanrwst, an abundance of Trinucleus concentricus , Leptæna sericea, and “other common Bala fossils”. Ramsay considered the upper ashes of

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