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The last mentioned strata consist of dark-coloured micaceous flags, frequently calcareous, with a great thickness of shales, generally black, below them. The same beds are also seen at Builth, in Radnorshire, where they are interstratified with volcanic matter. Diplograpsus pristis, Hisinger. Rastrites peregrinus, Barrande. Scotland; Bohemia; Saxony. Diplograpsus folium, Hisinger.
According to Professor Ramsay, their thickness is about 3300 feet in North Wales, including those of the Lower Llandeilo. The lavas are feldspathic, and of porphyritic structure, and, according to the same authority, of an aggregate thickness of 2500 feet. Arenicolites linearis, Hall. Arenig beds, Stiper-Stones. a. Didymograpsus geminus, Hisinger, sp.
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