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Abstract

Tectonic and stratigraphical relationships of the Snowdon Volcanic Series are described from a critical locality near Beddgelert. Here a Caradocian pre-cleavage fault of Caledonoid trend is identified and it is suggested that this structure was important in determining the disposition of the largely subaerial Caradocian volcanic rocks. Regional considerations suggest that while basic Ordovician volcanic rocks of Wales are largely pillow lavas and therefore submarine, the acid volcanic rocks are mainly ignimbrites and therefore subaerial. It is suggested that such subaerial vulcanicity owes its origin to the build up of magma blisters.