The stratigraphy of Ordovician volcanic rocks on the southern and eastern flanks of the Harlech Dome in Merionethshire

The stratigraphy of nine areas of Ordovician volcanic rocks flanking the Harlech Dome is dcscribed and a correlation of the Ordoyician volcanic succession from Arenig to Cader Idris proposed. Nine formations are distinguished which together form a distinct lithostratigraphic unit termed the Aran Volcanic Group. At least six disconformities can bc recognised within the Group.