BALA

See also:

• SERIES (a Latin word from serere, to join)

SERIES  , in geology, a series of dark slates and sand-stones with beds of limestone which occurs in the neighbourhood of Bala, Merionethshire, North Wales . It was first described by A . Sedgwick, who considered it to be the upper part of his Cambrian System . The series is now placed at the top of the Ordovician System, above the Llandeilo beds . The Bala limestone is from 20 to 40 ft. thick, and is recognizable over most of North Wales; it is regarded as the equivalent of the Coniston limestone of the Lake District . The series in the type area consists of the Hirnant limestone, a thin inconstant bed, which is separated by 1400 ft. of slates from the Bala limestone, below this are more slates and volcanic rocks . The latter are represented by large contemporaneous deposits of tuff and felsitic lava which in the Snowdon District are several thousand feet thick . In South Wales the Bala Series contains the following beds in descending order:—the Trinucleus seticornis beds (Slade beds, Redhill shales and Sholeshook limestone), the Robeston Wathen beds, and the Dicranograptus shales . The typical graptolites are, in the upper part, Dicellograptus anceps and D. complanatus; in the lower part, Pleurograptus linearis and Dicranograptus Clingani . In Shropshire this series is represented by the Caradoc and Chirbury Series; in southern Scotland by the Hartfell and . Ardmillan Series, and by similar rocks in Ireland . See CARADOC SERIES and ORDOVICIAN SYSTEM .