of County Down. In the Lough Mask district beds of this age are found, as in Wales, interstratified with volcanic lavas and tuffs. Other localities are known in counties Tyrone, Meath and Louth, also in Lambay Island. Early descriptions of the Bala series The Bala Series was described as a group of dark slates and sandstones with beds of limestone which occurs in the neighborhood of Bala, Gwynedd, in north Wales. It was first described by Adam Sedgwick, who considered it to be the upper part of his Cambrian System. The Bala limestone is from 20 to thick, and is recognizable over most of North Wales. The series in the type area consists of the Hirnant limestone, a thin inconstant bed, which is separated by . 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 Dicranograplus shales. The typical graptolites are, in the upper part, Dicellograptus anceps and D. complanatus; in the lower part, Pleurograptus linearis and Dicranograptus clingani. References Bibliography Nat. Hist., 5th series, vol. vi., 1880; Geol. Mag., 1889; C. Lapworth and W. W. Watts, Proc. Geol. Assoc., xiii., 1894; J. E. Marr, Geol. Meg., 1892; J. B. Marr and T. Roberts, Q. J. G. S., 1885; B. N. Peach and J. Home, Silurian Rocks of Great Britain, vol. I., 1899 Geologic formations of the United Kingdom Stratigraphy of the United Kingdom