In the other North Welsh tract, around Bala Lake, the volcanic matter is much less conspicuous. The Arenig rocks are not seen nearer than the Arenig mountains which form the western boundary of this second tract. The Llandeilo beds consist of shaley deposits with a well-marked limestone, the Llandeilo limestone, in the centre, whilst the Caradoc beds consist chiefly of muddy sediments with some thin ashes and a limestone, the Bala limestone, at the top. The Ashgill series contains a basal limestone, the Rhiwlas limestone, succeeded by shales, and another thin limestone called the Hirnant limestone at the summit. In South Wales the Arenig beds[74] are chiefly composed of slates, and are divisible into an upper and lower group. The total thickness is about 2000 feet. The Llandeilo beds contain three series:-- Upper Llandeilo Slates 1000 Llandeilo Limestone 200 Lower Llandeilo Slates 800. [Footnote 74: A remarkable fauna, fairly well represented in Britain and exceedingly well developed on the continent, exists in the Uppermost Arenig and Lower Llandeilo beds, and it is well separated from the dominant Arenig fauna below and Llandeilo fauna above. To the beds which contain it Dr Hicks has given the name Llanvirn series.] The Caradoc beds consist of black graptolitic shales of no great thickness, succeeded by an impure limestone on the horizon of the Bala limestone, while the Ashgill series like that of North Wales is separated into upper and lower limestone stages with an intervening stage composed of shales. The deposits of the Welsh borderland are well developed in Shropshire, where there is practically a repetition of the Caernarvon-Merioneth development, with variations in detail. The Arenig and Caradoc volcanic rocks are not so thick as those of the Welsh district, but are nevertheless of considerable importance[75]. [Footnote 75: For information concerning these beds see Lapworth, C. and Watts, W. W., "The Geology of South Shropshire," _Proc. Geol. Assoc._, vol. XIII. p. 297.]