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Geographical Extent.—Ranging on each side of the Vale of Clwydd, the Silurian system continues by Llangollen, widening southwards to the valley of the Severn, which runs in it from Newton to the plain of Shrewsbury: it borders on the south the coal fields near Shrewsbury, and the Longmont and Stiperstones hills (of older rocks), and enters between these hills and the Clee hills in a long tongue directed N. E. to the Severn at Buildwas. This strike of the Silurian rocks, prolonged in the other direction to the S. W., passes by Knighton and Builth to Llandovery, Llangadock, and Llandeilo: it hence turns in the vale of Towy in a narrow course nearly west to Caermarthen, and with the same range passes Haverfordwest to St. Bride's Bay. From this central line the system expands on the south-east to Ludlow, Aymestry, and Knighton; and this straight south-eastern border extends parallel to the range from Builth to Llandovery, into a curious narrow tongue or broken anticlinal ridge, which crosses the Wye between Builth and Hay, and ranges towards Trecastle. (From Mr. Murchison's Observations.)

About Dudley and Walsall the Ludlow formation is admirably exhibited in singular narrow and short anticlinal ridges, rising in the midst of the coal formation of South Staffordshire, near the hills of basalt called Rowley Rag. These anticlinal ridges run north and south, in parallel courses (Sedgley, Hurst Hill, the Wren's Nest, and Dudley Castle Hill make four such ridges, the two latter being extremely clear), and against them all the coal strata rest at considerable angles of inclination. The diagrams fig. 40. and fig. 41. are intended to illustrate the curious structure of this region.

In Westmoreland and Yorkshire the upper Ludlow formation occurs near Kirkby Lonsdale, yielding fossils, and what I have supposed to represent the Llandeilo rocks in Ribblesdale, a conclusion placed on good evidence by a late investigation of Sedgwick (Geol. Proc. 1852).

Ireland contains the Silurian system, especially in