The mode of formation of the underclays is not certainly known. They do not appear to be soils or of terrestrial origin, but according to Arber (24) correspond closely to the black oozes of marine and semi-marine estuarine deposits of tropical swamps, or to the muds surrounding the stumps of trees in the buried forests of our coast-lines. They thus appear to be quite distinct from the shales above them, both in origin and physical characters. The more silicious portions, known as _Ganister_[9], possess comparatively few of the characteristics of clay though used, like all the more refractory clays of the Coal Measures, for all purposes for which fireclay is employed. The term _fireclay_ is, in fact, frequently applied to all the refractory deposits in the Coal Measures, without much regard to their composition (see Chapter V). [Footnote 9: The Dinas rock used in the Vale of Neath (Wales) is an even more silicious material found in the Millstone Grit immediately below the Coal Measures. It is largely employed for firebricks.] Valuable Coal Measure clays occur in enormous quantities in Northumberland, Durham, Yorkshire, Nottinghamshire, Derbyshire, Staffordshire, near Stourbridge, in Warwickshire, Shropshire, North and South Wales and South West Scotland. In Ireland, on the contrary, the Coal Measure clays are of little value except in the neighbourhood of Coal Island, co. Tyrone. The position of the 'Sagger Marls' of North Staffordshire (Keele Series and Etruria Marls), relative to the 'Farewell Rock' or Millstone Grit, is shown in fig. 8 in which the horizontal lines represent coal-seams and ironstone veins.