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]).
[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.
Fig. 8. Coal Measures sequence in North Staffordshire.
The dissimilarities in the fossils of the Coal Measure clays and shales in the Northern and Southern Hemispheres suggest that there is a considerable difference in their formation, but the number of clays and shales which have been examined is too small for any accurate conclusion to be drawn.
For many industrial purposes, particularly for the manufacture of refractory goods, the clays and shales of the Carboniferous System are highly important. The less valuable burn to a reddish colour, often spoiled with many grey spots of ferrous silicate derived from the pyrites in the clay, but the purer varieties burn to a delicate primrose or pale buff tint and are amongst the most heat-resisting materials known. The Coal Measure clays of Yorkshire are particularly esteemed for their refractory properties; for the manufacture of glazed bricks and for blocks for architectural purposes somewhat ambiguously termed 'glazed terra-cotta.' The inferior qualities are largely used for the manufacture of red engineering bricks, some of them competing successfully with the more widely known 'blue bricks' of Staffordshire.