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. The Coal Measure clays of Shropshire are noted for the manufacture of red roofing tiles, especially in the neighbourhood of Broseley. Agriculturally, the Coal Measure clays are usually poor, but are occasionally of good quality. The shales produce heavy, cold clays and the yellow subsoil produces soils of a light, hungry character so that the two should, if possible, be mixed together. =Permian clays= are of little value except for the manufacture of red building bricks. The Nottinghamshire Permian clays make excellent roofing tiles, flower pots and red bricks. Agriculturally, the Permian clays are a free working loam yielding large crops of most of the ordinary farm products. =Triassic clays= are of great importance in the Midlands, those upper portions of them known as the Keuper Marls being much used for the manufacture of bricks. They are specially known amongst clayworkers as the material from which the Midland red bricks of Nottinghamshire and Leicestershire and the Somersetshire tiles are prepared. =Jurassic clays= are an important group, of marine origin, occurring in close association with limestone. For this reason they form a valuable source of material for the manufacture of Portland cement, but are of less value to the brick and tile manufacturer. The Jurassic System contains so large a variety of clays, of such widely different ages and characteristics, that no general description of them can be given in the present volume. [Illustration: Fig. 9. Lias clay being worked for the manufacture of hand-made sand-faced roofing tiles. (_By courtesy of Messrs Webb Bros. Ltd., Cheltenham._)] The '_Lias clays_'--the lowest of the Jurassic formation--are chiefly dark, bituminous shales, including the 'alum shales,' and are often seriously contaminated with pyrites and ironstone. When carefully selected they may be used to advantage in the production of most red articles such as bricks, tiles, chimney pots, etc. They shrink less in the kiln than do most clays, and are easily fusible on account of the lime they contain, but on the whole this formation is of great value for the manufacture of the articles just mentioned.