The clays used in Great Britain for the manufacture of the best stoneware are the Devonshire and Dorset ball clays, the upper portions of these deposits being used for this purpose as they are somewhat less pure than the lower portions used in the manufacture of white ware. For coarser grades of stoneware, clays of other geological formations are employed, especially where the finished ware may be coloured, as the purity of the clay is of less importance. Providing a clay has a sufficiently long vitrification range, a suitable colour when burned, and that it is capable of being readily formed into the desired shapes, its composition and origin are of small importance to the stoneware manufacturer. In actual practice, however, the number of sources of good stoneware clay is distinctly limited, and many manufacturers are thus compelled to add suitable fluxes to refractory clays in order to meet some of their customers' requirements. For this purpose a mixture of fireclay with finely powdered felspar or Cornish stone is used. Chalk--which is a cheaper and more powerful flux--or powdered glass cannot be used as the range of vitrification of the mixture would be too short. Some manufacturers take the opposite course and add fireclay, flint, or other refractory material to a readily fusible clay. This is satisfactory if the latter clay is relatively low in lime and owes its fusibility to potash, soda or magnesia in the form of mica or felspar. The mica and felspar grains enter so slowly into combination with the clay that a long range of vitrification occurs, whereas with lime, or with some other soda and potash compounds, the combination occurs with great rapidity and the shape of the ware is spoiled. The =refractory clays= are commonly known as _fireclays_ on account of their resistance to heat. The china clays and kaolins are also refractory, but are too expensive and are not sufficiently plastic to be used commercially in the same manner as fireclays, except to a very limited extent, though bricks have been made for many years from the inferior portions of china clay rock at Tregoning Hill in Cornwall. The geological occurrence of the fireclays of the Coal Measures has already been described on p. 53. In addition, there are the refractory clays occurring in pockets or depressions in the Mountain Limestone of North Wales, Staffordshire, Derbyshire and Ireland, which consist of siliceous clays and sands, the insoluble residue of the local dissolution of the limestone, intermixed with the débris of the overlying Millstone Grit (see p. 54). These clays and sands can be mixed to produce bricks of remarkably low shrinkage, but the pockets are only large enough to enable comparatively small works to be erected and the clays are so irregular both in composition and distribution as to render their use somewhat speculative.