A third type of refractory clay--termed _flint clay_--is used in large quantities in the United States, but is seldom found in Great Britain. When moistened, flint clays do not soften, but remain hard and flint-like with a smooth shell-like fracture. For use they are ground extremely fine, but even then they develop little plasticity. They are considered by Ries (6) to have been formed by solution and re-precipitation of the clay subsequent to its primary formation, in a manner similar to flint. They are somewhat rich in alumina and many contain crystals of pholerite (Al2O3·2SiO2·3H2O). The Coal Measure fireclays (p. 53)--which are by far the most important--are divided into two sections by the coal seams, those above the coal being shaly and fissile in structure whilst those below (_underclays_) are without any distinct lamination. Both these clays may be equally refractory, but the underclays are those to which the term fireclay is usually applied. The lowest portions are usually more silicious and in some areas are so rich in silica as to be more appropriately termed silica rock or _ganister_. Fireclays may, in fact, be looked upon as a special term for the grey clays of the Coal Measures, interstratified with and generally in close proximity to the seams of coal. They are known locally as _clunches_ and _underclays_ and were at one time supposed to represent the soil that produced the vegetation from which the coal was formed, but are now considered by many authorities to be of estuarine origin. It is important to notice that whilst the coals almost invariably occur in association with underclays, some fireclays are found at a considerable distance from coal. The fireclays of the Coal Measures have a composition varying within comparatively wide limits even in contiguous strata; those chiefly used having an average of 20 to 30 per cent. of alumina and 50 to 70 per cent. of silica. They appear to consist of a mixture of clay and quartz with a small proportion of other minerals, but in some of them a portion of the clay is replaced by halloysite--another hydro-alumino-silicate with the formula H6Al2Si2O10 or Al2O3.2SiO2.3H2O. Their grey colour is largely due to vegetable (carbonaceous) matter and to iron compounds. The latter--usually in the form of pyrites--is detrimental to the quality of the goods as it forms a readily fusible slag. Unlike the iron in red-burning clays it can seldom be completely oxidized and so rendered harmless. The fireclays must therefore be carefully selected by the miners. On the Continent, and to a much smaller extent in Great Britain, refractory articles are made from mixtures of grog or burned fireclay with just sufficient raw clay to form a mass of the required strength. For this purpose a highly plastic, refractory clay is required and the Tertiary ball clays of Devon and Dorset (p. 64) are particularly suitable.