The clays in the =Carboniferous Limestone= are not, as a whole, of much importance, but the occurrence in this formation of pockets of white refractory clays in Staffordshire, North Wales (Mold) and Derbyshire is interesting, especially as these are used for the manufacture of firebricks and furnace linings. These clays are highly silicious and in composition are intermediate between the Yorkshire fireclays and ganister. Their origin is uncertain, but it is generally considered that they have been produced by the action of the weather and streams on the shales and grits of the Coal Measures which formerly occupied the higher ground around them, though Maw (16) states that 'it is scarcely open to question that they are the remnants of the subaerial dissolution of the limestone' (see 'Fireclays,' Chapter V). In the =Upper Carboniferous System= the clays are highly important because of their general refractory nature, though they differ greatly in this respect, some red-burning shales of this formation having no greater power to resist heat than have some of the surface clays. Those of the Coal Measures are of two main kinds--shales, or laminated rocks which readily split along the planes of deposition, and unstratified underclays. The _shales_ usually occur above the seams of coal and are either of lacustrine or marine origin, differences in their fossils and lithological character supporting one origin for some deposits and the other for the remainder. Some of them are fairly uniform in composition, but others vary so greatly in their physical characters, that they are divided by miners into 'binds' or relatively pure shales, 'rock-binds,' or sandy shales, and sandstones. They also vary greatly in thickness in different localities, and whilst they form the main feature in some districts, in others they are replaced by sandstones. The _underclays_ are so called from their usually lying beneath the coal seams. They are not noticeably stratified and vary greatly in character from soft unctuous materials to hard, sandy rocks. In composition they vary enormously, the percentage of silica ranging from 50 per cent., or less, to as high as 97 per cent.