Clay consists of kaolinite nearly always with large admixtures of other substances, such as exceedingly fine grains of x quartz, felspathic mud, and the like. When moist, clay is plastic, differing in this respect from mud. The particles of clay and mud are extremely fine and are carried for long distances before settling to the bottom. Hence the muds and clays are distributed over wider areas than the gravels and sands, and deposits of them indicate quieter and, usually, but not always, deeper waters than the conglomerates and sandstones.

Clay is found in very different conditions of purity. Kaolin, or porcelain clay, is nearly pure, while Potter's and Brick Clay contain finely divided quartz, and the latter in addition, lime, magnesia, iron, and alkalies. Clays with considerable percentage of iron burn red on firing.

Fire-clay is a nearly pure mixture of sand and clay, with only traces of iron, magnesia, or lime, and therefore burns to white or buff-coloured bricks, which will resist very high temperatures. Fire-clays occur frequently beneath coal seams, representing the ancient soil in which the coal plants grew. Such ancient fire-clays are often hard rocks, and must be ground up before using.

Mudstone is a rock which is composed of solidified clay or felspathic mud, or a mixture of the two, and which crumbles rapidly into mud when exposed to the action of the weather.

Shale is a finely stratified or laminated clay rock, formed from the solidification of mud and silt. In some of the paper shales there are as many as thirty or forty laminae to the inch, each representing a separate stage of deposition. Shales ordinarily contain more or less sand, and as this increases in quantity, they shade gradually into arenaceous shales and argillaceous sandstones, or by the increase of calcareous matter into limestones. Bituminous shale is coloured very dark or black by the carbonaceous matter with which it is saturated. When distilled, the bituminous shales yield hydrocarbons, and are of considerable economic importance; the carbonaceous matter may be of either animal or vegetable origin. Shales of this class grade into coals.

Marl is clay containing carbonate of lime, which rapidly crumbles on exposure to the weather.