The various types of shales and seat earths which are mined with our coals are extracted as reject in our coal preparation plants. The following are some of the more important types.
Shale
Shale is one to the rocks most commonly found in coal measure strata. It is usually found at the roof of coal seams, having been deposited in the seas which were formed after subsidence of the original forest. Shale is usually a dull (lusterless) grey in color and consists of very fine particles of clay-like material. Shales forming the roof of coal seams often contain plant impressions as illustrated in Lesson 1 Fig.1: tree trunk remains have occasionally been found.
Other types of shales include ironstone shales, which are brown in color, due to their iron content. All shales are heavy compared with coal. This property is of fundamental importance in coal preparation, and will be described in a later lesson.
Other refuse-making material Seat earths
The rocks immediately beneath the coal seams are called seat earths and were the soil in which the vegetation grew. Often soft, greyish clay-like rocks, they are sometimes mined with the coal. When it is clay-like this seat earth material is called fireclay, but when sandy it is called ganister. In either case the rock often has fossilized plant roots running through it.