a. A layer of fine-grained detrital material, usually clay, lying immediately beneath a coalbed or forming the floor of a coal seam. It represents the old soil in which the plants (from which the coal was formed) were rooted, and it commonly contains fossil roots (esp. of the genus Stigmaria). It is often a fireclay, and some underclays are commercial sources of fireclay. Syn: underearth; seat earth; seat clay; root clay; thill; warrant; coal clay; warrant clay. AGI
b. A bed of clay, in some cases highly siliceous, in many others highly aluminous, occurring immediately beneath a coal seam, and representing the soil in which the trees of the Carboniferous swamp forests were rooted. Stigmarian roots commonly occur as fossils in underclays, many of which are used as fireclays. See also: fireclay
Source:
Dictionary of Mining, Mineral, and Related Terms












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