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Underclay, un′dėr-klā, n. the bed of clay almost always found under coal-seams, considered as the soil in which grew the plants that formed the coal.

Underclay, with rootlets of Stigmaria, resting on gray shale, with two thin coaly seams.

Underclay with Stigmaria rootlets. a, Calamites. b, Stem of plant undetermined. c, Stigmaria roots. d, Erect trunk, 9 feet high.

The seams of coal, varying in thickness from a few inches to three or four feet, always rest on a bed of clay, known technically as the “underclay,” which represents the soil on which the plants originally grew.

Marsh gas is an inflammable gas which becomes explosive when mixed with air and ignited; it often escapes with great violence during the working of coal seams, the jets blowing out from the coal or underclay with a rushing noise, indicative of the high pressure under which the hydrocarbon gas has accumulated.