• Webster's Revised Unabridged Dictionary
    • n Underclay (Geol) A stratum of clay lying beneath a coal bed, often containing the roots of coal plants, especially the Stigmaria.
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Century Dictionary and Cyclopedia
    • n underclay Beds of clay frequently found immediately underlying beds of coal. They are generally believed to be the soil in which the vegetation of the coal grew, and they often contain stigmaria or roots of trees. Also called seat-earth, pounson, etc.
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Chambers's Twentieth Century Dictionary
    • n Underclay un′dėr-klā the bed of clay almost always found under coal-seams, considered as the soil in which grew the plants that formed the coal.
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In literature:

I wish Ehrenberg would undertake a microscopical hunt for infusoria in the underclay and shales; it might reveal something.
"More Letters of Charles Darwin Volume II" by Charles Darwin
But all underclays agree in two points: they are all unstratified.
"Scientific American Supplement, No. 447, July 26, 1884" by Various