• synonyms


  1. a grey or whitish clay rock containing fossilized plant roots and occurring beneath coal seams. When used as a refractory, it is known as fireclay
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Examples from the Web for underclay

Historical Examples

  • These roots were Stigmaria, and the stuff into which they penetrated was an underclay.

    Scientific American Supplement, No. 447, July 26, 1884


  • It rests on the underclay b b', filled with roots of trees in situ, and it supports a growing forest (C D).

  • I wish Ehrenberg would undertake a microscopical hunt for infusoria in the underclay and shales; it might reveal something.