- 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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Examples from the Web for underclay
These roots were Stigmaria, and the stuff into which they penetrated was an underclay.
It rests on the underclay b b', filled with roots of trees in situ, and it supports a growing forest (C D).A Manual of Elementary Geology
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