- 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
Examples from the Web for underclay
Historical Examples of 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