underclay

/ ˈʌndəˌkleɪ /

noun

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

Example Sentences

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

From Project Gutenberg

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

From Project Gutenberg

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

From Project Gutenberg