from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A stratum of clay lying beneath a coal bed.
from the GNU version of the Collaborative International Dictionary of English
- n. A stratum of clay lying beneath a coal bed, often containing the roots of coal plants, especially the Stigmaria.
from The Century Dictionary and Cyclopedia
- n. Beds of clay frequently found immediately underlying beds of coal.
under- + clay (Wiktionary)