Wordnet 3.0
NOUN (1)
1.
a fine-grained metamorphic rock formed by the action of heat on clay rocks;
[syn: hornfels, hornstone]
WordNet (r) 3.0 (2006):
hornfels
n 1: a fine-grained metamorphic rock formed by the action of
heat on clay rocks [syn: hornfels, hornstone]