hornfels
/ˈhɔːrnfɛlz/
noun
- A hard, fine-grained metamorphic rock formed by the heating of clay or shale by an igneous intrusion.
- The hikers noticed a dark band of hornfels where the lava had baked the surrounding shale.
- Geologists collected samples of hornfels to study its mineral composition under a microscope.
- The quarry exposed a layer of hornfels that was much harder to break than the surrounding sandstone.