from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Metamorphic rock having porphyritic texture.
from The Century Dictionary and Cyclopedia
- n. A sedimentary rock, originally (in some cases at least) a clay slate, or quartzite, which has been altered by dynamic metamorphism or by some other metamorphic agency so as to take on a slaty and more or less perfectly developed porphyritic structure.
Sorry, no etymologies found.