HORNFELS, in petrology, the group designation for a series of rocks that have been partially or wholly recrystallized by the heat of intrusive igneous masses. Most hornfelses are fine textured and massive, recrystallization having effaced the struc tures and textures of the original rocks and replaced original minerals by new assemblages. They are the characteristic rocks forming the inner zone of the thermal aureole of alteration sur rounding bosses or laccoliths of plutonic rocks. The commonest types of hornfelses are derived from shales, impure limestones and basic igneous rocks. Shales give rise to brown or black horn felses rich in biotite mica, andalusite and cordierite ; while impure limestones give the "calc-silicate hornfelses" rich in pyroxene, garnet, idocrase or wollastonite. The hornfelses derived from basic igneous rocks are dark green types rich in felspar and pyroxene or monoclinic amphibole. These are the basalt or dolerite hornfelses. (See also METAMORPHISM.) (C. E. T.)