Meta-igneous rock—a metamorphosed igneous rock.
Metamorphic rock—a rock which has been altered, particularly one which has been rendered crystalline, or recrystallized by heat and pressure.
Meta-sedimentary rock—a metamorphosed sedimentary rock.
Microgranite—a very fine-grained granite.
Microlites—incipient crystals found in glassy lavas; usually needle-shaped, or rod-like; occurring singly and in aggregates.
Millstone—see buhrstone.
Minette (mica-syenite)—a rock consisting essentially of orthoclase and mica, or a syenite in which mica replaces hornblende or predominates over it.
Monzonite—a granitic rock composed of orthoclase and plagioclase in nearly equal proportions, with ferromagnesian minerals; a rock intermediate between syenite and diorite.
Mudstone—solidified mud or silt, shale.
Nephelinite—a rock composed essentially of nepheline and augite, with magnetite and other accessories.