Mastodon. A genus of fossil extinct quadrupeds allied to the elephants; so called from the form of the hind teeth or grinders, which have their surface covered with conical mammillary crests. Etym., μαστος, mastos, pap, and οδων, odon, tooth.
Matrix. If a simple mineral or shell, in place of being detached, be still fixed in a portion of rock, it is said to be in its matrix. Matrix, womb.
Mechanical Origin, Rocks of. Rocks composed of sand, pebbles, or fragments, are so called to distinguish them from those of a uniform crystalline texture, which are of chemical origin.
Medusæ. A genus of marine radiated animals, without shells; so called, because their organs of motion spread out like the snaky hair of the fabulous Medusa.
Megalosaurus. A fossil gigantic amphibious animal of the saurian or lizard and crocodile tribe. Etym., μεγαλη, megale, great, and σαυρα, saura, lizard.
Megatherium. A fossil extinct quadruped, resembling a gigantic sloth. Etym., μεγα, mega, great, and θηριον, therion, wild beast.
Melastoma. A genus of Melastomacea, an order of exotic plants of the evergreen tree and shrubby kinds. Etym., μελας, melas, black, and στομα, stoma, mouth; because the fruit of one of these species stains the lips.
Mesotype. A simple mineral, white, and needle-shaped, one of the Zeolite family, frequently met with in the Trap-rocks.
Metamorphic Rocks. A stratified division of hypogene rocks, highly crystalline, such as gneiss and mica-schist, and so named because they have been altered by plutonic action. Etym., μετα, meta, trans, and μορφη, morphe, form.
Mica. A simple mineral, having a shining silvery surface, and capable of being split into very thin elastic leaves or scales. It is often called talc in common life; but mineralogists apply the term talc to a different mineral. The brilliant scales in granite are mica. Etym., mico, to shine.