Magnetite is found in association with igneous or metamorphic rocks, and often represents limonite or hematite which has been altered as the result of high temperatures. Some of it, in the igneous rocks especially, was undoubtedly in the molten magma and has crystallized out from the magma while it was still hot. It is the form of iron always indicative of former high temperatures. It is an ore mineral for about 3% of the iron in this country, but in Scandinavia and some other countries, it plays a leading role as the source of iron. It is found in the Adirondack Mountains, in New Jersey, Pennsylvania, Arkansas, North Carolina, New Mexico, and California. Siderite FeCO₃ Pl. 13 & 14 _Spathic iron_ Occurs in fibrous botryoidal masses or rhombohedral crystals, sometimes with curved faces; hardness 3.5; specific gravity 3.8; color gray-brown; streak white; luster vitreous; translucent on thin edges. Like hematite this mineral belongs to the hexagonal system, and crystallizes in hemihedral form, making the rhombohedron. Its faces are often curved, which is rare in minerals, only a few forms like this and dolomite having other than plane faces. When siderite crystals grow in clusters, the crowding often results in growth on one face only, making a mass of fibrous character, and in such cases the surface of the mass is botryoidal in contour. The mineral is likely to oxidize, losing its gray-brown color, and becoming limonite. In the United States it is scarcely ever used as an ore for iron, but in Germany and England a great deal of iron is smelted from this mineral. It occurs in Massachusetts, Connecticut, New York, throughout the Appalachian Mountains, and also in Ohio. Pyrite FeS₂ Pl. 15 & 16 _iron pyrites_ Occurs as cubes, octahedrons and pyritohedrons, or in compact masses, scales or grains; hardness 6; specific gravity 5.1; color brassy yellow; streak greenish-black; luster metallic; opaque on thin edges.