Iron was discovered by man later than gold or silver or copper, about 1000 B.C.; but once found it was so much more abundant than any of these that it soon dominated over copper, and from Roman times to the present has been the basis of progress in civilization, and these times are well called “the iron age.” Iron unites freely with the non-metals, and occurs as sulphides, oxides, carbonates, etc., and is also present as a secondary metal in that great group of minerals known as the silicates (see page 97). It alloys with a wide range of other metals, every combination altering the properties of the iron, and thus making it useful in a still greater range of manufacture. The introduction of ¼ to 2½% of carbon into iron makes steel, which is harder (in proportion to the amount of carbon) and stronger than the pure iron. Iron compounds are among the most numerous and important of the colors in Nature’s paint box, limonite furnishing the browns which color the soil and so many of the rocks, hematite giving the red color to other abundant rocks, and magnetite often coloring igneous rocks black, while the chlorophyll which gives the green color to plants is an iron compound, as is also the hemoglobin which gives the red to our blood. Iron is present in all igneous rocks, and secondarily in the sedimentary and metamorphic rocks. It is soluble in water, and so is being constantly transferred from place to place, and changes from one compound to another, according to the circumstances in which it is placed. The primary forms are pyrite, magnetite and the silicates. When in weathered rocks the iron is changed to limonite, siderite or hydrated silicates. Hematite is an intermediate oxide from which the water contained in limonite has been driven off by moderate heat or bacterial action. Limonite 2Fe₂O₃·3H₂O Pl. 12 Never crystalline, occurs in mammillary, botryoidal and stalactitic forms, or in fibrous, compact, oolitic, nodular or earthly masses; hardness 5.5; specific gravity 3.8; color yellow-brown to black; streak yellow-brown; luster metallic to dull; opaque.