Limonite is a very common mineral, the color, streak and hardness identifying it readily. Iron rust is its most familiar form. When powdered it is the ochre yellow used in paints. Being so universally distributed, it is to be expected it will occur in a variety of ways. First, there is the fibrous type found lining cavities, in geodes, or hanging in stalactites in caves. This has a silky luster, an opalescent, glazed or black surface, and is in mammillated or botryoidal masses. Second, it may occur in compact masses in veins, where it was deposited by waters; which, circulating through the adjacent rocks, gathered it from the rocks, and, on reaching the open seams, gave it up again. Third, it may occur in beds on the bottom of ponds, where it was deposited by waters which gathered it as they flowed over the surface of the country rocks. Measurements in Sweden show that it may accumulate in such places as much as six inches in the course of twenty years. In ponds and swamps, the decaying vegetation forms organic compounds, which cause the precipitation of the iron from the water, as it is brought in by the streams. This sort of iron in the bottom of ponds or swamps is also known as “bog iron.” Another form in which limonite may occur in ponds, lakes, or even the sea, is in oolitic masses. In this case the iron forms in tiny balls, with perhaps a grain of sand at the center, and one coat of iron after another formed around it, like the layers of an onion. If the resulting balls are tiny this is called oolitic (like fish eggs), but if the balls are larger it is pisolitic (like peas). Bacteria probably have a good deal to do with the precipitation of limonite in this manner. Fourth, limonite occurs in earthy masses, usually mixed with impurities like clay and sand, which are the residue left behind, where limestones have been dissolved by weathering. The fifth mode of occurrence is known as gossan, or “the iron hat,” which is a mass of limonite capping a vein of some sulphide mineral, like pyrite, chalcopyrite or pyrrhotite, which has been exposed to weathering; and in these minerals the sulphur has been removed, leaving a mass of limonite over the vein. This is particularly common in the west. Limonite is quite easily fusible and so was probably the first ore from which early man extracted iron.