Next to the fossil fuels, the most important products of the rocks in North America are the iron ores. Although certain igneous rocks are rich in iron, and in some instances contain it even in a pure or metallic state, none of the rocks that have cooled from fusion carry iron in any form in sufficient quantities to be of commercial importance. Most of the iron in igneous rocks is contained in mineral, usually silicates, and would be difficult to separate. When exposed to the air and to percolating water, the iron-bearing minerals of the igneous or other rocks decay and the iron enters into various new combinations. When organic acids are present, and especially carbon dioxide, ferrous carbonate is formed, which is quite soluble, and is taken into solution by percolating water, some of which emerges as springs, and joins the surface run-off, which may also take up ferrous carbonate in solution. One of the most common methods by which iron ore is accumulated is when water carrying ferrous carbonate in solution forms swamps and lakes, and in many instances as the water is exposed to the air and aided by evaporation it parts with a portion of its carbon dioxide, and the hydrated sesquioxide of iron or limonite results. When, under similar conditions, an excess of organic matter is present, beds of ferrous carbonate are formed. In other instances iron oxide is precipitated in swamps and lakes through the action of low forms of plant life. The ores of iron concentrated in these ways are in many instances in well-defined layers, or lenticular bodies, which are thickest in the central portion and thin out in all directions. Their forms are determined mainly by the shapes of the depressions they occupy. Both ferrous carbonate and limonite, however, occur in irregular surface deposits.