In our study of the igneous rocks, we learned that deep-seated molten masses in the slow process of cooling and consolidation frequently undergo differentiation, so that different parts of the same continuous magma consolidate into rocks of very different composition (see p. 290). Many basic rocks contain considerable quantities of metals, and there is good reason to believe that by segregation these metallic constituents may be so concentrated as to form ore bodies. The commonest ores which are referred to this mode of origin are magnetic iron oxide, generally containing titanium, such as those of the Adirondack Mountains, New York, and many other regions. Iron sulphides containing nickel in paying quantities occur in Pennsylvania and Canada, and nickeliferous olivines in Oregon, all of which are regarded as due to magmatic segregation. Chromite, the oxide of iron and chromium, also forms ore bodies of probably similar origin, and the great body of zinc ores at Franklin Furnace, New Jersey, has been referred to the same category.