(6) The reasons given in the last paragraph seem in themselves sufficient to account for the much greater production of metals in mountains than in plains, but there is a still more fundamental reason.
In regions of rugged relief there has usually been greater opportunity for the formation of ores than in level regions. The formation of ores usually requires that the earth's crust be pierced by molten masses of igneous rock, or broken by faults, or shattered by the folding and bending of the rocks. Such movements on a large scale occur only where the earth's crust is being upheaved into mountains.
How Ores are Formed.—In order to understand why metals are produced in places where the earth's crust is broken by volcanoes and faults, let us see how ores are formed. Most of the igneous rocks which constitute the main mass of the earth's crust contain very small quan tities of many metals. Except in the case of iron and aluminum, the two most abundant metals, the quantities found originally in the igneous rocks are seldom great enough to be profitably extracted until the metals have been segregated or concentrated by some process of nature. Even with iron and aluminum a process of segregation is usually needed whereby the tiny hits of metallic compounds scattered through a great mass of rock may be brought together into ore bodies of appreciable size. This process of segregation is extremely important, for without it the metals could scarcely have been used by man. It takes place in two chief ways, both of which occur much more readily in a mountain ous region than in a plain. The first, but less common way, is in masses of molten rock which push up through the earth's crust and often, although not always, reach the surface in volcanoes. Occasionally when such a mass of igneous rock is slowly cooling below the surface, certain metallic minerals which form crystals at high temperatures may solidify before the others and thus collect near the margins of the molten mass and later be carried to a position where they finally come to rest as a body of ore. The magnetic form of iron known as magnetite, which occurs abundantly in Sweden, is the commonest mineral of this type. At other times the gases which accompany a rising mass of molten rock come in contact with the cool rocks on all sides. The gases
frequently contain metals, and if the surrounding rock is of the right kind, especially if it is limestone, the metals may be deposited to form a zone of ore bordering the igneous intrusion. Iron and copper ores are the commonest minerals formed in this way, but lead, zinc, and gold may also occur.
The second and more important way in which the segregation of ores occurs is through the action of water. As water percolates through the rocks it dissolves the widely scattered bits of the oxides, sulphides, or other compounds of the metals, and later deposits them after the solution has become concentrated by the addition of material from a wide area. Water-formed ores are of two classes. One is formed at considerable depths by hot water given off from the igneous rocks themselves. This magmatic water, as it is usually called, generally carries the metals upward, and this process has given rise to much of the world's metallic wealth, including for example, practically all the copper of the United States, the gold veins of California, the silver deposits of Cobalt in Ontario, and the lead of Germany. The other class of water-formed ores is of much less importance. It consists of ores formed by water which percolates downward from the sur face. The large lead and zinc deposits of the Mississippi Valley in Wisconsin, Illinois, and Missouri were formed in this way. The surface water may produce another effect upon any of the three kinds of ores already mentioned. By trickling clown through the deposits it may dissolve metals from their upper parts and deposit them lower down, thus bringing about what is called secondary enrichment. This process produces relatively small bodies of extremely rich ore such as the upper part of the copper deposits of Arizona and of the famous Comstock silver lode in Nevada. It also produces great bodies of low-grade ore whose aggregate value is enormous. The great iron ore deposits of the Lake Superior region are supposed to be an example of deposits due to the action of water in a relatively dry and much faulted and folded mountain region of pre-Cambrian times.