Most of the raw materials used in trade and industry have their source in the earth. A few of these substances, such as gold, are found in a free state, but as noted before, the more common substances, such as iron, lead, tin, zinc, etc., are found combined with oxygen, sulphur, and dirt. To understand why these are found in this state, it is necessary to study the condition of the earth.
The interior of the earth is a hot, molten mass, from which constantly issues, on various parts of the earth, a stream of hot, molten stone or hot steam, gases, and so on. The gases are steam, carbonic acid, burning carbon, hydrogen, and hydrogen sulphide. The surface of the earth is in a comparatively cold condition. As we dig below the surface we find masses of stone and rock within which valuable metallic particles are embedded. These particles are called minerals. These combinations of mineral and rock are due to the mixing of hot masses. As a result, the metals that are acted upon by oxygen, acids (carbonic acid), hydrogen sulphide, etc., are found in the earth as oxides, sulphides, carbonates, etc. As gold is not acted upon by any of the ordinary gases it is found in a free state.
The earth appears to be composed of twelve main elements: oxygen, silicon, aluminum, calcium, magnesium, potassium, sodium, carbon, hydrogen, sulphur, chlorine, and iron. Of course many other elements, such as the precious metals, are present but are found in small quantities only. Most of the rocks found in the earth are mixtures of two or more minerals. Granite formed from volcanic eruption, for example, is a mixture of three minerals - feldspar, quartz, and mica; sandstone consists of particles of silica or sand; limestone consists of a carbonate of lime; slates consist of silicates of aluminum; and clay consists principally of aluminum compounds. The minerals are held together in the stone by some binding substance, like carbonate of lime, iron oxide, or silica. The color of the clay, rocks, and different parts of the earth is due to the presence of small quantities of iron and other metals. Changes in temperature cause the rocks to expand and contract and consequently they gradually split and crack. The rain then washes into the valley the loose parts of the rocks. Thus the soft, loose soil found on the surface of the earth is the result of the breaking up of the rocks in this way, and the process by which such soil is made is termed weathering or erosion.
Stones or rocks are designated as sedimentary, igneous, or metamorphic, the classification depending upon their origin.
Sedimentary rocks are remains of older rocks which have been deposited under water, layer by layer. Limestone and sandstone are examples of this class. Igneous rocks are formed by the solidifying in a crystalline state of lava from a volcano. Granite and allied stones are examples of this kind of rock. Metamorphic rocks are rocks that have, after formation, changed their original forms because of the movement or pressure of the earth. Slates and marbles are examples of this class.