We are more or less familiar with the division of all materials of nature into the animal, vegetable, and mineral kingdoms. With slight exceptions minerals are the materials which make up the known part of the earth. In a very real sense, then, mineralogy is the most fundamental of the various branches of the great science of geology because the events of earth history, as interpreted by the geologist, are recorded in the mineral matter (including most rocks) of the earth. When we examine the rocky material or mineral matter of the earth in any region we find that it consists of various kinds of substances each of which may be recognized by certain characteristics. Each definite substance (barring those of organic origin) is called a mineral. Or, more specifically, a mineral is a natural, inorganic, homogeneous substance of definite chemical composition. According to this definition a mineral must be found ready made in nature, must not be a product of life, must be of the same nature throughout, and its composition must be so definite that it can be expressed by a chemical formula. All artificial substances, such as laboratory and furnace products, are excluded from the category of minerals. Coal is not a mineral because it is both organic and of indefinite composition. A few examples of very common substances which perfectly satisfy the definition of a mineral are quartz, feldspar, mica, calcite, and magnetite. Only two substances--water and mercury--are ordinarily liquid minerals. There are nearly a thousand distinct mineral species, and to them and their varieties several thousand names have been applied. It is a surprising fact that of the eighty or more chemical elements, that is substances which cannot be subdivided into simpler ones, only eight make up more than 98 per cent of the weight of the crust of the earth, though, with one very slight exception, none of the eight exist as such in mineral form. The eight elements are oxygen (nearly 50 per cent), silicon (over 25 per cent), aluminum (over 7 per cent), iron (over 5 per cent), calcium (or "lime"), magnesium (or "magnesia"), sodium (or "soda"), and potassium (or "potash").