Certain rock formations are made up essentially of but one mineral in the form of numerous grains as, for example, limestone, which consists of calcite (carbonate of lime). Most of the ordinary rocks are, however, made up of two or more minerals mechanically bound together. Thus, in a specimen of granite on the author's desk several distinct mineral substances are distinguishable by the naked eye. These mineral grains are from one to five millimeters across. Most common among them are hard, clear, glassy grains called quartz; nearly white, hard grains, with smooth faces, called feldspar; small, silvery white plates, easily separable into very thin flakes, called mica; and small, hard, black grains, called magnetite. It is the business of the mineralogist to learn the characters of each mineral, how they may be distinguished from each other, how they may be classified, how they are found in nature, and what economic value they may have. It is an important part of the business of the geologist to learn what individual minerals combine to form the many kinds of rocks, how such rocks originate, what changes they have undergone, and what geological history they record. It is thus clear that the great science of geology is much broader in its scope than mineralogy. One of the most remarkable facts about minerals is that most of them by far have a crystalline structure, that is they are built up of tiny particles known as molecules. Such crystalline minerals are often more or less regular solid forms bounded by plane faces and sharp angles, such forms being known as "crystals." How do crystals develop such regularity of form? Any solid is considered to be made up of many very tiny (submicroscopic) molecules held together by an attractive force called cohesion. In liquids the molecules may more or less freely roll over each other, thus altering the shape of the mass without disrupting it. In gases the molecules are considered to be relatively long distances apart and moving rapidly. During the process of change of a substance from the condition of a liquid or gas to that of a solid, due to lowering of temperature or evaporation, the cohesive force pulls the particles (molecules) together into a rigid mass. Under favorable conditions such a solid has a regular polyhedral form. "This results from the fact that the particles or molecules of the substance which, while it was liquid or gaseous, rolled about on one another, have been in some way arranged, grouped and built up. To illustrate this, suppose a quantity of small shot to be poured into a glass: the shot will represent the molecules of a substance in the liquid state, as for example a solution of alum. If, now, we suppose these same shot to be coated with varnish or glue so that they will adhere to each other, and imagine them grouped as shown in Figure 70a, they will represent the arrangement of the molecules of the alum after it has become solid or crystallized.