Since the rocks are aggregates of minerals, it is best to take up the minerals first. A mineral may be defined as _a natural inorganic substance of definite chemical composition_. It is usually solid, generally has crystalline structure, and may or may not be bounded by crystal faces. _A crystal is a mineral, bounded by symmetrically grouped faces, which have definite relationships to a set of imaginary lines called axes._ There are between 1100 and 1200 minerals, of which 30 are so frequently present, and so dominant in making up the rocks, that they are termed _rock-forming minerals_. About 150 more occur frequently enough so that they can be termed common minerals, and one may expect to find a fairly large proportion of them. Some of these are abundant in one part of the country and rare in others, but this book is written to cover the United States, and so all those which have a fair abundance are included, though some will only be found in the west and others mostly in the east. Then there are some more minerals which are really rare, but which are cherished because of their beauty of color, and are used as gems. These are mentioned, and many of the gems are simply clear and beautiful examples of minerals, which in dark or cloudy forms are much more common. If one finds any of these rare minerals which are not mentioned in this book, he must turn to one of the larger mineralogies mentioned in the literature list to determine them. Crystal Structure A crystal is a mass of molecules, all of the same composition. A molecule in its turn is made up of atoms, and each atom is a unit mass of an element. Thus the calcite molecule is made up of one unit or atom of calcium, one of carbon, and three of oxygen (CaCO₃). These atoms are held together by an attraction, and make a molecule, and for the study of minerals the molecule is the unit. The mineral, calcite, is a mass of molecules all like the one above, and each molecule so small as to be invisible even with the aid of the most powerful microscope. When calcite is in crystal form, the molecules, like ranks of soldiers, are arranged each in its place, each at a definite distance from the other. While each molecule may vibrate or wiggle within certain limits it does not leave its place. (The comparison with soldiers is a good one for the molecules of one layer, but it must be remembered that in a crystal there are also like spacings and ranks up and down as well.) As long as the molecules remain in fixed ranks, up and down, forward and back, and sideways, the crystal is perfect. Calcite may be heated until it melts and becomes liquid. Then the molecules leave their definite arrangement and move about in all sorts of directions, like the soldiers after ranks have broken. So long as the molecules are thus free to move about but keep together, the substance is a liquid. There are cases when the molecules in this disorder take fixed positions without falling into ranks.