_Chlorine_ is found principally as an ingredient of rock-salt, which contains sixty per cent, of it, and of sea-water, which contains one and a half per cent. _Fluorine_ is found, though very sparingly, in nearly all the unstratified rocks. It forms nearly half of the mineral known as Derbyshire spar. Of the metals, _Iron_ is the only one that is found abundantly. It enters into the composition of nearly all mineral substances. It is generally combined with oxygen, and occurs less frequently as a carbonate or sulphuret. Of volcanic rocks it forms about twenty per cent. Its ores are sometimes found in the form of dikes or seams, having been injected from below; at other times, in the form of nodules or stratified masses, like other rocks of mechanical origin. _Manganese_ is likewise extensively diffused, but in very small quantity. The other metals are often met with, but their localities are of very limited extent. Of the metallic bases of the earths and alkalies, _Silicium_ is the most abundant. It generally occurs in the form of silex, which is an oxide of the metal. There are but few rocks in which it is not found in considerable amount. _Aluminium_ generally occurs as an oxide, in which form it is alumina. It is the base of the different varieties of clay and clay-slate. It is also a constituent of felspar and mica. _Potassium_ is an ingredient of felspar and mica, and hence is found in all the primary and in most of the volcanic rocks, as well as in the stratified rocks derived from them. _Sodium_ is a constituent of a variety of felspar which is somewhat abundant in volcanic rocks. Its principal source is the extensive beds of rock-salt, and the same substance in a state of solution in the waters of the ocean. _Calcium_ constitutes about forty per cent, of limestone, and is an ingredient in nearly all igneous rocks. This metal, in the state of an oxide, is lime. _Magnesium_ is somewhat abundant, but less so than calcium. It is one of the bases of dolomite and magnesian limestone, and is an ingredient of talc and all talcose rocks. The substances now enumerated constitute nearly the entire mineral mass of the crust of the earth. They may be arranged in the following order:-- I. NON-METALLIC SUBSTANCES. Oxygen. Hydrogen. Nitrogen. Carbon. Sulphur. Chlorine. Fluorine. II. METALS. Iron. Manganese. III. METALLIC BASES OF THE EARTHS AND ALKALIES. Silicium. Aluminium. Potassium. Sodium. Calcium. Magnesium. These substances, chemically combined, form _Simple Minerals_. SECTION II.--SIMPLE MINERALS.