5. Magnetite.—Protoxide and sesquioxide of iron: oxygen, 27.6; iron, 72.4; = 100. Isometric system, usually in octahedrons or dodecahedrons. Most abundant variety is coarsely to finely granular, sometimes dendritic. Hardness, 5.5-6.5, same as hematite. Sp. gr., 4.9-5.2. Lustre, metallic. Color and streak, iron-black, and hence color essential. Strongly magnetic; some specimens have distinct polarity, and are called loadstones. Specimen 14. The three iron-oxides just described—limonite, hematite, and magnetite—are all important ores of iron, and form a well-marked natural series. Thus limonite is never, hematite is usually, and magnetite is always, crystalline. Again, limonite with 60 per cent. of iron is never magnetic, hematite with 70 per cent. is sometimes magnetic, while magnetite with 72.4 per cent. is always magnetic. As the iron increases so does the magnetism. We have here an excellent illustration of the principle that the properties of the elements can be traced in those minerals in which they predominate. Iron is the only strongly magnetic element: magnetite contains more iron than any other mineral, and it is the only strongly magnetic mineral. These three iron-ores are easily distinguished from each other by the color of their powders or streak,—limonite yellow, hematite red, and magnetite black,—and from all other common minerals by their high specific gravity. 6. Quartz.—Oxide of silicon or silica: oxygen, 53.33; silicon, 46.67; = 100. Hexagonal system. The most common form is a hexagonal prism terminated by a hexagonal pyramid. Also coarsely and finely granular to perfectly compact, like flint; the compact or cryptocrystalline varieties often assuming botryoidal, stalactitic, and concretionary forms. It has no cleavage, but usually breaks with an irregular, conchoidal fracture like glass. Hardness, 7, being No. 7 of the scale; scratches glass easily. Sp. gr., 2.5-2.8. Lustre, vitreous. Pure quartz is colorless or white, but by admixture of impurities it may be of almost any color. Streak always white or light colored. Quartz is usually, as in specimen 15, transparent and glassy, but may be translucent or opaque. It is almost absolutely infusible and insoluble. The varieties of quartz are very numerous, but they may be arranged in two great groups:— 1. _Phenocrystalline_ or _vitreous_ varieties, including rock-crystal, amethyst, rose quartz, yellow quartz, smoky quartz, milky quartz, ferruginous quartz, etc. 2. _Cryptocrystalline_ or _compact_ varieties, including chalcedony, carnelian, agate, onyx, jasper, flint, chert, etc. Only three varieties, however, are of any great geological importance; these are: common glassy quartz (spec. 15), flint (spec. 16), and chert.