7, 8, 9, and 10.--Harder than any ordinary substance and represented in order by quartz, topaz, corundum, and diamond. [Illustration: Plate 17.--Skeleton of the Great Two-Legged, Carnivorous Dinosaur Reptile, Called "Tyrannosaurus," Which Lived During Cretaceous Time. (_Courtesy American Museum of Natural History._) Small Picture.--Restoration of the Earliest Known Bird of Which Several Nearly Perfect Skeletons Have Been Found. This feathered creature with reptilian characteristics lived at least 5,000,000 years ago. It had a long vertebrated tail, claws on the ends of the wings, and teeth. (_By E. W. Berry._)] Plate 18.--(_a_) Skeleton of the Largest Known Creature That Ever Flew. It was a flying reptile with spread wings of nearly twenty-five feet, and lived during the Cretaceous period several million years ago. (_Courtesy of the American Museum of Natural History._) Plate 18.--(_b_) Skeleton of a Remarkable Swimming Reptile of the Mesozoic Era. Length about twelve feet. Parts of skeletons of unborn young are seen. (_Courtesy of the American Museum of Natural History._) Minerals also show a great variety of colors. Many of them like quartz and calcite are colorless or white, others like galena (steel-gray) and pyrite (brass-yellow) show inherently characteristic colors, while still others like amethyst (purple) and sapphire (blue) are colored by impurities. There is also a great range in relative weights or density of minerals, commonly called the specific gravity, which range from less than one for ice to 21.5 for platinum, and even somewhat higher. The average specific gravity of all minerals of the earth is about 2.6. In the light of the above discussion of the general properties of minerals, we shall now proceed to name and briefly describe some of the minerals which are either very common, or of special interest, or of special economic importance. Only those features are listed by which the mineral species may be recognized at sight, or by the aid of very simple nonchemical tests. [Illustration: Fig. 74.--Drawings showing forms of crystals of common minerals: a and b, garnet (Isometric); c and d, feldspars (Monoclinic); e, f, and g, quartz (Trigonal); h, i, and j, calcite (Hexagonal); k, augite (Monoclinic); l. hornblende (Monoclinic); m, pyrite (Isometric).]