2.—Just scratched by the finger nail (e.g., gypsum).
3.—Just scratched by a copper coin (e.g., calcite).
4.—Easily cut by a knife, but does not cut glass (e.g., fluorite).
5.—Just scratches soft glass, and is cut by a knife (e.g., apatite).
6.—Harder than steel, and scratches glass easily (e.g., orthoclase).
7, 8, 9, and 10.—Harder than any ordinary substance and represented in order by quartz, topaz, corundum, and diamond.
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.)