[Illustration: A piece of gypsum looking on the surface of the perfect cleavage, and showing the two other cleavages as lines, intersecting at 66°. Twinning is also shown] [Illustration: A simple crystal of gypsum] [Illustration: Twin crystals of gypsum] Plate 51 [Illustration: Sulphur crystals] [Illustration: Ice crystals, the top one, the end of a hexagonal prism; the two lower figures multiple twins as in snow flakes] Plate 52 [Illustration: The Devil’s Tower, Wyoming, an example of igneous rock with columnar structure, and resting on sedimentary rocks. Courtesy of the U. S. Geological Survey] Plate 53 [Illustration: A coarse granite] [Illustration: Graphic granite] Plate 54 [Illustration: Syenite] [Illustration: Gabbro] Plate 55 [Illustration: Basalt-porphyry. The large white crystals are phenocrysts of plagioclase feldspar] [Illustration: Basalt-obsidian] Plate 56 [Illustration: Amgydoloid] Plate 57 [Illustration: The north face of Scott’s Bluff, Neb., showing sedimentary sandstones above and clays below. The type of erosion is characteristic of arid regions. Courtesy of the U. S. Geological Survey] Plate 58 [Illustration: Breccia] [Illustration: Conglomerate] Plate 59 [Illustration: Calcareous shale] [Illustration: Coquina] Plate 60 [Illustration: Foramenifera from Chalk; enlarged about 25 diameters] [Illustration: Encrinal Limestone; fragments of the stems, arms and body of Crinoids] Plate 62 [Illustration: _A_ diatomaceous earth magnified 50 times] [Illustration: _B_ and _C_ two diatoms from the above enlarged 250 times. After Gravelle, by the courtesy of Natural History] Plate 63 [Illustration: A metamorphic rock, showing the contortion of layers due to expansion under heat] Plate 64 [Illustration: A conglomerate partly metamorphosed to a gneiss. Note the flattened pebbles and the alternation of the intermediate material to mica scales, etc.] [Illustration: A typical gneiss] Plate 66 [Illustration: Phyllite] [Illustration: A white marble, with black streaks due to graphite] Plate 68 [Illustration: Claystones, simple and compound] [Illustration: A line concretion, which on splitting disclosed a fern leaf of the age of the coal measures] Plate 69 [Illustration: A septeria from Seneca Lake, N. Y.] [Illustration: Pisolite] Plate 70 [Illustration: A geode filled with quartz crystals] Plate 71 [Illustration: A quartz pebble from the bed of a New England brook] [Illustration: A pebble of schist and granite from the foot of Mt. Toby, Mass.] Plate 72 [Illustration: An iron-nickel meteorite, of 23 lbs. which fell in Claiborne Co., Tenn.] [Illustration: An etched slice of an iron meteorite which fell in Reed City, Osceola Co., Mich.] Plate 73