[Illustration: _C_ cross section to show how the form may appear six-sided]
[Illustration: Typical form of the anhydrite crystal]
[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: Ice crystals, the top one, the end of a hexagonal prism; the two lower figures multiple twins as in snow flakes]
[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]
[Illustration: Basalt-porphyry. The large white crystals are phenocrysts of plagioclase feldspar]
[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]
[Illustration: Foramenifera from Chalk; enlarged about 25 diameters]
[Illustration: Encrinal Limestone; fragments of the stems, arms and body of Crinoids]
[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]
[Illustration: A metamorphic rock, showing the contortion of layers due to expansion under heat]
[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 white marble, with black streaks due to graphite]
[Illustration: Claystones, simple and compound]
[Illustration: A line concretion, which on splitting disclosed a fern leaf of the age of the coal measures]
[Illustration: A septeria from Seneca Lake, N. Y.]
[Illustration: A geode filled with quartz crystals]
[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.]
[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.]
[Illustration: A stony meteorite, about natural size, which fell in 1875, in Iowa Co., Iowa]