CHAPTER IV.

THE VARIOUS ROCKS AS SOIL-FORMERS, 47.—General Classification, 47.—Sedimentary, Metamorphic, Eruptive, 47.—Sedimentary Rocks; Limestones, Sandstones, Clays, Claystones, Shales, 47.—Metamorphic Rocks: Formed from Sedimentary, 48.—Igneous or Eruptive Rocks, Basic and Acidic, 49.—_Generalities Regarding Soils Derived from Various Rocks_, 49.—Variations in Rocks themselves. Accessory Minerals, 50.—Granites; not always True to Name; Sierra Granites, 51.—Gneiss. Mica-schist, 51.—Diorites, 51.—Diabases, 51.—Eruptive Rocks; Glassy ones Weather Slowly; Basaltic Oxidize Rapidly, 52.—Red Soils of Hawaii, Pacific Northwest, 52.—Trachyte Soils; Light-colored, rich in Potash. Rhyolites generally make Poor Soils, 53.—_Sedimentary Rocks_, 53.—Limestones, 53.—“A Limestone Country is a Rich Country,” 53.—Residual Limestone Soils; from “Rotten Limestone” of Mississippi; Table, 54.—Shrinkage of Surface, 55.—Sandstone Soils, 55.—Vary According to Cement, and Nature of Sand, 55.—Calcareous, Dolomitic, Ferruginous, Zeolitic, 56.—Clay-sandstones, Claystones, 57.—_Natural Clays_, 57.—Great Variety, Enumeration and Definition, 58.—Colors of Clays, 58.—_Colloidal Clay_, Nature and Properties, 59.—Plasticity; Kaolinite Non-plastic, 59.—Causes of Plasticity, 60.—Separation of Colloidal Clay, its Properties, 61.—Effects of Alkali Carbonates on Clay, 62.