Fig. 10. The Structures of Rocks as shown in their Sections1, Granite, acid rock, all crystalline. Q, quartz = excess of acid ; F, felspar; M, white mica; B, black mica; P, apatite, lime phosphate. 2, Quartz-porphyry. The quartz grains alone have had time to form; they have been corroded, and the matrix is volcanic glass, virtually all the substances that form the other crystals in granite melted up. 3, Dolerite, basic rock, all crystalline. M, Magnetite (black) iron ore = excess of base; o, olivine altering to serpentine; A, augite; F, felspar. 4. Marble composed of crystals of calcite; a sediment recrystallized. 5, A sandstone. Sandgrains water-worn, held together by a small amount of iron oxide, which coats the grains and colours the sandstone brown. Open spaces between the grains. 6, Chlorite schist. A recrystallized rock, scales of chlorite all in more or less parallel position, with grains of magnetite and quartz between.
