3. The Amphibole Group: 3a. Hornblende. 3b. Tremolite. 3c. Actinolite. 3d. Uralite. 4. Muscovite (sericite). 5. The Chlorites: 5a. Jefferisite. 5b. Ripidolite. 5c. Penninite. 5d. Prochlorite. 6. Calcite (and aragonite). 7. Wollastonite. 8. Scapolite. 9. Garnet. 10. Epidote. 11. Zoisite. 12. Serpentine. 13. Talc. 14. Kaolin. 15. The Zeolites: 15a. Pectolite. 15b. Laumontite. 15c. Prehnite. 15d. Thomsonite. 15e. Natrolite. 15f. Analcite. 15g. Datolite. 15h. Chabazite. 15i. Stilbite, 15k. Heulandite. 15l. Harmotome. 16. Magnetite. 17. Hematite. 18. Limonite. 19. Siderite. 20. Pyrite. 21. Pyrrhotite. ROCKS AND ROCK DECAY. =27. Types Of Rocks.=—Rocks may be divided in reference to their structure into four types: First, crystalline; second, vitreous; third, colloidal; fourth, fragmental. Of these classes there may be selected, as types of the first order, granite and crystalline limestone. The second class is typically represented by obsidian. Rocks of this kind are confined to a volcanic origin. The third class of rocks is completely amorphous in its structure and is less common than the others. It is found only in rocks of chemical origin. Types of this class are the siliceous sinters, opals, flint nodules, and many serpentines. Of the fourth class of rocks, sandstone is typical, being comprised wholly of fragments of rocks pre-existing. The particles may be held together either by cohesion or by a cement composed of silica, iron oxids, carbonate of lime or clayey matter. =28. The Microscopical Structure of Rocks.=—A great deal more light is thrown upon the nature of rock materials by microscopical study than by their study in bulk. The requisites for a microscopical study of rock are that the material should be cut into extremely thin laminae with parallel sides and polished so as to transmit the light freely. The study of the crystalline structure of the material is then conducted by means of a microscope furnished with polarizing and analyzing appliances. The light before passing through the mineral film is polarized by a Nicol prism. After passing through the film it is analyzed by a second Nicol prism. In this way the crystalline structure of the rock as affecting polarized light is distinctly brought out. The thickness of the films examined should be from ¹⁄₅₀₀ to ¹⁄₆₀₀ of an inch. [Illustration: FIG. 1. Microstructure of granite. FIG. 2. Microstructure of micropegmatite. FIG. 3. Microstructure of quartz porphyry. FIG. 4. Microstructure of porphyritic obsidian. FIG. 5. Microstructure of trachyte. FIG. 6. Microstructure of serpentine. ]