▄Scapolite Group▄, 59 Scheme of classification into systems by optical determinations, 141 Scheme, optical, 145 Schiller structure, 25 Schrœder van der Kolk’s method for determining refractive indices, 22 Sections, methods of preparing, 117 thickness of rock, 124 Selenite plate, 33 Separation, by chemical means, 134 by electro-magnet, 134 by specific gravity, 133 ▄Sericite▄, 92 ▄Serpentine▄, 114 Shagreened surface, 17 ▄Shorl▄, 70 ▄Sillimanite▄, 73 Simple twinning, 37 Single image, 2 Skeleton forms, 17 Slides, glass, 125 Slower and faster rays, test for vibration directions of, 33 ▄Sodalite▄, 56 ▄Sodalite Group▄, 56 Sodium, test for, by Borichy’s method, 136 test for, by Behren’s method, 138 Sorby’s method for determining index of refraction, 19 Special micro-chemical tests, 139 Specific gravity separation, indicators for, 133 Sphærulitic structure, 39 ▄Sphene▄, 96 ▄Spinel▄, 53 Stage of microscope, rotating, 10 ▄Staurolite▄, 74 Strained or broken crystals, 15 Strength of double refraction, measure of, 30, 35 Structure, 37 Symmetrical extinction, 31 Systems, classification into, by optical determinations, 141 Table, double refraction, 142 Table, indices of refraction, 142 ▄Talc▄, 94 Test for vibration plane of polarizer, 9 Tetragonal crystals, 4 Thickness of rock sections, 124 Thickness of section and minerals, determined by table and diagram, 36 ▄Titanite▄, 96 Transmitted light, characters observed by, 13 effect on crystals, 2 Transparent minerals, investigation of characters of, 13 ▄Tremolite▄, 84 Triclinic crystals, 4 principal vibration directions in, 5 ▄Tridymite▄, 65 ▄Topaz▄, 74 ▄Tourmaline▄, 70 Twinning, 37 Uniaxial crystals, vibration directions in, 3 Uniaxial interference figures, 40 ▄Uralite▄, 88 Uralitization, 88 Uses of parallel and convergent light, 49 van der Kolk’s, Schrœder, method for determining refractive indices, 22 ▄Vesuvianite▄, 60 Vibration direction, 2 directions of faster and slower rays, test for, 33 directions, principal, 3, 4 plane of nicol, 9 plane of polarizer, test for, 9 von Federow mica wedge, 35 Wavy extinction, 16, 31 Wedge, mica, 35 quartz, 34 ▄Wernerite▄, 59 Xenomorphic, 15 ▄Zeolites▄, 79 ▄Zircon▄, 58 ▄Zoisite▄, 96 α and β, 96 Zonal structure, 38 ERRATA AND ADDENDA. P. 33, line 23, _for_ beat _read_ be at. P. 66, line 10, _after_ appearance _add_ (sometimes called “twinkling”). Corrections for mean index of refraction _n′_: p. 60, line 1, 1.551 to 1.584; p. 62, line 29, 1.766; p. 66, line 3, 1.601; p. 67, line 24, 1.622; p. 71, line 10, 1.633 to 1.674. Corrections for Indices of Refraction (mean) in Table on p. 142: Corundum 1.766; Tourmaline 1.674; Tourmaline (precious) 1.633; Dolomite 1.622; Scapolite (Meionite) 1.584; Scapolite (Marialite) 1.551; Calcite 1.601.