The more common minerals, or those which are important petrographically, are printed in heavy-faced type; the minerals of less importance in small capitals. ABBREVIATIONS AND CONVENTIONS USED. A. = Amorphous. I. = Isometric system. T. = Tetragonal system. O. = Orthorhombic system. M. = Monoclinic system. Tri. = Triclinic system. H. = Hexagonal system. M(H). = Monoclinic, with hexagonal form or characters, as in the case of biotite. ⟂ = At right angles to. El. = Elongation. Ex. = Extinction. ∥ Ex. = Parallel extinction, as when the crystals extinguish parallel to cleavage lines or crystal edges. Extinction which is symmetrical to intersecting cleavage lines is also included under this term. The mean refractive indices are printed in heavy-faced type. The term “grains” is used to describe not only minerals which occur in typically granular form, but also those which have coarser allotriomorphic form, as elæolite and sodalite in plutonic rocks, such as syenite, etc. GENERAL RULES FOR USE OF SCHEME. The division of the scheme into two vertical columns is based on the values of the mean refractive indices, as determined by the “relief” and appearance of the surface. When the refractive index is above 1.60, the relief is fairly well to distinctly marked and the surface rough to very rough, depending on the value of the index. Most of the rock-forming minerals with indices below 1.60 show no relief and a smooth surface, except in the case of a few of the rarer minerals (mostly isometric), which have very low indices and hence rough surface. Mistakes may easily be made in the case of minerals near the limit; but practice and the use of the Becke test should soon make possible the classification into the two groups suggested by the scheme, and the appended descriptions will help to check errors. When an unknown mineral lies adjacent to one that is known, use the Becke test for obtaining the relative refractive index of the unknown mineral; focus sharply on the line of contact between the known and unknown minerals,[158] then raise the objective slightly and the “bright line” will appear on the side of the mineral having the higher index. The method of Schrœder van der Kolk may also be employed, see p. 22. The horizontal divisions of the scheme depend on the relative strength of the double refraction based on the observed interference colors. These colors to be of use in classification must be correctly recognized. The lower and middle colors of the 1° order, from bluish-gray through white to yellow, are easily known. The bright red, blue, green, etc., colors of the 1°, 2° and 3° orders can also be differentiated without trouble from the very high order colors (4° and above), which are essentially white in tone with no decided color tint.