1.576 Chlorite (Penninite) 1.572 Talc 1.584 Scapolite (Meionite) 1.564 Phlogopite 1.56 Serpentine 1.559 Labradorite, Ab_{1}An_{1} 1.55 Kaolin 1.547 Quartz 1.551 Scapolite (Marialite) 1.601 Calcite 1.541 Oligoclase, Ab_{4}An_{1} ▄1.54▄ + ▄Canada balsam▄ 1.539 Nephelite (Elæolite) 1.539 Iolite (Cordierite) 1.535 Albite 1.525 Gypsum 1.525 Microcline 1.523 Orthoclase 1.509 Leucite 1.503 Haüynite 1.488 Analcite 1.483 Natrolite 1.483 Sodalite 1.477 Tridymite 1.46 Opal [Illustration: Diagram showing relation between strength of Double Refraction, Interference Colors and Thickness of Section.[156] ] ORDER OF CONSOLIDATION OF THE CONSTITUENT MINERALS IN PLUTONIC ROCKS. “There is in plutonic rocks a normal order of consolidation for the several constituents, which holds good with a high degree of generality. It is in the main, as pointed out by Rosenbusch, a law of ‘decreasing basicity.’ The order is briefly as follows: “1. Minor accessories (apatite, zircon, sphene, garnet, etc.) and iron ores. “2. Ferro-magnesian minerals—olivine, rhombic pyroxenes, augite, ægirine, hornblende, biotite, muscovite. “3. Felspathic minerals—plagioclase felspars (in order from anorthite to albite), orthoclase (and anorthoclase). “4. Quartz, and finally microcline. “In most rocks such minerals as are present follow the above order. The most important exceptions are the intergrowth of orthoclase and quartz and the crystallization of quartz in advance of orthoclase in some acid rocks, and the rather variable relations between groups 2 and 3 in some more basic rocks. The order laid down applies in general to parallel intergrowths of allied minerals; thus when augite is intergrown with ægirine or hornblende the former mineral forms the kernel of the complex crystal and the latter the outer shell; when a plagioclase crystal consists of successive layers of different compositions the layers become progressively more acid from the center to the margin. “Certain constituents having variable relations are omitted from the foregoing list. Thus nepheline (elæolite) and sodalite belong to group 3, but may crystallize out either before or after the feldspar.”[157] OPTICAL SCHEME. INTRODUCTION. The scheme is designed to furnish the student with a practical method of recognizing the common minerals in rock sections. The arrangement followed has been to group minerals having general optical characters in common, at the same time giving their specific characters so as to make it possible to distinguish one from another. In each rectangle the minerals are arranged in order of their indices of refraction. A tabulation of the minerals, with a list of optical characters appended, is of aid to the skilled investigator, but of very little assistance to the beginner.