PART I. CLASSIFICATION. Introduction. Defects of present system. Basis of classification. The facts to be considered; The existence of petrographic provinces; The principles; A brief discussion of various principles. Chemico-mineralogical c assification. Standard mineral composition. Construction of the system. Outline of the system. Class; Subclass; Order; Suborder; Rang; Subrang; Grad; Subgrad; Section. The role of actual mineral composition and texture in rock classification. Actual mineral composition. Complete or almost complete accord between the mode and norm; Appreciable difference between norm and mode; Varieties; Indeterminable modes. Texture. Crystallinity; Granularity; Fabric; Heterogeneous textures.