The metamorphic cycle becomes a logical and convenient geologic basis for correlating, interpreting, and classifying mineral products. Because of the great variety of materials and conditions represented in mineral deposits, prodigious efforts are required to remember them as independent entities; but as incidents or stages in the well-known progress of the metamorphic cycle, their essential characteristics may be easily remembered and kept in some perspective. Ores of certain metals, such as iron, occur in almost every phase of the metamorphic cycle,--as igneous after-effects, as weathered products, as sediments, and as schists. The ores of each of these several phases have group characteristics which serve to distinguish them in important particulars from ores belonging to other phases of the cycle. Having established the position of any particular ore in the metamorphic cycle, a number of safe inferences are possible as to mineralogical composition, shape, extent, and other conditions, knowledge of which is necessary for an estimate of commercial possibilities. FOOTNOTES: [1] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol. Survey_, 1920, p. 35. [2] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol. Survey_, 1920, p. 33. [3] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol. Survey_, 1920, pp. 22-23. CHAPTER III SOME SALIENT FEATURES OF THE GEOLOGY AND CLASSIFICATION OF MINERAL DEPOSITS VARIOUS METHODS OF CLASSIFICATION Mineral products may be classified according to use, commercial importance, geographic distribution, form and structure, mineralogical and chemical composition, or origin. Each of these classifications is useful for some purposes. The geologist usually prefers a classification based on origin or genesis. In the following chapters on mineral resources, however, such a classification is not the primary one, because of the desire to emphasize economic features. The mineral commodities are treated as units and by group uses. Some mineral commodities have so many different kinds of origin in different regions that to distribute them among several genetic groups in description would make it impossible to preserve the unity necessary for consideration of the economic features. While in the descriptive chapters many references are made to origin, it may be difficult for the reader to assemble them in perspective; for this reason we summarize at the outset some of the salient features of origin of mineral deposits and of their geologic classification.