A simple quantitative method of classification and chemical comparison of the main volcanic rock species has been devised using the three-axis orthogonal plot Na2O + K2O versus FeO + Fe2O3 + 1/2 (MgO + CaO) versus Al2O3/SiO2, in weight percent. The method combines a new index of basicity with some of the important aspects of variation diagrams proposed by Tilley and Murata. The resulting generalized treatment of major oxide data, which stresses the gross differences in rock composition, has the important advantages of de-emphasizing some of the possible effects of analytical error and requiring a minimum of manipulation and processing of data.

Graphs of large numbers of published and unpublished analyses readily discriminate between magma types and trends and reaffirm classical volcanic rock nomenclature. Good correlation is established with the arc fusion determinative procedure of Mathews, thereby increasing the scope and general utility of the method.

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