Classification of Igneous Rock Series

Some rock series cannot be properly classified either as alkalic or as sub-alkalic. To meet this situation a simple extension of the present twofold classification is proposed; this takes the form of a fourfold division into alkalic, alkali-calcic, calc-alkalic, and calcic groups. It is shown, from a partial graphical representation of thirteen rock series, that the silica value at which the curves for total alkalis and for lime intersect (the alkali-lime index) is characteristic of a rock series and is of classificatory value. From a consideration of the curves of the thirteen rock series and their mineral assemblages, limits are suggested for the proposed four groups in terms of the alkali-lime index and of characteristic minerals. Other classifications of rock series are briefly noticed, and it is observed that they are dependent on a chemical and mineralogical classification. Since an ideal genetic classification is not in sight, it is concluded that a somewhat closely defined, descriptive fourfold grouping on a chemical and mineralogical basis may be more useful than the present rather vague twofold division.