Classification of Igneous Rocks.—Igneous rocks according to their composition, chemical and mineralogical, their structures and textures, or their mode of occurrence, are subdivided into many groups. Between allied rocks there are, however, no hard and fast boundaries, for by increase or decrease in the proportions of their constituent minerals, they pass by every gradation one into the other. Similarly the distinctive textures of one kind of rock may often be traced gradually merging into those of another. Hence the definitions adopted in establishing rock nomenclature merely correspond to more or less arbitrarily selected points in a continuously graduated series. This is frequently urged as a rea son for reducing rock classification to its simplest possible terms and using only a few generalized rock designations.
The earliest attempts at the classification of rocks on modern lines are to be found in the works of Von Leonhard (1823) and Brongniart (1827). These and other efforts, principally by Nau mann (185o), Von Cotta (1855) and Roth 0860, while being built on the criteria of composition and structure of rocks, were largely developed from purely macroscopic investigation or ulti mate chemical analysis. The pioneer work of Sorby (1858) in preparing transparent microsections of rocks opened the way for the unravelling of the mineral constituents and textures of the finer grained igneous rocks, and rapid advances in the knowl edge of the constitution of igneous rocks followed. The classifica
tions of Zirkel (1873) and Rosenbusch (1877) were the first products of the application of the new method of petrographic research, and in these mineral constitution was accorded first place as a basis of subdivision. The effect of microscopic study was to stress the importance of mineralogical and textural char acters of rocks to the neglect of other characteristics. Rosen busch in particular, influenced by the contention principally of Lossen (who urged the significance of the geological relations of igneous rocks, their occurrence and manner of formation), fur ther developed his classification to express the relationship of geological occurrence and texture, and introduced in the second edition of his work a fundamental change.
The new classification is based first on geological position, texture and finally mineral and chemical composition. Igneous rocks are divided thus into three great groups :—( I) Deep seated rocks, (2) Dike rocks and (3) Effusive rocks. Like others before it, it is still essentially a qualitative classification, and at the present day is the one most commonly adopted by petrographers. The latest exposition of Rosenbusch's classification is set forth in his Elemente der Gesteinslehre revised by A. Osann (1923).