Origin of the American Quantitative Igneous Rock Classification: Part 1

In 1902, four American petrologists, C. Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry S. Washington co-authored a lengthy paper in which they proposed an extremely complex, quantitative classification of igneous rocks. Taking advantage of developments in microscopic petrography, the theory of magmatic differentiation, and knowledge of the chemical composition of igneous rocks, the ‘CIPW’ classification marked a radical break with all previous igneous rock classification schemes such as those of Rosenbusch and Zirkel.

The present paper is the first in a series that explores the genesis and conceptual development of the American quantitative classification. In 1893 Iddings posed a simple question to three young American petrologist friends, Cross, Pirsson, and George H. Williams, about the legitimacy of the use of the terms ‘porphyry’ and ‘porphyrite’. Henry Washington was to join the group later, but Williams was one of the original four. From this question there emerged an expanded discussion, conducted primarily via correspondence and mutual criticism of lengthy essays written by each of the four, dealing with larger questions of principles of classification and nomenclature. Any future classification, they agreed, should be based on igneous rock chemical composition and must reject geological age and geological occurrence as factors. Despite calls within the group for construction of a totally new igneous rock classification, the process evolved cautiously to a decision to publish a statement of general principles of igneous rock classification together with suggestions for changes in nomenclature for the benefit of petrologists. Eventually the project foundered because of health problems for Williams and the press of other obligations and interests on all four. In 1894, the project ground to a halt with the untimely death of Williams from typhoid fever.