Origin of the American Quantitative Igneous Rock Classification: Part 4

Abstract

After several failed attempts to construct a new chemico-mineralogical igneous rock classification on the basis of three or more factors, C. Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry S. Washington decided to establish a practical and logically consistent scheme on the basis of only two factors, namely, two primary groups consisting of hypothetical minerals with ideal chemical compositions, the amounts of which were to be calculated from the chemical compositions of igneous rocks. One primary group consisted mainly of alkali alumina silicates and quartz (salic minerals), whereas the other consisted mainly of calcic ferromagnesian minerals (femic). A list of the amounts of these calculated standard minerals within an igneous rock was termed the norm, and the list of the proportions of actual minerals in a rock was termed its mode.In the Fall of 1901 the quartet decided to publish a preliminary paper that presented the major themes of their two-factor classification scheme. Iddings was charged with writing a draft of the scheme, and Washington was assigned the task of producing an essay on nomenclature to be published later. The team continually refined the definitions of specific subdivisions of the scheme proposed by Iddings as well as the corresponding nomenclature proposed by Washington. They also refined the norm calculation algorithm and developed methods for calculating the norm from the mode of rocks containing aluminous ferromagnesian minerals such as hornblende and biotite.After a few months of evaluating and revising a succession of drafts, the team eventually determined to publish their work of classification and nomenclature all at once by combining the refined drafts into one large comprehensive manuscript in three parts: Part I on classification, Part II on nomenclature, and Part III on methods of calculation. Iddings coordinated and edited the various criticisms and drafts contributed by team members into one large manuscript and shepherded the project to final publication in late October 1902 in Journal of Geology. The massive 139-page article entitled ‘A quantitative chemico-mineralogical classification and nomenclature of igneous rocks’ was preceded in earlier issues of the same journal by a two-part series authored by Cross on the historical development of systematic petrography. In early 1903, both of Cross's papers and the CIPW article were combined and published as a book, Quantitative Classification of Igneous Rocks.