After evaluating two contrasting proposals, four American petrographers, Cross, Iddings, Pirsson, and Washington, meeting in late March 1901, formulated a preliminary quantitative classification of igneous rocks on a chemico-mineralogical basis. The team agreed that five different mineral groups should serve as factors for subdivision of the rocks into different orders. They also defined several further categories of subdivisions and established chemical and/or mineralogical criteria for those taxonomic levels. Washington began work on appropriate nomenclature for the various subdivisions.
During the ensuing month, Iddings and Washington suggested several modifications to the group proposal. After intense discussion by way of letter, the quartet struggled to work out further details of their scheme logically even as they encountered a host of difficulties in applying the scheme. Washington recognized that their preliminary scheme was too complex and impractical for use by working petrographers.
To meet the challenges, Washington and Pirsson met in late April 1901 and proposed that the team abandon its scheme and substitute one based on two factors only: ‘light minerals’ rich in Si, Al, K, and Na and ‘dark minerals’ rich in Si, Ca, Mg, Fe, and Al. All four agreed in principle to the new scheme and worked feverishly on the identity of and criteria for the subdivisions. Meeting in July 1901, Iddings, Pirsson, and Washington decided that the time had come to move toward publication of a statement of principles of the new two-factor scheme. Iddings was commissioned to write a rough draft describing the scheme and Washington was charged with working on nomenclature.