- GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of Geology Exclusive of North America, Geological Society of America, Boulder, CO, United States
"The term 'granite' could be usefully reserved for massive or weakly oriented plutonic rocks of colour index less than 20 per cent and quartz content between 20 per cent and 40 per cent by volume. A symbolic classification of rocks meeting these requirements is presented. The various classes are based on the relative proportions of plagioclase and alkali feldspar, and sub-classes based on the ternary dominance ratio quartz-alkali feldspar-plagioclase are also proposed."