This book represents the results of a lengthy study which Professor ALFRED RITTMANN began some thirty years ago. The relationship between the chemical and mineralogical composition of igneous rocks is established as far as is possible. Petrographers will appreciate that this problem is extremely complex, particularly since this relationship forms the basis of the classification and nomenclature of igneous rocks. The ingenious scheme of calculation of the CI.P.W. norm system is essentially chemical in nature. The compositions of the stoichiometrically ideal "normative minerals" do not...