THE CALCULATION OF THE NORM IN IGNEOUS 



ROCKS 



GEORGE I. FINLAY 

 Colorado College 



In presenting'^ to successive classes of students in petrography the 

 method of calculating the norm for igneous rocks under the quantita- 

 tive classification of Cross, Iddings, Pirsson, and Washington, the 

 writer has found such a selection of calculated analyses as is here given, 

 accompanied by discussion, of great service. For practice in calcula- 

 tion the collection of analyses with their norms by Dr. H. S. Washing- 

 ton given in Professional Papers 14 and 28 of the U. S. Geological 

 Survey is invaluable. Of the calculated analyses which follow, all 

 but two are taken from Professional Paper 14. By using the tables 

 given at the end of the Quantitative Classification of Igneous Rocks^ 

 the arithmetical work in calculating the norms is very greatly lessened. 

 The first set of tables, prepared by Professor J. F. Kemp and originally 

 published in the School of Mines Quarterly,^ gives the molecular pro- 

 portions for the percentage figures of the several oxides recorded 

 in rock analyses. The second set of tables in the Quantitative Classi- 

 fication of Igneous Rocks gives the percentage weights for various 

 proportions of molecules of the standard rock-making minerals. The 

 molecular proportions may be calculated by dividing the percentage 

 figures for each oxide by the molecular weight of the oxide. Thus for 

 65 . 70 per cent, silica the molecular proportion is i .095, the molecular 

 weight of SiOa being 60. The molecular proportion for 15.40 per 

 cent, soda is .248, the molecular weight of Na^O being 62. As a 

 preliminary step in the calculation of an analysis the molecular pro- 

 portions for each oxide must be looked up in the tables. Small 

 amounts of MnO (.001 to .005), and NiO are to be used as FeO; 



I Cross, Iddings, Pirsson, and Washington, Quantitative Classification of Igneous 

 Rocks (Chicago: The University of Chicago Press, 1903), pp. 237-59. 



^ J. F. Kemp, "The Recalculation of the Chemical Analyses of Rocks," School 

 of Mines Quarterly, XXVII, 75-88. 



