THE NORM IN IGNEOUS ROCKS 59 



and in the same manner small amounts of BaO and SrO are to be 

 added in with the CaO. If CraOj does not amount to .002 it is to be 

 added in with Fe203. For the calculation of a rock analysis we then 

 start with the molecular proportions of the ten oxides Si02, Al^Oj, 

 Fe,03, FeO, MgO, CaO, Na,0, K^O, TiO„ and V,0„ which are 

 contained in nearly every rock, and we may also have present ZrOj, 

 SO3, CI, F, and CO^, besides H^O; and, in smaller amounts, MnO 

 and NiO to be summed in with FeO; BaO and SrO, to be added to 

 CaO; and CrjOj which is to be counted as Fe^Og. ZrO^, Cr^O, 

 in amount more than. 002, TiOa, P2O5, SO3, CI, COj, and F, when 

 they are present, are first calculated as minor inflexible molecules. 

 Their calculation as zircon, chromite, ilmenite, apatite, noselite, 

 sodalite, calcite, and fiuorite presents no difficulty, for the method of 

 procedure is always the same. The eight oxides SiOj, AI2O3, Fe^Oj, 

 FeO, MgO, CaO, Na^O, and K2O are of much greater importance 

 in the calculation of the norm, for any one of these oxides in a given 

 analysis is disposed of with regard to the relative quantities of all the 

 others. The difficulty in presenting to the student the method of 

 procedure in its entirety lies in the fact that a rather long series of 

 considerations is to be put before him at the very outset. The aim 

 of the writer in the present paper is to develop little by little with the 

 aid of examples and discussions the condensed, precise statement of 

 the authors of the Quantitative Classification of Igneous Rocks, pp. 

 188-96. 



The simplest cases are those in which SiOj and AI2O3 are 

 present in relatively large amounts so that they meet all claims 

 upon them and are not exhausted. Al^O, remaining over is cor- 

 undum, and SiOa remaining over after all the allotments is quartz. 

 With SiOj present in abundance Al^Oj may meet all the claims 

 of K2O, NajO, and CaO upon it; or it may satisfy KjO, NaaO, 

 and part of the CaO. Again it may satisfy only K2O and part 

 of the Na^O; or, rarely, only part of the K2O. So the treatment 

 varies. 



The norm minerals of the two groups, which figure in the calcula- 

 tion, with the abbreviations for their names, and their formulas, are 

 as follows. The table is intended to set forth the relative importance 

 of their several rdles in the norm. 



