92 GEORGE I. FINLAY 



calculated norms. Where additions as those of small amounts of 

 BaO and SrO to CaO have not been made, slight discrepancies may 

 result all along the line. Numerically the allotment to such minerals 

 as diopside, hypersthene, and olivine may vary by a single unit one 

 way or the other in MgO and FeO. The application of the methods 

 of calculation given in the Quantitative Classification of Igneous 

 Rocks should however be precise, the end in view being the correct 

 placing of a given rock where it belongs in the scheme of classifica- 

 tion. The method of calculating the norm is necessarily arbitrary in 

 order that concordant results may be obtained by all who make use 

 of it. It should be borne in mind, however, that it agrees with the 

 great body of our observations on the occurrence of minerals in the 

 igneous rocks. The work of calculation has therefore a peculiar 

 value for the student, aside from his needs in classifying rocks, for it 

 directs his thought toward the relations obtaining among the phenom- 

 ena in cooling rock magmas. It brings home to him why it is that 

 we do not have such an occurrence as that of quartz and nephelite 

 together. It points out to him, for instance, the significance of the 

 presence in a rock of such minerals as corundum, acmite, or perofskite 

 rather than titanite, and it does much to make clear to his mind the 

 significance of each of the mineral molecules occurring in the igneous 

 rocks. 



Note. — In "The Roman Comagmatic Region," Publication jy of the Carnegie 

 Institution of Washington, Dr. H. S. Washington has stated on p. 15 a modification of the 

 method of calculation proposed by the authors of the quantitative system. This has 

 to do with the normative minerals, sodalite and noselite. These mineral molecules are 

 split up, and in their stead a statement is made in the norm of the amounts of halite 

 (HI), NaCl, and thenardite (Th), Na,S04. SO3 takes an equivalent number of 

 molecular units of Na^O to form thenardite, and CI takes one-half its number of 

 molecular units of NajO to form halite. "The soda which was previously combined 

 with the sodium chloride and sulphate remains with the rest in calculating the norm, 

 and, if necessary, is distributed between albite and nephelite in the usual way. An 

 advantage of this method of procedure is that it minimizes the influence of the small 

 amounts of CI and SO3 usually found, which is very great if they bind up in the norm 

 a much greater amount of soda and silica." 



