CLA SSIFICA TION OF IGNEO US RO CKS 575 



The simple metasilicate molecule Na 3 . Si0 2 , analogous to 

 that of wollastonite, is assumed to be present in rocks in which 

 there is an excess of alkalies over A1 2 3 and Fe 3 3 . It 

 appears in the arfvedsonite molecule, which develops in such 

 rocks. But this mineral, being alferric, is not included among 

 the standard minerals for reasons already given. 



In like manner the simplified akermanite molecule, 4Ca0.3SiO„, 

 is included among the standard minerals in place of melilite, 

 because of the alferric character of the actual melilite, our 

 uncertain knowledge of its real composition, and its complexity. 

 The Na 2 and-Al 2 3 with 2Si0 3 of melilite are calculated as 

 entering into a nephelite molecule, the Fe 3 3 is referred to 

 magnetite or hematite, leaving the CaO, partly replaced by 

 MgO, and the remaining Si0 2 in approximately the aker- 

 manite ratio of 4:3. The introduction of this molecule into 

 the calculation is necessary in rocks so low in SiO a that a sub- 

 silicate must develop. 



The molecule (Mg, Fe)0. Fe 2 3 .Si0 2 , which is character- 

 istic of augite, has been omitted for the reason that the ferric 

 oxide is largely replaced by alumina, and because its introduc- 

 tion complicates the problem of calculation. 



Metlwd of calculation. — In order to determine the kinds and 

 amounts of standard minerals that may express the composition 

 of a given rock, to establish its place in the system, we may 

 proceed by a consideration of its chemical composition as given 

 by chemical analysis, or by a consideration of its actual mineral 

 composition as determined by optical investigation, or we may 

 compare it with known rocks whose compositions have been 

 previously determined. 



And since it is advisable to select the same mineral combina- 

 tion for the basis of comparison of chemically similar rocks, as 

 already stated, and since there are gradual transitions among 

 igneous rocks, it is necessary to follow the same method of 

 procedure in calculating the kinds and amounts of the standard 

 minerals for all rocks. The method adopted by us is explained 

 in detail in a later part of this paper. It is based upon certain 



