W. Cross — Petrograjpkic Classification. 661 



ferent degrees and in different ways the specific mineral con- 

 stitution but not the chemical character of the whole. 



If this view of chemical classification is correct the criticism 

 by Lindgren and others that the C. I. P. W. system classifies 

 analyses and not rocks is not true. It appears to be an incor- 

 rect generalization based on tiie fact that the system leading to 

 the new nomenclature is founded on proportions of normative 

 molecules named after minerals of the same composition, 

 while the crystalline rock analyzed usually does not have 

 exactly that composition. But the C. I. P. W. system recog- 

 nizes this fact most explicitly. It makes no assumption that 

 the mode of a rock is, or, deploring the vagaries of Nature, 

 that it ought to be, like the norm. The norm is merely a 

 restatement of the chemical analysis in terms of mineral mole- 

 cules, by which the major significance of complex variations in 

 analyses may be expressed and more clearly comprehended. 



The authors of the C. I. P. "W. system call it "chemico-min- 

 eralogical,"* referring to the statement of the analysis in min- 

 eral molecules, and call the systematic terms "magmatic names"f 

 since they may be considered as applying to the magma before 

 the rock has formed from it. The C. I. P. W. system is 

 extended to the actual rock by modifying terms expressing its 

 motexj (mode and texture), but if one has in mind simply the 

 chemical classification the magmatic name clearly applies also 

 to the rock. 



The criticisms of the C. I. P. W. system here considered 

 really go back of that system to fundamental questions, in one 

 case to petrographic system in general and in the other to 

 methods of chemical classification. It is not the object of this 

 discussion to consider the particular norm adopted by the 

 authors of the C. I. P. W. system or the classification founded 

 on it. I wish to say, however, that there are reasons for that 

 norm, and that it is made up mainly of mineral molecules used 

 by Michel-Levy, Posenbusch, Brdgger, Harker and many 

 other petrographers, in studies of the relation between the 

 chemical and mineral constitution of igneous rocks. But the 

 C. I. P. W. system is not assumed by its authors to be the 

 "ultimate" classification of igneous rocks, although they 

 believe it to be a step toward that end. Discussion of its 

 merits and demerits and the test of use will determine its 

 future, but criticisms in which fundamental principles or con- 

 ditions of petrographic system are disregarded can not affect 

 the issue. 



*Op. cit.,p. 111. fOp. cit., p. 168. 



{Op. cit., pp. 168-172. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIX, No. 234.— June, 1915. 

 43 



