658 W. Cross — Petrographic Classification. 



nor separate types that are very ranch alike. I suspect that 

 this idea is entertained by many petrographers, with whom " the 

 wish is father to the thought." 



A principle is defined as " a truth which is evident and 

 general." I should state the real principle involved in this 

 matter as follows : Since igneous rocks form a continuous 

 series or field, as to chemical, mineral, and textural characters, 

 the divisions of their systematic classification by these factors 

 necessarily separate types that are nearly alike and include 

 types that are relatively very unlike. Petrographic system is 

 in this respect analogous to any regular division of mass, space 

 or time. The conception I attribute to Daly is antithetical 

 to this principle. The C. I. P. W. system operates as Daly 

 has pointed out, but this result is in harmony with the nature 

 of things. 



The criticism by Daly is strongly inconsistent as well as 

 untenable, for the same " defect " he abhors in the C. I. P. W. 

 system is clearly present even in the one he himself uses and 

 advocates. For example, certain monzonites are very closely 

 related to certain syenites but are relatively very different 

 from those monzonites which are near relatives of essexite or 

 gabbro. 



Perhaps the prevailing vagueness of the current modal or 

 mineralogical system of rock classification, whose partitions 

 are elastic like rubber and permit some dilation or complementary 

 contraction of its subdivisions according to the desires of the 

 individual petrographer regarding certain rocks, may blind 

 Daly to the existence in it of the same " defect " found by 

 him in the C. I. P. W. classification. 



An inconsistency in Daly's criticism, suggesting the control 

 of prejudice, is further strikingly shown on the page following 

 his denunciation of the C. I. P. W. system. He there states 

 his adoption of the modal or Rosenbusch scheme, which he 

 thinks will be improved in the course of time. To this Daly 

 adds that " To become an ideal Mode classification it should 

 be made quantitative." * But the divisions of this " ideal " 

 modal, quantitative system would have sharpedly defined 

 boundaries and necessarily include contrasting and separate 

 similar rock types, under the general principle enunciated 

 above. 



Another fallacious comment on the C. I. P. W. system has 

 been recently repeated by Lindgren f in a note on Iddings' 

 review of Daly's book, above referred to. Lindgren says that 

 Daly " does not stand quite alone " in his criticisms of 



*Op. cit., p. 12. f Science, vol. xli, p. 166, 1915. 



