W. Cross — Petrographic Classification. 657 



Art. XLYIII. — On certain Points in Petrographic Classifica- 

 tion ; b}' Whitman Cross. 



The Quantitative Classification of Igneous Rocks, proposed by 

 Cross, Icldings, Pirsson and Washington,* is based on principles 

 and conceptions which, at the time the system was first 

 published, in 1902, had been given but little prominence 

 in petrography. Both principles and essential features have 

 been discussed in a scientific spirit by petrographers of various 

 countries. This discussion and the criticism or suggestion 

 based on it have been helpful to the backward science of 

 systematic petrography and are, of course, welcomed by the 

 authors of the Quantitative system. 



The name "Quantitative System" was used because it was 

 desired to emphasize the fact that definite quantitative factors 

 were used in it throughout, making it contrast with earlier 

 systems. But manifestly there may be many other quantitative 

 systems and I shall here refer to this first one as the C. I. P. W. 

 classification or system, following a precedent established by 

 several writers. 



Some of the criticisms and comments which have been freely, 

 and in certain cases casually, made concerning the C. I. P. W. 

 system, have seemed to its authors too evidently fallacious to 

 need reply. But when such critical comments are repeated 

 by petrographers with the reputation enjoyed by P. A. Daly 

 and Walclemar Lindgren we must take a different attitude, for 

 their remarks should certainly be free from the suspicion of 

 prejudice or flippancy. 



Dr. Daly has severely assailed the C. I. P. W. classification 

 in his interesting work " Igneous rocks and their origin "f 

 and while the criticism appears to me clearly fallacious, 

 I must assume that it was carefully considered and deliberately 

 made. The classification is said to be fatally defective in that 

 " even its minor subdivisions individually include rock types 

 which are strongly contrasted chemically and separate others 

 which are almost alike chemically, mineralogically and geneti- 

 cally. 7 ' The system is then stigmatized, on account of this 

 " defect," as one which " disregards vital principles of scientific 

 classification." These principles are not formulated. 



The main conception, quite erroneously called a "principle," 

 which is involved in Daly's criticism, may, I hope, be fairly 

 expressed thus : A division of a classification of rocks 

 should not include types that differ very markedly in character 



* Whitman Cross, Joseph P. Iddings, Louis V. Pirsson and Henry S. 

 Washington, " Quantitative Classification of Igneous Rocks," Jour. Geol., 

 vol. x, 1902, pp. 555-690 and in book form, Univ. of Chicago Press, 1903. 



fR. A. Daly, " Igneous Rocks and their Origin," New York, pp. 10, 11, 

 1914. 



