62 SCIENCE PROGRESS 



The American Quantitative Classification starts from the 

 hypothesis that there are no "natural" lines of division in 

 igneous rocks on which a classification may be based; that 

 igneous rocks constitute a continuous field in all directions, and 

 are only capable of an arbitrary division into compartments of 

 equal value, just as, for example, is the scale of temperature. 1 

 Dr. Cross's discussion of the adequacy of certain factors as a 

 basis for classification on " natural lines" seems to me conclusive 

 that there are no suitable factors save, perhaps, two. 2 The 

 factors of geographic distribution, magmatic differentiation, 

 eutectics, mineral composition and texture, are passed in review 

 and dismissed as affording no suitable " natural " basis for rock- 

 classification. The chemical composition, however, is treated as 

 the most fundamental character of igneous rocks, and as the one 

 most susceptible of arbitrary subdivision. The American Quan- 

 titative Classification has therefore been based on chemical com- 

 position so manipulated as to give a mineralogical expression. 

 The authors of this classification have never, in the writer's 

 opinion, given sufficient weight to the possibilities of a classifica- 

 tion by actual mineral composition, or, in other words, a modal 

 classification on quantitative lines. It is to be admitted that the 

 presence of minutely-crystallised or uncrystallised matter, and 

 the occupation of certain minerals in others, are grave difficul- 

 ties ; but these could probably be overcome by certain expedients 

 of which an outline is given later. In any case, the difficulties 

 so caused would probably not be so great as those caused by the 

 omission of the alferric minerals, for example, from the norm on 

 which the American Quantitative Classification is based. 



From the standpoint of a utilitarian classification it is 

 certainly better to accept the American view of igneous rocks as 

 a "continuous series of chemical solutions and their solidified 

 phases " rather than continue to apply " the misleading bio- 

 logical concepts of ' families ' and ' descent ' " ; although it must 

 be admitted that many of the solidified solutions are related by 

 processes of differentiation from a common magma, and that 

 there may be a concentration of rock-types in certain parts of 

 the classificatory field. 



1 Quantitative Classification of Igneous Rocks, by W. Cross, J. P. Iddings, L. V. 

 Pirsson, and H. S. Washington. Chicago, 1903. 



'The Natural Classification of Igneous Rocks," Quart. Jour. Geo/. Soc. lxvi. 

 iyio, pp. 470-506. 



