A REVIEW OF IGNEOUS ROCK CLASSIFICATION 65 



the system is arbitrary, but that it is necessarily so since 

 igneous rocks are insusceptible to a quantitative treatment on 

 any " natural" or genetic factor as yet discovered, and constitute 

 a uniform field in all directions, only capable of arbitrary sub- 

 division into compartments of equal value. The discussion thus 

 shifts to the question of the possibility of a " natural " quantitative 

 classification expressing the genetic relations of igneous rocks. 

 As the result of a rigorous discussion, Dr. Cross concludes that 

 a natural basis for classification, whilst desirable, is impossible 

 in view of the unsuitability of most of the factors proposed. 1 

 It must also be admitted that at present no suitable factors 

 for quantitative treatment have emerged, save chemical and 

 mineralogical composition, and that it is highly improbable any 

 further " natural " basis for classification will be discovered in 

 the future. But since the needs of petrographers are immediate, 

 it appears necessary to fall back on an arbitrary mode of classi- 

 fication, even if only for purposes of reference and comparison. 2 

 Apart from the question as to whether there are any natural 

 bases for classification, petrographers cannot afford to wait for 

 their discovery. In the meantime, for want of an exact classi- 

 fication by which rocks may be compared, the science of com- 

 parative petrology, and the consideration of such important 

 problems as that of petrographic provinces, may come to a 

 standstill. It is of the utmost importance, therefore, to come to 

 a decision on this question of classification, if only from the 

 utilitarian point of view. 



Another class of criticism of the American Quantitative 

 Classification is that which centres round the norm and its use 

 in the system. The norm is a set of standard minerals calculated 

 from the analysis of a rock, and has been criticised as " hypo- 

 thetical," "artificially selected," "ideal," even as "imaginary." 

 To this Dr. Cross replies that the minerals of the norm are 

 largely those which are believed by most petrographers to be 

 present as definite compounds in the magma, and which singly, 

 or variously combined, form the minerals actually present in 

 the rock. There is, however, some point in the criticism, since 

 petrographers have to deal with rocks composed of minerals, 

 and not with magmatic molecules which are necessarily some- 

 what hypothetical. Dr. Cross also points out that the norm 



1 Quart. Jour. Geo/. Soc. vol. 66, 1910, pp. 470-506. 



- J. W. Evans, in discussion of Cross's paper, ibid. p. 504. 



