66 SCIENCE PROGRESS 



" is primarily a means of comparison, and has in itself nothing 

 to do with system." 1 Many petrographers have found it 

 extremely useful for this and other purposes, quite apart from 

 classification. 



The mode of calculation of the norm has received very little 

 criticism save that of Evans. In the main it corresponds with 

 the well-understood principles of the crystallisation of minerals 

 from igneous magmas. Evans considers the operation of 

 calculating the norm instructive but unreal, and as not com- 

 paring in educative value with that of calculating the mode. 

 He also points out the undue influence of the amount of alumina 

 and the state of oxidation of the iron on the result of the 

 calculation. In fact the bulk of Evans's criticism is that the 

 mode of calculation and subsequent subdivision of the norm 

 causes certain constituents to have an undue effect on the 

 classification, and therefore that " the lines of division of the 

 Quantitative Classification do not stand in any logical relation 

 to the chemical composition." 2 



Other criticism concerns the selection of certain minerals to 

 form the " salic " and " femic " groups of the norm. Evans 

 believes that " the salic group [is] in fact a collection of minerals 

 which [have] nothing essential in common, and that the funda- 

 mental lines of the classification [are] accordingly practically 

 meaningless." He instances especially the cases of corundum 

 and anorthite. In the calculation of the norm the excess of 

 alumina over that necessary for the formation of normative 

 felspars and felspathoids is regarded as corundum and placed 

 in the salic group. Generally, however, this excess enters 

 pyroxenes, amphiboles, or micas, and should thus be regarded 

 as femic. Anorthite, too, is considered as a basic silicate which 

 should enter the femic group, in spite of the fact that it generally 

 occurs in isomorphous admixture with albite. Nevertheless, 

 the division of the norm into salic and femic groups expresses 

 a common variation in igneous rocks which gives rise to what 

 are known as leucocratic and melanocratic facies. It is doubt- 

 ful, however, whether this factor should be given first place, as 

 it is in the American Quantitative Classification. 



A further line of criticism is that the Quantitative Classifica- 

 tion does not fulfil its declared purpose of bringing like rocks 



1 Quart. Jour. Ceol, Soc. vol. 66, 1910, p. 496. 



2 Science Progress, vol. i. 1907, p. 275, 



