﻿-106 DK. WHITMAN CROSS ON THE NATURAL [Aug. I9IO, 



the norm, is found to agree very closely with the actual mineral 

 composition (mode) of many comparatively simple rocks. Neither 

 the norm nor any other calculated set of molecules representing 

 minerals can correspond with the mode of most intermediate and 

 femic rocks, for mineral composition is not a function of chemical 

 composition alone, but is controlled largely by the varying con- 

 ditions under which a given magma consolidates in different places. 

 The molecules of the magma may form a considerable number of 

 complex compounds, and these undoubtedly vary as the equilibrium 

 in the solution is disturbed by changing conditions, and magmatic 

 reactions occur. 



To repeat, the norm is primarily a means of comparison, and has 

 in itself nothing to do with system. We seem to have made a 

 mistake in assuming that, provided we were explicit in definition, 

 this standard could be called a norm and its adjective normative 

 without confusion with the old term normal. Norm and norma- 

 tive had not been used in petrographic discussion, as far as I am 

 aware, before their introduction by us for a special purpose. When 

 proposing the terms norm and normative, we stated that 



' the standard minerals which make up the norm are to be called the 

 normative minerals, not the normal ones, since the latter adjective has 

 the meaning of usual or common.' ' 



But it has been one of the commonest criticisms of the Quantitative 

 System that we assume as normal to a rock a composition which 

 it does not exhibit. 2 Such is not the case. The norm is simply 

 a standard form of expressing the chemical analysis of a rock in 

 comprehensible terms, which also apply to its magma. The calcu- 

 lation of the norms for a set of rock analyses in no wise classifies the 

 rocks. It does assist more than any other expedient, in my opinion, 

 in understanding the complex relation between chemical analysis as 

 usually .stated and mineral composition. An example of its use 

 independent of system is afforded by the discussion of the analyses 

 of camptonite in this paper. 



The 'norm' in quantitative classification. — A quanti- 

 tative classification of rocks according to chemical factors may be 

 based on the usual analytical statement of composition, or on some 

 other expression calculated from the analysis. If a proper expres- 

 sion of chemical composition in mineral terms can be found, it 

 seems desirable to use that expression in classification. The norm 

 was of course planned for this ultimate purpose, as well as for 



1 ' Quantitative Classification of Igneous Rocks ' 1903, p. 147. 



2 The recently published brief statement of the Quantitative System by 

 Mr. L. Fletcher, which I know must have been intended for a fair resume 

 ('An Introduction to the Study of Rocks,' 4th ed., 1909, p. 146), refers to the 

 molecules of the norm as ' imaginary ' (!). I can but deprecate the use of a 

 term serving rather to prejudice the reader than to explain the system. 



