﻿Vol. 66. .] CLASSIFICATION OF IGNEOUS EOCKS. 495 



This would be a quantitative system, and no such scheme has been 

 presented. 



The representation by diagrams of the average composition of 

 rocks, to which certain names are given in the current system, 

 is valuable as illustrating the influence of chemical on mineral 

 character. Certain characteristics of selected centre-points are 

 thus brought out.. But the intermediate centre-points* have also 

 their characteristic diagrams, and no systematic solution is reached 

 until a quantitative basis for the units has been formulated. 



The connexion between mineral and chemical composition is 

 definite for some rocks, and approximate for most others. Hence, 

 if the varying chemical analyses can be interpreted in terms of 

 minerals, we can appreciate the significance of their differences. 

 An attempt to provide such a basis of comparison has been made by 

 Iddings, Pirsson, Washington, and mj'self ; and I wish to discuss 

 that proposition the more fully, because it has not been understood 

 by many petrographers, who have viewed it entirely as a part of 

 the Quantitative System. 



The 'norm' as a means of interpreting and compar- 

 ing rock- analyses. — An analysis of a fresh igneous rock gives 

 the composition of the magma which it represents as closely as 

 possible, the lost volatile constituents being no longer determinable. 

 A magma on crystallization yields certain minerals ; and, while the 

 micas, amphiboles, pyroxenes, and many felspars are complex 

 mixed crystals, they can be regarded as composed of simpler 

 mineral molecules, most of which may occur alone. It is possible 

 to select a series of comparatively simple mineral molecules, into 

 which all rock-analyses may be translated by calculation. If this 

 is done by an invariable rule, the meaning of chemical differences 

 in analyses is more clearly brought out than by comparing the 

 oxides or any chosen ratios. 



A definite set or series of mineral molecules was selected for this 

 purpose by my colleagues and myself, and called by us the norm, 

 meaning a standard of measurement or comparison. This series of 

 molecules was not a haphazard selection from among those that 

 might be used, nor was the procedure in calculation adopted without 

 special reason. Chemical affinities and the laws which seem to 

 control crystallization were regarded as far as possible, and the 

 norm is a kind of statement of potential mineral composition of a 

 magma. To an extent greater than we realized at first, it now 

 seems probable that the molecules of the norm may actually be the 

 molecules present in molten solution. Molecules with the com- 

 position of quartz, orthoclase, albite, nepheline, leucite, anorthite, 

 corundum, diopside, hypersthene, olivine, magnetite, apatite, and 

 others are not unlikely to exist as such in the magmas. The 

 metasilicates of the alkalis may also be there, though entering 

 into acmite or a similar mineral on crystallization. 



The chemico-mineralogical expression of an analysis, obtained in 



