﻿494 DE. WHITMAN CROSS ON THE NATURAL [Allg. 1 9 1 0, 



history of the objects is too complex. The only remaining basis 

 for systematic classification is in the characters of the objects 

 themselves. 



The chemical, mineral, and textural characters of igneous rocks 

 are each gradational as regards several elements., No systematic 

 division can be made except along arbitrary or artificial lines, and 

 in this sense petrographic classification must be unnatural, as the 

 units are not obvious, though no reproach necessarily attaches to 

 the artificial scheme, since it is unavoidable in the nature of 

 things. 



Any systematic classification of rocks worthy of the terms logical 

 and scientific must be constructed with due regard for the facts of 

 the rocks and the relative importance of their characters. Chemical 

 composition is the fundamental character of igneous rocks, and it 

 seems inevitable that it must be the basis of their scientific classifi- 

 cation. In magmas, a few chemical elements are sufficiently 

 abundant to influence materially the character of the minerals 

 developed. The number of minerals is somewhat greater. Whether 

 ultimate chemical or actual mineral components are considered, 

 the differences exhibited by rocks are mainly due to the relative 

 proportions of the components. Any system expressing these 

 relative proportions may be called a quantitative system, and such 

 would appear to be the logical one for petrography under the 

 circumstances. 



This is the age of quantitative investigation, statement, and 

 definition. Everywhere the effort of scientific men is to determine 

 constants and make definition precise, to discard estimate, approxi- 

 mation, assumption, where greater exactness may be attained. 

 Petrographic system should not lag behind. 



The use of chemical composition in quantitative 

 classification. — There are several ways of expressing the chemical 

 differences of rocks in classification. The elements themselves, 

 the molecules in which an analysis is commonly stated, and com- 

 binations of the latter, may be used. $ince the rock, generally 

 crystalline in some degree, is the object of classification, a scheme 

 based on elements or simple molecules as such, is less satisfactory, 

 less logical, than one expressing the connexion between chemical 

 and mineral composition. 



The use of the ' Atomzahl ' by Eosenbusch, in certain discussions, 

 has not appealed to many petrographers. Ratios of certain oxides 

 or groups of molecules stated in the analysis are all more or less 

 inadequate for the purpose, because their bearing on mineral com- 

 position is more or less indirect. 



The various efforts of recent years to determine certain molecular 

 ratios for rock-units, primarily established on a mineral basis, 

 serve to characterize the special rocks considered ; but no true 

 system can be reached in this way, without the ultimate necessity 

 of constructing a rigid chemical framework on some logical basis. 



