﻿Vol. 66.] THE NATURAL CLASSIFICATION OF IGNEOUS ROCKS. 505 



before augite, and in some cases even before olivine. Anorthosite 

 (which consisted mainly of anorthite) was more closely allied to 

 a peridotite than to a quartz-orthoelase rock, with which the 

 American classification associated it. Similar considerations applied 

 in the case of an excess of alumina, which usually appeared as a 

 constituent of augite or hornblende, 1 and should have been classed 

 with the femic minerals. The salic group was in fact a collection 

 of minerals which had nothing essential in common, and the 

 fundamental lines of division of the classification were accordingly 

 practically meaningless. Its general adoption in the description 

 of rocks would, therefore, hinder rather than advance our recog- 

 nition of their relation to one another. 



Mr. T. Crook contended that the primary requirement of a system 

 of classification of igneous rocks was, that it should be natural. 

 The formation of igneous rocks took place by differentiation, and 

 was essentially an evolutionary process. Experience proved that 

 differentiation-products required to be classified qualitatively and 

 not quantitatively, since it was only by this means that the natural 

 order could be reflected in a scheme of classification. Classifications 

 based on mineral composition and structure were admittedly im- 

 perfect ; but they were natural, inasmuch as they sacrificed sharp- 

 ness of grouping in their attempt to copy the natural order of 

 arrangement. It was in this sense that the quantitative-chemical 

 system was rightly stigmatized as unnatural when compared with 

 mineral-textural systems, since it insisted upon the essentiality 

 of ' sharp artificial boundaries ' which did not exist in Nature. 

 The chemical composition of an igneous rock, which the Quanti- 

 tative System treated as the most fundamental character, was less 

 fundamental than mineral composition. The chemical composition 

 of any given few grams of a holocrystalline rock was determined 

 by the minerals actually present, and at no period of differentia- 

 tion could it be truly said that this chemical composition was an 

 inherent feature of the magma. On the other hand, the mineral 

 composition was not determined by the chemical composition of the 

 particular piece of rock analysed, but by that of the magma during 

 the various stages of crystallization. By a development of this 

 argument it could be proved that the natural mineral composition 

 of a rock was more fundamental than its chemical composition or 

 any artificial norm calculated therefrom, and was therefore a 

 sounder basis of classification. 



Dr. L. L. Fermor said that most of the previous speakers had spoken 

 disapprovingly of the Quantitative Classification of igneous rocks. 

 He did not advocate its adoption, but drew attention to one good 

 feature, namely the idea of the norm. In dealing with a rock the 

 speaker found it very helpful to be able to compare one rock with 

 another, not on the basis of their actual mineral composition, nor 

 even of their chemical analyses, but preferably by converting the 



[Mica might also have been mentioned in this connexion. — J. W. E.] 

 Q. J. G. S. No. 263. 2 u 



