﻿Vol. 66J\ CLASSIFICATION OF IOXEOUS ROCKS. 499 



he would include sodalite, noselite (nosean), and zircon. These 

 mineral molecules are the only ones in the salic 

 division of the norm, by which three classes of igneous 

 rocks are classified as far as the subrang. 



Of the femic normative silicates Mr. Harker recognizes the 

 chief: acmite, diopside, hypersthene, and olivine, as magmatic 

 molecules, and the general rule that meta- or orthosilicates of 

 calcium, magnesium, and iron are present in the magma in relation 

 to the silica contents. In short, if Mr. Harker should attempt to 

 compare magmas, in a consistent manner, by study of rock-analyses, 

 and to express their character in terms of the mineral molecules 

 that he believes to be in solution (see quotation above), his list must 

 apparently be very nearly that of the norm. If the metasilicates 

 are combined in regular proportions in diopside, hypersthene, and 

 acmite, he will occasionally have an excess of sodium and calcium 

 metasilicates. The latter he might prefer to call pseudo-wollastonite, 

 the enantiotropic form of CaSi0 3 existing above 11S0 . 1 He 

 would also encounter the necesshty of recognizing some subsilicate 

 molecule for high lime contents in the magmas of melilite rocks, 

 for the latter would be a difficult complex molecule to insert into 

 his standard scheme. 



Mr. Harker might call his standard scheme of magmatic mole- 

 cules ' the norm ' or apply some other term ; he might use it in 

 his ideal classification or not ; but its usefulness for comparison 

 and interpretation would, I think, be considered greater than diagrams 

 or arbitrary ratios of oxides could be. 



Apparently, Mr. Harker's reluctance to recognize the significance 

 of the norm comes from his inference that 



' the underlying idea seems to be that the constituents exist in a rock-magma 

 in the form of free oxides, a conception which necessarily obscures natural 

 relationships.' 



This inference is perhaps not unnatural, in view of the statement 

 by Iddings in 1892, 2 that oxides and not definite silicate com- 

 pounds were present in the magmatic solution, and because this 

 idea was not specifically disclaimed in the presentation of the 

 Quantitative System. It is true that in 1902, when the System 

 was issued, the components of magmatic solution were by no 

 means so clearly known or indicated as at the present time. 

 But the idea that mineral molecules were the important ones, 

 under the conditions immediately preceding crystallization, was 

 by no means unknown to the authors of the Quantitative System, 

 nor were they at that time advocates of the hypothesis that the 

 magma contained oxides and silicate compounds. 



1 E. T. Allen & W. P. White, ' On Wollastonite & Pseudo- Wollastonite, 

 Polymorphic Forms of Calcium Metasilicate ; with Optical Study by Fred 

 Eugene Wright' Amer. Journ. Sci. ser. 4, vol. xxi (1906) pp. 89-108. 



2 'The Origin of Igneous Kecks ' Bull. Phil. Soc. Wash. vol. xii (1892- 

 95) p. 158. 



