﻿Vol. 66.] CLASSIFICATION OF IGNEOUS KOCKS. 501 



assumed ' Kerne ' of Rosenbusch. He specifies orthoclase, albite, 

 anorthite, nepkelinc, as molecules in which alumina is combined 

 with silica and alkali or lime. 1 



In his recent work (1909) on igneous rocks, Iddings says : — 



'They [the compounds in solution] are mostly silicates, with a few oxides, 

 Si0 2 , Fe 2 3 , A1 2 3 , and other non-silicates in some cases. The components 

 do not exist as uncombined oxides, with limited exceptions.' (Vol. i, p. 268.) 



The dichotomous principle on which the Quantitative System is 

 worked out is a practical necessity, as I venture to think will be 

 the conclusion of Mr. Harker — who objects to it — or any one else 

 attempting to deal with a number of variables in a quantitative 

 scheme. That the result in the Quantitative System is unfortunate 

 is not, I believe, the general opinion of those who have applied it. 

 Its primary effect is certainly to give appropriate form 

 and bounds to five vague magmatic divisions ordinarily 

 referred to as ultra-acid, acid, intermediate, basic, 

 and ultra-basic. The course of magmatic differentiation is 

 commonly conceived as tending to separate the femic (basic) from 

 the salic (acid or alkalic) magmas. In this first division of common 

 use Mr. Harker would surely not put corundum, a molecule which 

 he recognizes as existing in some magmas, with the femic or basic 

 constituents, although he criticizes our placing it in the salic group. 2 



The assertion that a unit of a quantitative system should bring 

 together things which are alike, must have regard for the basis on 

 which that unit is made. Mr. Harker criticizes the Quantitative 

 System in this way by reference to the subrang Toscanose. :i The 

 citation of the names which petrographers have given to rocks found 

 under Toscanose in Washington's tables is not, as Harker uses it, 

 proof of the heterogeneous character of the assemblage belonging 

 to this subrang. The larger number of different names assigned 

 by the current system to rocks under Toscanose Ave re given on 

 account of texture or occurrence, features not entering into the 

 chemico-mineralogical definition of Toscanose. Other names cited 

 by Mr. Harker are not properly applied to the rocks in question. 

 Thus the ' trachytes ' have a notable amount of free silica and 

 of lime-felspar : they should be called quartz-latite in most cases. 

 The ' hypersthene-andesite ' has in its norm 7*8 per cent, of 

 femic molecules, 8"3 per cent, of anorthite, and 2d-(j per cent, of 

 quartz, the balance being alkali felspar ; and its mode can vary 

 from this to no great extent. The * granites ' were named before 

 quartz-monzonite came into use, and are rich in the anorthite 

 molecule. The ' rhyolites ' all carry more anorthite than typical 

 rhyolites should. 



Instead of showing the heterogeneity of Toscanose, this list of 

 rocks illustrates the lax and inappropriate use of terms under the 



1 'Die Eruptivgesteine des Kristianiagebietes, III: Das Gansjgefolge des 

 Laurdalits ' 1898, p. 332. 



2 « The Natural History of Igneous Hocks' 1909, p. 304. ;i Ibid. p. 363. 



