﻿Yol. 66.1 CLASSIFICATION OF IGNEOUS E0CKS. 487 



with unknown quantities and processes. The parallelism between 

 differentiation and crystallization on which Yogt depends is not an 

 established law, attractive as it appears as a working hypothesis. 

 The cause of molecular migration in differentiation is unknown, and 

 there may- be several causes leading to dissimilar products. Xo 

 eutexia of rock-nrinerals are known. Yogt may have reached 

 approximately correct proportions for some of them, but in view of 

 the unknown influence of water and other mineralizers, of im- 

 purities of various kinds, and of a long series of changing conditions, 

 one may be pardoned for lack of confidence in his results. 



If we assume that there are ten or more important eutexia of 

 rock-making minerals, and that these are little affected by various 

 conditions and impurities, it is of course plain that each eutectic 

 mixture represents a centre-point about which many rocks cluster. 

 This must be the case whether the nearly pure eutexia are more 

 numerous than other mixtures or not. The statistical studies of 

 Yogt fail, it appears to me, to demonstrate that degree of quanti- 

 tative dominance of the eutexia which must exist to warrant the 

 systematic proposition made by him. The range of variation 

 covered by the term ' anchi ' (meaning almost), as used by Yogt, 

 is so great as to destroy its significance. 



^Yhen one considers the probable influence of some of the variable 

 factors ignored by Yogt — and especially that of water — it seems 

 not impossible that the whole fabric of Yogt's speculations may fall, 

 with the advance of knowledge on these points. 



Ytaginatic classification by eutectics is fundament- 

 all}' weak, because it rests on hypothesis, because it 

 does not apply to all rocks, and because it does not 

 consider the entire magma of most rocks. As developed 

 by Yogt it does not adequately recognize the very important rocks 

 representing parent magmas which are not anchi-eutectic, nor the 

 numerous intermediate rocks ('Zwischenstufen '). 



Turning from the magma to the rock, what stamp has the 

 eutectic placed on the consolidation-product, justifying its use in 

 classification? Becker's conception that the ground-mass of por- 

 phyries is practically the eutectic must be extensively qualified ; 

 and, if it were true, the conclusion that the phenocrysts are of little 

 systematic importance is not admissible from the petrographic 

 standpoint. 



The porphyritic texture is determined by conditions of con- 

 solidation, of which approach to the eutectic proportion in the still 

 liquid part of the mass is but one. A vast number of porphyries, 

 both extrusive and intrusive, possess ground-masses that are simply 

 that part of the magma which was liquid at the time of eruption. 

 The eruptive act takes place independently of the stage of crystal- 

 lization reached in the magmatic reservoir. The change of physical 

 conditions is, in many cases, sufficient to prevent further pheno- 

 crystic growth. 



Doubtless the ground-mass of many porphyries is nearly of 



