﻿486 DR. WHITMAN CROSS ON THE NATUKAL [Aug. 1 9 10, 



Eutectic classification as outlined by Becker is plainly not adapted 

 to general petrographic system. It ignores the fact that the rock 

 is the result of the consolidation of the whole magmatic solution, 

 except the escaped volatile constituents, and not of the eutectic 

 alone. It is not applicable to all rocks, and would be primarily of 

 use in purely physical discussions, such as the influence of varying 

 fluidity of lavas upon the form of volcanic cones. 1 



Yogt's proposition. — A proposition by J. H. L. Yogt 2 to 

 utilize eutectics in the genetic classification of igneous rocks is 

 combined with an hypothesis of magmatic differentiation. It must 

 be assumed that the reader is familiar with the notable series of 

 experimental investigations, statistical studies, and theoretical 

 discussions which led Yogt to his conclusions as to the existence 

 and approximate character of the eutexia of magmas. By suppos- 

 ing that the parent magmas of various centres are not themselves 

 very nearly of any eutectic composition, Yogt pictures the process 

 of differentiation to consist in the concentration of components 

 which are in excess of the eutectic. By long-continued migration 

 of these substances and their concentration in certain places there 

 may result partial magmas rich in plagioclase, pyroxene, olivine, 

 titanium and iron ores, spinel, or some other mineral. These 

 approach the simple composition of a single mineral in many 

 cases. The complementary result of such a process is an approach 

 on the part of the residue of the parent magma to the eutectic 

 mixture. 



Theoretically, the differentiation pictured by Yogt should end 

 with the production of a eutectic magma. Practically it does not, 

 owing to disturbance of equilibrium by some external influence, 

 and several anchi- eutectic magmas may be formed from a single 

 parent magma, as Yogt admits. 



Yogt believes that most magmas may be practically classified 

 either as anchi-monomineralic or anchi-eutectic, and that such 

 groups may serve as the basis of a genetic system for igneous 

 rocks. 



Although the extremely suggestive studies which have led to this 

 preliminary proposition are of the highest importance, there are 

 many reasons for questioning whether the genetic factors concerned 

 are applicable to petrographic system. It is universally recognized 

 that many genetic factors are not of systematic value. 



Objection to eutectic classification may be made on several 

 grounds. In the first place, the genetic importance of eutexia in 

 magmas remains to be established and defined. It is no disparage- 

 ment of Yogt's work in this direction to say that he deals chiefly 



1 G. F. Becker, ' The Geometrical Form of Volcanic Cones & the Elastic 

 Limit of Lava' Am. Journ. Sci. ser. 3, vol. xxx (1885) pp. 283-93. 



- ' Ueber anchi- monomineralische & anchi - eutektische Eruptivgesteine ' 

 Vidensk.-Selsk. Skrifter, I. Math.-Naturv. Klasse, 1908, No. 10; also under a 

 similar title in an earlier publication. 



