CA USES OF MA GMA TIC DIFFERENTIA T10N. 779 



minerals — are naturally more than two, and therefore the liqua- 

 tion must become very complicated, being not only a function of 

 temperature but also dependent on the original proportions. 

 Therefore, in other places, where the original, magma had another 

 composition, relatively stable andesitic magmas might be formed, 

 but this was evidently not the case in Iceland. 



Liquation is no doubt also a function of the pressure, but 

 experimental data are wanting. Still it may be considered as 

 probable that, if liquation would augment the volume of the 

 magma, then pressure would act the same as increase in tem- 

 perature, and inversely. The first is most frequently the case 

 with liquid-mixtures. 



The purpose of this communication is to give to liquation 

 and not to diffusion its place as the working hypothesis, upon 

 which the theory of differentiation is to be constructed. How 

 far this theory may differ from the approximation to it, given by 

 Rosenbusch in his " Kern "-theory, the future will show. 



In conclusion, I wish to express my best thanks to my 

 friend and colleague Dr. S. Arrhenius for much valuable infor- 

 mation furnished me in numerous discussions on this and other 

 subjects which lie on the border between petrology and physical 

 chemistry. 



Helge Backstrom. 



