248 /. H. L. VOGT 



with the mix-crystals of the succeeding crystallization) and the cor- 

 responding components in the liquid phase/ 



If we take plagioclase as an example, there will first crystal- 

 lize from a magma with the plagioclase components in the proportion, 

 e.g., 58 An 142 Ab — not only in melts consisting of pure An+Ab, 

 but also in magmas with more or less of other components — ^ an 

 embryonal mix-crystal with a composition about 84 An: 16 Ab. 

 The succeeding mix-crystals will contain gradually decreasing 

 amounts of An and increasing amounts of Ab with absolutely 

 lacking equilibrium say up to 30 An 170 Ab, and at last we may 

 get thin zones with still less An and still more Ab. These last 

 zones, as a rule, however, are in such small quantity that they may 

 be entirely overlooked or may be seen only with difficulty under 

 the microscope. 



In order to decide if the equilibrium between the solid and the fluid 

 phase is absolutely lacking or only more or less incomplete, we may 

 start with a rock having an average proportion An: Ab, determined 

 by the chemical analysis, and examine the composition of the 

 "first" mix-crystal. For this purpose we may with some correc- 

 tions, however, use the data given in my study on "Die physikalisch- 

 chemischen Gesetze der Krystallisationsfolge in Eruptivgesteine,"^ 

 where I calculated from chemical analyses, the proportions of 

 Or:Ab:An in many rocks (especially andesites), as well as the 

 composition of the "first" phenocryst in the same rocks. I will 

 give an extract from this table, recalculating Or:Ab:An to only 

 Ab:An. The values following are the calculated quantities of 

 An in the sum of Ab and An, both in the rock and the in phenocrysts. 

 Further, I give the theoretical composition (or the An content) 

 of the first plagioclase crystallizing in an Ab-fAn melt, according 

 to the scheme delivered by Bowen (19 13). 



Percentage of An in Ab+An, (a) in the total rock, (b) in the 

 phenocryst,'' and (c) theoretically. 



' I refer to the general explanation in Die Silikat-Schmelzldsungen, I and II (1903 

 and 1904), and to numerous later treatises by various authors. 



* See this Journal for 1921, pp. 325, 327. 



3Tscherm. Min. Petrogr. Mitt., XXIV (1905), pp. 503-4 and 512. 



4 The niunbers refer to my table in 1905. 



