"Vol. 65.] WITH PORPHYRINIC LABRADORITE- CRYSTALS. 



07 



method used by me and the synthetical method followed by Messrs. 

 Arthur L. Day & E. T. Allen, 1 both of which entirely ' dif- 

 ferent methods of research lead to the same result, the binary 

 mixed-crystal combination An : Ab belongs to Roozeboom's Type I. 

 For explanation I reproduce, from my paper in Tscherm. Min. 

 Petrogr. Mittb. vol. xxiv (1905) fig. 15 on p. 514, the accompanying 

 fig. 4. The upper unbroken line indicates the curve of solidification, 

 and the lower broken line the curve of composition of the first 



mo 



1300' 



1100 



l M 



separated mixed crystal; if melted An + Ab of composition N is 

 cooled to the point P, then, no account being taken of super- 

 saturation, a plagioclase of composition Q first crystallizes out. 



The first mixed crystal crystallizing out of a molten solution 

 with given proportions of An : Ab has approximately the following 

 composition 2 : — 



Out of solutio7is of: 

 60 An : 40 Ab 

 40 An : 60 Ab 



the first crystal contains : 

 about 80 An : 20 Ab. 

 about 65-60 An : 35-40 Ab. 



1 ' Isomorphism & Thermal Properties of the Felspars ' Carnegie Insti- 

 tution of Washington, Publ. 31, 1905. 



2 See my paper in Tscherm. Min. Petrogr. Mitth. vol. xxiv (1905) pp. 512-14. 



Q. J. G. S. No. 258. h 



