100 



PROF. J. H. L. VOGT ON LABRADORITE-NORITE [May I9OC 



ceased. This may probably depend on the considerable size of the 

 phenocrysts in connexion with the viscosity of the solution in- 

 creasing as the crystallization-temperature diminished. 



In order to explain the result, I give here a graphic illustration, 

 leaving the small quantity of orthoclase unconsidered ; in fig. 7 

 P represents the original relative proportion An : Ab (58'5 An : 

 41*5 Ab) of the solution ; Q, the composition of the first mixed 

 crystal ; B, the composition (65 An : 35 Ab) of the phenocrysts or 

 most of them ; and S, that of the plagioclase in the ground-mass 

 (55-3 An : 44-7 Ab). As the outer shell of the phenocrysts frequently 

 shows a zone richer in albite, the plane B has been continued by a 

 fine line in the direction of S. 



On the curve Q — .E, that is between about 78 An : 22 Ab and 

 65 An : 35 Ab, there existed equilibrium — or nearly equilibrium — 

 between solution and mixed crystal ; but then the equilibrium 

 ceased, and thus at last plagioclase of composition S was evolved. 



Fit 



100An 

 OAb 



"What has been said here of the plagioclase may be applied in 

 the essential points also to the hypersthene. The two chief com- 

 ponents of this mineral, namely silicate of magnesia and silicate 

 of iron, pretty certainly belong, as I have pointed out in earlier 

 papers, to mixed crystals of Type I. The mixed crystal first 

 separated out is richer in magnesia and poorer in iron than the 

 later crystals. The border of hypersthene comparatively richer in 

 silicate of iron round that which is somewhat poorer in this com- 

 ponent, sometimes observed in the labradorite-norite here described, 

 shows that the equilibrium of the components in question in the 

 solid and liquid phase was not complete. 



