ANORTHITE (AN). 8 1 



less than o.oi mm. thick, probably about o.ooi mm., a difference in 

 thickness when compared with anorthite of about 5,000 to 1. This 

 as shown elsewhere is due to the greater viscosity of the liquid 

 feldspars near their solidifying point as they approach the albitc end 

 of the series. 



Any comparison of the grain of rocks, that is, the size of the con- 

 stituent crystals, with a view to determining the physical conditions 

 attending the solidification of the magma, must be based in the first 

 instance on a knowledge of the behavior of the various rock-making 

 minerals under similar physical conditions, both separately and in 

 combination, that is, in solution with one another. The granu- 

 larity of rocks is clearly a function of the chemical composition. 



With respect to the homogeneity of the crystals separating from 

 the liquid, it is observed that the great part of each crystal aggregation 

 appears to be of one composition, but that in some cases a small 

 proportion, probably less than 1 per cent, is different from the bulk of 

 the feldspar, both in composition and habit. In one instance this 

 small variant differed in composition but not in habit from the main 

 mass of crystals. 



In the first case it appears that crystallization began with feldspar 

 richer in the anorthite molecule than the solution and developed 

 cuboidal forms. These were prolonged into prismatic bundles, the 

 prisms having the composition of the main mass of crystals. 



In the second case the small variant crystallized toward the end of 

 the crystallization and contained more albite molecules than the main 

 mass of feldspar crystals. It had the same habit as the other more 

 calcic portion, and appears to have crystallized at the same time with 

 it, the crystals with different optical properties being by the side of 

 one another and not in zonal relation. Neither of the feldspars 

 represents the end member of the series, An or Ab. 



The detailed description of the thin sections of these laboratory 

 preparations of lime-soda feldspars follows: 



Anorthite (An). 



(19). This aggregation consists of tabular crystals 3 to 5 mm. 

 thick in somewhat radial arrangement, and between these are smaller 

 tabular crystals in similar radial clusters. The clusters are twinned 

 according to the albite law in lamellae, o. 1 5 mm. thick and less. The 

 thinnest lamellae are not always continuous throughout the length of 

 a crystal. 



The optical orientation is uniform throughout the length of each 



