OUGOCLASE-ANDESINE (AB 3 AN,). 93 



the same composition as the preparation, since the mass is holocrystal- 

 line in places. 



(25 s ). The preparation consists of glass with spherulitic aggre- 

 gations. These consist of bundles about 0.4 mm. long, of prismatic 

 crystals, which are definite in the middle and spread out to plumose 

 forms at both ends, merging with others in crossed position to form 

 spherulites in favorable positions (Plate XIV). 



In the more spherulitic groups the terminal parts are made up of 

 the most delicate fiber-like crystals. In other parts of the preparation 

 the sheaf-like bundles are composed of more distinct, thicker prisms, 

 which are somewhat broader in one direction than in another; that 

 is, they are somewhat blade-shaped. In some cases these are twinned 

 according to the albite law. In the central parts of some of the 

 bundles there is a rectangularly jointed development of the stouter 

 feldspar crystals which are continuous with the more prismatic 

 crystals forming the main part of the bundles. 



The optical behavior of those rectangular parts which show albite 

 twinning corresponds to that of AbiAni, there being symmetrical ex- 

 tinction angles of 30 . In the long prisms the angle of extinction is 

 small, and the length of the prisms is the direction of vibration of the 

 fastest ray. This corresponds to the optical behavior of a more sodic 

 feldspar with a prismatic development parallel to the crystal axis a. 

 In parts of the preparation the feldspars have crystallized in thin 

 tabular plates parallel to the second pinacoid (010). 



(24). This preparation is similar to (25 a ). It is partly glass, 

 partly spherulitic aggregations of bundles of prismatic crystals, 

 spreading out at the ends. The central portions of some aggrega- 

 tions are of stout prisms and rectangular groups with symmetrical 

 extinction angles of 30 (Plate XV). As in preparation (25 s ) there 

 was a first crystallization of feldspar with higher content in anor- 

 thite molecules than the average of the mixture. This formed a 

 small fraction, probably less than 1 per cent of the whole. The 

 principal crystallization appears to be homogeneous and must have 

 essentially the composition of the mixture Ab3An!. 



The outline of the bladed crystals in the plane (010), judging from 

 the optical orientation, and the elongation of the blades parallel to a, 

 is determined by the pinacoids of the second kind (201), (304) in 

 some cases furnishing an angle of about 8o, the angle between the 

 two being nearly bisected by the trace of the third pinacoid (001). 

 In other cases the blades appear to be bounded by (201) and (203), 

 with the apex angle truncated by (201). Such plates are nearly 



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