OLIGOCUASE-ANDESINU (aB 3 AN,). 



91 



to be alike in composition, there are rectangular sections, which at 

 first appear to be cross-sections of square prisms with hollow centers, 

 but are nearly equidimensional crystals which in some cases have 

 prismatic prolongations. These rectangular crystals have rectangular 

 spaces at the center, are twinned, and exhibit symmetrical extinction 

 angles of about 30 . As in other preparations of the more sodic 

 feldspars, there are comparatively few small crystals of more calcic 

 feldspar, approximately AbiAni, which began to form in cuboidal 

 shapes, but were followed by the crystallization of the bulk of the 

 mixture in feldspars of the average composition. 



Ougoclase-AndESinE (Ab 3 Ani). 



(21). Colorless glass, without crystals in the thin section studied 

 microscopically. 



(32). Colorless glass, with feldspar microlites and aggregates in the 

 form of bundles about o. 1 mm. long and in crossed bundles and to 

 some extent in spherulitic arrangements. The isolated microlites 



i 



Fig. 25. Microlites of oligoclase-andesine (AbgAnj). 



are instructive both on account of the exhibition of the habit of the 

 feldspar crystals in these preparations and also as an evidence of the 

 changes in habit during the short period of their growth. There are 

 two types of microscopic crystals, one tabular, the other prismatic. 

 These occur near one another intermingled in the glass. They are 

 0.03 mm. long and smaller. 



In many cases there appears to be a nucleus of feldspar within 

 feldspar; in some of these there are also small, irregularly shaped, 

 colorless grains with rather strong index of refraction whose compo- 

 sition is not determinable. These are extremely minute and not 

 abundant. 



The feldspar nucleus exhibits stronger refraction than the marginal 

 feldspar, but the direction of extinction is the same in both parts, 

 proving like optical orientation and showing that the central part of 

 the microlite is thicker than the margin and of the same composition. 

 The initial crystal in these cases is thicker, that is, the crystallization 



