ITAIJAN PETROI.OGICAL SKETCHES 549 



difference in refractive index is also marked. Inclusions are 

 not very abundant and are usually small spots of glass, with 

 some augite, magnetite and apatite crystals. In the alkali feld- 

 spar phenocrysts inclusions of plagioclase of some size are 

 occasionally seen, but the reverse was not noticed. 



The examination of plagioclase phenocrysts in the slides was 

 made with care and resulted in establishing the fact that they 

 are anorthite. In a section cut approximately parallel to c (001 ) 

 an optic axis emerges almost perpendicularly, and the angle of 

 its extinction is 38° with the trace of the plane b (010). The 

 alkali feldspar border (to be described presently), in this case 

 extinguishes parallel to the same plane of the anorthite crystal. 

 In another section cut parallel to b (010) the plane of the optic 

 axes (determined by the emergence of an axis at the border of 

 the field), formed an angle of — 34° with the basal cleavage 

 cracks, while the individuals of the Carlsbad twin extinguished 

 at 29° and 30j^° on each side of the twinning plane. The 

 border around this crystal extinguished at an angle of 11° with 

 the basal plane of the anorthite. This determination of the 

 plagioclase as anorthite confirms Klein's observation of its 

 basicity. 



Apart from the composition of the plagioclase, the most 

 interesting feature of the feldspars is brought out between 

 crossed nicols. It is then seen that both alkali feldspar and anor- 

 thite phenocr^'sts are surrounded, almost without exception, by a 

 border or mantle of alkali feldspar of late growth. This forms 

 one individual around the phenocryst proper, as shown by the 

 simultaneous extinction of the whole border. The outer edge is 

 in general very irregular, ending with an uneven and often uncer- 

 tain line against the groundmass ; though here and there there 

 has been an attempt to fill out the crystal form, resulting in quite 

 sharply defined straight edges. 



The mutual extinctions prove that the mantle is orientated 

 like the nuclear crystal, and if the inner orthoclase crystal be 

 twinned the twinning is continued uninterruptedly in the outer 

 mantle. This is also true to some extent of the Carlsbad twin- 



