ANDESINE-I.ABRADORITE (AB,ANi). 89 



small bundles of fibers losing their optical orientation and finally their 

 distinct outline. Their place is occupied by spherulitic feldspar rays 

 whose orientation is independent of the position of the former bundles 

 of fibers. The former position of the bundles is shown in places by 

 pairs of stouter fibers, the outside members of the bundles of fibers; 

 in other places by clusters of minute inclusions resembling air spaces 

 between a network of fibers. These fade out in passing from the 

 margin of the spherulite inward through the millimeter-thick shell. 

 They have entirely disappeared in the central part of the spherulite. 

 The feldspar of the spherulite is homogeneous and the small bundles 

 appear homogeneous. They must have the composition of the pre- 

 pared mixture. The small angle of extinction of the bundles of fibers 

 is difficult to account for. It behaves more like oligoclase than 

 labradorite. 



(20). A branching spherulitic aggregation with numerous cavities 

 elongated in the direction of the radiating fibers. The section is too 

 thick to show well the microscopic structure of the parts. They 

 appear to be prisms and blades composed of minute subparallel 

 parts. The feldspar appears to be homogeneous. 



Andesine-Labradorite (ABjAn,). 



(26). The section of this preparation is extremely fine-grained 

 at one end and glassy at the other, the lower end of the crucible. 

 The crystals and spherulites become larger toward the glassy end of 

 the section (Plate XII). The glass has lower refraction than balsam. 

 There are many opaque white lumps, which appear to be unmelted 

 powder. They constitute 5 to 10 per cent of the mass. 



The feldspar crystals form fibrous bundles, single and in sets, cross- 

 ing one another at various angles, spreading out into plumes and 

 spherulitic groups. 



In the central portion of some fibrous bundles there are rectangu- 

 lar, lath-shaped, and block-like crystals with albite twinning, which 

 exhibit symmetrical extinction angles of 45 . These parts must have 

 the composition of AbjAn^, at least. The thinner prisms or fibers 

 outside of these show lower extinction angles, about 25 . If these 

 correspond to maxima in each case, the fibrous feldspar is about 

 AbiAni. The feldspars, except the central portions, which are com- 

 paratively few, appear to be homogeneous. Some of the plumose 

 aggregates which are albite twins are very beautiful. 



(27). This preparation consists of glass with twinned prisms of 

 feldspar in radiating groups about 1 mm. long which are not properly 

 spherulites. They are shown in Plate XIII. They are in most cases 



