go ISOMORPHISM AND THERMAL, PROPERTIES OF FELDSPARS. 



prisms, and not plates or blades. In places these are thin plates or 

 blades in twinned aggregations, as in (58 a_b ). The prisms are more 

 distinctly developed at the ends of some of the radiating aggregates, 

 where they are distinctly twinned according to the albite law and 

 yield symmetrical extinction angles of 30 . 



(59 a-b )- This preparation is glass, with spherulitic aggregations 

 about 2 mm. in diameter grading into smaller radiating bundles cross- 

 ing one another in groups of 2, 3, 4, and more, as in other cases already 

 described. The middle part of the bundles consists of stout prisms 

 passing into extremely thin fibers. The stouter portion yields ex- 

 tinction angles of 30 . The feldspars appear to be homogeneous 

 crystals. 



(64 a_b ). This preparation consists of spherulitic aggregations simi- 

 lar to (59), and also short rectangular prisms with almost square 

 cross-section. They are 0.04 to 0.07 mm. long and 0.007 to 0.010 mm. 

 wide. The forms are similar to the lath-shaped feldspar microlites 

 common in andesites. They are in some cases twinned, in others not. 

 They yield extinction angles of 30 . 



Andesine (Ab^ANj). 



(54 a ~ b ). This preparation is extremely minutely crystallized. The 

 main mass appears to be holocrystalline, composed of flake-like 

 microlites of feldspar overlapping one another at all angles, so as to 

 produce weak double refraction. The crystals are larger in patches 

 and in shells about isotropic spaces, as though the crystallization was 

 coarse about small spaces, like the walls of cavities of geodes. The 

 feldspar crystals project into the spaces. But these spaces are 

 filled with colorless isotropic material with refraction considerably 

 lower than balsam, presumably glass. It amounts to several per cent 

 of the whole. This residual glass probably has a different composi- 

 tion from the feldspar mass, otherwise it should not have solidified as 

 glass, for the larger crystallization of the feldspar in juxtaposition with 

 it indicates that the controlling conditions became more favorable to 

 the crystallization of the feldspar. 



(66 a_b ) . The small thin sections of this preparation are holocrystal- 

 line, without glass. The preparation consists of crossed bundles of 

 prisms and blades, without true spherulites. The bundles vary in 

 width from 0.0 1 mm. to less and in length from 0.5 mm. to less. Some 

 of the crystals are in albite twins, others not. While the prismatic 

 sections exhibit nearly parallel extinction in all cases, the long axis of 

 the prism being the direction of vibration of the fastest ray, and appear 



