86 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



in places reached the composition of Ab^Ani. But there is no evi- 

 dence that the crystallization of the more calcic feldspar had ceased 

 before that of the more sodic feldspar began. On the contrary, they 

 appear, judging from the shape of the aggregates, to have grown 

 simultaneously, but toward the end of the act of solidification of the 

 solution. 



(30) and (31). These are thin sections from one preparation, which 

 is glassy below and crystalline above. (31) is from the upper part of 

 the preparation at right angles to the upper surface. It consists of 

 a minutely crystallized aggregation with more coarsely crystallized 

 parts composed of lath-shaped and bladed crystals, the largest being 

 1.8 mm. long and 0.18 mm. thick, there being all gradations in size 

 from the largest to the smallest (Plate IV). The crystals lie at all 

 angles, sometimes radiating, but spherulitic aggregates have not been 

 developed. In many cases the crystals have rectangular outlines; in 

 others they taper at the extremities, or wedge out by reason of the 

 interference of adjacent crystals. Twinning is common, but some 

 are not twinned. The albite law prevails, and in some crystals the 

 symmetrical extinction angles are 45 . The habit of the crystals is 

 not definitely determinable, whether tabular plates or elongated 

 blades. The crystals are of different thicknesses within the thin 

 section, the plates or blades being thinner than the section of the 

 preparation. For this reason the Becke method of testing relative 

 refringence of adjacent crystals is not applicable, as the thicker 

 parts of crystals in glass or balsam always appear in higher relief 

 than the thinner parts. 



In (30), from the middle of the preparation, just above the glassy 

 bottom half, there are aggregations of delicate tabular crystals in par- 

 allel and subparallel groups in various angular positions (PI. V). In 

 some places the tabular crystals in cross-section are sharply outlined 

 and straight-edged. In most cases the outline is indefinite and the 

 larger plates consist of multitudes of parallel and subparallel plates, 

 whose outline in the plane of flattening, however, is often sharply 

 defined. They appear to be plates parallel to (010), bounded by the 

 pinacoids (001), (201), and (I01). The angle between the traces of 

 (001) and (201) is about 8i, and that between the traces of (001) 

 and (I01) about 52 . In some cases the plates are nearly equi- 

 dimensional, in others they are elongated into blades parallel to (001) 

 or to (201). Owing to the aggregation of subparallel crystals of 

 extreme thinness, the optical behavior is that of aggregates, and 

 confused. In one plate of a thicker crystal cut parallel to (010) 



