BRANCHING FELDSPAR MICR0L1TBS. 415 



developed than the others. In some cases the sections are twinned parallel 

 to the broad plane. In the rhombic sections the extinction is diagonal and 

 no twinning is noticeable. The long prisms or needles are twinned in 

 nearly all instances, the composition plane being parallel to the length of 

 the prism. The direction of vibration of the fastest ray is nearly parallel 

 to the direction of the length of the prisms, proving them to be elongated 

 parallel to the clinoaxis. The twinning is after the Manebach law; and 

 the cross sections show that the crystals have a clinodome with basal plane 

 and possibly with elinopinacoids. 



These crystals, branch out in two different ways. In some cases they 

 appear to split, the parts being slightly inclined to one another at first, and 

 becoming more divergent farther on. This is shown in the right-hand half 

 of PL LIII, fig. 2. Both parts are twinned in the same manner and have 

 the same optical orientation — that is, each is a prism parallel to the clino- 

 axis. By this method a succession of branchings takes place, the prisms 

 becoming more and more numerous and thinner, often terminating in a 

 spherical surface, the aggregation in thin section resembling a rounded 

 bush of branching stems, especially when the prisms terminate in broad 

 leaf-like fronds, which often happens (PI. LIV, fig. 3). In other cases the 

 branching of the long, twinned prisms is seen to obey a crystallographic 

 law. Short prisms project from opposite sides of the Manebach twin at an 

 angle corresponding to that between the vertical axis and clinoaxis, about 

 64° (left-hand half of PI. LIII, fig. 2). These branches are prisms parallel 

 to the vertical crystallographic axis. They are sometimes long and slender, 

 and are curved to a position parallel to that of the prism or stem from which 

 they branch; in such cases the bundle of feldspar needles will consist of 

 numerous prisms elongated parallel to the vertical axis, and a central one 

 elongated parallel to the clinoaxis. The optical orientation of prisms 

 parallel to the vertical axis will differ according as the plane of the optic 

 axes lies in the plane of symmetry or is at right angles to it. In the first 

 case the direction of vibration of the slowest ray will always make a small 

 angle, with the axis of the prism, or be parallel to it. In the second case # 

 the speed of transmission of the ray vibrating in the direction of the prism 

 axis will sometimes be less and at other times greater than that of the ray 

 vibrating at right angles to it, according to which side it is viewed from. 

 Closely allied to these growths and associated with them are delicate rays 



