Washington and Wright — Feldspar from Linosa. 67 



lamellae, the extinction angles in the zone of symmetry varied 

 with the position of the sections, and the symmetrical extinc- 

 tion angles most commonly observed ranged between 35° and 

 40°, the ellipsoidal axis c being next the twinning junction line 

 in each case. 



In many of the sections showing apparently albite twinning 

 lamellae, a second set of polysynthetic lamellae often appeared, 

 making usually angles of 57° to 63° with the first, and so dis- 

 tributed in the lamellae of the first set as to be symmetrical to 



Fig. 2. 



Fig. 2. Polysynthetic mierocline-like twinning on soda anorthite. Nicols 

 crossed. Mag. 75 diameters. 



its planes of twinning. Thus, if the lamellae of the first set 

 (albite lamellae) are placed in a north-south direction, then in 

 lamella No. 1 of this set the lamellae of the second set trend N". 

 60° E. approximately, while in the adjacent lamellae No. 2 of the 

 first set the lamellae of the second set trend N. 60° W., the JST.-S. 

 direction acting as a plane of symmetry. The positions of 

 extinction of the interposed lamellae in any given twinning 

 band of the first set agree closely with the extinction positions 

 of the immediately adjoining second lamellae of the first set. 



