66 Washington and Wright — Feldspar from Linosa. 



Wright at the time. It varied in granularity and, except for 

 occasional patches of more crystalline material, was too fine 

 for optic determinative work. No separate crystals were 

 obtained and no goniometric measurements could therefore be 

 made. Since 1905 repeated experiments have been made with 

 the crystallization of this substance from the melt, and also 

 by heating the glass at different temperatures, but the results 

 of the optic examinations have been practically the same in 

 each case, and the different experiments need not be con- 

 sidered in detail here. 



Fig. 1. 



Fig. 1. Polysynthetic albite-like twinning on artificial soda anorthite. 

 Nicols crossed. Magnification 85 diameters. 



The most characteristic feature of this form of Na a Al 2 Si 2 8 

 is its polysynthetic twinning, which in many instances closely 

 resembles that of the plagioclases (fig. 1). On other sections 

 the cross grating twinning of microcline is developed, and if 

 encountered in a rock section might easily be mistaken for this 

 (figs. 2 and 3). The extinction angles on such polysyntheti- 

 cally twinned individuals ranged up to 44° on symmetrically 

 extinguishing sections, the angles between the (c) ellipsoidal 

 axes of adjacent lamellae being 88°. As in the plagioclase 



