10 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 64 



inclosing rock. Optical examination of the clear crystals from hol- 

 low heulandite amygdules showed them to be transparent and color- 

 less and devoid of any visible pigmenting material to which the red 

 color might be attributed. Optically the crystals lining the small 

 cavities or making up the flesh-red amygdules of the amygdaloid 

 rock are biaxial positive ( + ) with 2V = 52°. The indices of refrac- 

 tion are a =1.482, /3 = 1.485, 7 = 1.489, 7- « = 0.007. The acute 

 bisectrix (Z) is perpendicular to the 6(010) face, while the extinction, 

 measured from the c crystallographic axis averages 35°, the optic 

 plane being nearly parallel (measured 5° to 8°) to the trace of one of 

 the terminal clinodomes, either t{101) or s(TOl). The crystals as 

 seen between crossed nicols are frequently not entirely simple, being 



1. 2. 3. 4. 



Figs, l to 4.— Heulandite Crystals from Challis, Idaho. 



divided by sutures into areas which differ slightly in extinction angle. 

 Thus the crystal measured and illustrated by the drawing (fig. 2), 

 was found to be divided vertically by a straight line, on one side of 

 which the extinction, measured from the vertical cleavage lines, 

 gave 38^° while on the other side it was 44^° and a wedge entered 

 at the termination of the crystal in which the extinction was 33°, 

 the latter being about the normal extinction angle of homogeneous 

 crystals. In addition to this peculiarity the crystals frequently 

 show a very narrow outer border of a material of decidedly higher 

 index of refraction. 



The crystals of this heulandite which were measured did not give 

 overly satisfactory angles, mainly because of the poor development, 



