64 JOURNAL OF THE 
vicinal so that the reflections were not very perfect. 
The angle of the apparent prism J/Ad twinned is approx- 
imately 90°. Also the angle mAm over the twinning 
plane 011 could be measured only appreximately, varying 
from 0°49 to 1°25. The approximate angles are given 
below, and from those marked with an asterisk the fol- 
lowing axial ratio was calculated: 
@ :b:¢ = "768 21: 1°245; B=53°27’ =001A100 
Measured Calculated. 
bAb, 010A010 *90° (Cover twinning plane) 
ada, 1OOALO0 *73°O (over twinning plane) 
bam, 010A110 *58°19' 
cho, 001A100:, 53°27 =p 
cam, 001A110 60°, 59°45', 59°57’ 59°sa 
Physical properties.—The crystals are brittle and 
show no apparent cleavage. The luster is vitreous. 
Many of the crystals are colorless and transparent while 
others are white. The hardness is cetween 4 and 4°5. 
The specific gravity taken on a number of separate cry- 
stals, by means of the heavy solution, varied between 
2:278 and 2°366. This variation was probably due to the 
difference in the ratio of the barium to the calcium in the 
different crystals. 
A section parallel to the pinacoid 
faces 6, 010, the apparent prism, 1e- 
.ealed in polarized hght the struct- 
ure shown in hg. 3. The parts I 
and I extinguish simultaneously, as 
also I] and II; while portions III, 
which are parallel to the basal 
plane, show parallel extinction. 
The section showed something of a 
zonal structure, so that the extinct- 
ion could only be measured approxi- 
