DESCRIPTION OF ROCKS AND ANALYSES 19 
developed cleavage-plane. Cleavage-flakes give good biaxial figures, 
with central emergence of the acute bissectrix y, and the plane of 
optical axes is normal to the elongation, consequently £=c with 
nearly straight extinction. The other cleavage-direction, neaily of 
equal perfectness and perpendicular to the just mentioned, gives 
the central emergence of the obtuse bissectrix «. Thus the optical 
orientation can be noted as nearly: 
a=«e 
Dy, 
c= 
Further on the following measurements in daylight were executed on 
the mineral with immersion liquid on free splinters of good orientation: 
a = 1.525 + 0.001 
= ‚| @=1.527-++0.001 | from these terms calculated: 
6 = 1.527 + 0.0005 Na | Fa dl 2) 
tel y = 1.539 + 0.0005 2 Ey = 82° 34’; 
with the compensator of Babinet on free splinter: 
| 7: 8—a= 0.0032, thickness = 0.1326 mm.; 
on thin sections: 
ees 7— 8 0.0113, i — 754) 
| B:y—0 = 0.0145, 5 — (ORD ae Pees 
which measurements agree well with the birefraction values calculated 
from the refractive indexes. 
By means of the Wright micrometric ocular-scale there was found 
2Ey= 81°40’, 
and a control measurement with camera lucida and rotation disc gave 
DE 82: ? 
in good agreement with the above calculation. 
A closer examination of thin sections developes a feeble striation 
quite normal to c, the sections normal to y and « being broad vic. 
narrow columnar shaped, and normal to £ elongated rectangular, 
and the extinctions sometimes sensibly deviating from the crystallo- 
graphical axes assumed as orthorhombic by the cleavages / (010) 
