284 The American Geologist. November, lauo 
/ig = greenish brown dark brown 
«m = brown brown 
wp = pale greenish yellow yellowish brown 
The absorption formula is therefore: //g > ^'/m > //p • 
The refringence is lower than that of olivine ; the birefring- 
ence is masked by compensation, and apparently does not ex- 
ceed .015. The mineral usually seems nearly uniaxial, but 
when no superposition and compensation interferes 2E may 
attain at least 2i'^° , as measured (2c/) in the troctolyte. It is 
often possible to see clearly that the fibers mutually compen- 
sate, and it is believed that to such a cause can properly be at- 
tributed the low birefringence, the small apparent optic angle 
and the weak pleochroism often seen. The direction of the fi- 
bers is positive or negative with parallel extinction. Bowlin- 
gite alters sometimes to chlorite. 
Pe?minitc occurs very rarely in minute flakes as an altera- 
tion product of the labradorite. It may be sometimes derived 
from bowlingite. 
Calcite is a rare alteration product of labradorite. 
The CHEMICAL COMPOSITION of the troctolyte shows a high 
per cent of titanium. The protoxide of iron and the magnesia 
are naturally high. 
I 
SiO, 35.81 
TiOg 2.30 
AI2O3 14-32 
F2O3 7-38 
FeO 15-25 
MnO 18 
MgO 10.49 
CaO 17-23 
Na,0 2.06 
K,b 37 
H.O 5.25 
100.62 
Sp. Gr. 3.07 
-3.10 
I. Troctolyte from near Duluth, Minn. No appreciable 
BaO, nor SrO. 
