1/2 The American Geologist. September, 1900 
mains inferior to that of the apatite («g or even //p), though 
it is nearly always superior to the latter, when the halo is very 
marked. Thus other occurrences show- 
Apatite ?/p < 7ig and 7?m biotite. 
Apatite ng < wg and wm biotite. 
The indices of apatite have been carefully measured several 
times ; the results follow : 
Heusser Schrauf Lattermann Zim^nyi 
i«g = 1.6460 1.6390 1.6388 1-637 
«p = 1. 64 1 7 1.6345 1.6346 1.633 
The indices of phlogopite have been determined by Michael 
Levy and Lacroix* as follows : 
I «g ^, 1 .606 
Phlogopite, of Templeton, Canada. •] «m == 1.606 
( Wp = 1.562 
It must be added that the indices of biotite are somewhat 
variable, as is shown by the variations in the value of the 
birefringence; the birefringence of the biotite in question here 
does not exceed .050. 
Michael Levyf has shown that the refringence generally in- 
creases in pleochroic halos, and estimated from the intensity of 
the white line between the halo and the surrounding mineral 
(Becke method) that this increase may exceed .01, notably in 
halos in cordierite about zircon.. But the same method ap- 
plied to the present case indicates that the increase in the value 
of the refringence due to the halo is comparatively slight, so 
that the facts stated above can be accounted for only on the as- 
sumption, either that the apatite in question has a refringence 
abnormally low, or that the biotite has a refringence very un- 
usually high. The fact that repeated measures of the indices 
of apatite have given values very concordant and the fact that 
the indices of biotite are known to vary render it probable that 
the second assumption is the correct one. 
The birefringence in the halos above studied is increased 
probably about .007, at least .006, which is equivalent to an 
increase of at least 12%. 
*Michel Levy and Lacroix; Mineraux des Roches, 1888, p. 240. 
tMichel Levy: Sur la refringence des aureoles polychroiques: C. 
R. CXX. 
