2o6 The American Geologist. October, 1900 
irregular in outline. Good crystal outline has not been seen 
in the mineral of the Pigeon point rock; in the gabbro from 
Birch lake the faces ;/z(iio) and h^{\QO) form a complete out- 
line in one case. (See Plate XVII, Fig. 6.) 
The color and pleochroism are intense and characteristic 
with: 
^% = pale brown slightly greenish greenish brown 
nxD. = dark brown to opaque dark greenish brown 
«p = dark yellow brown greenish yellow 
The absorption is strong: ;/g > ;/p > ;/m. 
The cleavages are indistinct or wanting; irregular coarse 
fractures exist frequently. The refringence is very high, ex- 
ceeding that of all the minerals seen surrounding it, namely 
biotite, brown and green hornblende and augite. The bire- 
fringence is extremely variable and may vary even in a single 
crystal ; it has been observed as low as .007, in which case the 
mineral is of a paler brown or green color and only feebly 
pleochroic. In other cases the birefringence reaches .027 at 
least, in which case the color and pleochroism are so intense as 
to nearly mask the birefringence. The dispersion is usually 
strong and prevents complete extinction. The extinction is 
sensibly parallel to the elongation, usually very slight, which 
is positive or negative. 
Allanite produces very intense pleochroic halos in all the 
colored minerals in which it has been observed; these halos 
sometimes have a diameter more than twice as long as the di- 
ameter of the included allanite. In brown biotite it causes 
intense greenish brown halos strongly pleochroic; in brown 
and green hornblende it causes intense brown halos, strongly 
pleochroic to pale yellow. The refringence is distinctly in- 
creased in these halos and in one case the birefringence is in- 
creased at least .003. This is in green hornblende. The halo 
is usually only approximately circular; one peculiar double 
halo in brown hornblende consists of two rings exactly circu- 
lar, the outer one being very much lighter; both are very 
clearly defined. (See Plate XVII, Fig. 7.) 
Pyrite occurs rarely and sparingly as a primary constituent. 
Its time of crystallization was long or varied. Indeed it 
sometimes occurs in automorphic forms enclosed by augite or 
labradorite, and at other times it is wholly irregular in outline 
