204 The Americati Geologist. October, i90o 
the sodium found. A test, not very delicate, was made for 
chromic acid, and a delicate test was made for vanadic acid; 
both gave negative results. The qualitative test for titanium 
was, of course, very strong. The pink color is probably in 
part due to this element. 
The analysis then shows that the mineral is a titaniferous 
diopside, possessing no very marked chemical peculiarities. 
(See Plate XX, figure lo.) The abnormal optical properties, 
therefore, can scarcely be attributed to the chemical composi- 
tion. Though pyroxene is not among the minerals whose 
optic angle is notably afifected by heat, it seems to the writer 
that the peculiar condition here described must be ascribed 
to peculiar physical conditions during or after the crystalliza- 
tion. Since the anomalous condition was first discovered 
in the rocks from Pigeon point, it would be appropriate to call 
pyroxene thus optically abnormal pigeoniie. 
The mineral has been found also in two gabbroid rocks 
from near Duluth. at the other extremity of the same great 
gabbro area. 
Olivine is usually abundant. It sometimes forms rounded 
automorphic crystals, but is often penetrated by the labradorite 
crystals. The cleavages are occasionally very distinct when 
alteration has set in; oftener they are entirely lacking. 
The birefringence gives very clear colors rising often into 
the second order; it is at least .034. 
In the process of separating the pyroxene from this rock, 
the coarse powder containing pyroxene, olivine, and a little 
highly titaniferous and wholly non-magnetic ilmenite was di- 
gested in hydrochloric acid at a gentle heat for several hours. 
An examination with the microscope showed that the ilmen- 
ite and the pyroxene were not perceptibly attacked. From 
the solution thus obtained the iron and magnesia were pre- 
cipitated: 
FeO ^= .0522 mg. 
MgO = .0534 mg. 
From this the theoretical composition of the olivine would 
be: 
SiO» = 33.57 
FeO = 48.74 
MgO = 17.69 
