Gabbroid Rocks of Mi?mesota. — IVmchell. 203 
acid it must be ilmenite. These grains were mechanically 
separated out with no difficulty, though the operation is trying 
to the patience and perseverence of the operator. A qualita- 
tive test for titanium confirmed the presence of ilmenite, as it 
showed much Tio2 in these grains. A small part of the powder 
obtained mounted in Canada balsam was sufficient to deter- 
mine that the same material used for the analysis showed the 
peculiarly small optic angle. 
The material, purified as described, gave the results in the 
column I ; in the second column are the results of the only 
other analysis known of pyroxene from Pigeon point: 
I II 
Si02 45-05 48.34 
TiOg 4 39 1-98 
AlzOa .16 2.QO 
FesOs 5-5° 4-68 
FeO 14.90 14.15 
MnO 1.58 
MgO 15.15 n-34 
CaO 10.72 15.10 
NasO 1.27 
K^O .78 
H,0 13 
99-63 98.49 
I. Pyroxene from the olivine diabase (1843) from Pigeon point, 
Minn. 
II. Pink titaniferous diallage from the olivine diabase from Pigeon 
point. Minn.; partial analysis by Dr. R. B. Riggs. (See Bayley: Bull. 
109, U. S. Geol. Survey.) 
The specific gravity of the pyroxene is greater than 3.316, 
but only slightly; grains containing the least alteration or im- 
purity were thereby rendered lighter than 3.316 and thus 
separated from the rest. 
The amount of titanium oxide found is very notable, but 
so far as known this element has no effect on the optic angle, 
titanaugite having as large an angle as ordinary augite. The 
alumina was determined by difference as given, but careful 
and repeated microchemical tests show no trace of alumina. 
The manganese gives a good quantitative test, but is not in 
important amount. The potassium is high as compared with 
