Gabbroid Rocks of Mt?i?icsota. — Winclicll. 367 
The chemical composition does not differ essentially from 
that expressed by the formula Ab3An4. The feldspar Ab3xA.n* 
has been studied much less than the plagioclase Abi Ani. Its 
crystallographic characters have never received adequate at- 
tention, the great majority of the measures heretofore made on 
labradorite having been obtained from material of the com- 
position AbjAnj, or Ab^ Anj^. It is for this reason that the 
crystallographic measures, though necessarily less exact in 
proportion to the total possible variations, are of unusual in- 
terest. Moreover, they present a value for the angle Z*^^ (100) A 
(010) which is larger than 94° 10', the value of this angle in 
anorthite, and the supposed limiting value according to the 
theory of Tschermak. In this connection it is shown that the 
angle/^ (001) A(oio) increases with remarkable rapidity as the 
labradorite becomes more basic, and both extremes of the 
labradorite series seem to show values at variance with the 
theory of Tschermak. This appears not only from the meas- 
ures made by the author on a series of labradorites from vari- 
ous localities, but also from a critical examination and com- 
parative study of all the measures of the angle/^i (ooi)A(oio) 
heretofore made on labradorite whose exact composition was 
determined. 
The measures of the indices of refraction show that a dis- 
tinct increase occurs between AbiAu: and Al33An4, in perfect 
harmony with the values obtained, and the steady increase 
shown by all the members of the plagioclase series, as the 
basicity increases. The values obtained are, moreover slight- 
ly greater than those obtained from the same mineral derived 
from the volcanic rocks. The direct measure of the bire- 
fringence likewise shows a slightly greater value than that in 
labradorite of the volcanic rocks. But these differences are 
wholly negligible when compared with the difference in the 
optic angle, which is about 6° greater in the Minnesota feld- 
spar than in the labradorites of the same composition from 
volcanic rocks studied by Fouque. Therefore, there seems 
to exist certain differences in the character of the mineral in 
the two series of rocks. 
Cordierite occurs in only one rock of the series ; but in that 
it is abundant and characteristic. Its presence is very signifi- 
cant as it indicates an unusual chemical condition, and fur- 
