Gabbroid Rocks of Minnesota. — WinchelL 243 
extinction in sections perpendicular to each one of the bisec- 
trices. His numbers are indeed entirely characteristic for any 
given feldspar, and constitute therefore an iijiportant diagnos- 
tic. In the case of the Carlton feldspar they are all the more 
important since all the plagioclases studied by Fouque were 
derived from volcanic rocks, and therefore differ in origin 
from the one here studied. 
Two sections of the labradorite from Carlton peak perpen- 
dicular to n<y and wp were prepared M. Ivan Werlein. Per- 
pendicular to wp the extinction is at an angle of 57° to 57° 30', 
but the bisectrix is not absolutely centered, and the measure is 
therefore slightly inaccurate. On the contrary, the bisectrix 
??g is very well centered, and the extinction angle is 37° to 37° 
30'. 
A comparison with the results of Fouque can be tabu- 
lated as follows; 
Extinction angle 
imposition* Sp. Gr. Locality parallel to perpendicular to 
grMOlO) p(OOl) ng np 
;,An^ 2.698 Besseyre, Hte. Loire 31° 59 
jAn^ 2.710 Capello, Fayal ■ 32° 58 
,An^ 2.709 He St. Jorge, Azores 210-22 - 8= 33° 59^ 
jAn^ 2.705 Capello, Fayal 26-28 12°-13 35° 58 
.,An^ 2.701 Carlton Peak, Minn. 26 12° 37° 57 30, 
■jAn^ Theoretical angles deduced from the 
diagrams of Michel Levy. 24° 9° 30° 58 
The extinction angles parallel to ^{p\o) and /(ooi) are 
therefore entirely comparable to the higher values found by 
Fouque. The extinctiont perpendicular to Tz/p is slightly 
different, but it is not strictly accurate; the extinction perpen- 
dicular to S//g is distinctly higher than the values obtained 
from the volcanic rocks. Can this be due again to a difference 
*For exact composition see page 238; the composition of the Min- 
nesota labradorite will be given later. 
fF. Fouque: Bull. Soc. Min. France, vol. XVIL 1894. M. Fouque 
designates by S and T the sections respectively perpendicular to the 
acute and to the obtuse bisectri.x, and these letters are followed by 
11^ or ;7p , according as the bisectrix is positive or negative. S^t; 
therefore means the section perpendicular to the acute bisectrix, 
which is positive. 
