Gabbroid Rocks of Minnesota. — Wincliell. 241 
measures. This extraordinary difference can not be explained 
on the ground of difference in chemical composition since the 
Minnesota plagioclase is not appreciably more basic than 
those studied by Fouque.^' Since his measures were all made 
on feldspars from the volcanic rocks, it seems probable that 
the optic angle is distinctly greater in labradorite of the deep- 
seated rocks than in the corresponding labradorite of the vol- 
canic rocks. It will at least be interesting to examine this 
question further as the measures of the optic angle on ma- 
terial of known composition increase in number. It is well 
known that for orthoclasef the angle of the optic axes is gener- 
ally very small in the mineral derived from the volcanic rocks, 
and that it may be, on the contrary, quite large in orthoclase of 
the granitic rocks. 
Dispersion. The values of the optic angles for the different 
colors determine at once that p > l' about //g the acute bi- 
sectrix, and p<v about ^/p . 
*The following measures on ihe labradorite AbiAn, and on material 
of composition not exactly known, have been made: 
Corap. Sp. Gr. Locality Author 2Va 2Ha 2G} 
.\biAni(?) 2.694 Chenavari Fouque 77°0' 
do 2.696 Pico (Azores) Foiique T7'6' 
do 2.695 St. Lucia, Pico Fouque 77"36' 
do 2.698 Lavaofl720,Pic. Fouque 77 38' 
do (?) 2.700 Peak of Pico Fouque 78 34' 
do (?) 2.688 Rochesauve Fouque 80"40' 
82^25' 
Ab3An4(?) - Labrador DesCloizeaux 82°6' 89 10' 
AbAn (?) Djupivogur, 
Iceland DesCloizeaux 8144' 88"15' 
Ab7An,(?) Ojamo, Russia DesCloizeaux 80°4r 87'^5' 
do do do 79°12' 85°29' 
do do ' do 78°34' 84 43' 
Ab,An, Ojamo, Russia Schuster 79°24' 82°36' 
do do do 78^6' 81=24' 
It should be noted that the measures of Des Cloizeaux were made 
in red light and are therefore not exactly comparable with the others 
which were made in yellow light. However the difference could 
scarcely exceed 30'. Fouque's material was all from volcanic rocks, 
witli the possible exception of that from Rochesauve: the others were 
all from deep-seated rocks, with the possible exception of that from 
Iceland. These measures tend to confirm the view that the optic 
Jingle in labradorite of the deep-seated rocks is notably greater than in 
the corresponding labradorite of the volcanic rocks. 
tSee Lacroix: Mineralogie de la France, II. p. 68. 
