Gabbroid Rocks of Minnesota. — Winchell. 263 
passes into diallage. Twinning occurs parallel to /^'(loo). 
The mineral is very dark green to black in mass, but in thin 
section it is colorless. In sections however, as thick as .1 mm 
a distinct pleochroism can be observed as follows: 
Ug = greenish 
n^ ^ greenish yellow 
Wp = pale greenisn 
The absorption formula is Wg ^ w„i > ?/p 
The refringence is very high producing a marked relief and 
a shagreen surface; the latter also shows that the mineral pos- 
sesses considerable hardness. The birefringence is about .022, 
and gives bright colors usually red or blue. The acute posi- 
tive bisectrix is wg about which the optic angle is large. 
Magnetite. The iron oxide as a primary element of the 
rock is often even rarer than the augite. Good crystal outline 
is uncommon, but the mineral may be enclosed within either 
the augite or the labradorite. The octohedral parting parallel 
to the pyramidal crystal faces has been noted well developed. 
Magnetite occurs oftener as a decomposition product, in the 
fornroffine particles and masses, than as a primary mineral. 
Apatite. This mineral is very rare in the plagioclasyte 
from Carlton peak. It shows sharply defined hexagonal crys- 
tals in which cleavage can sometimes be detected parallel to 
the base (0001), and more imperfectly parallel to one or more 
prismatic faces. In this rock it is strictly uniaxial and nega- 
tive. 
Inclusions of the Primary Minerals, hi the labradorite. 
As early as 1845, Sheerer * made a study of the inclusions in 
labradorite from Hitteroe, Norway, and reached the conclu- 
sion that they consisted of hematite, ilmenite, and one or two 
more undetermined minerals. , 
Vogelsang,t in 1868, studied the inclusions in labradorite 
from the coast of Labrador, but chiefly with reference to their 
possible connection with the play of colors seen on certain sur- 
faces. He showed that they were practically unattacked after 
remaining four days in hot hydrochloric acid. 
*T. Sheerer: Annalen der Physik und Chemie. Poggendorff. 
LXIV. 1845, p. 163- 
tH. Vogelsang: Sur le labradorite colore de la cote du Labrador. 
Archives Neerlandaises. II. 1868. 
