Gabbroid Rocks of Minnesota. — Wi?ichell. 223 
and related gabbros in Minnesota, Canada, Newfoundland, 
New York, Finland, and Russia. 
Physical Characters of Labradorite. The albite twinning 
is very common and shows itself megascopically as a series of 
striations, usually very fine, on the cleavage face /(ooi). An- 
hedra showing no twinning are decidedly rare. It is easily seen, 
even without the microscope, that this twinning has its com- 
position plane parallel to g^ (010) and its twinning axis per- 
pendicular to this plane. It has been noted that one set of 
twinning lamellae is much narrower than the other; the finer 
lamellae may, however, become wide enough to give distinct 
images with the goniometer, and then the angle pp'{po\) f\ 
(001) can be measured. The lines separating the twinning 
lamellae are generally very straight, and usually continuous, the 
lamellae occasionally terminate abruptly, but oftener taper out 
to a fine point. This twinning is never made up of less than 
three bands. The only other twinning visible without the mi- 
croscope is that according to the pericline law, which may oc- 
casionally be seen forming very fine striations on the face 
^'(010) nearly parallel to the cleavage/ (001). Under the mi- 
croscope it appears that this twinning is much less highly de- 
veloped than the albite, and appears rather rarely crossing the 
latter at varying angles depending upon the orientation of the 
section. Unfortunately it does not appear in the sections 
which have been made parallel to ^''^(oio), nor in those perpen- 
dicular to ;/g and n^ so that its position cannot be exactly de- 
termined. 
Twinning according to the Carlsbad law is decidedly rare. 
It occurs, however, combined with the pericline twinning as 
well as with the common albite twinning. Its composition 
plane is the same as that of the albite type, (in the cases exam- 
ined) but its axis of rotation is the vertical crystallographic 
axis. In no case is it composed of more than two individuals. 
No other twinning has been found in the plagioclasyte, butin 
the diabases, where all the types of twinning are beautifully 
well developed, a single example of the Baveno type has been 
found. A single anhedron shows four types of twinning, 
namely the Carlsbad, albite, pericline and Baveno. The sec- 
tion shows the albite and pericline twinning at an angle of 
94^° and is therefore approximately parallel to /^ '(100); but 
