Gabbroid Rocks of Minnesota. — Winchell. 297 
More rarely the feldspar is slightly more acid, and is re- 
ferable to andesine of the composition Abs An2 about. The 
extinction perpendicular to iig is then about 10°, and per- 
pendicular to //p from 66° to 69°. In cleavage pieces parallel 
to/(ooi) the extinction is sensibly parallel; parallel to ^'(010) 
it is about — 9°. 
The plagioclase very rarely shows the zonal structure; un- 
dulatory extinction is uncommon. 
'Inclusions in the feldspar are rare; liquid inclusions occur, 
as well as a few solid particles. 
Both bi'onzite and cnstatitc exist in the rock, though the 
former greatly predominates in sections studied. Both occur 
in rounded grains sometimes showing a tendency to crystal 
form, but with the angles always rounded. The prismatic 
cleavages are large and interrupted. In thin sections of not 
more than .03 mm. thickness enstatite is wholly colorless; 
bronzite is sometimes colorless and sometimes tinted very pale 
yellowish green with very feeble pleochroism as follows: 
n^ = pale green. 
;/m= paie yellowish i^reen. 
;/p = pale yellowish to colorles.s. 
The absorption is % > //,„ > //j,. 
The refringence is very high, but the birefringence is 
Aveak, with: 
Bronzite. Enstatite. 
«g — ;/,n — .004 at least. .005 at least. 
«m — i'/p = .007 at least, .003 at least. 
«g - 11^^ = .012 at least. .008 at least. 
The ordinary pyroxene (bronzite) of the rock is negative, 
and would theref(jre be called hypersthene, if the very pal'e 
color did not show that it is decidedly low in iron. Enstatite. 
on the contrary, is positive. Both have large optic angles. 
The pyroxenes contain many liquid inclusions, sometimes 
{|uite large. They contain more rarely masses, and needles of 
magnetite. Mnally they sometimes enclose anhedra of 
cordierite and minute flakes of biotite. They alter to antho- 
])hyllite, as will be described later. 
Biotite is abundant; it never shows crystal outline, but en- 
closes other minerals, never bending around them. It pos- 
