TJic Keweenawan in Mirmesota. — Elftnian. 141 
are all gradations from a narrow rim of hornblende around a 
plate of pyroxene, or a few small areas of hornblende within 
the pyroxene to a plate of hornblende with a few scattering 
areas or a single core of pyroxene. Some sections of the 
highly altered rock contain only hornblende, but in several 
sections from the same hand specimens there is nearly always 
some trace of diallage. Since the hornblende is always closely 
associated with the pyroxene and since it is absent from the 
unaltered rock it is concluded that the hornblende in the gab- 
bro is always of secondary origin. The mineral has a light 
to dark green color. Pleochroism is strong and the maxi- 
mum extinction angle is about 22 degrees. The columnar 
structure of uralite is prominent. Frequently the mineral is 
finely fibrous. 
Biotite is common in the altered parts and absent in the 
fresh portions of the gabbro. It occurs as thin scales and 
irregular crystals sometimes two to three inches across. The 
outer parts of some of the large pyroxene crystals consist of 
biotite. ■ In thin section it is seen surrounding plates of py- 
roxene and hornblende. These three minerals are some- 
times found in the same individual, the biotite forming the 
outer rim, the hornblende the middle and the pyroxene the 
inner core. The biotite also forms a mosaic with highly al- 
tered pyroxene and hornblende. It occurs as a reaction 
product between magnetite and plagioclase. Small flakes of 
the mineral within the feldspars indicate the former presence 
of magnetite. Instead of single plates replacing the pyroxene 
there are in some sections beautiful rosettes of biotite. The 
mineral varies from a dark brown to reddish brown. The 
latter color is found chiefly in the biotite surrounding the 
magnetite. Cleavage is nearly always prominent. From its 
association of biotite with the other minerals it is undoul:>tedly 
always secondary. 
Chlorite occurs as an alteration product derived largely 
from the ferro-magnesian constituents of the rock. It is pres- 
ent in minute greenish flakes and needles sometimes arranged 
in rosettes and fibrous aggregates, which, between crossed 
nicols, show a dark cross. 
Epidote, like chlorite, is not abundant, and occurs as an 
alteration product. It is present in the altered feldspar, and 
other minerals, as minute yellow flakes. 
