The Koochiching Granite. — WincheU. 
297 
zonal structure is ver}^ noticeable; in the larger part three 
zones occur, though the inner one may be part of a ditferent 
Fig. 2.— a twin (hornblende?) 
crystal growth. The cleavage lines are uninterrupted by the 
twinning plane, although they make an angle of about 19 de- 
grees with it. The larger part of the twin gives an extinction 
angle of about 26 degrees on the cleavage; the other part one 
of about 13 degrees. The larger part gives an interference 
ligure which is near an optic axis; the smaller part, one near 
a bisectrix. One twanning lamella is seen for a short distance, 
namely, across the outer zone in one place. Inclusions of 
quartz and epidote are distinct, while the twin is surrounded 
by biotite, hornblende, quartz and epidote. The microscopic 
characters agree, in the main, with those of hornblende, but 
the extinction angle is inexplicably large. Further, it is diffi- 
cult to reconcile the position of the twinning plane with re- 
spect to the cleavage with any known manner of twinning in 
hornblende. 
Tabulating, to compare the results of the specific gravity 
determination with the microscopical investigation, we have: 
Per cent. 
12.00 
Mineral. Sp. Gr. 
Biotite 2.8 —3.2 
Muscovite?.. 2.75— 3.2 
Above 3. 
Hornblende.. 3.2 —3.3 
Epidote 3.25—3.36 
Zoisite? 3.25—3.36 
Apatite 3.16—3.22 
Sphene 3.3 —3.7 
9.29 
