Gabbroid Rocks of Minnesota. — W'lnchelL 295 
gabbro only in the replacement of the monoclinic by a rhombic 
pyroxene. 
The noryte studied came from Sec. 15, T. 63 - 9, a short 
distance northwest 'from Snowbank Lake. It is at the north- 
ern edge of the gabbro area, where the latter comes in con- 
tact with Archean clastic "greenstones." Furthermore, this 
type of rock is not uncommon along the northern line of con- 
tact of the gabbro mass with the older schists, and it is never 
found away from the contact zone. This rock has been termed 
"muscovadyte"* or latterly, biotite noryte, by the Minne- 
sota Survey. 
It is usually very dense, and fine grained, and shows very 
little evidence of schistosity. On the other hand, it occurs 
in beds often of great size, which sometimes have a recog- 
nizable dip and strike, and are associated with "greenstones" 
(Keewatin) probably of sedimentary origin. The strike and 
dip of these masses of noryte always follow the direction of 
the contact plane between the gabbro and the earlier Keewa- 
tin sediments. That is, they are nearly horizontal and extend 
from N. E. to S. W. These masses occur quite commonly at 
many difTerent points along the northern edge of the gabbro 
area, e. g., at Gun flint lake, Birch lake, Gabimichigama lake. 
Disappointment lake, near Little Saganaga lake, etc. The 
transition from the ordinary gabbro to the "muscovadyte" 
has been repeatedly described, as well as the transition from 
mica schist to "muscavadyte," as they occur in the field, and in 
both cases the changes are very gradual, by imperceptible 
degrees. The type which we are studying forms a part of the 
altered gabbro rather than the metamorphosed schist. It is, 
further, of interest to note that similar poikilitic and granular 
rocks (containing cordierite?) have been found as contact 
phases of the gabbro in the Adirondacks of New York state by 
H. P. Gushing. 
The color of the rock on fresh surfaces is a dark gray, 
sometimes slightly greenish; on such surfaces the bright scales 
of biotite are very conspicuous, in a finely granular glassy 
background. Occasionally a yellowish green mineral can be 
*The name rauscovadyte is derived from the Spanish "muscovado" 
(brown sugar) and refers tn the l)rown color of surfaces exposed to wea- 
thering. 
