632 FRANK F. GROUT 
THE RED ROCK 
Introduction.—The “red rock”’ has purposely been left out of 
the discussion of variations from the gabbro, not because it differs 
from the gabbro more radically than anorthosite differs from peri- 
dotite, but because its geologic relations are very different. It has 
not been seen as bands in banded gabbro. The change from gabbro 
to red rock is somewhat abrupt and without alternation. The gray 
gabbro rapidly gives place to a bright red rock very different from 
the gabbro in mineral, chemical, and physical characters. 
Fic. 5.—Thin section of Duluth gabbro Fic. 6.—Spotted anorthosite. The dark 
showing diabasic texture. White to light areas are olivine crystals, which poikiliti- 
gray, basic labradorite; dark gray, augite; cally inclose thousands of smaller plagio- 
black, magnetite which is late to crystal- clase grains. 
lize. Plain light, 20. 
The ‘‘red rock”? has become widely known under this name 
because of its brilliant color and the difficulty of giving it a more 
accurate classification. Some confusion may arise also from the 
fact that red felsitic flows appear in the Keweenawan series. The 
rock here discussed is intrusive and granitoid. 
The chief outcrops near Duluth are irregular patches at the top 
of the main gabbro and apophyses into its roof; it occurs also near 
the top of the earlier feldspathic gabbro, in a large sill close above 
the gabbro, and in some small dikes near the bottom of the gabbro. 
Description.—The texture varies from sugary near contacts to 
very coarse in certain patches. The rock is peculiarly friable, so that 
