A TYPE OF IGNEOUS DIFFERENTIATION 631 
sills along the shore of Lake Superior. The rock is a conspicuously 
spotted one (Fig. 6). The darker spots are large poikilitic olivine 
grains inclosing plagioclase and the white ground mass is feldspar 
with only a small trace of magnetite. Unfortunately the bound- 
aries of this spotted rock are concealed. 
Magnetite phase.—It is possible to find specimens and thin bands 
of gabbro near Duluth bearing as much as 36 per cent of titaniferous 
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Fic. 4.—Diagram of the norms of analyses of red rock of the Duluth gabbro 
formation. 
magnetite. The bands are near the center of the gabbro, which is 
nearly three miles thick. ‘There is no olivine in the magnetite rock, 
but augite is more abundant than in the average gabbro. The 
band shows fluxion structure, but the minerals seem to have crystal- 
lized about simultaneously, magnetite late in some (Fig. 5). Simi- 
lar bands in Cook County, one hundred miles northeast, much 
larger and richer in magnetite, make up the titaniferous iron ores 
for which the gabbro is famous. 
