634 FRANK F. GROUT 
A somewhat different gradation is observed in Lincoln Park and 
near the top of the inclined railroad to Duluth Heights. In these 
places it is possible to select samples showing all stages between 
gabbro and red rock, but the relations are not those of a regular 
zone. The upper part of the banded gabbro shows many local 
patches with interstitial red granophyr, grading into dikelike 
stringers and patches of red rock of complex form and relations 
(Fig. 7). Many of these stringers with sharply defined walls can 
be traced along their length into less sharply defined markings and 
finally grade imperceptibly into the black gabbro which formed the 
walls a few feet away. Both the gabbro and the red rock intrude 
the roof, sometimes in the same crack, sometimes more distinctly. 
Although a considerable part of the red rock is so much later in 
time of solidification that it could intrude the gabbro, the texture 
of the red rock is coarse up to its contacts and grades into that of 
the gabbro without a break, indicating that they were about 
equally hot. The irregularity in the form of the stringers may also 
be a sign that the gabbro was not wholly solid (see Fig. 7). Such 
a relation may be properly described as that of an aplite. 
Similar relations of gabbro to red rock, both gradational and 
aplitic, are easily traced for many miles along the belt at the north- 
east end of the gabbro in Cook County, where the combined thick- 
ness is so reduced as to make the mass more like a sill, and the 
red rock constitutes a larger proportion of the intrusion than at 
Duluth. The same relation may be expected in the central, 
thicker part of the gabbro mass,’ but this has not been mapped 
in detail as yet (see Fig. 1). 
A third gradation from red rock to gabbro is that in the pegma- 
tites near the base.” 
Origin of the red rock.—All three of these occurrences of red rock 
and gradations would seem from field studies to be clearly attrib- 
utable to a differentiation. However, this sweeping assignment of 
the granophyr to differentiation ignores a whole group of occurrences 
«This is not wholly in agreement with the brief statements in U.S. Geol. Sur. 
Mon. 52, pp. 374-75: : 
2 Frank F. Grout, ‘“‘The Pegmatites of the Duluth Gabbro,” Econ. Geol., XIII, 
(1918), 185. 
