444 FRANK F. GROUT 
a hundred miles long is insignificant. The relations are well 
exposed in the western part of Duluth. By detailed study it is 
found that the intrusion of gabbro occurred at two or more times, 
for at Lincoln Park and elsewhere the chilled contact and apophyses 
of one show that an older gabbro had already cooled. Banding 
(Fig. 1) is shown chiefly by the later mass, which is much the larger 
of the two. 
In some places two rock types alternate, but in most there are 
several minor rock varieties in irregular alternation. The bands 
vary in thickness from a fraction of an inch to many feet. It is 
likely that in the average the gabbro does not show such minute or 
intimate lamination as some associated sediments,’ but while there 
may be a general difference, each varies to resemble the other. 
Some contacts between adjacent bands are abrupt, but more com- 
monly there is a complete gradation between them. Some neigh- 
boring bands contrast strongly in color, while others are visible 
only on careful scrutiny; some are intensified by weathering, pro- 
ducing black, brown, gray, and white colors; some are conspicuous 
only from a difference in the degree of glacial polish (Fig. 2). Some 
large outcrops at Duluth show faint bands as much as fifty feet 
wide. Itis therefore evident that smaller outcrops a few feet wide 
may not reveal a banded structure even if it really exists. The 
whole area has been mapped as banded, because the outcrops 
which did not show the structure were smal! and not numerous; 
they may represent other variations of the mass, but are here 
considered as probably thick bands. .Most of the bands are 
regular, parallel, and fairly continuous along the strike and dip. 
However, there are locally lenticular bands, and spots or bunches 
along the bands, as shown in Fig. 2. Rarely the bands curve and 
finger out into each other (Fig. 3) and are as complex in structure 
as the ancient metamorphic gneisses. This irregularity is not as 
prominent as in the gabbro of the Isle of Skye (1); but locally the 
average dip of about 25° to the east increases to 80° with some 
variation also in strike. Although these outcrops may resemble 
metamorphic gneisses enough to be deceptive, a thorough study of 
™U. S. Grant, ‘Contact Metamorphism of a Basic Igneous Rock,” Bull. Geol. 
Soc. Amer., XI (1900), 508. 
