294 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
of the Greenstone of Keweenaw Point. It consists chiefly of very fresh augite 
in relatively large areas, inclosing 
numbers of tabular plagioclases (an- 
orthite), and having in the inter- 
spaces, which are chiefly occupied 
by the same tabular plagioclases, 
many small altered olivines and 
' particles of magnetite. The dike is 
strongly cross-jointed, save at the 
edges, which are traversed by joints 
parallel to the walls, and are com- 
posed of an aphanitic rock, much 
altered to chlorite. The rocks tra- 
versed are the usual stratiform 
Fic. 15.—Dike traversing stratiform amygdaloid and amygdaloid and columnar mela- 
columnar melaphyr, two miles below Lester River, 
Minnesota coast. phyr of the Agate Bay Group. 
Equivalents of the Duluth, Lester River, and Agate Bay Groups at the east 
end of the Minnesota coast—At the eastern end of the Minnesota coast there 
intervenes, between the Huronian slates and the base of the Beaver Bay 
Group, which next overlies the Agate Bay beds, a space only three and a half 
miles wide, measured at right angles to the east and west strike. With the 
flat dip prevalent in this region this width cannot include a total thickness of 
more than 3,000 feet, while at the Duluth end of the coast there lie between 
the same horizons the whole of the Duluth, Lester River, and Agate Bay 
groups, a thickness of some 9,000 feet; not to speak of the Duluth gabbros, 
which must add several thousand feet more. Forty miles west of Grand 
Portage, however, about Brulé Lake, in T. 63, R. 2 W. and R. 3 W., the 
gabbros are present in full force, while between them and the lower limit 
of the Beaver Bay Group there is a width of 10 miles, within which space, 
supposing the dip to be not more than 10°, a figure which the observations 
along Cascade River show to be closely right, there is room for a much 
greater thickness of the Duluth, Lester River, and Agate Bay groups. The 
small thickness on the lake shore near Grand Portage is probably to be 
