44 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
River and Grand Portage Bay—all on the Minnesota coast. Other places 
are the Lake shore at Duluth (small areas); Brewery Creek, Duluth; the 
north shore of Nipigon Bay, near the mouth of Nipigon River, Ontario, Can- 
ada; the west side of the mouth of Nipigon Straits; and the beds immedi- 
ately over “The Greenstone,” in the Eagle River section of Keweenaw 
Point, especially the beds numbered 107 and 96 by Marvine.’ These rocks 
are also common in the country back of the Minnesota coast, where they 
form most of the steep-backed ridges so commonly encountered in trav- 
ersing the woods of this region. The Duluth gabbro falls for the most part 
under the next head, 7. e., is orthoclase-gabbro; butin the western extension 
of the Duluth gabbro belt towards Fond du Lac, and again to the north- 
ward, on the Cloquet River, very coarse olivine-gabbros are abundant. 
There is again an immense development of these rocks, often with exces- 
sively coarse grain, in the Bad River country of Wisconsin, where they 
underlie the greater part of a belt of country forty miles in length and two 
to four in width. ° 
The following tabulation presents in a compact way the observations 
made on a number of specimens of these rocks brought from different parts 
of the Lake Superior basin. The angular measurements by which the feld- 
spars were determined are included. The numbers of the specimens, when 
unaccompanied by any letter; are the collection numbers of the specimens 
gathered especially for this work. Numbers with the letter ““W ” attached 
belong to a collection made by Col. Charles Whittlesey in the Bad River 
region of Wisconsin, and now, with the thin sections, in the cabinet of 
the Wisconsin State University. Those with the letters 5, SW, and I 
attached were collected respectively by Messrs. Strong, E. T. Sweet, and 
myself, for the Wisconsin State Geological Survey, and are also now, with 
the thin sections, in the cabinet of the Wisconsin State University. For 
further statements as to the four last-named collections, with descriptions of 
many specimens, see the Geology of Wisconsin, Vol. III., pp. 27.to 49, 53 
to 238, 305 to 362, and 365 to 428. 
1Geological Survey of Michigan, Vol. I, 1869-1873, Chapter VIII., pp. 117-140. 
