26 ; COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
all of which places they form mere edgings between the lake and the older 
rocks, the only exception being at Mamainse, where a great thickness is 
exposed; and, finally, constitute the isolated reef between Keweenaw 
Point and Marquette, known as Stannard’s Rock. Between Gros Cap and 
Stannard’s Rock the rocks of this series do not appear, but there can be 
little doubt that they are continuous underneath the lake and underneath 
the newer horizontal beds of the eastern part of the upper peninsula of 
Michigan. 
The main dimensions of Lake Superior are as follows, in air-line dis- 
tances:! from Fond du Lae to Sault Sainte Marie, 377 miles; from Duluth 
to Batchewanung Bay, the extreme straight-line length of the lake, 368.5 
miles; extreme width from South Bay, near Grand Island, to the mouth of 
Nipigon River, 195.5 miles; width from Presq/isle River, on the Michigan 
coast, to Poplar River, on the Minnesota coast, 73.5 miles; from the eastern 
extremity of Keweenaw Point to Nipigon Point, 116 miles; from the same to 
Otter Head, 91.5 miles; from the same to Mamainse Point, 140.5 miles; from 
the same to Whitefish Point, 137.3 miles; from the same to Sault Sainte 
Marie, 170.5 miles; and from the same to Duluth, 212 miles. A measure- 
ment, first, on a line from Fond du Lac, N. 664° E. to a point in the middle 
of the lake on the national boundary line, about 45 miles north of Kewee- 
naw Point, and thence 8. 554° E. to Sault Sainte Marie, gave for the two 
distances, respectively, 229 miles and 191 miles, or, for the extreme length of 
the lake, measured along its middle line, 420 miles. The approximate length 
of the coast line of the lake, exclusive of the more minute irregularities, but 
including all the larger bays, is 1,700 miles. The coasts of the greater islands 
will increase this to 1,900 miles. The total area of the lake, exclusive of 
all the larger islands, but inclusive of all bays, is about 30,880 square miles. 
Deducting from the last figure 2,500 square miles for lake area underlaid 
by older rocks in the extreme northeast, in Thunder, Black, and Nipigon 
bays, and, again, between Keweenaw Bay and Marquette, we obtain 
28,380 square miles as the approximate total extent of the group under the 
waters of Lake Superior. 
To obtain its whole geographical extent we must add to the last figure 
1 All of these figures were obtained by careful measurements on the U. 8. Lake Survey charts. 
