AREA OF THE KEWEENAW SERIES. ; AAT 
as follows: for Michigan, in the area between Keweenaw Point and the 
Montreal River (the “Main Trap Range,” and Porcupine Mountains region) 
1,744 square miles; for the area of horizontal sandstone south of the Main 
Range of Michigan, and west of the head of Keweenaw Bay, which area 
is undoubtedly underlaid by rocks of this group, 1,400 square miles; for 
Wisconsin, including the area in the immediate vicinity of Lake Superior, 
and again on the Saint Croix River, where the Keweenawan rocks are cov- 
ered with horizontal sandstones, about 5,000 square miles; for Minnesota, 
south of the Saint Louis, including portions buried beneath horizontal sand- 
stone, 1,000 square miles; for Minnesota, north of Lake Superior, about 
3,200 square miles; for Isle Royale, 210 square miles; for Isle Saint Ignace 
and adjoining islands, 180 square miles; for Michipicoten Island, 75 square 
miles; and for small areas along the east shore between Cape Gargantua 
and the Sault, 90 square miles. These rocks are also undoubtedly buried 
beneath the horizontal formations of the south shore of the lake west of 
the Sault, but not to any very great extent. So that we may be safe in 
placing the entire geographical extent of the series at 41,000 square miles 
for the immediate basin of Lake Superior. This is exclusive of an exten- 
sion northward of some of the lower beds into the basin of Lake Nipigon 
through the valleys of Black-Sturgeon and Nipigon rivers. The thick- 
ness of this extension, judging from Bell’s descriptions,' is inconsiderable, 
the rocks lying often nearly horizontal. In the valleys of Black-Sturgeon 
and Nipigon rivers they appear to form strips between older rocks on either 
side, but in the Nipigon Lake basin to have a wider extent. Bell’s map 
makes the total area in this basin as much as 5,000 square miles. 
Throughout all of this wide extent, though local peculiarities are to be 
noted, the general characteristics of the group are wonderfully constant. 
The predominating rocks belong to the basic crystalline class. They are, 
as a rule, in distinct, but, for the most part, heavy layers, their bedded 
structure being due, as I believe, to their having been spread out as suc- 
cessive molten flows, and more rarely, perhaps, as injections between the 
previously formed layers. These basic rocks belong wholly to the augite- 
1 Report of Progress of Geological Survey of Canada, 1866—69 ; 1872~73. 
