CHAPTER V. 
GENERAL STRATIGRAPHY OF THE KEWEENAW SERIES. 
Separation of the series into a Lower and an Upper Division.—The Upper Division; its thickness; its 
wide spread underneath the waters of Lake Superior; its surface spread and general characters 
between Keweenaw Point and the Montreal River.—In Northern Wisconsin.—The sandstones of 
the Apostle Islands and the Bayfield coast do not belong here.—The Upper Division on Isle Roy- 
ale and Caribou Island.—The Lower Division; from the nature of the case no subordinate strati- 
graphy can be laid down for the whole extent.—Characteristics of certain broad horizons.—The 
very coarse gabbros of Bad and Saint Louis rivers.—The sandstones of the east side of Thunder 
Bay.—Enormous total thickness of the Lower Division.—Variations in total thickness connected 
with the origin of the constituent beds.—Remarkable thinning on Bad River.—Thickness on 
Keweenaw Point.—Between Keweenaw Point and the Ontonagon; west of the Ontonagon.—On 
Bad River.—The thickness between Bad River and the Saint Croix.—In the Saint Croix Valley. 
—On the Minnesota coast.—On Isle Royale.—About Black and Nipigon bays.—In the valleys of 
Black Sturgeon, and Nipigon rivers.—On Michipicoten.—At Cape Gargantua.—At Mamainse. 
The most prominent fact in regard to the stratigraphy of the Kewee- 
naw Series is its separation into two grand divisions: an Upper member, 
made up wholly of detrital material, for the most part red sandstone and 
shale; and a Lower member, made up chiefly of a succession of flows of 
basic rocks, but including layers of conglomerate and sandstone nearly to 
the base, and more or less of original acid rocks. 
The line of separation between these two divisions has to be adopted 
somewhat arbitrarily, since the sandstone gradually increases in quantity 
upward; but placing it at the base of the outer conglomerate of Keweenaw 
Point—which corresponds to the top of the upper amygdaloid of the Por- 
cupines, and to the base of the great conglomerate of the Montreal, and 
which is above any known occurrence of eruptive matter—I estimate the 
Upper Division to attain a maximum thickness of about 15,000 feet in the 
middle portion of the Lake Superior basin. Towards the eastern and west- 
ern ends of the basin the thickness must be much less than this. 
The sandstones of the Upper Division are largely concealed beneath the 
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