Some Pomts in the Oeology of Northern Wisconsin. 115 



again, these sandstones reappear (at points marked lY), on the 

 Lead waters of the St. Croix, from where they can be traced 

 uninteriiptedly until they disappear beneath the light-colored 

 sandstones of the Mississippi valley. From these latter the 

 horizontal red sandstones of the west end of lake Superior 

 differ much in appearance and composition, being always of a 

 dark red or reddish brown color, and always carrying a large 

 percentage of sesquioxide of iron, and of alumina. These 

 peculiarities have led many geologists to assign these red 

 sandstones to a much later date than the Lower Silurian, and 

 it has always been wondered why they should so differ from 

 the light- colored sandstones of the Mississippi, if they are of 

 the same age. The explanation is, however, very simple, as 

 shown further on. 



On the east side of Keweenau Point there are horizontal 

 reddish sandstones which are, without doubt, the exact 

 equivalents of those just described, because: 1st. they bear 

 the same relation to the associated copper bearing and Huro- 

 nian rocks ; 2d. They graduate upward into light-colored 

 sandstones, which themselves pass underneath limestones of 

 Trenton age ; and, 3d. They extend westward nearly as far as 

 the Montreal river, being thus separated from the sandstones 

 of the west end of the lake by only a few miles. 



Having given thus briefly an outline sketch of the geology 

 of northern Wisconsin, with the main features of its four great 

 groups of rocks, it remains for me to draw your attention es- 

 pecially to those points bearing on the age of the copper-bear- 

 ing series. The age of this series, as well as that of the accom- 

 panying horizontal sandstones, now proved to be Lower Silu- 

 rian, has for years given rise to discussion, the earlier geolo- 

 gists calling them all Triassic, Foster and Whitney making 

 them the equivalents cf the Potsdam sandstones of New Y'ork, 

 whilst the Canada geologists have regarded them as the equiv- 

 alents of the Quebec group of Canada East. Still more lately 

 Mr. Bell of the Canada Geological Survey, has revived the 



