THICKNESS OF THE LOWER DIVISION. 159 
it on either side, until they had accumulated sufficiently to overflow its 
upper surface. 
Twenty miles west of Bad River the coarse gabbro has thinned out, 
and beyond it to the west and southwest, all the way to the Saint Croix 
Valley, the Lower Division must have its old thickness of 25,000 feet, to 
judge from the country covered and dip angles observed. 
On the west side of the Saint Croix Valley the thickness must be as 
great, the surface width from east to west on Snake and Kettle rivers being 
as much as nine miles, which, with the high easterly dips in the lower layers 
(45° to 70°), must mean a thickness of nearly 25,000 feet, without reaching 
either the upper or lower limit of the division. 
The Copper Range of Douglas County, Wisconsin, again, appears to 
indicate by its exposures nearly as great a thickness, for, although the belt 
of exposures is relatively narrow, its course is oblique to the courses of the 
constituent beds. The dip of these beds is southeasterly, and in going along 
the range eastward one is steadily descending in horizon. Here again the 
lower limit must be several thousand feet below the lowest rock seen. 
On the Minnesota coast I estimate the total thickness above the Huro- 
nian or Animikie slates to be some 22,000 to 24,000 feet on the west end 
of the coast, and not more than 16,000 at the east end. This difference is 
due in large measure to the total disappearance of the coarse gabbro belt 
of the Saint Louis—a case analogous to that of the coarse gabbros of Bad 
River in Wisconsin—but also to the thinning eastward of higher layers. 
Several thousand feet must intervene between the highest rocks of the Min- 
nesota coast and the summit of the Lower Division. 
Isle Royale is formed of a succession of beds dipping southward at an 
angle which increases in amount, as the series is crossed from south to 
north, from 8° or 10° toa much higher but undetermined figure. The 
southernmost or highest layers belong to the Upper Division, while most of 
the island is made up of layers of the Lower Division, with a total thickness 
which cannot be much less than 10,000feet, and may be much more. To 
judge from the courses of the rock belts on Isle Royale and the eastern end 
of the Minnesota coast, the higher beds of the latter are the same as the 
