412 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
diate vicinity of the Dalles of the Saint Croix, of the similar southerly 
course with high easterly dip of the Keweenawan beds of Snake River, 
Minnesota, and, as a consequence, of the southerly direction of the axis of 
the synclinal near its final termination; and (4) the determination by Sweet 
of the existence of a southward dip in the Saint Louis River slates, and the 
consequent probability that the Huronian rocks form the bottom beds of 
the synclinal. 
In Vol. IIT of the Geology of Wisconsin, published in 1880, I embod- 
ied these points in a brief discussion of the structure of Northern Wis- 
consin, accompanied by a map, which I now modify only as to the exact 
extent of the upper sandstones of the Keweenawan, and as to the western 
extension of the horizontal sandstone of the lake shore, which, on the map 
of 1880, was made by misprint to extend to the north side of the Saint 
Louis River at Duluth. No such sandstone is to be seen near Duluth. 
At the beginning of my study for the present memoir, North Wisconsin 
had been shown to be traversed by a broad synclinal in the Keweenawan 
rocks, possibly also in the Huronian, which was presumably the continua- 
tion of the Isle Royale-Keweenaw Point depression. The exact nature 
and position of the western termination of the synclinal, the relation to the 
synclinal of the rocks of the Minnesota coast, and of the Porcupine Mount- 
ains, and the behavior of the depression to the eastward of Isle Royale, 
were all points left in doubt, though it appeared exceedingly probable that 
the entire western half of the Lake Superior basin is a synclinal depression 
affecting both Huronian and Keweenawan rocks. 
Now, however, I feel able to announce with confidence that the entire 
lake basin, including not only the western half, but the eastern half as well, 
is a synclinal depression; that this depression certainly affects the Kewee- 
nawan rocks throughout their entire extent; that it as certainly affects in 
very large measure the underlying Huronian rocks, which, while they are 
greatly folded where extending without the limits of the depression, within 
its limits form without folds its bottom layers; that the axis of the depres- 
sion has, like the lake itself, at first a northwesterly and then a southwest- 
erly direction, with minor bends corresponding to the several bends in the 
axis of the lake; that the eastern termination of the depression is buried 
