384. COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
R. 26 W., and again on Prairie River in sections 33 and 34, T. 56, R. 25 W. 
The rocks at these points are described as arenaceous quartzites with mag- 
netite, dipping gently southward, by Mr. Bailey Willis, of the United States 
Geological Survey. 
The line drawn from the Mesabi Iron Range to Pokegoma Falls indi- 
cates approximately the northern limit of the Animikie rocks. What be- 
comes of these rocks still farther west is unknown. South of this line, 
however, are immense exposures of slates on the Saint Louis River, begin- 
ning one mile above Knife Falls, in Sec. 14, T. 49, R. 17 W., and contin- 
uing thence down stream to the 8. W. 4, Sec. 11, T. 48, R. 16 W., several 
miles below ‘Thompson, where they disappear underneath the red sand- 
stones, as previously described. These slates dip constantly southward, 
more often at a higher angle than 45° than at a lower. The strike is not 
exactly east, but more or less north of east, the northing increasing in 
amount to the eastward. Throughout, the slate is affected by a strong 
cleavage, which stands vertically, and trends with the strike of the rocks. 
Across these slates at right angles to the trend is a distance of several miles, 
which, with the high angle of dip, would indicate an enormous thickness, 
unless there are some folds here, which, from the presence of slaty cleavage, 
would seem probable. South and west from the Saint Louis the slates are 
known to continue for some miles, but their extent in that direction has not 
yet been worked out. These slates are clay-slates, light- and dark-gray and 
greenish being the prevailing colors. Often they merge into and include 
beds of quartz-slate, which under the microscope shows the quartz largely 
as a fragmental material, but also, in considerable measure, as an original 
constituent. Great dikes are seen at several points cutting the slates, trend- 
ing northward, and composed of a moderately coarse, strongly augitic dia- 
base or gabbro. 
The Saint Louis River slates are plainly the same as the Thunder Bay 
slates, but affected by a slaty cleavage. They are lithologically the same, 
and are cut by the same great dikes. They are the upper portion of the 
series of which the Mesabi iron beds form the lower portion. 
The rock series thus described under the name of the Animikie 
Group—a name first used by Hunt,! and referring to the Indian name of 
‘Trans, Amer. Inst. Mining Engineers, I, 339. 
