1884.] The Crystalline Rocks of the Northwest. 987 
diorite. In Minnesota this is the iron-bearing horizon at Ver- 
milion lake. Itis Nos. vı to xıv at Marquette, Nos. rv to vr at 
Penokee, and Nos. vı to xı at Menominee. Maximum thickness 
4450 feet. 
FIFTH GROUP. 
This is the group of gray quartzite and marble. It is repre- 
sented by No. v at Marquette, Nos. 11 to v at Menominee and 
Nos. 1 to 11 at Penokee. In Minnesota this horizon seems to 
run along the south side of Ogishke Muncie lake, near the inter- 
national boundary, and perhaps includes the great slate-conglom- 
erate which is there represented. Normal thickness from 400 to 
1000 feet ; but if the great conglomerate of Ogishke Muncie be in 
cluded here, the thickness of this group in Northern Minnesota 
will exceed 6000 feet. 
SIXTH GROUP. 
Granite and syenite with hernblendic schists —This lowest recog- 
nized horizon has frequently been styled Laurentian. In Minne- 
sota it is found on the international boundary at Saganaga lake, 
and large boulders from it are included in the overlying conglom- 
erate at Ogishke Muncie lake, showing an important break in 
the stratigraphy.» Thickness unknown but very great. 
These six great groups compose, so far as can be stated now, 
the crystalline rocks of the Northwest. Their geographic rela- 
tions to the non-crystalline rocks, if not their stratigraphic, have 
been so well ascértained, that it can be stated confidently that 
they are all older than the Cupriferous series of Lake Superior, 
and hence do not consist of nor include metamorphosed sedi- 
ments of Silurian or any later age.’ 
This statement of the grand grouping of the crystalline ter- 
ranes of the Northwest may be varied by the addition of detailed 
and minor distinctions and by subdivisions, but its correctness 
resis upon careful observations and reports of competent geolo- 
gists and cannot at present be gainsayed. 
Examining these groups more closely we find: 
I. We have, beneath the red tilted shales and sandstones, a 
Steat granite and gabbro group. This has been variously regarded 
by different geologists. While by many early observers it was 
"The term Silurian here is understood to cover nothing below the base of the 
Trenton. 
