986 The Crystalline Rocks of the Northwest. (October, 
Omitting the igneous rocks, which in the form of dikes cut 
through the shales and sandstones of the Cupriferous formation 
and are interbedded with them in the form of overflows, we may 
concisely arrange the crystalline rocks, disregarding minor difer- 
ences and collating only the broad stratigraphic distinctions, in 
the following manner in descending order : 
There are six groups: 
First GROUP. 
Granite and gneiss with gabbro—This group is represented in 
Minnesota by the gabbro and red syenite at Duluth, and by the 
extension of this range of hills north-eastwardly nearly to the 
international boundary. Its thickness is unknown, but certainly 
reaches several hundred feet. The outcrop of red granite near 
New Ulm, lying under the conglomerate and red quartzite, is 
probably in the south-westward line of extension of this group. 
This group is represented by No. xx south-west of Lake Michi- 
gamme, by No. xx at Menominee and by No. 1 and 1a at Black 
river. 
SEconD GROUP. 
Mica schist—This group consists of schists that are micaceous 
- and often staurolitic as well as garnetiferous. It can be seen 18 
Minnesota on the Mississippi river at Little Falls, and at Pike 
rapids. The schists are variously associated with beds and veils 
of granite and gneiss. This is No. xıx at Marquette, XVII ton 
at Menominee, xx to xxi at Penokee, and has a maximum thick- 
ness of 5000 feet. 
THIRD GROUP. ; 
Carbonaceous and arenaceous black slates, and black mica sclas 
—These sometimes pass into roofing slates, with beds of iron 
ore, quartzite and diorite. This group includes the black ee 
of the Animikie group in Northern Minnesota, of Knife lake, 
Knife portage on the St: Louis river, and carbonaceous slates 
lately discovered near Aitkin on the Mississippi river- It incl 
Nos, XIV to XVII at Marquette, Nos. vi to xvii at Penokee, 3% 
Nos. xv and xvi at Menominee. Thickness 2600 feet. 
FOURTH Group. 
; Hydro-mica and magnesian schists-—Soft and obscuré, we of 
ing quartzose and also hematitic, also with numerous 
