The Kc%vccnaivan in Minnesota. — Elftnmn. 183 
them had some points which could be recognized over the 
entire area. The "terra incognita" usually did not show the 
phenomena which had been predicted for it. The following 
brief outline is given in order that the reader may better un- 
derstand the detailed descriptions as they are given in the suc- 
ceeding chapters. The results given below are based entirely 
upon the writer's investigation and the petrographical descrip- 
tions of the groups are taken from the series of rocks collected 
by the writer for the Minnesota Survey. 
An important obstacle in the way of getting a satisfactory 
subdivision of the Keweenawan has been the failure to recog- 
nize the extent of the faulting. It is evident that there is a 
belt near the lake Superior coast which contains a series of 
faults, some of which show a displacement of over 1,000 feet. 
This belt is conspicuous for its peculiar topography and is 
known as the Sawteeth mountains. 
The proposed subdivision of the Keweenawan series is 
based upon the chronologic succession, the stratigraphic con- 
tinuity and the distinctive lithologic characters of each 
member. The eruptive rocks of each member possess a 
strong similarity in lithologic characters and are closely al- 
lied in their genetic relationship. This suddivision elimi- 
nates the supposed promiscuous chronologic relations of 
the acid, basic and intermediate eruptive rocks. The mem- 
bers, here proposed, are, in order of their age, the Gabbro, 
the Beaver Bay Diabase, the Red Rock, the Temperance 
River and the Later Diabase. 
The Gabbro /neniber. This includes essentially the basal 
gabbro of Irving and the gabbro of the Mesabi hills of Win- 
chell. It is entirely of an intrusive nature and appears to be 
one mass, the proportion of whose mineral constituents vary 
so that locally well defined varieties of the rock are recog- 
nized. On its northern side the gabbro is in contact with pre- 
Keweenawan formations, and on its southern border it is asso- 
ciated with later members of the Keweenawan series. 
The Beaver Bay Diabase member. This member has not 
l)een found in contact with the preceding member. The area 
Ijetween the two is occupied by parts of the later members. 
In the vicinity of Brule lake are some rocks which may pos- 
siblv be older than the gabbro, but these are not associated 
