342  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
miles  to  125  miles,  and  here  are  the  important  districts  of  the  region. 
This  distribution  shows  that  this  series  once  occupied  the  major  por- 
tion of  the  Lake  Superior  basin  and  from  it  extended  varying  dis- 
tances, although  for  much  of  the  basin  at  the  present  time  the  Ke- 
weenawan  rocks  may  be  overlain  by  the  Cambrian  sandstone. 
The  chief  areas  of  the  Keweenawan  are  (1)  those  of  Black  and 
Nipigon  bays  and  Lake  Nipigon,  (2)  northern  Minnesota,  (3)  north- 
ern Wisconsin,  and  (4)  northern  Michigan.  Subordinate  areas  are 
(1)  Isle  Royal,  (2)  Michipicoten  Island,  (3)  Mamainse  Peninsula, 
and  other  points  on  the  east  shore  of  Lake  Superior.  Irving  esti- 
mated the  area  of  the  Keweenawan  without  the  intrusives  in  the 
older  series  at  41,000  square  miles. 
It  is  thus  evident  that  Lake  Superior  in  Keweenawan  time  was  an 
area  of  regional  volcanic  activity  extending  east-west  for  more  than 
400  miles  and  north-south  scarcely  a  less  distance. 
SUCCESSION. 
A  broad  study  of  the  various  districts  of  the  Keweenawan  leads  to 
the  conclusion  that  a  threefold  division  of  the  series  as  a  whole  may 
be  made  as  follows,  beginning  at  the  bottom:  (1)  Lower  Keweena- 
wan, comprising  conglomerates,  sandstones,  dolomitic  sandstones, 
shales,  and  marls;  (2)  middle  Keweenawan,  comprising  extrusive 
and  intrusive  igneous  rocks,  with  important  amounts  of  interstrati- 
fied  sandstones  and  conglomerates  and  subordinate  amounts  of  shale ; 
and  (3)  upper  Keweenawan,  comprising  conglomerates,  sandstones, 
and  shales.  In  the  region  of  Black  and  Nipigon  bays  and  in  north- 
ern Minnesota  the  lower  and  middle  groups  appear.  In  Michigan 
and  Wisconsin  the  lower,  middle,  and  upper  groups  are  found.  On 
Isle  Royal  and  Michipicoten  Island  only  the  middle  group  is  known, 
and  to  the  eastern  side  of  Lake  Superior  the  lower  and  middle  groups 
appear.     Thus  in  only  one  district  is  the  full  succession  known. 
LOWER  KEWEENAWAN. 
The  lower  Keweenawan  is  found  in  all  of  those  districts  in  which 
the  succession  extends  to  the  bottom  of  the  series.  However,  by  far 
the  most  important  area  of  the  lower  Keweenawan  is  that  on  Black 
and  Nipigon  bays.  The  rocks  are  here  a  series  of  quartzose  sand- 
stones, dolomitic  sandstones,  and  red  marls,  which  have  a  thickness 
estimated  by  Logan  to  be  from  800  to  900  feet  and  by  Bell  to  be  from 
1,300  to  1,400  feet.  In  northern  Minnesota,  northern  Wisconsin,  and 
Michigan,  and  at  the  east  end  of  Lake  Superior,  the  lower  Kewee- 
nawan is  represented  by  a  conglomerate  and  sandstone  of  moderate 
thickness,  none  of  the  exposed  measured  section  being  more  than  300 
