LAKE  SUPERIOR  REGION.  341 
trict  of  Minnesota,  have  been  observed  at  several  localities.  In  the 
mapping  of  the  Vermilion  district  by  the  United  States  geologists 
the  lower-middle  Huronian  was  traced  continuously  to  the  north- 
east for  some  15  miles  into  Hunters  Island.  Along  the  line  of  the 
Canadian  Northern  Railway  similar  sediments  may  be  observed  in 
the  Mattawan  and  Atikokan  districts.  The  published  Canadian 
maps  of  this  great  region  for  the  most  part  discriminate  only  between 
the  granites  and  the  green  rocks,  the  latter  including  both  Keewatin 
and  Huronian,  as  the  terms  are  here  used. 
International  boundary  and  north  shore  of  Lake  Superior. — Ex- 
tending from  the  east  end  of  the  Vermilion  district  in  the  vicinity  of 
Gunfiint  Lake  eastward  to  Pigeon  Point  and  northeastward  to  the 
head"  of  Thunder  Bay  on  Lake  Superior,  and  thence  in  discontinuous 
areas  past  Nipigon  Bay,  is  a  great  area  of  upper  Huronian  or  Animi- 
kie  rocks  similar  in  all  respects  to  the  upper  Huronian  rocks  of  the 
Mesabi  district  and  of  the  south  shore,  except  that  the  iron-bearing 
and  quartzite  members  are  not  so  well  developed  and  the  series  is 
more  largely  intruded  parallel  to  the  bedding  by  Keweenawan  sills  of 
basic  igneous  rock,  known  as  the  Logan  sills.  A  narrow  belt  of 
loAver-middle  Huronian  sediments  has  been  recently  found  by  W.  N. 
Smith  north  of  the  Animikie  rocks  to  the  east  of  Thunder  Bay. 
Michipieoten  district. — Rocks  of  lower-middle  Huronian  age,  con- 
sisting of  conglomerate  (Dore  conglomerate),  graywacke,  and  slate, 
associated  with  basic  and  acidic  eruptives,  similar  in  all  respects  to 
the  lower-middle  Huronian  of  the  Vermilion  district,  are  here  pres- 
ent. They  are  intruded  by  granites  and  gneisses  mapped  as  "  Lau- 
rentian  "  under  the  old  usage  of  the  Canadian  geologists. 
KEWEENAWAN    SERIES. 
The  Keweenawan  is  the  upper  of  the  two  series  of  the  Algon- 
kian  system  in  the  Lake  Superior  region.  The  most  characteristic 
feature  of  the  Keweenawan  is  that  abundant  effusive  rocks  are  as 
widespread  as  the  series  itself ;  indeed,  they  probably  compose  from  a 
third  to  a  half  of  the  series.  The  Keweenawan  contrasts  with  the 
Huronian  in  that  in  the  latter  series  the  effusive  rocks  are  largely 
concentrated  in  a  number  of  localities,  although  in  these  areas  they 
may  be  of  very  great  thickness.  In  short,  the  Keweenawan  was  a 
period  of  regional  volcanic  activity,  while  the  Huronian  was  a  period 
of  local  volcanism. 
DISTRIBUTION. 
The  Keweenawan  rocks  border  the  major  part  of  the  shore  of  the 
western  half  of  Lake  Superior,  occupy  islands  in  the  eastern  half,  and 
are  found  on  the  mainland  at  the  extreme  eastern  end  of  the  lake. 
For  the  western  half  of  the  lake  these  rocks  extend  inland  from  a  few 
