LAKE  SUPERIOR  REGION.  325 
Superior  and  Lake  Huron  regions.  The  conclusions  of  the  party  con- 
cerning individual  districts  are  summarized  in  the  sections  devoted 
to  those  districts.  General  conclusions  concerning  the  regions  as  a 
whole  are  given  in  the  following  statement,  quoted  from  their  report : 
We  do  not  feel  that  our  examination  of  the  Lake  Superior  region  was  suf 
ficiently  detailed  to  warrant  an  attempt  at  correlation  of  the  individual  for- 
mations of  the  various  districts.  There  are,  however,  certain  general  points 
which  seem  to  be  reasonably  clear  and  about  which  there  is  no  difference  of 
opinion  between  us.    These  are  as  follows : 
There  is  an  important  structural  break  at  the  base  of  the  Keweenawan.  The 
term  "  Keweenawan  "  should  include  substantially  all  of  the  areas  which  have 
been  thus  mapped,  or  mapped  as  Nipigon,  by  the  Canadian  and  United  States 
surveys  and  the  State  surveys  of  Michigan,  Minnesota,  and  Wisconsin. 
Below  the  Keweenawan  is  the  Huronian  system,  which,  in  our  opinion,  should 
include  the  following  series :  In  the  Marquette  district  the  Huronian  should 
include  the  Upper  and  Lower  Marquette  series  as  defined  in  the  monographs 
of  the  United  States  Geological  Survey,  or  the  Upper,  Middle,  and  Lower  Mar- 
quette series  as  given  in  the  previous  paragraphs.  In  the  Penokee-Gogebic  dis- 
trict the  Huronian  should  include  the  series  which  has  been  called  the  Penokee- 
Gogebic  series  proper  and  the  limestone  and  quartzite  which  have  local  develop- 
ment and  which  we  visited  east  of  Presque  Isle  River.  In  the  Mesabi  district 
the  Huronian  should  include  the  Mesabi  series  proper  and  the  slate  graywacke 
conglomerate  series  unconformably  below  the  Mesabi  series.  In  the  Vermilion 
district  the  Huronian  should  include  the  Knife  slates  and  the  Ogishke  conglom- 
erates. In  the  Rainy  Lake  district  the  Huronian  should  include  that  part  of 
the  Coutchiching  of  the  south  part  of  Rainy  Lake  which  is  limited  below  by 
basal  conglomerate,  as  shown  at  Shoal  Lake.  In  the  Thunder  Bay  district  the 
Huronian  should  include  the  Animikie  and  the  graywacke  series  in  the  Loon 
Lake  area.  In  the  Original  Huronian  area  the  Huronian  should  include  the 
area  mapped  by  Logan  and  Murray  as  Huronian,  except  that  the  Thessalon 
greenstones  should  probably  be  excluded. 
Unconformably  below  the  Huronian  is  the  Keewatin.  The  Keewatin  includes 
the  rocks  so  defined  for  the  Lake  of  the  Woods  area  and  their  equivalents.  We 
believe  the  Kitchi  and  Mona  schists  of  the  Marquette  district,  the  green  schist 
(Mareniscan)  of  the  Penokee-Gogebic  district,  the  greenstone  series  of  the 
Mesabi  district,  the  Ely  greenstones  and  Soudan  formation  of  the  Vermilion 
district,  the  part  of  the  area  mapped  as  Keewatin  by  Lawson  in  the  Rainy 
Lake  district  not  belonging  structurally  with  the  Coutchiching,  and  probably 
the  Thessalon  greenstone  series  on  the  north  shore  of  Lake  Huron  to  be  equiva- 
lent to  the  Keewatin  of  the  Lake  of  the  Woods,  and,  so  far  as  this  is  true,  they 
should  be  called  Keewatin. 
For  the  granites  and  gneissoid  granites  which  antedate,  or  protrude  through, 
the  Keewatin,  and  which  are  pre-Huronian,  the  term  "  Laurentian  "  is  adopted. 
In  certain  cases  this  term  may  also  be  employed,  preferably  with  an  explanatory 
phrase,  for  associated  granites  of  large  extent  which  cut  the  Huronian  or  whose 
relations  to  the  Huronian  can  not  be  determined. 
