370  PRE-CAMBBIAN    GEOLOGY    OF    NORTH    AMERICA. 
mediate  position.  It  is  most  significant  that  these  three  men,  starting 
with  different  views,  finally  reached  like  conclusions.  For  a  long 
time  Rominger  denied  the  existence  of  the  basement  granite-gneiss- 
schist  complex.  Irving  was  slow  to  recognize  the  presence  of  later  in- 
trusive granite.  In  Brooks's  earlier  work  in  the  Marquette  district 
he  did  not  find  the  evidence  of  intrusive  granite  gneiss  which  he 
found  later  in  the  Menominee  district. 
The  subsequent  detailed  study  and  mapping  of  the  iron-bearing 
districts  of  Michigan,  Minnesota,  and  Wisconsin  for  the  United  States 
Geological  Survey  by  Van  Hise,  Bayley,  Clements,  Smyth,  Leith, 
Merriam,  and  others,  and  for  the  Michigan  Geological  Survey  by 
Seaman,  Lane,  and  others,  have  brought  out  abundant  evidence  of 
the  essential  correctness  of  the  succession  of  Brooks,  Pumpelly,  Irving, 
Chamberlin,  Rominger,  Sweet,  and  Wright,  and  has  of  course  resulted 
in  further  subdivision  of  the  series.  The  general  succession  of  this 
group  is :  Cambrian  ;  unconformity ;  Keweenawan ;  unconformity ; 
Huronian,  consisting  of  two  and  in  Michigan  three  unconformable 
groups;  unconformity;  Keewatin  green  schists,  and  Laurentian  gran- 
ites and  gneisses  intrusive  into  the  Keewatin.  A  principal  contribu- 
tion of  these  groups  has  been  the  recognition  of  unconformities  within 
the  Huronian  and  the  subdivision  of  the  Huronian  series  into  forma- 
tions and  the  determination  of  their  order  of  superposition.  Wads- 
worth's  succession  for  the  Marquette  district,  though  differing  in 
names,  is  also  essentially  the  same  as  that  of  the  United  States  Geo- 
logical Survey  for  this  district,  but  he  places  the  "  Eastern  "  sandstone 
below  rather  than  above  the  Keweenawan  series  where  the  two  come 
together  on  Keweenaw  Point.  In  north-central  Wisconsin  Weidman 
finds  unconformable  series  which  he  correlates  with  the  Huronian, 
Laurentian,  and  Keewatin  series  of  the  remainder  of  the  Lake 
Superior  region. 
Up  to  1902  the  United  States  geologists  placed  much  emphasis  on 
the  dual  nature  of  the  Huronian  series.  The  succession  and  correla- 
tion for  the  region  as  a  whole  was  based  on  this  conception.  Seaman 
was  the  first  to  establish  the  facts  that  the  Huronian  groups  of  the 
Marquette  district,  where  they  are  best  represented,  are  three  instead 
of  two  in  number;  that  the  lower  Huronian  of  the  United  States 
Geological  Survey  really  consists  of  two  unconformable  groups,  and 
that  the  unconformity  between  these  groups  is  perhaps  as  great  as 
between  those  that  had  previously  been  called  the  upper  and  lower 
Huronian.  This  discovery  introduced  a  disturbing  factor  which  re- 
quired some  revision  of  the  general  correlation.  In  no  district  outside 
of  the  Marquette  district  have  all  three  groups  been  found,  but  it  has. 
been  necesary  to  determine  with  which  of  three,  instead  of  two,  groups 
a  given  group  in  any  district  is  to  be  correlated.  The  new  possibili- 
ties in  correlation  of  the  Marquette  rocks  with  the  Crystal  Falls, 
Menominee,  and  Minnesota  rocks  are  discussed  on  pages  359,  362, 
