LAKE    SUPERIOR   REGION.  385 
early  recognized  the  existence  of  an  unconformity  beneath  what  is 
now  known  as  the  Huronian  series,  but  at  that  time  they  believed 
the  iron  formation  underlying  this  unconformity  to  be  itself  Huro- 
nian, and  therefore  the  unconformity  to  be  an  inter-Huronian  uncon- 
formity. The  break  between  the  Huronian  sediments  and  the  base- 
ment complex  in  Minnesota  was  not  recognized  as  such  prior  to  the 
detailed  work  of  the  United  States  Geological  Survey  in  the  later 
part  of  the  last  century. 
In  the  Mesabi  district  lower-middle  Huronian  sediments  were  not 
discriminated  from  the  green  schists  of  the  basement  complex  by  the 
Minnesota  Survey — both  were  included  under  "  Keewatin."  Grant 
suggested  their  possible  separation,  and  Leith  discriminated  a  lower 
Huronian  sedimentary  series  of  considerable  magnitude  and  found 
decisive  evidence  of  its  unconformity  on  the  green  schists  of  the 
basement  complex. 
The  Marquette  district  on  the  south  shore,  next  to  be  taken  up 
in  geographic  order,  should  perhaps  have  been  discussed  first  because 
it  was  the  first  district  in  which  the  unconformity  between  the  Algon- 
kian  and  the  Archean  was  clearly  recognized. 
Unconformable  contacts  have  been  found  at  many  localities,  but 
here  the  clastic  series  are  folded,  and  certain  of  the  contacts  between 
the  elastics  and  the  crystalline  complex,  by  overlapping,  are  below 
upper  members  of  the  clastic  series  rather  than  truly  basal  contacts. 
Of  the  two  localities  cited  by  Brooks  for  the  unconformity  between 
the  Huronian  and  the  Laurentian,  that  at  Republic  Mountain  is 
clearly  between  the  lower  part  of  the  Huronian  series  and  the 
granite-gneiss-schist  complex,  while  that  at  Plumbago  Creek,  in  the 
L'Anse  district,  is  certainly  below  a  high  horizon.  Of  the  many  local- 
ities cited  by  Rominger  and  Irving  in  which  there  are  contacts  be- 
tween the  granite-gneiss-schist  complex  and  the  overlying  elastics,  if 
Brooks's  succession  be  accepted,  several  belong  well  down  in,  if  not 
actually  at  the  base  of,  the  Huronian  series.  At  these  contacts  the 
elastics  are  generally  conglomerates,  built  up  chiefly  of  the  debris 
of  the  underlying  rocks,  and  oftentimes  so  thoroughly  cemented  as 
closely  to  resemble  them  and  to  lead  to  the  conclusion,  if  not  carefully 
examined,  that  there  is  a  real  transition  between  the  clastic  and  crys- 
talline rocks.  As  already  indicated,  this  was  at  first  Rominger's  opin- 
ion, and  the  apparent  transition  was  taken  to  indicate  a  gradual  meta- 
morphism  from  the  conglomerates  to  the  granite  or  schists,  as  the  case 
might  be.  But  Rominger's  later  studies  led  him  to  see,  as  did  also 
Irving,  that  these  basal  conglomerates  are  rocomposed  rocks  resting 
upon  an  earlier  formed  crystalline  and  often  granitic  base.  Brooks, 
Rominger,  and  Irving  all  hold  that  in  the  Menominee  district  con- 
tacts are  found  between  very  low,  or  the  lowest,  members  of  the  clastic 
*  55721— Bull.  360—09 25 
