LAKE  SUPERIOR  REGION.  359 
The  predominating  member  of  the  upper  Huronian,  slate,  appears 
in  many  exposures  eastward  from  the  Gogebic  district  through  the 
great  triangular  area  connecting  with  the  Crystal  Falls,  Marquette, 
and  Menominee  districts  to  the  east.  There  is  little  room  for  doubt 
that  the  upper  slate  series  in  each  of  these  districts  is  directly  con- 
tinuous with  the  slate  of  the  upper  Huronian  of  the  Gogebic  district. 
In  the  Marquette  district,  beneath  the  upper  predominantly  slaty 
group,  are  two  unconformable  sedimentary  groups  formerly  con- 
sidered a  unit.  These  may  be  called  for  convenience  in  discussion 
the  middle  and  lower  Huronian.  As  it  has  already  been  noted  that 
the  Marquette  district  is  the  only  district  in  which  this  division  of  the 
sedimentary  rocks  beneath  the  upper  Huronian  has  been  made,  the 
lower  and  middle  groups  of  this  district  may  for  the  present  be  corre- 
lated with  the  undivided  lower  Huronian  of  other  parts  of  the  Lake 
Superior  region.  Beneath  the  Huronian  series  is  a  complex  of  green- 
stones and  green  schists,  of  typical  Keewatin  aspect,  bearing  a  small 
amount  of  iron  formation  and  acidic  igneous  rocks  characteristic  of 
the  Laurentian. 
In  the  Crystal  Falls  district  lithological  representatives  of  the 
middle  and  lower  Huronian  appear  beneath  the  upper  Huronian  and 
may  be  correlated  in  the  same  manner  as  in  the  Marquette  district. 
In  the  Menominee  district  the  upper  Huronian  again  appears,  con- 
nected with  the  Animikie  of  the  Gogebic  district  by  isolated  exposures. 
Beneath  this,  unconformably,  is  a  sedimentary  group  lithologically 
similar  to  and  probably  to  be  correlated  with  the  lower  Huronian  of 
the  Marquette  district. 
In  the  Iron  River  district  upper  Huronian  iron-bearing  slates, 
similar  in  all  respects  to  those  in  the  Menominee  district  and  prob- 
ably continuous  with  them,  are  underlain  by  dolomite  and  quartzite 
which  may  be  correlated  with  the  lower  Huronian  in  the  Menominee 
district.  This  lower  group  rests  upon  a  basement  of  green  schist 
and  granite  connecting  directly  with  the  Archean  of  the  Menominee 
district. 
In  the  Crystal  Falls,  Felch,  Menominee,  and  Iron  River  districts 
the  upper  Huronian  is  nearly  conformable  structurally  with  the 
rocks  below,  a  relation  contrasting  strongly  with  that  between  the 
upper  Huronian  and  underlying  groups  in  the  Marquette  district. 
The  significance  of  this  relation  is  given  on  page  362. 
Beneath  the  Huronian  series  of  the  Menominee,  Crystal  Falls,  and 
Iron  River  districts  is  again  a  complex  of  greenstones  and  green 
schists,  the  Keewatin,  and  acidic  igneous  rocks,  the  Laurentian.  Cer- 
tain of  these  greenstones  have  volcanic  textures  and  are  similar  to 
greenstones  and  green  schists  of  the  Crystal  Falls  district  which  have 
been  mapped  as  Huronian. 
