LAKE    SUPERIOR   REGION.  339 
schist  and  overlain  by  slate,  which  is  continuous  with  that  at  Carlton 
and  Cloquet.  Diabase,  gabbro,  and  granite  are  found  in  close  asso- 
ciation. They  are  probably  intrusive,  although  development  work 
has  not  proceeded  far  enough  to  prove  this.  The  rocks  are  weathered 
deeply  and  covered  by  Cretaceous  conglomerate,  indicating  a  correla- 
tion of  the  weathering  with  the  Cretaceous  peneplain.  It  is  believed 
that  these  rocks  are  to  be  correlated  with  the  upper  Huronian  of  the 
Gogebic  district  and  that  they  are  in  general  along  the  southern 
margin  of  the  Mesabi  syncline.  The  district  is  complexly  folded. 
Similar  rocks  are  found  by  drilling  to  the  southwest,  west,  and 
northwest. 
Along  Minnesota  River  are  quartzites  similar  to  the  Huronian 
quartzites  of  Michigan,  but  to  which  part  of  the  Huronian  is 
unknown. 
Mesabi  district. — In  this  district  a  twofold  classification  of  the 
Huronian  series  is  possible.  The  upper  part  corresponds  with  the 
upper  Huronian  of  Michigan  and  Wisconsin,  while  the  lower  part 
might  be  equivalent  to  either  the  lower  or  the  middle  Huronian  of 
Michigan,  or  to  both.  It  will  be  here  referred  to  as  the  lower-middle 
Huronian. 
The  unconformity  between  the  upper  and  the  lower  groups  is,  as 
usual,  a  great  one.  The  lower-middle  Huronian  consists  of  con- 
glomerates, graywackes,  and  slates,  with  bedding  and  schistosity 
vertical  and  striking  east-northeast.  It  occurs  in  but  small  areas. 
It  is  intruded  by  the  Giants  Range  granite,  which  constitutes  the 
main  topographic  feature  of  the  Mesabi  range.  The  upper  Huronian 
(or  Animikie  group)  comprises  from  the  base  up  the  Pokegama 
quartzite  (200  feet),  the  iron-bearing  Biwabik  formation  (1,000 
feet),  and  the  Virginia  slate,  of  great  but  unknown  thickness.  This 
group  rests  on  the  south  slope  of  the  Giants  range,  strikes  east- 
northeast,  dips  for  the  most  part  southerly  and  southeasterly  at 
angles  varying  from  5°  to  20°,  and  is  gently  cross  folded,  the  axes 
of  the  cross  folds  pitching  in  the  direction  of  the  general  dip  noted. 
At  the  east  end  of  the  district  granite  and  basic  igneous  rocks  of  Ke- 
weenawan  age  are  intrusive  into  this  group  in  the  form  of  sills  and 
dikes.  Here  the  upper  Huronian  is  highly  metamorphosed  and 
tilted  to  an  angle  of  45°  S. 
Vermilion  district. — In  the  Vermilion  district  only  a  twofold  classi- 
fication of  the  Huronian  is  possible,  the  groups  corresponding  very 
closely  in  lithology  and  structure  with  the  groups  of  the  Mesabi 
district,  although  varying  greatly  in  the  magnitude  of  the  respective 
areas.  The  lower-middle  Huronian  sedimentary  rocks  consist  of 
conglomerate,  slate,  and  graywacke  and  occupy  large  areas  in  this 
district.  Usually  the  group  appears  in  belts  between  and  surround- 
ing the  cores  of  greenstone  belonging  to  the  basement  complex.     The 
