LAKE   SUPERIOR   REGION.  355 
series,  present  conclusive  evidence  of  the  existence  of  a  great  pre- 
Huronian  unconformity  in  these  districts. 
In  the  Gogebic  district  an  unconformity  beneath  the  cherty  lime- 
stone mapped  by  Irving  and  Van  Hise  as  the  bottom  of  the  Huronian 
series,  Avas  inferred  from  the  general  field  distribution  and  the  con- 
trast in  lithology,  deformation,  and  metamorphisni.  This  belt  of 
cherty  limestone  was  found  to  extend  with  uniform  strike  across  va- 
rious highly  metamorphosed  and  deformed  members  of  both  the  Lau- 
rentian  and  the  Keewatin  series.  They  found  no  actual  contacts. 
Subsequently  Seaman  and  Sutton  found  quartzite  and  basal  con- 
glomerate beneath  the  limestone,  resting  upon  steeply  tilted  schistose 
Keewatin  rocks,  presenting  proof  diagrammatic  in  its  clearness. 
In  the  Vermilion  district  of  Minnesota  the  great  Stuntz  and  Ogishke 
conglomerates  at  the  base  of  the  Huronian  series  are  found  to  rest 
against  all  the  members  of  the  Laurentian  and  Keewatin  of  this  dis- 
trict. Here  again  the  general  difference  in  deformation,  metamor- 
phism,  and  intrusion  by  igneous  rocks  exhibited  by  rocks  above  and 
below  the  unconformity  is  striking.  But  locally  the  fragmental  rocks 
of  the  lower-middle  Huronian  can  scarcely  be  distinguished  from  the 
igneous  rocks,  acidic  or  basic,  from  which  they  have  been  derived. 
There  is  an  especial  difficulty  in  distinguishing  some  of  the  greenstone 
conglomerates  of  the  lower-middle  Huronian  from  the  volcanic 
breccias  and  tuffs  of  the  Keewatin.  Indeed,  the  possibility  must  be 
distinctly  recognized  that  there  are  localities  in  the  Vermilion  district 
where  there  may  have  been  continuous  sedimentation  from  the  Kee- 
watin to  the  Huronian,  volcanic  action  dying  out  toward  the  end  of 
the  Keewatin  and  normal  aqueous  sedimentation  becoming  more  im- 
portant in  the  lower-middle  Huronian. 
In  the  Mesabi  district  may  be  seen  similar  evidence  of  unconform- 
ity between  the  lower-middle  Huronian  and  the  Keewatin.  The 
Laurentian  is  present  in  but  very  small  quantity. 
In  the  Rainy  Lake  district  conglomerates  mapped  as  Keewatin  are 
believed  to  represent  the  base  of  the  Huronian  series.  These  are 
found  in  contact  with  Laurentian  granite  and  Keewatin  green 
schists.  In  the  latter  case  both  are  highly  metamorphosed,  and  paral- 
lel schistosity  has  been  developed,  but  there  is  still  difference  in  lith- 
ology and  texture,  and  an  unconformity  was  not  doubted  when  the 
contact  was  observed  by  the  members  of  the  joint  committee  of  Can- 
adian and  United  States  geologists. 
In  the  Steep  Rock  Lake  area  Pumpelly  and  Smyth  described  the 
unconformable  contact  of  the  Huronian  rocks  upon  Laurentian 
granites. 
In  the  Thunder  Bay  district  schistose  conglomerates  are  found  in 
contact  with  Keewatin  green  schists  in  the  Loon  Lake  region. 
