246  t>RE-CAMBRIAN   GEOLOGY    OF    NORTH   AMERICA. 
discordance  in  dip.  difference  in  amount  of  deformation  and  of  meta- 
morphism,  distribution  of  the  series,  and  relations  to  intrusives. 
The  Keweenawan  rocks  consist  of  gabbro,  diabase,  and  granite,  all 
of  which  are  intrusive  into  the  rocks  with  which  they  come  into  con- 
tact. The  north  edge  of  the  gabbro  runs  diagonally  across  the  east 
end  of  the  district  from  southwest  to  northeast,  resting  upon  the  edges 
of  each  of  the  members  of  the  Upper  Huronian  series,  and  at  Birch 
Lake  against  the  Lower  Huronian  granite.  North  of  the  gabbro 
margin  in  range  12  are  isolated  exposures  of  diabase  which  may 
represent  sills  associated  with  gabbro  intrusion.  The  granite  forms 
the  crest  of  the  Giants  range  through  Rs.  12  and  13.  This  granite 
has  not  heretofore  been  discriminated  from  the  Lower  Huronian 
granite.  The  exomorphic  effect  of  the  gabbro  and  the  granite  upon 
the  Upper  Huronian  series  has  been  profound. 
Clements,211  in  1903,  describes  the  geology  of  the  Vermilion  iron- 
bearing  district  of  Minnesota.  Elaborate  general  and  detailed  maps 
accompanying  this  report  are  based  on  field  work  by  Clements,  Van 
Hise,  Bayley,  Merriam,  and  Leith. 
The  district  ranges  from  2  to  18  miles  in  width,  and  extends  from 
a  little  west  of  Lake  Vermilion,  in  a  direction  a  little  north  of  east  to 
Gunflint  Lake,  on  the  international  boundary,  a  distance  of  about 
100  miles. 
The  rocks  of  the  district  are  described  under  the  headings,  Archean, 
Lower  Huronian,  and  Upper  Huronian,  representing  series  separated 
by  marked  unconformities. 
The  Archean  of  the  Vermilion  district  is  divided  as  follows,  from 
the  base  up :  The  Ely  greenstone,  the  iron-bearing  Soudan  formation, 
and  the  granites  of  Vermilion,  Trout,  Burntside,  Basswood,  and 
Saganaga  lakes. 
The  Ely  greenstones  consist  of  basic  and  intermediate  igneous 
rocks  widely  distributed  in  anticlinal  areas,  as  shown  by  the  distri- 
bution of  the  overlying  sediments.  They  were  originally  rocks  cor- 
responding in  character  to  intermediate  andesites  and  basic  basalts. 
They  have  been  extremely  altered,  but  retain  in  many  cases  in  strik- 
ing perfection  the  original  structures,  such  as  ellipsoidal  parting  and 
spherulitic  and  amygdaloidal  structures.  A  study  of  their  various 
textures  and  structures  shows  that  these  greenstones  are  unquestion- 
ably of  igneous  origin  and  largely  of  volcanic  character.  Many  of 
them  have  been  rendered  schistose  by  pressure.  The  greenstones  have 
also  been  strongly  affected  by  the  contact  metamorphism  due  to 
the  intrusion  of  great  granite  masses.  As  a  result  of  this  intrusion 
there  have  been  produced  from  the  greenstones  amphibole  schists, 
which  form  a  marginal  facies  of  the  greenstones,  lying  between  them 
and  the  adjacent  granites.  The  greenstones  have  also  been  meta- 
morphosed by  the  Duluth  gabbro,  of  Keweenawan  age,  and  granular 
