244  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
The  Keweenawan  gabbro  lies  diagonally  across  the  east  end  of  the 
district. 
The  Archean  rocks  consist  principally  of  green  rocks  of  great 
variety,  including  dolerites,  metadolerites,  basalts,  metabasalts,  dio- 
rites,  and  hornblendic,  micaceous,  and  chloritic  schists.  The  more 
massive  rocks  frequently  have  an  ellipsoidal  structure  which  is  char- 
acteristic of  the  green  igneous  rocks  of  other  parts  of  the  Lake 
Superior  region.  In  addition  to  the  green  basic  rocks  there  are 
present  small  areas  of  granite  and  porphyritic  rhyolite. 
The  Lower  Huronian  series  consists  of  sediments  and  granite. 
The  sediments  are  graywackes,  slates,  and  conglomerates,  all  meta- 
morphosed, with  bedding  and  schistosity  practically  vertical.  They 
may  be  as  thick  as  10,000  feet,  but  it  is  thought  more  probable  that 
the  thickness  does  not  exceed  5,000  feet.  The  Lower  Huronian  sedi- 
ments rest  unconformable  upon  the  Archean  rocks,  as  shoAvn  by  basal 
conglomerates  containing  fragments  of  all  the  varieties  of  rocks 
found  in  the  Archean.  The  Lower  Huronian  granite  forms  the 
main  mass  of  the  Giants  range  westward  from  a  point  near  the 
east  line  of  R.  14  W.  It  is  intrusive  into  both  the  Archean  rocks 
and  the  Lower  Huronian  sediments  and  has  produced  strong  ex- 
omorphic  effects  in  both. 
The  Upper  Huronian  or  Animikie  series  consists  of  three  forma- 
tions— the  Pokegama  quartzite  at  the  base,  above  this  the  Biwabik 
formation  (iron  bearing),  and  above  this  the  Virginia  slate. 
The  Pokegama  quartzite  comprises  vitreous  quartzite,  micaceous 
quartz  slate,  and  conglomerate.  The  thickness  ranges  from  0  to  500 
feet,  averaging  about  200  feet.  The  conglomerate  at  the  base  indi- 
cates unconformable  relations  of  the  Pokegama  formation  to  the 
Archean  and  Lower  Huronian  rocks. 
The  Biwabik  formation,  the  iron-bearing  formation,  comprises 
ferruginous,  amphibolitic,  sideritic,  and  calcareous  cherts,  siliceous, 
ferruginous,  and  amphibolitic  slates,  paint  rocks,  "  greenalite  "  rocks, 
sideritic  and  calcareous  rocks,  conglomerates  and  quartzites,  and 
iron  ores.  Cherts  make  up  the  bulk  of  the  formation.  The  original 
rock  of  the  formation  is  shown  to  consist  largely  of  minute  granules 
of  green  ferrous  silicate,  thus  confirming  Spurr's  conclusion.  The 
material  was  called  glauconite  by  Spurr,  but  is  here  determined  to 
be  a  hydrous  ferrous  silicate  entirely  lacking  potash,  and  thus  not 
glauconite.  It  is  named  "  greenalite  "  for  convenience  in  discussion. 
The  cherts  and  iron  ores  are  shown  to  develop  mainly  from  the  altera- 
tion of  the  greenalite  granules.  The  slates  are  in  thin  layers  inter- 
bedded  with  the  other  phases  of  the  iron  formation.  The  paint  rocks 
result  from  the  alteration  of  the  slates.  The  conglomerates  and 
quartzites  form  a  thin  layer  from  a  few  inches  to  perhaps, 15  feet  or 
more  in  thickness  at  the  base  of  the  formation.     They  pass  upward 
