168  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
has  been  changed  into  a  ferruginous  quartzite.  The  typical  occur- 
rence of  this  quartzite  is  at  the  end  of  the  Felch  Mountain  range.  It 
also  appears  between  the  Archean  ovals  in  the  northeastern  part  of 
the  district.  If  distinct  conglomerates  were  formed  at  the  bottom  of 
this  quartizite,  they  are  buried  under  glacial  deposits  or  have  disap- 
peared as  the  result  of  metamorphism. 
In  the  western  part  of  the  district  the  rocks  of  the  Lower  Huronian 
at  the  surface  are  the  great  Hemlock  formation,  and  here  the  basal 
horizon  of  the  Upper  Huronian  is  a  slate  or  slaty  conglomerate,  the 
fragments  of  which  are  derived  mainly  from  the  underlying  Hemlock 
formation.  The  sandstones  and  conglomerates  varied  upward  into 
shales  and  grits,  which  have  been  altered  into  mica  slates  and  mica 
schists.  After  a  considerable  thickness  of  mudstone  and  grit  was 
deposited,  there  followed  a  layer  of  combined  clastic  and  nonclastic 
sediments,  the  latter  including  iron-bearing  carbonates.  These  appear 
to  be  at  a  somewhat  persistent  horizon,  and  in  this  belt  are  found  the 
iron-formation  rocks  and  iron  ores  in  the  Upper  Huronian  in  the 
vicinity  of  Crystal  Falls.  Above  these  ferruginous  rocks  there  was 
deposited  a  great  thickness  of  shales  and  grits  which  have  been  trans- 
formed into  mica  slates  and  mica  schists. 
Since  the  deposition  of  the  Upper  Huronian  the  rocks  of  the  dis- 
trict have  been  folded.  The  more  complex  folds  vary  from  a  north- 
south  to  an  east-west  direction.  The  closer  folds  in  the  northeastern 
part  of  the  area  are  nearly  north-south.  In  the  central  part  of  the 
area  the  closer  folds  strike  northwest-southeast.  In  the  eastern  and 
southeastern  parts  of  the  district  the  closer  folds  are  nearly  east-west. 
All  of  these  folds  have  steep  pitches. 
Subsequent  to  or  during  the  late  stage  of  this  time  of  folding  there 
was  a  period  of  great  igneous  activity,  probably  contemporaneous 
with  the  Keweenawan.  At  this  time  there  were  introduced  into  both 
the  Lower  and  the  Upper  Huronian  vast  bosses  and  numerous  dikes. 
The  intrusives  A^ary  from  those  of  an  ultrabasic  character,  such  as 
peridotites,  through  those  of  a  basic  character,  such  as  gabbros  and 
dolerites,  to  those  of  an  acidic  character,  such  as  granites.  These 
intrusives,  while  altered  metasomatically,  do  not  show  marked  evi- 
dence of  dynamic  metamorphism;  therefore  the  conclusion  that  they 
were  introduced  later  than  the  period  of  intense  folding  already 
described. 
Cambrian  rocks  overlie  unconformably  the  rocks  above  described. 
