172  IMiK-f'AMIilUAX    ROCKS    OF    NORTH    AMERICA.  [buu.86. 
of  subsequent  changes.  The  ore  bodies,  for  instance,  are  secondary 
concentrations,  generally  in  troughs,  due  to  the  action  of  percolating 
water.  The  same  tacts  are  apparent  as  to  the  Upper  Marquette  and 
Upper  Menominee  ores  ami  jaspers. 
In  the  case  of  the  iron  formations  of  the  Lower  Marquette,  Lower 
Vermilion,  and  similar  districts  the  question  can  not  be  so  decisively 
answered.  The  Lower  Marquette  iron-bearing  formation  is  generally, 
if  not  always,  the  uppermost  member  found  in  the  lower  series,  when  ero- 
sion has  not  carried  it  away,  and  therefore  apparently  occupies  a  definite 
horizon,  although  as  the  structure  of  this  district  has  not  been  worked 
out  in  detail  this  can  not  be  positively  asserted.  In  the  Vermilion  lake 
district  the  great  masses  of  ore  and  jasper  of  immense  thickness  seem 
to  extend  only  for  a  short  distance  along  the  strike  of  the  rocks,  then 
disappear  and  reappear  at  intervals  to  the  northeastward  at  Long  lake, 
at  Hunter's  island,  and  other  points,  probably  also  north  of  Port  Arthur 
in  the  Kamiuistiquia  district.  This  lack  of  persistence  may  be  due  to 
the  fact  that  the  fold  or  folds  are  not  horizontal,  but  have  a  varying  pitch. 
Theore  formation  at  the  swells  of  the  pitchingfblds  may  have  been  removed 
by  erosion.  While  it  is  not  proved,  it  is  probable  that  the  ore  forma- 
tion is  at  a  definite  horizon,  since  the  different  outcrops  appear  to  be  in 
the  same  part  of  the  series  and  along  the  same  general  line  of  strike 
for  considerable  distances.  These  ore  and  jasper  formations  contain 
abundant  iron  carbonate,  are  inter] aminated  with  graphitic  schists  con- 
taining iron  carbonate,  the  graphite  of  which  is  so  abundant  and  widely 
disseminated  in  minute  particles  that  it  can  hardly  be  believed  to  be 
other  than  of  organic  origin,  and  from  these  forms  there  are  gradations 
to  the  other  forms  of  rock  found  in  the  iron-bearing  formation.  In  the 
deeper  workings  of  some  of  the  larger  mines  of  the  Marquette  and  Me- 
nominee districts,  the  ore  bears  much  residual  carbonate  of  iron  and 
also  carbonates  of  calcium  and  magnesium.  At  Iron  Mountain  and 
Metropolitan  the  iron  formation  grades  downward  by  insensible  de- 
grees into  the  limestone.  All  of  these  points,  and  the  remarkable  lith- 
ological  likenesses  between  the  phases  of  rocks  found  at  these  horizons 
and  those  occurring  in  the  iron  formations  of  the  Penokee  and  Animikie, 
demonstrably  of  detrital  origin,  are  cited  as  evidence  that  these  ores 
are  derived  from  an  originally  impure  cherty  carbonate  of  iron.  A 
study  of  the  Vermilion  iron  formation  by  N.  H.  and  11.  V.  Winchell 
lias  led  them  to  the  conclusion  that  it  is  of  direct  chemical  detrital 
origin  rather  than  derived  from  an  impure  cherty  .carbonate.  Which 
of  these  views  is  the  correct  one  does  not  concern  the  present  question, 
for  if  the  ore-bearing  formation  is  detrital  it  may  be  used  for  the  pur- 
poses of  stratigraphy.  We  thus  have  in  favor  of  an  origin  for  these 
lower  ore  formations  similar  to  those  of  the  Penokee  and  Animikie  a 
large  amount  of  positive  evidence,  while  the  only  adverse  positive  facts 
are  those  cited  by  Wadsworth,  and  these,  as  has  already  been  said,  may 
be  brought  into  accordance  with  the  sedimentary  theory  by  consider- 
