﻿ADIRONDACK 
  MAGNETIC 
  IRON 
  ORES 
  13 
  

  

  included 
  impurities 
  by 
  regional 
  and 
  contact 
  metamorphism. 
  

   Pyroxene, 
  amphibole, 
  mica, 
  graphite, 
  pyrite 
  and 
  scapolite 
  are 
  

   common 
  associates. 
  With 
  an 
  increase 
  in 
  the 
  proportion 
  of 
  the 
  

   silicate 
  minerals, 
  the 
  limestones 
  pass 
  into 
  micaceous, 
  pyroxenic 
  

   or 
  amphibole 
  schists. 
  By 
  secondary 
  alteration 
  of 
  the 
  pyroxene 
  a 
  

   serpentinous 
  limestone 
  or 
  more 
  rarely 
  a 
  massive 
  serpentine 
  may 
  be 
  

   developed. 
  

  

  The 
  limestones 
  and 
  associated 
  schists 
  are 
  found 
  generally 
  in 
  long 
  

   narrow 
  belts 
  bordered 
  by 
  the 
  sedimentary 
  gneisses. 
  They 
  are 
  

   most 
  widespread 
  on 
  the 
  northwestern 
  side 
  of 
  the 
  Adirondacks 
  in 
  

   St 
  Lawrence, 
  Jefferson 
  and 
  Lewis 
  counties. 
  Four 
  main 
  belts, 
  with 
  

   a 
  length 
  of 
  from 
  15 
  to 
  35 
  miles, 
  and 
  a 
  great 
  number 
  of 
  smaller 
  ones 
  

   have 
  been 
  mapped 
  in 
  this 
  region. 
  On 
  the 
  east 
  side 
  they 
  occur 
  most 
  

   abundantly 
  in 
  Essex 
  county, 
  but 
  they 
  are 
  here 
  less 
  extensive. 
  In 
  

   the 
  interior 
  and 
  on- 
  the 
  northern 
  and 
  southern 
  borders, 
  the 
  lime- 
  

   stones 
  are 
  encountered 
  in 
  disconnected 
  patches, 
  occasionally 
  

   interfolded 
  with 
  the 
  igneous 
  rocks 
  in 
  which 
  they 
  were, 
  no 
  doubt, 
  

   involved 
  during 
  the 
  intrusion. 
  

  

  Gneiss. 
  The 
  sedimentary 
  gneisses 
  are 
  an 
  extremely 
  varied 
  

   class. 
  Their 
  many 
  phases 
  comprise 
  light 
  colored 
  acid 
  types 
  made 
  

   up 
  purely 
  of 
  quartz 
  and 
  feldspar, 
  gray 
  or 
  dark 
  gneisses 
  in 
  which 
  

   the 
  ferromagnesian 
  minerals 
  are 
  represented 
  more 
  or 
  less 
  abund- 
  

   antly, 
  and 
  black 
  basic 
  varieties 
  with 
  only 
  subordinate 
  feldspar 
  or 
  

   quartz. 
  Wide 
  differences 
  in 
  composition 
  are 
  often 
  observable 
  

   within 
  the 
  limits 
  of 
  a 
  single 
  outcrop, 
  particularly 
  in 
  passing 
  across 
  

   the 
  foliation. 
  The 
  transitions 
  from 
  one 
  variety 
  to 
  another 
  take 
  

   place 
  quickly 
  and 
  lend 
  the 
  appearance 
  of 
  a 
  banded 
  arrangement 
  

   comparable 
  to 
  that 
  of 
  bedding 
  among 
  unaltered 
  sediments. 
  Still 
  

   there 
  are 
  districts 
  in 
  which 
  the 
  gneisses 
  show 
  a 
  fair 
  degree 
  of 
  uni- 
  

   formity, 
  and 
  their 
  relations 
  are 
  only 
  to 
  be 
  established 
  after 
  careful 
  

   investigation 
  in 
  the 
  field 
  and 
  laboratory. 
  The 
  presence 
  of 
  graphite 
  

   is 
  common 
  and 
  suggestive. 
  Garnet, 
  sillimanite 
  and 
  pyrite 
  are 
  also 
  

   characteristic 
  minerals. 
  Where 
  pyrite 
  occurs 
  the 
  beds 
  weather 
  

   rapidly, 
  taking 
  on 
  a 
  peculiar 
  rusty 
  appearance. 
  The 
  texture 
  of 
  

   the 
  gneisses 
  is 
  always 
  granular, 
  as 
  a 
  rule 
  finely 
  so, 
  due 
  to 
  the 
  

   intense 
  crushing 
  they 
  have 
  undergone. 
  

  

  The 
  distribution 
  of 
  the 
  sedimentary 
  gneisses 
  corresponds 
  in 
  a 
  

   general 
  way 
  to 
  that 
  of 
  the 
  limestones, 
  being 
  most 
  widely 
  developed 
  

   on 
  the 
  borders 
  of 
  the 
  region. 
  They 
  occur, 
  however, 
  over 
  consid- 
  

   erable 
  areas 
  where 
  limestone 
  may 
  be 
  relatively 
  scarce. 
  In 
  the 
  

   northern 
  Adirondacks, 
  Cushing 
  has 
  found 
  them 
  to 
  be 
  of 
  small 
  

   importance, 
  as 
  the 
  main 
  formations 
  are 
  igneous 
  or 
  of 
  so 
  question- 
  

  

  