﻿ADIRONDACK 
  MAGNETIC 
  IRON 
  ORES 
  65 
  

  

  As 
  contrasted 
  with 
  the 
  basic 
  members 
  acidic 
  varieties 
  are 
  also 
  

   to 
  be 
  found 
  most 
  significantly 
  in 
  the 
  cores 
  but 
  also 
  in 
  the 
  natural 
  

   ledges. 
  In 
  the 
  acidic 
  varieties 
  the 
  dark 
  silicates 
  retreat, 
  it 
  may 
  be 
  

   even 
  to 
  the 
  vanishing 
  point, 
  while 
  quartz 
  enters 
  and 
  the 
  rock 
  

   reaches 
  well 
  into 
  the 
  mineralogy 
  of 
  the 
  granites. 
  The 
  feldspar 
  is 
  

   most 
  commonly 
  microperthite 
  as 
  before 
  and 
  when 
  mingled 
  with 
  

   abundant 
  quartz 
  it 
  yields 
  the 
  "21 
  gneiss" 
  of 
  the 
  writer's 
  earlier 
  

   article. 
  Dark 
  silicates 
  almost 
  entirely 
  fail, 
  plagioclase 
  is 
  rare, 
  but 
  

   magnetite 
  is 
  invariably 
  present 
  in 
  scattered 
  grains, 
  which 
  in 
  the 
  

   cores, 
  unless 
  care 
  is 
  taken, 
  might 
  readily 
  be 
  taken 
  for 
  a 
  dark 
  

   silicate. 
  This 
  rock 
  is 
  one 
  of 
  great 
  importance 
  in 
  the 
  geology 
  of 
  the 
  

   ores 
  since 
  it 
  is 
  the 
  common 
  hanging 
  wall 
  of 
  the 
  Old 
  Bed 
  group. 
  

   Although 
  in 
  coarseness 
  of 
  crystallization 
  it 
  does 
  not 
  vary 
  in 
  texture 
  

   from 
  the 
  typical 
  syenite, 
  yet 
  both 
  in 
  mineralogy 
  and 
  in 
  rare 
  asso- 
  

   ciated 
  minerals 
  it 
  suggests 
  the 
  suspicion 
  that 
  it 
  has 
  affinities 
  with 
  

   pegmatites, 
  or 
  that 
  some 
  influence 
  such 
  as 
  the 
  presence 
  of 
  vapors 
  

   or 
  mineralizers 
  aided 
  in 
  its 
  development 
  from 
  the 
  normal 
  syenitic 
  

   magma. 
  From 
  this 
  highly 
  acidic 
  and 
  light 
  colored 
  rock 
  the 
  transi- 
  

   tion 
  is 
  abrupt 
  to 
  the 
  dark 
  basic 
  masses 
  of 
  iron 
  ore. 
  

  

  Another 
  light 
  colored 
  phase 
  consists 
  of 
  oligoclase 
  and 
  quartz 
  

   with 
  a 
  few 
  magnetites 
  and 
  zircons. 
  This 
  was 
  specifically 
  found 
  

   above 
  the 
  Barton 
  Hill 
  group, 
  and 
  was 
  described 
  in 
  the 
  writer's 
  for- 
  

   mer 
  paper 
  as 
  the 
  "Orchard 
  gneiss." 
  It 
  is 
  after 
  all 
  a 
  not 
  very 
  

   different 
  rock 
  from 
  the 
  "21 
  gneiss." 
  Microperthitic 
  orthoclase 
  

   implies 
  rich 
  soda 
  because 
  of 
  its 
  albite 
  spindles, 
  whereas, 
  if 
  the 
  

   potash 
  of 
  the 
  orthoclase 
  fails 
  and 
  a 
  slight 
  increase 
  of 
  lime 
  takes 
  its 
  

   place, 
  we 
  have 
  the 
  necessary 
  components 
  of 
  oligoclase. 
  

  

  A 
  still 
  further 
  variation 
  from 
  the 
  normal 
  augite 
  syenite, 
  is 
  one 
  in 
  

   which 
  the 
  feldspar 
  and 
  the 
  dark 
  silicates, 
  augite 
  and 
  hornblende, 
  

   are 
  in 
  the 
  usual 
  proportions 
  of 
  say 
  two 
  thirds 
  feldspar 
  and 
  one 
  

   third 
  dark 
  silicates, 
  and 
  yet 
  oligoclase 
  takes 
  the 
  place 
  of 
  the 
  usual 
  

   microperthite. 
  A 
  rather 
  acidic 
  diorite 
  results, 
  but 
  yet 
  so 
  involved 
  

   with 
  the 
  syenites 
  as 
  to 
  prevent 
  one 
  drawing 
  any 
  distinctions 
  

   between 
  them, 
  as 
  being 
  separate 
  intrusive 
  masses. 
  

  

  In 
  the 
  above 
  condensed 
  outline 
  of 
  the 
  rocks, 
  the 
  characteristic 
  

   names 
  of 
  igneous 
  types, 
  syenite, 
  gabbro, 
  diorite 
  etc., 
  have 
  been 
  

   employed, 
  implying 
  that 
  the 
  rocks 
  themselves 
  are 
  igneous. 
  This 
  

   is 
  opposed 
  to 
  the 
  older 
  idea 
  which 
  is 
  generally 
  still 
  held 
  by 
  the 
  

   engineers 
  and 
  others 
  engaged 
  in 
  the 
  mining 
  industry. 
  The 
  latter 
  

   view 
  the 
  ores 
  and 
  their 
  inclosing 
  rocks 
  as 
  sediments, 
  which 
  conform 
  

   in 
  a 
  pronounced 
  degree 
  to 
  the 
  sedimentary 
  structures 
  shown 
  by 
  

   strata 
  in 
  parallel 
  arrangement 
  and 
  in 
  folds 
  and 
  faults. 
  The 
  writer 
  

  

  