﻿Adirondack 
  magnetic 
  iron 
  ores 
  109 
  

  

  Gneiss 
  series. 
  The 
  gneisses 
  are 
  the 
  most 
  widespread 
  of 
  the 
  

   rocks 
  represented 
  in 
  this 
  region. 
  With 
  the 
  exception 
  of 
  the 
  

   numerous 
  but 
  comparatively 
  limited 
  exposures 
  of 
  dike 
  rocks 
  which 
  

   are 
  hereafter 
  described, 
  they 
  comprise 
  practically 
  all 
  of 
  the 
  crystal- 
  

   lines 
  adjacent 
  to 
  the 
  ore 
  bodies 
  and 
  occupy 
  as 
  well 
  most 
  of 
  the 
  

   surrounding 
  country. 
  The 
  occurrence 
  of 
  gabbro 
  and 
  augite 
  

   syenite 
  has 
  been 
  noted 
  at 
  Rand 
  hill, 
  east 
  of 
  Dannemora 
  mountain, 
  

   but 
  so 
  far 
  as 
  observed 
  they 
  do 
  not 
  appear 
  anywhere 
  in 
  the 
  imme- 
  

   diate 
  vicinity 
  of 
  Lyon 
  Mountain. 
  

  

  In 
  this 
  series 
  is 
  included 
  a 
  complex 
  of 
  rocks 
  differing 
  in 
  com- 
  

   position 
  and 
  physical 
  characters. 
  The 
  study 
  of 
  their 
  field 
  rela- 
  

   tionships 
  is 
  attended 
  with 
  much 
  difficulty 
  owing 
  to 
  the 
  heavy 
  

   mantle 
  of 
  drift 
  - 
  over 
  the 
  area 
  affording 
  limited 
  opportunity 
  for 
  

   observation, 
  and 
  to 
  the 
  variations 
  in 
  the 
  rocks 
  from 
  place 
  to 
  place. 
  

   Practically 
  all 
  of 
  the 
  numerous 
  specimens 
  taken 
  from 
  typical 
  

   exposures 
  in 
  the 
  vicinity 
  of 
  the 
  ore 
  bodies 
  may 
  be 
  classed, 
  how- 
  

   ever, 
  in 
  the 
  following 
  groups. 
  

  

  Augite 
  gneiss. 
  This 
  is 
  a 
  reddish 
  or 
  grayish 
  granular 
  rock 
  char- 
  

   acterized 
  by 
  the 
  presence 
  of 
  augite. 
  It 
  is 
  mentioned 
  by 
  Cushing 
  

   as 
  one 
  of 
  the 
  predominant 
  types 
  in 
  the 
  group 
  of 
  gneisses 
  designated 
  

   as 
  the 
  Saranac 
  formation. 
  

  

  In 
  its 
  prevailing 
  development 
  it 
  consists 
  essentially 
  of 
  feldspar, 
  

   quartz 
  and 
  augite, 
  with 
  subordinate 
  hornblende, 
  titanite, 
  mag- 
  

   netite 
  and 
  apatite. 
  The 
  augite 
  is 
  an 
  emerald-green 
  variety 
  and 
  

   is 
  always 
  abundant, 
  sometimes 
  composing 
  as 
  much 
  as 
  20 
  per 
  cent 
  

   of 
  the 
  rock 
  mass. 
  The 
  feldspar 
  is 
  mostly 
  orthoclase, 
  but 
  small 
  

   amounts 
  of 
  plagioclase 
  (oligoclase) 
  may 
  be 
  present. 
  The 
  ortho- 
  

   clase 
  shows 
  a 
  microperthitic 
  intergrowth 
  with 
  albite. 
  The 
  quartz 
  

   is 
  a 
  fluctuating 
  constituent, 
  though 
  the 
  relative 
  quantity 
  is 
  below 
  

   rather 
  than 
  above 
  the 
  proportions 
  found 
  in 
  typical 
  granites. 
  The 
  

   greenish, 
  strongly 
  pleochroic 
  hornblende 
  occurs 
  in 
  skeletal 
  or 
  very 
  

   irregularly 
  bounded 
  anhedra 
  and 
  may 
  be 
  in 
  part 
  derived 
  from 
  the 
  

   augite 
  with 
  which 
  it 
  is 
  intimately 
  associated. 
  Of 
  the 
  other 
  com- 
  

   ponents 
  titanite 
  alone 
  has 
  importance. 
  Most 
  specimens 
  exhibit 
  

   this 
  mineral 
  abundantly 
  distributed 
  in 
  the 
  form 
  of 
  rounded 
  grains 
  

   varying 
  from 
  light 
  yellow 
  to 
  reddish 
  brown 
  in 
  color. 
  It 
  also 
  

   occurs 
  as 
  rims 
  surrounding 
  the 
  magnetite. 
  In 
  some 
  specimens 
  

   taken 
  from 
  the 
  walls 
  of 
  the 
  ore 
  bodies, 
  the 
  titanite 
  constitutes 
  

   fully 
  five 
  per 
  cent 
  of 
  the 
  rock 
  mass. 
  

  

  The 
  field 
  appearance 
  of 
  the 
  gneiss 
  is 
  usually 
  massive, 
  with 
  but 
  

   faint 
  tendency 
  toward 
  a 
  parallel 
  grouping 
  of 
  the 
  constituents. 
  

   Though 
  it 
  has 
  undergone 
  more 
  or 
  less 
  crushing 
  which 
  has 
  broken 
  

  

  