﻿CRYSTALLINE 
  ROCKS 
  OP 
  THE 
  WESTERN 
  ADIRONDACK 
  REGION 
  473 
  

  

  The 
  prominent 
  features 
  of 
  this 
  type 
  of 
  the 
  rock 
  have 
  been 
  

   presented 
  in 
  the 
  paper 
  referred 
  to 
  and 
  need 
  be 
  only 
  briefly 
  re- 
  

   stated 
  here. 
  Feldspar 
  is 
  the 
  predominant 
  constituent, 
  giving 
  to 
  

   the 
  normal 
  gray 
  variety 
  the 
  appearance 
  of 
  anorthosite. 
  A 
  deep 
  

   green, 
  non-pleochroic 
  pyroxene, 
  apatite 
  and 
  magnetite, 
  with 
  a 
  

   varying 
  amount 
  of 
  quartz 
  make 
  up 
  the 
  rest 
  of 
  the 
  rock. 
  In 
  the 
  

   field 
  (see 
  pi. 
  1, 
  2, 
  and 
  3) 
  it 
  varies 
  from 
  almost 
  entirely 
  massive 
  

   to 
  distinctly 
  gneissoid, 
  the 
  latter 
  variety 
  being 
  by 
  far 
  the 
  most 
  

   common. 
  Under 
  the 
  microscope 
  the 
  feldspar 
  is 
  shown 
  to 
  be 
  a 
  

   very 
  fine 
  microperthite, 
  with 
  more 
  or 
  less 
  plagioclase, 
  the 
  former 
  

   often 
  building 
  a 
  rim 
  around 
  the 
  latter. 
  The 
  feldspars 
  are 
  in 
  

   large 
  individuals 
  while 
  the 
  other 
  constituents 
  are 
  in 
  small 
  scat- 
  

   tered 
  grains 
  or 
  crystals. 
  Some 
  cataclastic 
  structure 
  is 
  nearly 
  al- 
  

   ways 
  shown 
  and 
  often 
  it 
  is 
  beautifully 
  developed. 
  

  

  The 
  role 
  of 
  the 
  quartz 
  is 
  not 
  easily 
  determined 
  as 
  in 
  some 
  cases 
  

   it 
  is 
  rather 
  abundant 
  while 
  in 
  others 
  it 
  is 
  quite 
  lacking. 
  In 
  aver- 
  

   age 
  specimens 
  it 
  is 
  to 
  be 
  regarded 
  as 
  an 
  accessory 
  mineral 
  and 
  is 
  

   not 
  important 
  in 
  classifying 
  the 
  rock. 
  

  

  The 
  mineralogical 
  composition 
  of 
  the 
  rock 
  indicates 
  its 
  affinity 
  

   with 
  the 
  syenites 
  as 
  stated 
  for 
  this 
  variety 
  in 
  the 
  paper 
  above 
  

   cited, 
  and 
  this 
  idea 
  is 
  supported 
  by 
  the 
  chemical 
  composition 
  as 
  

   shown 
  by 
  the 
  following 
  analysis:® 
  

  

  Si0 
  2 
  

  

  65.65 
  

  

  CaO 
  

  

  2.47 
  

  

  A1 
  2 
  G 
  3 
  

  

  16.84 
  

  

  K 
  2 
  

  

  5.04 
  

  

  FeO 
  

  

  4.01 
  

  

  Ka 
  2 
  

  

  5.27 
  

  

  MgO 
  

  

  .13 
  

  

  H 
  2 
  

  

  .30 
  

  

  .Total 
  99.71 
  

  

  The 
  figures 
  given 
  for 
  the 
  alkalis 
  suggest 
  the 
  possible 
  presence 
  

   of 
  anorthoclase, 
  but 
  no 
  separation 
  of 
  the 
  feldspars 
  for 
  more 
  ac- 
  

   curate 
  determination 
  has 
  been 
  attempted, 
  as 
  their 
  ever-present 
  

   intergrowth 
  would 
  make 
  a 
  separation 
  difficult 
  if 
  not 
  impossible. 
  

   However 
  if 
  the 
  prevailing 
  microperthite 
  is 
  an 
  intergrowth 
  of 
  

   potash 
  and 
  soda 
  feldspars 
  as 
  is 
  doubtless 
  the 
  case, 
  it 
  would 
  suf- 
  

   ficiently 
  explain 
  the 
  alkali 
  content 
  of 
  the 
  rock. 
  And 
  the 
  composi- 
  

  

  a 
  Crystalline 
  limestones 
  and 
  associated 
  rocks 
  of 
  the 
  northwestern 
  Adirondack 
  region. 
  Bull., 
  

   geol. 
  soc. 
  6:274. 
  

  

  