﻿506 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  well 
  as 
  of 
  the 
  experience 
  which 
  has 
  been 
  gained 
  in 
  Essex 
  co. 
  and 
  

   of 
  the 
  work 
  of 
  Prof. 
  Cushing 
  on 
  the 
  north, 
  some 
  important 
  ad- 
  

   vances 
  have 
  been 
  made 
  regarding 
  the 
  nature 
  and 
  origin 
  of 
  some 
  

   of 
  these 
  puzzling 
  gneisses. 
  We 
  have 
  been 
  impressed 
  specially 
  

   with 
  the 
  following 
  facts 
  : 
  

  

  1 
  Crushing 
  and 
  granulation 
  of 
  what 
  were 
  formerly 
  massive 
  

   and 
  rather 
  coarsely 
  crystalline 
  rocks 
  have 
  been 
  excessive. 
  The 
  

   region 
  of 
  the 
  crystalline 
  rocks 
  in 
  Washington 
  and 
  Warren 
  co. 
  

   must 
  have 
  suffered 
  extreme 
  dynamic 
  metamorphism. 
  Finely 
  

   granular 
  gneisses 
  were 
  met 
  repeatedly 
  that 
  were 
  taken 
  for 
  

   quartzites 
  at 
  first, 
  but 
  which 
  microscopic 
  study 
  shows 
  to 
  be 
  

   crushed 
  eruptives. 
  The 
  gneissoid 
  structure 
  has 
  been 
  super-in- 
  

   duced 
  upon 
  them 
  by 
  these 
  processes, 
  and 
  even 
  uncrushed 
  nuclei 
  

   or 
  lenticular 
  eyes 
  (Augen) 
  have 
  been 
  destroyed. 
  Apparently 
  

   biotite 
  has 
  been 
  prone 
  to 
  develop 
  along 
  some 
  surfaces 
  of 
  displace- 
  

   ment. 
  In 
  the 
  exposures 
  about 
  Whitehall 
  the 
  crushing 
  and 
  

   granulation 
  are 
  specially 
  marked. 
  

  

  2 
  Our 
  knowledge 
  of 
  the 
  geology 
  of 
  the 
  region 
  will 
  be 
  greatly 
  

   advanced 
  as 
  soon 
  as 
  we 
  can 
  find 
  the 
  necessary 
  evidence 
  to 
  prove 
  

   what 
  the 
  gneisses 
  were 
  before 
  they 
  suffered 
  metamorphism. 
  In 
  

   former 
  reports 
  mention 
  has 
  been 
  made 
  of 
  gneisses 
  of 
  a 
  general 
  

   greenish 
  color 
  which 
  have 
  been 
  found 
  under 
  the 
  microscope 
  to 
  

   consist 
  of 
  microperthite 
  in 
  largest 
  part, 
  with 
  which 
  are 
  varying 
  

   amounts 
  of 
  quartz, 
  brown 
  hornblende, 
  emerald 
  green 
  augite, 
  

   hypersthene, 
  plagioclase 
  and 
  small 
  accessories. 
  Varieties 
  rich 
  

   in 
  quartz 
  have 
  been 
  met 
  and 
  again 
  others 
  with 
  no 
  quartz 
  but 
  

   with 
  abundance 
  of 
  dark 
  silicates. 
  The 
  commonest 
  variety 
  is 
  

   near 
  syenite 
  in 
  composition. 
  These 
  rocks 
  have 
  been 
  very 
  puz- 
  

   zling, 
  because 
  in 
  the 
  field 
  they 
  resembled 
  gabbros 
  so 
  closely, 
  

   whereas 
  in 
  the 
  laboratory 
  thin 
  sections 
  showed 
  mostly 
  micro- 
  

   perthite, 
  i. 
  e. 
  orthoclase, 
  thickly 
  penetrated 
  with 
  albite. 
  They 
  

   were 
  specially 
  cited 
  when 
  Ticonderoga 
  township 
  was 
  described 
  

   in 
  the 
  report 
  for 
  1893, 
  p. 
  452. 
  All 
  the 
  varieties 
  on 
  the 
  south 
  were 
  

  

  a 
  See 
  Report, 
  N. 
  Y 
  .state 
  geol. 
  for 
  1893, 
  p. 
  452: 
  petrography 
  of 
  series 
  1 
  in 
  Ticonderoga; 
  

   p. 
  470, 
  North 
  Elba, 
  series 
  1. 
  

  

  