﻿iS 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  term 
  in 
  the 
  east 
  necessitates 
  holding 
  either 
  that 
  the 
  Grenville 
  

   series 
  is 
  also 
  older 
  than 
  the 
  Lower 
  Huronian, 
  or 
  that 
  the 
  Lauren- 
  

   tian 
  granite 
  is 
  younger 
  than 
  about 
  Lake 
  Superior, 
  since 
  the 
  granite 
  

   is 
  certainly 
  younger 
  than 
  the 
  Grenville. 
  

  

  GRENVILLE 
  SCHISTS 
  

  

  As 
  has 
  been 
  stated, 
  Grenville 
  schists 
  exist 
  in 
  such 
  great 
  variety 
  

   and 
  with 
  such 
  rapid 
  alternations 
  as 
  to 
  defy 
  detailed 
  mapping 
  and 
  

   to 
  render 
  detailed 
  description 
  laborious 
  and 
  profitless. 
  The 
  schists 
  

   are 
  everywhere 
  intricately 
  involved 
  with 
  hard, 
  white, 
  garnetif- 
  

   erous 
  gneisses 
  which, 
  heretofore 
  regarded 
  as 
  sediments, 
  seem 
  to 
  

   the 
  writer 
  to 
  be 
  plainly 
  igneous 
  rocks. 
  They 
  cut 
  the 
  schists 
  intru- 
  

   sively 
  and 
  develop 
  pegmatites. 
  In 
  the 
  majority 
  of 
  exposures 
  they 
  

   are 
  merely 
  injected 
  along 
  the 
  foliation 
  planes 
  of 
  the 
  schist, 
  form- 
  

   ing 
  injection 
  gneisses 
  and 
  looking 
  extremely 
  like 
  interbanded 
  

   sediments. 
  The 
  more 
  common 
  of 
  the 
  schists 
  are 
  mica 
  schists, 
  and 
  

   the 
  prevailing 
  Grenville 
  combination 
  of 
  the 
  quadrangle 
  consists 
  

   of 
  the 
  interbanded 
  mica 
  schist 
  and 
  white 
  granite. 
  

  

  These 
  mica 
  schists 
  vary 
  'from 
  very 
  weak 
  rocks 
  with 
  abundant 
  

   mica 
  to 
  much 
  firmer 
  ones 
  in 
  which 
  mica 
  is 
  scant. 
  Because 
  of 
  

   weakness, 
  the 
  former 
  variety 
  is 
  seldom 
  seen 
  in 
  outcrop, 
  but 
  several 
  

   cuts 
  through 
  such 
  schists 
  expose 
  them 
  well 
  along 
  the 
  Adirondack 
  

   Railroad 
  a 
  mile 
  north 
  -of 
  Saratoga. 
  The 
  firmer 
  varieties 
  outcrop 
  

   everywhere. 
  

  

  These 
  schists 
  are 
  feldspar-quartz-mica 
  combinations, 
  and 
  nearly 
  

   everywhere 
  contain 
  in 
  addition 
  pink 
  garnets. 
  The 
  mica 
  is 
  biotite 
  

   and 
  the 
  bulk 
  of 
  the 
  feldspar 
  is 
  plagioclase, 
  oligoclase 
  to 
  andesine. 
  

   Quartz 
  forms 
  in 
  general 
  from 
  10 
  to 
  25 
  per 
  cent 
  of 
  the 
  rock. 
  Folia- 
  

   tion 
  is 
  thorough 
  and 
  even. 
  

  

  On 
  the 
  one 
  hand 
  these 
  mica 
  schists 
  grade 
  over 
  into 
  amphibolites, 
  

   which 
  are 
  heavy 
  black 
  gneisses 
  composed 
  essentially 
  of 
  plagioclase 
  

   feldspar 
  and 
  hornblende, 
  with 
  usually 
  black 
  mica 
  and 
  pyroxene 
  in 
  

   addition; 
  on 
  the 
  other 
  hand 
  they 
  grade 
  into 
  hard, 
  light 
  colored 
  

   feldspar-quartz 
  gneisses, 
  by 
  diminution 
  in 
  the 
  mica 
  present. 
  

  

  The 
  garnets 
  seem 
  to 
  owe 
  their 
  origin 
  to 
  contact 
  action 
  of 
  the 
  

   white 
  granite 
  upon 
  the 
  schists, 
  as 
  will 
  be 
  later 
  shown. 
  Graphite 
  is 
  

   a 
  frequent 
  mineral 
  in 
  the 
  schists. 
  

  

  The 
  schists 
  are 
  in 
  chief 
  part 
  metamorphosed 
  shales, 
  as 
  indicated 
  

   clearly 
  by 
  their 
  composition 
  and 
  structure. 
  They 
  have 
  been 
  entirely 
  

   recrystallized, 
  injected 
  in 
  complex 
  fashion 
  by 
  granite, 
  and 
  vastly 
  

   changed 
  in 
  appearance 
  and 
  character. 
  Originally 
  they 
  varied 
  

  

  