﻿GEOLOGY 
  OF 
  SARATOGA 
  SPRINGS 
  AND 
  VICINITY 
  21 
  

  

  PRECAMBRIC 
  IGNEOUS 
  ROCKS 
  

  

  General 
  statement. 
  In 
  order 
  of 
  age 
  the 
  Precambric 
  igneous 
  

   rocks 
  of 
  the 
  quadrangles 
  are 
  the 
  Laurentian 
  (?) 
  granite, 
  the 
  syen- 
  

   ite, 
  and 
  the 
  trap 
  (diabase) 
  dikes. 
  There 
  are 
  in 
  addition 
  very 
  

   small 
  and 
  occasional 
  masses 
  of 
  amphibolite 
  which 
  are 
  probably 
  

   original 
  gabbros 
  and 
  younger 
  than 
  the 
  syenite, 
  but 
  they 
  merit 
  only 
  

   passing 
  notice 
  because 
  of 
  their 
  small 
  bulk 
  and 
  problematic 
  

   character. 
  

  

  Laurentian 
  ( 
  ?) 
  granite. 
  A 
  previous 
  statement 
  makes 
  reference 
  

   to 
  a 
  white, 
  granitic 
  rock, 
  intricately 
  involved 
  with 
  the 
  Grenville 
  

   schists, 
  which 
  is 
  a 
  common 
  constituent 
  of 
  the 
  Grenville 
  throughout 
  

   the 
  southern 
  Adirondacks 
  and 
  which 
  we 
  have 
  heretofore 
  regarded 
  

   as 
  a 
  Grenville 
  rock. 
  Thus 
  Cushing, 
  reporting 
  upon 
  similar 
  rocks 
  

   from 
  the 
  Little 
  Falls 
  quadrangle, 
  classes 
  them 
  as 
  Grenville. 
  1 
  Kemp 
  

   and 
  Hill 
  describe 
  the 
  similar 
  rock 
  from 
  the 
  " 
  Noses 
  " 
  in 
  the 
  

   Mohawk 
  valley 
  as 
  a 
  Grenville 
  sediment. 
  2 
  The 
  problem 
  is 
  an 
  

   involved 
  one 
  since 
  the 
  granite 
  is 
  seldom 
  pure 
  but 
  has 
  everywhere 
  

   taken 
  in 
  a 
  considerable 
  amount 
  of 
  the 
  adjacent 
  Grenville 
  rocks, 
  

   giving 
  rise 
  to 
  a 
  mixed 
  rock 
  composed 
  of 
  varying 
  amounts 
  of 
  

   granite 
  and 
  schist. 
  The 
  recognition 
  of 
  pegmatitic 
  phases 
  of 
  the 
  

   rock 
  threw 
  the 
  first 
  doubt 
  upon 
  its 
  sedimentary 
  character; 
  after- 
  

   wards 
  it 
  came 
  to 
  be 
  recognized 
  that 
  the 
  granite 
  itself, 
  as 
  well 
  as 
  its 
  

   pegmatites, 
  was 
  intrusive 
  as 
  regards 
  the 
  schists. 
  The 
  chemical 
  

   analysis 
  eventually 
  settled 
  the 
  question. 
  

  

  The 
  granite 
  is 
  a 
  difficult 
  rock 
  to 
  describe 
  because 
  it 
  is 
  nearly 
  

   everywhere 
  so 
  involved 
  with 
  the 
  schists, 
  or 
  with 
  material 
  from 
  

   the 
  schists, 
  as 
  to 
  make 
  rock 
  free 
  from 
  such 
  contamination 
  difficult 
  

   of 
  recognition. 
  The 
  mica 
  is 
  the 
  most 
  obvious 
  of 
  the 
  contributions 
  

   from 
  the 
  schist, 
  which 
  is 
  certainly 
  much 
  richer 
  in 
  that 
  mineral 
  

   than 
  the 
  original 
  granite. 
  Hence 
  arises 
  a 
  tendency 
  to 
  infer 
  that 
  

   those 
  portions 
  of 
  the 
  granite 
  freest 
  from 
  mica 
  are 
  those 
  least 
  con- 
  

   taminated. 
  Such 
  portions 
  show 
  a 
  very 
  white 
  rock 
  composed 
  of 
  

   white 
  feldspar 
  and 
  quartz 
  and 
  a 
  very 
  little 
  black 
  mica 
  (biotite). 
  

   In 
  addition 
  are 
  small 
  amounts 
  of 
  apatite, 
  titanite, 
  zircon, 
  and 
  

   magnetite. 
  Pink 
  garnets 
  are 
  always 
  present 
  but 
  are 
  not 
  regarded 
  

   as 
  original 
  minerals 
  of 
  the 
  rock. 
  They 
  are 
  most 
  abundant 
  in 
  the 
  

   schists 
  which 
  have 
  been 
  thoroughly 
  impregnated 
  with 
  granite, 
  and 
  

   have 
  in 
  most 
  cases 
  resulted 
  from 
  corrosive 
  interaction 
  between 
  the 
  

   minerals 
  of 
  the 
  two 
  rocks. 
  In 
  other 
  words, 
  most 
  of 
  the 
  garnet 
  is 
  

  

  1 
  N. 
  Y. 
  State 
  Mus. 
  Bui. 
  77, 
  p. 
  17-19. 
  

  

  2 
  N. 
  Y. 
  State 
  Geol. 
  19th 
  Ann. 
  Rep't 
  p. 
  r32-r35. 
  

  

  