﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  l8l 
  

  

  Picton 
  granite 
  

  

  The 
  specimen 
  of 
  Picton 
  granite 
  analyzed 
  was 
  selected 
  as 
  an 
  

   average 
  representative 
  of 
  the 
  rock, 
  and 
  bears 
  out 
  the 
  impression 
  

   gained 
  in 
  the 
  field 
  that 
  as 
  a 
  whole 
  it 
  is 
  less 
  acid 
  than 
  the 
  Lauren- 
  

   tian 
  granites 
  where 
  uncontaminated 
  by 
  Grenville 
  material. 
  It 
  

   seems 
  less 
  quartzose, 
  and 
  always 
  shows 
  considerable 
  hornblende, 
  

   which 
  is 
  relatively 
  scarce 
  in 
  the 
  granite 
  gneiss. 
  The 
  thin 
  section 
  

   shows 
  it 
  to 
  be 
  fairly 
  rich 
  in 
  accessory 
  minerals, 
  titanite 
  and 
  apatite 
  

   especially 
  being 
  frequent 
  and 
  fairly 
  coarse, 
  the 
  former 
  particularly 
  

   so. 
  Some 
  pyrite 
  is 
  present, 
  zircon 
  also, 
  little 
  hematite 
  inclusions 
  

   in 
  the 
  feldspars, 
  and 
  ilmenite 
  or 
  rutile 
  needles 
  in 
  the 
  quartz. 
  A 
  few 
  

   minute 
  tourmalin 
  crystals 
  also 
  occur. 
  The 
  green 
  hornblende 
  is 
  alter- 
  

   ing 
  to 
  biotite, 
  and 
  there 
  is 
  additional 
  biotite 
  in 
  the 
  rock 
  as 
  well. 
  For 
  

   the 
  feldspars, 
  microperthite, 
  microcline, 
  microcline-microperthite 
  

   and 
  oligoclase 
  are 
  all 
  present 
  in 
  considerable 
  amount, 
  and 
  all 
  with 
  

   strongly 
  marked 
  characters. 
  A 
  good 
  deal 
  of 
  micropegmatite, 
  some 
  

   of 
  it 
  quite 
  coarse, 
  is 
  also 
  to 
  be 
  seen. 
  Altogether, 
  in 
  its 
  minor 
  

   mineralogy, 
  the 
  rock 
  presents 
  considerable 
  contrast 
  to 
  the 
  granite 
  

   gneiss. 
  

  

  The 
  norm 
  of 
  the 
  rock 
  is 
  a 
  follows: 
  

  

  Or... 
  

  

  • 
  33-3^' 
  

  

  

  

  Class 
  I, 
  persalane 
  

  

  Ab... 
  

   An.. 
  

  

  . 
  26.20 
  

   . 
  8.62 
  

  

  .89. 
  

  

  00 
  

  

  Order 
  4, 
  britannare 
  

  

  Qz... 
  

  

  . 
  20.82 
  ^ 
  

  

  

  

  Rang 
  2, 
  toscanase 
  

  

  Hy.. 
  

  

  . 
  . 
  4.48^ 
  

  

  

  

  Subrang 
  3, 
  toscanos( 
  

  

  Mt.. 
  

  

  . 
  3-48 
  

  

  

  

  

  11... 
  

  

  . 
  1.52 
  

  

  ' 
  10 
  

  

  .64 
  

  

  

  Py... 
  

  

  . 
  0.15 
  

  

  

  

  

  Ap.. 
  

  

  . 
  I.OI> 
  

  

  

  

  

  It 
  thus 
  falls 
  in 
  the 
  same 
  rock 
  group 
  as 
  the 
  granite 
  gneisses, 
  

   but 
  is 
  much 
  nearer 
  the 
  border 
  of 
  the 
  group 
  than 
  they 
  are. 
  The 
  

   greater 
  variety 
  and 
  abundance 
  of 
  the 
  femic 
  and 
  alferric 
  minerals, 
  

   hornblende, 
  biotite 
  and 
  titanite, 
  would 
  cause 
  the 
  mode 
  to 
  depart 
  

   somewhat 
  more 
  widely 
  from 
  the 
  norm 
  than 
  in 
  the 
  previous 
  cases, 
  

   the 
  lime 
  to 
  form 
  titanite 
  being 
  deducted 
  from 
  the 
  anorthite, 
  re- 
  

   leasing 
  alumina 
  for 
  the 
  biotite 
  and 
  diminishing 
  the 
  quartz 
  per- 
  

   centage. 
  

  

  The 
  dike 
  phases 
  of 
  this 
  granite 
  range 
  more 
  acid 
  than 
  this, 
  but 
  

   with 
  this 
  exception 
  it 
  is 
  thought 
  that 
  the 
  average 
  of 
  the 
  rock 
  com- 
  

   position 
  is 
  well 
  represented 
  by 
  the 
  analysis. 
  The 
  composition 
  is 
  

  

  