﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  4I 
  

  

  Picton 
  granite. 
  This 
  is 
  the 
  latest, 
  most 
  extensive, 
  most 
  inter- 
  

   esting, 
  and 
  most 
  important 
  of 
  the 
  intrusives 
  of 
  the 
  region. 
  It 
  is 
  

   named 
  from 
  Picton 
  island 
  (called 
  Robbins 
  island 
  on 
  the 
  map) 
  

   where 
  it 
  is 
  most 
  extensively 
  quarried. 
  It 
  is, 
  however, 
  best 
  and 
  

   most 
  extensively 
  exposed 
  on 
  Grindstone 
  island 
  and 
  would 
  have 
  

   been 
  named 
  after 
  it 
  except 
  for 
  the 
  fact 
  that 
  the 
  whole 
  name 
  was 
  

   too 
  long, 
  and 
  the 
  term 
  " 
  Grindstone 
  granite 
  " 
  possibly 
  misleading. 
  

   It 
  is 
  extensively 
  exposed 
  also 
  on 
  the 
  west 
  end 
  of 
  Wellesley 
  

   island. 
  Abundant 
  dikes 
  of 
  it 
  appear 
  on 
  the 
  mainland 
  of 
  the 
  

   Alexandria 
  sheet, 
  cutting 
  the 
  Alexandria 
  granite 
  gneiss 
  and 
  the 
  

   Grenville 
  schists, 
  but 
  the 
  main 
  mass 
  falls 
  short 
  of 
  reaching 
  the 
  

   shore. 
  It 
  does 
  reach 
  the 
  mainland 
  on 
  the 
  Clayton 
  sheet, 
  how- 
  

   ever, 
  judging 
  from 
  the 
  exposures 
  of 
  the 
  Precambric 
  inlier 
  up 
  

   French 
  creek, 
  and 
  may 
  have 
  wide 
  extent 
  here 
  under 
  the 
  Paleo- 
  

   zoic 
  rocks. 
  Across 
  the 
  border 
  in 
  Canada 
  it 
  seems 
  to 
  have 
  large 
  

   extent, 
  though 
  it 
  has 
  not 
  yet 
  been 
  differentiated 
  from 
  the 
  

   Laurentian 
  in 
  mapping. 
  If, 
  however, 
  we 
  are 
  correct 
  in 
  correlat- 
  

   ing 
  the 
  granite 
  at 
  Kingston 
  with 
  this 
  rock, 
  a 
  bathylith 
  of 
  consider- 
  

   able 
  extent 
  is 
  implied. 
  

  

  The 
  general 
  rock 
  is 
  a 
  rather 
  bright 
  red 
  granite 
  of 
  quite 
  coarse 
  

   grain. 
  It 
  varies 
  much 
  in 
  this 
  respect 
  however, 
  and 
  much 
  of 
  the 
  

   border 
  rock 
  is 
  of 
  much 
  finer 
  grain, 
  as 
  is 
  also 
  true 
  of 
  the 
  general 
  

   run 
  of 
  the 
  dikes 
  which 
  radiate 
  out 
  from 
  the 
  mass. 
  To 
  a 
  certain 
  

   extent 
  this 
  diminution 
  in 
  apparent 
  size 
  of 
  grain 
  is 
  due 
  to 
  mash- 
  

   ing, 
  but 
  certainly 
  the 
  major 
  part 
  of 
  it 
  is 
  a 
  primary 
  difiference. 
  

  

  Red 
  feldspars 
  (microperthite, 
  microcline 
  and 
  oligoclase) 
  con- 
  

   stitute 
  75^ 
  or 
  more 
  of 
  the 
  rock. 
  Considerable 
  quartz 
  is 
  usually 
  

   present 
  and 
  is 
  frequently 
  characterized 
  by 
  a 
  slightly 
  bluish 
  cast, 
  

   which 
  makes 
  a 
  helpful 
  diagnostic 
  feature 
  of 
  the 
  rock. 
  Horn- 
  

   blende 
  and 
  biotite 
  are 
  sufficiently 
  abundant 
  to 
  show 
  prominent 
  

   black 
  spots 
  in 
  the 
  otherwise 
  red 
  rock. 
  In 
  the 
  finer 
  grained 
  border 
  

   varieties 
  and 
  dikes, 
  these 
  black 
  minerals 
  retreat, 
  quartz 
  becomes 
  

   somewhat 
  more 
  prominent, 
  and 
  the 
  rock 
  appears 
  more 
  acid. 
  

   The 
  general 
  rock, 
  however, 
  does 
  not 
  impress 
  one 
  as 
  a 
  particularly 
  

   acid 
  rock 
  for 
  a 
  granite, 
  and 
  this 
  impression 
  is 
  borne 
  out 
  on 
  

   analysis 
  (given 
  in 
  a 
  later 
  section). 
  

  

  The 
  rock 
  of 
  the 
  inlier 
  to 
  the 
  south 
  of 
  Clayton, 
  and 
  that 
  at 
  

   Kingston 
  are 
  correlated 
  with 
  this 
  granite 
  with 
  some 
  reserve. 
  

   The 
  Kingston 
  rock 
  is 
  a 
  red 
  granite 
  of 
  almost 
  identical 
  appear- 
  

   ance 
  with 
  this, 
  agrees 
  closely 
  in 
  composition, 
  and 
  the 
  only 
  hesi- 
  

   tancy 
  felt 
  in 
  the 
  matter 
  is 
  owing 
  to 
  the 
  distance 
  separating 
  the 
  

   two 
  areas. 
  In 
  all 
  likelihood 
  the 
  rock 
  can 
  be 
  carried 
  across 
  on 
  

  

  