﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  1 
  3 
  

  

  here 
  as 
  farther 
  east, 
  in 
  fact 
  there 
  seems 
  to 
  be 
  a 
  slow 
  but 
  progres- 
  

   sive 
  increase 
  in 
  severity 
  of 
  metamorphism 
  in 
  passing 
  east. 
  The 
  

   differences 
  in 
  this 
  respect 
  are 
  not 
  so 
  prominent 
  in 
  the 
  Grenville 
  

   and 
  Laurentian 
  rocks 
  as 
  in 
  the 
  later 
  igneous 
  rocks, 
  but 
  character- 
  

   ize 
  all. 
  Even 
  here, 
  however, 
  the 
  character 
  of 
  the 
  metamorphism 
  in- 
  

   dicates 
  a 
  considerable 
  depth 
  for 
  the 
  rocks 
  concerned 
  during 
  the 
  

   time 
  when 
  it 
  took 
  place. 
  But 
  it 
  also 
  suggests 
  a 
  less 
  depth 
  of 
  over- 
  

   lying 
  material 
  than 
  is 
  possessed 
  by 
  the 
  region 
  farther 
  east. 
  

  

  This 
  overlying 
  material 
  has 
  since 
  been 
  removed 
  by 
  slow 
  surface 
  

   erosion. 
  Greater 
  thickness 
  has 
  been 
  removed 
  on 
  the 
  east 
  than 
  on 
  

   the 
  west 
  apparently, 
  the 
  differences 
  in 
  metamorphism 
  being 
  thus 
  

   most 
  readi!y 
  explained. 
  Further, 
  this 
  removal 
  by 
  erosion 
  took 
  place 
  

   wholly 
  in 
  Precambric 
  time 
  indi 
  :ating 
  that 
  the 
  region 
  w^as 
  a 
  land 
  

   area 
  for 
  a 
  long 
  period. 
  Precambric 
  time 
  however 
  was 
  very 
  long, 
  

   the 
  Grenville 
  sediments 
  were 
  deposited 
  early 
  in 
  it, 
  the 
  district 
  sub- 
  

   sequently 
  rose 
  above 
  sea 
  level 
  and 
  remained 
  as 
  land 
  during 
  the 
  

   long 
  ages 
  of 
  the 
  middle 
  and 
  late 
  Precambric. 
  The 
  large 
  amount 
  

   of 
  rock 
  thickness 
  removed 
  not 
  only 
  argues 
  for 
  a 
  long 
  erosion 
  in- 
  

   terval 
  but 
  likely 
  indicates 
  renewal 
  of 
  uplift 
  on 
  one 
  or 
  more 
  oc- 
  

   casions, 
  since 
  it 
  is 
  not 
  probable 
  that 
  the 
  region 
  ever 
  attained 
  an 
  

   altitude 
  as 
  great 
  as 
  that 
  represented 
  by 
  the 
  thickness 
  of 
  rock 
  re- 
  

   moved. 
  

  

  Late 
  in 
  Precambric 
  time, 
  and 
  toward 
  the 
  close 
  of 
  this 
  long, 
  

   erosion 
  period, 
  came 
  renewed 
  igneous 
  activity, 
  an 
  upward 
  move- 
  

   ment 
  of 
  heavy, 
  black, 
  basic 
  lava 
  taking 
  place. 
  Not 
  improbably 
  

   some 
  of 
  this 
  material 
  reached 
  the 
  land 
  surface 
  of 
  the 
  time 
  and 
  

   spread 
  out 
  as 
  lava 
  flows. 
  If 
  so 
  subsequent 
  w^ear 
  has 
  removed 
  

   every 
  trace 
  of 
  their 
  presence, 
  cutting 
  awa}^ 
  the 
  surface 
  sufficiently 
  

   so 
  that 
  the 
  only 
  sign 
  of 
  this 
  igneous 
  activity 
  which 
  remains 
  on 
  

   the 
  surface 
  of 
  today 
  is 
  the 
  trap 
  dikes, 
  the 
  lava-filled 
  channels 
  of 
  

   ascent 
  of 
  the 
  molten 
  rock. 
  The 
  trap 
  is 
  absolutely 
  unmetamorphosed 
  

   and 
  gives 
  every 
  indication 
  of 
  having 
  solidified 
  at 
  quite 
  shallow 
  

   depth. 
  Hence 
  the 
  conclusion 
  is 
  forced 
  that 
  the 
  eruption 
  occurred 
  

   toward 
  the 
  close 
  of 
  the 
  long 
  Precambric 
  erosion 
  period 
  previously 
  

   described, 
  and 
  since 
  only 
  a 
  comparatively 
  slight 
  amount 
  of 
  v/ear 
  

   followed, 
  that 
  these 
  dikes 
  are 
  of 
  very 
  late 
  Precambric 
  age 
  ; 
  in 
  fact 
  

   it 
  is 
  by 
  no 
  means 
  impossible 
  that 
  they 
  may 
  be 
  as 
  young 
  as 
  the 
  

   early 
  Cambric. 
  

  

  If 
  we 
  could 
  follow 
  these 
  dikes 
  down 
  into 
  the 
  earth 
  beneath 
  the 
  

   surface 
  of 
  today, 
  no 
  doubt 
  we 
  should 
  find 
  that 
  they 
  lead 
  upward 
  

   from 
  underground 
  masses 
  of 
  trap 
  of 
  considerable 
  size, 
  quite 
  analo- 
  

   gous 
  to 
  the 
  bathyliths 
  of 
  the 
  earlier 
  granites. 
  

  

  