﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  53 
  

  

  is 
  of 
  light 
  brown 
  color, 
  in 
  the 
  coarser 
  varieties 
  very 
  light 
  brown, 
  

   resembling 
  muscovite, 
  though 
  it 
  seems 
  undoubted 
  phlogopite.^ 
  

   Scapolite 
  is 
  also 
  an 
  abundant 
  mineral 
  in 
  these 
  zones, 
  a 
  phlogopite- 
  

   scapolite-calcite 
  rock 
  being 
  the 
  usual 
  combination. 
  This 
  is 
  not 
  

   one 
  of 
  the 
  customarv- 
  types 
  of 
  Grenville 
  contact 
  rocks 
  in 
  the 
  general 
  

   region, 
  though 
  the 
  common 
  one 
  here. 
  

  

  There 
  are 
  two 
  other 
  types 
  of 
  contact 
  rocks 
  which 
  occur 
  in 
  small 
  

   quantity 
  within 
  the 
  area 
  here, 
  though 
  common 
  enough 
  elsewhere, 
  

   which 
  call 
  for 
  brief 
  attention. 
  They 
  occur 
  in 
  the 
  district 
  east 
  of 
  

   Redwood 
  where 
  Grenville 
  rocks 
  of 
  all 
  types 
  are 
  cut 
  by 
  small 
  gran- 
  

   ite 
  masses. 
  One 
  is 
  a 
  heavy, 
  basic, 
  black 
  rock, 
  weathering 
  rapidly, 
  

   and 
  composed 
  chiefly 
  of 
  green 
  pyroxene 
  and 
  black 
  hornblende, 
  with 
  

   a 
  little 
  graphite, 
  considerable 
  pyrite, 
  and 
  some 
  15;^ 
  of 
  calcite 
  re- 
  

   maining. 
  Heavy, 
  pyroxenic 
  rocks 
  of 
  this 
  type 
  occur 
  throughout 
  

   the 
  Adirondack 
  region 
  at 
  limestone 
  contacts, 
  though 
  usually 
  not 
  

   so 
  hornblendic 
  as 
  this 
  rock. 
  

  

  The 
  other 
  rock 
  consists 
  of 
  large, 
  gray 
  green 
  pyroxenes 
  set 
  in 
  a 
  

   felt 
  of 
  tremolite 
  needles, 
  with 
  rather 
  abundant 
  pyrite 
  as 
  the 
  only 
  

   accessory 
  mineral. 
  Such 
  tremolite 
  rocks 
  occur 
  not 
  infrequently 
  in 
  

   the 
  Grenville, 
  the 
  tremolite 
  quite 
  commonly 
  altering 
  to 
  talc. 
  The 
  

   especial 
  interest 
  attaching 
  to 
  this 
  particular 
  exposure 
  is 
  that 
  the 
  

   tremolite 
  rock 
  is 
  developed 
  at 
  the 
  contact 
  of 
  granite 
  against 
  Gren- 
  

   ville 
  rusty 
  gneiss, 
  and 
  seems 
  quite 
  certainly 
  a 
  result 
  of 
  the 
  con- 
  

   tact 
  action 
  of 
  the 
  one 
  upon 
  the 
  other. 
  So 
  far 
  as 
  we 
  recall, 
  just 
  that 
  

   type 
  of 
  contact 
  action 
  has 
  not 
  heretofore 
  been 
  noted 
  in 
  the 
  region. 
  

  

  Great 
  Precambric 
  erosion 
  

  

  The 
  Grenville 
  rocks 
  are 
  the 
  only 
  Precambric 
  sediments 
  in 
  the 
  

   region, 
  and 
  are 
  of 
  very 
  early 
  Precambric 
  age. 
  The 
  remaining 
  rocks 
  

   of 
  this 
  age 
  in 
  the 
  district 
  are 
  all 
  igneous, 
  and 
  there 
  is 
  no 
  evidence 
  

   that 
  any 
  later 
  Precambric 
  sediments 
  were 
  ever 
  deposited 
  here- 
  

   abouts, 
  though 
  it 
  is 
  possible 
  that 
  some 
  such 
  were 
  deposited 
  and 
  

   subsequently 
  worn 
  away. 
  The 
  Precambric 
  rocks 
  of 
  the 
  present 
  

   surface, 
  both 
  sedimentary 
  and 
  igneous, 
  present 
  characters 
  which, 
  

   so 
  far 
  as 
  we 
  know, 
  are 
  only 
  given 
  to 
  rocks 
  under 
  conditions 
  of 
  high 
  

   pressure, 
  and 
  at 
  least 
  moderately 
  high 
  temperature, 
  conditions 
  which 
  

   in 
  general 
  prevail 
  only 
  at 
  considerable 
  depths 
  below 
  the 
  surface. 
  

   A'll 
  the 
  igneous 
  rocks 
  except 
  the 
  diabases 
  give 
  evidence 
  that 
  they 
  

   solidified 
  well 
  beneath 
  the 
  surface, 
  and 
  the 
  deformation 
  of 
  both 
  

   these 
  and 
  the 
  sediments 
  is 
  of 
  deep-seated 
  type. 
  It 
  is, 
  however, 
  not 
  

  

  1 
  It 
  is 
  of 
  the 
  second 
  order 
  and 
  with 
  very 
  small 
  axial 
  angle. 
  

  

  