﻿36 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  It 
  should 
  be 
  stated 
  that 
  this 
  period 
  of 
  igneous 
  activity 
  is 
  by 
  no 
  

   means 
  confined 
  to 
  the 
  Adirondacks. 
  Similar 
  intrusions 
  are 
  known 
  

   in 
  the 
  Highlands-of-the-Hudson 
  and 
  in 
  Canada. 
  The 
  covering 
  of 
  

   Paleozoic 
  strata 
  in 
  southwestern 
  New 
  York 
  prevents 
  direct 
  -observa- 
  

   tion, 
  but 
  all 
  things 
  considered, 
  it 
  is 
  more 
  than 
  likely 
  that 
  much 
  or 
  

   all 
  of 
  that 
  part 
  of 
  the 
  State, 
  stripped 
  of 
  the 
  Paleozoics, 
  would 
  also 
  

   show 
  Grenville 
  cut 
  up 
  by 
  granite 
  and 
  syenite. 
  

  

  FOLDING 
  OF 
  THE 
  ROCKS 
  AND 
  UPLIFT 
  OF 
  THE 
  ADIRONDACKS 
  

  

  At 
  some 
  time 
  after 
  these 
  great 
  periods 
  of 
  igneous 
  activity 
  and 
  cool- 
  

   ing 
  of 
  the 
  rocks, 
  the 
  whole 
  Adirondack 
  region 
  was 
  subjected 
  to 
  an 
  

   enormous 
  pressure 
  as 
  a 
  result 
  of 
  which 
  the 
  rocks 
  were 
  highly 
  folded 
  

   and 
  compressed. 
  The 
  Grenville 
  strata 
  now 
  seldom 
  lie 
  horizontally 
  

   but 
  are 
  tilted 
  at 
  all 
  sorts 
  of 
  angles, 
  and 
  even 
  the 
  igneous 
  rocks 
  thus 
  

   far 
  described, 
  show 
  unmistakable 
  evidence 
  of 
  having 
  been 
  greatly 
  

   compressed 
  because 
  the 
  minerals 
  are 
  flattened 
  out 
  or 
  arranged 
  in 
  

   parallel 
  fashion, 
  often 
  exhibiting 
  a 
  crude, 
  banded 
  structure. 
  Rocks 
  

   which 
  have 
  thus 
  been 
  changed 
  are 
  known 
  as 
  gneisses 
  and 
  may 
  be 
  

   either 
  igneous 
  or 
  sedimentary. 
  

  

  Still 
  later 
  than 
  the 
  granites 
  and 
  syenites, 
  there 
  occurred 
  intrusions 
  

   of 
  gabbros'in 
  the 
  form 
  of 
  dikes 
  or 
  fissures 
  filled 
  with 
  igneous 
  rock. 
  

   This 
  gabbro 
  is 
  a 
  very 
  dark 
  gray, 
  rather 
  coarse-grained, 
  plutonic 
  

   rock, 
  and 
  it 
  has 
  been 
  found 
  in 
  numerous 
  small 
  masses 
  throughout 
  

   the 
  Adirondacks, 
  but 
  especially 
  in 
  the 
  eastern 
  portion. 
  Its 
  age, 
  

   younger 
  than 
  the 
  granite-syenite 
  series, 
  is 
  demonstrated 
  by 
  the 
  fact 
  

   that 
  it 
  often 
  breaks 
  through 
  those 
  rocks, 
  and 
  also 
  because 
  it 
  is 
  not 
  

   nearly 
  so 
  much 
  metamorphosed. 
  

  

  The 
  Grenville 
  sediments 
  were 
  completely 
  crystallized 
  as 
  a 
  result 
  

   of 
  this 
  metamorphism 
  so 
  that 
  all 
  beds 
  of 
  limestone 
  were 
  converted 
  

   into 
  marble, 
  sandstone 
  into 
  quartzite, 
  and 
  the 
  shales 
  into 
  gneisses 
  of 
  

   varying 
  character. 
  Thus 
  the 
  Grenville 
  strata 
  have 
  been 
  very 
  greatly 
  

   altered 
  from 
  their 
  original 
  condition, 
  which 
  explains 
  why 
  they 
  do 
  

   not 
  look 
  like 
  the 
  more 
  typical 
  and 
  familiar 
  sediments 
  of 
  later 
  age. 
  

   The 
  manner 
  in 
  which 
  the 
  Grenville 
  strata, 
  especially 
  the 
  limestones, 
  

   were 
  crumbled 
  and 
  folded 
  shows 
  that 
  the 
  rocks 
  must 
  have 
  been 
  in 
  a 
  

   more 
  or 
  less 
  plastic 
  condition 
  when 
  the 
  pressure 
  was 
  exerted 
  (see 
  

   plate 
  13). 
  Heat 
  and 
  moisture, 
  no 
  doubt, 
  aided 
  in 
  this 
  process 
  

   which 
  we 
  call 
  metamorphism. 
  Such 
  a 
  process 
  can 
  take 
  place 
  only 
  

   at 
  thousands 
  of 
  feet 
  below 
  the 
  surface 
  of 
  the 
  earth 
  where 
  the 
  rocks, 
  

   under 
  the 
  enormous 
  weight 
  of 
  overlying 
  material, 
  would 
  act 
  like 
  

  

  