﻿140 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  such 
  dykes 
  cutting 
  across 
  or 
  tlirougli 
  it 
  for 
  a 
  long 
  distance, 
  and 
  

   to 
  an 
  unknown 
  depth. 
  These 
  represent 
  cracks 
  or 
  clefts 
  by 
  which 
  

   the 
  country 
  has 
  been 
  riven 
  and 
  which 
  have 
  been 
  filled 
  by 
  the 
  rise 
  

   of 
  melted 
  matter 
  from 
  below. 
  They 
  are 
  all 
  sizes, 
  from 
  half 
  an 
  

   inch 
  to 
  100 
  feet 
  or 
  more 
  in 
  thickness. 
  

  

  Plutonic 
  dykes 
  are 
  not 
  confined 
  to 
  Archaean 
  regions. 
  Dykes 
  

   of 
  granite 
  are 
  seen 
  in 
  many 
  places 
  on 
  New 
  York 
  island, 
  penetrat- 
  

   ing 
  in 
  every 
  direction 
  the 
  Lower 
  Silurian 
  mica-schist 
  which 
  forms 
  

   the 
  masses 
  of 
  its 
  territory. 
  

  

  These 
  are 
  examples 
  of 
  a 
  phenomenon 
  frequently 
  observed, 
  viz.: 
  

   a 
  plutonic 
  rock 
  penetrating 
  strata 
  of 
  Paleozoic 
  or 
  later 
  age. 
  

   They 
  are 
  similar 
  in 
  their 
  origin 
  to 
  the 
  out-flows 
  of 
  lava 
  from 
  

   volcanoes. 
  

  

  A 
  prominent 
  example 
  of 
  a 
  late 
  plutonic 
  intrusion 
  is 
  seen 
  in 
  

   the 
  ^palisades 
  ' 
  of 
  the 
  Hudson, 
  which 
  is 
  described 
  under 
  the 
  Tri- 
  

   assie 
  rocks. 
  

  

  The 
  plutonic 
  and 
  metamorphic 
  rocks 
  generally 
  decompose 
  

   slowly 
  and 
  produce 
  a 
  poor 
  or 
  barren 
  soil. 
  The 
  districts 
  formed 
  of 
  

   these 
  rocks 
  are 
  the 
  least 
  fertile 
  in 
  our 
  state, 
  except 
  where 
  over- 
  

   lying 
  deposits 
  of 
  glacial 
  drift 
  and 
  alluvium 
  furnish 
  a 
  soil 
  which 
  

   is 
  adapted 
  to 
  tillage 
  and 
  the 
  support 
  of 
  vegetation. 
  

  

  Typical 
  Localities 
  of 
  the 
  Archaean 
  

  

  The 
  most 
  southern 
  locality 
  of 
  Archaean 
  rock 
  in 
  New 
  York 
  

   state 
  is 
  on 
  New 
  York 
  island, 
  between 
  7th 
  and 
  8th 
  avenues 
  

   south 
  of 
  155th 
  street. 
  This 
  is 
  a 
  good 
  exposure 
  and 
  is 
  typical 
  

   of 
  the 
  Archaean 
  gneiss 
  of 
  southeastern 
  New 
  York. 
  This 
  gneiss 
  

   is 
  well 
  shown 
  throughout 
  Westchester 
  county 
  along 
  the 
  shore 
  of 
  

   the 
  Hudson, 
  though 
  at 
  a 
  few 
  points 
  Lower 
  Silurian 
  limestone 
  

   and 
  mica-schist 
  occur. 
  A 
  little 
  north 
  of 
  Peekskill 
  may 
  be 
  seen 
  

   the 
  granite 
  mountains 
  of 
  the 
  Highlands, 
  which 
  traverse 
  Orange 
  

   and 
  Putnam 
  counties. 
  These 
  are 
  chiefly 
  massive, 
  though 
  on 
  

   their 
  flanks 
  are 
  some 
  gneissoid 
  rocks 
  and 
  in 
  many 
  of 
  the 
  valleys 
  

   are 
  Palaeozoic 
  limestones 
  and 
  schists. 
  Other 
  localities 
  are 
  seen 
  

   in 
  Dover 
  mountEiin 
  and 
  in 
  Stissing 
  mountain 
  in 
  Dutchess 
  county. 
  

   North 
  of 
  this 
  southeastern 
  area, 
  the 
  Archaean 
  rocks 
  are 
  chiefly 
  

   confined 
  to 
  the 
  region 
  known 
  as 
  the 
  Adirondack 
  wilderness. 
  

  

  