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  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  Igneous 
  rocks 
  

  

  The 
  igneous 
  rocks 
  are 
  very 
  numerous, 
  but 
  may 
  be 
  classified 
  

   in 
  a 
  few 
  groups 
  by 
  mineral 
  composition 
  and 
  texture. 
  Tbe 
  tex- 
  

   ture 
  indicates 
  usually 
  the 
  conditions 
  of 
  their 
  cooling. 
  If 
  the 
  

   cooling 
  occurred 
  at 
  a 
  considerable 
  depth, 
  the 
  process 
  was 
  grad- 
  

   ual, 
  crystals 
  of 
  the 
  component 
  minerals 
  formed 
  slowly 
  and 
  freely, 
  

   and 
  the 
  resulting 
  texture 
  is 
  coarse. 
  If 
  the 
  cooling 
  was 
  in 
  the 
  

   open 
  air, 
  as 
  in 
  a 
  lava 
  bed, 
  the 
  process 
  was 
  more 
  rapid; 
  there 
  was 
  

   not 
  sufficient 
  time 
  for 
  crystals 
  to 
  forin, 
  and 
  the 
  resulting 
  

   texture 
  is 
  fine 
  or 
  glassy. 
  

  

  The 
  first 
  class 
  is 
  called 
  plutonic, 
  the 
  second 
  volcanic. 
  Plu- 
  

   tonic 
  rocks 
  abound 
  in 
  the 
  regions 
  where 
  old 
  geologic 
  formations 
  

   are 
  exposed, 
  since 
  there, 
  either 
  the 
  intrusions 
  did 
  not 
  reach 
  the 
  

   surface 
  or 
  the 
  surface 
  material 
  which 
  cooled 
  as 
  lava 
  was 
  re- 
  

   moved 
  by 
  long 
  erosion, 
  and 
  we 
  see 
  only 
  those 
  parts 
  which 
  were 
  

   deeply 
  covered 
  while 
  cooling. 
  Examples 
  of 
  this 
  are 
  seen 
  in 
  the 
  

   Palisades 
  of 
  the 
  Hudson; 
  the 
  granite 
  mountains, 
  Anthony's 
  

   Nose, 
  Storm 
  King, 
  Breakneck 
  and 
  other 
  peaks 
  of 
  the 
  highlands, 
  

   and 
  in 
  Mt. 
  Marcy, 
  Whiteface, 
  etc., 
  of 
  the 
  Adirondack 
  chain. 
  The 
  

   volcanic 
  rocks 
  are 
  chiefly 
  exposed 
  in 
  regions 
  of 
  the 
  newer 
  forma- 
  

   tions 
  because 
  of 
  the 
  deep-seated 
  plutonic 
  masses 
  have 
  not 
  yet 
  

   been 
  brought 
  to 
  view 
  by 
  erosion. 
  The 
  only 
  good 
  exposure 
  of 
  this 
  

   character 
  in 
  New 
  York 
  is 
  the 
  mass 
  of 
  red 
  porphyry 
  or 
  trachyte 
  

   at 
  Cannon's 
  Pt., 
  near 
  Essex, 
  on 
  Lake 
  Champlain. 
  

  

  This 
  statement 
  involves 
  the 
  theory 
  that 
  every 
  volcanic 
  mass 
  

   has 
  beneath 
  it, 
  or 
  oonnected 
  with 
  it, 
  a 
  plutonic 
  mass 
  of 
  the 
  same 
  

   general 
  chemical 
  composition.^- 
  

  

  The 
  names 
  of 
  a 
  few 
  important 
  igneous 
  rocks 
  and 
  their 
  essential 
  

   compositions 
  are 
  given 
  below 
  according 
  to 
  the 
  classification 
  of 
  

   Kosenbusch.* 
  

  

  a 
  The 
  accurate 
  classification 
  of 
  rocks 
  dates 
  from 
  about 
  1873. 
  with 
  the 
  develop- 
  

   ment 
  of 
  methods 
  of 
  study 
  with 
  the 
  microscope. 
  Most 
  of 
  the 
  older 
  hooks 
  in 
  English 
  

   are 
  much 
  behind 
  the 
  present 
  German 
  standard 
  of 
  progress. 
  

  

  b 
  Mikroskopische 
  Physiographic 
  der 
  Mineralien 
  Tind 
  Gesteine. 
  

  

  