﻿28 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  Megascopic 
  features 
  

  

  The 
  gabbro 
  and 
  its 
  derivatives 
  present 
  a 
  truly 
  remarkable 
  number 
  

   of 
  facies 
  or 
  varieties 
  clearly 
  visible 
  to 
  the 
  naked 
  eye. 
  The 
  coarse- 
  

   ness 
  of 
  grain 
  varies 
  from 
  the 
  merest 
  fraction 
  of 
  a 
  millimeter 
  to 
  fully 
  

   an 
  inch 
  (for 
  example, 
  the 
  stock 
  on 
  the 
  south 
  side 
  of 
  Loon 
  lake). 
  

   The 
  fine-grained 
  portions 
  are 
  confined 
  to 
  the 
  borders 
  of 
  the 
  stocks 
  

   or 
  the 
  few 
  branching 
  tongues 
  and 
  were 
  caused 
  by 
  the 
  more 
  rapid 
  

   chilling 
  of 
  the 
  rock 
  in 
  those 
  positions. 
  Even 
  the 
  finest 
  grained 
  rocks, 
  

   however, 
  are 
  noncrystalline. 
  As 
  a 
  rule 
  the 
  coarseness 
  of 
  grain 
  in- 
  

   creases 
  toward 
  the 
  interior 
  of 
  the 
  masses, 
  though 
  often 
  medium 
  to 
  

   coarse 
  grained 
  rocks 
  extend 
  to 
  the 
  very 
  contact. 
  The 
  typical 
  or 
  

   prevailing 
  gabbro 
  shows 
  a 
  medium 
  grain; 
  that 
  is, 
  the 
  grains 
  are 
  

   from 
  i 
  to 
  5 
  millimeters 
  across. 
  

  

  The 
  texture 
  varies 
  from 
  coarse 
  to 
  medium 
  to 
  fine-grained 
  gran- 
  

   itoid, 
  to 
  medium 
  to 
  coarse-grained 
  diabasic 
  (ophitic). 
  The 
  gabbro 
  

   from 
  the 
  stock 
  on 
  the 
  south 
  side 
  of 
  Loon 
  lake 
  is 
  an 
  excellent 
  example 
  

   of 
  diabase 
  texture 
  in 
  which 
  the 
  feldspar 
  laths 
  attain 
  a 
  length 
  of 
  an 
  

   inch 
  or 
  more. 
  The 
  typical 
  gabbro 
  always 
  exhibits 
  the 
  diabasic 
  

   texture. 
  

  

  In 
  color, 
  the 
  gabbro 
  and 
  its 
  derivatives 
  range 
  from 
  nearly 
  black 
  

   through 
  dark 
  to 
  light 
  gray, 
  the 
  darker 
  varieties 
  often 
  showing 
  a 
  

   slight 
  reddish 
  tinge 
  due 
  to 
  the 
  presence 
  of 
  garnets. 
  The 
  gray 
  rocks 
  

   all 
  belong 
  to 
  the 
  more 
  acidic 
  (dioritic 
  and 
  syenitic) 
  facies 
  described 
  

   below. 
  In 
  one 
  case 
  a 
  greenish 
  gray 
  color 
  was 
  noted. 
  The 
  very 
  dark 
  

   color 
  of 
  the 
  typical 
  gabbro 
  is 
  due 
  to 
  the 
  fact 
  that 
  the 
  feldspars 
  are 
  

   so 
  charged 
  with 
  tiny 
  black 
  inclusions. 
  

  

  In 
  the 
  typical 
  gabbros 
  the 
  minerals 
  commonly 
  recognizable 
  with 
  

   the 
  naked 
  eye 
  or 
  hand 
  lens 
  are 
  plagioclase, 
  pyroxene, 
  hornblende, 
  

   garnet, 
  biotite, 
  and 
  ilmenite, 
  while 
  in 
  addition 
  to 
  these 
  orthoclase 
  

   and 
  quartz 
  may 
  often 
  be 
  seen 
  in 
  the 
  more 
  acidic 
  phases. 
  

  

  Except 
  for 
  the 
  rather 
  common 
  presence 
  of 
  highly 
  gneissoid 
  to 
  

   even 
  schistose 
  amphibolite 
  borders, 
  the 
  stocks 
  of 
  typical 
  gabbro 
  are 
  

   practically 
  devoid 
  of 
  gneissoid 
  structure. 
  Some 
  of 
  the 
  lighter 
  

   colored, 
  more 
  acidic 
  phases, 
  however, 
  show 
  fairly 
  well-developed 
  

   foliation. 
  

  

  It 
  is 
  important 
  to 
  note 
  that 
  many 
  of 
  the 
  above 
  described 
  varia- 
  

   tions 
  may 
  be 
  found 
  within 
  a 
  single 
  stock 
  as, 
  for 
  example, 
  south 
  of 
  

   Mountain 
  Spring 
  lake. 
  The 
  following 
  statements 
  from 
  Smyth's 
  

   description 
  of 
  a 
  similar 
  western 
  Adirondack 
  gabbro 
  J 
  fittingly 
  apply 
  

   here 
  : 
  " 
  These 
  [primary] 
  changes 
  in 
  character 
  take 
  place 
  very 
  sud- 
  

   denly, 
  and 
  the 
  different 
  phases 
  are 
  most 
  irregularly 
  distributed, 
  

  

  ^mer. 
  Jour. 
  Sci., 
  April 
  1806, 
  p. 
  273-74. 
  

  

  