﻿64 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  tiny 
  zircons 
  do 
  not 
  fail. 
  Quartz 
  is 
  not 
  entirely 
  lacking, 
  but 
  in 
  

   typical 
  specimens 
  it 
  is 
  a 
  minor 
  component. 
  The 
  syenite 
  was 
  called 
  

   in 
  the 
  writer's 
  earlier 
  paper, 
  the 
  " 
  Barton 
  gneiss." 
  A 
  number 
  of 
  

   analyses 
  have 
  been 
  made 
  for 
  Professor 
  Cushing 
  from 
  specimens 
  

   gathered 
  in 
  the 
  northern 
  Adirondacks 
  and 
  they 
  uniformly 
  run 
  

   below 
  65 
  per 
  cent 
  Si0 
  2 
  , 
  the 
  percentage 
  at 
  which 
  quartz 
  begins 
  to 
  

   be 
  an 
  important 
  mineral 
  in 
  the 
  eruptive 
  rocks. 
  

  

  In 
  the 
  cores 
  as 
  well 
  as 
  in 
  the 
  hand 
  specimens 
  the 
  syenite 
  is 
  a 
  

   blotchy, 
  black 
  and 
  green 
  rock, 
  which 
  always 
  has 
  a 
  pronounced 
  

   green 
  cast 
  when 
  fresh. 
  On 
  ledges 
  that 
  have 
  been 
  long 
  exposed 
  to 
  

   weathering 
  it 
  is 
  often 
  decidedly 
  rusty, 
  especially 
  in 
  the 
  basic 
  

   phases. 
  While 
  the 
  percentage 
  in 
  iron 
  is 
  not 
  so 
  very 
  high, 
  yet 
  this 
  

   element 
  must 
  be 
  combined 
  in 
  one 
  or 
  more 
  of 
  the 
  minerals 
  in 
  some 
  

   unstable 
  form, 
  such 
  that 
  it 
  readily 
  oxidizes. 
  It 
  is 
  often 
  necessary 
  

   to 
  break 
  into 
  good 
  sized 
  blocks 
  before 
  the 
  reasonably 
  fresh 
  green 
  

   rock 
  appears 
  at 
  the 
  core. 
  It 
  has 
  also 
  been 
  our 
  experience 
  in 
  the 
  

   field 
  to 
  find 
  the 
  syenite 
  sometimes 
  developing 
  on 
  exposure 
  a 
  dead 
  

   white 
  crust 
  that 
  resembles 
  the 
  anorthosites 
  and 
  that 
  is 
  deceptive. 
  

   In 
  these 
  varieties 
  the 
  iron 
  must 
  be 
  in 
  small 
  amount 
  or 
  else 
  limited 
  

   to 
  some 
  stable 
  compound 
  that 
  resists 
  decay. 
  In 
  fact 
  with 
  the 
  

   variations 
  to 
  be 
  next 
  outlined 
  and 
  the 
  protean 
  appearance 
  on 
  

   weathering, 
  it 
  is 
  not 
  surprising 
  that 
  the 
  syenitic 
  rocks 
  have 
  so 
  long 
  

   escaped 
  identification 
  as 
  such. 
  

  

  As 
  a 
  departure 
  from 
  the 
  normal 
  proportions 
  of 
  feldspar 
  and 
  

   dark 
  silicate, 
  we 
  sometimes 
  find 
  the 
  latter 
  developing 
  in 
  great 
  ty 
  

   increased 
  amount. 
  The 
  feldspar 
  is 
  far 
  less 
  prominent 
  and 
  a 
  dark 
  

   basic 
  rock 
  ensues 
  which 
  on 
  slight 
  acquaintance 
  one 
  would 
  consider 
  

   a 
  basic 
  gabbro 
  or 
  diorite. 
  But 
  the 
  characteristic 
  feldspar, 
  as 
  well 
  

   as 
  the 
  normal 
  dark 
  silicates 
  of 
  the 
  syenite, 
  are 
  still 
  present, 
  and 
  both 
  

   in 
  the 
  drill 
  cores, 
  as 
  well 
  as 
  in 
  the 
  field, 
  we 
  find 
  a 
  quick 
  passage 
  from 
  

   the 
  usual 
  variety 
  to 
  the 
  basic 
  with 
  no 
  eruptive 
  contact 
  that 
  would 
  

   indicate 
  a 
  separate 
  intrusive 
  mass 
  or 
  an 
  included 
  sediment. 
  For 
  

   these 
  dark 
  bands 
  we 
  can 
  adduce 
  no 
  other 
  reasonable 
  conception 
  

   than 
  that 
  the 
  original 
  intrusive 
  mass 
  was 
  in 
  parts 
  more 
  basic 
  than 
  

   elsewhere, 
  and 
  that 
  if 
  its 
  parent 
  magma 
  were 
  homogeneous, 
  it 
  

   separated, 
  as 
  has 
  been 
  so 
  often 
  observed 
  in 
  later 
  years 
  in 
  large 
  

   eruptive 
  masses, 
  into 
  portions 
  of 
  contrasted 
  composition 
  although 
  

   of 
  common 
  parentage. 
  This 
  basic 
  syenite 
  was 
  not 
  recognized 
  as 
  

   such 
  in 
  the 
  writer's 
  previous 
  paper, 
  but 
  was 
  esteemed 
  to 
  be 
  a 
  

   gneissoid 
  representative 
  of 
  the 
  gabbro. 
  While 
  it 
  resembles 
  this 
  

   rock 
  in 
  the 
  closest 
  w 
  T 
  ay, 
  yet 
  the 
  drill 
  cores 
  now 
  available 
  prove 
  

   its 
  affinity 
  with 
  the 
  syenites. 
  

  

  