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  Phillijjs 
  — 
  Miner 
  alogical 
  Structure 
  and 
  Chemical 
  

  

  cracks 
  ; 
  following 
  this 
  system, 
  they 
  form 
  parallel 
  lines 
  of 
  fine 
  

   granules 
  of 
  magnetite, 
  limonite, 
  etc., 
  which 
  seem 
  to 
  be 
  also 
  an 
  

   important 
  factor 
  in 
  producing 
  the 
  schiller-like 
  luster, 
  and 
  they 
  

   would 
  evidently 
  be 
  a 
  result 
  of 
  surface 
  action. 
  These 
  secondary 
  

   granules 
  are 
  very 
  numerous 
  since 
  the 
  percentage 
  of 
  ferrous 
  

   iron 
  in 
  all 
  these 
  crystals 
  is 
  verj^ 
  high. 
  

  

  As 
  decomposition 
  advances, 
  other 
  products 
  are 
  formed 
  and 
  

   become 
  mixed 
  with 
  the 
  oxide 
  of 
  iron 
  granules, 
  such 
  as 
  fibrous 
  

   chlorite, 
  biotite 
  and 
  epidote 
  ; 
  even 
  when 
  this 
  has 
  reached 
  such 
  

   an 
  advanced 
  stage 
  that 
  the 
  original 
  crystal 
  is 
  replaced 
  by 
  a 
  

   crystalline 
  aggregate, 
  the 
  position 
  of 
  the 
  fine 
  interposition 
  

   planes 
  of 
  the 
  mother 
  crystal 
  is 
  still 
  marked 
  by 
  lines 
  of 
  dark 
  

   granules. 
  The 
  oxidation 
  of 
  the 
  large 
  amount 
  of 
  iron 
  contained 
  

   in 
  the 
  diallage 
  is 
  the 
  first 
  step 
  in 
  the 
  rock's 
  decomposition 
  and 
  

   has 
  commenced 
  even 
  before 
  the 
  clouding 
  of 
  the 
  feldspars. 
  

  

  Biotite^ 
  as 
  a 
  primary 
  mineral, 
  is 
  absent, 
  and 
  is 
  found 
  only 
  as 
  

   a 
  secondary 
  product 
  derived 
  from 
  the 
  decomposition 
  of 
  the 
  

   diallage. 
  It 
  is 
  present 
  only 
  in 
  those 
  slides 
  wdiich 
  show 
  the 
  

   diallage 
  in 
  an 
  advanced 
  stage 
  of 
  decomposition 
  ; 
  its 
  formation 
  

   begins 
  after 
  a 
  considerable 
  amount 
  of 
  the 
  iron 
  has 
  been 
  

   oxidized. 
  

  

  Quartz.— 
  ^\\2X 
  has 
  been 
  said 
  of 
  biotite 
  may 
  also 
  be 
  said 
  of 
  

   quartz, 
  to 
  a 
  great 
  extent, 
  with 
  this 
  exception, 
  that 
  in 
  specimens 
  

   from 
  quarry 
  No. 
  3 
  there 
  are 
  a 
  small 
  number 
  of 
  quartz 
  grains 
  

   vrhicli 
  are 
  evidently 
  primary, 
  judging 
  from 
  their 
  size 
  and 
  the 
  

   freshness 
  of 
  the 
  surrounding 
  crystals. 
  Secondary 
  quartz 
  is 
  

   formed 
  in 
  connection 
  witli 
  the 
  formation 
  of 
  chlorite 
  in 
  the 
  

   decomposition 
  of 
  the 
  diallage 
  ; 
  this 
  is 
  best 
  represented 
  in 
  the 
  

   Hopewell 
  quarries, 
  where 
  quartz 
  granules 
  can 
  be 
  picked 
  out 
  

   of 
  the 
  mass 
  of 
  chlorite. 
  

  

  Amjphihole^ 
  in 
  the 
  form 
  of 
  hornblende, 
  is 
  represented 
  in 
  a 
  

   few 
  slides 
  from 
  the 
  central 
  portions 
  of 
  the 
  dike, 
  but 
  is 
  most 
  

   frequent 
  in 
  all 
  slides 
  from 
  quarry 
  JN^o. 
  3 
  and 
  here 
  stands 
  in 
  a 
  

   very 
  intimate 
  relation 
  and 
  is 
  closely 
  connected 
  with 
  the 
  history 
  

   of 
  the 
  diallage, 
  from 
  which 
  it 
  has 
  been 
  derived 
  by 
  para- 
  

   morphism. 
  It 
  occurs 
  in 
  tw^o 
  distinct 
  varieties: 
  one 
  compact, 
  

   yellow-brown 
  in 
  transmitted 
  light, 
  the 
  other 
  fibrous 
  and 
  light- 
  

   green, 
  both 
  of 
  w-hich 
  are 
  vei'y 
  pleochroic. 
  In 
  the 
  green 
  variety, 
  

   the 
  ray 
  vibrating 
  parallel 
  to 
  c 
  is 
  decidedly 
  bluish 
  green 
  ; 
  that 
  

   parallel 
  to 
  a 
  and 
  h 
  yellow. 
  In 
  the 
  compact 
  form, 
  the 
  pleochro- 
  

   ism 
  is 
  shown 
  only 
  in 
  shades 
  of 
  brown 
  and 
  yellow 
  ; 
  the 
  absorp- 
  

   tion 
  is 
  very 
  marked, 
  especially 
  in 
  the 
  compact 
  variety, 
  and 
  

   sections 
  with 
  the 
  light 
  rays 
  vibrating 
  parallel 
  to 
  c 
  are 
  very 
  

   dark. 
  The 
  absorption 
  in 
  both 
  varieties 
  is 
  as 
  usual 
  c 
  > 
  b 
  > 
  a. 
  

   The 
  .prismatic 
  cleavage 
  is 
  perfectly 
  developed 
  and 
  is 
  strikingly 
  

   in 
  contrast 
  to 
  that 
  of 
  the 
  neighboring 
  diallage 
  crystals. 
  The 
  

   niaximum 
  extinction-angle 
  measured 
  from 
  cleavage 
  cracks 
  in 
  

  

  