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  PldlUps 
  — 
  Mine 
  ralog 
  leal 
  Structure 
  and 
  Cheniical 
  

  

  with 
  the 
  olivine 
  of 
  basalts. 
  Prof. 
  J. 
  W. 
  Judd," 
  in 
  connection 
  

   with 
  the 
  older 
  peridotites 
  of 
  Scotland, 
  concludes 
  that 
  it 
  is 
  a 
  

   phenomenon 
  which 
  takes 
  place 
  only 
  at 
  a 
  great 
  depth 
  from 
  the 
  

   surface, 
  and 
  consequently 
  at 
  considerable 
  pressure. 
  From 
  this 
  

   we 
  may 
  conclude 
  that 
  originally 
  this 
  trap 
  was 
  buried 
  far 
  below 
  

   the 
  surface 
  at 
  the 
  time 
  of 
  its 
  intrusion, 
  and 
  has 
  since 
  been 
  

   elevated 
  to 
  its 
  present 
  position, 
  and 
  the 
  strata 
  originally 
  cover- 
  

   ing 
  it 
  have 
  been 
  carried 
  away. 
  

  

  The 
  second 
  method 
  of 
  the 
  olivine 
  decomposition 
  is 
  that 
  

   generally 
  found 
  at 
  the 
  surface, 
  in 
  which 
  an 
  olivine 
  rich 
  in 
  

   iron, 
  such 
  as 
  this 
  seems 
  to 
  be, 
  by 
  oxidation 
  and 
  formation 
  of 
  

   small 
  scales 
  of 
  limonite 
  and 
  hematite, 
  becomes 
  dark 
  yellow, 
  

   brown 
  or 
  red. 
  The 
  greater 
  number, 
  and 
  especially 
  the 
  

   smaller 
  rounded 
  grains, 
  have 
  become 
  deeply 
  colored 
  by 
  this 
  

   formation 
  of 
  iron 
  oxide, 
  while 
  the 
  larger, 
  some 
  at 
  least, 
  are 
  

   still 
  perfectly 
  fresh, 
  with 
  the 
  exception 
  of 
  the 
  magnetite 
  

   grains. 
  Their 
  rounded 
  shape 
  is 
  due 
  to 
  a 
  partial 
  resolution 
  in 
  

   the 
  magma, 
  as 
  is 
  shown 
  by 
  the 
  corrosive 
  cavities.''' 
  There 
  are 
  

   present, 
  also, 
  large 
  inclusions 
  of 
  glass, 
  so 
  characteristic 
  of 
  

   olivine. 
  The 
  absence 
  of 
  serpentine 
  as 
  a 
  decomposition 
  product 
  

   would 
  also 
  point 
  to 
  an 
  olivine 
  with 
  large 
  percentage 
  of 
  iron. 
  

  

  In 
  size 
  the 
  olivines 
  attain 
  a 
  maximum 
  diameter 
  of 
  about 
  

   04"^™. 
  

  

  Augite. 
  — 
  The 
  crystallization 
  of 
  the 
  diallage-like 
  augite 
  is 
  

   varied. 
  In 
  slides 
  of 
  the 
  finer 
  or 
  diabase 
  structure, 
  the 
  ground- 
  

   mass 
  is 
  composed 
  of 
  a 
  collection 
  of 
  microscopical 
  crj^stals, 
  with 
  

   now 
  and 
  then 
  a 
  lath-shaped 
  section 
  of 
  a 
  larger 
  augite 
  lying 
  

   among 
  them. 
  In 
  sections 
  at 
  right 
  angles 
  to 
  the 
  d-axis 
  the 
  

   characteristic 
  prismatic 
  cleavage 
  is 
  well 
  developed, 
  as 
  is 
  also 
  

   the 
  cleavage 
  parallel 
  to 
  the 
  orthopinacoid 
  ; 
  crystals 
  from 
  quarry 
  

   No. 
  1 
  show 
  a 
  maximum 
  extinction 
  angle 
  of 
  46°. 
  

  

  These 
  diallagic 
  crystals 
  constantly 
  increase 
  in 
  length 
  as 
  we 
  

   pass 
  in 
  from 
  the 
  border 
  ; 
  in 
  quarry 
  No. 
  2 
  they 
  are 
  8 
  to 
  10™°' 
  

   in 
  length 
  ; 
  in 
  quarry 
  No. 
  3 
  they 
  reach 
  their 
  maximum 
  length. 
  

   Tliey 
  are 
  all 
  elongated 
  in 
  the 
  direction 
  of 
  the 
  c-axis, 
  and 
  the 
  

   pinacoids 
  are 
  developed 
  at 
  the 
  expense 
  of 
  the 
  prism 
  faces, 
  

   giving 
  them 
  a 
  flattened 
  appearance. 
  The 
  color 
  constantly 
  

   deepens, 
  and 
  in 
  reflected 
  light 
  they 
  appear 
  about 
  black, 
  

   except 
  on 
  cleavage 
  faces 
  where 
  a 
  characteristic 
  schiller 
  is 
  

   developed. 
  The 
  cleavage 
  faces 
  parallel 
  to 
  the 
  prism 
  show 
  the 
  

   trace 
  of 
  a 
  twinning 
  plane 
  parallel 
  to 
  the 
  orthopinacoid, 
  running 
  

   directly 
  through 
  Uie 
  middle 
  of 
  the 
  crystals, 
  each 
  twin 
  being 
  

   equally 
  developed. 
  This 
  twinning 
  is 
  very 
  constant 
  in 
  the 
  

   larger 
  crystals, 
  and 
  between 
  crossed 
  nicols, 
  in 
  some 
  instances, 
  

   polysyntlietic 
  twinning 
  is 
  indicated 
  along 
  this 
  line. 
  In 
  trans- 
  

   mitted 
  light, 
  their 
  color 
  is 
  uniformly 
  brown, 
  with 
  little 
  pleo- 
  

   chroism. 
  Exthicrion 
  is 
  undulatory, 
  indicating 
  a 
  shell-like 
  

   structure, 
  the 
  angle 
  of 
  extinction 
  being 
  greater 
  for 
  external 
  than 
  

  

  