﻿542 
  Wright 
  — 
  Determination 
  of 
  the 
  Plagioclase 
  Feldspars. 
  

  

  were 
  plotted 
  to 
  scale 
  and 
  a 
  smooth 
  curve 
  passed 
  through 
  the 
  

   computed 
  points. 
  After 
  having 
  computed 
  the 
  set 
  of 
  extinc- 
  

   tion 
  angles 
  for 
  each 
  of 
  the 
  accepted 
  plagioclase 
  feldspars 
  whose 
  

   optical 
  properties 
  are 
  known 
  with 
  a 
  fair 
  degree 
  of 
  accuracy, 
  

   the 
  different 
  sets 
  were 
  plotted, 
  each 
  to 
  scale 
  on 
  the 
  ordinate 
  

   passing 
  through 
  the 
  proper 
  plagioclase 
  composition 
  on 
  the 
  

   abscissa 
  axis. 
  The 
  variation 
  in 
  the 
  extinction 
  angles 
  of 
  each 
  

   of 
  the 
  poles 
  was 
  then 
  obtained 
  by 
  passing 
  a 
  curve 
  through 
  the 
  

   extinction 
  angles 
  for 
  any 
  given 
  pole. 
  On 
  this 
  plot, 
  when 
  

   completed, 
  there 
  were 
  curves 
  representing 
  the 
  variation 
  in 
  

   extinction 
  angle 
  with 
  composition 
  for 
  all 
  poles 
  at 
  10° 
  intervals 
  

   from 
  +90° 
  to 
  —90°. 
  Now 
  if 
  for 
  any 
  given 
  plagioclase 
  sec- 
  

   tion 
  whose 
  normal 
  is 
  in 
  the 
  plane 
  010 
  and 
  includes 
  an 
  angle 
  X 
  x 
  

   with 
  the 
  pole 
  of 
  the 
  projection 
  (e. 
  g., 
  +20°), 
  the 
  extinction 
  

   angle 
  is 
  S 
  x 
  , 
  then 
  the 
  extinction 
  angle 
  S 
  2 
  of 
  the 
  albite 
  lamellae 
  in 
  

   Carlsbad 
  twinning 
  relations 
  to 
  the 
  first 
  set 
  is 
  that 
  of 
  the 
  sec- 
  

   tion 
  whose 
  normal 
  is 
  in 
  the 
  plane 
  010 
  and 
  includes 
  an 
  angle 
  

   -~\ 
  (e. 
  g., 
  —20°) 
  with 
  the 
  pole 
  of 
  the 
  projection. 
  But 
  

   on 
  the 
  plot 
  prepared 
  as 
  above 
  such 
  extinction 
  angles 
  can 
  

   be 
  read 
  off 
  directly, 
  and 
  for 
  every 
  extinction 
  angle 
  in 
  the 
  

   one 
  set 
  the 
  proper 
  extinction 
  angle 
  of 
  the 
  second 
  set 
  at 
  the 
  

   proper 
  composition 
  can 
  be 
  ascertained. 
  The 
  curves 
  of 
  Plate 
  X 
  

   were 
  determined 
  in 
  this 
  manner. 
  They 
  are 
  smooth 
  empirical 
  

   curves 
  passing 
  through 
  known 
  points 
  determined 
  by 
  definite 
  

   construction 
  from 
  the 
  data 
  of 
  observation. 
  Although 
  similar 
  

   to 
  the 
  Michel-Levy 
  curves 
  in 
  form, 
  they 
  are 
  not 
  theoretical 
  

   curves, 
  but 
  are 
  strictly 
  empirical. 
  Their 
  accuracy 
  depends, 
  

   therefore, 
  directly 
  on 
  the 
  data 
  of 
  observation. 
  With 
  the 
  

   accumulation 
  of 
  more 
  precise 
  optical 
  data 
  on 
  the 
  plagioclases, 
  

   and 
  especially 
  with 
  increase 
  in 
  knowledge 
  of 
  the 
  effect 
  of 
  solid 
  

   solution 
  with 
  orthoclase, 
  carnegieite 
  or 
  nephelite, 
  and 
  possibly 
  

   kaliophillite 
  and 
  other 
  substances, 
  these 
  empirical 
  curves 
  will 
  

   be 
  changed 
  somewhat. 
  At 
  present, 
  however, 
  they 
  are 
  as 
  

   accurate 
  as 
  it 
  is 
  possible 
  to 
  draw 
  them 
  from 
  the 
  available 
  data. 
  

  

  A 
  comparison 
  of 
  these 
  curves 
  with 
  the 
  Michel-Levy 
  set 
  

   shows 
  on 
  the 
  whole 
  fairly 
  good 
  agreement, 
  although 
  in 
  certain 
  

   spots 
  the 
  two 
  plots 
  disagree 
  by 
  over 
  20 
  per 
  cent. 
  This 
  is 
  

   especially 
  the 
  case 
  between 
  the 
  plagioclases 
  of 
  the 
  andesine 
  and 
  

   labradorite 
  groups. 
  

  

  In 
  view 
  of 
  the 
  detailed 
  descriptions 
  of 
  the 
  methods 
  for 
  

   determining 
  the 
  plagioclase 
  feldspars 
  in 
  all 
  textbooks 
  on 
  micro- 
  

   scopic 
  petrography, 
  further 
  explanation 
  of 
  Plate 
  X 
  seems 
  

   unnecessary. 
  

  

  