﻿Lane 
  and 
  Keller— 
  Notes 
  on 
  Michigan 
  Minerals. 
  501 
  

  

  = 
  65°-l 
  [P.E. 
  = 
  0°-0 
  

   tptn 
  = 
  57° 
  3 
  

   tpb 
  — 
  5 
  7°-6 
  [P.E. 
  = 
  0°-374] 
  

   p 
  : 
  t 
  (? 
  the 
  best 
  cleavage) 
  = 
  80°^ 
  ^ 
  2 
  q 
  \ 
  (cleavage 
  faces) 
  

  

  :b/ 
  =83i 
  

  

  imf 
  =84° 
  

  

  + 
  4° 
  

  

  — 
  / 
  

   + 
  10 
  

  

  — 
  10 
  

  

  These 
  angles 
  from 
  the 
  basis 
  to 
  the 
  lateral 
  cleavage 
  were 
  

   taken 
  with 
  a 
  reflection 
  goniometer. 
  The 
  lateral 
  faces 
  gave 
  

   only 
  a 
  " 
  schimmer." 
  

  

  The 
  sense 
  of 
  the 
  angle 
  from^? 
  to 
  t, 
  m 
  and 
  b, 
  is 
  still 
  uncer- 
  

   tain, 
  i. 
  e. 
  it 
  is 
  possible, 
  though 
  not 
  probable, 
  that 
  one 
  or 
  more 
  

   of 
  them 
  may 
  be 
  on 
  the 
  other 
  side 
  of 
  p. 
  The 
  angle 
  from 
  the 
  

   negative 
  extinction 
  to 
  the 
  trace 
  of 
  b. 
  in 
  basal 
  sections, 
  is 
  14° 
  

   [P.E. 
  = 
  0°"42]. 
  The 
  angle 
  from 
  the 
  positive 
  extinction 
  to 
  the 
  

   trace 
  of 
  t 
  should 
  be, 
  by 
  calculation 
  11°. 
  Certain 
  observations 
  

   give 
  12 
  0, 
  3. 
  The 
  directions 
  of 
  extinction 
  in 
  basal 
  cleavage 
  

   fragments 
  vary 
  from 
  being 
  nearly 
  parallel 
  to 
  the 
  trace 
  of 
  t, 
  to 
  

   being 
  nearly 
  perpendicular 
  to 
  the 
  trace 
  of 
  b, 
  which 
  last 
  position 
  

   agrees 
  best 
  with 
  the 
  observations 
  of 
  previous 
  writers.- 
  This 
  

   variation 
  is 
  doubtless 
  due 
  to 
  superposed 
  twinned 
  lamellae. 
  The 
  

   thinner 
  fragments 
  which 
  have 
  least 
  signs 
  of 
  twinning, 
  i. 
  e. 
  

   sharpest 
  extinctions 
  and 
  purest 
  pleochroism, 
  give 
  an 
  extinction 
  

   angle 
  about 
  as 
  drawn. 
  

  

  For 
  t, 
  i. 
  e. 
  in 
  cleavage 
  bits 
  that 
  have 
  a 
  strong 
  pleochroism, 
  

   blue 
  to 
  yellow, 
  and 
  appear 
  to 
  be 
  parallel 
  to 
  t, 
  the 
  extinction 
  

   angle 
  against 
  the 
  trace 
  of 
  p 
  averages 
  18°-6, 
  but 
  there 
  is 
  a 
  

   strong 
  dispersion 
  of 
  the 
  extinction 
  ; 
  ex. 
  p 
  <^ 
  ex. 
  v. 
  Values 
  as 
  

   high 
  as 
  21° 
  occur 
  often. 
  Such 
  cleavage 
  fragments 
  are 
  very 
  

   frequent. 
  

  

  In 
  cleavage 
  bits 
  that 
  are 
  apparently 
  parallel 
  to 
  b, 
  and 
  have 
  a 
  

   pleochroism 
  from 
  green 
  to 
  yellow, 
  the 
  extinction 
  angle 
  is 
  

   almost 
  inappreciable. 
  A 
  section 
  artificially 
  cut 
  nearly 
  per- 
  . 
  

   pendicular 
  to 
  p 
  and 
  parallel 
  to 
  m 
  (probably) 
  gave 
  an 
  extinction 
  

   angle 
  of 
  8°. 
  The 
  best 
  cleavage 
  follows 
  t, 
  the 
  next 
  b, 
  a 
  very 
  

   poor 
  one 
  m, 
  and 
  there 
  seem 
  to 
  be 
  traces 
  of 
  a 
  cleavage 
  perpen- 
  

   dicular 
  to 
  b. 
  

  

  The 
  refraction 
  by 
  the 
  de 
  Chaulnes 
  method 
  is 
  about 
  1*75, 
  

   the 
  bi-refraction, 
  judging 
  from 
  the 
  brightest 
  polarization 
  

   colors, 
  about 
  0*007, 
  not 
  above 
  that 
  of 
  quartz. 
  [According 
  to 
  

   Lacroix 
  it 
  is 
  0-015, 
  but 
  doubtless 
  it 
  varies 
  with 
  the 
  ratio 
  of 
  

   Mg 
  : 
  Fe.] 
  

  

  In 
  convergent 
  light 
  the 
  positive 
  acute 
  bisectrix 
  emerges 
  

   doubly 
  obliquely, 
  i. 
  e. 
  so 
  that 
  when 
  the 
  cross 
  is 
  formed, 
  neither 
  

   arm 
  passes 
  exactly 
  through 
  the 
  center 
  of 
  the 
  field 
  of 
  view. 
  

  

  