﻿S. 
  L. 
  Penfield 
  — 
  Graftonite, 
  a 
  new 
  Mineral. 
  

  

  31 
  

  

  the 
  plane 
  of 
  lamination. 
  In 
  the 
  section 
  represented 
  by 
  fig. 
  1 
  

   the 
  extinction 
  angle 
  of 
  the 
  triphylite 
  is 
  about 
  27° 
  ; 
  hence, 
  if 
  the 
  

   hypothesis 
  which 
  is 
  proposed 
  is 
  the 
  correct 
  one, 
  the 
  section 
  

   should 
  be 
  nearly 
  though 
  not 
  quite 
  at 
  right 
  angles 
  to 
  the 
  clino- 
  

   axis 
  of 
  the 
  graftonite 
  and 
  the 
  macro-axis 
  of 
  the 
  triphylite. 
  

   Furthermore, 
  according 
  to 
  the 
  investigation 
  of 
  Penfield 
  and 
  

  

  10 
  

  

  ii 
  

  

  Pratt," 
  it 
  may 
  be 
  assumed 
  that 
  in 
  triphylite 
  having 
  about 
  33 
  

   per 
  cent, 
  of 
  FeO 
  the 
  acute 
  bisectrix 
  is 
  the 
  J-axis, 
  the 
  plane 
  of 
  

   the 
  optical 
  axes 
  being 
  100, 
  as 
  indicated 
  by 
  fig. 
  10, 
  and 
  the 
  

   divergence 
  of 
  the 
  optical 
  axes, 
  2 
  V, 
  about 
  80°. 
  Hence 
  in 
  a 
  

   section 
  similar 
  to 
  fig. 
  10, 
  at 
  right 
  angles 
  to 
  the 
  5-axis 
  of 
  the 
  

   triphylite, 
  the 
  latter 
  should 
  show 
  a 
  bisectrix 
  ; 
  while 
  if 
  slightly 
  

   inclined, 
  as 
  may 
  be 
  assumed 
  to 
  be 
  the 
  case 
  in 
  the 
  section 
  repre- 
  

   sented 
  by 
  fig. 
  1, 
  some 
  portion 
  of 
  the 
  interference 
  figure 
  ought 
  

   to 
  be 
  visible. 
  This 
  in 
  reality 
  is 
  the 
  case 
  when 
  the 
  tryphylite 
  

   lamellae 
  are 
  examined, 
  although, 
  owing 
  to 
  the 
  character 
  of 
  the 
  

   material, 
  the 
  interference 
  figures 
  are 
  not 
  very 
  distinct. 
  Thus 
  

   the 
  optical 
  properties, 
  as 
  far 
  as 
  they 
  have 
  been 
  made 
  out, 
  con- 
  

   firm 
  the 
  hypothesis 
  that 
  a 
  dome 
  face 
  of 
  the 
  triphylite 
  is 
  in 
  con- 
  

   tact 
  with 
  a 
  clinopinacoid 
  face 
  of 
  the 
  graftonite. 
  Fig. 
  11 
  

   represents 
  two 
  crystals 
  of 
  triphylite 
  orientated 
  in 
  twin 
  position 
  

   to 
  one 
  another 
  upon 
  a 
  clinopinacoid 
  face 
  of 
  graftonite. 
  Crys- 
  

   tals 
  thus 
  orientated, 
  provided 
  they 
  increased 
  in 
  size, 
  would 
  have 
  

   to 
  adapt 
  themselves 
  to 
  circumstances 
  as 
  they 
  grew 
  together, 
  

   and 
  the 
  surface 
  of 
  contact 
  between 
  them 
  would 
  probably 
  be 
  

   irregular 
  and 
  not 
  conformable 
  to 
  any 
  definite 
  crystal 
  plane. 
  

   Such 
  a 
  condition 
  may 
  have 
  existed 
  when 
  lamella 
  5, 
  fig. 
  1, 
  was 
  

   forming, 
  for 
  in 
  one 
  portion 
  the 
  extinction 
  is 
  to 
  the 
  right, 
  in 
  

   another 
  to 
  the 
  left, 
  while 
  the 
  two 
  portions 
  unite 
  along 
  an 
  

  

  *This 
  Journal, 
  III, 
  1, 
  p. 
  387, 
  1895. 
  

  

  