﻿30 
  8. 
  L. 
  Pen 
  field 
  — 
  Graftonite, 
  a 
  new 
  Mineral. 
  

  

  is 
  almost 
  parallel 
  to 
  the 
  prismatic 
  face 
  m, 
  1 
  10, 
  of 
  the 
  grafton- 
  

   ite, 
  as 
  shown 
  by 
  the 
  fact 
  that 
  the 
  edges 
  between 
  m 
  and 
  d 
  and 
  

   W 
  and 
  D, 
  fig. 
  9, 
  form 
  almost 
  a 
  continuous 
  line, 
  or 
  from 
  fig. 
  

   8, 
  that 
  the 
  angle 
  between 
  d 
  and 
  m 
  (0L1 
  ^ 
  110 
  = 
  54:° 
  56') 
  is 
  

   almost 
  the 
  same 
  as 
  that 
  between 
  W 
  and 
  D 
  (102^021 
  = 
  53° 
  54/) 
  

  

  The 
  foregoing 
  demonstrations 
  furnish 
  a 
  working 
  hypothesis 
  

   for 
  explaining 
  how 
  the 
  two 
  minerals 
  might 
  possibly 
  have 
  

   grown 
  together 
  because 
  of 
  similarity 
  in 
  certain 
  prominent 
  crys- 
  

   tallographic 
  features, 
  and 
  it 
  remains 
  to 
  be 
  shown 
  to 
  what 
  

   extent 
  the 
  hypothesis 
  conforms 
  to 
  the 
  structural 
  features 
  pre- 
  

   sented 
  by 
  the 
  intergrowth 
  of 
  the 
  two 
  minerals. 
  Iu 
  fig. 
  10 
  

   there 
  is 
  represented 
  in 
  the 
  center 
  an 
  ideal 
  cross-section 
  of 
  a 
  

   graftonite 
  crystal, 
  at 
  right 
  angles 
  to 
  the 
  clino-axis, 
  and 
  showing 
  

   the 
  outlines 
  of 
  the 
  forms 
  d 
  and 
  o. 
  Such 
  a 
  section 
  should 
  

   exhibit 
  in 
  polarized 
  light 
  parallel 
  extinction, 
  as 
  is 
  the 
  case 
  

   with 
  the 
  graftonite 
  in 
  tig. 
  1. 
  On 
  either 
  side 
  of 
  the 
  graftonite 
  

   in 
  fig. 
  10 
  are 
  represented 
  sections 
  of 
  triphylite 
  crystals 
  at 
  right 
  

   angles 
  to 
  the 
  macro-axis 
  b 
  and 
  so 
  orientated 
  that 
  faces 
  of 
  the 
  

   macrodome 
  W 
  are 
  parallel 
  to 
  and 
  in 
  contact 
  with 
  the 
  clino- 
  

   pinacoid 
  h 
  of 
  the 
  graftonite; 
  compare 
  fig. 
  9. 
  Such 
  sections 
  

   should 
  show 
  extinction 
  angles 
  of 
  31° 
  to 
  the 
  right 
  and 
  to 
  the 
  left 
  

   of 
  the 
  interposed 
  graftonite, 
  as 
  indicated 
  by 
  the 
  arrows. 
  The 
  

   structure 
  indicated 
  by 
  the 
  ideal 
  section, 
  fig. 
  10, 
  corresponds 
  

   to 
  the 
  optical 
  properties 
  of 
  the 
  minerals 
  in 
  the 
  section 
  repre- 
  

   sented 
  by 
  fig. 
  1, 
  where 
  the 
  interlaminated 
  triphylite 
  (compare 
  

   lamellae 
  1, 
  2 
  and 
  3) 
  shows 
  inclined 
  extinction, 
  symmetrically 
  

   disposed 
  to 
  the 
  right 
  and 
  to 
  the 
  left 
  of 
  the 
  plane 
  of 
  lamination. 
  

   If 
  a 
  cross 
  section 
  of 
  the 
  composite 
  crystal 
  were 
  at 
  right 
  angles 
  

   to 
  the 
  symmetry 
  plane 
  of 
  the 
  graftonite, 
  but 
  not 
  at 
  right 
  angles 
  

   to 
  the 
  clino-axis, 
  then 
  the 
  extinction 
  angles 
  of 
  the 
  triphylite 
  

   would 
  be 
  less 
  than 
  31°, 
  but 
  still 
  remain 
  equal 
  on 
  either 
  side 
  of 
  

  

  