﻿L. 
  V. 
  Pirsson 
  — 
  Gmelinite 
  from 
  Nova 
  Scotia. 
  59 
  

  

  signal 
  were 
  extremely 
  good 
  and 
  it 
  is 
  not 
  far 
  from 
  the 
  average 
  

   of 
  them 
  all. 
  From 
  this 
  we 
  derive 
  the 
  axial 
  ratio 
  : 
  

  

  a 
  : 
  c 
  : 
  : 
  1 
  : 
  0*734486 
  

  

  If 
  we 
  refer 
  gmelinite 
  to 
  the 
  axial 
  ratio 
  of 
  chabazite, 
  commonly 
  

   accepted 
  where 
  r^r=S5° 
  14' 
  and 
  

  

  a 
  : 
  c 
  : 
  : 
  1 
  : 
  1-0860 
  

  

  the 
  prominent 
  rhombohedron 
  of 
  gmelinite 
  becomes 
  -§, 
  2023, 
  

   and 
  this 
  requires 
  a 
  length 
  on 
  the 
  vertical 
  axis 
  of 
  chabazite 
  of 
  

   *7240 
  and 
  a 
  polar 
  angle 
  of 
  67° 
  28'. 
  These 
  Pinnacle 
  Island 
  

   crystals 
  would 
  not 
  permit 
  of 
  so 
  great 
  an 
  error 
  in 
  the 
  determi- 
  

   nation, 
  if 
  they 
  were 
  referable 
  to 
  the 
  axis 
  of 
  chabazite. 
  The 
  

   discussion 
  of 
  this 
  point 
  will 
  be 
  referred 
  to 
  later. 
  The 
  follow- 
  

   ing 
  table 
  gives 
  the 
  calculated 
  and 
  measured 
  angles 
  which 
  show 
  

   the 
  identification 
  of 
  the 
  forms. 
  The 
  first 
  column 
  of 
  figures 
  

   gives 
  the 
  theoretical 
  angles 
  calculated 
  for 
  this 
  species, 
  the 
  

   second 
  gives 
  the 
  theoretical 
  ones 
  calculated 
  by 
  DesCloizeaux, 
  

   and 
  the 
  third 
  those 
  calculated 
  from 
  the 
  axes 
  of 
  chabazite 
  : 
  

  

  For 
  reasons 
  stated 
  before, 
  none 
  of 
  these 
  angles 
  could 
  be 
  meas- 
  

   ured 
  with 
  great 
  accuracy, 
  yet 
  the 
  averages 
  agree 
  better 
  with 
  

   the 
  theory 
  presented 
  for 
  these 
  crystals 
  than 
  that 
  given 
  by 
  Des 
  

   Cloizeaux. 
  The 
  angles 
  of 
  the 
  scalenohedron, 
  as 
  given 
  above, 
  

   was 
  measured 
  on 
  a 
  crystal 
  from 
  Two 
  Islands 
  where 
  it 
  was 
  

   present 
  almost 
  without 
  striations. 
  This 
  crystal 
  is 
  shown 
  in 
  

   lig. 
  1. 
  In 
  all 
  of 
  the 
  figures 
  the 
  crystals 
  are 
  shown 
  revolved 
  

   60° 
  into 
  the 
  position 
  of 
  a 
  minus 
  rhombohedron, 
  it 
  having 
  been 
  

   found 
  that 
  this 
  gave 
  a 
  better 
  view 
  of 
  them. 
  'With 
  the 
  excep- 
  

   tion 
  noted, 
  all 
  the 
  measurements 
  given 
  in 
  the 
  foregoing 
  are 
  

   upon 
  crystals 
  from 
  Five 
  Islands. 
  

  

  Tvnnning. 
  — 
  The 
  twinning 
  of 
  gmelinite 
  has 
  never 
  to 
  our 
  

   knowledge 
  been 
  observed, 
  beyond 
  a 
  brief 
  note 
  as 
  to 
  its 
  possi- 
  

   bility 
  in 
  an 
  article 
  by 
  Howe, 
  mentioned 
  later. 
  In 
  examining 
  

   a 
  series 
  of 
  specimens 
  I 
  have 
  discovered, 
  however, 
  numerous 
  

   instances 
  of 
  a 
  twinning 
  on 
  the 
  basal 
  plane. 
  All 
  that 
  have 
  

   been 
  observed 
  were 
  penetration 
  twins. 
  They 
  are 
  often 
  shown 
  

   by 
  the 
  growth 
  of 
  the 
  scalenohedron 
  <p 
  and 
  small 
  p 
  face, 
  as 
  pre- 
  

   sented 
  in 
  fig. 
  3, 
  directly 
  out 
  from 
  the 
  plane 
  of 
  the 
  positive 
  

   rhombohedron. 
  This 
  method 
  of 
  twinning 
  is 
  shown 
  in 
  fig. 
  2, 
  an 
  

  

  Forms. 
  

  

  Calc. 
  Dx. 
  

  

  Chab. 
  

  

  Meas. 
  

  

  

  /• 
  *r 
  1011*1101 
  

  

  *68°08' 
  67°31 
  / 
  

  

  67°2S' 
  

  

  See 
  above. 
  

  

  

  r 
  *??i 
  1011 
  * 
  1010 
  

  

  49 
  42 
  *50 
  03 
  

  

  50 
  m\ 
  

  

  49°40'-50°02' 
  

  

  av. 
  of 
  8 
  =49° 
  46' 
  

  

  r 
  *p 
  1011 
  *0U1 
  

  

  37 
  44£ 
  37 
  27 
  

  

  37 
  244. 
  

  

  37 
  30-37 
  51 
  

  

  av. 
  of 
  7=37 
  39 
  

  

  m~q 
  1010*3032 
  

  

  38 
  10 
  

  

  ■AS 
  341 
  

  

  38 
  48 
  

  

  

  O 
  *<& 
  4377 
  * 
  7347 
  

  

  29 
  21i 
  

  

  29 
  04f 
  

  

  29 
  33 
  29 
  58 
  

  

  

  r 
  ^<t> 
  1011 
  *4377 
  

  

  16 
  04£ 
  

  

  15 
  56t 
  

  

  16 
  14 
  16 
  00 
  

  

  

  m*a 
  1010*1120 
  

  

  30 
  

  

  

  

  29 
  56 
  

  

  

  m^l 
  1010*5270 
  

  

  16 
  06 
  

  

  .... 
  

  

  16 
  30 
  

  

  

  