﻿L. 
  V. 
  Pirsson 
  — 
  Gmelinite 
  from 
  JVova 
  Scotia. 
  61 
  

  

  over 
  the 
  twinning 
  plane 
  in 
  two 
  cases 
  and 
  found 
  to 
  be 
  2° 
  21/ 
  

   and 
  2° 
  55', 
  while 
  theory 
  wonld 
  demand 
  from 
  our 
  gmelinite 
  

   ratios 
  4° 
  16 
  ; 
  and 
  for 
  the 
  chabazite 
  2° 
  39'. 
  It 
  should 
  be 
  stated, 
  

   however, 
  that 
  the 
  two 
  p 
  faces 
  on 
  each 
  pair. 
  of 
  the 
  measured 
  

   twins 
  showed 
  the 
  low 
  vicinal 
  scalenohedron, 
  characteristic 
  of 
  

   this 
  face, 
  and 
  this 
  of 
  course 
  tended 
  to 
  diminish 
  the 
  measured 
  

   angles. 
  

  

  Indices 
  of 
  refraction. 
  — 
  Three 
  prisms 
  were 
  cut 
  from 
  differ- 
  

   ent 
  crystals, 
  by 
  using 
  the 
  plane 
  m 
  for 
  one 
  face 
  of 
  the 
  prism 
  and 
  

   grinding 
  another 
  in 
  the 
  prismatic 
  zone. 
  In 
  the 
  first 
  one 
  only 
  

   was 
  any 
  well 
  defined 
  double 
  refraction 
  detected 
  by 
  the 
  eye 
  ; 
  in 
  

   the 
  other 
  two 
  the 
  image 
  of 
  the 
  slit 
  was 
  measured 
  by 
  holding 
  

   the 
  analyzer 
  in 
  front 
  with 
  the 
  shorter 
  diagonal 
  vertical 
  and 
  

   horizontal. 
  The 
  three 
  gave 
  : 
  

  

  &?Na 
  1-4760 
  1-4646 
  1*4770 
  

  

  fNa 
  ' 
  1-4674 
  ]-4637 
  1-4765 
  

  

  There 
  is 
  therefore 
  a 
  very 
  weak 
  negative 
  double 
  refraction 
  

   which 
  varies 
  in 
  different 
  crystals, 
  the 
  averge 
  was 
  wNa 
  — 
  sNa 
  

   for 
  the 
  above 
  =-0033, 
  while 
  Negri* 
  found 
  ^Na— 
  sNa 
  ='0018 
  

   in 
  crystals 
  from 
  Montecchio 
  Maggiore. 
  

  

  Optical 
  characters. 
  — 
  In 
  a 
  section 
  cut 
  normal 
  to 
  the 
  vertical 
  

   axis 
  it 
  is 
  seen 
  under 
  the 
  microscope, 
  between 
  crossed 
  nicols, 
  

   that 
  the 
  section 
  is 
  not 
  uniformly 
  dark, 
  but 
  that 
  slight 
  optical 
  

   anomalies 
  present 
  themselves, 
  somewhat 
  as 
  in 
  leucite. 
  There 
  

   does 
  not 
  seem 
  to 
  be 
  any 
  definite 
  separation 
  into 
  parts, 
  which 
  

   would 
  show 
  the 
  crystal 
  composed 
  of 
  several 
  individuals. 
  In 
  

   strongly 
  convergent 
  light 
  the 
  uniaxial 
  interference 
  figure 
  is 
  

   seen, 
  and 
  at 
  some 
  places, 
  in 
  revolving, 
  this 
  generally 
  opens 
  a 
  

   trifle, 
  with 
  the 
  arms 
  of 
  the 
  cross 
  assuming 
  the 
  position 
  of 
  

   hyperbolas. 
  This 
  is 
  most 
  marked 
  in 
  the 
  hard 
  outer 
  shell, 
  men- 
  

   tioned 
  before, 
  where 
  a 
  small 
  but 
  distinct 
  separation 
  can 
  be 
  

   seen. 
  These 
  characters 
  explain 
  very 
  clearly 
  the 
  variation 
  in 
  

   the 
  indices 
  of 
  refraction 
  in 
  different 
  crystals 
  noted 
  above. 
  

  

  Cleavage. 
  — 
  The 
  prismatic 
  cleavage, 
  first 
  noted 
  by 
  Rose, 
  is 
  

   easily 
  produced 
  but 
  is 
  never 
  very 
  perfect. 
  In 
  a 
  basal 
  section, 
  

   under 
  the 
  microscope, 
  it 
  is 
  seen 
  as 
  a 
  series 
  of 
  cracks 
  parallel 
  to 
  

   the 
  prism 
  edges. 
  An 
  endeavor 
  to 
  determine 
  whether 
  a 
  rhom- 
  

   bohedral 
  cleavage 
  existed, 
  or 
  not, 
  met 
  with 
  only 
  partial 
  success. 
  

   A 
  series 
  of 
  fragments, 
  with 
  the 
  faces 
  of 
  the 
  prism 
  and 
  unit 
  

   rhombohedron 
  upon 
  them 
  for 
  orientation, 
  were 
  placed 
  on 
  the 
  

   goniometer. 
  Upon 
  revolving, 
  the 
  prismatic 
  cleavage 
  always 
  

   gave 
  a 
  reflection 
  and 
  in 
  a 
  number 
  of 
  cases 
  there 
  were 
  reflected 
  

   faint 
  but 
  distinct 
  signals 
  in 
  the 
  zone, 
  from 
  small 
  faces, 
  which 
  

   gave 
  measurements 
  from 
  the 
  prismatic 
  cleavage, 
  as 
  follows 
  : 
  

  

  49° 
  42', 
  49° 
  36', 
  49° 
  50', 
  49° 
  43', 
  49° 
  55', 
  50° 
  06', 
  49° 
  02' 
  

   * 
  Zeitschr. 
  f. 
  Kryst, 
  xiv, 
  p. 
  584, 
  1888. 
  

  

  