﻿60 
  

  

  L. 
  V. 
  Pirsson 
  — 
  Gmelinite 
  from 
  Nova 
  Scotia. 
  

  

  example 
  which 
  did 
  not 
  have 
  the 
  centers 
  of 
  the 
  two 
  individuals 
  

   coincident. 
  Further, 
  the 
  figure 
  shows 
  a 
  common 
  habit 
  in 
  the 
  

   development 
  of 
  the 
  forms. 
  The 
  second 
  method 
  of 
  twinning 
  is 
  

   that 
  in 
  which 
  the 
  J- 
  rhombohedron 
  becomes 
  the 
  twinning 
  plane. 
  

  

  This 
  was 
  first 
  seen 
  on 
  a 
  specimen 
  from 
  Parsborough, 
  which 
  

   presented 
  a 
  number 
  of 
  examples 
  ; 
  it 
  was 
  afterwards 
  observed 
  

   on 
  a 
  number 
  of 
  other 
  specimens. 
  In 
  all 
  of 
  these 
  the 
  twins 
  

   were 
  large 
  sized 
  crystals. 
  An 
  example 
  of 
  this 
  method 
  of 
  twin- 
  

   ning 
  is 
  shown 
  in 
  fig. 
  4, 
  as 
  well 
  as 
  another 
  modification 
  of 
  the 
  

   habit. 
  The 
  angle 
  r^r 
  was 
  measured 
  over 
  the 
  twinning 
  plane 
  

   in 
  eight 
  cases 
  with 
  the 
  following 
  results 
  : 
  

  

  26° 
  04', 
  26° 
  05', 
  25° 
  58', 
  25° 
  59', 
  25° 
  48', 
  25° 
  42', 
  25° 
  45', 
  25° 
  33' 
  

  

  the 
  average 
  of 
  which 
  would 
  give 
  an 
  angle 
  of 
  r 
  on 
  the 
  twinning 
  

   plane 
  of 
  77° 
  0', 
  and 
  this 
  shows 
  the 
  latter 
  to 
  be 
  the 
  f 
  rhombo- 
  

   hedron. 
  If 
  we 
  use 
  the 
  elements 
  already 
  given 
  (and 
  the 
  angle 
  

   r^r 
  was 
  measured 
  on 
  one 
  of 
  the 
  best 
  of 
  these 
  crystals 
  as 
  

   68° 
  09' 
  agreeing 
  closely 
  with 
  that 
  given 
  as 
  theoretical) 
  the 
  

   angles 
  given 
  above 
  would 
  be 
  in 
  theory 
  25° 
  04'. 
  If, 
  however, 
  

   we 
  use 
  the 
  elements 
  of 
  chabazite 
  and 
  consider 
  the 
  prominent 
  

   rhombohedron 
  on 
  the 
  gmelinite 
  as 
  -§, 
  our 
  twinning 
  plane 
  

   becomes 
  the 
  unit 
  rhombohedron 
  of 
  chabazite 
  and 
  the 
  theoreti- 
  

   cal 
  re-entrant 
  angle 
  between 
  the 
  f 
  rhombohedrons 
  twinning 
  on 
  

   this 
  plane 
  would 
  be 
  26° 
  18f 
  r 
  . 
  The 
  last 
  three 
  measured 
  angles, 
  

   which 
  were 
  the 
  best, 
  are 
  then 
  about 
  half-way 
  between 
  these 
  

   two 
  calculated 
  angles. 
  In 
  like 
  manner 
  p^p 
  was 
  measured 
  

  

  