﻿Rogers 
  — 
  Sphalerite 
  Crystals 
  of 
  a 
  Peculiar 
  Habit. 
  135 
  

  

  goniometer 
  _ 
  were 
  possible. 
  The 
  angle 
  o-/\cr 
  over 
  the 
  edge 
  

   £ 
  833 
  /\ 
  833) 
  was 
  measured 
  with 
  a 
  contact 
  goniometer 
  giving 
  

   the 
  following 
  results 
  : 
  

  

  Limits. 
  Average. 
  Calculated. 
  

  

  5- 
  O 
  - 
  H< 
  

  

  55° 
  43' 
  55° 
  524' 
  

  

  <j/\(t 
  (833 
  /s 
  833) 
  12 
  measurements 
  

  

  Angle 
  measurements 
  for 
  the 
  same 
  form 
  on 
  crystals 
  of 
  normal 
  

   habit 
  from 
  other 
  localities 
  in 
  this 
  district 
  confirm 
  these 
  results. 
  

   They 
  are 
  as 
  follows 
  : 
  

  

  Limits. 
  Average. 
  Calculated. 
  

  

  (T/s 
  o- 
  (833 
  /s 
  833) 
  18 
  measurements 
  55°-56° 
  30' 
  55° 
  46' 
  55°52£' 
  

  

  Occasionally 
  the 
  negative 
  tetrahedron 
  is 
  present. 
  The 
  

   rough 
  surface 
  which 
  it 
  exhibits 
  is 
  characteristic 
  of 
  all 
  the 
  sphal- 
  

   erite 
  crystals 
  of 
  the 
  district. 
  

  

  Twins 
  are 
  more 
  common 
  than 
  the 
  simple 
  crystals, 
  which 
  is 
  

   also 
  true 
  of 
  all 
  the 
  sphalerite 
  crystals 
  of 
  the 
  district. 
  The 
  

   twinning 
  plane 
  is 
  the 
  ordinary 
  one 
  for 
  sphalerite, 
  o 
  (111) 
  1. 
  

   As 
  no 
  reentrant 
  angles 
  are 
  present, 
  the 
  twins 
  may 
  easily 
  be 
  mis- 
  

   taken 
  for 
  simple 
  crystals, 
  but 
  they 
  may 
  be 
  recognized 
  as 
  such 
  

   by 
  the 
  fact 
  that 
  the 
  prominent 
  dodecahedral 
  faces 
  have 
  no 
  

   other 
  dodecahedral 
  faces 
  parallel 
  to 
  them, 
  and 
  by 
  the 
  further 
  

   fact 
  that 
  in 
  the 
  positive 
  octants 
  only 
  two 
  of 
  the 
  a 
  faces 
  are 
  

   united 
  at 
  one 
  point 
  while 
  in 
  the 
  simple 
  crystals 
  three 
  meet 
  

   around 
  this 
  point. 
  

  

  Becke- 
  has 
  shown 
  that 
  the 
  positive 
  and 
  negative 
  character 
  

   of 
  the 
  forms 
  on 
  sphalerite 
  may 
  be 
  distinguished 
  by 
  means 
  of 
  

   etch-figures. 
  He 
  used 
  hydrochloric 
  acid, 
  but 
  fortunately 
  some 
  

   of 
  the 
  crystals 
  here 
  described 
  exhibit 
  natural 
  etch-figures 
  on 
  

   the 
  faces 
  of 
  the 
  dodecahedron. 
  One 
  large 
  twinned 
  crystal 
  

   measuring 
  25 
  mm 
  by 
  30 
  mm 
  shows 
  them 
  very 
  well. 
  

  

  The 
  etch-figures 
  consist 
  of 
  " 
  etch-hills 
  "f 
  which 
  in 
  plan 
  

   appear 
  as 
  monosymmetric 
  four 
  sided 
  figures, 
  fig. 
  3, 
  with 
  the 
  

  

  *Becke, 
  Tschermak's 
  Min. 
  u. 
  Petr. 
  Mitth., 
  vol. 
  v, 
  p. 
  457, 
  1883. 
  

  

  f 
  For 
  a 
  recent 
  discussion 
  of 
  the 
  nomenclature 
  of 
  etch-figures 
  see 
  R. 
  A. 
  Daly, 
  

   A 
  Comparative 
  Study 
  of 
  Etch-Figures; 
  The 
  Amphiboles 
  and 
  Pyroxenes. 
  Proc. 
  

   Am. 
  Acad, 
  of 
  Arts 
  and 
  Sciences, 
  vol. 
  xxxiv, 
  pp. 
  375 
  et 
  seq. 
  

  

  