﻿44 
  Iddings 
  and 
  Penfield 
  — 
  Minerals 
  in 
  hollow 
  Sphemdites 
  

  

  crystals 
  from 
  Traversella. 
  This 
  face 
  is 
  always 
  brilliant 
  but 
  so 
  

   much 
  rounded 
  that 
  upon 
  measuring 
  upon 
  z 
  one 
  can 
  indiffer- 
  

   ently 
  arrive 
  at 
  a 
  number 
  of 
  approximations 
  according 
  as 
  one 
  

   stops 
  at 
  the 
  upper, 
  central 
  or 
  lower 
  part 
  of 
  the 
  broad 
  reflection 
  

   which 
  it 
  furnishes." 
  On 
  the 
  crystals 
  from 
  Glade 
  Creek, 
  N 
  

   and 
  N' 
  and 
  L 
  and 
  1/ 
  are 
  faultless 
  as 
  regards 
  luster 
  and 
  

   absence 
  of 
  striations 
  and 
  rounding. 
  An 
  idea 
  of 
  the 
  promi- 
  

   nence 
  of 
  these 
  faces 
  may 
  be 
  obtained 
  from 
  figs. 
  1, 
  2 
  and 
  3 
  

   where 
  their 
  relative 
  size 
  and 
  development 
  on 
  three 
  of 
  the 
  

   measured 
  crystals 
  has 
  been 
  preseived 
  as 
  far 
  as 
  possible. 
  . 
  They 
  

   undoubtedly 
  occur 
  both 
  as 
  positive 
  r 
  and 
  negative 
  r, 
  and 
  as 
  

   positive 
  I 
  and 
  negative 
  I 
  ; 
  their 
  persistency 
  in 
  replacing 
  all 
  of 
  

   the 
  edges 
  between^' 
  and 
  <r 
  would 
  indicate 
  this 
  as 
  well 
  as 
  the 
  

   results 
  of 
  an 
  experiment 
  in 
  etching 
  one 
  of 
  the 
  crystals 
  with 
  

   hydrofluoric 
  acid. 
  The 
  crystal 
  represented 
  in 
  fig. 
  1, 
  was 
  thus- 
  

   proved 
  to 
  be 
  a 
  right-handed 
  twin. 
  The 
  greater 
  part 
  of 
  r 
  and 
  

   j 
  in 
  front, 
  and 
  all 
  of 
  !N 
  were 
  positive, 
  as 
  was 
  also 
  the 
  greater 
  

   portion 
  of 
  the 
  faces 
  lettered 
  z 
  and 
  <r, 
  the 
  twinning 
  boundaries 
  

   running 
  very 
  unequally 
  over 
  these 
  faces 
  ; 
  while 
  the 
  face 
  lettered 
  

   !N' 
  was 
  both 
  positive 
  and 
  negative, 
  the 
  positive 
  part 
  being 
  deeply 
  

   etched 
  while 
  the 
  acid 
  had 
  almost 
  no 
  action 
  on 
  the 
  negative 
  

   portion. 
  Left-handed 
  crystals 
  were 
  not 
  etched, 
  but 
  it 
  is 
  safe 
  to 
  

   infer 
  from 
  the 
  development 
  of 
  L 
  and 
  L', 
  that 
  they 
  are 
  both 
  

   positive 
  and 
  negative. 
  Right 
  and 
  left 
  forms 
  were 
  not 
  observed 
  

   in 
  the 
  same 
  crystal. 
  On 
  the 
  goniometer 
  the 
  reflections 
  from 
  

   all 
  of 
  the 
  faces 
  except 
  m 
  were 
  very 
  perfect, 
  and 
  the 
  following 
  

   measurements 
  were 
  made. 
  

  

  35' 
  

  

  We 
  have 
  also 
  examined 
  the 
  quartz 
  crystals 
  in 
  the 
  litli- 
  

   ophysse 
  of 
  Obsidian 
  Cliff, 
  Yellowstone 
  National 
  Park, 
  and 
  find 
  

   that 
  they 
  too 
  have 
  the 
  habit 
  which 
  we 
  have 
  just 
  described. 
  

   They 
  are 
  always 
  very 
  small, 
  seldom 
  over 
  ^ 
  mm 
  in 
  diameter, 
  but 
  

   some 
  were 
  found 
  which 
  were 
  so 
  perfect 
  that 
  they 
  gave 
  excel- 
  

   lent 
  reflections 
  and 
  could 
  be 
  accurately 
  measured 
  on 
  the 
  goni- 
  

   ometer. 
  They 
  generally 
  have 
  the 
  habit 
  represented 
  in 
  figs. 
  1 
  

   and 
  2, 
  although 
  sometimes^' 
  and 
  a 
  were 
  as 
  fully 
  developed 
  as 
  

   in 
  Oig. 
  3. 
  The 
  N 
  and 
  L 
  faces 
  seldom 
  failed. 
  The 
  crystals 
  

   were 
  so 
  small 
  that 
  the 
  positive 
  and 
  negative 
  character 
  of 
  the 
  

   rhombohedrons 
  could 
  not 
  well 
  be 
  distinguished. 
  In 
  most 
  

   cases 
  however 
  a 
  § 
  rhombohedron 
  was 
  observed 
  between 
  the 
  

   unit 
  rhombohedron 
  and 
  prism. 
  Measurements 
  were 
  mostly 
  

  

  

  Calculated. 
  

  

  

  Measured. 
  

  

  

  2Af 
  

  

  46° 
  15' 
  52" 
  

  

  46° 
  15' 
  

  

  

  

  r, 
  N 
  (NorL) 
  

  

  17° 
  22' 
  43" 
  

  

  17° 
  24' 
  

  

  

  

  j 
  * 
  & 
  

  

  52° 
  33' 
  20" 
  

  

  52° 
  32' 
  

  

  52° 
  33' 
  52° 
  32^' 
  

  

  52 
  

  

  j 
  a 
  (N 
  or 
  L) 
  

  

  16° 
  56' 
  

  

  16° 
  55+'' 
  

  

  16° 
  57' 
  16° 
  57' 
  

  

  

  r 
  ^jorz^ff 
  

  

  10° 
  31' 
  

  

  10° 
  33'" 
  

  

  10° 
  32' 
  

  

  

  