﻿of 
  Rhyolitefrom 
  Glade 
  Creek, 
  Wyoming. 
  45 
  

  

  made 
  in 
  the 
  vertical 
  zone 
  m, 
  j, 
  r, 
  over 
  the 
  apex 
  of 
  the 
  crystal 
  

   on 
  z, 
  <r, 
  m. 
  The 
  prism 
  faces 
  were 
  always 
  so 
  much 
  striated 
  that 
  

   no 
  satisfactory 
  measurements 
  could 
  be 
  made 
  from 
  them. 
  The 
  

   measurements 
  are 
  as 
  follows 
  : 
  

  

  Calculated. 
  Measured. 
  

   r 
  a 
  z 
  over 
  base, 
  103° 
  34' 
  103° 
  35£ 
  103° 
  33' 
  103° 
  34' 
  103° 
  35 
  103° 
  35' 
  

   fAjorzAd, 
  10 
  3L 
  10 
  42 
  10 
  47 
  10 
  35 
  10 
  29 
  10 
  45 
  10° 
  35' 
  10° 
  19' 
  

   r 
  a 
  z 
  adjoining, 
  46 
  16 
  46 
  17 
  

   »-aN, 
  17 
  23 
  17 
  19 
  

  

  On 
  several 
  crystals 
  a 
  second 
  rhombohedron 
  having 
  the 
  

   symbol 
  ^f-, 
  (10*0. 
  10*7) 
  was 
  observed, 
  occurring 
  either 
  alone 
  

   with 
  7*, 
  z 
  and 
  m, 
  or 
  between 
  j 
  and 
  r 
  and 
  a 
  and 
  z. 
  It 
  had 
  a 
  

   relatively 
  large 
  size 
  and 
  gave 
  distinct 
  reflections 
  ; 
  its 
  measure- 
  

   ment 
  on 
  to 
  r 
  and 
  z, 
  is 
  as 
  follows 
  : 
  

  

  Calculated. 
  Measured. 
  

  

  9° 
  21' 
  9° 
  45', 
  9° 
  25', 
  9° 
  38' 
  9° 
  52', 
  9° 
  43'. 
  

  

  The 
  occurrence 
  in 
  the 
  hollow 
  spherulites 
  of 
  this 
  very 
  unusual 
  

   development 
  of 
  quartz, 
  as 
  well 
  as 
  its 
  association 
  with 
  the 
  rare 
  

   mineral 
  fayalite, 
  may 
  be 
  taken 
  to 
  indicate 
  that 
  the 
  crystals 
  were 
  

   formed 
  under 
  conditions 
  which 
  do 
  not 
  usually 
  prevail. 
  On 
  

   the 
  highly 
  modified 
  quartz 
  crystals 
  from 
  Alexander 
  Co., 
  N". 
  C, 
  

   j, 
  a 
  and 
  L 
  were 
  frequently 
  observed 
  by 
  vom 
  Rath,* 
  but 
  the 
  

   crystals 
  from 
  Glade 
  Creek, 
  and 
  Obsidian 
  Cliff, 
  are 
  very 
  different 
  

   in 
  showing 
  these 
  rare 
  forms 
  well- 
  developed 
  on 
  otherwise 
  very 
  

   simple 
  crystals. 
  

  

  Tridymite 
  is 
  present 
  in 
  some 
  of 
  the 
  cavities 
  in 
  characteristic 
  

   crossed 
  twins, 
  and 
  is 
  abundant 
  in 
  thin 
  sections 
  of 
  the 
  rock. 
  

  

  The 
  most 
  noticeable 
  mineral 
  next 
  to 
  quartz 
  is 
  fayalite. 
  It 
  

   forms 
  stout 
  crystals 
  about 
  l 
  mm 
  long 
  with 
  very 
  much 
  the 
  same 
  

   habit 
  as 
  those 
  represented 
  by 
  fig. 
  2, 
  in 
  our 
  paper 
  " 
  On 
  the 
  occur- 
  

   rence 
  of 
  Fayalite 
  in 
  the 
  lithophysse 
  of 
  obsidian 
  and 
  rhyolite 
  in 
  

   the 
  Yellowstone 
  National 
  Park,"f 
  or 
  by 
  fig. 
  54 
  in 
  the 
  paper 
  

   on 
  Obsidian 
  Cliff 
  already 
  cited.J 
  They 
  have 
  undergone 
  more 
  

   or 
  less 
  alteration 
  to 
  iron 
  oxide 
  and 
  are 
  now 
  opaque 
  and 
  black. 
  

   Some 
  are 
  still 
  transparent 
  at 
  the 
  centre. 
  When 
  tested 
  chemi- 
  

   cally 
  they 
  give 
  decided 
  reactions 
  for 
  both 
  iron 
  and 
  magnesium. 
  

   This 
  may 
  indicate 
  that 
  the 
  unaltered 
  fayalite 
  is 
  rich 
  in 
  mag- 
  

   nesium. 
  There 
  is 
  not 
  sufficient 
  unaltered 
  material 
  at 
  hand 
  to 
  

   undertake 
  a 
  complete 
  chemical 
  analysis. 
  The 
  occurrence 
  of 
  

   the 
  fayalite 
  at 
  G-lade 
  Creek 
  is 
  quite 
  the 
  same 
  as 
  that 
  in 
  other 
  

   hollow 
  spherulites 
  in 
  the 
  rhyolites 
  at 
  various 
  localities 
  in 
  the 
  

   Yellowstone 
  National 
  Park. 
  

  

  *Zeitsclir. 
  Kryst., 
  x, 
  p. 
  156. 
  % 
  L. 
  c. 
  p. 
  271. 
  

  

  f 
  This 
  Journal, 
  vol. 
  xxx, 
  July, 
  1885, 
  p. 
  59. 
  

  

  