﻿of 
  Rhyolite 
  from 
  Glade 
  Creek, 
  Wyoming. 
  41 
  

  

  lites 
  are 
  distinctly 
  radially 
  fibrous, 
  the 
  rays 
  being 
  relatively 
  

   coarse 
  or 
  prismatic. 
  The 
  outline 
  of 
  the 
  spherules 
  is 
  not 
  evenly 
  

   circular, 
  but 
  irregularly 
  jagged, 
  especially 
  when 
  they 
  adjoin 
  

   areas 
  of 
  tridymite 
  and 
  quartz. 
  Here 
  the 
  rays 
  of 
  the 
  spherules 
  

   develop 
  into 
  definite 
  prismatic 
  crystals, 
  and 
  have 
  the 
  optical 
  

   characters 
  of 
  orthoclase 
  in 
  prisms 
  elongated 
  parallel 
  to 
  the 
  

   inclined 
  axis, 
  a. 
  They 
  have 
  a 
  slight 
  extinction 
  angle, 
  reach- 
  

   ing 
  10°, 
  and 
  have 
  the 
  axis 
  of 
  greatest 
  elasticity 
  parallel 
  to 
  the 
  

   length 
  of 
  the 
  prism. 
  The 
  spherulites, 
  therefore, 
  behave 
  as 
  

   though 
  made 
  up 
  of 
  optically 
  negative 
  prisms. 
  In 
  one 
  rock 
  

   section 
  they 
  appear 
  to 
  have 
  more 
  of 
  a 
  granophyric 
  structure, 
  

   with 
  a 
  feather-like 
  texture 
  within 
  the 
  feldspar 
  prisms. 
  The 
  

   fine 
  fibers 
  producing 
  this 
  effect 
  do 
  not 
  reach 
  the 
  end 
  of 
  

   the 
  best 
  developed 
  prisms, 
  leaving 
  them 
  terminated 
  by 
  clear 
  

   feldspar 
  substances, 
  as 
  in 
  the 
  case 
  of 
  the 
  granophyric 
  pheno- 
  

   cryst 
  in 
  the 
  rhyolite 
  of 
  Eureka, 
  Nevada, 
  described 
  in 
  the 
  

   article 
  on 
  Obsidian 
  Cliff 
  already 
  referred 
  to.* 
  In 
  these 
  

   spherulites 
  the 
  presence 
  of 
  quartz 
  within 
  the 
  feldspar 
  is 
  

   indicated 
  by 
  this 
  micro-structure, 
  but 
  in 
  the 
  first 
  mentioned 
  

   spherulites 
  there 
  is 
  nothing 
  to 
  suggest 
  its 
  presence, 
  except 
  

   the 
  highly 
  siliceous 
  nature 
  of 
  the 
  rock. 
  Since 
  it 
  is 
  only 
  

   the 
  marginal 
  terminations 
  of 
  the 
  feldspar 
  prisms 
  which 
  are 
  

   determinable 
  as 
  such, 
  the 
  central 
  portion 
  of 
  the 
  spherulites 
  

   may 
  be 
  more 
  complex 
  without 
  its 
  being 
  recognized, 
  for 
  a 
  

   small 
  amount 
  of 
  quartz 
  would 
  not 
  materially 
  affect 
  the 
  optical 
  

   character 
  of 
  the 
  feldspar. 
  The 
  light 
  colored, 
  crystalline 
  por- 
  

   tions 
  of 
  the 
  rock 
  with 
  the 
  minute 
  pits 
  are 
  seen 
  under 
  the 
  

   microscope 
  to 
  be 
  more 
  highly 
  crystallized 
  parts 
  of 
  the 
  ground- 
  

   mass. 
  They 
  combine 
  the 
  spherulitic 
  structure 
  with 
  a 
  more 
  or 
  

   less 
  granular 
  one. 
  The 
  little 
  cavities 
  are 
  found 
  to 
  be 
  hollows 
  

   at 
  the 
  centre 
  of 
  small 
  feldspar 
  spherulites, 
  which 
  are 
  made 
  up 
  

   of 
  feldspar 
  prisms 
  whose 
  ends 
  project 
  irregularly 
  inward 
  into 
  

   the 
  cavities 
  and 
  outward 
  into 
  the 
  adjoining 
  minerals. 
  The 
  

   cavities 
  appear 
  to 
  be 
  minute 
  spots 
  once 
  occupied 
  by 
  vapor 
  or 
  

   some 
  liquid, 
  around 
  which 
  feldspar 
  crystallized 
  in 
  prisms 
  radia- 
  

   ting 
  outward. 
  In 
  the 
  crystalline 
  patches 
  the 
  tridymite 
  lies 
  in 
  

   various 
  orientations, 
  and 
  through 
  it 
  in 
  all 
  directions 
  run 
  what 
  

   look 
  like 
  transparent 
  needles, 
  which 
  in 
  some 
  cases 
  also 
  radiate 
  

   out 
  from 
  the 
  coarser 
  micro-spherulites. 
  They 
  are 
  dull 
  between 
  

   crossed 
  nicols, 
  and 
  might 
  easily 
  be 
  mistaken 
  for 
  apatite, 
  but 
  

   their 
  optical 
  characters 
  are 
  also 
  those 
  of 
  sanidine 
  prisms 
  that 
  

   have 
  developed 
  parallel 
  to 
  the 
  axis 
  of 
  greatest 
  elasticity. 
  This 
  

   is 
  shown 
  to 
  be 
  the 
  case 
  in 
  a 
  thin 
  section 
  of 
  another 
  rhyolite 
  in 
  

   which 
  the 
  same 
  structure 
  has 
  been 
  developed 
  on 
  a 
  somewhat 
  

   larger 
  scale. 
  In 
  the 
  rhyolite 
  from 
  Grlade 
  Creek, 
  quartz 
  some- 
  

  

  * 
  L. 
  c, 
  p. 
  275, 
  Plate 
  XT, 
  fig. 
  5. 
  

  

  