﻿40 
  Iddings 
  and 
  Penfield 
  — 
  Minerals 
  in 
  hollow 
  Spherulites 
  

  

  indications 
  of 
  spherical 
  zones 
  near 
  the 
  outer 
  margin 
  of 
  the 
  

   shell, 
  but 
  no 
  radial 
  iibration 
  can 
  be 
  observed 
  macroscopically. 
  

   There 
  is 
  nothing 
  in 
  the 
  arrangement 
  of 
  the 
  comparatively 
  

   large 
  crystals 
  within 
  the 
  cavity 
  which 
  suggests 
  either 
  a 
  radia- 
  

   tion 
  from 
  the 
  center, 
  or 
  the 
  concentric 
  shelly 
  structure 
  of 
  

   lithophysse. 
  

  

  An 
  examination 
  with 
  the 
  microscope 
  proves 
  that 
  there 
  is 
  a 
  

   radial 
  fibration 
  in 
  the 
  outer 
  shell 
  of 
  these 
  spherulites. 
  And 
  since 
  

   certain 
  of 
  the 
  minerals 
  which 
  are 
  characteristic 
  of 
  the 
  central 
  

   portion 
  are 
  found 
  in 
  the 
  shell 
  also, 
  4 
  it 
  is 
  evident 
  that 
  the 
  forma- 
  

   tion 
  of 
  the 
  outer 
  and 
  inner 
  parts 
  of 
  these 
  spherulites 
  was 
  con- 
  

   temporaneous. 
  There 
  are 
  also 
  small 
  irregular 
  cavities 
  contain- 
  

   ing 
  the 
  same 
  well- 
  developed 
  crystals, 
  which 
  have 
  no 
  definite 
  

   spherulitic 
  walls, 
  but 
  are 
  surrounded 
  by 
  white 
  crystalline 
  

   margins, 
  which 
  extend 
  irregularly 
  into 
  the 
  surrounding 
  rock. 
  

   The 
  same 
  thing 
  also 
  occurs 
  in 
  small 
  crystalline 
  patches 
  and 
  

   streaks 
  in 
  the 
  ground-mass 
  of 
  the 
  rock, 
  like 
  the 
  more 
  crystalline 
  

   portions 
  of 
  the 
  laminated 
  lithoidite 
  at 
  Obsidian 
  Cliff.* 
  The 
  

   massive 
  rhyolite 
  at 
  Glade 
  Creek 
  also 
  passes 
  to 
  the 
  westward 
  

   into 
  laminated 
  lithoidal 
  rhyolite 
  with 
  open 
  layers 
  filled 
  with 
  

   the 
  same 
  minerals 
  as 
  those 
  in 
  the 
  hollow 
  spherulites. 
  

  

  The 
  light 
  colored 
  crystalline 
  portions 
  just 
  mentioned, 
  when, 
  

   examined 
  with 
  a 
  lens, 
  are 
  found 
  to 
  be 
  dotted 
  with 
  minute 
  round 
  

   pits 
  about 
  as 
  large 
  as 
  the 
  point 
  of 
  a 
  pin. 
  At 
  first 
  sight 
  they 
  

   appear 
  to 
  be 
  small 
  colorless 
  grains 
  of 
  some 
  mineral 
  like 
  

   quartz, 
  but 
  closer 
  investigation 
  shows 
  -them 
  to 
  be 
  hollow. 
  Their 
  

   relation 
  to 
  the 
  crystalline 
  material 
  about 
  them 
  is 
  revealed 
  by 
  a 
  

   microscopical 
  study 
  of 
  the 
  rock. 
  

  

  Thin 
  sections 
  of 
  the 
  rock 
  show 
  it 
  to 
  be 
  a 
  rhyolite 
  similar 
  to 
  

   much 
  of 
  the 
  lithoidal 
  rhyolite 
  of 
  the 
  neighboring 
  region, 
  except 
  

   for 
  a 
  greater 
  amount 
  of 
  angite 
  phenocrysts. 
  The 
  porphy- 
  

   ritical 
  quartz, 
  sanidine 
  and 
  plagioclase 
  need 
  no 
  special 
  mention, 
  

   being 
  like 
  those 
  of 
  most 
  rhyolites. 
  Magnetite 
  forms 
  quite 
  

   large 
  grains, 
  associated 
  with 
  the 
  augite, 
  often 
  having 
  zircon 
  

   crystals 
  attached 
  to 
  them. 
  The 
  augite 
  is 
  light 
  greenish 
  yellow, 
  

   and 
  is 
  somewhat 
  rounded. 
  It 
  is 
  partly 
  altered 
  to 
  brown 
  iron 
  

   oxide, 
  which 
  penetrates 
  cracks 
  in 
  the 
  crystals. 
  In 
  some 
  

   instances 
  it 
  is 
  entirely 
  decomposed, 
  leaving 
  a 
  pseudomorph 
  of 
  

   brown 
  iron 
  oxide. 
  

  

  The 
  ground-mass 
  of 
  the 
  rock 
  is 
  spherulitic 
  throughout, 
  with 
  

   here 
  and 
  there 
  spaces 
  between 
  groups 
  of 
  spherulites 
  which 
  are 
  

   composed 
  of 
  crystals 
  of 
  feldspar 
  with 
  tridymite 
  or 
  quartz. 
  

   Short 
  opaque 
  trichites 
  and 
  sharply 
  defined 
  crystals 
  of 
  magnetite 
  

   are 
  scattered 
  uniformly 
  through 
  the 
  mass, 
  or 
  are 
  arranged 
  in 
  

   lines 
  which 
  mark 
  the 
  flow 
  structure. 
  The 
  microscopic 
  spheru- 
  

  

  * 
  L. 
  c, 
  p. 
  264. 
  

  

  