﻿42 
  Iddings 
  and 
  Penfield 
  — 
  Minerals 
  in 
  holloiv 
  Spherulites 
  

  

  times 
  occupies 
  the 
  place 
  of 
  tridymite 
  between 
  the 
  feldspar 
  

   crystals. 
  

  

  The 
  mineral 
  which 
  is 
  most 
  abundant 
  in 
  the 
  hollow 
  spheru- 
  

   lites 
  is 
  quartz, 
  occurring 
  in 
  stout 
  crystals, 
  seldom 
  over 
  2 
  mm 
  in 
  

   diameter 
  (in 
  one 
  instance 
  5 
  mm 
  ), 
  very 
  transparent 
  and 
  with 
  a 
  

   pale 
  smoky 
  color 
  ; 
  also 
  in 
  slender 
  white 
  prisms, 
  10 
  mm 
  long. 
  

   The 
  latter 
  are 
  sometimes 
  clear 
  and 
  transparent 
  in 
  part, 
  but 
  

   are 
  mostly 
  full 
  of 
  cracks, 
  and 
  many 
  of 
  'them 
  are 
  covered 
  with 
  

   a 
  crust 
  of 
  hyalite. 
  The 
  hyalite 
  is 
  isotropic, 
  and 
  has 
  minute 
  

   microlites 
  of 
  feldspar 
  scattered 
  through 
  it. 
  Both 
  the 
  stout, 
  

   clear 
  quartz 
  crystals, 
  and 
  the 
  slender 
  white 
  prisms 
  occur 
  to- 
  

   gether 
  in 
  the 
  same 
  spherulite, 
  and 
  in 
  a 
  number 
  of 
  instances 
  

   it 
  was 
  observed 
  that 
  the 
  clear 
  crystals 
  are 
  deposited 
  on 
  a 
  

   nearly 
  flat 
  side 
  of 
  the 
  cavity, 
  while 
  the 
  white 
  prisms, 
  inter- 
  

   secting 
  in 
  all 
  directions, 
  make 
  up 
  a 
  sort 
  of 
  net 
  work 
  which 
  

   rises 
  above 
  it 
  in 
  a 
  dome-shaped 
  mass. 
  The 
  first 
  impression 
  

   is 
  that 
  the 
  transparent 
  quartz 
  crystallized 
  in 
  a 
  shallow 
  basin 
  

   in 
  a 
  liquid 
  while 
  the 
  upper 
  portion 
  crystallized 
  in 
  a 
  vapor. 
  

   This 
  hypothesis 
  is, 
  however, 
  untenable, 
  since 
  in 
  some 
  cases, 
  

   the 
  transparent 
  crystals 
  in 
  the 
  hollows 
  of 
  one 
  rock 
  specimen, 
  

  

  

  

  coat 
  walls 
  which 
  are 
  not 
  symmetrically 
  disposed 
  to 
  one 
  

   another 
  and 
  hence 
  could 
  not 
  represent 
  the 
  same 
  water 
  level. 
  

   Transparent, 
  stout, 
  quartz 
  crystals 
  are 
  attached 
  to 
  the 
  walls 
  

   of 
  the 
  cavity 
  so 
  that 
  only 
  one 
  termination 
  or 
  one 
  side 
  of 
  

   the 
  prism 
  is 
  free 
  ; 
  in 
  the 
  net 
  work 
  of 
  slender 
  white 
  prisms, 
  

   however, 
  doubly 
  terminated 
  crystals 
  occur. 
  These 
  quartz 
  

   crystals 
  proved 
  on 
  examination 
  to 
  be 
  very 
  interesting. 
  They 
  

   are 
  not 
  highly 
  modified, 
  but 
  possess 
  some 
  faces 
  with 
  very 
  

   simple 
  indices 
  which 
  are 
  exceedingly 
  rare, 
  even 
  on 
  highly 
  

   modified 
  quartz 
  crystals, 
  giving 
  therefore 
  a 
  type 
  of 
  crystalliz- 
  

   ation 
  which, 
  to 
  our 
  knowledge, 
  is 
  altogether 
  new 
  for 
  this 
  

   common 
  mineral. 
  They 
  all_ 
  show, 
  in 
  addition 
  to 
  the 
  common 
  

   quartz 
  forms 
  (prism 
  m, 
  1010, 
  I_, 
  always 
  horizontally 
  striated, 
  

   and 
  the 
  rhombohedrons 
  r, 
  lOll, 
  1 
  and 
  z, 
  0111, 
  -1) 
  steep 
  

  

  