﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  355 
  

  

  it 
  seems 
  that 
  the 
  direction 
  of 
  elongation 
  is 
  always 
  that 
  of 
  cubic 
  

   axes. 
  In 
  many 
  cases 
  the 
  direction 
  of 
  growth 
  has 
  been 
  influ- 
  

   enced 
  by 
  twinning. 
  This 
  is 
  evident 
  at 
  once 
  in 
  snch 
  a 
  form 
  as 
  

   shown 
  at 
  c 
  and 
  is 
  the 
  natural 
  explanation 
  wherever 
  the 
  angles 
  

   between 
  axes 
  differ 
  from 
  90°. 
  The 
  best 
  measurements 
  that 
  

   could 
  be 
  made 
  witli 
  the 
  microscope 
  show 
  that 
  angles 
  which 
  do 
  

   not 
  differ 
  sensibly 
  from 
  45° 
  and 
  60° 
  occur. 
  The 
  mode 
  of 
  

  

  Fig. 
  4. 
  

  

  Fig. 
  4. 
  Cristobalite 
  crystals; 
  size 
  0*l-0 
  - 
  

  

  twinning 
  which 
  would 
  give 
  such 
  results 
  as 
  regards 
  the 
  direc- 
  

   tions 
  of 
  the 
  axes 
  and 
  which 
  is 
  in 
  accord 
  with 
  the 
  observed 
  

   positions 
  of 
  the 
  faces 
  is 
  twinning 
  after 
  the 
  octahedron 
  (111) 
  or 
  

   spinel 
  twinning. 
  If, 
  after 
  twinning 
  has 
  occurred 
  at 
  some 
  

   point 
  during 
  the 
  growth 
  along 
  an 
  axis, 
  the 
  same 
  axis 
  continues 
  

   to 
  grow, 
  an 
  angle 
  of 
  60° 
  is 
  formed. 
  If, 
  however, 
  a 
  second 
  

   axis, 
  which 
  would 
  normally 
  assume 
  a 
  90° 
  position, 
  starts 
  

   growth 
  in 
  the 
  twinned 
  position, 
  an 
  angle 
  of 
  45° 
  results. 
  

  

  