﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  345 
  

  

  decrease 
  of 
  viscosity 
  by 
  the 
  retention 
  of 
  volatile 
  substances 
  

   (mineralizers) 
  — 
  conditions 
  implying 
  a 
  crystallization 
  of 
  the 
  

   magma 
  under 
  pressure 
  — 
  tridymite 
  if 
  once 
  formed 
  would 
  

   become 
  unstable 
  at 
  870° 
  and 
  would 
  pass 
  over 
  into 
  quartz. 
  

   This 
  matter 
  will 
  be 
  taken 
  up 
  a 
  little 
  later. 
  

  

  Through 
  the 
  kindness 
  of 
  Dr. 
  E. 
  S. 
  Larsen, 
  of 
  the 
  U. 
  S. 
  Geo- 
  

   logical 
  Survey, 
  my 
  attention 
  has 
  been 
  called 
  to 
  an 
  interesting 
  

   occurrence 
  of 
  cristobalite 
  in 
  a 
  basalt 
  found 
  by 
  Dr. 
  Whitman 
  

   Cross 
  in 
  the 
  Hawaiian 
  Islands. 
  In 
  the 
  calculation 
  of 
  the 
  norm 
  

   from 
  the 
  chemical 
  analysis 
  of 
  the 
  rock, 
  no 
  olivine 
  appeared, 
  

   while 
  examination 
  of 
  thin 
  sections 
  showed 
  abundant 
  olivine. 
  

   The 
  explanation 
  was 
  found 
  in 
  the 
  discovery 
  of 
  small 
  crystals 
  

   of 
  cristobalite 
  in 
  cavities. 
  In 
  another 
  set 
  of 
  rocks 
  from 
  

   Colorado 
  which 
  Dr. 
  Larsen 
  brought 
  to 
  my 
  attention, 
  the 
  flow- 
  

   structure 
  of 
  acid 
  effusives 
  is 
  well 
  developed 
  and 
  certain 
  of 
  the 
  

   bands 
  show 
  innumerable 
  tridymite 
  crystals 
  whose 
  arrangement 
  

   with 
  respect 
  to 
  the 
  other 
  constituents 
  of 
  the 
  rocks 
  indicates 
  

   their 
  simultaneous 
  crystallization 
  from 
  the 
  melt. 
  It 
  seems 
  

   probable 
  that 
  these 
  minerals 
  are 
  not 
  so 
  rare 
  as 
  has 
  been 
  gener- 
  

   ally 
  supposed 
  and 
  that 
  with 
  careful 
  search 
  they 
  might 
  often 
  be 
  

   found. 
  

  

  Emphasis 
  should 
  be 
  laid 
  upon 
  the 
  fact 
  that 
  the 
  presence 
  of 
  

   cristobalite 
  or 
  tridymite 
  in 
  a 
  rock 
  does 
  not 
  necessarily 
  imply 
  

   that 
  at 
  the 
  time 
  of 
  formation 
  of 
  these 
  minerals 
  the 
  temperature 
  

   was 
  above 
  the 
  respective 
  inversion-points 
  (1470° 
  and 
  870°). 
  

   Any 
  set 
  of 
  conditions 
  which 
  will 
  bring 
  together 
  quantities 
  of 
  

   ungrouped 
  Si0 
  2 
  molecules 
  in 
  such 
  a 
  manner 
  as 
  to 
  favor 
  their 
  

   rapid 
  assemblage 
  in 
  definite 
  groupings 
  without 
  giving 
  time 
  for 
  

   perfect 
  equilibrium 
  to 
  be 
  established 
  (as 
  in 
  the 
  reactions 
  of 
  

   vapors) 
  ; 
  or 
  which 
  will 
  suddenly 
  bring 
  a 
  system 
  in 
  which 
  equi- 
  

   librium 
  prevails 
  into 
  new 
  conditions, 
  at 
  the 
  same 
  time 
  introduc- 
  

   ing 
  obstacles 
  to 
  the 
  establishment 
  of 
  a 
  new 
  equilibrium 
  (as 
  in 
  

   the 
  sudden 
  chilling 
  of 
  a 
  melt), 
  will 
  favor 
  the 
  deposition 
  of 
  

   unstable 
  forms. 
  It 
  is 
  evident 
  that 
  there 
  will 
  be 
  two 
  factors 
  to 
  

   be 
  considered 
  ; 
  first, 
  the 
  question 
  of 
  whether 
  change 
  of 
  condi- 
  

   tions 
  has 
  been 
  too 
  rapid 
  for 
  equilibrium 
  to 
  follow, 
  and 
  second, 
  

   the 
  question 
  as 
  to 
  what 
  was 
  the 
  previous 
  condition 
  from 
  which 
  

   the 
  state 
  in 
  question 
  has 
  been 
  reached. 
  

  

  Certain 
  phenomena 
  which 
  Lacroix 
  and 
  others* 
  have 
  de- 
  

   scribed, 
  where 
  quartzose 
  inclusions 
  in 
  volcanic 
  rocks 
  have 
  

   been 
  partly 
  or 
  wholly 
  transformed 
  to 
  tridymite 
  f 
  or 
  to 
  tridymite 
  

  

  point 
  has 
  been 
  exceeded. 
  Some 
  uncertainty 
  on 
  this 
  point 
  

   arises 
  from 
  the 
  observation 
  which 
  Lacroix 
  makes 
  f 
  that, 
  in 
  the 
  

  

  * 
  A. 
  Lacroix, 
  Les 
  Enclaves 
  des 
  Koches 
  Yolcaniques, 
  1893 
  ; 
  Bull. 
  Soc. 
  Min., 
  

   xiv, 
  185, 
  1891 
  ; 
  K. 
  v. 
  Chrustschoff, 
  Tschermak 
  Min. 
  Pet. 
  Mitth., 
  vii, 
  295, 
  

   1886. 
  

  

  f 
  Les 
  Enclaves 
  des 
  Koches 
  Yolcaniques, 
  p. 
  570. 
  

  

  