﻿348 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  unreliability 
  of 
  the 
  results 
  and 
  the 
  danger 
  that 
  they 
  will 
  be 
  

   misinterpreted. 
  The 
  data 
  for 
  such 
  a- 
  calculation 
  would 
  be 
  of 
  

   great 
  value, 
  but 
  until 
  they 
  are 
  available, 
  we 
  can 
  hardly 
  make 
  a 
  

   more 
  positive 
  statement 
  than 
  to 
  say 
  that 
  pressure 
  will 
  raise 
  the 
  

   inversion-point 
  by 
  some 
  unknown 
  amount. 
  

  

  Information 
  to 
  be 
  Obtained 
  from 
  the 
  Study 
  of 
  Tridymite- 
  

   bearing 
  Rocks. 
  

  

  A 
  careful 
  study 
  of 
  the 
  relations 
  and 
  characteristics 
  of 
  the 
  

   minerals 
  in 
  tridymite-bearing 
  rocks 
  should 
  give 
  us 
  important 
  

   information 
  on 
  certain 
  problems 
  connected 
  with 
  the 
  processes 
  

   of 
  volcanic 
  activity. 
  The 
  position 
  of 
  the 
  quartz-tridymite 
  

   inversion-point 
  is 
  within 
  a 
  critical 
  region 
  as 
  regards 
  the 
  

   temperatures 
  of 
  vulcanism, 
  and 
  the 
  history 
  of 
  the 
  tridymite, 
  as 
  

   revealed 
  by 
  petrologic 
  study, 
  may, 
  with 
  the 
  accumulation 
  of 
  

   evidence, 
  be 
  able 
  to 
  settle 
  a 
  number 
  of 
  debated 
  points. 
  The 
  

   kind 
  of 
  evidence 
  to 
  which 
  I 
  refer 
  may 
  be 
  illustrated 
  by 
  an 
  

   example. 
  In 
  the 
  set 
  of 
  rocks 
  from 
  Colorado 
  which 
  Dr. 
  Larsen 
  

   kindly 
  placed 
  at 
  my 
  disposal, 
  I 
  have 
  found 
  certain 
  very 
  sug- 
  

   gestive 
  features, 
  which 
  appear 
  to 
  show 
  that 
  quartz 
  phenocrysts 
  

   which 
  were 
  formed 
  in 
  the 
  magma 
  at 
  depth, 
  became 
  converted 
  

   into 
  tridymite 
  during 
  the 
  process 
  of 
  extrusion. 
  The 
  nature 
  of 
  

   the 
  evidence 
  is 
  as 
  follows 
  : 
  When 
  in 
  laboratory 
  experiments 
  

   ground 
  quartz 
  is 
  converted 
  into 
  tridymite 
  in 
  a 
  sodic 
  tungstate 
  

   flux, 
  it 
  is 
  frequently 
  found 
  that 
  many 
  of 
  the 
  quartz 
  grains 
  

   retain 
  their 
  individuality 
  during 
  the 
  process, 
  but* 
  are 
  replaced 
  

   by 
  an 
  aggregate 
  of 
  tridymite 
  crystals. 
  If 
  the 
  replacement 
  has 
  

   not 
  been 
  quite 
  complete, 
  very 
  irregular 
  nuclei 
  of 
  quartz 
  

   remain, 
  corroded 
  by 
  the 
  encroaching 
  tridymite. 
  Similar 
  

   phenomena 
  have 
  been 
  remarked 
  in 
  the 
  transformation 
  of 
  

   quartz 
  bricks 
  into 
  tridymite 
  in 
  metallurgical 
  establishments 
  or 
  

   glass 
  furnaces.* 
  In 
  the 
  Colorado 
  rocks 
  certain 
  tridymite 
  

   aggregates 
  suggest 
  the 
  same 
  sort 
  of 
  process 
  carried 
  to 
  com- 
  

   pletion. 
  They 
  are 
  distinctive 
  units, 
  quite 
  sharply 
  set 
  off 
  from 
  

   the 
  surrounding 
  matrix, 
  which 
  frequently 
  bends 
  around 
  them 
  

   in 
  flow 
  lines. 
  They 
  are 
  not 
  spherulites, 
  as 
  the 
  component 
  

   crystals 
  are 
  disposed 
  at 
  random 
  instead 
  of 
  in 
  a 
  radial 
  form, 
  and 
  

   are 
  much 
  larger 
  than 
  ordinarily 
  found 
  in 
  spherulites. 
  Perhaps 
  

   most 
  important 
  of 
  all 
  is 
  the 
  observed 
  fact 
  that 
  in 
  several 
  in- 
  

   stances 
  the 
  outlines 
  of 
  the 
  nodules 
  are 
  those 
  of 
  slightly 
  rounded 
  

   hexagons. 
  Al] 
  in 
  all, 
  the 
  appearance 
  suggests 
  a 
  derivation 
  from 
  

   quartz 
  phenocrysts. 
  The 
  rocks 
  in 
  which 
  the 
  phenomena 
  occur 
  

   may 
  be 
  called 
  tridymite-latites 
  ; 
  that 
  is, 
  rocks 
  corresponding 
  in 
  

   mineralogical 
  make-up 
  to 
  quartz-latites 
  but 
  in 
  which 
  the 
  role 
  

   of 
  quartz 
  as 
  an 
  essential 
  constituent 
  is 
  taken 
  by 
  tridymite. 
  

  

  *E. 
  Mallard, 
  Bull. 
  Soc. 
  Min., 
  xiii, 
  172, 
  1890; 
  P. 
  J. 
  Holmquist, 
  Geol. 
  

   Foren. 
  Forhand, 
  xxxiii, 
  4, 
  245-260, 
  1911 
  ; 
  K. 
  Endell, 
  Stahl 
  u. 
  Eisen, 
  Nr. 
  10, 
  

   1912. 
  

  

  