﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals, 
  347 
  

  

  A 
  question 
  of 
  considerable 
  importance 
  is, 
  whether 
  the 
  gen- 
  

   eral 
  absence 
  of 
  tridymite 
  in 
  rocks 
  which 
  have 
  cooled 
  slowly 
  

   (such 
  as 
  large 
  bodies 
  of 
  deep-seated 
  intrusives) 
  must 
  be 
  under- 
  

   stood 
  as 
  proving 
  that 
  the 
  temperature 
  of 
  crystallization 
  was 
  

   below 
  the 
  tridymite-quartz 
  inversion-point. 
  Considered 
  solely 
  

   from 
  the 
  experimental 
  evidence 
  on 
  the 
  quartz-tridymite 
  rela- 
  

   tions, 
  it 
  may 
  probably 
  be 
  said 
  that 
  the 
  temperature 
  during 
  the 
  

   final 
  stages 
  of 
  crystallization 
  of 
  the 
  quartz 
  was 
  below 
  this 
  

   point, 
  but 
  nothing 
  is 
  implied 
  regarding 
  the 
  first 
  stages 
  ; 
  for 
  if 
  

   tridymite 
  were 
  precipitated 
  at 
  an 
  earlier 
  stage, 
  but 
  remained 
  in 
  

   contact 
  with 
  a 
  fluid 
  portion 
  of 
  the 
  magma 
  after 
  the 
  temper- 
  

   ature 
  dropped 
  below 
  the 
  inversion-point, 
  it 
  would 
  probably 
  

   pass 
  over 
  into 
  quartz 
  within 
  a 
  short 
  time 
  (probably 
  within 
  a 
  

   few 
  days). 
  Examples 
  are 
  not 
  lacking 
  in 
  which 
  traces 
  of 
  such 
  

   inversion 
  appear 
  to 
  survive. 
  The 
  peculiar 
  form 
  of 
  quartz 
  in 
  

   some 
  eruptives 
  has 
  been 
  thus 
  explained. 
  Professor 
  Lacroix* 
  

   lias 
  found 
  a 
  number 
  of 
  instances 
  of 
  such 
  relations, 
  and 
  Dr. 
  

   Per 
  Geijerf 
  has 
  recently 
  described 
  others. 
  

  

  In 
  the 
  examples 
  just 
  referred 
  to, 
  the 
  peculiar 
  form 
  of 
  the 
  

   quartz 
  bears 
  testimony 
  to 
  the 
  history 
  through 
  which 
  it 
  has 
  

   passed, 
  but 
  under 
  different 
  conditions, 
  especially 
  those 
  obtain- 
  

   ing 
  during 
  the 
  crystallization 
  of 
  a 
  coarsely 
  granular 
  rock, 
  the 
  

   newly 
  formed 
  quartz 
  would 
  undoubtedly 
  tend 
  to 
  assume 
  its 
  

   proper 
  ciwstallographic 
  structure, 
  and 
  no 
  evidence 
  of 
  the 
  inter- 
  

   mediate 
  steps 
  of 
  the 
  process 
  could 
  be 
  found 
  in 
  the 
  final 
  product. 
  

  

  It 
  is 
  hardly 
  necessary 
  to 
  consider 
  in 
  detail 
  the 
  various 
  nat- 
  

   ural 
  occurrences 
  of 
  tridymite 
  and 
  cristobalite 
  which 
  have 
  been 
  

   described 
  in 
  the 
  literature. 
  So 
  far 
  as 
  I 
  have 
  been 
  able 
  to 
  ascer- 
  

   tain, 
  the 
  descriptions 
  given 
  bear 
  out 
  the 
  principles 
  of 
  origin 
  

   which 
  have 
  been 
  outlined. 
  

  

  Effect 
  of 
  Pressure 
  upon 
  the 
  Quartz-Tridtmite 
  Inversion. 
  

  

  In 
  the 
  preliminary 
  paper 
  on 
  the 
  silica 
  minerals 
  which 
  the 
  

   writer 
  published, 
  some 
  inquiry 
  was 
  made 
  into 
  the 
  effect 
  which 
  

   pressure 
  would 
  have 
  in 
  displacing 
  the 
  inversion 
  point. 
  By 
  em- 
  

  

  (dT 
  T 
  \ 
  

  

  — 
  — 
  — 
  (t> 
  2 
  — 
  <y 
  a 
  ) 
  \ 
  and 
  

  

  assuming 
  probable 
  values 
  of 
  L, 
  the 
  heat 
  of 
  inversion, 
  and 
  

   v^—Vj, 
  the 
  volume 
  change, 
  a 
  displacement 
  (rise) 
  of 
  0'10537° 
  per 
  

   atmosphere 
  was 
  deduced. 
  It 
  is 
  doubtful, 
  however, 
  if 
  a 
  cal- 
  

   culation 
  of 
  this 
  kind 
  serves 
  any 
  useful 
  purpose 
  because 
  of 
  the 
  

   lack 
  of 
  certainty 
  of 
  the 
  values 
  assumed 
  and 
  the 
  consequent 
  

  

  *A. 
  Lacroix: 
  Sur 
  la 
  tridymite 
  du 
  Vesuve 
  et 
  sur 
  la 
  genese 
  de 
  ce 
  mineral 
  

   par 
  fusion, 
  Bull. 
  Soc. 
  Min., 
  xxxi, 
  323, 
  1908. 
  

  

  f 
  Per 
  Geijer: 
  Geol. 
  Foren. 
  Forhandl., 
  xxxiv, 
  1, 
  pp. 
  51-80, 
  1913. 
  Eefer- 
  

   ences 
  are 
  given 
  in 
  this 
  paper 
  to 
  instances 
  cited 
  "by 
  other 
  writers. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XXXVI, 
  No. 
  214.— 
  October, 
  1913. 
  

   23 
  

  

  