﻿230 
  JOURNAL 
  OF 
  THE 
  WASHINGTON 
  ACADEMY 
  OE 
  SCIENCES 
  VOL. 
  12, 
  NO. 
  5 
  

  

  is 
  Ty". 
  If 
  crystallization 
  be 
  now 
  induced, 
  there 
  will 
  be 
  a 
  sudden 
  de- 
  

   velopment 
  of 
  a 
  pressure, 
  corresponding 
  to 
  the 
  saturation 
  pressure 
  at 
  

   Ty"; 
  the 
  water 
  introduced 
  into 
  the 
  melt 
  at 
  a 
  pressure 
  head 
  of 
  4 
  at- 
  

   mospheres 
  will 
  be 
  released 
  at 
  a 
  much 
  higher 
  pressure, 
  in 
  this 
  case 
  

   about 
  11 
  atmospheres. 
  If 
  an 
  undercooled 
  magma 
  were 
  to 
  come 
  into 
  

   contact 
  with 
  percolating 
  waters, 
  or 
  the 
  vapor 
  generated 
  therefrom, 
  as 
  

   previously 
  explained, 
  a 
  similar 
  introduction 
  of 
  water 
  at 
  a 
  low 
  pres- 
  

   sure 
  might 
  take 
  place. 
  Introduction 
  of 
  this 
  water 
  might 
  of 
  itself 
  

   induce 
  crystallization 
  in 
  virtue 
  of 
  the 
  lowered 
  viscosity 
  of 
  the 
  resulting 
  

   magmatic 
  solution, 
  and 
  it 
  is 
  conceivable 
  that 
  the 
  result 
  would 
  be 
  a 
  sud- 
  

   den 
  and 
  violent 
  outburst 
  of 
  steam 
  and 
  ash, 
  at 
  a 
  comparatively 
  low 
  

   temperature. 
  

  

  SUMMARY 
  

  

  It 
  has 
  been 
  shown 
  that 
  when 
  a 
  system 
  composed 
  of 
  volatile 
  and 
  

   non-volatile 
  components 
  such 
  as 
  water 
  and 
  KNO3 
  is 
  cooled, 
  crystal- 
  

   lization 
  will 
  take 
  place 
  at 
  a 
  temperature 
  lower 
  than 
  the 
  freezing 
  point 
  

   of 
  the 
  pure 
  non-volatile 
  salt 
  by 
  an 
  amount 
  corresponding 
  to 
  the 
  amount 
  

   of 
  volatile 
  material 
  present, 
  and 
  that 
  the 
  corresponding 
  three-phase 
  

   pressure 
  increases 
  rapidly 
  as 
  the 
  temperature 
  is 
  lowered 
  from 
  the 
  

   melting 
  point 
  of 
  the 
  salt. 
  This 
  increase 
  is 
  rapid 
  whether 
  measured 
  

   in 
  terms 
  of 
  the 
  decrease 
  in 
  temperature 
  of 
  the 
  three-phase 
  equilibrium 
  

   or 
  in 
  terms 
  of 
  the 
  content 
  of 
  volatile 
  material 
  in 
  the 
  solution. 
  From 
  

   the 
  latter 
  fact 
  it 
  follows 
  that 
  in 
  systems 
  of 
  the 
  type 
  of 
  magmas, 
  in 
  

   which 
  the 
  non-volatile 
  material 
  is 
  composed 
  of 
  such 
  substances 
  as 
  the 
  

   silicates, 
  and 
  in 
  which 
  the 
  pressure 
  required 
  to 
  retain 
  any 
  considerable 
  

   proportion 
  of 
  water 
  in 
  solution 
  must 
  be 
  large, 
  a 
  comparatively 
  small 
  

   amount 
  of 
  crystallization 
  will 
  result 
  in 
  a 
  large 
  increase 
  in 
  pressure. 
  

   When 
  a 
  magma 
  containing 
  water 
  cools, 
  with 
  consequent 
  crystalliza- 
  

   tion 
  and 
  development 
  of 
  high 
  pressure, 
  under 
  an 
  incompetent 
  crust, 
  

   a 
  release 
  of 
  pressure 
  will 
  take 
  place, 
  which 
  may 
  be 
  catastrophic 
  in 
  

   violence 
  or 
  take 
  the 
  form 
  of 
  a 
  succession 
  of 
  mildly 
  explosive 
  outbursts. 
  

   In 
  case 
  the 
  magma 
  cools 
  under 
  a 
  competent 
  crust 
  the 
  pressure 
  will 
  

   rise 
  to 
  a 
  maximum, 
  and 
  then 
  decrease, 
  probably 
  without 
  at 
  any 
  time 
  

   showing 
  critical 
  phenomena. 
  

  

  