﻿Becker 
  — 
  Some 
  Queries 
  on 
  Rock 
  Difere?itiation. 
  31 
  

  

  as 
  salt 
  and 
  the 
  hypothetical 
  lava 
  50 
  times 
  as 
  much 
  as 
  the 
  sul- 
  

   phate. 
  Diffusion 
  of 
  matter 
  in 
  the 
  lava, 
  therefore, 
  takes 
  over 
  

   200,000 
  times 
  as 
  long 
  as 
  the 
  diffusion 
  of 
  heat 
  in 
  solid 
  rock 
  at 
  

   ordinary 
  temperatures. 
  Even 
  if 
  melted 
  lava 
  conducts 
  heat 
  

   many 
  thousand 
  times 
  worse 
  than 
  solid 
  rock, 
  so 
  that 
  the 
  con- 
  

   ductivity 
  of 
  the 
  fluid 
  might 
  be 
  neglected, 
  the 
  temperature 
  in 
  

   an 
  unequally 
  heated 
  mass 
  of 
  melted 
  lava 
  would 
  be 
  sensibly 
  

   equalized 
  by 
  the 
  conduction 
  of 
  the 
  solid 
  walls 
  of 
  the 
  reservoir 
  

   before 
  any 
  tendency 
  to 
  molecular 
  flow 
  which 
  difference 
  of 
  tem- 
  

   perature 
  might 
  have 
  induced 
  would 
  have 
  had 
  time 
  to 
  produce 
  

   sensible 
  effects. 
  

  

  Now 
  in 
  any 
  case 
  of 
  the 
  segregation 
  of 
  homogeneous, 
  mis- 
  

   cible, 
  fluid 
  magmas 
  by 
  molecular 
  flow, 
  the 
  available 
  osmotic 
  

   force 
  is 
  only 
  the 
  difference 
  between 
  two 
  osmotic 
  forces, 
  and 
  the 
  

   transfer 
  of 
  a 
  given 
  quantity 
  of 
  matter 
  to 
  a 
  given 
  distance 
  will 
  

   be 
  much 
  slower 
  than 
  in 
  simple 
  diffusion. 
  Hence, 
  so 
  far 
  as 
  I 
  

   can 
  see, 
  a 
  mass 
  of 
  lava 
  of 
  volume, 
  say 
  1 
  cubic 
  kilometer, 
  would 
  

   not 
  have 
  had 
  time 
  to 
  segregate 
  into 
  distinctly 
  different 
  rocks 
  

   by 
  molecular 
  flow 
  if 
  it 
  had 
  been 
  kept 
  melted 
  since 
  the 
  close 
  of 
  

   the 
  Archaean, 
  even 
  if 
  the 
  temperature 
  of 
  the 
  top 
  could 
  have 
  

   been 
  kept 
  sensibly 
  above 
  the 
  temperature 
  of 
  the 
  bottom. 
  But 
  

   it 
  is 
  very 
  difficult 
  to 
  imagine 
  how 
  a 
  mass 
  of 
  lava 
  could 
  be 
  

   more 
  highly 
  heated 
  at 
  the 
  top 
  than 
  at 
  the 
  bottom, 
  since 
  in 
  

   general 
  temperature 
  increases 
  with 
  depth. 
  If 
  the 
  bottom 
  

   were 
  more 
  highly 
  heated 
  than 
  the 
  top, 
  of 
  course 
  convection 
  

   currents 
  would 
  be 
  set 
  up 
  and 
  those 
  would 
  effectually 
  prevent 
  

   any 
  segregation 
  on 
  Soret's 
  method. 
  I 
  do 
  not 
  think 
  this 
  method 
  

   should 
  be 
  invoked 
  in 
  explanation 
  of 
  rock 
  differences 
  unless 
  

   it 
  can 
  be 
  shown 
  how 
  heating 
  from 
  the 
  top 
  can 
  occur. 
  

  

  JN"o 
  such 
  difficulties 
  present 
  themselves 
  in 
  such 
  cases 
  as 
  that 
  

   of 
  the 
  growth 
  of 
  a 
  crystal. 
  A 
  supersaturated 
  solution 
  of 
  a 
  

   given 
  substance 
  cannot 
  exist 
  in 
  immediate 
  contact 
  with 
  the 
  

   solid 
  form 
  of 
  the 
  same 
  substance, 
  but 
  a 
  solution 
  may 
  be 
  super- 
  

   saturated 
  at 
  a 
  very 
  short 
  distance 
  from 
  the 
  solid 
  mass. 
  In 
  

   cases 
  of 
  crystallization 
  there 
  is 
  thus 
  an 
  osmotic 
  pressure-dif- 
  

   ference 
  directed 
  toward 
  the 
  growing 
  crystal, 
  and 
  molecular 
  

   flow 
  results. 
  The 
  molecular 
  flow 
  attending 
  the 
  formation 
  of 
  

   phenocrysts 
  is 
  usually 
  confined 
  to 
  distances 
  of 
  a 
  few 
  milli- 
  

   meters, 
  or 
  at 
  most 
  a 
  few 
  centimeters, 
  and 
  is 
  clearly 
  a 
  process 
  

   involving 
  no 
  unreasonable 
  amount 
  of 
  time. 
  Similarly 
  the 
  

   formation 
  of 
  aureoles 
  of 
  diffusion 
  around 
  blebs 
  to 
  distances 
  of 
  

   a 
  few 
  centimeters 
  is 
  not 
  a 
  very 
  lengthened 
  process. 
  Thus 
  if 
  

   such 
  a 
  bleb 
  had 
  the 
  properties 
  of 
  the 
  hypothetical 
  lava 
  dis- 
  

   cussed 
  above, 
  an 
  aureole 
  5 
  cm 
  in 
  depth 
  might 
  form 
  around 
  it 
  in 
  

   about 
  a 
  year, 
  and 
  sensible 
  diffusion 
  would 
  extend 
  to 
  a 
  distance 
  

   of 
  l 
  mm 
  in 
  the 
  comparatively 
  short 
  period 
  of 
  3-| 
  hours. 
  

  

  Convection 
  unavoidable. 
  — 
  It 
  has 
  been 
  shown 
  above 
  that 
  

  

  