﻿30 
  Becker 
  — 
  Some 
  Queries 
  on 
  Rock 
  Differentiation. 
  

  

  supposes 
  the 
  diffusivity 
  of 
  such 
  a 
  lava 
  to 
  be 
  ^ 
  that 
  of 
  sulphate 
  

   of 
  copper 
  solution, 
  then 
  the 
  time 
  needful 
  to 
  give 
  a 
  certain 
  satura- 
  

   tion 
  at 
  a 
  certain 
  point 
  will 
  be 
  50 
  times 
  as 
  great. 
  It 
  would 
  

   take 
  50 
  years 
  instead 
  of 
  1 
  to 
  establish 
  the 
  conditions 
  given 
  in 
  

   the 
  second 
  column 
  of 
  the 
  table. 
  A 
  million 
  years 
  is 
  20,000 
  X 
  

   50 
  or 
  141 
  2 
  X50 
  and 
  consequently 
  in 
  this 
  vast 
  period 
  sensible 
  

   impregnation 
  of 
  the 
  lava 
  would 
  have 
  extended 
  to 
  only 
  about 
  

   49 
  m 
  from 
  contact 
  (i. 
  e. 
  141x35 
  cm 
  ) 
  and 
  semi-saturation 
  to 
  

   some 
  12 
  m 
  . 
  

  

  It 
  may 
  seem 
  to 
  some 
  readers 
  that 
  I 
  have 
  exaggerated 
  the 
  

   viscosity 
  of 
  lava. 
  Certainly 
  literature 
  contains 
  some 
  accounts 
  

   of 
  lavas 
  said 
  to 
  " 
  run 
  like 
  water," 
  but 
  I 
  have 
  been 
  able 
  to 
  find 
  

   no 
  approximately 
  precise 
  data 
  indicating 
  such 
  fluidity. 
  That 
  

   most 
  lava 
  streams, 
  those 
  of 
  Vesuvius, 
  for 
  example, 
  advance 
  

   even 
  on 
  steep 
  declivities 
  at 
  a 
  small 
  fraction 
  of 
  a 
  mile 
  an 
  hour 
  

   is 
  certain. 
  It 
  will 
  be 
  seen, 
  however, 
  that 
  the 
  arguments 
  of 
  

   this 
  paper 
  would 
  not 
  be 
  essentially 
  changed 
  if 
  lava 
  were 
  sup- 
  

   posed 
  no 
  more 
  viscous 
  than 
  a 
  bluestone 
  solution. 
  But 
  it 
  may 
  

   also 
  be 
  asked 
  why 
  even 
  greater 
  fluidity 
  may 
  not 
  be 
  assumed 
  in 
  

   lavas 
  prior 
  to 
  eruption, 
  a 
  fluidity 
  sufficient 
  to 
  allow 
  of 
  segrega- 
  

   tion 
  in 
  a 
  moderate 
  time. 
  There 
  seem 
  to 
  me 
  abundant 
  grounds 
  

   for 
  refusing 
  assent 
  to 
  such 
  an 
  assumption. 
  Hypogeal 
  magmas 
  

   must 
  be 
  under 
  great 
  pressure 
  and 
  they 
  must 
  be 
  close 
  to 
  their 
  

   melting 
  points 
  ; 
  for 
  if 
  they 
  were 
  considerably 
  superheated 
  the 
  

   surrounding 
  rock 
  masses 
  would 
  melt 
  and 
  the 
  temperature 
  

   would 
  fall 
  to 
  the 
  melting 
  point. 
  To 
  bring 
  about 
  considerable 
  

   superheating 
  would 
  be 
  almost 
  as 
  difficult 
  as 
  to 
  boil 
  water 
  in 
  a 
  

   vessel 
  of 
  ice. 
  The 
  less 
  viscous 
  the 
  magma 
  the 
  more 
  difficult 
  

   would 
  superheating 
  be. 
  Now 
  liquids 
  which, 
  like 
  lava, 
  condense 
  

   in 
  solidifying 
  are 
  most 
  viscous 
  at 
  the 
  melting 
  point, 
  and 
  pres- 
  

   sure 
  increases 
  their 
  viscosity. 
  Hence 
  hypogeal 
  lavas 
  must 
  be 
  

   more 
  viscous 
  than 
  they 
  are 
  when 
  they 
  reach 
  the 
  surface. 
  The 
  

   relief 
  of 
  pressure 
  is 
  equivalent 
  to 
  superheating. 
  It 
  is, 
  there- 
  

   fore, 
  irrational 
  to 
  assume 
  that 
  lavas 
  prior 
  to 
  eruption 
  are 
  at 
  all 
  

   more 
  fluid 
  than 
  they 
  are 
  at 
  eruption. 
  All 
  indications 
  point 
  to 
  

   the 
  opposite 
  conclusion. 
  

  

  These 
  illustrations 
  show 
  that 
  diffusion 
  of 
  fluids, 
  particularly 
  

   viscous 
  ones, 
  is 
  an 
  excessively 
  slow 
  process. 
  It 
  is 
  instructive 
  

   to 
  compare 
  its 
  rate 
  with 
  that 
  of 
  the 
  diffusion 
  of 
  heat. 
  Accord- 
  

   ing 
  to 
  Lord 
  Kelvin, 
  the 
  time 
  required 
  for 
  the 
  diffusion 
  of 
  com- 
  

   mon 
  salt 
  in 
  water 
  to 
  be 
  represented 
  by 
  a 
  given 
  curve 
  is 
  more 
  

   than 
  870 
  times 
  as 
  long 
  as 
  that 
  needful 
  to 
  diffuse 
  heat 
  through 
  

   underground 
  strata 
  in 
  such 
  a 
  way 
  as 
  to 
  be 
  represented 
  by 
  the 
  

   same 
  curve.* 
  Copper 
  sulphate 
  requires 
  five 
  times 
  as 
  much 
  time 
  

  

  * 
  The 
  diffusivity 
  of 
  heat 
  in 
  underground 
  strata 
  has 
  an 
  average 
  value 
  of 
  0*01. 
  

   In 
  Lord 
  Kelvin's 
  article 
  on 
  heat, 
  Enc. 
  Brit 
  , 
  9th 
  ed., 
  vol. 
  Xr, 
  p. 
  582, 
  Table 
  B 
  (auto- 
  

   graph 
  issue), 
  the 
  time 
  required 
  for 
  the 
  diffusion 
  of 
  heat 
  in 
  underground 
  strata 
  

   should 
  be 
  given 
  as 
  3,170,000 
  years 
  instead 
  of 
  as 
  -^ 
  of 
  this 
  period. 
  

  

  