﻿32 
  Becker 
  — 
  Some 
  Queries 
  on 
  Rock 
  Differentiation. 
  

  

  segregation 
  of 
  magmas 
  by 
  the 
  method 
  of 
  Ludwig 
  and 
  Soret 
  

   would 
  occupy 
  a 
  stupendous 
  time 
  even 
  if 
  a 
  mass 
  of 
  melted 
  lava 
  

   could 
  be 
  kept 
  free 
  from 
  convection 
  currents. 
  This 
  freedom, 
  

   however, 
  could 
  only 
  be 
  secured 
  by 
  permanent, 
  regular 
  decrease 
  

   of 
  temperature 
  from 
  the 
  upper 
  surface 
  of 
  the 
  magma 
  down- 
  

   wards. 
  In 
  any 
  fluid 
  of 
  only 
  moderate 
  viscosity 
  even 
  a 
  very 
  

   small 
  rise 
  of 
  temperature 
  at 
  the 
  bottom 
  would 
  cause 
  convec- 
  

   tion 
  currents 
  which 
  in 
  a 
  day 
  would 
  undo 
  the 
  segregation 
  it 
  had 
  

   taken 
  thousands 
  of 
  years 
  to 
  accomplish 
  by 
  Soret's 
  method. 
  

   Lavas 
  of 
  very 
  great 
  viscosity 
  would 
  also 
  mingle 
  by 
  convection 
  

   far 
  more 
  rapidly 
  than 
  the 
  most 
  diffusible 
  solutions 
  could 
  segre- 
  

   gate 
  through 
  differences 
  of 
  temperature. 
  Mingling 
  again 
  

   might 
  occur 
  in 
  the 
  absence 
  of 
  bottom-heating 
  by 
  any 
  mechan- 
  

   ical 
  disturbance 
  of 
  surrounding 
  rock 
  masses. 
  

  

  That 
  in 
  general 
  the 
  temperature 
  of 
  the 
  globe 
  increases 
  with 
  

   depth 
  is 
  perhaps 
  the 
  best 
  established 
  generalization 
  of 
  geology. 
  

   Hence 
  even 
  if 
  it 
  be 
  granted 
  for 
  the 
  sake 
  of 
  argument 
  that 
  in 
  

   some 
  particular 
  locality 
  the 
  temperature 
  decreases 
  with 
  depth, 
  

   it 
  is 
  clear 
  that 
  such 
  a 
  thermal 
  distribution 
  is 
  a 
  case 
  of 
  unstable 
  

   equilibrium. 
  It 
  can, 
  therefore, 
  only 
  be 
  temporary 
  and 
  it 
  

   would 
  surely 
  be 
  a 
  strange 
  exception 
  were 
  such 
  an 
  abnormal 
  

   distribution 
  of 
  temperature 
  to 
  last 
  for 
  1000 
  years. 
  Yet 
  in 
  that 
  

   time 
  no 
  segregation 
  worth 
  mentioning 
  as 
  an 
  origin 
  of 
  rock 
  

   differences 
  could 
  occur. 
  The 
  normal 
  condition 
  of 
  a 
  hypogeal 
  

   molten 
  magma 
  must 
  be 
  that 
  in 
  which 
  temperature 
  increases 
  

   with 
  depth 
  and 
  in 
  which 
  convection 
  effectually 
  precludes 
  any 
  

   process 
  of 
  segregation 
  by 
  molecular 
  flow. 
  

  

  Immiscible 
  fluids. 
  — 
  Another 
  method 
  of 
  segregation, 
  which 
  

   is 
  quite 
  distinct 
  from 
  that 
  discussed 
  above, 
  depends 
  upon 
  

   changes 
  in 
  the 
  mutual 
  solubility 
  of 
  fluids. 
  Some 
  fluids 
  which 
  

   at 
  certain 
  temperatures 
  mingle 
  in 
  all 
  proportions 
  dissolve 
  one 
  

   another 
  only 
  in 
  certain 
  proportions 
  at 
  other 
  temperatures. 
  

   Thus 
  benzol 
  and 
  acetic 
  acid 
  mix 
  without 
  limit 
  at 
  15°, 
  but 
  below 
  

   this 
  temperature 
  separate 
  out 
  into 
  two 
  layers, 
  one 
  of 
  which 
  con- 
  

   tains 
  nearly 
  twice 
  as 
  much 
  acetic 
  acid 
  as 
  the 
  other. 
  So, 
  too, 
  

   phenol 
  and 
  water 
  mingle 
  freely 
  above 
  69°, 
  but 
  not 
  below 
  this 
  

   temperature 
  ; 
  and 
  there 
  are 
  many 
  similar 
  instances. 
  The 
  phe- 
  

   nomena 
  were 
  studied 
  by 
  Mr. 
  Alexejew, 
  who 
  concluded 
  that 
  in 
  

   all 
  cases 
  where 
  the 
  solutions 
  do 
  not 
  react 
  chemically 
  upon 
  one 
  

   another 
  they 
  become 
  miscible 
  above 
  a 
  certain 
  temperature.* 
  

  

  Though 
  Mr. 
  Alexejew 
  studied 
  some 
  of 
  the 
  physical 
  relations 
  

   of 
  solutions 
  of 
  fluids 
  in 
  fluids, 
  he 
  did 
  not 
  determine 
  whether 
  

   the 
  passage 
  from 
  complete 
  to 
  partial 
  miscibility 
  is 
  accompanied 
  

   by 
  expansion 
  or 
  contraction. 
  This 
  step, 
  however, 
  has 
  been 
  

   taken 
  by 
  Mr. 
  Herman 
  Pfeiffer 
  in 
  Prof. 
  Ostwald's 
  laboratory. 
  

   He 
  finds 
  that 
  this 
  change 
  is 
  accompanied 
  by 
  a 
  sudden 
  sharp 
  

  

  * 
  Wied. 
  Ann., 
  vol. 
  xxviii, 
  1886, 
  p. 
  327. 
  

  

  