﻿38 
  Becker 
  — 
  Some 
  Queries 
  on 
  Rock 
  Differentiation. 
  

  

  a 
  given 
  magma 
  passed 
  very 
  gradually 
  through 
  the 
  temperature 
  

   at 
  which 
  it 
  has 
  more 
  recently 
  been 
  remelted. 
  As 
  for 
  the 
  

   pressure, 
  it 
  seems 
  possible 
  that 
  under 
  continental 
  areas 
  a 
  given 
  

   fusing 
  subterranean 
  mass 
  may 
  exist 
  under 
  a 
  somewhat 
  smaller 
  

   load 
  than 
  that 
  to 
  which 
  it 
  was 
  subjected 
  at 
  primal 
  consolida- 
  

   tion, 
  for 
  the 
  general 
  tendency 
  of 
  continents 
  is 
  to 
  upheaval 
  and 
  

   degradation 
  by 
  erosion. 
  If 
  this 
  change 
  of 
  pressure 
  is 
  of 
  any 
  

   consequence 
  at 
  all, 
  it 
  will 
  tend 
  to 
  make 
  the 
  refusing 
  mass 
  more 
  

   fusible 
  and 
  more 
  miscible. 
  

  

  If 
  now 
  the 
  mass 
  were 
  both 
  homogeneous 
  and 
  in 
  molecular 
  

   equilibrium 
  before 
  the 
  primal 
  consolidation, 
  it 
  may 
  be 
  in 
  equi- 
  

   librium 
  after 
  refusion. 
  If 
  the 
  pressure 
  is 
  smaller 
  than 
  the 
  

   original 
  one, 
  this 
  difference 
  would 
  have 
  no 
  tendency 
  to 
  promote 
  

   segregation. 
  If 
  by 
  some 
  almost 
  inconceivable 
  coincidence, 
  the 
  

   upper 
  portion 
  of 
  the 
  refused 
  mass 
  were 
  heated 
  to 
  a 
  higher 
  

   temperature 
  than 
  the 
  lower 
  part, 
  this 
  temperature 
  would 
  be 
  

   equalized 
  by 
  conduction 
  through 
  the 
  walls, 
  if 
  not 
  through 
  the 
  

   liquid, 
  before 
  any 
  sensible 
  segregation 
  could 
  occur. 
  

  

  If 
  the 
  mass 
  were 
  heterogeneous 
  in 
  consequence 
  of 
  primal 
  

   segregation, 
  fusion 
  would 
  again 
  tend 
  to 
  restore 
  molecular 
  equi- 
  

   librium 
  and 
  the 
  only 
  chance 
  of 
  a 
  new 
  segregation 
  would 
  lie 
  in 
  

   the 
  possible 
  difference 
  between 
  primitive 
  and 
  ultimate 
  pressure, 
  

   which, 
  if 
  positive, 
  would 
  tend 
  to 
  mixture 
  rather 
  than 
  to 
  separa- 
  

   tion. 
  

  

  If 
  the 
  mass 
  were 
  heterogeneous 
  because' 
  the 
  primal 
  fusion 
  

   had 
  not 
  continued 
  long 
  enough 
  to 
  bring 
  about 
  homogeneity, 
  

   refusion 
  would 
  be 
  accompanied 
  by 
  a 
  tendency 
  to 
  continue 
  the 
  

   process 
  of 
  molecular 
  flow 
  and 
  to 
  decrease 
  the 
  heterogeneity 
  of 
  

   the 
  mass; 
  but 
  even 
  if 
  the 
  refused 
  mass 
  were 
  kept 
  molten 
  for 
  a 
  

   million 
  years, 
  this 
  tendency 
  would 
  probably 
  have 
  only 
  insig- 
  

   nificant 
  results. 
  

  

  Mixture 
  by 
  eruption. 
  — 
  Little 
  or 
  nothing 
  is 
  known 
  of 
  the 
  

   process 
  of 
  refusion 
  of 
  subterranean 
  masses 
  to 
  eruptive 
  magmas. 
  

   Supposing 
  a 
  mass 
  which 
  is 
  fused 
  and 
  near 
  its 
  melting 
  point 
  

   to 
  remain 
  in 
  its 
  subterranean 
  reservoir, 
  it 
  must 
  in 
  general 
  

   receive 
  or 
  lose 
  heat. 
  In 
  the 
  latter 
  case 
  it 
  will 
  reconsolidate, 
  in 
  

   the 
  former 
  the 
  mass 
  of 
  melted 
  material 
  will 
  increase. 
  In 
  the 
  

   case 
  of 
  rocks, 
  fusion 
  is 
  accompanied 
  by 
  expansion 
  and 
  the 
  

   magma 
  must 
  have 
  more 
  space 
  than 
  it 
  occupied 
  in 
  a 
  solid 
  con- 
  

   dition. 
  Any 
  elastic 
  strains 
  in 
  surrounding 
  masses 
  will 
  also 
  

   tend 
  to 
  expel 
  it 
  and 
  it 
  would 
  seem 
  to 
  me 
  most 
  probable 
  that 
  

   magmas 
  are 
  expelled 
  as 
  soon 
  as 
  the 
  mass 
  of 
  melted 
  material 
  

   had 
  increased 
  to 
  a 
  certain 
  limit 
  dependent 
  upon 
  local 
  condi- 
  

   tions. 
  If 
  so, 
  there 
  must 
  be 
  little 
  time 
  for 
  the 
  fulfillment 
  on 
  a 
  

   large 
  scale 
  of 
  a 
  process 
  so 
  slow 
  as 
  molecular 
  flow. 
  Doubtless 
  

   fusion 
  may 
  be 
  confined 
  to 
  a 
  nearly 
  homogeneous 
  portion 
  of 
  the 
  

   earth's 
  lithoid 
  shell. 
  If 
  the 
  hypothesis 
  explained 
  above 
  of 
  

  

  