﻿and 
  its 
  Significance 
  as 
  to 
  their 
  Origin. 
  43 
  

  

  In 
  this 
  connection, 
  it 
  is 
  of 
  interest 
  to 
  note 
  that 
  Suess* 
  says 
  : 
  

   •• 
  Thus 
  the 
  question 
  arises 
  whether 
  diminution 
  of 
  calcium 
  and 
  

   magnesium 
  in 
  the 
  Atlantic 
  hemisphere 
  may 
  not 
  stand 
  in 
  some 
  

   connection 
  with 
  the 
  progress 
  of 
  consolidation." 
  

  

  These 
  suggestions, 
  whose 
  exceedingly 
  hypothetical 
  nature 
  

   can 
  not 
  be 
  too 
  strongly 
  emphasized, 
  obviously 
  are 
  based 
  

   upon 
  the 
  view, 
  adopted 
  throughout 
  the 
  discussion, 
  that, 
  in 
  

   every 
  region 
  of 
  alkaline 
  magmas, 
  large 
  reservoirs 
  of 
  subalka- 
  

   line 
  magmas 
  exist 
  from 
  which 
  the 
  former 
  were 
  derived. 
  

   Whether 
  or 
  not 
  intrusion 
  and 
  extrusion 
  of 
  such 
  subalkaline 
  

   magmas 
  would 
  precede 
  the 
  formation 
  of 
  the 
  alkaline 
  magmas 
  

   would 
  depend 
  upon 
  local 
  conditions. 
  A 
  period 
  of 
  active 
  

   mountain-making, 
  with 
  attendant 
  intrusion 
  and 
  extrusion 
  of 
  

   subalkaline 
  rocks, 
  might 
  be 
  followed 
  by 
  a 
  period 
  of 
  relative 
  

   stability 
  with 
  the 
  delicate 
  adjustment 
  of 
  conditions 
  necessary 
  

   for 
  the 
  elaboration 
  of 
  alkaline 
  magmas. 
  Or, 
  on 
  the 
  other 
  

   hand, 
  it 
  might 
  equally 
  well 
  happen 
  that 
  the 
  subalkaline 
  mag- 
  

   mas 
  would 
  remain 
  at 
  depth, 
  while 
  the 
  alkaline 
  magmas 
  reached 
  

   higher 
  levels. 
  In 
  dealing 
  with 
  phenomena 
  of 
  such 
  a 
  large 
  

   order 
  of 
  magnitude, 
  involving 
  not 
  only 
  great 
  bodies 
  of 
  magma, 
  

   but, 
  also, 
  long 
  periods 
  of 
  time, 
  determinable 
  relationships 
  can 
  

   hardly 
  be 
  expected. 
  

  

  In 
  brief, 
  the 
  association 
  of 
  alkaline 
  and 
  subalkaline 
  rocks, 
  

   with 
  distinct 
  types 
  of 
  crustal 
  disturbance, 
  in 
  so 
  far 
  as 
  it 
  may 
  

   exist, 
  is 
  ascribed 
  to 
  the 
  presence 
  or 
  absence 
  of 
  opportunity 
  for 
  

   the 
  mineralizers 
  to 
  exert 
  their 
  influence 
  as 
  agents 
  of 
  differen- 
  

   tiation. 
  The 
  mineralizers 
  are 
  regarded 
  as 
  the 
  fundamental 
  

   factor 
  in 
  the 
  process, 
  the 
  mechanical 
  conditions 
  as 
  affording 
  

   secondary 
  control, 
  which, 
  though 
  perhaps 
  in 
  part 
  subject 
  to 
  

   Harker 
  and 
  Becke's 
  generalization 
  as 
  to 
  distribution, 
  may 
  ulti- 
  

   mately 
  prove 
  wholly 
  independent 
  of 
  it. 
  Much 
  more 
  thorough 
  

   investigation 
  is 
  required 
  before 
  the 
  extent 
  and 
  nature 
  of 
  the 
  

   relations 
  between 
  petrologic 
  and 
  tectonic 
  types 
  can 
  be 
  estab- 
  

   lished, 
  and. 
  while 
  some 
  connection 
  doubtless 
  exists, 
  there 
  can 
  

   be 
  no 
  question 
  that 
  the 
  relation 
  is 
  less 
  direct 
  and 
  vastly 
  more 
  

   complicated 
  than 
  is 
  implied 
  by 
  the 
  foregoing 
  bald 
  statement 
  or 
  

   than 
  could 
  be 
  expressed 
  by 
  any 
  statement 
  at 
  the 
  present 
  time. 
  

  

  If, 
  as 
  here 
  maintained, 
  the 
  magmas 
  contain, 
  within 
  them- 
  

   selves, 
  the 
  essential 
  agents 
  of 
  differentiation, 
  it 
  must 
  follow 
  

   that 
  when 
  these 
  agents 
  are 
  allowed, 
  by 
  surrounding 
  conditions, 
  

   to 
  operate, 
  their 
  effect 
  is 
  cumulative 
  and. 
  thus, 
  intensified 
  with 
  

   the 
  passage 
  of 
  time. 
  The 
  separation 
  of 
  an 
  alkaline 
  fraction 
  

   from 
  a 
  subalkaline 
  magma 
  implies 
  a 
  concentration, 
  in 
  the 
  for- 
  

   mer, 
  of 
  the 
  agents 
  which 
  caused 
  the 
  differentiation. 
  It 
  neces- 
  

   sarily 
  results 
  that 
  the 
  derivative 
  magma 
  has 
  a 
  greater 
  tendency 
  

   to 
  differentiate 
  than 
  the 
  original 
  subalkaline 
  magma 
  had. 
  

  

  * 
  Op. 
  cit., 
  iv, 
  p. 
  590. 
  

  

  