﻿40 
  Smyth, 
  Jr. 
  — 
  Composition 
  of 
  the 
  Alkaline 
  Rooks 
  

  

  tration 
  from 
  the 
  main 
  magma 
  by 
  a 
  process 
  in 
  which 
  the 
  min- 
  

   eralizers 
  are 
  universally 
  recognized 
  as 
  having 
  played 
  a 
  leading 
  

   part. 
  

  

  Not 
  only 
  are 
  the 
  rare 
  elements 
  thus 
  concentrated 
  in 
  the 
  peg- 
  

   matites, 
  but 
  the 
  alkalies 
  themselves 
  appear 
  in 
  muscovite, 
  lepi- 
  

   dolite, 
  microcline, 
  albite, 
  etc. 
  In 
  other 
  words, 
  the 
  elements 
  

   whose 
  relative 
  abundance 
  is 
  a 
  characteristic 
  feature 
  of 
  the 
  alka- 
  

   line 
  rocks 
  in 
  general, 
  are 
  concentrated 
  in 
  pegmatites 
  through 
  the 
  

   agency 
  of 
  mineralizers 
  in 
  virtue 
  of 
  the 
  power 
  of 
  the 
  latter 
  to 
  

   nnite 
  with 
  these 
  elements 
  and 
  yield 
  mobile 
  compounds 
  which, 
  

   from 
  their 
  distinctive 
  physical 
  character, 
  are 
  capable 
  of 
  being 
  

   segregated 
  from 
  the 
  average 
  alkaline 
  magma. 
  Now, 
  since 
  the 
  

   alkaline 
  rocks, 
  as 
  a 
  whole, 
  differ 
  from 
  the 
  subalkaline 
  rocks 
  in 
  

   containing 
  larger 
  proportions 
  of 
  these 
  same 
  elements, 
  is 
  it 
  not 
  a 
  

   reasonable 
  conclusion 
  that 
  the 
  same 
  agents 
  have 
  effected 
  the 
  

   concentration 
  in 
  both 
  oases 
  ? 
  In 
  other 
  words, 
  the 
  facts 
  stated 
  

   above 
  suggest 
  that 
  the 
  alkaline 
  rocks 
  represent 
  magmas 
  of 
  

   exceptional 
  composition 
  derived 
  from 
  ordinary 
  subalkaline 
  

   magmas 
  through 
  the 
  agency 
  of 
  mineralizers, 
  and, 
  thus, 
  the 
  

   relation 
  between 
  the 
  two 
  branches 
  of 
  rock 
  is, 
  to 
  some 
  extent, 
  

   analogous 
  to 
  that 
  existing 
  between 
  a 
  local 
  magma 
  and 
  its 
  peg- 
  

   matitic 
  phases. 
  

  

  According 
  to 
  this 
  view, 
  the 
  pegmatites 
  of 
  an 
  alkaline 
  region 
  

   represent 
  a 
  final 
  stage 
  in 
  the 
  process 
  of 
  concentration 
  of 
  certain 
  

   elements, 
  which 
  started 
  in 
  a 
  subalkaline 
  magma, 
  and 
  was 
  effected 
  

   through 
  the 
  agency 
  of 
  mineralizers. 
  The 
  power 
  of 
  these 
  agents 
  

   is 
  due 
  to 
  their 
  own 
  mobility, 
  which 
  enables 
  them 
  to 
  permeate 
  

   a 
  magma, 
  together 
  with 
  their 
  capacity 
  to 
  form 
  mobile 
  com- 
  

   pounds 
  with 
  certain 
  elements. 
  As 
  a 
  result 
  of 
  these 
  two 
  proper- 
  

   ties, 
  the 
  mineralizers 
  are 
  able, 
  under 
  favorable 
  conditions, 
  to 
  

   extract 
  these 
  elements 
  from 
  the 
  magma 
  and 
  to 
  transfer 
  them 
  

   elsewhere. 
  The 
  elements 
  peculiarly 
  subject 
  to 
  such 
  extraction 
  

   are 
  clearly 
  shown 
  by 
  the 
  pegmatites, 
  and 
  the 
  same 
  elements 
  are 
  

   extracted 
  to 
  form 
  alkaline 
  rocks. 
  

  

  So 
  far 
  as 
  the 
  abundant 
  elements 
  of 
  the 
  alkaline 
  rocks 
  are 
  con- 
  

   cerned, 
  there 
  is 
  no 
  difficulty 
  in 
  accepting 
  their 
  concentration 
  as 
  

   the 
  result 
  of 
  diffusion, 
  fractional 
  crystallization, 
  gravity 
  or 
  what- 
  

   ever 
  agents 
  may 
  effect 
  the 
  usual 
  differentiations 
  of 
  magmas. 
  

   But 
  such 
  an 
  explanation 
  of 
  the 
  concentration 
  of 
  the 
  rare 
  ele- 
  

   ments 
  in 
  alkaline 
  magmas 
  seems 
  wholly 
  inadequate, 
  and 
  we 
  are 
  

   forced 
  to 
  have 
  recourse 
  to 
  some 
  agent 
  capable 
  of 
  extracting 
  

   minute 
  quantities 
  of 
  rare 
  elements 
  from 
  the 
  subalkaline 
  magmas 
  

   and 
  concentrating 
  them 
  in 
  the 
  alkaline 
  magmas. 
  The 
  high 
  

   atomic 
  weights 
  of 
  many 
  of 
  these 
  rare 
  elements 
  and 
  their 
  

   extreme 
  degree 
  of 
  dilution 
  would 
  both, 
  presumably, 
  tend 
  to 
  

   prevent 
  differentiation 
  by 
  diffusion, 
  and 
  the 
  only 
  agents 
  appar- 
  

   ently 
  capable 
  of 
  performing 
  the 
  task 
  are 
  the 
  mineralizers. 
  

  

  