﻿38 
  Smyth, 
  Jr. 
  — 
  Composition 
  of 
  the 
  Alkaline 
  Rocks 
  

  

  cause, 
  the 
  mineralizers 
  being 
  the 
  active 
  agents, 
  which, 
  through 
  

   their 
  affinity 
  for 
  the 
  rarer 
  elements, 
  with 
  which 
  they 
  form 
  

   mobile 
  compounds, 
  concentrate 
  the 
  latter 
  in 
  the 
  magmatic 
  

   extracts 
  which 
  furnish 
  the 
  materials 
  for 
  pegmatitic 
  intrusions. 
  

  

  As 
  a 
  result 
  of 
  this 
  process, 
  the 
  rarer 
  elements 
  of 
  a 
  normal 
  

   magma, 
  which 
  may 
  originally 
  have 
  been 
  so 
  diffused 
  through 
  

   the 
  mass 
  as 
  to 
  be 
  hardly 
  perceptible, 
  become 
  concentrated 
  in 
  a 
  

   comparatively 
  small 
  amount 
  of 
  magma, 
  which, 
  being 
  tapped 
  

   off 
  separately, 
  solidifies 
  as 
  a 
  rock 
  different 
  in 
  composition 
  from 
  

   that 
  representing 
  the 
  original, 
  magma, 
  and 
  characterized 
  by 
  the 
  

   presence, 
  in 
  relatively 
  large 
  amounts, 
  of 
  the 
  rare 
  elements 
  and 
  

   mineralizers, 
  although, 
  as 
  a 
  rule, 
  the 
  latter 
  are 
  in 
  large 
  part 
  

   dissipated. 
  

  

  Indeed, 
  but 
  for 
  this 
  process, 
  many 
  of 
  the 
  rare 
  elements 
  

   would 
  doubtless 
  be 
  unknown 
  to 
  us. 
  Even 
  with 
  such 
  a 
  method 
  

   of 
  concentration 
  in 
  existence, 
  radium, 
  in 
  spite 
  of 
  its 
  striking 
  

   properties, 
  was 
  discovered 
  only 
  through 
  the 
  exercise 
  of 
  extra- 
  

   ordinary 
  skill 
  and 
  patience 
  ; 
  and 
  there 
  can 
  be 
  little 
  doubt 
  that 
  

   there 
  are 
  other 
  elements 
  with 
  such 
  limited 
  tendency 
  toward 
  

   natural 
  concentration 
  as 
  to 
  put 
  their 
  detection 
  beyond 
  our 
  

   present 
  means 
  of 
  accomplishment. 
  

  

  Thus, 
  there 
  is 
  a 
  very 
  general 
  tendency 
  in 
  magmas 
  toward 
  

   the 
  segregation 
  of 
  the 
  rare 
  elements 
  and 
  the 
  mineralizers 
  in 
  a 
  

   distinct 
  part, 
  usually 
  erupted 
  separately 
  and 
  always 
  in 
  much 
  

   smaller 
  quantity 
  than 
  the 
  average 
  magmas. 
  The 
  composition 
  

   of 
  the 
  latter, 
  moreover, 
  is 
  not 
  perceptibly 
  changed 
  by 
  the 
  

   withdrawal 
  of 
  the 
  rarer 
  elements, 
  since, 
  • 
  compared 
  with 
  the 
  

   total 
  mass, 
  their 
  quantity 
  is 
  entirely 
  negligible. 
  This, 
  how- 
  

   ever, 
  is 
  not 
  necessarily 
  true 
  of 
  the 
  gaseous 
  constituents, 
  as 
  they 
  

   may 
  have 
  been 
  present 
  in 
  considerable 
  amount, 
  and 
  subse- 
  

   quently 
  dissipated. 
  

  

  Between 
  these 
  phenomena 
  and 
  those 
  presented 
  by 
  the 
  alka- 
  

   line 
  and 
  subalkaline 
  rocks 
  as 
  a 
  whole, 
  the 
  analogy 
  is 
  too 
  strik- 
  

   ing 
  to 
  be 
  accidental, 
  in 
  spite 
  of 
  the 
  great 
  difference 
  in 
  order 
  of 
  

   magnitude. 
  This 
  difference 
  of 
  scale 
  is 
  such 
  that 
  no 
  close 
  asso- 
  

   ciation, 
  either 
  in 
  time 
  or 
  in 
  space, 
  can 
  be 
  expected 
  in 
  the 
  case 
  

   of 
  the 
  two 
  groups 
  of 
  rocks, 
  like 
  that 
  existing 
  between 
  pegma- 
  

   tites 
  and 
  their 
  associated 
  rocks, 
  but 
  contemporaneous 
  or 
  

   successional 
  association 
  of 
  the 
  two 
  branches, 
  though 
  less 
  inti- 
  

   mate, 
  is 
  practically 
  universal 
  ; 
  while 
  the 
  quantitative 
  relation 
  

   between 
  the 
  alkaline 
  and 
  subalkaline 
  rocks 
  on 
  the 
  one 
  hand, 
  

   and 
  the 
  pegmatites 
  and 
  their 
  associated 
  rocks, 
  on 
  the 
  other, 
  are 
  

   analogous, 
  just 
  as 
  are 
  the 
  chemical 
  relations. 
  With 
  reference 
  

   to 
  the 
  latter, 
  it 
  is 
  true 
  that 
  definite 
  analytical 
  data, 
  as 
  to 
  the 
  

   quantities 
  of 
  the 
  rarer 
  elements 
  mentioned 
  above, 
  are 
  meager, 
  

   but, 
  so 
  far 
  as 
  they 
  exist, 
  they 
  justify 
  the 
  conclusion 
  reached 
  ; 
  

   and 
  substantiation 
  is 
  afforded 
  by 
  field 
  relations, 
  the 
  rare 
  ele- 
  

  

  