﻿36 
  Smyth, 
  Jr. 
  — 
  Composition 
  of 
  the 
  Alkaline 
  Rocks 
  

  

  separated 
  by 
  gravity, 
  during 
  a 
  gaseous 
  stage 
  of 
  the 
  earth, 
  into 
  

   an 
  upper, 
  subalkaline, 
  and 
  a 
  lower, 
  alkaline, 
  layer 
  which 
  

   furnish 
  the 
  respective 
  types 
  of 
  rock, 
  as 
  well 
  as 
  intermediate 
  

   varieties 
  due 
  to 
  mixing. 
  

  

  Jensen* 
  advances 
  the 
  hypothesis 
  " 
  that 
  alkaline 
  rocks 
  are 
  

   derived 
  from 
  . 
  . 
  . 
  Archaean 
  saline 
  beds 
  which, 
  by 
  chemical 
  

   attacks 
  on 
  the 
  adjacent 
  sediments, 
  have 
  given 
  rise 
  to 
  an 
  alka- 
  

   line 
  magma 
  in 
  the 
  process 
  of 
  metamorphosis. 
  This 
  magma 
  

   has 
  been 
  squeezed 
  laterally 
  into 
  continental 
  areas 
  and 
  has 
  

   undergone 
  differentiation, 
  or 
  it 
  has 
  mixed 
  with 
  other 
  magmas, 
  

   chiefly 
  basic, 
  and 
  then 
  differentiated." 
  

  

  Daly,f 
  as 
  previously 
  stated, 
  regards 
  alkaline 
  magmas 
  as 
  

   derived 
  from 
  subalkaline 
  magmas, 
  but 
  finds 
  the 
  cause 
  of 
  differ- 
  

   entiation 
  in 
  the 
  assimilation 
  of 
  limestone, 
  both 
  the 
  carbon 
  

   dioxide 
  of 
  the 
  latter, 
  and 
  the 
  lime, 
  being 
  active 
  agents. 
  These 
  

   agents 
  he 
  views 
  as 
  disturbing 
  the 
  chemical 
  equilibrium 
  of 
  the 
  

   magma, 
  thus 
  tending 
  to 
  differentiation 
  and 
  the 
  resultant 
  pro- 
  

   duction 
  of 
  the 
  alkaline 
  rocks. 
  

  

  Harker+ 
  differs 
  from 
  Daly 
  in 
  seeking 
  the 
  chief 
  cause 
  of 
  the 
  

   special 
  differentiation 
  that 
  produces 
  alkaline 
  rocks 
  in 
  mechan- 
  

   ical, 
  rather 
  than 
  chemical, 
  conditions 
  and, 
  noting 
  an 
  association 
  

   of 
  this 
  " 
  branch 
  " 
  of 
  rocks 
  with 
  the 
  Atlantic 
  type 
  of 
  structure, 
  

   as 
  defined 
  by 
  Suess, 
  concludes 
  that 
  both 
  are 
  products 
  of 
  the 
  

   same 
  mechanical 
  conditions. 
  

  

  Thus, 
  the 
  hypotheses 
  mentioned 
  explain 
  alkaline 
  magmas 
  as 
  

   primordial 
  or 
  as 
  due 
  to 
  melting 
  of 
  saline 
  sediments, 
  with 
  some 
  

   assimilation, 
  to 
  assimilation 
  and 
  consequent 
  differentiation, 
  or 
  

   to 
  differentiation 
  caused 
  by 
  crustal 
  disturbances. 
  

  

  As 
  already 
  stated, 
  the 
  present 
  writer 
  considers 
  the 
  alkaline 
  

   magmas 
  to 
  be 
  derived 
  from 
  the 
  subalkaline 
  magmas, 
  as 
  indicated 
  

   not 
  only 
  by 
  the 
  relatively 
  small 
  amount 
  and 
  local 
  occurrence 
  

   of 
  the 
  former, 
  but 
  also 
  by 
  the 
  association 
  of 
  the 
  two 
  types, 
  the 
  

   existence 
  of 
  intermediate 
  varieties 
  and 
  the 
  successive 
  appear- 
  

   ance 
  of 
  both 
  in 
  a 
  given 
  region. 
  This 
  view 
  is 
  further 
  supported 
  

   by 
  certain 
  peculiarities 
  of 
  chemical 
  composition 
  of 
  the 
  alkaline 
  

   rocks 
  which, 
  taken 
  in 
  conjunction 
  with 
  their 
  small 
  quantity, 
  

   are 
  thought 
  to 
  be 
  very 
  suggestive 
  as 
  to 
  the 
  agents 
  and 
  condi- 
  

   tions 
  of 
  their 
  origin. 
  

  

  As 
  their 
  name 
  implies, 
  and 
  as 
  shown 
  by 
  the 
  analyses 
  given 
  

   above, 
  the 
  alkaline 
  rocks 
  are, 
  as 
  a 
  rule, 
  high 
  in 
  alkalies, 
  par- 
  

   ticularly 
  sodium, 
  but, 
  for 
  the 
  present 
  purpose, 
  attention 
  is 
  

   directed, 
  not 
  to 
  the 
  dominant 
  constituents, 
  but 
  rather 
  to 
  those 
  

  

  * 
  Jensen, 
  H. 
  I. 
  : 
  The 
  Distribution, 
  Origin 
  and 
  Relationships 
  of 
  Alkaline 
  

   Rocks, 
  Proc. 
  Linn. 
  Soc. 
  N. 
  S. 
  Wales, 
  xxxiii, 
  pp. 
  585-586, 
  1908. 
  

  

  fDaly, 
  R. 
  A. 
  : 
  Origin 
  of 
  the 
  Alkaline 
  Rocks, 
  Bull. 
  Geol. 
  Soc. 
  Am., 
  xxi, 
  

   pp. 
  87-118, 
  1910. 
  

  

  JHarker, 
  A. 
  : 
  The 
  Natural 
  History 
  of 
  Igneous 
  Rocks, 
  pp. 
  102 
  and 
  330 
  

   et 
  seq, 
  1909. 
  

  

  