﻿audits 
  Significance 
  as 
  to 
  their 
  Origin. 
  35 
  

  

  to 
  the 
  truth, 
  while 
  the 
  nature 
  of 
  the 
  data 
  used 
  by 
  Clarke 
  is 
  

   such 
  as 
  to 
  inspire 
  particular 
  confidence 
  in 
  his 
  results. 
  

  

  The 
  figures 
  of 
  I, 
  then, 
  may 
  be 
  taken 
  as 
  representing 
  the 
  

   mean 
  composition 
  of 
  the 
  igneous 
  rocks, 
  or 
  the 
  average 
  magma, 
  

   including 
  not 
  only 
  the 
  abundant 
  subalkaline 
  rocks, 
  but 
  the 
  less 
  

   common 
  alkaline 
  rocks. 
  Indeed, 
  since 
  the 
  latter, 
  as 
  stated 
  

   above, 
  always 
  receive 
  special 
  attention, 
  it 
  is 
  probable 
  that 
  a 
  

   disproportionately 
  large 
  number 
  of 
  their 
  analyses 
  are 
  used 
  in 
  

   making 
  up 
  the 
  average, 
  thus 
  tending 
  to 
  bring 
  the 
  results 
  

   slightly 
  closer 
  than 
  they 
  should 
  be 
  to 
  the 
  average 
  for 
  alkaline 
  

   rocks. 
  But 
  whether 
  or 
  not 
  this 
  is 
  so, 
  it 
  is 
  evident 
  that 
  the 
  

   mean 
  composition, 
  as 
  given, 
  is 
  distinctly 
  subalkaline 
  in 
  charac- 
  

   ter, 
  as 
  is 
  clearly 
  shown 
  by 
  comparison 
  with 
  Daly's 
  average 
  of 
  

   the 
  composition 
  of 
  eighty-nine 
  diorites, 
  given 
  in 
  IV, 
  and 
  

   representing 
  a 
  typical 
  subalkaline 
  magma, 
  slightly 
  less 
  siliceous 
  

   than 
  the 
  average 
  rock, 
  and 
  with 
  corresponding 
  differences 
  in 
  

   other 
  respects, 
  but, 
  on 
  the 
  whole, 
  showing 
  fairly 
  close 
  agree- 
  

   ment 
  with 
  I. 
  

  

  On 
  the 
  other 
  hand, 
  the 
  same 
  writer's 
  average 
  of 
  twenty- 
  

   three 
  alkaline 
  syenites, 
  given 
  in 
  V, 
  while 
  having 
  nearly 
  the 
  

   same 
  silica 
  content 
  as 
  the 
  average 
  rock, 
  differs 
  from 
  it 
  markedly 
  

   in 
  other 
  respects, 
  particularly 
  in 
  having 
  higher 
  alumina, 
  lower 
  

   magnesia 
  and 
  lime 
  and 
  much 
  higher 
  alkalies, 
  features 
  generally 
  

   characteristic 
  of 
  alkaline 
  rocks. 
  In 
  view 
  of 
  the 
  fact, 
  stated 
  

   above, 
  that 
  the 
  alkaline 
  rocks 
  are 
  included 
  in 
  the 
  general 
  mean, 
  

   it 
  is 
  clear 
  that, 
  if 
  the 
  figures 
  of 
  V 
  are 
  at 
  all 
  typical, 
  the 
  alka- 
  

   line 
  rocks 
  must 
  exist 
  in 
  comparatively 
  small 
  quantity, 
  while 
  it 
  

   is 
  equally 
  clear 
  that 
  the 
  average 
  magma 
  must 
  be 
  distinctly 
  

   subalkaline. 
  The 
  latter 
  magma 
  is, 
  obviously, 
  of 
  world-wide 
  

   extent, 
  while 
  the 
  former 
  occurs 
  in 
  relatively 
  limited 
  amount, 
  

   but 
  at 
  widely 
  scattered 
  points. 
  Differing 
  from 
  the 
  average 
  

   rock 
  only 
  in 
  the 
  relative 
  percentages 
  of 
  elements 
  common 
  to 
  

   both, 
  the 
  alkaline 
  rocks 
  are 
  to 
  be 
  regarded 
  as 
  derivatives 
  of 
  the 
  

   subalkaline 
  magma 
  rather 
  than 
  as 
  something 
  essentially 
  dis- 
  

   tinct. 
  

  

  In 
  view 
  of 
  the 
  small 
  amounts 
  of 
  alkaline 
  rocks, 
  it 
  is 
  evi- 
  

   dent 
  that 
  the 
  moderate 
  concentration 
  in 
  them 
  of 
  such 
  

   abundant 
  constituents 
  as 
  alumina 
  and 
  the 
  alkalies 
  would 
  not 
  

   materially 
  affect 
  the 
  composition 
  of 
  the 
  greatly 
  preponderant 
  

   average 
  magma, 
  while 
  less 
  abundant 
  constituents 
  may 
  exist 
  in 
  

   the 
  latter 
  in 
  such 
  small 
  quantities 
  as 
  to 
  escape 
  detection, 
  and 
  

   yet 
  be 
  markedly 
  concentrated 
  in 
  the 
  derivative 
  alkaline 
  frac- 
  

   tion. 
  

  

  Quite 
  different 
  from 
  this 
  conception 
  of 
  the 
  derivation 
  of 
  

   alkaline 
  from 
  subalkaline 
  magmas 
  is 
  the 
  view 
  of 
  Becke,* 
  who 
  

   suggests 
  that 
  the 
  constituents 
  of 
  the 
  two 
  types 
  of 
  magma 
  were 
  

  

  * 
  Becke, 
  F. 
  : 
  Die 
  Eruptivgebiete 
  des 
  bohm. 
  Mittelgebirges 
  und 
  der 
  arnerik. 
  

   Andes, 
  Tscherrnak's 
  Min. 
  und 
  Petr. 
  Mitt., 
  xxii, 
  p. 
  247, 
  1903. 
  

  

  