﻿130 
  N. 
  L. 
  Bowen 
  — 
  Sodium- 
  Potassium 
  Nephelites. 
  

  

  plagioclase 
  and 
  those 
  of 
  the 
  latter 
  group 
  from 
  solutions 
  of 
  

   calcic 
  plagioclase, 
  but 
  it 
  is 
  impossible 
  to 
  be 
  sure 
  that 
  this 
  is 
  

   true 
  from 
  the 
  data 
  at 
  hand. 
  The 
  nephelites 
  average 
  upwards 
  

   of 
  12 
  per 
  cent 
  plagioclase 
  in 
  solid 
  solution. 
  

  

  The 
  peculiar 
  twinned 
  nephelite 
  described 
  by 
  Esch, 
  from 
  

   Etinde 
  volcano, 
  West 
  Africa, 
  seems 
  to 
  correspond 
  definitely 
  

   with 
  the 
  orthorhombic 
  form 
  of 
  the 
  potassium-rich 
  nephelites.* 
  

   Esch 
  observed 
  slightly 
  inclined 
  extinction 
  and 
  therefore 
  con- 
  

   siders 
  his 
  mineral 
  triclinic, 
  but 
  the 
  correspondence 
  is 
  too 
  great 
  

   in 
  other 
  respects 
  to 
  render 
  it 
  likely 
  that 
  his 
  form 
  is 
  distinct 
  

   from 
  the 
  artificial 
  form. 
  It 
  is 
  to 
  be 
  noted 
  that 
  the 
  nepheiin- 
  

   ites 
  described 
  by 
  him, 
  though 
  dominantly 
  soda-rich, 
  are 
  some- 
  

   times 
  rather 
  rich 
  in 
  potash 
  as 
  well, 
  running 
  to 
  leucitites. 
  Pos- 
  

   sibly, 
  then, 
  some 
  of 
  the 
  nephelite 
  is 
  potash-rich 
  and 
  closely 
  

   related 
  to 
  the 
  artificial 
  varieties 
  showing 
  the 
  same 
  twinning. 
  

   It 
  was 
  suggested 
  in 
  a 
  former 
  paper 
  that 
  Esch's 
  mineral 
  might 
  

   be 
  carnegieite 
  with 
  its 
  properties 
  modified 
  by 
  solid 
  solution, 
  but 
  

   this 
  suggestion 
  now 
  seems 
  quite 
  unjustified. 
  + 
  

  

  General 
  Considerations. 
  

  

  The 
  mineral 
  nephelite 
  proves 
  to 
  be 
  of 
  rather 
  complex 
  consti- 
  

   tution, 
  a 
  common 
  feature 
  of 
  a 
  number 
  of 
  rock-forming 
  min- 
  

   erals. 
  The 
  micas, 
  pyroxenes 
  and 
  amphiboles 
  exhibit 
  this 
  

   characteristic 
  in 
  even 
  more 
  marked 
  form 
  ; 
  indeed 
  their 
  consti- 
  

   tution 
  has 
  not 
  yet 
  been 
  satisfactorily 
  explained. 
  The 
  difficulty 
  

   is 
  the 
  result 
  of 
  the 
  binding 
  up, 
  within 
  one 
  crystalline 
  phase 
  or 
  

   mineral, 
  of 
  several 
  of 
  the 
  components^: 
  of 
  the 
  magma. 
  While 
  

   the 
  complexity 
  of 
  the 
  individual 
  crystalline 
  phases 
  is 
  greatly 
  

   increased 
  their 
  number 
  is 
  correspondingly 
  decreased 
  and, 
  in 
  

   some 
  respects, 
  a 
  simplification 
  of 
  the 
  process 
  of 
  crystallization 
  

   results. 
  This 
  fact 
  is 
  the 
  key 
  to 
  the 
  problem 
  of 
  the 
  " 
  gesteins- 
  

   serie." 
  It 
  is 
  the 
  crystallization 
  from 
  the 
  magma 
  of 
  these 
  min- 
  

   erals 
  of 
  continuously 
  varying 
  composition 
  which 
  results 
  in 
  the 
  

   formation 
  from 
  a 
  single 
  magma 
  of 
  a 
  series 
  of 
  rock-types 
  show- 
  

   ing 
  that 
  consanguinity 
  which 
  is 
  found 
  to 
  characterize 
  the 
  petro- 
  

   graphic 
  province. 
  

  

  Nephelite 
  syenite 
  is 
  undoubtedly 
  the 
  most 
  important 
  of 
  the 
  

   alkaline 
  rocks. 
  Alkalic 
  feldspar 
  is 
  a 
  prominent 
  constituent 
  

   and 
  through 
  it 
  nephelite 
  syenite 
  is 
  related 
  to 
  sub-alkaline 
  types. 
  

   The 
  suggestion 
  has 
  been 
  offered 
  in 
  another 
  paper 
  that 
  the 
  

   nephelite 
  syenites 
  are 
  intimately 
  related 
  to 
  the 
  mica-bearing 
  

   sub-alkaline 
  rocks, 
  biotite 
  granites. 
  § 
  They 
  are 
  considered 
  to 
  

  

  *Sitzb. 
  Berl. 
  Akad., 
  xviii, 
  400, 
  1901. 
  

  

  fN. 
  L. 
  Bowen, 
  this 
  Journal 
  (4), 
  xxxiii, 
  572. 
  1912. 
  

  

  j 
  Components 
  in 
  the 
  specialized 
  phase 
  rule 
  sense, 
  not 
  synonymous 
  with 
  

   constituents. 
  

  

  § 
  The 
  Later 
  Stages 
  of 
  the 
  Evolution 
  of 
  the 
  Igneous 
  Bocks, 
  Jour. 
  Geol. 
  

   Supplement 
  vol. 
  xxiii, 
  55, 
  1915. 
  

  

  