﻿Ran 
  some 
  — 
  Neio 
  Occiirrence 
  of 
  Nepheline 
  Syenite. 
  421 
  

  

  geiieity 
  and 
  the 
  indefinite 
  patchy 
  intergrowths 
  frequently 
  

   observed 
  in 
  syenitic 
  rocks 
  containing 
  considerable 
  amounts 
  of 
  

   both 
  potash 
  and 
  soda. 
  

  

  The 
  nepheline 
  originally 
  occupied 
  areas 
  between 
  the 
  feldspars, 
  

   often 
  of 
  triangular 
  form, 
  but 
  it 
  is 
  almost 
  wholly 
  decomposed 
  

   and 
  replaced 
  by 
  fine 
  aggregates 
  of 
  secondary 
  products, 
  or 
  by 
  

   transparent 
  areas 
  of 
  analcite. 
  The 
  aggregates 
  are 
  usually 
  too 
  

   fine 
  to 
  allow 
  of 
  the 
  microscopic 
  determination 
  of 
  their 
  compo- 
  

   nent 
  minerals. 
  In 
  many 
  cases 
  however 
  they 
  consist 
  chiefly 
  of 
  

   a 
  sericitic 
  mica 
  (possibly 
  paragonite) 
  and 
  analcite, 
  the 
  latter 
  

   forming 
  a 
  mati'ix 
  in 
  which 
  the 
  minute 
  mica 
  scales 
  lie. 
  

  

  The 
  amphibole, 
  which 
  is 
  black 
  in 
  the 
  hand-specimens, 
  agrees 
  

   in 
  its 
  optical 
  character 
  with 
  the 
  hornblende 
  common 
  in 
  many 
  

   nepheline 
  syenites."^ 
  It 
  occurs 
  in 
  prisms 
  more 
  or 
  less 
  auto- 
  

   morphic 
  in 
  the 
  prism 
  zone. 
  The 
  optical 
  orientation 
  is 
  the 
  

   usual 
  one 
  a 
  = 
  a, 
  & 
  = 
  b, 
  and 
  cac 
  = 
  about 
  9°. 
  The 
  pleochro- 
  

   ism 
  is 
  a 
  greenish 
  yellow, 
  b 
  brown 
  or 
  dark 
  green, 
  and 
  c 
  dark 
  

   brownish 
  green. 
  The 
  absorption 
  is 
  b> 
  c> 
  a. 
  The 
  amphibole 
  

   is 
  not 
  optically 
  homogeneous, 
  a 
  single 
  crystal 
  frequently 
  show- 
  

   ing 
  considerable 
  variation 
  in 
  color 
  and 
  pleochroism. 
  It 
  is 
  

   often 
  intergrown 
  with 
  biotite, 
  the 
  latter 
  commonly 
  forming 
  an 
  

   outer 
  fringe 
  or 
  border 
  to 
  the 
  amphibole. 
  More 
  rarely 
  it 
  is 
  

   intergrown 
  with 
  a 
  pyroxene 
  which 
  is 
  apparently 
  an 
  segirine- 
  

   augite 
  ; 
  but 
  the 
  latter 
  mineral 
  is 
  never 
  abundant, 
  and 
  is 
  wholly 
  

   wanting 
  in 
  some 
  thin 
  sections. 
  

  

  The 
  biotite 
  possesses 
  no 
  features 
  of 
  special 
  interest 
  beyond 
  a 
  

   tendency 
  toward 
  poikilitic 
  structure 
  and 
  frequent 
  intergrowths 
  

   with 
  amphibole. 
  

  

  Cancrinite, 
  with 
  its 
  relatively 
  strong 
  double 
  refraction, 
  is 
  a 
  

   rather 
  conspicuous 
  constituent 
  in 
  some 
  thin-sections 
  when 
  

   viewed 
  with 
  crossed 
  nicols. 
  It 
  is 
  clear 
  and 
  colorless, 
  and 
  

   occurs 
  in 
  irregular 
  areas 
  or 
  in 
  forms 
  partly 
  idiomorphic 
  in 
  the 
  

   prism 
  zone. 
  It 
  is 
  found 
  sometimes 
  in 
  little 
  nests 
  associated 
  

   with 
  calcite 
  and 
  analcite, 
  or 
  completely 
  filling 
  interstices 
  

   between 
  the 
  feldspars, 
  or 
  even 
  embedded 
  in 
  the 
  latter. 
  The 
  

   prismatic 
  cleavage 
  is 
  marked 
  by 
  fine 
  straight 
  lines, 
  and 
  the 
  

   extinction 
  is 
  parallel. 
  Sections 
  approximately 
  normal 
  to 
  

   the 
  23rism 
  axis 
  give 
  a 
  negative 
  uniaxial 
  interference 
  figure. 
  

   The 
  index 
  of 
  refraction 
  is 
  distinctly 
  lower 
  than 
  the 
  balsam. 
  

   The 
  interference 
  colors 
  are 
  more 
  brilliant 
  than 
  those 
  of 
  the 
  

   other 
  minerals 
  in 
  the 
  thin 
  sections, 
  with 
  the 
  exception 
  of 
  the 
  

   muscovite. 
  However 
  they 
  rarely 
  exceed 
  blue 
  of 
  the 
  first 
  order, 
  

   and 
  are 
  usually 
  orange. 
  Confusion 
  with 
  muscovite 
  is 
  some- 
  

   times 
  possible 
  in 
  a 
  hasty 
  examination, 
  but 
  the 
  differences 
  in 
  

  

  *Rosenbascb, 
  Massige 
  Gesteine, 
  1896, 
  p. 
  158. 
  

  

  