﻿Lindgren 
  — 
  Granodiorite 
  and 
  other 
  intermediate 
  rocks. 
  275 
  

  

  spars 
  nearly 
  always 
  partly 
  idiomorphic 
  ; 
  these 
  minerals 
  are 
  

   usually 
  cemented 
  by 
  a 
  later 
  consolidated 
  mass 
  of 
  orthoclaseand 
  

   quartz. 
  The 
  orthoclase 
  may, 
  in 
  types 
  relatively 
  rich 
  in 
  K 
  2 
  0, 
  

   assume 
  the 
  form 
  of 
  microcline 
  ; 
  albite 
  and 
  perthitic 
  growths 
  

   are 
  ordinarily 
  absent, 
  but 
  the 
  latter 
  have 
  been 
  noted 
  in 
  small 
  

   amounts. 
  The 
  soda-lime 
  feldspar 
  generally 
  ranges 
  from 
  Ab 
  7 
  An 
  3 
  

   to 
  Ab^jAiij 
  ; 
  and 
  is 
  always 
  either 
  a 
  basic 
  oligoclase 
  or 
  an 
  ande- 
  

   sine 
  ; 
  acid 
  oligoclase 
  or 
  varieties 
  containing 
  more 
  lime 
  than 
  

   andesine 
  are 
  absent. 
  The 
  biotite 
  is 
  a 
  dark 
  brown 
  normal 
  

   variety 
  with 
  small 
  but 
  distinct 
  angle 
  between 
  the 
  optic 
  axes. 
  

   The 
  hornblende 
  is 
  green 
  or 
  brownish-green 
  with 
  a 
  maximum 
  

   extinction 
  of 
  18°-20°. 
  Pyroxene 
  (augite) 
  is 
  known 
  as 
  kernels 
  

   in 
  hornblende, 
  though 
  the 
  latter 
  is 
  certainly 
  not 
  of 
  secondary 
  

   origin. 
  It 
  is 
  by 
  no 
  means 
  impossible 
  that 
  granodiorites 
  with 
  a 
  

   notable 
  amount 
  of 
  pyroxene 
  may 
  be 
  found, 
  but 
  they 
  certainly 
  

   are 
  not 
  very 
  common 
  and 
  represent 
  no 
  widely 
  spread 
  type. 
  In 
  

   the 
  normal 
  types 
  both 
  biotite 
  and 
  hornblende 
  are 
  usually 
  pres- 
  

   ent. 
  But 
  either 
  one 
  may 
  occasionally 
  be 
  sparingly 
  represented 
  

   or 
  even 
  absent. 
  Titanite 
  is 
  an 
  always 
  present 
  constituent 
  of 
  

   the 
  rock. 
  Magnetite 
  is 
  only 
  present 
  in 
  small 
  quantities. 
  

  

  C. 
  Table 
  of 
  Mineralogical 
  Composition 
  of 
  Granodiorite. 
  

   Numbers 
  correspond 
  to 
  Table 
  A. 
  

  

  II 
  III 
  IV 
  

  

  Potassium 
  feldspar 
  15* 
  18* 
  17*75 
  

  

  j 
  Sodium 
  ieldspar 
  29 
  ,a 
  28* 
  a 
  ' 
  29'41 
  a 
  " 
  

  

  ( 
  Calcium 
  feldspar 
  15- 
  b 
  12'10 
  b 
  ' 
  ll-91 
  b 
  " 
  

  

  Biotite 
  ) 
  ) 
  , 
  

  

  Hornblende 
  \ 
  lb 
  ' 
  16-60 
  \ 
  lZ 
  ' 
  W 
  

  

  Quartz 
  22- 
  20'80 
  25'71 
  

  

  Apatite 
  -24 
  -30 
  -30 
  

  

  Magnetite 
  1-50 
  1-50 
  -84 
  

  

  Titanite 
  1- 
  1-40 
  1*40 
  

  

  Total 
  99 
  74 
  98*70 
  100-11 
  

  

  a 
  + 
  b 
  = 
  44- 
  =Ab 
  2 
  Au 
  1 
  , 
  with 
  34 
  per 
  cent 
  An. 
  

  

  a,+ 
  b 
  / 
  = 
  40-l 
  =Ab 
  7 
  An 
  a 
  , 
  with 
  30 
  per 
  cent 
  An. 
  

  

  a 
  /y 
  4- 
  b 
  tt 
  = 
  41-32 
  = 
  Ab 
  7 
  An 
  3 
  , 
  with 
  29 
  per 
  cent 
  An. 
  

  

  The 
  chemical 
  composition 
  of 
  the 
  granodiorites 
  in 
  the 
  Sierra 
  

   Nevada 
  is 
  characterized 
  by 
  a 
  percentage 
  of 
  silica 
  neither 
  very 
  

   high 
  nor 
  very 
  low, 
  ranging 
  from 
  59 
  to 
  69 
  per 
  cent. 
  The 
  iron 
  

   and 
  magnesia 
  are 
  relatively 
  low, 
  the 
  lime 
  on 
  the 
  contrary 
  rang- 
  

   ing 
  between 
  3 
  per 
  cent 
  and 
  6*50 
  per 
  cent, 
  though 
  the 
  latter 
  

   limit 
  is 
  rarely 
  reached. 
  The 
  sum 
  of 
  the 
  alkalies 
  ranges 
  from 
  

   4-92 
  to 
  7*13 
  and 
  thus 
  may 
  notably 
  exceed 
  the 
  lime, 
  but 
  never 
  

   fall 
  short 
  of 
  it 
  more 
  than 
  one 
  per 
  cent. 
  As 
  to 
  the 
  relation 
  

   between 
  K 
  a 
  O 
  and 
  Na 
  a 
  O 
  the 
  latter 
  is 
  apt 
  to 
  predominate, 
  rang- 
  

  

  