﻿Lindgren 
  — 
  Granodiorite 
  and 
  other 
  intermediate 
  rocks. 
  277 
  

  

  should 
  be 
  stated 
  that 
  the 
  granodiorites 
  of 
  the 
  Sierra 
  Nevada 
  

   vary 
  between 
  59 
  and 
  69 
  per 
  cent 
  ; 
  but 
  just 
  as 
  granites 
  and 
  

   quartz-diorites 
  include 
  rocks 
  with 
  up 
  to 
  74 
  per 
  cent 
  or 
  even 
  

   more 
  of 
  silica, 
  so 
  should 
  the 
  family 
  of 
  the 
  granodiorites 
  include 
  

   similar 
  acidic 
  types, 
  though 
  they 
  do 
  not 
  appear 
  to 
  be 
  of 
  very 
  

   common 
  occurrence. 
  

  

  In 
  regard 
  to 
  the 
  lime 
  it 
  may 
  be 
  said 
  that 
  the 
  upper 
  limit 
  

   (6-50 
  per 
  cent) 
  is 
  rarely 
  reached 
  and 
  when 
  at 
  the 
  same 
  time 
  the 
  

   potash 
  approaches 
  1 
  per 
  cent 
  the 
  rock 
  should 
  rather 
  be 
  classed 
  

   as 
  a 
  quartz- 
  diorite. 
  When 
  the 
  percentage 
  of 
  silica 
  falls 
  below 
  

   59 
  the 
  percentage 
  of 
  orthoclase 
  is 
  usually 
  also 
  lowered 
  to 
  such 
  a 
  

   degree 
  that 
  the 
  rocks 
  become 
  diorites. 
  

  

  With 
  high 
  silica 
  in 
  rocks 
  which 
  otherwise 
  correspond 
  exactly 
  

   to 
  the 
  composition 
  of 
  a 
  granodiorite 
  the 
  lime 
  is 
  apt 
  to 
  fall 
  low 
  

   and 
  may 
  in 
  rocks 
  closely 
  related 
  to 
  monzonites 
  even 
  descend 
  

   to 
  2*80 
  per 
  cent. 
  

  

  The 
  truly 
  characteristic 
  features 
  of 
  the 
  granodiorites 
  is 
  that 
  

   the 
  soda-lime 
  feldspar, 
  which 
  always 
  is 
  a 
  calcareous 
  oligoclase 
  

   or 
  an 
  andesine, 
  is 
  at 
  least 
  equal 
  to 
  double 
  the 
  amount 
  of 
  the 
  

   alkali 
  feldspar. 
  The 
  latter 
  may 
  be 
  taken 
  to 
  vary 
  from 
  8 
  per 
  

   cent 
  to 
  20 
  per 
  cent. 
  Below 
  the 
  lower 
  limit 
  the 
  rock 
  becomes 
  

   a 
  quartz-diorite 
  : 
  above 
  the 
  upper 
  a 
  quartz-monzonite. 
  

  

  Distribution 
  of 
  types. 
  — 
  As 
  with 
  all 
  other 
  classes 
  of 
  rocks, 
  

   granodiorite 
  presents 
  somewhat 
  varying 
  types 
  in 
  different 
  

   localities. 
  Many 
  of 
  the 
  intrusive 
  areas 
  in 
  the 
  foothill 
  region 
  

   and 
  on 
  the 
  middle 
  slopes 
  of 
  the 
  Sierra 
  Nevada 
  are 
  associated 
  

   with 
  normal 
  diorites, 
  quartz-diorites 
  and 
  gabbros. 
  As 
  a 
  con- 
  

   sequence 
  of 
  the 
  generally 
  more 
  basic 
  character 
  of 
  the 
  magma, 
  

   the 
  granodiorite 
  of 
  the 
  foothills 
  shows 
  a 
  basic 
  type 
  with 
  

   relatively 
  higher 
  percentage 
  of 
  lime 
  and 
  lower 
  of 
  potash 
  (see 
  

   IV, 
  V, 
  VI, 
  VII). 
  In 
  the 
  great 
  granitic 
  areas 
  of 
  the 
  Sierra 
  the 
  

   general 
  type 
  of 
  the 
  magma 
  is 
  more 
  acidic, 
  hence 
  the 
  granodio- 
  

   rite 
  tends 
  more 
  towards 
  an 
  acidic 
  type 
  without 
  exceeding 
  the 
  

   limits 
  of 
  the 
  family. 
  However, 
  extensive 
  masses 
  of 
  grano- 
  

   diorites 
  of 
  the 
  more 
  basic 
  type 
  (as 
  well 
  as 
  diorites) 
  exist 
  on 
  the 
  

   quadrangles 
  of 
  Truckee 
  and 
  Colfax, 
  in 
  the 
  High 
  Sierra. 
  These 
  

   two 
  types 
  resemble 
  each 
  other 
  so 
  much 
  that 
  their 
  separation 
  in 
  

   the 
  field 
  appears 
  entirely 
  impracticable. 
  From 
  a 
  cursory 
  

   examination 
  it 
  appears 
  probable 
  that 
  much 
  of 
  the 
  granitic 
  

   masses 
  of 
  Southern 
  California 
  belong 
  to 
  the 
  granodiorites. 
  A 
  

   large 
  part 
  of 
  those 
  of 
  the 
  peninsular 
  range 
  of 
  Lower 
  California 
  

   are 
  certainly 
  granodiorites, 
  as 
  I 
  have 
  shown 
  by 
  microscopical 
  

   examination 
  and 
  separation 
  by 
  Thoulet 
  Solution.* 
  

  

  Relation 
  to 
  allied 
  rocks. 
  — 
  In 
  1895 
  Prof. 
  W. 
  C. 
  Brdgger 
  

   established 
  the 
  new 
  family 
  of 
  the 
  monzonites,f 
  rocks 
  inter- 
  

   mediate 
  in 
  composition 
  between 
  granite 
  and 
  diorite. 
  

  

  *Proc. 
  Cal. 
  Acad. 
  Sci., 
  2nd 
  Ser.. 
  vol. 
  i, 
  part 
  2, 
  p. 
  6, 
  1888. 
  

   fDie 
  Eruptions 
  folge 
  der 
  triadischen 
  Eruptivgesteinen 
  bei 
  Predazzo 
  in 
  Sud- 
  

   tyrol. 
  Kristiania, 
  1895, 
  p. 
  19. 
  

  

  