﻿280 
  Lindgren 
  — 
  Granodiorite 
  and 
  other 
  intermediate 
  rocks. 
  

  

  In 
  this 
  manner 
  the 
  former 
  family 
  becomes 
  quite 
  extensive 
  and 
  

   certainly 
  includes 
  wider 
  limits 
  than 
  does 
  granodiorite. 
  In 
  a 
  

   postscript 
  to 
  his 
  discussion 
  of 
  the 
  rocks 
  from 
  Monzoni 
  (p. 
  182) 
  

   Prof. 
  Brogger 
  devotes 
  a 
  few 
  paragraphs 
  to 
  granodiorites, 
  stat- 
  

   ing 
  that 
  according 
  to 
  the 
  analysis 
  (No. 
  I 
  in 
  Table 
  A) 
  the 
  rock 
  

   clearly 
  belongs 
  to 
  the 
  qnartz-diorites 
  or 
  rather 
  to 
  the 
  small 
  

   group 
  between 
  the 
  diorites 
  and 
  the 
  quartz-diorites. 
  He 
  finally 
  

   concludes 
  that 
  granodiorite 
  is 
  not 
  used 
  in 
  the 
  same 
  sense 
  as 
  

   quartz-monzonite 
  but 
  rather 
  as 
  a 
  synonym 
  for 
  tonalite. 
  This 
  

   view 
  is 
  clearly 
  due 
  to 
  the 
  fact 
  that 
  Brogger 
  only 
  recognizes 
  one 
  

   intermediate 
  group, 
  placing 
  it 
  exactly 
  in 
  the 
  middle 
  between 
  

   the 
  granites 
  and 
  the 
  quartz-diorites. 
  The 
  above 
  discussion 
  has 
  

   sufficiently 
  set 
  forth 
  that 
  granodiorite 
  is 
  not 
  a 
  synonym 
  for 
  

   quartz-mica-diorite 
  or 
  tonalite, 
  which 
  is 
  a 
  typical 
  rock 
  of 
  that 
  

   kind, 
  except 
  in 
  this 
  sense 
  that 
  many 
  petrographers 
  have 
  formerly 
  

   used 
  the 
  term 
  quartz-mica-diorite, 
  in 
  absence 
  of 
  others, 
  for 
  rocks 
  

   comparatively 
  rich 
  in 
  orthoclase. 
  

  

  It 
  has 
  been 
  proposed* 
  by 
  Mr. 
  Turner 
  to 
  class 
  rocks 
  of 
  the 
  com- 
  

   position 
  of 
  IV 
  (and 
  consequently 
  also 
  rocks 
  like 
  III) 
  in 
  Table 
  A 
  as 
  

   quartz-monzonites. 
  This 
  can 
  only 
  be 
  done 
  by 
  disregarding 
  the 
  

   definition 
  given 
  by 
  Prof. 
  Brogger 
  of 
  this 
  latter 
  rock. 
  No. 
  IV 
  

   contains 
  17*75 
  per 
  cent 
  orthoclase 
  and 
  41*4:0 
  per 
  cent 
  soda-lime 
  

   feldspars 
  ; 
  this 
  is 
  clearly 
  a 
  rock 
  in 
  which 
  the 
  latter 
  feldspars 
  

   greatly 
  predominate 
  and 
  not 
  a 
  rock 
  with 
  approximately 
  equal 
  

   quantities 
  of 
  the 
  two 
  feldspars. 
  

  

  Taking 
  an 
  example 
  to 
  the 
  point, 
  there 
  are 
  in 
  Idaho 
  large 
  

   areas 
  of 
  a 
  granular 
  rock 
  having 
  the 
  following 
  composition 
  : 
  

  

  Si0 
  2 
  68-48 
  

  

  A1 
  2 
  3 
  _. 
  15-01 
  

  

  FeO 
  2-90 
  

  

  CaO 
  2-60 
  

  

  MgO 
  1-21 
  

  

  K 
  2 
  4-25 
  

  

  Na 
  2 
  3-22 
  

  

  Rest 
  2-33 
  

  

  100-00 
  

   From 
  the 
  data 
  obtained 
  by 
  microscopic 
  examination, 
  this 
  

   rock 
  may 
  be 
  calculated 
  as 
  follows 
  : 
  

  

  Alkali 
  feldspar 
  21* 
  

  

  Oligoclase 
  . 
  34* 
  

  

  Biotite 
  13- 
  

  

  Quartz 
  __ 
  29- 
  

  

  Accessories 
  3. 
  

  

  100- 
  

   * 
  H. 
  W. 
  Turner, 
  The 
  Granitic 
  Rocks 
  of 
  the 
  Sierra 
  Nevada. 
  Journal 
  of 
  Geology, 
  

   1899, 
  p. 
  141. 
  

  

  