﻿282 
  Lindgren 
  — 
  Granodiorite 
  and 
  other 
  intermediate 
  rocks. 
  

  

  composition 
  of 
  quartz, 
  oligoclase 
  or 
  andesine, 
  orthoclase 
  or 
  

   microcline, 
  hornblende 
  or 
  biotite 
  (usually 
  both); 
  the 
  acces- 
  

   sories 
  being 
  titanite, 
  apatite, 
  magnetite 
  and 
  zircon. 
  The 
  quartz 
  

   may 
  average 
  23 
  per 
  cent, 
  the 
  soda-lime 
  feldspars 
  44 
  per 
  cent, 
  

   the 
  orthoclase 
  (with 
  microcline 
  and 
  albite) 
  14 
  per 
  cent, 
  varying 
  

   from 
  8 
  per 
  cent 
  to 
  20 
  per 
  cent, 
  the 
  ferro-magnesian 
  silicates 
  14 
  

   per 
  cent. 
  These 
  figures 
  are 
  not 
  claimed 
  to 
  be 
  an 
  exact 
  aver- 
  

   age, 
  which 
  in 
  the 
  nature 
  of 
  the 
  case 
  is 
  difficult 
  to 
  obtain, 
  but 
  

   they 
  correspond 
  to 
  a 
  fairly 
  typical 
  rock. 
  The 
  family 
  is 
  pro- 
  

   posed 
  to 
  represent 
  a 
  very 
  important 
  and 
  very 
  widespread 
  type 
  

   of 
  rocks, 
  especially 
  common 
  along 
  the 
  Pacific 
  slopes 
  of 
  the 
  

   Cordilleran 
  Ranges. 
  This 
  family 
  has 
  been 
  clearly 
  defined 
  ; 
  the 
  

   name 
  has 
  for 
  many 
  years 
  been 
  in 
  use 
  in 
  the 
  folios 
  of 
  the 
  TJ. 
  S. 
  

   Geological 
  Survey. 
  It 
  has 
  found 
  rapid 
  and 
  general 
  acceptance 
  

   by 
  reason 
  of 
  its 
  simple 
  construction 
  and 
  its 
  implied 
  definition. 
  

   But 
  chiefly 
  and 
  above 
  all 
  it 
  deserves 
  to 
  stand 
  because 
  repre- 
  

   senting 
  a 
  natural 
  group 
  of 
  rocks. 
  

   Washington, 
  D. 
  C, 
  December, 
  1899. 
  

  

  