﻿Lindgren 
  — 
  Granodiorite 
  and 
  other 
  intermediate 
  rocks. 
  269 
  

  

  Art. 
  XXVII. 
  — 
  Granodiorite 
  and 
  other 
  intermediate 
  rocks 
  ; 
  

   by 
  Waldemar 
  Lindgren. 
  

  

  Introduction. 
  — 
  Along 
  the 
  great 
  mountain 
  chain 
  which 
  fol- 
  

   lows 
  the 
  Pacific 
  Coast 
  of 
  America 
  from 
  Cape 
  Horn 
  to 
  Alaska 
  

   the 
  intrusive 
  rocks 
  and 
  the 
  phenomena 
  accompanying 
  them 
  

   are 
  displayed 
  upon 
  a 
  scale 
  hardly 
  rivaled 
  elsewhere. 
  More- 
  

   over, 
  these 
  intrusions 
  have 
  taken 
  place 
  in 
  comparatively 
  recent 
  

   times, 
  during 
  Jurassic, 
  Cretaceous 
  and 
  even 
  Tertiary 
  times. 
  

   The 
  result 
  was 
  the 
  formation 
  or 
  the 
  accentuation 
  of 
  the 
  great 
  

   ranges 
  now 
  overlooking 
  the 
  Pacific 
  Ocean. 
  The 
  uplift 
  which 
  

   accompanied 
  the 
  displacing 
  intrusion 
  of 
  enormous 
  masses 
  of 
  

   magma, 
  was 
  followed 
  by 
  a 
  period 
  of 
  erosion 
  of 
  scarcely 
  appre- 
  

   ciated 
  magnitude. 
  This 
  erosion 
  brought 
  to 
  light 
  the 
  deeply 
  

   buried, 
  intruded 
  granular 
  rocks 
  as 
  great, 
  irregular 
  bodies, 
  

   oatholiths. 
  sometimes 
  continuous 
  for 
  hundreds 
  of 
  miles. 
  Before 
  

   these 
  remarkable 
  relations 
  had 
  been 
  elucidated, 
  the 
  granular 
  

   rocks 
  of 
  the 
  Pacific 
  Coast 
  were 
  often 
  regarded 
  as 
  Archean 
  ; 
  

   and 
  a 
  close 
  scrutiny 
  of 
  supposedly 
  older 
  areas 
  brings 
  almost 
  

   yearly 
  additions 
  to 
  the 
  Mesozoic 
  and 
  Tertiary 
  batholiths. 
  

  

  Looking 
  over 
  the 
  results 
  thus 
  far 
  obtained 
  by 
  investigations 
  

   of 
  these 
  rocks, 
  one 
  cannot 
  fail 
  to 
  note 
  the 
  prevalence 
  of 
  rocks 
  

   occupying 
  intermediate 
  places 
  between 
  granite 
  and 
  quartz- 
  

   diorite. 
  The 
  latter 
  are 
  fairly 
  common 
  ; 
  the 
  true 
  granites 
  are 
  

   relatively 
  rare 
  ; 
  most 
  common 
  appears 
  an 
  intermediate 
  rock 
  in 
  

   which 
  the 
  soda-lime 
  feldspars 
  predominate 
  over 
  the 
  orthoclase, 
  

   and 
  in 
  which 
  the 
  silica 
  varies 
  from 
  60 
  to 
  70 
  per 
  cent. 
  

  

  Granodiorite, 
  origin 
  of 
  name. 
  — 
  Soon 
  after 
  the 
  beginning 
  of 
  

   the 
  geological 
  mapping 
  of 
  the 
  Sierra 
  Nevada 
  in 
  California 
  in 
  

   1886, 
  it 
  became 
  apparent 
  that 
  the 
  rocks 
  of 
  that 
  region 
  which 
  

   had 
  previously 
  been 
  referred 
  to 
  as 
  granite 
  were, 
  to 
  a 
  large 
  

   extent, 
  of 
  intermediate 
  character 
  and 
  could 
  properly 
  be 
  referred 
  

   neither 
  to 
  the 
  granites, 
  nor 
  to 
  the 
  quartz-diorites. 
  Besides 
  

   these 
  intermediate 
  rocks, 
  quartz-diorites, 
  diorites, 
  and 
  gabbros 
  

   were 
  recognized. 
  The 
  prevailing 
  intermediate 
  rock 
  from 
  dif- 
  

   ferent 
  parts 
  of 
  the 
  Sierra 
  Xevada 
  was 
  studied 
  by 
  the 
  geologists 
  

   then 
  working 
  there 
  (G. 
  F. 
  Becker, 
  H. 
  W. 
  Turner, 
  and 
  W. 
  

   Lindgren) 
  and 
  was 
  found 
  to 
  have 
  a 
  fairly 
  constant 
  character, 
  

   consisting 
  of 
  a 
  medium 
  to 
  coarse-grained 
  aggregate 
  of 
  quartz, 
  

   oligoclase 
  or 
  andesine, 
  orthoclase, 
  biotite, 
  hornblende 
  and 
  

   various 
  accessory 
  constituents, 
  among 
  which 
  titanite 
  was 
  most 
  

   prominent. 
  

  

  A 
  series 
  of 
  separations 
  by 
  the 
  Thoulet 
  solution 
  were 
  made 
  

   by 
  the 
  writer 
  of 
  rocks 
  from 
  various 
  points 
  of 
  the 
  Sierra 
  Nevada, 
  

   partly 
  from 
  the 
  foothills, 
  partly 
  from 
  the 
  Yosemite 
  Valley, 
  and 
  

   partly 
  from 
  the 
  eastern 
  slope 
  of 
  the 
  Sierra, 
  in 
  the 
  State 
  of 
  

  

  Am. 
  Jour. 
  Sci. 
  — 
  Fourth 
  Series, 
  Vol. 
  IX, 
  Xo. 
  52.— 
  April, 
  1900. 
  

   19 
  

  

  