﻿W. 
  Lindgren 
  — 
  Granitic 
  Rocks 
  of 
  California. 
  309 
  

  

  erosion. 
  The 
  outcrops 
  are 
  of 
  rounded 
  form, 
  often 
  weathering 
  

   into 
  huge 
  detached 
  boulders 
  ; 
  the 
  color 
  is 
  very 
  light 
  gray. 
  The 
  

   granodiorite 
  is 
  a 
  medium 
  to 
  coarse-grained 
  rock, 
  the 
  average 
  

   diameter 
  of 
  the 
  grain 
  being 
  2 
  to 
  3 
  millimeters. 
  The 
  grayish 
  

   quartz 
  and 
  white 
  feldspar 
  grains 
  are 
  of 
  about 
  equal 
  size. 
  The 
  

   quartz 
  is 
  decidedly 
  less 
  prominent 
  than 
  in 
  the 
  granite, 
  and 
  the 
  

   feldspar 
  does 
  not 
  reach- 
  the 
  dimensions 
  attained 
  in 
  the 
  latter 
  

   rock. 
  Black 
  mica 
  and 
  hornblende 
  are 
  usually 
  present 
  in 
  about 
  

   equal 
  quantities. 
  The 
  foils 
  of 
  the 
  former 
  reach 
  2 
  or 
  3 
  milli- 
  

   meters 
  in 
  diameter, 
  while 
  the 
  hornblende 
  is 
  roughly 
  prismatic, 
  

   the 
  crystals 
  sometimes 
  attaining 
  1 
  centimeter 
  in 
  length. 
  By 
  

   reason 
  of 
  this 
  development 
  of 
  the 
  hornblende, 
  a 
  somewhat 
  por- 
  

   phyritic 
  aspect 
  may 
  occasionally 
  be 
  obtained. 
  Titanite 
  is 
  nearly 
  

   always 
  present 
  in 
  small 
  isolated 
  brownish 
  grains. 
  A 
  little 
  mag- 
  

   netite 
  is 
  also 
  a 
  constant 
  accessory 
  mineral. 
  The 
  appearance 
  and 
  

   composition 
  of 
  the 
  rock 
  is 
  very 
  constant 
  over 
  large 
  areas, 
  with 
  

   only 
  small 
  variations 
  in 
  grain 
  and 
  in 
  the 
  quantity 
  of 
  hornblende 
  

   and 
  biotite. 
  In 
  a 
  few 
  places 
  the 
  quantity 
  of 
  hornblende 
  dimin- 
  

   ishes 
  and 
  the 
  rock 
  then 
  assumes 
  a 
  habit 
  more 
  similar 
  to 
  that 
  of 
  

   granite 
  ; 
  thus, 
  for 
  instance, 
  at 
  Buck 
  Island 
  Lake, 
  between 
  Rubi- 
  

   con 
  Peak 
  and 
  Rubicon 
  Point 
  and 
  in 
  the 
  area 
  east 
  of 
  Fallen 
  

   Leaf 
  Lake. 
  Microscopical 
  and 
  chemical 
  investigation 
  shows 
  

   the 
  rock 
  at 
  this 
  point 
  to 
  be 
  a 
  granodiorite, 
  though 
  rather 
  rich 
  

   in 
  orthoclase. 
  I 
  

  

  Analysis 
  III 
  shows 
  the 
  composition 
  of 
  a 
  typical 
  granodiorite 
  

   from 
  the 
  northwestern 
  shore 
  of 
  Silver 
  Lake. 
  It 
  is 
  a 
  light-gray 
  

   granular 
  rock 
  composed 
  of 
  white 
  or 
  yellowish 
  feldspar 
  in 
  grains 
  

   and 
  imperfect 
  prisms, 
  grayish 
  quartz, 
  biotite 
  foils 
  up 
  to 
  1 
  mil- 
  

   limeter 
  in 
  diameter, 
  and 
  a 
  rather 
  abundant 
  dark-green 
  horn- 
  

   blende 
  in 
  well-defined 
  stout 
  prisms 
  up 
  to 
  8 
  millimeters 
  in 
  

   length. 
  Grains 
  of 
  brown 
  titanite 
  are 
  also 
  present. 
  

  

  The 
  microscrope 
  shows 
  the 
  structure 
  typical 
  for 
  granodior- 
  

   ites 
  : 
  Yery 
  plentiful, 
  roughly 
  idiomorphic 
  prismatic 
  crystals 
  

   of 
  an 
  acid 
  plagioclase, 
  sharply 
  outlined 
  foils 
  of 
  yellowish 
  brown 
  

   biotite 
  partly 
  decomposed 
  into 
  chlorite, 
  and 
  grains 
  of 
  imperfect 
  

   prisms 
  of 
  ordinary 
  brownish 
  green 
  hornblende, 
  accompanied 
  

   by 
  a 
  few 
  grains 
  of 
  magnetite. 
  These 
  constituents 
  are 
  cemented 
  

   by 
  anhedral 
  quartz, 
  orthoclase, 
  and 
  a 
  little 
  microline. 
  Titanite 
  

   occurs 
  in 
  small 
  grains 
  enclosed 
  in 
  biotite. 
  On 
  the 
  borders 
  of 
  

   the 
  plagioclase 
  and 
  orthoclase 
  a 
  little 
  micropegmatite 
  often 
  

   occurs. 
  Some 
  slight 
  evidences 
  of 
  crushing 
  are 
  present 
  in 
  this 
  

   rock, 
  though 
  such 
  phenomena 
  are 
  in 
  general 
  very 
  rare 
  in 
  the 
  

   granodiorites. 
  

  

  As 
  neither 
  the 
  hornblende 
  nor 
  the 
  biotite 
  has 
  been 
  analyzed, 
  

   it 
  is 
  clear 
  that 
  no 
  exact 
  calculation 
  of 
  this 
  analysis 
  can 
  be 
  made. 
  

   It 
  is, 
  however, 
  possible 
  to 
  arrive 
  at 
  the 
  approximate 
  composi- 
  

   tion 
  by 
  means 
  of 
  the 
  following 
  calculation 
  : 
  

  

  