﻿312 
  W. 
  Lindgren 
  — 
  Granitic 
  Rocks 
  of 
  California. 
  

  

  Meeks 
  Creek, 
  Truckee 
  sheet, 
  a 
  few 
  miles 
  north 
  of 
  the 
  northern 
  

   boundary 
  of 
  the 
  Pyramid 
  Peak 
  sheet. 
  Both 
  in 
  appearance 
  and 
  

   composition 
  the 
  rock 
  is 
  a 
  normal 
  granodiorite. 
  

  

  Comparing 
  the 
  analytical 
  and 
  microscopical 
  results 
  with 
  the 
  

   field 
  notes, 
  it 
  is 
  clear 
  that 
  the 
  granodiorite, 
  as 
  it 
  appears 
  in 
  the 
  

   High 
  Sierra, 
  is 
  a 
  rock 
  of 
  well-defined 
  and 
  fairly 
  constant 
  com- 
  

   position, 
  structure, 
  and 
  appearance. 
  It 
  is 
  neither 
  a 
  normal 
  

   diorite, 
  nor 
  is 
  it 
  a 
  granite 
  ; 
  it 
  is 
  clearly 
  an 
  intermediate 
  type, 
  

   occupying 
  a 
  place 
  between 
  normal 
  quartz-mica-diorite 
  and 
  

   quartz-monzonite 
  (Brogger).* 
  All 
  transitions 
  toward 
  diorite 
  

   and, 
  more 
  rarely, 
  toward 
  granitite, 
  may 
  be 
  found, 
  but 
  they 
  are 
  

   local 
  and 
  do 
  not 
  cover 
  large 
  areas, 
  while 
  the 
  normal 
  grano- 
  

   diorite 
  is 
  the 
  prevailing 
  rock 
  of 
  the 
  Sierras. 
  Comparing 
  the 
  

   type 
  here 
  described 
  with 
  the 
  granodiorites 
  of 
  the 
  many 
  smaller 
  

   areas 
  enclosed 
  in 
  the 
  slates 
  on 
  the 
  western 
  flank 
  of 
  the 
  range, 
  it 
  

   can 
  be 
  stated 
  that 
  the 
  latter 
  as 
  a 
  rule 
  approach 
  more 
  closely 
  to 
  

   the 
  quartz-diorites, 
  the 
  percentage 
  of 
  lime 
  being 
  higher 
  and 
  

   the 
  percentage 
  of 
  potash 
  more 
  often 
  smaller 
  than 
  equal 
  to 
  that 
  

   of 
  soda. 
  A 
  few 
  of 
  these 
  smaller 
  granitic 
  areas 
  could, 
  in 
  fact, 
  

   almost 
  as 
  well 
  be 
  indicated 
  as 
  quartz-mica-diorites. 
  In 
  the 
  

   general 
  habit, 
  however, 
  in 
  the 
  percentage 
  of 
  quartz, 
  horn- 
  

   blende, 
  and 
  biotite, 
  and 
  in 
  the 
  constant 
  presence 
  of 
  titanite, 
  

   they 
  are 
  entirely 
  similar 
  to 
  the 
  granodiorites 
  of 
  the 
  High 
  

   Sierra. 
  Microcline, 
  not 
  common 
  in 
  the 
  granodiorites 
  of 
  the 
  

   foothill 
  region, 
  occurs 
  abundantly 
  in 
  those 
  of 
  the 
  High 
  Sierra. 
  

  

  Diorite 
  and 
  gabbro. 
  — 
  When 
  the 
  amount 
  of 
  hornblende 
  and 
  

   biotite 
  in 
  a 
  granodiorite 
  increases, 
  it 
  is 
  usual 
  to 
  find 
  the 
  quartz 
  

   and 
  orthoclase 
  relatively 
  diminished 
  in 
  quantity 
  and 
  rock 
  types 
  

   more 
  closely 
  allied 
  to 
  normal 
  diorites 
  result. 
  At 
  the 
  same 
  time 
  

   pyroxene 
  frequently 
  appears, 
  and 
  transitions 
  into 
  gabbro 
  are 
  

   formed. 
  These 
  more 
  basic 
  rocks 
  in 
  places 
  form 
  smaller 
  areas 
  

   enclosed 
  in 
  granodiorite 
  or 
  granite 
  ; 
  more 
  frequently 
  they 
  lie 
  

   between 
  the 
  two 
  rocks 
  or 
  on 
  the 
  contacts 
  between 
  granite 
  or 
  

   granodiorite 
  and 
  the 
  schists. 
  The 
  rock 
  in 
  these 
  areas 
  is 
  of 
  a 
  

   very 
  variable 
  structure 
  and 
  composition, 
  ranging 
  from 
  a 
  quartz 
  - 
  

   diorite 
  to 
  a 
  gabbro, 
  almost 
  approaching 
  a 
  peridotite. 
  The 
  lat- 
  

   ter 
  type 
  is, 
  however, 
  rare. 
  The 
  normal 
  diorite, 
  such 
  as 
  occurs 
  

   in 
  the 
  canyon 
  of 
  the 
  south 
  fork 
  of 
  the 
  American 
  Piver, 
  is 
  

   medium 
  to 
  coarse-grained, 
  composed 
  nearly 
  entirely 
  of 
  horn- 
  

   blende 
  and 
  plagioclase. 
  A 
  little 
  quarz, 
  however, 
  very 
  fre- 
  

   quently 
  enters 
  into 
  the 
  composition. 
  Typical 
  coarse-grained 
  

   gabbros 
  with 
  large 
  reddish 
  gray 
  basic 
  feldspars 
  and 
  dark 
  green 
  

  

  * 
  Though 
  it 
  is 
  often 
  difficult 
  in 
  practice 
  to 
  separate 
  the 
  normal 
  quartz-mica- 
  

   diorite 
  from 
  the 
  granodiorite, 
  it 
  would 
  seem 
  suitable 
  to 
  restrict 
  granodiorite 
  to 
  the 
  

   following 
  limits: 
  Si0 
  2 
  59 
  — 
  69 
  per 
  cent, 
  A1 
  2 
  3 
  14—17 
  per 
  cent. 
  Fe 
  2 
  3 
  H— 
  2£ 
  per 
  

   cent, 
  FeO 
  1$ 
  — 
  4£ 
  per 
  cent, 
  CaO 
  3 
  — 
  6| 
  per 
  cent, 
  MgO 
  1 
  — 
  2-J- 
  per 
  cent, 
  K 
  2 
  1 
  — 
  3f 
  

   Na 
  2 
  1\— 
  4*. 
  

  

  