﻿304 
  W. 
  Lindgren 
  — 
  Granitic 
  Rocks 
  of 
  California. 
  

  

  but 
  they 
  are 
  accessible 
  only 
  with 
  difficulty. 
  The 
  schistosity 
  is 
  

   indicated 
  on 
  the 
  outcrop 
  by 
  lines, 
  straight 
  on 
  the 
  whole, 
  but 
  

   delicately 
  wavy 
  in 
  detail 
  ; 
  heavy 
  benches 
  alternate 
  with 
  streaks, 
  

   in 
  which 
  the 
  lamination 
  is 
  very 
  tine. 
  Nodules 
  and 
  nests 
  of 
  

   apparently 
  segregated 
  quartz 
  are 
  common. 
  On 
  the 
  ridges 
  and 
  

   slopes 
  satisfactory 
  outcrops 
  are 
  rarely 
  seen, 
  as 
  the 
  rock 
  here 
  

   weathers 
  to 
  a 
  dark-red 
  soil. 
  

  

  A 
  part 
  of 
  the 
  area 
  in 
  the 
  southwestern 
  corner, 
  is 
  also 
  

   intensely 
  altered 
  ; 
  micaceous 
  schists 
  and 
  a 
  striped 
  green 
  

   and 
  white 
  schist, 
  consisting 
  of 
  pyroxene, 
  quartz, 
  feldspar 
  and 
  

   wollastonite, 
  evidently 
  a 
  product 
  of 
  contact 
  metamorphic 
  

   action 
  on 
  limestone, 
  appear 
  in 
  this 
  vicinity. 
  

  

  The 
  stratification 
  can 
  only 
  rarely 
  be 
  observed 
  beyond 
  doubt, 
  

   as 
  for 
  instance 
  where 
  quartzite 
  and 
  black 
  clay-slate 
  alternate, 
  

   but 
  it 
  is 
  probable 
  that 
  in 
  most 
  cases 
  the 
  stratification 
  approxi- 
  

   mately 
  coincides 
  with 
  the 
  superimposed 
  schistosity. 
  In 
  the 
  

   northern 
  area 
  the 
  strike 
  of 
  the 
  schistosity 
  is 
  generally 
  due 
  

   north 
  and 
  the 
  dip 
  either 
  about 
  vertical 
  or 
  westward 
  at 
  a 
  steep 
  

   angle, 
  this 
  being 
  contrary 
  to 
  the 
  general 
  rule 
  further 
  down 
  the 
  

   slope. 
  South 
  of 
  the 
  south 
  fork 
  of 
  the 
  American 
  River 
  the 
  

   strike 
  is 
  more 
  irregular, 
  but 
  generally 
  east-west, 
  while 
  the 
  dip 
  

   is 
  always 
  within 
  20° 
  of 
  the 
  perpendicular 
  and 
  usually 
  to 
  the 
  

   north. 
  Comparing 
  with 
  the 
  S.E. 
  part 
  of 
  Placerville 
  sheet 
  and 
  

   the 
  N.E. 
  part 
  of 
  Jackson 
  sheet, 
  it 
  will 
  be 
  seen 
  that 
  the 
  series 
  

   in 
  these 
  regions 
  also 
  has 
  an 
  abnormal 
  east-west 
  strike 
  ; 
  the 
  

   cause 
  may 
  possibly 
  be 
  sought 
  in 
  the 
  mechanics 
  of 
  the 
  intru- 
  

   sion, 
  the 
  slates 
  in 
  this 
  vicinity 
  being 
  especially 
  torn 
  up 
  by 
  

   deeply 
  incised 
  bays 
  of 
  granitic 
  rock. 
  Horizontal 
  and 
  inclined 
  

   joints 
  also 
  traverse 
  the 
  schists, 
  separating 
  them 
  into 
  rhomboidal 
  

   fragments. 
  The 
  contact 
  of 
  the 
  schists 
  with 
  the 
  granitic 
  rock 
  

   is 
  usually 
  best 
  defined 
  where 
  that 
  line 
  runs 
  parallel 
  to 
  the 
  

   schistosity. 
  Wherever 
  the 
  contact 
  cuts 
  across 
  the 
  strike 
  a 
  

   stronger 
  metamorphism, 
  accompanied 
  by 
  a 
  feathering 
  out 
  of 
  

   the 
  schists 
  and 
  by 
  an 
  injection 
  of 
  granitic 
  magma, 
  is 
  often 
  

   noted. 
  The 
  cleavage 
  of 
  the 
  schists 
  has 
  not 
  been 
  produced 
  by 
  

   the 
  pressure 
  of 
  the 
  intruding 
  magma 
  ; 
  it 
  existed 
  before 
  the 
  

   granitic 
  irruption. 
  

  

  A 
  few 
  isolated 
  areas 
  of 
  schists, 
  quartzites 
  and 
  highly-altered 
  

   tuffs 
  referred 
  to 
  the 
  Jura-trias 
  are 
  scattered 
  on 
  both 
  sides 
  of 
  

   the 
  crest 
  in 
  the 
  northern 
  part 
  of 
  the 
  area 
  of 
  the 
  Pyramid 
  Peak 
  

   sheet. 
  One 
  of 
  the 
  principal 
  reasons 
  for 
  assigning 
  them, 
  with 
  

   doubt, 
  however, 
  to 
  the 
  Jura-trias 
  is 
  their 
  position 
  in 
  the 
  contin- 
  

   uation 
  of 
  strata 
  known 
  to 
  be 
  of 
  that, 
  age 
  in 
  the 
  area 
  of 
  the 
  

   Truckee 
  sheet 
  adjoining 
  northward 
  ; 
  another 
  is 
  that 
  the 
  prin- 
  

   cipal 
  mass, 
  near 
  Mount 
  Tallac, 
  is 
  intimately 
  connected 
  with 
  

   large 
  masses 
  of 
  dark-green 
  diabase-porphyrite 
  and 
  porphyrite 
  

   tuff, 
  which 
  is 
  characteristic 
  of 
  the 
  Jura-trias 
  at 
  Sailor 
  Canyon 
  

   (Colfax 
  sheet) 
  and 
  northward. 
  The 
  color 
  of 
  the 
  outcrops 
  of 
  

  

  