﻿PEALE] 
  ERUPTIVE 
  ROCKS 
  SIERRA 
  LA 
  SAL. 
  97 
  

  

  1)0 
  evidei)ci3 
  of 
  the 
  sedimentary 
  beds 
  continning 
  across 
  the 
  mouutains, 
  

   and 
  the 
  red 
  beds 
  form 
  the 
  surface 
  formation 
  to 
  the 
  northeast 
  and 
  north, 
  

   and 
  even 
  on 
  some 
  of 
  the 
  peaks. 
  This 
  may 
  be 
  explain 
  d 
  by 
  the 
  fact 
  

   that 
  the 
  center 
  of 
  greatest 
  force 
  was 
  at 
  the 
  north, 
  and 
  he 
  force 
  of 
  up- 
  

   heaval 
  being 
  greater, 
  broke 
  the 
  strata 
  to 
  a 
  greater 
  extent, 
  rendering 
  

   them 
  more 
  easily 
  acted 
  upon 
  by 
  eroding 
  influences. 
  Whether 
  any 
  of 
  

   the 
  sedimentaries 
  extended 
  across 
  the 
  mountains 
  uninterruptedly 
  at 
  all 
  

   points 
  to 
  the 
  north, 
  we 
  cannot 
  say. 
  They 
  did 
  so 
  at 
  the 
  south, 
  but 
  there 
  

   were 
  in 
  all 
  probability 
  places 
  where 
  the 
  eruptive 
  material 
  broke 
  entirely 
  

   through, 
  reaching 
  the 
  surface. 
  At 
  present 
  we 
  find 
  the 
  highest 
  peaks 
  in 
  

   the 
  middle 
  group, 
  where 
  there 
  is 
  a 
  capping 
  of 
  sedimentary 
  beds 
  on 
  the 
  

   principal 
  summit. 
  The 
  elevations 
  in 
  the 
  northern 
  group 
  differ 
  from 
  

   those 
  of 
  the 
  middle 
  group 
  by 
  a 
  few 
  hundred 
  feet. 
  Is 
  this 
  difference 
  due 
  

   simply 
  to 
  the 
  removal 
  of 
  the 
  sedimentaries 
  from 
  the 
  forajer? 
  If 
  so, 
  

   the 
  two 
  groups 
  may 
  have 
  had 
  the 
  same 
  original 
  elevation. 
  I 
  think 
  it 
  

   likely, 
  hovrever, 
  that 
  the 
  northern 
  group 
  was 
  originally 
  the 
  highest. 
  

  

  A 
  closer 
  investigation 
  than 
  we 
  were 
  able 
  to. 
  make 
  would 
  doubtless 
  

   reveal 
  dikes, 
  extending 
  from 
  the 
  main 
  masses 
  into 
  the 
  sedimentaries, 
  

   as 
  in 
  the 
  Eik 
  Mountains 
  and 
  adjacent 
  groups. 
  The 
  upheaval 
  would 
  

   naturally 
  cause 
  fractures 
  across 
  the 
  sedimentaries 
  and 
  separations 
  of 
  

   some 
  of 
  the 
  strata, 
  which 
  would 
  be 
  filled 
  with 
  the 
  eruptive 
  material. 
  

   This 
  would 
  be 
  of 
  more 
  frequent 
  occurrence 
  near 
  the 
  surface. 
  As 
  we 
  

   descend 
  we 
  naturally 
  expect 
  to 
  find 
  less 
  disturbance 
  in 
  the 
  sedimenta- 
  

   ries, 
  on 
  account 
  of 
  the 
  greater 
  resistance 
  the 
  eruptive 
  material 
  would 
  

   have 
  to 
  overcome. 
  As 
  it 
  nears 
  tbe 
  surface 
  this 
  becomes 
  less, 
  and 
  some 
  

   of 
  the 
  overlying 
  beds 
  may 
  be 
  carried 
  to 
  tbe 
  summit, 
  as 
  in 
  the 
  case 
  of 
  

   station 
  C8. 
  

  

  The 
  rock 
  of 
  the 
  Salt 
  Mountains 
  resembles 
  that 
  of 
  Mount 
  Marcellina. 
  

   It 
  is 
  light-gray 
  rock 
  — 
  a 
  feldspathic 
  matrix, 
  with 
  crystals 
  of 
  feldspar 
  

   and 
  hornblende, 
  giving 
  it 
  a 
  porphyritic 
  appearance. 
  In 
  Mr. 
  Holmes's 
  

   district 
  there 
  are 
  several 
  eruptive 
  areas, 
  in 
  which 
  the 
  rocks 
  are 
  of 
  the 
  

   same 
  general 
  character. 
  These 
  will 
  be 
  described 
  in 
  his 
  report. 
  The 
  

   Abajo 
  Mouutains 
  were 
  not 
  visited 
  during 
  the 
  season, 
  but 
  they 
  are 
  of 
  the 
  

   same 
  character 
  as 
  the 
  Sierra 
  la 
  Sal. 
  Dr. 
  Newberry 
  visited 
  them 
  in 
  

   iSot), 
  and 
  thus 
  refers 
  to 
  them:* 
  "The 
  Sierra 
  is 
  com[)osed 
  geologically 
  

   of 
  an 
  erupted 
  nucleus, 
  mainly 
  a 
  gray 
  or 
  bluish-white 
  trachyte, 
  some- 
  

   times 
  becoming 
  a 
  porphyry, 
  surrounded 
  by 
  the 
  upheaved, 
  partially 
  

   eroded, 
  sedimentary 
  rocks." 
  He 
  says, 
  also, 
  that 
  the 
  Lower 
  Cretaceous 
  

   sandstones 
  and 
  Middle 
  Cretaceous 
  shales 
  are 
  exposed 
  in 
  all 
  the 
  ravines 
  

   leading 
  down 
  from 
  the 
  mouutains, 
  and 
  that 
  the 
  sedimentary 
  strata 
  rise 
  

   on 
  to 
  tbe 
  trachyte 
  core, 
  as 
  though 
  it 
  had 
  been 
  pushed 
  up 
  through 
  them. 
  

   This 
  is 
  exactly 
  what 
  has 
  been 
  done 
  in 
  the 
  Sierra 
  la 
  Sal. 
  In 
  the 
  Sierra 
  

   Abajo 
  there 
  appears 
  to 
  have 
  been 
  little 
  erosion 
  as 
  compared 
  with 
  the 
  

   Sierra 
  la 
  Sal. 
  The 
  accompanying 
  sections 
  (Plate 
  VIII) 
  will 
  help 
  in 
  giv- 
  

   ing 
  an 
  idea 
  of 
  the 
  structure 
  of 
  the 
  Salt 
  Mountains. 
  

  

  Lone 
  Cone 
  Mountain 
  will 
  be 
  fully 
  described 
  in 
  Mr. 
  Holmes's 
  report, 
  as 
  

   it 
  is 
  an 
  extension 
  of 
  one 
  of 
  his 
  volcanic 
  groups. 
  

  

  As 
  to 
  the 
  age 
  of 
  the 
  eruption 
  that 
  caused 
  the 
  elevation 
  of 
  the 
  Sierra 
  

   la 
  Sal, 
  all 
  that 
  can 
  be 
  said 
  is, 
  that 
  it 
  was 
  Post-cretaceous, 
  and 
  probably 
  

   Pre-glacial. 
  When 
  on 
  one 
  of 
  the 
  peaks, 
  I 
  noticed 
  what 
  appeared 
  to 
  be 
  

   roches 
  moutonnees 
  forms 
  in 
  the 
  red 
  beds 
  some 
  distance 
  to 
  the 
  north- 
  

   west 
  of 
  the 
  mountains. 
  I 
  think 
  it 
  likely 
  that 
  much 
  of 
  the 
  erosion 
  of 
  the 
  

   mountains 
  was 
  effected 
  by 
  glacial 
  action. 
  The 
  elevation 
  of 
  the 
  mount- 
  

   ains 
  above 
  the 
  surrounding 
  country 
  and 
  the 
  steepness 
  of 
  the 
  slopes 
  are 
  

  

  * 
  ExplorinjT 
  Expedition 
  from 
  Santa 
  Fe 
  to 
  Junction 
  of 
  Grand 
  and 
  Green 
  Rivers, 
  p. 
  100. 
  

   7 
  G 
  s 
  

  

  