﻿96 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  around 
  the 
  western 
  side 
  of 
  the 
  group, 
  and 
  there 
  dip 
  to 
  the 
  westward. 
  

   The 
  central 
  peaks 
  of 
  this 
  northern 
  group 
  are 
  all 
  trachytic, 
  and 
  range 
  

   in 
  elevation 
  from 
  12,000 
  feet 
  to 
  12,600 
  feet. 
  

  

  Middle 
  group. 
  — 
  This 
  group 
  lies 
  due 
  south 
  of 
  the 
  center 
  of 
  the 
  north- 
  

   ern 
  group, 
  and 
  contains 
  four 
  principal 
  peaks 
  of 
  the 
  following 
  elevation 
  : 
  

   12,300 
  feet, 
  12,724 
  feet, 
  12,890 
  feet, 
  and 
  12,980 
  feet. 
  The 
  area 
  occupied 
  

   is 
  much 
  smaller 
  than 
  that 
  of 
  the 
  northern 
  group, 
  but 
  it 
  is 
  interesting 
  

   from 
  the 
  presence 
  of 
  sedimentary 
  beds 
  on 
  station 
  68, 
  the 
  highest 
  peak. 
  

   These 
  consist 
  of 
  sandstones 
  and 
  shales 
  very 
  much 
  metamorphosed, 
  

   forming 
  a 
  capping 
  to 
  the 
  peak. 
  There 
  is 
  at 
  present 
  no 
  connection 
  with 
  

   the 
  sedimentaries 
  at 
  the 
  base 
  of 
  the 
  peak. 
  This 
  remnant 
  is 
  horizontal 
  

   in 
  position, 
  and 
  appears 
  to 
  have 
  been 
  carried 
  up 
  by 
  the 
  eruptive 
  mate- 
  

   rial. 
  It 
  may 
  be 
  a 
  remnant 
  of 
  beds 
  that 
  once 
  extended 
  uninterruptedly 
  

   across 
  the 
  mountains, 
  or 
  it 
  may 
  be 
  that 
  in 
  the 
  upheaval 
  it 
  was 
  torn 
  

   abruptly 
  from 
  the 
  beds 
  whose 
  upturned 
  edges 
  rest 
  against 
  the 
  mount- 
  

   ains. 
  

  

  On 
  the 
  sides 
  of 
  the 
  peaks, 
  just 
  north 
  of 
  station 
  68, 
  fragments 
  of 
  black 
  

   shales 
  are 
  seen 
  included 
  in 
  the 
  volcanic 
  rock. 
  The 
  eruptive 
  material 
  

   appears 
  not 
  only 
  to 
  have 
  carried 
  up 
  fragments, 
  but 
  also 
  to 
  have 
  spread 
  

   laterally 
  and 
  included 
  portions 
  of 
  the 
  sedimentary 
  rocks. 
  These 
  beds 
  

   appear 
  to 
  be 
  remnants 
  of 
  the 
  Cretaceous, 
  and 
  the 
  beds 
  surrounding 
  the 
  

   group 
  are 
  probably 
  of 
  Lower 
  Cretaceous 
  age. 
  The 
  height 
  of 
  the 
  peaks 
  

   and 
  the 
  steepness 
  of 
  the 
  slopes 
  has 
  caused 
  the 
  accumulation 
  of 
  debris 
  

   at 
  the 
  base 
  of 
  the 
  mountains, 
  so 
  that 
  it 
  is 
  difficult 
  to 
  get 
  at 
  the 
  relations 
  

   without 
  more 
  time 
  than 
  was 
  at 
  our 
  disposal. 
  Wherever 
  the 
  beds 
  are 
  

   exposed, 
  they 
  are 
  seen 
  rising 
  toward 
  the 
  mountains. 
  

  

  Southern 
  group. 
  — 
  This 
  is 
  the 
  smallest 
  group 
  of 
  the 
  Sierra 
  la 
  Sal. 
  It 
  

   is 
  a 
  little 
  to 
  the 
  west 
  of 
  south 
  from 
  the 
  middle 
  group, 
  and 
  is 
  separated 
  

   from 
  it 
  by 
  a 
  saddle 
  10,200 
  feet 
  in 
  elevation, 
  across 
  which 
  sandstones 
  of 
  

   probable 
  Lower 
  Cretaceous 
  age 
  extend. 
  I 
  am 
  of 
  the 
  opinion 
  that 
  but 
  

   one 
  of 
  the 
  peaks 
  shows 
  the 
  volcanic 
  rock, 
  viz, 
  the 
  highest 
  peak, 
  the 
  ele- 
  

   vation 
  of 
  which 
  is 
  12,004 
  feet. 
  The 
  other 
  peaks 
  probably 
  have 
  sedi- 
  

   mentary 
  rocks 
  extending 
  over 
  them. 
  On 
  the 
  main 
  peak 
  they 
  extend 
  to 
  

   about 
  timber-line, 
  (11,000 
  feet,) 
  and 
  on 
  the 
  east, 
  south, 
  and 
  west 
  sides 
  • 
  

   of 
  the 
  group, 
  a 
  sandstone 
  (probably 
  Dakota) 
  is 
  seen 
  dipping 
  away 
  from 
  

   the 
  mass 
  50° 
  to 
  60°, 
  and 
  flattening 
  out 
  as 
  we 
  recede 
  from 
  the 
  group. 
  

  

  From 
  the 
  summits 
  of 
  the 
  mountains 
  little 
  can 
  be 
  determined, 
  except 
  

   that 
  the 
  sedimentary 
  beds 
  dip 
  away 
  from 
  them, 
  as 
  we 
  are 
  so 
  high 
  above 
  

   the 
  surrounding 
  country. 
  We 
  are, 
  therefore, 
  left 
  simply 
  to 
  conjecture 
  the 
  

   exact 
  age 
  of 
  the 
  beds 
  on 
  the 
  west 
  side, 
  being 
  guided 
  to 
  a 
  certain 
  extent 
  by 
  

   our 
  knowledge 
  of 
  the 
  eastern 
  side. 
  Dr. 
  i^ewberry, 
  when 
  with 
  Macomb's 
  

   expedition 
  in 
  1859, 
  was 
  at 
  Ojo 
  Verde, 
  about 
  twelve 
  miles 
  southwest 
  of 
  

   the 
  southern 
  group. 
  From 
  the 
  view 
  he 
  obtained 
  of 
  the 
  Sierra 
  la 
  Sal, 
  he 
  

   was 
  of 
  the 
  opinion 
  that 
  rocks 
  of 
  Cretaceous, 
  Jurassic, 
  and 
  Triassic 
  age 
  

   rise 
  on 
  the 
  base 
  of 
  the 
  mountains 
  from 
  the 
  east, 
  south, 
  and 
  southwest. 
  

   He 
  says, 
  also, 
  "It 
  is 
  evident, 
  therefore, 
  that 
  the 
  rocks 
  composing 
  the 
  

   Colorado 
  plateau 
  are 
  there 
  locally 
  upheaved, 
  precisely 
  as 
  around 
  the 
  

   Sierra 
  Abajo 
  and 
  the 
  other 
  isolated 
  mountains, 
  which 
  I 
  have 
  already 
  

   before 
  enumerated." 
  * 
  A 
  closer 
  investigation 
  confirms 
  the 
  truth 
  of 
  this 
  

   opinion. 
  

  

  The 
  only 
  difficulty 
  met 
  with 
  in 
  the 
  study 
  of 
  this 
  interesting 
  region 
  

   is 
  the 
  great 
  amount 
  of 
  dSbris 
  that 
  has 
  accumulated 
  at 
  the 
  base 
  of 
  

   the 
  mountains. 
  There 
  has 
  been 
  an 
  immense 
  amount 
  of 
  erosion 
  since 
  

   their 
  upheaval 
  — 
  probably 
  glacial, 
  at 
  least 
  in 
  part. 
  The 
  erosive 
  forces 
  

   appear 
  to 
  have 
  had 
  greater 
  play 
  toward 
  the 
  north, 
  for 
  we 
  find 
  there 
  

  

  * 
  Exploring 
  Expedition 
  from 
  Santa 
  F6 
  to 
  Junction 
  of 
  Grand 
  and 
  Green, 
  p. 
  93. 
  

  

  