﻿112 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  there 
  the 
  granite 
  continues 
  on 
  southward^ 
  overlaid 
  on 
  the 
  east 
  by- 
  

   trachyte, 
  and 
  on 
  the 
  west 
  by 
  a 
  small 
  patch 
  of 
  the 
  same 
  material. 
  

   Farther 
  on, 
  the 
  heavy 
  beds 
  of 
  red 
  Carboniferous 
  sandstone 
  set 
  in 
  on 
  

   both 
  sides, 
  so 
  that 
  in'indian 
  Creek 
  Pass, 
  only 
  a 
  small 
  outcrop 
  of 
  dark- 
  

   red 
  granite 
  is 
  to 
  be 
  found. 
  This, 
  too, 
  is 
  soon 
  covered 
  completely, 
  and 
  

   the 
  summit 
  of 
  the 
  pass 
  shows 
  nothing 
  but 
  the 
  red 
  sandstone. 
  Again 
  it 
  

   appears 
  on 
  the 
  main 
  divide, 
  leading 
  from 
  Trinchera 
  Peak 
  northward 
  to 
  

   station 
  119. 
  The 
  latter 
  is 
  located 
  at 
  the 
  northern 
  terminus 
  of 
  a 
  bold 
  

   ridge, 
  at 
  an 
  elevation 
  of 
  11,757 
  feet. 
  A 
  number 
  of 
  peaks, 
  granitic, 
  ly- 
  

   ing 
  between 
  it 
  and 
  Trinchera, 
  rise 
  to 
  an 
  altitude 
  of 
  over 
  32,000 
  feet. 
  

   Continuing 
  southward 
  in 
  a 
  narrow 
  strip, 
  flanked 
  both 
  east 
  and 
  west 
  by 
  

   Carboniferous 
  strata, 
  the 
  metamorphic 
  rocks 
  make 
  a 
  slight 
  bend 
  to 
  the 
  

   westward, 
  leaving 
  Trinchera 
  and 
  the 
  succeeding 
  highest 
  points 
  to 
  the 
  

   east. 
  Sedimentary 
  strata, 
  standing 
  nearly 
  on 
  end, 
  lie 
  tipped 
  

   up 
  against 
  the 
  granite, 
  and 
  these 
  form 
  the 
  highest 
  peaks 
  of 
  the 
  

   range. 
  So 
  far 
  as 
  could 
  be 
  determined, 
  the 
  visible 
  dip 
  of 
  the 
  heavy 
  

   granitic 
  layers 
  is 
  a 
  westerly 
  one 
  at 
  this 
  locality, 
  as 
  is 
  also 
  

   that 
  of 
  the 
  superincumbent 
  Carboniferous 
  strata. 
  Opposite 
  stations 
  

   115 
  and 
  117, 
  on 
  the 
  west 
  side 
  of 
  the 
  range, 
  granite 
  again 
  protrudes 
  

   from 
  under 
  the 
  Carboniferous. 
  Erosion 
  probably 
  has 
  carried 
  away 
  

   the 
  strata 
  that 
  were 
  gradually 
  thinning 
  out 
  as 
  they 
  neared 
  the 
  base 
  of 
  

   the 
  mountains, 
  thus 
  exposing 
  the 
  metamorphic 
  rocks 
  to 
  view. 
  Culebra 
  

   peak 
  (station 
  IIG), 
  14,079 
  feet 
  high, 
  is 
  within 
  the 
  metamorphic 
  area, 
  

   which, 
  soon 
  after 
  having 
  swerved 
  to 
  the 
  west, 
  returns 
  to 
  the 
  center 
  of 
  

   the 
  range. 
  A 
  general 
  dip 
  of 
  10° 
  to 
  15° 
  westward 
  may 
  here 
  be 
  observed 
  

   in 
  the 
  granite. 
  South 
  of 
  station 
  113, 
  located 
  at 
  an 
  elevation 
  of 
  13,719 
  

   feet, 
  the 
  granite 
  comes 
  to 
  a 
  close. 
  A 
  high 
  plateau 
  is 
  formed 
  by 
  an 
  

   overflow 
  of 
  tracl^yte, 
  which 
  extends 
  westward, 
  and 
  covers 
  the 
  

   granite 
  toward 
  the 
  south. 
  On 
  the 
  east 
  side 
  Carboniferous 
  sandstone 
  

   again 
  overlies 
  the 
  metamorphic 
  rocks. 
  At 
  this 
  locality 
  there 
  are 
  a 
  

   number 
  of 
  species 
  and 
  varieties 
  of 
  rocks 
  represented. 
  Similar 
  to 
  the 
  

   process 
  of 
  substitution 
  of 
  one 
  mineral 
  by 
  another, 
  as 
  observed 
  in 
  the 
  

   Blauca 
  group, 
  mica, 
  hornblende, 
  and 
  chlorite 
  occasionally 
  exchange 
  

   their 
  relative 
  functions, 
  and 
  the 
  result 
  produced 
  thereby 
  is 
  a 
  constant 
  

   variation 
  of 
  the 
  names 
  that 
  a 
  lithologist 
  would 
  apply 
  to 
  the 
  rock. 
  

   Granite, 
  however, 
  is 
  the 
  basis 
  of 
  all 
  these, 
  and 
  nearly 
  all 
  the 
  varieties 
  

   or 
  even 
  species 
  are 
  referable 
  to 
  it.' 
  True 
  gneiss 
  occurs 
  in 
  bands 
  or 
  

   large 
  masses, 
  changing 
  into 
  mica-schist 
  and 
  chloritic 
  schist. 
  Swinging 
  

   westward, 
  around 
  the 
  trachytic 
  plateau, 
  the 
  metamorphic 
  area 
  con- 
  

   tinues 
  south 
  again. 
  Stations 
  112 
  (12,840 
  feet) 
  and 
  110 
  (12,684 
  feet) 
  

   are 
  both 
  located 
  within 
  this 
  extension. 
  At 
  this 
  locality 
  the 
  range 
  has 
  

   separated, 
  the 
  one 
  spur 
  cont-inuing 
  on 
  farther 
  south, 
  while 
  the 
  other 
  

   ibllows 
  a 
  southwesterly 
  course. 
  On 
  the 
  latter 
  the 
  two 
  last 
  named 
  sta- 
  

   tions 
  were 
  made. 
  To 
  the 
  east 
  of 
  station 
  112 
  the 
  north 
  branch 
  of 
  Rio 
  

   Costilla 
  takes 
  its 
  rise, 
  in 
  a 
  valley 
  that 
  separates 
  the 
  two 
  spurs 
  of 
  the 
  

   range. 
  Starting 
  in 
  trachyte 
  it 
  flows 
  through 
  granite 
  for 
  a 
  large 
  portion 
  

   of 
  the 
  distance 
  it 
  travels 
  before 
  joining 
  the 
  smaller 
  south 
  fork. 
  Neither 
  

   spur 
  of 
  the 
  range 
  is 
  very 
  high 
  here, 
  so 
  that 
  a 
  very 
  good 
  pass 
  exists 
  

   north 
  of 
  Costilla 
  Peak. 
  An 
  old 
  Indian 
  trail 
  leads 
  over 
  it, 
  and 
  with 
  but 
  

   comparatively 
  a 
  small 
  amount 
  of 
  work 
  it 
  might 
  be 
  utilized 
  as 
  a 
  good 
  

   road. 
  South 
  of 
  station 
  110 
  the 
  granite, 
  which 
  here 
  is 
  of 
  a 
  coarse-grained 
  

   variety, 
  stained 
  dark 
  red 
  on 
  its 
  surface 
  by 
  the 
  oxidation 
  of 
  magnetite 
  it 
  

   contains, 
  is 
  again 
  covered 
  by 
  a 
  heavy 
  mass 
  of 
  trachyte 
  coming 
  from 
  the 
  

   south. 
  Costilla 
  Peak 
  (station 
  111, 
  12,634 
  feet 
  above 
  sea-level) 
  is 
  the 
  last 
  

   granitic 
  promontory. 
  Following 
  down 
  the 
  valley 
  of 
  the 
  Costilla, 
  which 
  

   several 
  times 
  closes 
  into 
  narrow 
  granitic 
  cauons, 
  we 
  reach 
  the 
  foot-hills 
  fall- 
  

  

  