﻿144 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  agent 
  in 
  the 
  decomposition 
  and 
  disintegration 
  of 
  rocks 
  tlian 
  the 
  aid 
  of 
  

   growing 
  vegetation. 
  The 
  action 
  in 
  this 
  instance 
  is 
  not 
  merely 
  a 
  physi- 
  

   cal 
  bnt 
  also 
  a 
  chemical 
  one, 
  au<l 
  although 
  atmospheric 
  influence 
  is 
  not 
  

   to 
  be 
  undervalued, 
  I 
  have 
  come 
  to 
  the 
  conclusion 
  that 
  the 
  compara- 
  

   tively 
  '' 
  fresh 
  " 
  appearance 
  at 
  this 
  locality 
  is 
  due 
  mainly 
  to 
  the 
  absence 
  

   of 
  densely 
  distributed 
  vegetation. 
  Besides 
  the 
  fact 
  that 
  rocks 
  consti- 
  

   tuted 
  as 
  basalt 
  is, 
  form 
  soil 
  but 
  very 
  slowly, 
  it 
  is 
  not 
  to 
  be 
  overlooked 
  

   that 
  the 
  scarcity 
  of 
  water 
  will 
  retard 
  the 
  growth 
  of 
  plants 
  very 
  much, 
  

   so 
  that 
  the 
  rocli 
  has 
  a 
  double 
  chance 
  of 
  retaining 
  its 
  original 
  physical 
  

   and 
  chemical 
  character. 
  Having 
  crossed 
  liio 
  San 
  Antonio, 
  we 
  stand 
  

   at 
  the 
  base 
  of 
  Mount^^ 
  San 
  Antonio, 
  10,883 
  feet 
  above 
  sea-level. 
  A 
  dome- 
  

   shaped 
  peak 
  rises 
  2,000 
  feet 
  above 
  the 
  valley 
  and 
  is 
  composed 
  entirely 
  

   of 
  basalt 
  and 
  melaphyr. 
  

  

  This 
  peak 
  is 
  one 
  of 
  the 
  main 
  points 
  of 
  outflow, 
  and 
  has 
  sent 
  its 
  lava 
  in 
  

   regular 
  flows 
  in 
  every 
  direction, 
  more 
  particularly 
  toward 
  the 
  west 
  and 
  

   south, 
  however. 
  Having 
  alluded 
  to 
  this 
  we 
  come 
  to 
  the 
  consideration 
  

   of 
  an 
  important 
  point. 
  Does 
  the 
  basalt 
  capping 
  the 
  trachytes 
  of 
  the 
  loot- 
  

   hills 
  on 
  the 
  west 
  side 
  of 
  San 
  Luis 
  come 
  from 
  the 
  west 
  or 
  east? 
  My 
  

   opinion 
  inclines 
  toward 
  the 
  latter 
  view. 
  Although 
  the 
  entire 
  slope 
  of 
  

   this 
  basalt 
  is 
  from 
  west 
  to 
  east, 
  at 
  an 
  angle 
  of 
  2° 
  to 
  8°, 
  I 
  have 
  rea- 
  

   sons 
  to 
  assert 
  that 
  an 
  uplift 
  — 
  very 
  gradual 
  — 
  took 
  place 
  a 
  little 
  west 
  of 
  

   the 
  central 
  line 
  of 
  that 
  plateau 
  range, 
  and 
  that 
  the 
  present 
  position 
  

   of 
  the 
  volcanic 
  strata 
  is 
  accounted 
  for 
  thereby. 
  (See 
  chapter 
  HI.) 
  So 
  

   far 
  as 
  could 
  be 
  observed, 
  there 
  is 
  either 
  none 
  or 
  very 
  little 
  diflerence 
  

   both 
  in 
  constitution 
  and 
  mode 
  of 
  occurrence 
  in 
  the 
  basalts 
  existing 
  

   , 
  along 
  the 
  western 
  border 
  of 
  San 
  Luis 
  Valley. 
  An 
  exception, 
  however, 
  

   is 
  found 
  at 
  the 
  isolated 
  hills 
  denoting 
  a 
  local 
  outflow, 
  which 
  among 
  

   themselves 
  are 
  very 
  similar. 
  South 
  of 
  Mount 
  San 
  Antonio 
  are 
  a 
  num- 
  

   ber 
  of 
  low 
  basaltic 
  blnfls, 
  which 
  I 
  did 
  not 
  visit 
  personally, 
  but 
  from 
  

   their 
  position 
  regard 
  tliem 
  as 
  remnants 
  of 
  subsequent 
  flows 
  fiom 
  the 
  

   original 
  point 
  of 
  emptier.. 
  Their 
  present 
  shape 
  may 
  be 
  owing 
  partly 
  

   to 
  rupture, 
  partly 
  to 
  eiosion. 
  

  

  Entering 
  San 
  Luis 
  Valley 
  from 
  the 
  western 
  mountains, 
  the 
  streams 
  

   run 
  through 
  almost 
  inaccessible, 
  narrow 
  caiions 
  in 
  the 
  basalt. 
  Farther 
  

   up-stream, 
  near 
  their 
  headwaters, 
  in 
  the 
  trachyte, 
  they 
  show 
  the 
  same 
  

   phenomenon, 
  although 
  the 
  causes 
  are 
  diflerent. 
  As 
  is 
  shown 
  in 
  chap- 
  

   ter 
  HI, 
  evidences 
  were 
  found 
  at 
  the 
  headwaters 
  of 
  Eio 
  Conejos 
  and 
  its 
  

   tributaries 
  of 
  very 
  extensive 
  glaciers. 
  These 
  latter 
  moved 
  along 
  the 
  

   sloping 
  plane 
  of 
  the 
  elevated 
  plateau 
  until 
  they 
  reached, 
  tarther 
  east- 
  

   ward, 
  the 
  readily 
  yielding 
  trachytic 
  conglomerate, 
  and 
  there 
  cut 
  in 
  

   deeply. 
  Alt 
  hough 
  it 
  is 
  possible 
  that 
  these 
  glaciers 
  extended 
  down-stream 
  

   ibr 
  some 
  distance, 
  I 
  have 
  found 
  no 
  prootof 
  their 
  h.iving 
  reached 
  the 
  val- 
  

   ley 
  proper. 
  ]So 
  erratic 
  bowlders 
  of 
  trachyte, 
  no 
  moraiual 
  deposits, 
  and 
  

   no 
  striation 
  of 
  rocks 
  appear 
  in 
  the 
  valley 
  itself. 
  Entering 
  one 
  of 
  the 
  ba- 
  

   saltic 
  canons, 
  that 
  of 
  Kio 
  San 
  Antonio 
  for 
  instance, 
  it 
  will 
  be 
  seen 
  that 
  

   the 
  basalt 
  is 
  depcisited 
  in 
  regular 
  layers, 
  from 
  4 
  to 
  20 
  feet 
  in 
  thickness. 
  

   Vertical 
  walls 
  inclose 
  either 
  the 
  cieek-bed 
  or 
  the 
  narrow 
  valley, 
  reach- 
  

   ing 
  more 
  than 
  100 
  feet 
  in 
  height. 
  The 
  creek 
  itself 
  flows 
  over 
  basaltic 
  

   bowlders, 
  that 
  do 
  not 
  seem 
  to 
  be 
  in 
  positu, 
  but 
  to 
  have 
  fallen 
  from 
  the 
  

   sides. 
  Comparing 
  the 
  single 
  layers 
  of 
  the 
  two 
  caiion-walls, 
  they 
  wdl 
  

   be 
  found 
  to 
  correspond 
  perfectly. 
  It 
  seems 
  to 
  me, 
  therefore, 
  that 
  in 
  the 
  

   absence 
  of 
  any 
  extensive 
  aqueous 
  erosion, 
  considering 
  the 
  narrow 
  width 
  

   of 
  the 
  canon, 
  and 
  the 
  constancy 
  of 
  stratigraphical 
  relations, 
  these 
  

   canons 
  \ve»e 
  formed 
  by 
  rupture. 
  As 
  the 
  most 
  effective 
  and 
  plausible 
  

   cause 
  i>roducing 
  this 
  elfect, 
  I 
  regard 
  volcanic 
  earthquakes. 
  It 
  seems 
  no 
  

   more 
  than 
  reasonable 
  to 
  suppose, 
  m 
  view 
  of 
  the 
  experience 
  of 
  our 
  pres- 
  

  

  