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  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  about 
  14 
  miles 
  in 
  length 
  forms 
  a 
  barrier 
  between 
  a 
  long 
  narrow 
  valley 
  

   (San 
  Pedro 
  Valley) 
  running 
  along 
  the 
  base 
  of 
  the 
  range 
  and 
  the 
  eastern 
  

   edge 
  of 
  San 
  Luis 
  Valley 
  proper. 
  This 
  basalt 
  is 
  but 
  a 
  continuation 
  or 
  

   vice 
  versa 
  of 
  the 
  group 
  in 
  the 
  valley 
  upon 
  which 
  stations 
  103 
  to 
  105 
  are 
  

   located. 
  I 
  mention 
  this 
  point 
  here, 
  as 
  it 
  will 
  subsequeutly 
  be 
  found 
  

   to 
  be 
  of 
  importance. 
  Heavy 
  beds 
  of 
  coarse 
  gravel 
  occur 
  north 
  of 
  Rio 
  

   Culebra, 
  belonging 
  properly 
  to 
  the 
  valley, 
  but 
  in 
  consequece 
  of 
  their 
  

   thickness 
  and 
  subsequent 
  erosion, 
  appearing 
  like 
  the 
  last 
  foot-hills 
  of 
  

   the 
  range. 
  Along 
  the 
  western 
  border 
  of 
  the 
  valley 
  the 
  mountains 
  are 
  

   but 
  a 
  continuation 
  of 
  the 
  great 
  volcanic 
  area 
  which 
  has 
  been 
  discussed 
  in 
  

   part 
  in 
  Uni'»3d 
  States 
  Geological 
  Survey, 
  (Report 
  1874, 
  page 
  10.3.) 
  Tra- 
  

   chyte 
  and 
  trachytic 
  conglomerate 
  compose 
  the 
  greater 
  portion 
  by 
  far, 
  

   and 
  basalt, 
  when 
  found, 
  is 
  merely 
  observed 
  in 
  the 
  character 
  of 
  a 
  super- 
  

   incumbent 
  Ktatum 
  or 
  strata. 
  These 
  latter 
  become 
  of 
  considerable 
  impor- 
  

   tance 
  south 
  of 
  Rio 
  Alamosa, 
  inasmuch 
  as 
  they 
  produce 
  a 
  gentle 
  east- 
  

   erly 
  slope 
  of 
  the 
  foot-hills 
  and 
  descend 
  into 
  the 
  valley, 
  forming, 
  for 
  

   many 
  miles, 
  its 
  level 
  plain. 
  Mountains 
  and 
  hills 
  in 
  the 
  southern 
  por- 
  

   tion 
  thereof 
  are 
  composed 
  of 
  the 
  same 
  material, 
  and 
  for 
  a 
  long 
  distance 
  

   the 
  unbroken 
  cover 
  of 
  basalt 
  effectually 
  prevents 
  any 
  insight 
  into 
  the 
  

   structure 
  beneath. 
  

  

  Comparing 
  both 
  absolute 
  and 
  relative 
  elevations 
  of 
  the 
  ranges 
  and 
  

   peaks 
  on 
  both 
  sides 
  of 
  San 
  Luis 
  Valley 
  we 
  find 
  that 
  those 
  on 
  the 
  eastern 
  

   side 
  are 
  the 
  higher 
  ones. 
  On 
  average 
  the 
  high 
  peaks 
  of 
  the 
  Sangre 
  de 
  

   Cristo 
  Range 
  may 
  be 
  said 
  to 
  reach 
  an 
  altitude 
  of 
  13,000 
  to 
  14,000 
  feet, 
  

   and 
  quite 
  a 
  number 
  of 
  them 
  exceed 
  the 
  latter 
  figure. 
  Comparing 
  this 
  

   with 
  those 
  mountains 
  of 
  the 
  west 
  side, 
  that 
  are 
  situated 
  at 
  about 
  an 
  

   equal 
  distance 
  from 
  the 
  edge 
  of 
  the 
  valley, 
  we 
  observe 
  that 
  the 
  eastern 
  

   mountains 
  are 
  from 
  3,000 
  to 
  4,000 
  feet 
  higher, 
  on 
  average. 
  While 
  the 
  

   data 
  of 
  elevation 
  obtained 
  throughout 
  the 
  valley 
  itself 
  cannot 
  but 
  be 
  

   too 
  meager 
  to 
  be 
  considered 
  satisfactory, 
  we 
  are 
  enabled 
  to 
  say, 
  that 
  its 
  

   western 
  side, 
  near 
  the 
  mountains, 
  is 
  higher 
  than 
  the 
  corresponding 
  one 
  

   on 
  the 
  east. 
  These 
  facts 
  become 
  important 
  in 
  the 
  question 
  of 
  the 
  trans- 
  

   portation 
  of 
  geological 
  material 
  within 
  the 
  boundaries 
  of 
  the 
  valley, 
  

   and 
  are 
  therefore 
  here 
  briefiy 
  alluded 
  to. 
  It 
  seems 
  natural, 
  of 
  course, 
  

   that 
  we 
  should 
  find 
  collected 
  in 
  San 
  Luis 
  Valley 
  specimens 
  representing 
  

   all 
  the 
  formations 
  above 
  giveu 
  as 
  forming 
  the 
  mountainous 
  regions 
  sur- 
  

   rounding 
  it. 
  Drainage 
  heading 
  in 
  any 
  one 
  stratum 
  might 
  carry 
  bowl- 
  

   ders 
  and 
  j)ebbles 
  into 
  the 
  valley, 
  thus 
  leaving 
  us 
  a 
  safe 
  guide 
  whereby 
  

   to 
  recognize 
  the 
  former 
  courses 
  of 
  rivers 
  and 
  streams. 
  Attrition, 
  how- 
  

   ever, 
  has 
  reduced 
  the 
  size 
  of 
  these 
  bowlders 
  and 
  pebbles 
  to 
  such 
  small 
  

   dimensions 
  that 
  but 
  little 
  if 
  any 
  satisfaction 
  can 
  be 
  gained 
  therefrom. 
  

   So 
  far 
  as 
  I 
  have 
  been 
  able 
  to 
  see, 
  it 
  is 
  only 
  the 
  distribntiou 
  of 
  one 
  spe- 
  

   cific 
  kind 
  of 
  sand 
  or 
  gravel 
  over 
  any 
  giveu 
  locality 
  that 
  is 
  of 
  impor- 
  

   tance 
  in 
  unravelling 
  the 
  geological 
  history 
  of 
  this 
  vast 
  valley. 
  In 
  the 
  

   subsequent 
  i)ages 
  this 
  view 
  will 
  be 
  maintained, 
  and 
  the 
  reasons 
  given 
  

   more 
  fully 
  therefor, 
  besides 
  the 
  deductions 
  arrived 
  at 
  from 
  the 
  observa- 
  

   tion 
  of 
  such 
  specific 
  distribution. 
  

  

  North 
  of 
  the 
  Rio 
  Grande, 
  San 
  Luis 
  Valley 
  presents 
  an 
  unbroken 
  plain, 
  

   slightly 
  depressed 
  in 
  the 
  center, 
  rising 
  more 
  on 
  the 
  west 
  than 
  on 
  the 
  

   east 
  side. 
  Sand 
  covers 
  the 
  entire 
  expanse. 
  Near 
  San 
  Luis 
  Lakes 
  the 
  

   progress 
  of 
  vegetation, 
  together 
  with 
  the 
  action 
  of 
  moisture, 
  has 
  been 
  

   productive 
  of 
  soil. 
  Should 
  a 
  similar 
  process 
  be 
  applicable 
  to 
  other 
  por- 
  

   tions, 
  the 
  sanu^ 
  desirable 
  result 
  would 
  probably 
  accrue. 
  Approaching 
  

   the 
  western 
  border 
  of 
  the 
  valley 
  bowlders 
  of 
  volcanic 
  rocks 
  — 
  mainly 
  

   trachyte 
  and 
  some 
  basalt 
  — 
  may 
  be 
  observed, 
  but 
  they 
  extend 
  only 
  

   a 
  short 
  distance 
  downward 
  from 
  the 
  foot-hills. 
  Aualoaous 
  to 
  this 
  we 
  

  

  