﻿194 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  liot 
  volcanic 
  matter, 
  the 
  strati 
  graphical 
  conditions 
  hare, 
  from 
  that 
  

   cause 
  alone, 
  suffered 
  greatly. 
  On 
  the 
  South 
  Fork 
  of 
  Rio 
  Pnrgatorio 
  the 
  

   last 
  outcrop 
  of 
  the 
  Colorado 
  shales 
  was 
  seen, 
  about 
  one 
  mile 
  riorth 
  of 
  

   station 
  125. 
  There 
  the 
  dark 
  shales, 
  with 
  characteristic 
  fossils, 
  appeared 
  

   in 
  the 
  bed 
  of 
  the 
  creek, 
  where 
  they 
  had 
  been 
  protected 
  from 
  erosion 
  by 
  

   a 
  basaltic 
  dike, 
  cutting 
  through 
  parallel 
  to 
  their 
  dip. 
  So 
  far 
  as 
  could 
  

   be 
  made 
  out 
  the 
  latter 
  was 
  about 
  20° 
  to 
  the 
  eastward. 
  I 
  cannot 
  regard 
  

   this 
  as 
  certain, 
  however, 
  for 
  reasons 
  above 
  indicated. 
  Forming 
  a 
  long- 
  

   continued 
  bluff 
  which 
  commences 
  south 
  of 
  Spanish 
  Peaks 
  and 
  extends 
  

   for 
  about 
  30 
  miles 
  almost 
  due 
  south, 
  we 
  observe 
  a 
  series 
  of 
  yellow 
  sand- 
  

   stones 
  and 
  shales. 
  This 
  bluff" 
  is 
  about 
  800 
  feet 
  above 
  the 
  level 
  of 
  the 
  

   valley, 
  immediately 
  west 
  of 
  it. 
  As 
  a 
  rule, 
  the 
  strata 
  composing 
  it 
  have 
  

   a 
  dip 
  of 
  4P 
  to 
  6° 
  toward 
  the 
  east. 
  Local 
  variations 
  therefrom 
  occur 
  

   frequently, 
  however. 
  The 
  latter 
  case 
  may 
  be 
  observed 
  near 
  station 
  125, 
  

   on 
  the 
  South 
  Fork 
  of 
  the 
  Purgatorio. 
  In 
  the 
  valley, 
  which 
  is 
  narrow, 
  

   trending 
  eastward, 
  the 
  Colorado 
  shales 
  appear, 
  as 
  stated 
  above, 
  while 
  

   the 
  sandstones 
  and 
  yellow 
  shales 
  dip 
  off, 
  locally, 
  on 
  either 
  side, 
  at 
  

   angles 
  of 
  15° 
  to 
  2(P. 
  This 
  small 
  anticlinal 
  fold 
  soon 
  disappears, 
  as 
  we 
  

   descend 
  along 
  the 
  stream, 
  and 
  with 
  it 
  the 
  Colorado 
  sliales. 
  At 
  that 
  

   l^oint 
  the 
  regular 
  easterly 
  dip, 
  diminished 
  to 
  about 
  3°, 
  again 
  sets 
  in, 
  

   until, 
  as 
  we 
  go 
  farther 
  eastward, 
  the 
  strata 
  become 
  horizontal 
  and 
  

   finally 
  show 
  a 
  westerly 
  inclination 
  of 
  1° 
  to 
  3°. 
  At 
  Trinidad 
  this 
  latter 
  

   dip 
  is 
  developed 
  more 
  than 
  elsewhere, 
  and 
  it 
  is 
  there, 
  too, 
  that 
  we 
  find 
  

   undoubted 
  Cretaceous 
  beds 
  in 
  the 
  lowest 
  portions 
  of 
  the 
  Purgatory 
  Val- 
  

   ley. 
  Thus 
  a 
  shallow 
  synclinal 
  fold 
  is 
  formed. 
  Going 
  north 
  from 
  Trini- 
  

   dad 
  along 
  the 
  edge 
  of 
  these 
  same 
  bluffs 
  we 
  can 
  observe 
  first 
  an 
  almost 
  

   horizontal 
  arrangement 
  of 
  the 
  strata, 
  until 
  we 
  have 
  passed 
  the 
  Cucha- 
  

   ras. 
  There 
  they 
  begin 
  to 
  dip 
  southward 
  at 
  a 
  small 
  anp;le, 
  which 
  does 
  

   not 
  increase 
  much 
  toward 
  the 
  northern 
  terminus 
  of 
  the 
  bluffs. 
  Combin- 
  

   ing 
  this 
  inclination 
  with 
  the 
  synclinal 
  from 
  west 
  to 
  east 
  we 
  have 
  a 
  per- 
  

   fect 
  " 
  trough," 
  the 
  deepest 
  portions 
  of 
  which 
  have 
  been 
  selected 
  by 
  the 
  

   Purgatorio 
  as 
  its 
  channel. 
  

  

  Whether 
  or 
  not 
  the 
  variation 
  of 
  the 
  dips 
  observed 
  between 
  the 
  Cre- 
  

   taceous 
  strata 
  and 
  the 
  younger, 
  yellow 
  sandstones 
  and 
  shales 
  forming 
  

   the 
  long 
  bluff", 
  should 
  be 
  regarded 
  as 
  evidence 
  of 
  an 
  uncouformability 
  

   becomes 
  a 
  difficult 
  question. 
  In 
  the 
  field 
  I 
  could 
  not 
  persuade 
  myself 
  

   that 
  the 
  Cretaceous 
  beds, 
  standing 
  either 
  vertical 
  or 
  dipping 
  westward, 
  

   could 
  change, 
  within 
  the 
  horizontal 
  distance 
  of 
  two 
  miles, 
  their 
  dip 
  so 
  

   as 
  to 
  conform 
  with 
  that 
  of 
  the 
  overlying 
  strata 
  (6° 
  east). 
  Nothing 
  was 
  

   found 
  to 
  prove 
  that 
  they 
  did, 
  unless 
  indeed 
  the 
  occurrence 
  at 
  Trinidad, 
  

   30 
  miles 
  distant, 
  where 
  there 
  is 
  no 
  unconformability' 
  noticeable, 
  be 
  ac- 
  

   cepted 
  as 
  evidence. 
  It 
  is 
  certainly 
  contrary 
  to 
  all 
  former 
  experience 
  

   that 
  we 
  should 
  expect 
  an 
  unconformability. 
  We 
  have 
  numerous 
  

   instances 
  along 
  the 
  Front 
  Range, 
  similar 
  to 
  this 
  one, 
  where 
  the 
  beds 
  in 
  

   the 
  immediate 
  vicinity 
  of 
  the 
  mountains 
  dip 
  very 
  steeply, 
  but 
  soon 
  lose 
  

   their 
  dip 
  and 
  become 
  almost 
  horizontal.* 
  

  

  Nevertheless, 
  I 
  am 
  inclined 
  to 
  regard 
  the 
  occurrence 
  in 
  the 
  Purgato- 
  

   rio 
  region 
  as 
  an 
  unconformability, 
  although 
  I 
  by 
  no 
  means 
  deny 
  that 
  

   subsequent 
  detail-study 
  may 
  prove 
  me 
  to 
  be 
  in 
  error. 
  In 
  assuming 
  that 
  

   this 
  unconformability 
  exists, 
  I 
  argue 
  that 
  tbe 
  strata 
  underlying 
  the 
  yel- 
  

   low 
  sandstones 
  and 
  shales 
  rapidly 
  lose 
  their 
  steep 
  dip 
  and 
  become 
  con- 
  

   formable 
  with 
  the 
  latter, 
  as 
  they 
  appear 
  at 
  Trinidad. 
  

  

  With 
  certainty 
  it 
  can 
  be 
  made 
  out 
  at 
  least 
  that 
  the 
  yellow 
  sandstones 
  

   and 
  shales 
  directly 
  overlie 
  undoubted 
  Cretaceous 
  beds 
  at 
  Trinidad. 
  

  

  * 
  Compare 
  United 
  States 
  Geological 
  Report, 
  ld73, 
  p. 
  136. 
  

  

  I 
  

  

  