﻿126 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  was 
  found, 
  although 
  at 
  a 
  few 
  isolated 
  spots 
  small 
  patches 
  of 
  the 
  shales 
  

   were 
  exposed, 
  only 
  partly 
  altered. 
  Beyond 
  the 
  dike 
  we 
  find 
  the 
  same 
  dip- 
  

   ping 
  at 
  the 
  base 
  of 
  the 
  sandstone 
  bluff 
  at 
  an 
  angle 
  of 
  about 
  30° 
  to 
  35°. 
  

   Here 
  we 
  are 
  once 
  more 
  in 
  an 
  easily-recognizable 
  horizon. 
  Shales 
  (e) 
  and 
  

   sandstones 
  (/) 
  compose 
  the 
  bluff, 
  the 
  strata 
  of 
  which 
  rest 
  apparently 
  

   unconformably 
  upon 
  the 
  underlying 
  dark-grey 
  shales. 
  In 
  the 
  same 
  

   way 
  as 
  has 
  been 
  done, 
  the 
  wall 
  or 
  the 
  stratum 
  forming 
  it 
  can 
  be 
  traced 
  

   southward 
  until 
  it 
  disappears 
  under 
  the 
  trachyte 
  immediately 
  north 
  of 
  

   Costilla 
  Peak. 
  It 
  has 
  been 
  mentioned 
  above 
  that 
  the 
  easterly 
  dip 
  

   changes 
  to 
  a 
  vertical 
  position 
  and 
  then 
  farther 
  south 
  gradually 
  turns 
  into 
  

   a 
  decidedly 
  western 
  one; 
  so 
  that 
  the 
  Carboniferous 
  strata 
  rest 
  mj^ow 
  the 
  

   Cretaceous. 
  Wherever 
  creeks, 
  coming 
  from 
  the 
  mountains, 
  have 
  cut 
  

   through 
  the 
  Carboniferous 
  and 
  then 
  the 
  Cretaceous 
  strata, 
  as, 
  for 
  instance, 
  

   the 
  three 
  main 
  forks 
  of 
  the 
  Purgatory, 
  good 
  sections 
  can 
  be 
  obtained 
  of 
  

   all 
  those 
  beds 
  that 
  resist, 
  to 
  a 
  certain 
  extent, 
  the 
  eroding 
  influence. 
  

   The 
  shales, 
  however, 
  have 
  been 
  cut 
  out 
  in 
  deep 
  troughs 
  by 
  the 
  waters 
  

   that 
  were 
  narrowed 
  in 
  their 
  flow, 
  and 
  suddenly 
  found 
  themselves 
  at 
  lib- 
  

   erty 
  to 
  expand. 
  In 
  the 
  stead 
  of 
  a 
  large 
  portion 
  of 
  the 
  material 
  carried 
  

   away, 
  erratic 
  bowlders, 
  &c., 
  were 
  deposited, 
  and 
  thus 
  all 
  examination 
  

   of 
  the 
  strata 
  really 
  existing 
  at 
  any 
  given 
  point 
  is 
  made 
  very 
  difficult. 
  

  

  Colorado 
  group. 
  — 
  {Cretaceous 
  No. 
  2.) 
  — 
  All 
  the 
  shales 
  alluded 
  to 
  above 
  

   as 
  occurring 
  in 
  Stonewall 
  Valley 
  and 
  both 
  north 
  and 
  south 
  of 
  it, 
  I 
  sup- 
  

   pose 
  to 
  belong 
  to 
  this 
  group. 
  Although 
  I 
  had 
  occasion 
  to 
  traverse 
  that 
  

   particular 
  region 
  very 
  carefully, 
  I 
  did 
  not 
  succeed 
  in 
  finding 
  more 
  than 
  

   one 
  locality 
  where 
  the 
  question 
  was 
  answered 
  beyond 
  all 
  doubt. 
  For- 
  

   tunately, 
  this 
  is 
  in 
  such 
  close 
  proximity 
  to 
  the 
  questionable 
  areas, 
  that 
  I 
  

   Lave 
  drawn 
  my 
  deductions, 
  and 
  have 
  considered 
  myself 
  justified 
  in 
  so 
  

   doing, 
  mainly 
  from 
  the 
  observations 
  made 
  there. 
  It 
  has 
  been 
  mentioned 
  

   that 
  the 
  yellow 
  sandstone 
  bluffs 
  at 
  the 
  eastern 
  edge 
  of 
  the 
  valley 
  contain 
  

   strata 
  resting 
  apparently 
  unconformably 
  upon 
  those 
  which 
  are 
  regarded 
  

   as 
  Cretaceous. 
  Station 
  125 
  is 
  located 
  about 
  a 
  mile 
  south 
  of 
  the 
  South 
  

   Purgatory, 
  on 
  one 
  of 
  the 
  younger 
  bluffs. 
  While 
  the 
  strata, 
  both 
  of 
  

   sandstone 
  and 
  shale, 
  of 
  the 
  latter 
  are 
  characterized 
  by 
  light 
  colors 
  of 
  

   great 
  uniformity, 
  I 
  observed 
  on 
  the 
  river 
  itself, 
  in 
  its 
  valley, 
  a 
  dark-grey 
  

   shale, 
  protected 
  from 
  erosion 
  by 
  several 
  basaltic 
  dikes. 
  Upon 
  examin- 
  

   ation 
  this 
  shale 
  proved 
  to 
  have 
  a 
  dip 
  of 
  about 
  23° 
  east, 
  and 
  to 
  contain 
  

   Inoceramus 
  and 
  Ostrea 
  complexa. 
  Its 
  identity, 
  therefore, 
  was 
  established. 
  

   Further 
  investigation 
  showed 
  that 
  it 
  was 
  merely 
  the 
  continuation 
  of 
  the 
  

   shales 
  in 
  the 
  valley 
  that 
  had 
  furnished 
  such 
  unsatisfactory 
  exposures. 
  

   It 
  was 
  apparently 
  unconformable 
  with 
  the 
  overlying 
  rocks, 
  but 
  con- 
  

   formable 
  with 
  the 
  gradually 
  diminishing 
  dip 
  of 
  the 
  shales 
  immediately 
  

   west 
  of 
  it, 
  and 
  in 
  direct 
  connection 
  with 
  them. 
  This 
  is 
  the 
  only 
  well- 
  

   established 
  occurrence 
  of 
  Cretaceous 
  No. 
  2 
  that 
  I 
  can 
  cite 
  for 
  the 
  valley 
  

   or 
  its 
  immediate 
  vicinity. 
  Observation, 
  however, 
  has 
  shown 
  that 
  the 
  

   character 
  of 
  this 
  depression 
  is 
  very 
  much 
  in 
  accordance 
  with 
  those 
  

   generally 
  formed 
  in 
  the 
  readily 
  yielding 
  shales 
  of 
  this 
  group, 
  and 
  in 
  my 
  

   own 
  mind 
  I 
  have 
  no 
  doubt 
  that 
  the 
  assumption 
  of 
  its 
  belonging 
  to 
  that 
  

   series 
  is 
  correct. 
  

  

  On 
  the 
  Huerfano 
  the 
  development 
  of 
  No. 
  2 
  is 
  by 
  far 
  greater. 
  Eeferring 
  

   to 
  section 
  VII, 
  two 
  strata 
  of 
  the 
  shales 
  will 
  be 
  noticed 
  — 
  the 
  one 
  {d) 
  of 
  a 
  

   yellowish 
  to 
  greenish 
  color; 
  the 
  upper 
  one 
  [d') 
  reddish. 
  Above 
  them 
  is 
  

   a 
  sandstone 
  (c) 
  containing 
  fragments 
  of 
  Inocerami 
  and 
  Ostrew. 
  This 
  por- 
  

   tion 
  of 
  the 
  Cretaceous 
  was 
  deposited 
  without 
  any 
  intermission 
  of 
  time 
  

   after 
  the 
  lower 
  sandstones, 
  because 
  no 
  erosion 
  of 
  the 
  latter 
  seems 
  to 
  

   have 
  taken 
  place. 
  A 
  large 
  bay 
  was 
  formed 
  by 
  the 
  waters 
  depositing 
  

   No. 
  2, 
  which 
  extended 
  for 
  a 
  considerable 
  distance 
  up 
  the 
  Huerfano. 
  At 
  

  

  