﻿186 
  EEPOKT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  of 
  the 
  San 
  Juan. 
  Inocerami 
  and 
  Ostrece 
  are 
  found 
  abundantly 
  in 
  

   them. 
  They 
  dip 
  both 
  north 
  and 
  south 
  at 
  angles 
  of 
  9°. 
  Above 
  them 
  

   occurs 
  a 
  stratum 
  (&,) 
  consisting 
  of 
  yellowish, 
  shaly 
  sandstone, 
  with 
  

   indistinct 
  remains 
  of 
  plants. 
  It 
  is 
  about 
  250 
  feet 
  in 
  thickness, 
  dipping 
  

   in 
  both 
  directions, 
  as 
  the 
  underlying 
  shales, 
  but 
  at 
  a 
  decreased 
  angle» 
  

   This 
  1 
  regard 
  as 
  the 
  same 
  that 
  we 
  found 
  on 
  station 
  47 
  of 
  1874,* 
  where 
  it 
  

   contained 
  innumerable 
  fragments 
  of 
  Inoceranms. 
  Overlaying 
  the 
  sand- 
  

   stone 
  there 
  are 
  850 
  feet 
  of 
  yellow-grey 
  shales, 
  sandy 
  in 
  part 
  (c), 
  that 
  I 
  

   regard 
  as 
  belonging 
  to 
  the 
  Colorado 
  group, 
  having; 
  found 
  fossils 
  charac- 
  

   teristic 
  of 
  that 
  horizon. 
  This 
  in 
  turn 
  is 
  capped 
  by 
  250 
  to 
  300 
  feet 
  of 
  a 
  

   yellow 
  to 
  reddish 
  sandstone 
  (d), 
  forming 
  a 
  prominent 
  bluff, 
  upon 
  which 
  

   fetation 
  75 
  was 
  located, 
  at 
  an 
  absolute 
  elevation 
  of 
  8,064 
  feet. 
  Here 
  the 
  

   southerly 
  dip 
  has 
  decreased 
  to 
  6°. 
  With 
  this 
  sandstone 
  I 
  commence 
  

   the 
  Fox 
  Hills 
  group. 
  At 
  the 
  north 
  end 
  of 
  the 
  section, 
  trachytic 
  con- 
  

   glomerate 
  (e) 
  overlies 
  the 
  shales, 
  and 
  trachyte 
  (/) 
  covers 
  the 
  former. 
  

   Traveling 
  northward 
  from 
  the 
  junction 
  of 
  Rio 
  Blanco 
  with 
  the 
  San 
  

   Juan, 
  I 
  at 
  first 
  assumed 
  the 
  occurrence 
  of 
  the 
  two 
  groups 
  of 
  shale, 
  a 
  

   and 
  G 
  to 
  indicate 
  a 
  fault. 
  Having 
  passed 
  the 
  anticlinal 
  axis, 
  however, 
  

   in 
  the 
  lower 
  shales, 
  I 
  found 
  the 
  series 
  c 
  repeated 
  farther 
  north, 
  and 
  no 
  

   evidence 
  of 
  any 
  other 
  disturbance 
  than 
  that 
  produced 
  by 
  the 
  fold. 
  On 
  

   the 
  San 
  Juan, 
  below 
  its 
  junction 
  with 
  the 
  Blanco, 
  this 
  section 
  can 
  be 
  

   studied 
  with 
  greater 
  success, 
  as 
  the 
  beds 
  there 
  become 
  nearly 
  horizon- 
  

   tal, 
  dipping 
  southward 
  only 
  3°. 
  Local 
  disturbances, 
  produced, 
  probably, 
  

   by 
  subsidences, 
  occur 
  quite 
  frequently 
  ; 
  are 
  of 
  small 
  extent 
  however. 
  

  

  Below 
  the 
  junction 
  of 
  the 
  two 
  rivers 
  the 
  beds 
  above 
  the 
  lower 
  shales 
  

   increase 
  in 
  thickness. 
  A 
  section 
  constructed 
  there 
  would 
  show 
  the 
  fol- 
  

   lowing 
  result: 
  

   4. 
  Yellow 
  and 
  white 
  sandstones. 
  Lower 
  strata 
  massive, 
  

  

  white 
  ; 
  upper 
  interstratified 
  with 
  shaly 
  sandstone 
  and 
  

  

  shales, 
  yellow 
  350 
  feet. 
  

  

  3. 
  Greyish-brown 
  and 
  yellowish 
  shales, 
  containing 
  Inoce- 
  

  

  ramus 
  and 
  Ostrea 
  within 
  400 
  of 
  the 
  ujjper 
  sandstone 
  1, 
  200 
  feet. 
  

  

  2. 
  Yellow 
  sandstone, 
  fine 
  to 
  middle-grained, 
  interbedded 
  

  

  with 
  thin 
  strata 
  of 
  shales 
  ... 
  230 
  feet. 
  

  

  1. 
  Dark 
  grey 
  shales, 
  weathering 
  lighter, 
  containing 
  fossils. 
  . 
  1, 
  100 
  feet. 
  

  

  We 
  have, 
  therefore, 
  in 
  this 
  region 
  a 
  total 
  thickness 
  of 
  about 
  2,500 
  

   feet 
  for 
  the 
  Colorado 
  group. 
  This 
  is 
  greater 
  than 
  farther 
  west, 
  but 
  it 
  

   has 
  frequently 
  been 
  observed 
  that 
  shales 
  of 
  this 
  character 
  either 
  grad- 
  

   ually 
  merge 
  into 
  sandstones, 
  or 
  that 
  sandstones, 
  losing 
  their 
  character 
  

   as 
  such, 
  may 
  turn 
  into 
  shales. 
  Difference 
  of 
  thickness, 
  even 
  within 
  

   comparatively 
  limited 
  districts, 
  can 
  consequently 
  not 
  be 
  regarded 
  as 
  a 
  

   safe 
  criterion 
  upon 
  which 
  to 
  base 
  or 
  reject 
  identification. 
  

  

  Leaving 
  Bio 
  Blanco, 
  and 
  traveling 
  southeast, 
  we 
  remain 
  within 
  the 
  

   borders 
  of 
  the 
  Colorado 
  group. 
  Station 
  63 
  is 
  located 
  east 
  of 
  Kio 
  

   Navajo, 
  on 
  a 
  remnant 
  of 
  trachorheites, 
  that 
  at 
  one 
  time 
  was 
  in 
  connec- 
  

   tion 
  with 
  the 
  main 
  body 
  farther 
  east. 
  It 
  has 
  an 
  altitude 
  of 
  9,905 
  feet, 
  

   and 
  is 
  surrounded 
  by 
  Cretaceous 
  beds. 
  A 
  section 
  (Section 
  XI), 
  taken 
  

   from 
  the 
  ridge 
  on 
  the 
  south 
  side 
  of 
  the 
  Navajo 
  through 
  station 
  63, 
  shows 
  

   an 
  anticlinal 
  fold 
  very 
  similar 
  to 
  that 
  observed 
  west 
  of 
  Eio 
  Blanco. 
  

   As 
  there, 
  the 
  Lower 
  Colorado 
  shales 
  (a) 
  form 
  the 
  base 
  of 
  the 
  valley, 
  dip- 
  

   ping 
  both 
  north 
  and 
  south 
  at 
  angles 
  of 
  8° 
  to 
  10°. 
  Overlying 
  them 
  we 
  

   find 
  a 
  yellow 
  sandstone 
  (&) 
  about 
  200 
  feet 
  in 
  thickness, 
  which 
  in 
  turn 
  

   is 
  covered 
  by 
  yellowish-grey, 
  partly 
  sandy 
  shales 
  (c). 
  Upon 
  these, 
  

   which 
  are 
  capped 
  unconformably 
  by 
  the 
  volcanic 
  rock 
  {d) 
  of 
  station 
  63, 
  

  

  * 
  Keport 
  United 
  States 
  Geoloo^ical 
  Survey, 
  1874, 
  Section 
  VI, 
  p. 
  224. 
  

  

  