﻿260 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  lying 
  agaiust 
  the 
  steep 
  slopes 
  north 
  of 
  the 
  Mancos 
  trail. 
  Farther 
  around 
  

   to 
  the 
  west, 
  about 
  the 
  sources 
  of 
  the 
  Rio 
  Mancos, 
  they 
  pass 
  beneath 
  the 
  

   black 
  shales 
  and 
  the 
  masses 
  of 
  trachyte, 
  and 
  are 
  only 
  seen 
  where 
  the 
  

   canons 
  penetrate 
  deepest. 
  

  

  West 
  of 
  the 
  Mancos, 
  where 
  the 
  conditions 
  of 
  the 
  strata 
  are 
  so 
  uniform, 
  

   it 
  will 
  not 
  be 
  necessary 
  to 
  speak 
  in 
  great 
  detail. 
  I 
  shall, 
  therefore, 
  de- 
  

   scribe 
  them 
  briefly 
  in 
  connection 
  with 
  the 
  several 
  valleys 
  in 
  which 
  they 
  

   occur. 
  

  

  On 
  the 
  San 
  Juan, 
  at 
  the 
  mouth 
  of 
  Red 
  Creek, 
  the 
  Dakota 
  sandstones 
  

   come 
  to 
  the 
  surface 
  and 
  rise 
  with 
  a 
  slight 
  inclination 
  toward 
  the 
  Sierra 
  

   Oarriso. 
  At 
  the 
  mouth 
  of 
  the 
  Mancos, 
  eight 
  miles 
  below, 
  they 
  crop 
  out 
  

   in 
  the 
  high 
  bluffs. 
  The 
  section 
  at 
  this 
  point 
  consists 
  of 
  about 
  200 
  feet 
  

   of 
  sandstones, 
  apparently 
  of 
  less 
  compact 
  structure 
  than 
  usual 
  ; 
  many 
  

   beds 
  of 
  shale 
  appear, 
  and 
  some 
  irregular 
  seams 
  of 
  coal. 
  

  

  The 
  Rio 
  Mancos, 
  after 
  entering 
  the 
  alluvial 
  valley 
  of 
  the 
  San 
  Juan, 
  

   flows 
  for 
  some 
  distance 
  along 
  the 
  base 
  of 
  a 
  steep 
  bluff, 
  in 
  the 
  lower 
  part 
  

   of 
  which 
  there 
  is 
  an 
  exposure 
  of 
  brilliantly 
  colored 
  beds, 
  consisting 
  of 
  

   shales, 
  marls, 
  and 
  many 
  thin 
  seams 
  of 
  hard 
  clay-slate 
  and 
  flne-grained 
  

   purple 
  quartzite. 
  

  

  From 
  the 
  mouth 
  of 
  the 
  Mancos 
  to 
  the 
  western 
  border 
  of 
  our 
  district, 
  

   a 
  distance 
  by 
  way 
  of 
  the 
  river 
  of 
  some 
  forty 
  miles, 
  I 
  found 
  the 
  upper 
  

   beds 
  of 
  the 
  Dakota 
  sandstones 
  capping 
  the 
  bluffs 
  on 
  all 
  sides. 
  There 
  is 
  

   no 
  perceptible 
  dip, 
  and 
  I 
  was 
  somewhat 
  astonished 
  to 
  find 
  that 
  the 
  dif- 
  

   ference 
  of 
  level 
  between 
  the 
  upper 
  surface 
  of 
  the 
  sandstones 
  at 
  Station 
  

   56, 
  near 
  the 
  Mancos, 
  and 
  Station 
  45, 
  on 
  our 
  western 
  line, 
  was 
  only 
  200 
  

   feet, 
  the 
  western 
  one 
  being 
  that 
  many 
  feet 
  higher. 
  The 
  dip 
  to 
  the 
  east, 
  

   therefore, 
  amounts 
  to 
  200 
  feet 
  in 
  40 
  miles. 
  The 
  river 
  in 
  that 
  distance 
  

   descends 
  500 
  feet, 
  so 
  that 
  there 
  are 
  500 
  feet 
  of 
  additional 
  strata 
  exposed. 
  

  

  The 
  section 
  taken 
  at 
  Station 
  45 
  is 
  as 
  follows 
  : 
  beginning 
  with 
  the 
  upper 
  

   stratum 
  of 
  the 
  outstanding 
  butte 
  on 
  which 
  the 
  station 
  was 
  made, 
  and 
  

   nearly 
  on 
  a 
  level 
  with 
  the 
  surface 
  of 
  the 
  Great 
  Sage 
  Plain 
  which 
  lies 
  to 
  

   the 
  north, 
  we 
  have 
  first 
  60 
  to 
  80 
  feet 
  of 
  yellowish, 
  moderately 
  hard 
  sili- 
  

   ceous 
  sandstone, 
  changing 
  into 
  a 
  massive, 
  closely 
  compacted 
  conglomer- 
  

   ate 
  at 
  the 
  base. 
  Following 
  this 
  are 
  200 
  to 
  300 
  feet 
  of 
  variegated 
  shales 
  

   and 
  marls, 
  with 
  thin 
  seams 
  of 
  argillaceous 
  slate 
  and 
  concretionary 
  layers 
  

   of 
  impure 
  limestone. 
  At 
  the 
  base, 
  these 
  thinly 
  laminated 
  beds 
  are 
  re- 
  

   placed 
  by 
  soft 
  sandstones, 
  and 
  following 
  these 
  is 
  a 
  series 
  of 
  rocks 
  some 
  

   500 
  feet 
  in 
  thickness 
  that 
  xjresents 
  many 
  curious 
  charaicters. 
  Sand- 
  

   stones, 
  marls, 
  clays, 
  shales, 
  and 
  conglomerates 
  are 
  thrown 
  together 
  

   without 
  apparent 
  regularity. 
  A 
  massive 
  sandstone, 
  20 
  or 
  30 
  feet 
  in 
  

   thickness, 
  will 
  often 
  be 
  found, 
  if 
  followed 
  a 
  few 
  yards 
  horizontally, 
  to 
  

   change 
  suddenly 
  into 
  an 
  uncompacted 
  mass 
  of 
  most 
  uncertain 
  composi- 
  

   tion, 
  that 
  weathers 
  into 
  a 
  thousand 
  fantastic 
  forms 
  and 
  presents 
  the 
  

   colors 
  of 
  the 
  rainbow. 
  The 
  color, 
  composition, 
  and 
  style 
  of 
  weathering 
  

   seem 
  to 
  change 
  with 
  every 
  turn 
  in 
  the 
  river. 
  

  

  Beneath 
  these 
  beds 
  come 
  a 
  series 
  of 
  laminated 
  sandstones 
  about 
  200 
  

   feet 
  in 
  thickness. 
  They 
  have 
  generally 
  a 
  dull 
  purplish 
  hue, 
  and 
  are 
  

   very 
  impure 
  and 
  coarse. 
  There 
  seems 
  to 
  be 
  a 
  very 
  regular 
  alternation 
  

   of 
  hard 
  and 
  soft 
  layers, 
  so 
  that 
  from 
  top 
  to 
  bottom 
  there 
  is 
  a 
  succession 
  

   of 
  prominent 
  edges 
  and 
  deep 
  grooves. 
  These 
  beds 
  lie 
  upon 
  a 
  massive 
  

   red 
  or 
  pinkish 
  sandstone, 
  which 
  extends 
  to 
  the 
  river-bed. 
  

  

  The 
  upper 
  member 
  of 
  this 
  section 
  is 
  the 
  strongest 
  layer 
  of 
  the 
  almost 
  

   universally 
  recognized 
  No. 
  1 
  Cretaceous. 
  The 
  variegated 
  series 
  which 
  

   succeeds 
  it 
  I 
  at 
  first 
  felt 
  inclined 
  to 
  call 
  Jurassic, 
  since 
  it 
  resembles 
  so 
  

   closely 
  the 
  variegated 
  beds 
  that 
  on 
  the 
  eastern 
  slopes 
  of 
  the 
  mountains 
  

   have 
  usually 
  been 
  credited 
  to 
  that 
  age. 
  I 
  observe, 
  however, 
  that, 
  

  

  