﻿ECTLicn.] 
  SAN 
  JUAN 
  EEGION 
  COLORADO 
  GROUP. 
  18,3 
  

  

  of 
  our 
  district. 
  It 
  seems, 
  judging 
  from 
  what 
  was 
  seen 
  from 
  some 
  dis- 
  

   tance, 
  to 
  extend 
  farther 
  south, 
  in 
  the 
  same 
  relative 
  position. 
  At 
  the 
  

   base 
  of 
  station 
  94 
  it 
  dips 
  steeply 
  to 
  the 
  westward, 
  but 
  is 
  soon 
  covered 
  

   by 
  the 
  grey 
  shales 
  which 
  form 
  the 
  level 
  of 
  the 
  valley. 
  Agricultural 
  

   pursuits 
  are 
  followed 
  by 
  the 
  Mexicans 
  who 
  have 
  settled 
  the 
  region, 
  and 
  

   we 
  were 
  informed 
  that 
  the 
  rich 
  soil 
  was 
  productive 
  of 
  good 
  results. 
  A 
  

   sufficiently 
  large 
  supply 
  of 
  water 
  is 
  at 
  hand 
  to 
  answer 
  for 
  purposes 
  of 
  

   irrigation. 
  

  

  Colorado 
  group. 
  — 
  This 
  group 
  is 
  by 
  far 
  more 
  varied 
  in 
  its 
  occurrence 
  

   as 
  well 
  as 
  in 
  its 
  stratigraphy 
  than 
  the 
  preceding 
  one. 
  It 
  covers 
  a 
  large 
  

   area 
  and 
  displays 
  many 
  features 
  of 
  interest 
  to 
  a 
  geologist. 
  In 
  1874* 
  

   the 
  sbales 
  belonging 
  to 
  this 
  division 
  of 
  the 
  Cretaceous 
  were 
  first 
  found 
  

   on 
  Junction 
  Creek. 
  From 
  there 
  they 
  extend 
  eastward, 
  formingin 
  the 
  region 
  

   ofthe 
  Nutria, 
  San 
  Juan, 
  and 
  Blanco, 
  the 
  bases 
  of 
  the 
  broad 
  valleys 
  through 
  

   which 
  these 
  streams 
  flow. 
  It 
  has 
  been 
  mentioned 
  above 
  that 
  the 
  shales 
  

   extend 
  to 
  the 
  very 
  edge 
  of 
  the 
  mountains 
  at 
  a 
  number 
  of 
  points, 
  and 
  

   directly 
  underlie 
  the 
  trachyte, 
  in 
  that 
  case 
  excluding 
  exposures 
  of 
  any 
  

   of 
  the 
  lower 
  strata. 
  A 
  general 
  southerly 
  dip 
  of 
  the 
  beds 
  can 
  be 
  ob- 
  

   served, 
  but 
  quite 
  a 
  number 
  of 
  minor 
  disturbances 
  have 
  locally 
  changed 
  

   this. 
  A 
  general 
  shallow 
  fold, 
  extending 
  from 
  west 
  to 
  east, 
  which 
  in- 
  

   volves 
  these 
  shales, 
  will 
  be 
  discussed 
  in 
  the 
  subjoined 
  pages. 
  Although 
  

   its 
  dimensions 
  vertically 
  are 
  but 
  small, 
  the 
  effect 
  it 
  has 
  had 
  upon 
  the 
  

   entire 
  region 
  cannot 
  be 
  overlooked, 
  and 
  the 
  knowledge 
  of 
  its 
  existence 
  

   is 
  necessary 
  for 
  the 
  proper 
  understanding 
  of 
  facts 
  observed. 
  

  

  We 
  first 
  found 
  the 
  shales 
  of 
  this 
  group 
  overlying 
  the 
  Dakota 
  sand- 
  

   stone 
  south 
  of 
  station 
  36. 
  They 
  are 
  exposed 
  all 
  along 
  the 
  valleys 
  and 
  

   low 
  divides 
  separating 
  the 
  northern 
  tributaries 
  of 
  the 
  Piedra, 
  west 
  of 
  

   station 
  3G, 
  Dark 
  grey 
  when 
  Iresh, 
  they 
  change 
  to 
  almost 
  white 
  upon 
  

   being 
  subjected 
  for 
  a 
  long 
  time 
  to 
  atmospheric 
  influences. 
  Efflorescence 
  

   of 
  alkali 
  at 
  many 
  localities 
  produces 
  " 
  saltlicks," 
  the 
  favorite 
  resort 
  for 
  

   game. 
  Station 
  37, 
  which 
  is 
  located 
  upon 
  an 
  isolated 
  remnant 
  of 
  the 
  

   trachyte 
  farther 
  east, 
  is 
  entirely 
  surrounded 
  by 
  these 
  shales; 
  creeks 
  

   and 
  streams 
  cut 
  down 
  steeply 
  into 
  the 
  yielding 
  material, 
  producing 
  pre- 
  

   cipitous 
  banks. 
  Frequently 
  very 
  fine 
  carving, 
  by 
  water, 
  can 
  be 
  observed 
  

   on 
  the 
  sides 
  of 
  blufi's 
  or 
  hills, 
  resembling 
  en 
  miniature 
  regular 
  mount- 
  

   ain 
  ridges. 
  Slight 
  changes 
  of 
  color, 
  varying 
  from 
  different 
  shades 
  of 
  

   grey 
  to 
  brown, 
  produce 
  in 
  such 
  an 
  instance 
  an 
  exceedingly 
  picturesque 
  

   effect. 
  Inoeerami 
  and 
  Ostrece 
  occur 
  in 
  abundance 
  in 
  the 
  shales 
  all 
  along 
  

   its 
  outcrop, 
  the 
  former, 
  however, 
  most 
  frequently 
  in 
  fragments 
  only. 
  

  

  Station 
  40, 
  8,374 
  feet 
  above 
  sea-level, 
  is 
  located 
  south 
  of 
  the 
  Piedra, 
  

   on 
  a 
  small 
  knoll 
  composed 
  of 
  Colorado 
  shales. 
  From 
  there 
  they 
  extend 
  

   westward 
  to 
  the 
  wooded 
  ridge 
  east 
  of 
  Piedra, 
  overlying 
  Dakota 
  sand- 
  

   stone, 
  which 
  composes 
  the 
  latter. 
  A 
  dip 
  of 
  4° 
  to 
  6° 
  was 
  noticeable 
  to 
  

   the 
  eastward, 
  forming 
  most 
  likely 
  a 
  shallow 
  synclinal, 
  the 
  eastern 
  edge 
  

   of 
  which 
  is 
  obscured 
  both 
  by 
  the 
  drift 
  at 
  the 
  base 
  of 
  the 
  mountains 
  and 
  

   by 
  the 
  superincumbent 
  volcanics. 
  Fossils 
  were 
  found 
  here 
  identifying 
  

   the 
  strata. 
  Crossing 
  a 
  low 
  divide 
  we 
  reach 
  Kio 
  Nutria, 
  and 
  find 
  that 
  it 
  

   flows 
  entirely 
  in 
  these 
  shales. 
  Isolated 
  strata 
  of 
  sandstone 
  occur 
  in 
  

   their 
  upper 
  members 
  and 
  form 
  cappings 
  for 
  low 
  bluff's 
  and 
  tables 
  that 
  

   occur 
  along 
  the 
  higher 
  portions 
  of 
  the 
  region. 
  About 
  nine 
  miles 
  south- 
  

   east 
  of 
  station 
  41 
  we 
  strike 
  the 
  Eio 
  San 
  Juan. 
  At 
  that 
  point 
  Pagosa 
  

   Springs 
  are 
  located. 
  Among 
  the 
  Indians 
  of 
  the 
  region 
  the 
  waters 
  of 
  

   these 
  springs 
  have 
  become 
  very 
  famous. 
  Their 
  high 
  temperature, 
  per- 
  

   haps, 
  too, 
  the 
  mineral 
  constituents 
  they 
  hold 
  in 
  solution, 
  have 
  won 
  for 
  

  

  * 
  Report 
  United 
  States 
  Geological 
  Survey 
  1874, 
  p. 
  224. 
  

  

  